Projection of X- and Y-cells of the cat's lateral geniculate nucleus to areas 17 and 18 of visual cortex

Characterization of extracellular spike waveforms recorded in wallaby primary visual cortex

Extracellular recordings were made from 642 units in the primary visual cortex (V1) of a highly visual marsupial, the Tammar wallaby. The receptive field (RF) characteristics of the cells were objectively estimated using the non-linear input model (NIM), and these were correlated with spike shapes. We found that wallaby cortical units had 68% regular spiking (RS), 12% fast spiking (FS), 4% triphasic spiking (TS), 5% compound spiking (CS) and 11% positive spiking (PS). RS waveforms are most often associated with recordings from pyramidal or spiny stellate cell bodies, suggesting that recordings from these cell types dominate in the wallaby cortex. In wallaby, 70-80% of FS and RS cells had orientation selective RFs and had evenly distributed linear and nonlinear RFs. We found that 47% of wallaby PS units were non-orientation selective and they were dominated by linear RFs. Previous studies suggest that the PS units represent recordings from the axon terminals of non-orientation selective cells originating in the lateral geniculate nucleus (LGN). If this is also true in wallaby, as strongly suggested by their low response latencies and bursty spiking properties, the results suggest that significantly more neurons in wallaby LGN are already orientation selective. In wallaby, less than 10% of recorded spikes had triphasic (TS) or sluggish compound spiking (CS) waveforms. These units had a mixture of orientation selective and non-oriented properties, and their cellular origins remain difficult to classify.

Immediate Impact of Acute Elevation of Intraocular Pressure on Cortical Visual Motion Processing

Purpose

To physiologically examine the impairment of cortical sensitivity to visual motion during acute elevation of intraocular pressure (IOP).

Methods

Motion processing in the cat brain is well characterized, its X and Y cell visual pathways being functionally analogous to parvocellular and magnocellular pathways in primates. Using this model, we performed ocular anterior chamber perfusion to reversibly elevate IOP over a range from 30 to 90 mm Hg while monitoring cortical activity with intrinsic signal optical imaging. Drifting random-dot fields and gratings were used to characterize cortical population responses to motion direction and orientation in early visual areas 17 and 18.

Results

We found that acute IOP elevations at 50 mm Hg and above, which is often observed in acute glaucoma, suppressed cortical motion direction responses. This suppression was more profound in area 17 than in area 18, and more profound in central than peripheral visual field (eccentricities 0°–4° vs. 4°–8°) within area 17. In addition, orientation responses were more suppressed than motion direction responses for the same IOP modulation.

Conclusions

In contrast to human chronic glaucoma that may cause greater dysfunction in large-cell magnocellular than in small-cell parvocellular visual pathways, our direct measurement of cortical processing networks implies that the small X-cell pathway shows greater vulnerability to acute IOP elevation than the large Y-cell pathway in visual motion processing. The results demonstrate that fine discrimination mechanisms for motion in the central visual field are particularly impacted by acute IOP attacks, suggesting a neural basis for immediate visual deficits in the fine motion perception of acute glaucoma patients.

Local organization of spatial frequency tuning dynamics in the cat visual areas 17 and 18

Spatial frequency (SF) is a prominent feature to which most neurons in cat areas 17 and 18 (area 17/18) exhibit tuning selectivity. Previous studies have shown that neurons with similar SF tunings are locally clustered into SF preference domains. However, the functional organization of SF tuning remains not fully understood. Neurons in these areas show a variety of SF tuning dynamics; however, it is unknown how neurons with diverse dynamics are locally organized to form the population dynamics of the domains. The laminar organization of SF dynamics is also unknown, knowledge of which may be useful for determining how SF tuning dynamics of cat area 17/18 neurons arise in cortical circuits. To address these issues, we recorded the activities of multiple neurons in the cat area 17/18 using microelectrode arrays and characterized the time courses of the SF tunings of these neurons by a subspace reverse correlation. A wide range of SF dynamics was already present in the input layer, suggesting that intracortical mechanisms contribute to generating SF dynamics inside this layer but do not further shape it outside this layer. Local neuronal pools with similar SF tunings contained diverse SF dynamics. The average preferred SF of a pool similarly increased with response time. Moreover, the range of single-neuron preferred SFs in a pool tended to increase with time. Our results suggest that, in the presence of organized tuning diversity within an SF domain, the cortical SF organization remains stable during response time in cat area 17/18.NEW & NOTEWORTHY In cat area 17/18, we found that a local pool of neurons with similar spatial frequency (SF) tunings shows diverse but organized dynamics. Our results suggest that, in the presence of organized tuning diversity within an SF domain, the cortical SF organization remains stable over response time in these areas. Laminar analysis suggests that intracortical mechanisms contribute to generating SF dynamics inside the input layer but do not further shape it outside this layer.

Nonlinear Y-Like Receptive Fields in the Early Visual Cortex: An Intermediate Stage for Building Cue-Invariant Receptive Fields from Subcortical Y Cells

Many of the neurons in early visual cortex are selective for the orientation of boundaries defined by first-order cues (luminance) as well as second-order cues (contrast, texture). The neural circuit mechanism underlying this selectivity is still unclear, but some studies have proposed that it emerges from spatial nonlinearities of subcortical Y cells. To understand how inputs from the Y-cell pathway might be pooled to generate cue-invariant receptive fields, we recorded visual responses from single neurons in cat Area 18 using linear multielectrode arrays. We measured responses to drifting and contrast-reversing luminance gratings as well as contrast modulation gratings. We found that a large fraction of these neurons have nonoriented responses to gratings, similar to those of subcortical Y cells: they respond at the second harmonic (F2) to high-spatial frequency contrast-reversing gratings and at the first harmonic (F1) to low-spatial frequency drifting gratings ("Y-cell signature"). For a given neuron, spatial frequency tuning for linear (F1) and nonlinear (F2) responses is quite distinct, similar to orientation-selective cue-invariant neurons. Also, these neurons respond to contrast modulation gratings with selectivity for the carrier (texture) spatial frequency and, in some cases, orientation. Their receptive field properties suggest that they could serve as building blocks for orientation-selective cue-invariant neurons. We propose a circuit model that combines ON- and OFF-center cortical Y-like cells in an unbalanced push-pull manner to generate orientation-selective, cue-invariant receptive fields.

SIGNIFICANCE STATEMENT

A significant fraction of neurons in early visual cortex have specialized receptive fields that allow them to selectively respond to the orientation of boundaries that are invariant to the cue (luminance, contrast, texture, motion) that defines them. However, the neural mechanism to construct such versatile receptive fields remains unclear. Using multielectrode recording, we found a large fraction of neurons in early visual cortex with receptive fields not selective for orientation that have spatial nonlinearities like those of subcortical Y cells. These are strong candidates for building cue-invariant orientation-selective neurons; we present a neural circuit model that pools such neurons in an imbalanced "push-pull" manner, to generate orientation-selective cue-invariant receptive fields.

Optic nerve, superior colliculus, visual thalamus, and primary visual cortex of the northern elephant seal (Mirounga angustirostris) and California sea lion (Zalophus californianus)

The northern elephant seal (Mirounga angustirostris) and California sea lion (Zalophus californianus) are members of a diverse clade of carnivorous mammals known as pinnipeds. Pinnipeds are notable for their large, ape-sized brains, yet little is known about their central nervous system. Both the northern elephant seal and California sea lion spend most of their lives at sea, but each also spends time on land to breed and give birth. These unique coastal niches may be reflected in specific evolutionary adaptations to their sensory systems. Here, we report on components of the visual pathway in these two species. We found evidence for two classes of myelinated fibers within the pinniped optic nerve, those with thick myelin sheaths (elephant seal: 9%, sea lion: 7%) and thin myelin sheaths (elephant seal: 91%, sea lion: 93%). In order to investigate the architecture of the lateral geniculate nucleus, superior colliculus, and primary visual cortex, we processed brain sections from seal and sea lion pups for Nissl substance, cytochrome oxidase, and vesicular glutamate transporters. As in other carnivores, the dorsal lateral geniculate nucleus consisted of three main layers, A, A1, and C, while each superior colliculus similarly consisted of seven distinct layers. The sea lion visual cortex is located at the posterior side of cortex between the upper and lower banks of the postlateral sulcus, while the elephant seal visual cortex extends far more anteriorly along the dorsal surface and medial wall. These results are relevant to comparative studies related to the evolution of large brains.

Neural Processing of Second-Order Motion in the Suprasylvian Cortex of the Cat

Neuronal responses to second-order motion, that is, to spatiotemporal variations of texture or contrast, have been reported in several cortical areas of mammals, including the middle-temporal (MT) area in primates. In this study, we investigated whether second-order responses are present in the cat posteromedial lateral suprasylvian (PMLS) cortex, a possible homolog of the primate area MT. The stimuli used were luminance-based sine-wave gratings (first-order) and contrast-modulated carrier stimuli (second-order), which consisted of a high-spatial-frequency static grating (carrier) whose contrast was modulated by a low-spatial-frequency drifting grating (envelope). Results indicate that most PMLS neurons responded to second-order motion and for the vast majority of cells, first- and second-order preferred directions were conserved. However, responses to second-order stimuli were significantly reduced when compared to those evoked by first-order gratings. Circular variance was increased for second-order stimuli, indicating that PMLS direction selectivity was weaker for this type of stimulus. Finally, carrier orientation selectivity was either absent or very broad and had no influence on the envelope's orientation selectivity. In conclusion, our data show that PMLS neurons exhibit similar first- and second-order response profiles and that, akin primate area MT cells, they perform a form-cue invariant analysis of motion signals.

Attention-Deficit Hyperactivity Disorder-Like Traits and Distractibility in the Visual Periphery

We examined the performance of nonclinical subjects with high and low levels of self-reported attention-deficit hyperactivity disorder (ADHD)-like traits in a novel distractibility paradigm with far peripheral visual distractors, the likely origin of many distractors in everyday life. Subjects were tested on a Sustained Attention to Response Task with distractors appearing before some of the target or nontarget stimuli. When the distractors appeared 80 ms before the targets or nontargets, participants with high levels of ADHD-like traits were less affected in their reaction times than those with lower levels. Reducing the distractor-target or nontarget interval to 10 ms removed the reaction time advantage for the high group. We suggest that at 80 ms, the distractors were cueing the arrival of the target or nontarget, and that those with high levels of ADHD-like traits were more sensitive to the cues. Increased sensitivity to stimuli in the visual periphery is consistent with hyperresponsiveness at the level of the superior colliculus.

Are visual peripheries forever young?

The paper presents a concept of lifelong plasticity of peripheral vision. Central vision processing is accepted as critical and irreplaceable for normal perception in humans. While peripheral processing chiefly carries information about motion stimuli features and redirects foveal attention to new objects, it can also take over functions typical for central vision. Here I review the data showing the plasticity of peripheral vision found in functional, developmental, and comparative studies. Even though it is well established that afferent projections from central and peripheral retinal regions are not established simultaneously during early postnatal life, central vision is commonly used as a general model of development of the visual system. Based on clinical studies and visually deprived animal models, I describe how central and peripheral visual field representations separately rely on early visual experience. Peripheral visual processing (motion) is more affected by binocular visual deprivation than central visual processing (spatial resolution). In addition, our own experimental findings show the possible recruitment of coarse peripheral vision for fine spatial analysis. Accordingly, I hypothesize that the balance between central and peripheral visual processing, established in the course of development, is susceptible to plastic adaptations during the entire life span, with peripheral vision capable of taking over central processing.

Zif268 mRNA Expression Patterns Reveal a Distinct Impact of Early Pattern Vision Deprivation on the Development of Primary Visual Cortical Areas in the Cat

Pattern vision deprivation (BD) can induce permanent deficits in global motion perception. The impact of timing and duration of BD on the maturation of the central and peripheral visual field representations in cat primary visual areas 17 and 18 remains unknown. We compared early BD, from eye opening for 2, 4, or 6 months, with late onset BD, after 2 months of normal vision, using the expression pattern of the visually driven activity reporter gene zif268 as readout. Decreasing zif268 mRNA levels between months 2 and 4 characterized the normal maturation of the (supra)granular layers of the central and peripheral visual field representations in areas 17 and 18. In general, all BD conditions had higher than normal zif268 levels. In area 17, early BD induced a delayed decrease, beginning later in peripheral than in central area 17. In contrast, the decrease occurred between months 2 and 4 throughout area 18. Lack of pattern vision stimulation during the first 4 months of life therefore has a different impact on the development of areas 17 and 18. A high zif268 expression level at a time when normal vision is restored seems to predict the capacity of a visual area to compensate for BD.

Binocular neurons in parastriate cortex: interocular 'matching' of receptive field properties, eye dominance and strength of silent suppression

Spike-responses of single binocular neurons were recorded from a distinct part of primary visual cortex, the parastriate cortex (cytoarchitectonic area 18) of anaesthetized and immobilized domestic cats. Functional identification of neurons was based on the ratios of phase-variant (F1) component to the mean firing rate (F0) of their spike-responses to optimized (orientation, direction, spatial and temporal frequencies and size) sine-wave-luminance-modulated drifting grating patches presented separately via each eye. In over 95% of neurons, the interocular differences in the phase-sensitivities (differences in F1/F0 spike-response ratios) were small (≤0.3) and in over 80% of neurons, the interocular differences in preferred orientations were ≤10°. The interocular correlations of the direction selectivity indices and optimal spatial frequencies, like those of the phase sensitivies and optimal orientations, were also strong (coefficients of correlation r ≥0.7005). By contrast, the interocular correlations of the optimal temporal frequencies, the diameters of summation areas of the excitatory responses and suppression indices were weak (coefficients of correlation r ≤0.4585). In cells with high eye dominance indices (HEDI cells), the mean magnitudes of suppressions evoked by stimulation of silent, extra-classical receptive fields via the non-dominant eyes, were significantly greater than those when the stimuli were presented via the dominant eyes. We argue that the well documented ‘eye-origin specific’ segregation of the lateral geniculate inputs underpinning distinct eye dominance columns in primary visual cortices of mammals with frontally positioned eyes (distinct eye dominance columns), combined with significant interocular differences in the strength of silent suppressive fields, putatively contribute to binocular stereoscopic vision.

The mechanism for processing random-dot motion at various speeds in early visual cortices

All moving objects generate sequential retinotopic activations representing a series of discrete locations in space and time (motion trajectory). How direction-selective neurons in mammalian early visual cortices process motion trajectory remains to be clarified. Using single-cell recording and optical imaging of intrinsic signals along with mathematical simulation, we studied response properties of cat visual areas 17 and 18 to random dots moving at various speeds. We found that, the motion trajectory at low speed was encoded primarily as a direction signal by groups of neurons preferring that motion direction. Above certain transition speeds, the motion trajectory is perceived as a spatial orientation representing the motion axis of the moving dots. In both areas studied, above these speeds, other groups of direction-selective neurons with perpendicular direction preferences were activated to encode the motion trajectory as motion-axis information. This applied to both simple and complex neurons. The average transition speed for switching between encoding motion direction and axis was about 31°/s in area 18 and 15°/s in area 17. A spatio-temporal energy model predicted the transition speeds accurately in both areas, but not the direction-selective indexes to random-dot stimuli in area 18. In addition, above transition speeds, the change of direction preferences of population responses recorded by optical imaging can be revealed using vector maximum but not vector summation method. Together, this combined processing of motion direction and axis by neurons with orthogonal direction preferences associated with speed may serve as a common principle of early visual motion processing.

Phase sensitivity of complex cells in primary visual cortex

Neurons in the primary visual cortex are often classified as either simple or complex based on the linearity (or otherwise) of their response to spatial luminance contrast. In practice, classification is typically based on Fourier analysis of a cell's response to an optimal drifting sine-wave grating. Simple cells are generally considered to be linear and produce responses modulated at the fundamental frequency of the stimulus grating. In contrast, complex cells exhibit significant nonlinearities that reduce the response at the fundamental frequency. Cells can therefore be easily and objectively classified based on the relative modulation of their responses - the ratio of the phase-sensitive response at the fundamental frequency of the stimulus (F₁) to the phase-invariant sustained response (F₀). Cells are classified as simple if F₁/F₀>1 and complex if F₁/F₀<1. This classification is broadly consistent with criteria based on the spatial organisation of cells' receptive fields and is accordingly presumed to reflect disparate functional roles of simple and complex cells in coding visual information. However, Fourier analysis of spiking responses is sensitive to the number of spikes available - F₁/F₀ increases as the number of spikes is reduced, even for phase-invariant complex cells. Moreover, many complex cells encountered in the laboratory exhibit some phase sensitivity, evident as modulation of their responses at the fundamental frequency. There currently exists no objective quantitative means of assessing the significance or otherwise of these modulations. Here we derive a statistical basis for objectively assessing whether the modulation of neuronal responses is reliable, thereby adding a level of statistical certainty to measures of phase sensitivity. We apply our statistical analysis to neuronal responses to moving sine-wave gratings recorded from 367 cells in cat primary visual cortex. We find that approximately 60% of complex cells exhibit statistically significant (α<0.01) modulation of their responses to optimal moving gratings. These complex cells are phase sensitive and reliably encode spatial phase.

Phase sensitivities, excitatory summation fields, and silent suppressive receptive fields of single neurons in the parastriate cortex of the cat

We have recorded single-neuron activity from cytoarchitectonic area 18 of anesthetized (0.4–0.7% isoflurane in 65% N2O-35% O2 gaseous mixture) domestic cats. Neurons were identified as simple or complex on the basis of the ratios between the phase-variant (F1) component and the mean firing rate (F0) of spike responses to optimized (orientation, direction, spatial and temporal frequencies, size) high-contrast, luminance-modulated, sine-wave drifting gratings (simple: F1/F0 spike-response ratios > 1; complex: F1/F0 spike-response ratios < 1). The predominance (∼80%) of simple cells among the neurons recorded from the principal thalamorecipient layers supports the idea that most simple cells in area 18 might constitute a putative early stage in the visual information processing. Apart from the “spike-generating” regions (the classical receptive fields, CRFs), the receptive fields of three-quarters of area 18 neurons contain silent, extraclassical suppressive regions (ECRFs). The spatial extent of summation areas of excitatory responses was negatively correlated with the strength of the ECRF-induced suppression of spike responses. Lowering the stimulus contrast resulted in an expansion of the summation areas of excitatory responses accompanied by a reduction in the strength of the ECRF-induced suppression. The spatial and temporal frequency and orientation tunings of the ECRFs were much broader than those of the CRFs. Hence, the ECRFs of area 18 neurons appear to be largely “inherited” from their dorsal thalamic inputs. In most area 18 cells, costimulation of CRFs and ECRFs resulted in significant increases in F1/F0 spike-response ratios, and thus there was a contextually modulated functional continuum between the simple and complex cells.

Connections of auditory and visual cortex in the prairie vole (Microtus ochrogaster): evidence for multisensory processing in primary sensory areas

In prairie voles, primary sensory areas are dominated by neurons that respond to one sensory modality, but some neurons also respond to stimulation of other modalities. To reveal the anatomical substrate for these multimodal responses, we examined the connections of the primary auditory area + the anterior auditory field (A1 + AAF), the temporal anterior area (TA), and the primary visual area (V1). A1 + AAF had intrinsic connections and connections with TA, multimodal cortex (MM), V1, and primary somatosensory area (S1). TA had intrinsic connections and connections with A1 + AAF, MM, and V2. Callosal connections were observed in homotopic locations in auditory cortex for both fields. A1 + AAF and TA receive thalamic input primarily from divisions of the medial geniculate nucleus but also from the lateral geniculate nucleus (LGd), the lateral posterior nucleus, and the ventral posterior nucleus (VP). V1 had dense intrinsic connections and connections with V2, MM, auditory cortex, pyriform cortex (Pyr), and, in some cases, somatosensory cortex. V1 had interhemispheric connections with V1, V2, MM, S1, and Pyr and received thalamic input from LGd and VP. Our results indicate that multisensory integration occurs in primary sensory areas of the prairie vole cortex, and this may be related to behavioral specializations associated with its niche.

Complex cell receptive fields: evidence for a hierarchical mechanism

Simple cells in the primary visual cortex have segregated ON and OFF subregions in their receptive fields, while complex cells have overlapping ON and OFF subregions. These two cell types form the extremes at each end of a continuum of receptive field types. Hubel and Wiesel in 1962 suggested a hierarchical scheme of processing whereby spatially offset simple cells drive complex cells. Simple and complex cells are often classified by their responses to moving sine wave gratings: simple cells have oscillatory responses while complex cells produce unmodulated responses. Here, using moving gratings as stimuli, we show that a significant number of cells that display low levels of response modulation at high contrasts demonstrate high levels of response modulation at low contrasts. Most often a drifting low contrast grating generates a large phasic response at the fundamental frequency of the grating (F(1)) and a smaller but significant phasic response that is approximately 180 deg out-of-phase with the F(1) component. We present several models capable of capturing the effects of stimulus contrast on complex cell responses. The model that best reproduces our experimental results is a variation of the classical hierarchical model. In our model several spatially offset simple cells provide input to a complex cell, with each simple cell exhibiting a different contrast response function. At low contrasts only one of these simple cells is sufficiently excited to reveal its receptive field properties. As contrast is increased additional spatially offset simple cells with higher contrast thresholds add their responses to the overall spiking activity.

Subcortical representation of non-Fourier image features

A fundamental goal of visual neuroscience is to identify the neural pathways representing different image features. It is widely argued that the early stages of these pathways represent linear features of the visual scene and that the nonlinearities necessary to represent complex visual patterns are introduced later in cortex. We tested this by comparing the responses of subcortical and cortical neurons to interference patterns constructed by summing sinusoidal gratings. Although a linear mechanism can detect the component gratings, a nonlinear mechanism is required to detect an interference pattern resulting from their sum. Consistent with in vitro retinal ganglion cell recordings, we found that interference patterns are represented subcortically by cat LGN Y-cells, but not X-cells. Linear and nonlinear tuning properties of LGN Y-cells were then characterized and compared quantitatively with those of cortical area 18 neurons responsive to interference patterns. This comparison revealed a high degree of similarity between the two neural populations, including the following: (1) the representation of similar spatial frequencies in both their linear and nonlinear responses, (2) comparable orientation selectivity for the high spatial frequency carrier of interference patterns, and (3) the same difference in their temporal frequency selectivity for drifting gratings versus the envelope of interference patterns. The present findings demonstrate that the nonlinear subcortical Y-cell pathway represents complex visual patterns and likely underlies cortical responses to interference patterns. We suggest that linear and nonlinear mechanisms important for encoding visual scenes emerge in parallel through distinct pathways originating at the retina.

Strabismus modifies intrinsic and inter-areal connections in cat area 18

The development of long-range horizontal connections depends on visual experience. Previous experiments have shown that in area 17 of strabismic but not in normal cats, horizontal fibers preferentially connect cell groups driven by the same eye indicating that fibers between coactive neurons are selectively stabilized. To test whether this is a general organizing principle of intracortical long-range circuitry we extended our analyses to both intrinsic horizontal connections within area 18 and to inter-areal connections between areas 17 and 18. To this end, we visualized the functional architecture of area 18 by intrinsic signal imaging. Horizontal circuitry was labeled by injecting fluorescent latex microspheres into functionally identified domains. Additionally, domains sharing the same ocular dominance as the neurons at the injection sites were visualized by 2-deoxyglucose autoradiography to allow comprehensive labeling of functional domains in regions far from the injection sites. Quantitative analyses revealed that in strabismic cats, 72% of the retrogradely labeled neurons in area 18 and 68% of the neurons in area 17 were located in the same ocular dominance domains as the injection sites. In contrast, these numbers were 52% and 54% in normal animals. These data show that experience modifies both intrinsic connections within area 18 and inter-areal projections from area 17 to area 18 as has been previously described for intrinsic and callosal connections in area 17. This provides further evidence for the hypothesis that the correlation of activity is a major selection criterion for the stabilization of neuronal circuits during postnatal development.

Models and Measurements of Functional Maps in V1

The organization of primary visual cortex has been heavily studied for nearly 50 years, and in the last 20 years functional imaging has provided high-resolution maps of its tangential organization. Recently, however, the usefulness of maps like those of orientation and spatial frequency (SF) preference has been called into question because they do not, by themselves, predict how moving images are represented in V1. In this review, we discuss a model for cortical responses (the spatiotemporal filtering model) that specifies the types of cortical maps needed to predict distributed activity within V1. We then review the structure and interrelationships of several of these maps, including those of orientation, SF, and temporal frequency preference. Finally, we discuss tests of the model and the sufficiency of the requisite maps in predicting distributed cortical responses. Although the spatiotemporal filtering model does not account for all responses within V1, it does, with reasonable accuracy, predict population responses to a variety of complex stimuli.

Influence of adapting speed on speed and contrast coding in the primary visual cortex of the cat

Adaptation is a ubiquitous property of the visual system. Adaptation often improves the ability to discriminate between stimuli and increases the operating range of the system, but is also associated with a reduced ability to veridically code stimulus attributes. Adaptation to luminance levels, contrast, orientation, direction and spatial frequency has been studied extensively, but knowledge about adaptation to image speed is less well understood. Here we examined how the speed tuning of neurons in cat primary visual cortex was altered after adaptation to speeds that were slow, optimal, or fast relative to each neuron's speed response function. We found that the preferred speed (defined as the speed eliciting the peak firing rate) of the neurons following adaptation was dependent on the speed at which they were adapted. At the population level cells showed decreases in preferred speed following adaptation to speeds at or above the non-adapted speed, but the preferred speed did not change following adaptation to speeds lower than the non-adapted peak. Almost all cells showed response gain control (reductions in absolute firing capacity) following speed adaptation. We also investigated the speed dependence of contrast adaptation and found that most cells showed contrast gain control (rightward shifts of their contrast response functions) and response gain control following adaptation at any speed. We conclude that contrast adaptation may produce the response gain control associated with speed adaptation, but shifts in preferred speed require an additional level of processing beyond contrast adaptation. A simple model is presented that is able to capture most of the findings.

Two streams of attention-dependent beta activity in the striate recipient zone of cat's lateral posterior-pulvinar complex

Local field potentials from different visual cortical areas and subdivisions of the cat's lateral posterior-pulvinar complex of the thalamus (LP-P) were recorded during a behavioral task based on delayed spatial discrimination of visual or auditory stimuli. During visual but not auditory attentive tasks, we observed an increase of beta activity (12-25 Hz) as calculated from signals recorded from the caudal part of the lateral zone of the LP-P (LPl-c) as well as from cortical areas 17 and 18 and the complex located at the middle suprasylvian sulcus (MSS). This beta activity appeared only in the trials that ended with a successful response, proving its relationship to the mechanism of visual attention. In contrast, no enhanced beta activity was observed in the rostral part of the lateral zone of the LP-P and in the pulvinar proper. Two subregions of LPl-c (ventromedial and dorsolateral) were distinguished by visually related, attentional beta activity of low (12-18 Hz) and high (18-25 Hz) frequencies, respectively. At the same time, area 17 exhibited attentional activation in the whole beta range, and an increase of power in low-frequency beta was observed in the medial bank of MSS, whereas cortical area 18 and the lateral bank of the MSS were activated in the high beta range. Phase-correlation analysis revealed that two distinct corticothalamic systems were synchronized by the beta activity of different frequencies. One comprised of cortical area 17, ventromedial region of LPl-c, and medial MSS, the second involved area 18 and the dorsolateral LPl-c. Our observations suggest that LPl-c belongs to the wide corticothalamic attentional system, which is functionally segregated by distinct streams of beta activity.

The squirrel as a rodent model of the human visual system

Over the last 50 years, studies of receptive fields in the early mammalian visual system have identified many classes of response properties in brain areas such as retina, lateral geniculate nucleus (LGN), and primary visual cortex (V1). Recently, there has been significant interest in understanding the cellular and network mechanisms that underlie these visual responses and their functional architecture. Small mammals like rodents offer many advantages for such studies, because they are appropriate for a wide variety of experimental techniques. However, the traditional rodent models, mice and rats, do not rely heavily on vision and have small visual brain areas. Squirrels are highly visual rodents that may be excellent model preparations for understanding mechanisms of function and disease in the human visual system. They use vision for navigating in their environment, predator avoidance, and foraging for food. Visual brain areas such as LGN, V1, superior colliculus, and pulvinar are particularly large and well elaborated in the squirrel, and the squirrel has several extrastriate cortical areas lateral to V1. Unlike many mammals, most squirrel species are diurnal with cone-dominated retinas, similar to the primate fovea, and have excellent dichromatic color vision that is mediated by green and blue cones. Owing to their larger size, squirrels are physiologically more robust than mice and rats under anesthesia, and some hibernating species are particularly tolerant of hypoxia that occurs during procedures such as brain slicing. Finally, many basic anatomical and physiological properties in the early visual system of squirrel have now been described, permitting investigations of cellular mechanisms. In this article, we review four decades of anatomical, behavioral, and physiological studies in squirrel and make comparisons with other species.

Contrast gain control is drift-rate dependent: an informational analysis

Neurons in the visual cortex code relative changes in illumination (contrast) and adapt their sensitivities to the visual scene by centering the steepest regions of their sigmoidal contrast response functions (CRFs: spike rate as a function of contrast) on the prevailing contrast. The influence of this contrast gain control has not been reported at nonoptimal drift rates. We calculated the Fisher information contained in the CRFs of halothane-anesthetized cats. Fisher information gives a measure of the accuracy of contrast representations based on the ratio of the square of the steepness of the CRF and the spike-rate dependency of the spiking variance. Variance increases with spike rate, so Fisher information is maximal where the CRF is steep and spike rates are low. Here, we show that the contrast at which the maximal Fisher information (C(MFI)) occurs for each adapting drift rate is at a fixed level above the adapting contrast. For adapting contrasts of 0 to 0.32 the relationship between C(MFI) and adapting contrast is well described by a straight line with a slope close to 1. The intercept of this line on the C(MFI)-axis is drift-rate dependent, although the slope is not. At high drift rates relative to each cell's peak the C(MFI) offset is higher than that for low drift rates. The results show that the contrast coding strategy in visual cortex maximizes accuracy for contrasts above the prevailing contrast in the environment for all drift rates. We argue that tuning the system for accuracy at contrasts above the prevailing value is optimal for viewing natural scenes.

'Simplification' of responses of complex cells in cat striate cortex: suppressive surrounds and 'feedback' inactivation

In mammalian striate cortex (V1), two distinct functional classes of neurones, the so-called simple and complex cells, are routinely distinguished. They can be quantitatively differentiated from each other on the basis of the ratio between the phase-variant (F1) component and the mean firing rate (F0) of spike responses to luminance-modulated sinusoidal gratings (simple, F1/F0 > 1; complex, F1/F0 < 1). We investigated how recurrent cortico-cortical connections affect the spatial phase-variance of responses of V1 cells in the cat. F1/F0 ratios of the responses to optimally oriented drifting sine-wave gratings covering the classical receptive field (CRF) of single V1 cells were compared to those of: (1) responses to gratings covering the CRFs combined with gratings of different orientations presented to the 'silent' surrounds; and (2) responses to CRF stimulation during reversible inactivation of postero-temporal visual (PTV) cortex. For complex cells, the relative strength of the silent surround suppression on CRF-driven responses was positively correlated with the extent of increases in F1/F0 ratios. Inactivation of PTV cortex increased F1/F0 ratios of CRF-driven responses of complex cells only. Overall, activation of suppressive surrounds or inactivation of PTV 'converted' substantial proportions (50 and 30%, respectively) of complex cells into simple-like cells (F1/F0 > 1). Thus, the simple-complex distinction depends, at least partly, on information coming from the silent surrounds and/or feedback from 'higher-order' cortices. These results support the idea that simple and complex cells belong to the same basic cortical circuit and the spatial phase-variance of their responses depends on the relative strength of different synaptic inputs.

Relationship Between Contrast Adaptation and Orientation Tuning in V1 and V2 of Cat Visual Cortex

Previous studies investigating the response properties of neurons in the primary visual cortex of cats and primates have shown that prolonged exposure to optimally oriented, high-contrast gratings leads to a reduction in responsiveness to subsequently presented test stimuli. We recorded from 119 neurons in cat V1 and V2 and found that in a high proportion of cells contrast adaptation also occurs for gratings oriented orthogonal to a neuron's preferred orientation, even though this stimulus did not elicit significant increases in spiking activity. Approximately 20% of neurons adapted equally to all orientations tested and a further 46% showed at least some adaptation to orthogonally oriented gratings, whereas 20% of neurons did not adapt to orthogonal gratings. The magnitude of contrast adaptation was positively correlated with adapting contrast, but was not related to the spiking activity of the cells. Highly direction selective neurons produced stronger adaptation to orthogonally oriented gratings than other neurons. Orientation-related adaptation was correlated with the rate of change of orientation tuning in consecutive cells along electrode penetrations that traveled parallel to the cortical layers. Nonoriented adaptation was most common in areas where orientation preference changed rapidly, whereas orientation-selective adaptation was most common in areas where orientation preference changed slowly. A minority of neurons did not show contrast adaptation (14%). No major differences were found between units in different cortical layers, V1 and V2, or between complex and simple cells. The relevance of these findings to the current understanding of adaptation within the context of orientation column architecture is discussed.

Optical imaging in cat area 18: strabismus does not enhance the segregation of ocular dominance domains

While early-onset strabismus leads to clearly segregated domains of the left and the right eye in cat primary visual cortex (area 17), far less is known about experience-dependent plasticity of ocular dominance in area 18. We therefore used optical imaging of intrinsic signals to analyze the influence of strabismus on cortical maps in cat area 18. Monocular visual stimulation of the left and right eye with moving square wave gratings of four different orientations induced patchy activity maps. Unlike our previous observations in cat area 17, the monocular activity maps in area 18 of strabismic cats were rather similar so that functional ocular dominance domains were not clearly segregated. Imaging of the 17/18 border region confirmed this observation and revealed a sudden change in the segregation of the left and right eye domains across the border. Our results demonstrate that modified visual input can have different consequences for different visual areas: while the decorrelation of activity between the two eyes (as induced by strabismus) clearly enhances the segregation of ocular dominance domains in cat area 17, area 18 does not show this effect although electrophysiological studies have confirmed that the percentage of binocularly driven neurons is as reduced as in area 17.

What the brain stem tells the frontal cortex. I. Oculomotor signals sent from superior colliculus to frontal eye field via mediodorsal thalamus

Neuronal processing in cerebral cortex and signal transmission from cortex to brain stem have been studied extensively, but little is known about the numerous feedback pathways that ascend from brain stem to cortex. In this study, we characterized the signals conveyed through an ascending pathway coursing from the superior colliculus (SC) to the frontal eye field (FEF) via mediodorsal thalamus (MD). Using antidromic and orthodromic stimulation, we identified SC source neurons, MD relay neurons, and FEF recipient neurons of the pathway in Macaca mulatta. The monkeys performed oculomotor tasks, including delayed-saccade tasks, that permitted analysis of signals such as visual activity, delay activity, and presaccadic activity. We found that the SC sends all of these signals into the pathway with no output selectivity, i.e., the signals leaving the SC resembled those found generally within the SC. Visual activity arrived in FEF too late to contribute to short-latency visual responses there, and delay activity was largely filtered out in MD. Presaccadic activity, however, seemed critical because it traveled essentially unchanged from SC to FEF. Signal transmission in the pathway was fast ( approximately 2 ms from SC to FEF) and topographically organized (SC neurons drove MD and FEF neurons having similarly eccentric visual and movement fields). Our analysis of identified neurons in one pathway from brain stem to frontal cortex thus demonstrates that multiple signals are sent from SC to FEF with presaccadic activity being prominent. We hypothesize that a major signal conveyed by the pathway is corollary discharge information about the vector of impending saccades.

Area 21a of cat visual cortex strongly modulates neuronal activities in the superior colliculus

We have examined the influence of cortico-tectal projections from one of the pattern-processing extrastriate visual cortical areas, area 21a, on the responses to visual stimuli of single neurones in the superior colliculi of adult cats. For this purpose area 21a was briefly inactivated by cooling to 10 degrees C using a Peltier device. Responses to visual stimuli before and during cooling as well as after rewarming ipsilateral area 21a were compared. In addition, in a subpopulation of collicular neurones we have studied the effects of reversible inactivation of ipsilateral striate cortex (area 17, area V1). When area 21a was cooled, the temperature of area 17 was kept at 36 degrees C and vice versa. In the majority of cases (41/65; 63%), irrespective of the velocity response profiles of collicular neurones, inactivation of area 21a resulted in a significant decrease in magnitude of responses of neurones in the ipsilateral colliculus and only in a small proportion of cells (2/65; 3.1%) was there a significant increase in the magnitude of responses. Inactivation of area 21a resulted in significant changes in the magnitude of responses of collicular cells located not only in the retino-recipient layers but also in the stratum griseum intermediale. In most cases, reversible inactivation of area 17 resulted in a greater reduction in the magnitude of responses of collicular cells than inactivation of area 21a. Reversible inactivation of area 21a also affected the direction selectivity indices and length tuning of most collicular cells tested.

Topographic reorganization in area 18 of adult cats following circumscribed monocular retinal lesions in adolescence

Circumscribed laser lesions were made in the nasal retinae of one eye in adolescent cats. Ten to sixteen months later, about 80 % of single neurones recorded in the lesion projection zone (LPZ) of contralateral area 18 (parastriate cortex, area V2) were binocular but when stimulated via the lesioned eye had ectopic discharge fields (displaced to normal retina in the vicinity of the lesion). Although the clear majority of binocular cells recorded from the LPZ responded with higher peak discharge rates to stimuli presented via the non-lesioned eye, the orientation and direction selectivities as well as preferred and upper cut-off velocities for stimuli presented through either eye were very similar. Furthermore, the sizes of the ectopic discharge fields of binocular cells recorded from the LPZ were not significantly different from those of their counterparts plotted via the non-lesioned eye. Thus, monocular retinal lesions performed in adolescent cats induce topographic reorganization in the LPZ of area 18. Although a similar reorganization occurs in area 17 (striate cortex, area V1) of cats in which monocular retinal lesions were made either in adulthood or adolescence, in view of the very different velocity response profiles of ectopic discharge fields in areas 17 and those in area 18, it appears that ectopic discharge fields in area 17 are largely independent of excitatory feedback input from area 18.

Postnatal dendritic development of Y-like geniculocortical relay neurons

We describe the dendritic development of neurons in the dorsal lateral geniculate nucleus (LGNd) projecting to cortical area 18 in the postnatal cat. LGN neurons were identified by retrograde labeling from area 18 with fluorescent latex microspheres and injected in the fixed slice with Lucifer yellow (LY) and horseradish peroxidase (HRP) to visualize their dendritic arborizations. Both topological (measures of the patterns of dendritic branching and their territorial coverage) and metric parameters (measures of the quantitative parameters describing the size, length, extent and diameter of the dendritic arbors) were measured in three-dimensions for 25 LGN neurons in cats between 1 and 18 postnatal weeks. In addition, dendritic growth was compared to the changing dimensions of the LGNd. At all ages, neurons projecting to area 18 have large somata and radiate dendrites. From 1 to 18 weeks neurons increase in size--both soma area and the length of all dendritic segments double during this period. Intermediate and terminal dendritic segments show comparable growth until 5 weeks. However, only terminal segments continue to grow significantly from 5 until 18 weeks. Dendrites become straighter during development, the angle between daughter branches decreases and dendritic segment diameter increases, with terminal segments showing a greater increase relative to intermediate segments. The density of dendritic appendages increases transiently at 5 weeks and a differential redistribution occurs, so that by 18 weeks dendrites further from the soma have a greater density of appendages than those near the soma. Some dendritic relationships remain invariant during development--intermediate segments are always shorter, thicker and straighter than terminal segments. During these changes however, area 18 projecting neurons maintain a constant number of primary dendrites and have, on average, a constant branching pattern. The relative volume of the LGNd occupied by an area 18 projecting neuron increases 2.4-fold between 1 and 18 weeks as the dendrites grow with the result that the coverage of a given point of the LGNd by dendrites of area 18 projecting nearly doubles from 24 to 45 neurons per unit volume. This increased net dendritic overlap provides a substrate for enhanced numerical synaptic divergence of the Y-cell pathway from a point source in the retina to the visual cortex.

Functional plasticity in extrastriate visual cortex following neonatal visual cortex damage and monocular enucleation

Neonatal lesions of primary visual cortex (areas 17, 18 and 19; VC) in cats lead to significant changes in the organization of visual pathways, including severe retrograde degeneration of retinal ganglion cells of the X/beta class. Cells in posteromedial lateral suprasylvian (PMLS) cortex display plasticity in that they develop normal receptive-field properties despite these changes, but they do not acquire the response properties of striate neurons that were damaged (e.g., high spatial-frequency tuning, low contrast threshold). One possibility is that the loss of X-pathway information, which is thought to underlie striate cortical properties in normal animals, precludes the acquisition of these responses by cells in remaining brain areas following neonatal VC damage. Previously, we have shown that monocular enucleation at the time of VC lesion prevents the X-/beta-cell loss in the remaining eye. The purpose of the present study was to determine whether this sparing of retinal X-cells leads to the development of striate-like response properties in PMLS cortex. We recorded the responses of PMLS neurons to visual stimuli to assess spatial-frequency tuning, spatial resolution, and contrast threshold. Results indicated that some PMLS cells in animals with a neonatal VC lesion and monocular enucleation displayed a preference for higher spatial frequencies, had higher spatial resolution, and had lower contrast thresholds than PMLS cells in cats with VC lesion alone. Taken together, these results suggest that preserving X-pathway input during this critical period leads to the addition of some X-like properties to PMLS visual responses.

Recent models and findings in visual backward masking: a comparison, review, and update

Visual backward masking not only is an empirically rich and theoretically interesting phenomenon but also has found increasing application as a powerful methodological tool in studies of visual information processing and as a useful instrument for investigating visual function in a variety of specific subject populations. Since the dual-channel, sustained-transient approach to visual masking was introduced about two decades ago, several new models of backward masking and metacontrast have been proposed as alternative approaches to visual masking. In this article, we outline, review, and evaluate three such approaches: an extension of the dual-channel approach as realized in the neural network model of retino-cortical dynamics (Ogmen, 1993), the perceptual retouch theory (Bachmann, 1984, 1994), and the boundary contour system (Francis, 1997; Grossberg & Mingolla, 1985b). Recent psychophysical and electrophysiological findings relevant to backward masking are reviewed and, whenever possible, are related to the aforementioned models. Besides noting the positive aspects of these models, we also list their problems and suggest changes that may improve them and experiments that can empirically test them.

Organization of efferent neurons in area 19: the projection to extrastriate area 21a

The organization of efferent neurons in area 19 of the cat was examined by bulk injections of retrograde tracers, WGA-HRP and CTX-Au, into extrastriate area 21a. In one case, the cortex was cut coronally and retrogradely labeled cells in area 19 were present in columnar register throughout layers 2 to 6, with the majority of labeled cells in layers 2/3. The number of columns per tissue section ranged from 0 to 4 and had a centre-to-centre spacing ranging from 0.6 to 0.9 mm. A few lightly labeled cells were found between the columns. In six other cases, the visual cortex was flattened, and cut in the tangential plane to reveal a pattern of irregular, widely spaced bands that were elongated in the mediolateral direction with a mean centre-to-centre spacing of 2.6 mm. The density of labeled cells within these bands fluctuated such that dense aggregates of cells were found, on average, at 0.9 mm intervals along the bands. This tangential heterogeneity in density, along with the patchy columnar staining witnessed in the coronal plane, suggests that the widely spaced efferent projection bands may have a patchy substructure with a spacing of approximately 1 mm. The pattern of efferent projection bands and its substructure in area 19 is reminiscent of the stripe-like organization of V2 found in primates.

Effects of feedback projections from area 18 layers 2/3 to area 17 layers 2/3 in the cat visual cortex

In the absence of a direct geniculate input, area 17 cells in the cat are nevertheless able to respond to visual stimuli because of feedback connections from area 18. Anatomic studies have shown that, in the cat visual cortex, layer 5 of area 18 projects to layer 5 of area 17, and layers 2/3 of area 18 project to layers 2/3 of area 17. What is the specific role of these connections? Previous studies have examined the effect of area 18 layer 5 blockade on cells in area 17 layer 5. Here we examine whether the feedback connections from layers 2/3 of area 18 influence the orientation tuning and velocity tuning of cells in layers 2/3 of area 17. Experiments were carried out in anesthetized and paralyzed cats. We blocked reversibly a small region (300 microm radius) in layers 2/3 of area 18 by iontophoretic application of GABA and recorded simultaneously from cells in layers 2/3 of area 17 while stimulating with oriented sweeping bars. Area 17 cells showed either enhanced or suppressed visual responses to sweeping bars of various orientations and velocities during area 18 blockade. For most area 17 cells, orientation bandwidths remained unaltered, and we never observed visual responses during blockade that were absent completely in the preblockade condition. This suggests that area 18 layers 2/3 modulate visual responses in area 17 layers 2/3 without fundamentally altering their specificity.

Self-organization of shift-invariant receptive fields

This paper proposes a new learning rule by which cells with shift-invariant receptive fields are self-organized. With this learning rule, cells similar to simple and complex cells in the primary visual cortex are generated in a network. To demonstrate the new learning rule, we simulate a three-layered network that consists of an input layer (or the retina), a layer of S-cells (or simple cells), and a layer of C-cells (or complex cells). During the learning, straight lines of various orientations sweep across the input layer. Here both S- and C-cells are created through competition. Although S-cells compete depending on their instantaneous outputs, C-cells compete depending on the traces (or temporal averages) of their outputs. For the self-organization of S-cells, only winner S-cells increase their input connections in a similar way to that for the neocognitron. In other words, the winner S-cells have LTP (long term potentiation) in their input connections. For the self-organization of C-cells, however, loser C-cells decrease their input connections (LTD=long term depression), while winners increase their input connections (LTP). Here both S- and C-cells are accompanied by inhibitory cells. Modification of inhibitory connections together with excitatory connections is important for creation of C-cells as well as S-cells.

Response component analysis of simple and complex cells of area 18 during depression of area 17

Simple and complex cells of visual areas of cats may be reliably classified according to the modulatory index (MI) of their responses. This investigation is aimed at analysing the MI in area 18 when a small region (about 200-400 µm in diameter) of area 17 was inactivated with a microinjection of GABA, in anesthetized cats. Cells were stimulated with sine-wave gratings whose orientation, spatial, and temporal frequencies were optimal for the studied unit. The AC and DC response components, and the MI were computed along with fast Fourier transforms of evoked discharges recorded as peristimulus time histograms. Results showed that these response components were relatively unaffected in simple cells, whereas complex cells exhibited large changes when area 17 was silenced. In particular, a large proportion of complex cells showed a MI greater than 1, thereby adopting a response pattern resembling simple cells. It is suggested that this subpopulation of complex cells receives a direct input from geniculate X cells.Key words: simple cells, complex cells, visual cortex, corticocortical influences, cats.

Visual responses of neurones in the second visual area of flying foxes (Pteropus poliocephalus) after lesions of striate cortex

1. The first (V1) and second (V2) cortical visual areas exist in all mammals. However, the functional relationship between these areas varies between species. While in monkeys the responses of V2 cells depend on inputs from V1, in all non-primates studied so far V2 cells largely retain responsiveness to photic stimuli after destruction of V1. 2. We studied the visual responsiveness of neurones in V2 of flying foxes after total or partial lesions of the primary visual cortex (V1). The main finding was that visual responses can be evoked in the region of V2 corresponding, in visuotopic co-ordinates, to the lesioned portion of V1 ('lesion projection zone'; LPZ). 3. The visuotopic organization of V2 was not altered by V1 lesions. 4. The proportion of neurones with strong visual responses was significantly lower within the LPZs (31.5 %) than outside these zones, or in non-lesioned control hemispheres ( > 70 %). LPZ cells showed weak direction and orientation bias, and responded consistently only at low spatial and temporal frequencies. 5. The data demonstrate that the functional relationship between V1 and V2 of flying foxes resembles that observed in non-primate mammals. This observation contrasts with the 'primate-like' characteristics of the flying fox visual system reported by previous studies.

Spatial and temporal matching of receptive field properties of binocular cells in area 19 of the cat

The spatial and temporal properties of single neurons were investigated in area 19 of the cat. We evaluated the matching of binocular receptive field properties with regard to the respective strength of the ipsilateral and contralateral inputs. Results indicate that most cells in area 19 are well tuned to spatial and temporal frequencies and exhibit relatively low contrast threshold (mean=6.8%) when assessed using optimal parameters and tested through the dominant eye. Spatial resolution (mean=0.75 c/degree), optimal spatial frequencies (mean=0.16 c/degree) were relatively low and spatial bandwidths (mean=2.1 octaves) were broader as compared to those of cells in area 17 but comparable to those of cells in other extrastriate areas. On the other hand temporal resolution (mean=10.7 Hz), optimal temporal frequency (mean=4.5 Hz) and temporal bandwidths (mean=2.9 octaves) were higher and broader than in primary visual cortex. A significant relationship exists between most of the cell's properties assessed through either eye. For some parameters, such as spatial and temporal resolution, ocular dominance was shown to be significantly related to the extent of matching between the two eyes. For these parameters, binocular cells that exhibited a balanced ocular dominance were generally well matched with regard to the receptive field properties of each eye whereas the largest mismatches were found in cells that were more strongly dominated by one eye. These results suggest that visual input contributes to the activation of cells in area 19 in a redundant manner, possibly attesting to the multiplicity of parallel pathways to this area in the cat.

Influences of area 17 on neuronal activity of simple and complex cells of area 18 in cats

To understand the influence of the ascending path linking area 17 to area 18 of visual cortices, experiments were carried out in which a small neuronal population of area 17 was inactivated with GABA, while unitary responses were recorded in area 18. In the latter, cells are identified as belonging to the simple or complex family according to their firing pattern evoked in response to sine-wave gratings scrolling through the receptive fields. Anesthetized cats were prepared for single-cell recordings. In area 17, a GABA-containing pipette was placed in superficial layers in order to inactivate reversibly a small neuronal population. Prior to blockade, the orientation tuning curves were obtained in both areas and the difference in optimal orientation between areas 17 and 18 was recorded. In area 18, cells were classified as simple or complex. The strategy was to study the reaction of neurons in area 18 prior to, during and after area 17 depression. In most simple cells, whenever the difference in orientation was in the iso-range, that is when the difference in optimal orientations of the injected site (in area 17) and of the neuron in area 18 was less than 30 degrees, the GABA application produced a decline of the evoked discharges, whereas GABA injection augmented the evoked firing rate when the difference was in the cross-range (>60 degrees). In contrast to simple cells, GABA depression enhanced the responses in the majority of complex cells with like orientations in both areas. When the difference between recording sites was in the cross-range, then area 17 depression produced weaker evoked firing. A tangential penetration of the injecting pipette, allowing injection of different orientation sites while testing the same unit in area 18, revealed that the latter could react with an enhancement or a decline of the responses as the injecting pipette shifted from iso (or cross) to cross (or iso) disparity in optimal orientations between areas 17 and 18. These results suggest that the path connecting area 17 to area 18 may be functionally discriminated on the basis of the orientation domain and cell types. In addition, our data suggest that the ascending visual streams are required to generate orientation specificity in area 18.

The response of cat visual cortex to flicker stimuli of variable frequency

We examined the possibility that neurons or groups of neurons along the retino-cortical transmission chain have properties of tuned oscillators: To this end, we studied the resonance properties of the retino-thalamo-cortical system of anaesthetized cats by entraining responses with flicker stimuli of variable frequency (2-50 Hz). Responses were assessed from multi-unit activity (MUA) and local field potentials (LFPs) with up to four spatially segregated electrodes placed in areas 17 and 18. MUA and LFP responses were closely related, units discharging with high preference during LFP negativity. About 300 ms after flicker onset, responses stabilized and exhibited a highly regular oscillatory patterning that was surprisingly similar at different recording sites due to precise stimulus locking. Fourier transforms of these steady state oscillations showed maximal power at the inducing frequency and consistently revealed additional peaks at harmonic frequencies. The frequency-dependent amplitude changes of the fundamental and harmonic response components suggest that the retino-cortical system is entrainable into steady state oscillations over a broad frequency range and exhibits preferences for distinct frequencies in the theta- or slow alpha-range, and in the beta- and gamma-band. Concomitant activation of the mesencephalic reticular formation increased the ability of cortical cells to follow high frequency stimulation, and enhanced dramatically the amplitude of first- and second-order harmonics in the gamma-frequency range between 30 and 50 Hz. Cross-correlations computed between responses recorded simultaneously from different sites revealed pronounced synchronicity due to precise stimulus locking. These results suggest that the retino-cortical system contains broadly tuned, strongly damped oscillators which altogether exhibit at least three ranges of preferred frequencies, the relative expression of the preferences depending on the central state. These properties agree with the characteristics of oscillatory responses evoked by non-temporally modulated stimuli, and they indicate that neuronal responses along the retino-cortical transmission chain can become synchronized with precision in the millisecond range not only by intrinsic interactions, but also by temporally structured stimuli.

Cortical projections of the parvocellular laminae C of the dorsal lateral geniculate nucleus in the cat: an anterograde wheat germ agglutinin conjugated to horseradish peroxidase study

The areal and laminar distributions of the projection from the parvocellular part of laminae C of the dorsal lateral geniculate nucleus (Cparv) were studied in visual cortical areas of the cat with the anterograde tracing method by using wheat germ agglutinin conjugated to horseradish peroxidase. A particular objective of this study was to examine the central visual pathways of the W-cell system, the precise organization of which is still unknown. Because the Cparv in the cat is said to receive W-cell information exclusively from the retina and the superior colliculus, the results obtained would provide an anatomical substrate for the W-cell system organization in mammals. The results show that the cortical targets of the Cparv are areas 17, 18, 19, 20a, and 21a and the posteromedial lateral suprasylvian (PMLS) and ventral lateral suprasylvian(VLS) areas. In area 17, the projection fibers terminate in the superficial half of layer I; the lower two-thirds of layer III, extending to the superficial part of layer IV; and the deep part of layer IV, involving layer Va. These terminations form triple bands in area 17. The projection terminals in layer I are continuous, whereas those in layers III, IV, and Va distribute periodically, exhibiting a patchy appearance. In areas 18 and 19, the projection fibers terminate in the superficial half of layer I and in the full portions of layers III and IV, forming double bands. In these areas, the terminals in layer I are continuous, whereas those in layers III and IV distribute periodically, exhibiting a patchy appearance. In area 20a, area 21a, PMLS, and VLS, projection fibers terminate in the superficial part of layer I, in part of layer III, and in the full portion of layer IV, although they are far fewer in number than those seen in areas 17, 18, and 19. The present results demonstrate that the Cparv fibers terminate in a localized fashion in both the striate and the extrastriate cortical areas and that these W-cell projections are quite unique in their areal and laminar organization compared with the X- and Y-cell systems.

Excitatory convergence of Y and non-Y information channels on single neurons in the PMLS area, a motion area of the cat visual cortex

We analysed the receptive field properties of neurons in the posteromedial lateral suprasylvian (PMLS) visual cortical area of anaesthetized cats in which there was selective conduction block of the largest (Y-type) fibres in one optic nerve. As in normal cats, in cats with selective block of one optic nerve the great majority of PMLS cells could be activated by photic stimulation through either eye. However, the responses evoked by stimulation via the eye with the selectively pressure-blocked optic nerve ('Y-blocked eye') were significantly weaker than those of the same cells evoked by the stimulation via the normal eye. Accordingly, eye dominance histograms were shifted markedly in favour of the normal eye. Furthermore, there was a significant shift towards lower preferred velocities when PMLS cells were photically stimulated via the Y-blocked eye. Finally, when stimulated via the Y-blocked eye, PMLS cells responded poorly or not at all to high stimulus velocities (> or = 100 degrees/s). On the other hand, a number of receptive field properties, such as the spatial organization of receptive fields, the size of the discharge fields, orientation tuning and direction selectivity indices, were not significantly affected by the removal of the Y input. We conclude that virtually all neurons in the PMLS area of the cat receive excitatory input from both Y and non-Y information channels, although the Y channel provides the dominant input and makes the principal contribution to the detection of high-velocity motion.

Limits of parallel processing: excitatory convergence of different information channels on single neurons in striate and extrastriate visual cortices

1. It has been postulated that the distinct parallel retino-geniculo-cortical information channels characterizing visual pathways of virtually all mammals are selectively linked to parallel motion, colour and/or form information processing 'streams' distinguishable within the primary visual cortices, extrastriate cortical areas of occipital lobes and the temporal and parietal visual cortices. 2. Using selective pressure-blocking of the large-fibre channel (the so-called Y-channel) in the optic nerve of the cat, we have experimentally examined the 'selective excitatory parallel links' hypothesis. We conclude that the majority of neurons in the primary visual cortices (areas 17, 18) as well as in the two 'higher order' visual areas, area 21a and posteromedial lateral suprasylvian (PMLS) area, constituting, respectively, part of the 'form' and part of the 'motion' processing streams, receive their excitatory inputs from both Y- and non-Y-information channels. In areas 17, 18 and 21a (but not in PMLS area), there are, however, subpopulations of cells that apparently receive excitatory inputs from only one information channel. 3. Review of the relevant work on the macaque monkey suggests that the situation is similar in the primate: that is, there is a substantial degree of excitatory convergence of different retino-geniculo-cortical information channels on single neurons in the primary visual cortices and the extrastriate cortices constituting parts of the form/colour or the motion processing streams. 4. Despite this high degree of excitatory convergence of different information channels, the large-fibre channels (the Y-channel in the cat and the magnocellular or Y-like channel in macaque), are in both carnivores and primates the principal contributors to the motion processing cortical streams.

Long-range horizontal connections between supragranular pyramidal cells in the extrastriate visual cortex of the rat

In this study, we examined the morphological structure and synaptic physiology of long-range axon projections among supragranular pyramidal cells in the extrastriate visual cortex of the rat. Intra- and extracellular recordings from layer II/III pyramidal cells were performed in brain slices of area 18a following extracellular stimulation of either the underlying white matter or within layer II/III. Neurons were injected with biocytin for two-dimensional reconstruction of their axon arborizations. The conduction velocity of afferent fibers (0.58 m/s) was twice as high as that of intracortical tangential fibers (0.28 m/s). Layer II/III cells were mainly di- or polysynaptically driven by afferent activation, but predominantly monosynaptically driven from intracortical stimulation sites. The afferent as well as intracortically evoked postsynaptic potentials showed a very similar time course and shape. From both stimulation sites, suprathreshold action potentials could be elicited. The current threshold for a postsynaptic response and the slope and width of excitatory postsynaptic potentials (EPSPs) increased with the distance of lateral stimulation. The morphological properties of layer II/III pyramidal cell axon collaterals closely corresponded to the electrophysiological results. Long-range intraareal axon collaterals could be followed up to 1 mm within the supragranular layers. Their length-distance distribution showed an inverse relationship to the threshold currents of EPSPs. Pyramidal cells exhibited regularly spaced patches of horizontal axon collaterals with an interpatch distance of about 250 microns. We concluded that the supragranular horizontal network in the extrastriate visual cortex of the rat is qualitatively very similar to that of cats and monkeys. However, quantitative differences exist in its spatial extent and physiological characteristics.

Orientational influences of layer V of visual area 18 upon cells in layer V of area 17 in the cat cortex

We examined the orientation tuning curves of 86 cells located in layer V of area 17, before, during, and after focal blockade of a small (300-microns diameter) region of near-retinotopic register in layer V of area 18 of quantitatively established orientation preference. Such focal blockade revealed three distinct populations of area 17 layer V cells-cells with decreased responses to stimuli of some orientations (21%), cells with increased responses to stimuli of some orientations (43%), and cells unaffected by the focal blockade (36%). These effects were clearcut, reproducible, and generally directly related to the known receptive field properties of the cell recorded in area 18 at the center of the zone of blockade. These effects were also analyzed in terms of alterations in orientation bandwidth in the cells in area 17 as a result of the blockade-bandwidth increases (22%) and decreases (24%) were found; however, these changes were essentially unrelated to the measured receptive field properties. Inhibitory and excitatory effects were most pronounced when the regions in areas 17 and 18 were of like ocular dominance and were of similar orientation preference. Inhibitory effects (suggesting a normally excitatory input) were most dependent upon the similarity of receptive fields; excitatory effects (suggesting a normally inhibitory input) were less heavily dependent.