Lateral interactions at direction-selective striate neurones in the cat demonstrated by local cortical inactivation

Demonstrations of Spatiotemporal Integration and what they Tell us about the Visual System

Five sets of displays are presented on the journal website to be viewed in conjunction with the text. We concentrate on the factors that give rise to the integration and disruption of the direction of apparent motion in two-dimensional and three-dimensional space. In the first set of displays we examine what factors contribute to the integration and disruption of apparent motion in the Ramachandran/Anstis clustered bistable quartets. In the second set we examine what factors give rise to the perception of the direction of motion in rotating two-dimensional wheels and dots. In the third and fourth sets we examine how the depth cues of shading and disparity contribute to the perception of apparent motion of opaque displays, and to the perception of rotating unoccluded displays, respectively. In the fifth set we examine how the depth cue of motion parallax influences the perception of apparent motion. Throughout, we make inferences about the roles which various parallel pathways and cortical areas play in the perceptions produced by the displays shown.

Mislocalization of near-threshold tactile stimuli in humans: a central or peripheral phenomenon?

Principles of brain function can be disclosed by studying their limits during performance. Tactile stimuli with near-threshold intensities have been used to assess features of somatosensory processing. When stimulating fingers of one hand using near-threshold intensities, localization errors are observed that deviate significantly from responses obtained by guessing - incorrectly located stimuli are attributed more often to fingers neighbouring the stimulated one than to more distant fingers. Two hypotheses to explain the findings are proposed. The 'central hypothesis' posits that the degree of overlap of cortical tactile representations depends on stimulus intensity, with representations less separated for near-threshold stimuli than for suprathreshold stimuli. The 'peripheral hypothesis' assumes that systematic mislocalizations are due to activation of different sets of skin receptors with specific thresholds. The present experiments were designed to decide between the two hypotheses. Taking advantage of the frequency tuning of somatosensory receptors, their contribution to systematic misclocalizations was studied. In the first experiment, mislocalization profiles were investigated using vibratory stimuli with frequencies of 10, 20 and 100 Hz. Unambiguous mislocalization effects were only obtained for the 10-Hz stimulation, precluding the involvement of Pacinian corpuscles in systematic mislocalization. In the second experiment, Pacinian corpuscles were functionally eliminated by applying a constant 100-Hz vibratory masking stimulus together with near-threshold pulses. Despite masking, systematic mislocation patterns were observed rendering the involvement of Pacinian corpuscles unlikely. The results of both experiments are in favor of the 'central hypothesis' assuming that the extent of overlap in somatosensory representations is modulated by stimulus intensity.

Lack of orientation and direction selectivity in a subgroup of fast-spiking inhibitory interneurons: cellular and synaptic mechanisms and comparison with other electrophysiological cell types

Neurons in cat area 17 can be grouped in 4 different electrophysiological cell classes (regular spiking, intrinsically bursting, chattering, and fast spiking [FS]). However, little is known of the functional properties of these different cell classes. Here we compared orientation and direction selectivity between these cell classes in cat area 17 and found that a subset of FS inhibitory neurons, usually with complex receptive fields, exhibited little selectivity in comparison with other cell types. Differences in occurrence and amplitude of gamma-range membrane fluctuations, as well as in numbers of action potentials in response to optimal visual stimuli, did not parallel differences observed for orientation and direction selectivity. Instead, differences in selectivity resulted mostly from differences in tuning of the membrane potential responses, although variations in spike threshold also contributed: weakly selective FS neurons exhibited both a lower spike threshold and more broadly tuned membrane potential responses in comparison with the other cell classes. Our results are consistent with the hypothesis that a subgroup of FS neurons receives connections and possesses intrinsic properties allowing the generation of weakly selective responses. The existence of weakly selective inhibitory neurons is consistent with orientation selectivity models that rely on broadly tuned inhibition.

Posteromedial lateral suprasylvian motion area modulates direction but not orientation preference in area 17 of cats

In visual cortices of cats there are two major, largely parallel, feedforward processing streams which conduct visual information from the primary visual cortices to the parietal and temporal visual cortices, processing motion and form information, respectively. In addition to the feedforward streams, there exist many feedback projections from higher-order visual cortices to lower-order visual cortices. Using the intrinsic signal optical imaging, this study has examined the influence of feedback signals originating from area posteromedial lateral suprasylvian (PMLS), the dominant motion-processing region of the parietal cortex, on responses of neurons, orientational maps, and directional maps in cats' area 17 (striate cortex). The inactivation of area PMLS by local application of GABA resulted in the reduction of the magnitude of responses of area 17 cells though area 17 of the cat is mainly involved in form information processing rather than motion. Furthermore, inactivation of area PMLS abolished the global layout of direction maps in area 17 but did not affect the basic structure of the orientation maps in area 17. Thus, it appears that higher-order cortical areas along one information processing stream may exert cross-stream modulatory effects on fundamental properties of neurons located in the lower-order areas along distinct information processing streams.

Cuing the dimension of a distractor: Verbal cues of target identity also benefit same-dimension distractor singletons

Cuing the identity of an upcoming target speeds its detection. This effect is generally assumed to operate on the level of the target dimension, not of its feature identity. Here, we investigated whether that is the case, in a design in which preparing for a cued dimension would incur costs as well as benefits. Participants searched for targets that could be defined on several dimensions, but were also presented with distractors that were defined on the same dimensions. Cuing the identity of an upcoming target increased the effect of distractors defined on the same dimension as the target. This suggests that cuing a target's identity has effects that operate at least partly at the level of the target dimension.

Visual Cortex: Two-Photon Excitement

Current in vivo methods for imaging the visual cortex lack the ability to map response properties at the level of single cells. A new technique using two-photon imaging of calcium signals has now overcome this limitation.

Spatial distribution of inhibitory synaptic connections during development of ferret primary visual cortex

Intracortical inhibition in the primary visual cortex plays an important role in creating properties like orientation and direction selectivity. However, the development of the spatial pattern of inhibitory connections is largely unexplored. This was investigated in the present study. Tangential slices of layers 2/3 of ferret striate cortex were prepared for whole-cell patch clamp recordings, and presynaptic inhibitory inputs to pyramidal neurons were scanned by local photolysis of Nmoc-caged glutamate. Inhibitory synaptic currents (IPSCs) were first detected around postnatal day (P) 17. They originated locally around the recorded cells. Both the number and the total areas supplying the inhibitory inputs increased thereafter and peaked at the time around and shortly after eye opening (P29-37). A refinement period then followed in which the areas providing the majority of inhibitory inputs shrank from 600 microm around the recorded neurons to 200-300 microm in more mature animals (>/=P38). The amplitude of IPSCs increased progressively with increasing age. Long-range inhibitory inputs (>600 microm) were present around eye opening and they often developed into a clustered patchy pattern in more mature animals (>/=P38). In summary, our results show a refinement and clustering in the spatial pattern of inhibitory connections during postnatal development of ferret visual cortex.

The role of feedback in shaping neural representations in cat visual cortex

In the primary visual cortex, neurons with similar response preferences are grouped into domains forming continuous maps of stimulus orientation and direction of movement. These properties are widely believed to result from the combination of ascending and lateral interactions in the visual system. We have tested this view by examining the influence of deactivating feedback signals descending from the visuoparietal cortex on the emergence of these response properties and representations in cat area 18. We thermally deactivated the dominant motion-processing region of the visuoparietal cortex and used optical and electrophysiological methods to assay neural activity evoked in area 18 by stimulation with moving gratings and fields of coherently moving randomly distributed dots. Feedback deactivation decreased signal strength in both orientation and direction maps and virtually abolished the global layout of direction maps, whereas the basic structure of the orientation maps was preserved. These findings could be accounted for by a selective silencing of highly direction-selective neurons and by the redirection of preferences of less selective neurons. Our data suggest that signals fed back from the visuoparietal cortex strongly contribute to the emergence of direction selectivity in early visual areas. Thus we propose that higher cortical areas have significant influence over fundamental neuronal properties as they emerge in lower areas.

Topographical Aspects of Intracortical Excitation and Inhibition Contributing to Orientation Specificity in Area 17 of the Cat Visual Cortex

Intracortical mechanisms contributing to orientation and direction specificity were investigated with a method of local cortical inactivation. Single-unit activity was recorded in area 17 of the anaesthetized cat while a small volume of cortical tissue 400 - 2900 microm lateral to the recorded cell was inactivated by gamma-aminobutyric acid (GABA) microiontophoresis. Cells were stimulated with moving bars of variable orientation and changes of the response were monitored. Recording and inactivation sites were histologically verified. Statistically significant changes in orientation tuning during GABA-induced remote inactivation were observed in 80 of 145 cells (55%), and consisted in a reduced orientation specificity due to either increased (36%) or decreased (19%) responses. Increases of responses were more pronounced for the non-optimal orientations. This effect mainly occurred with GABA application at distances around 500 microm and is interpreted as loss of inhibition. Reduced orientation specificity as a result of decreasing response mainly to the optimal orientation was interpreted as loss of excitation. This effect most frequently occurred with inactivation at distances around 1000 microm. Loss of inhibition was also elicited from a distance of 1000 microm; such inhibition, however, affected only directionality, without inducing changes in orientation tuning. For several cells at distances >1000 microm from the inactivation site a temporal sequence consisting of a change in direction specificity followed by a reduction of orientation specificity, and finally by direct GABAergic inhibition of the cell under study, could be induced with gradually increasing ejecting currents. The results indicate that excitation and inhibition originating from populations of neurons at different horizontal distances differentially contribute to direction and orientation specificity of a given visual cortical cell.

Axonal topography of cortical basket cells in relation to orientation, direction, and ocular dominance maps

The axonal (bouton) distributions of a layer 4 clutch cell (CC), two layer 3 medium-sized basket cells (MBC), and a layer 3 large basket cell (LBC) to orientation, direction, and ocular dominance maps were studied quantitatively. 1) The CC provided exclusively local projections (<380 microm from the soma) and contacted a narrow "niche" of functional representations. 2) The two MBCs emitted local projections (75% and 79% of all boutons), which were engaged with isoorientations (61% and 48%) and isodirections, and long-range projections (25% and 21%, >313 microm and >418 microm), which encountered cross-orientation sites (14% and 12%) and isoorientation sites (7% and 5%). Their direction preferences were mainly perpendicular to or opposite those of local projections. 3) The LBC provided the majority (60%) of its boutons to long-range distances (>437 microm). Locally, LBC boutons showed a rather balanced contribution to isoorientations (19%) and cross-orientations (12%) and preferred isodirections. Remotely, however, cross-orientation sites were preferred (31% vs. 23%) and the directional output was balanced. 4) Monte Carlo simulations revealed that the differences between the orientation specificity of local and long-range projections cannot be explained by a homogeneous lateral distribution of the boutons. 5) There was a similar eye preference in the local and long-range projection fields of the MBCs. The LBC contacted both contra- and ipsilateral eye domains. 6) The basket axons showed little laminar difference in orientation and direction topography. The results suggest that an individual basket cell can mediate a wide range of effects depending on the size and termination pattern of the axonal field.

Pattern and component motion selectivity in cortical area PMLS of the cat

Visual motion perception is one of the most prominent functions performed by the mammalian cerebral cortex. The moving images are commonly considered to be processed in two stages. The first-stage neurons are sensitive to the motion of one-dimensional orientated components, and their outputs are combined at the second stage to perceive the global motion of the whole pattern. Alternatively, the pattern motion may be signalled by monitoring a distinctive feature of the image, such as a line-end or a corner. In the present study, a series of 'random-line' patterns were used to measure the direction-tuning responses of 138 neurons in the posteromedial lateral suprasylvian area of the cat. The novel stimuli comprised identical thin line segments, with a length : width ratio no less than 10 : 1, which were moved perpendicularly or obliquely to their common orientation during the recordings. When the component lines were much shorter than the size of receptive field, the majority of cells were selective to the direction of pattern motion while only a small subset was sensitive to the direction of component motion. However, the response profiles of most cells became more component-motion selective with the increment of orientation element in stimulus by elongating the component lines in the patterns. These findings imply that the two-stage theory might be incomplete for modelling the visual motion analysis. Even at relatively low levels of the visual system, some kind of nonorientation-based processing may coexist with the orientation-sensitive processing in a dynamic competition, where one rises as the other falls depending upon the strength of the orientation element in the stimulus, so that under some circumstances it becomes possible to signal the veridical direction of pattern motion.

Combined physiological-anatomical approaches to study lateral inhibition

In the visual cortex, large basket cells form the cellular basis of long-range lateral inhibition. The present paper focuses on combinations of methods with which large basket cells can be studied in the context of extensive neuronal representations. In the first approach, the topographic relationship between large basket axons and known functional representations such as orientation, direction, and ocular dominance is analysed. Functional mapping is carried out using extracellular electrode recordings or optical imaging of intrinsic signals followed by 3-dimensional anatomical reconstruction of biocytin stained large basket cells in the same regions. In the second approach, the contribution of lateral inhibition to orientation and direction selectivity is assessed using the GABA inactivation paradigm and direct inhibitory projections from the inactivation to recording sites are demonstrated with biocytin staining and injections of [3H]nipecotic acid, a radioactive marker for GABAergic cells. The limitation of these approaches is that they can only be used in cortical regions which lie on the surface of the brain.

Context, state and the receptive fields of striatal cortex cells

Visual cortical cells are commonly characterized by their receptive-field structure. Originally, a visual receptive field was defined in a purely spatial way as that retinal area from which a change in spiking response of the regarded cell could be elicited by visual stimulation. The first attempts to understand receptive-field structure were based entirely on the anatomical connectivity of the primary visual pathway. More recently, however, it has been discovered that the spatial and temporal context in which a stimulus is presented to a cell can strongly influence its receptive field, and this in turn is dependent on the state of arousal and attention. Accordingly, new concepts recognize that cortical receptive fields are highly dynamic entities embracing more than the sum of the full spatial and temporal response properties of a cell.

Spatial and temporal parameters of cortical inactivation by GABA

Inactivation by GABA is a powerful tool for studying the function of specific cortical regions. It is especially useful in electrophysiology, because inactivation is reversible within short time periods, and because the extent of the inactivated region can be accurately controlled. Iontophoresis of GABA inactivates neurons up to 300 microm around the micropipette. Pressure injection of GABA inactivates neurons further away, but the spatial and temporal characteristics of inactivation by this method have been poorly studied. In order to address this question, we built devices made of micropipettes and microelectrodes glued at various distances. We experienced that repetition of small injections of 100 mM GABA inactivate cortex in a more homogenous way than bolus injections. Diffusion of GABA after pressure injection does not seem to follow a point spread diffusion model as in the case of iontophoresis: GABA probably goes up along the micropipette shaft, and the volume of inactivation has an ellipsoidal form. In order to precisely determine the extent of the inactivated region, we built a mathematical model to fit the experimental data of inactivations obtained above and below the pipette tip. The model provides estimates of the inactivated region for volumes smaller than 60 nl of GABA 100 mM. Limits of inactivation are between 250 and 500 microm lateral to the tip of the pipette. The geometry of inactivation is difficult to predict beyond 60 nl and it seems hazardous to try to inactivate neurons beyond 800 microm with pressure injections of GABA 100 mM.

Evidence for a contribution of lateral inhibition to orientation tuning and direction selectivity in cat visual cortex: reversible inactivation of functionally characterized sites combined with neuroanatomical tracing techniques

We have previously reported that cells in cat areas 17 and 18 can show increases in response to non-optimal orientations or directions, commensurate with a loss of inhibition, during inactivation of laterally remote, visuotopically corresponding sites by iontophoresis of gamma-aminobutyric acid (GABA). We now present anatomical evidence for inhibitory projections from inactivation sites to recording sites where 'disinhibitory' effects were elicited. We made microinjections of [3H]-nipecotic acid, which selectively exploits the GABA re-uptake mechanism, < 100 microm from recording sites where cells had shown either an increase in response to non-optimal orientations during inactivation of a cross-orientation site (n = 2) or an increase in response to the non-preferred direction during inactivation of an iso-orientation site with opposite direction preference (n = 5). Retrogradely labelled GABAergic neurons were detected autoradiographically and their distribution was reconstructed from series of horizontal sections. In every case, radiolabelled cells were found in the vicinity of the inactivation site (three to six within 150 microm). The injection and inactivation sites were located in layers II/III-IV and their horizontal separation ranged from 400 to 560 microm. In another experiment, iontophoresis of biocytin at an inactivation site in layer III labelled two large basket cells with terminals in close proximity to cross-orientation recording sites in layers II/III where disinhibitory effects on orientation tuning had been elicited. We argue that the inactivation of inhibitory projections from inactivation to recording sites made a major contribution to the observed effects by reducing the strength of inhibition during non-optimal stimulation in recurrently connected excitatory neurons presynaptic to a recorded cell. The results provide further evidence that cortical orientation tuning and direction selectivity are sharpened, respectively, by cross-orientation inhibition and iso-orientation inhibition between cells with opposite direction preferences.

Review : Inhibitory Processes in the Visual Cortex

Theoretical and experimental studies have predicted and confirmed, respectfully, the presence of inhibitory processes in the visual cortex. To date, however, the precise role of inhibition in shaping these processes remains unclear. Numerous studies provide evidence that inhibition acts at the single-neuron level, endowing selectivity in these neurons for various stimulus characteristics. Similarly, other studies seem to suggest that inhibition is employed by larger ensembles of neurons, endowing individual neuronal characteristics only through the behavior of the entire network. This article addresses previous views of inhibitory processes and the ways they may be used in developing characteristic properties of neurons in the visual cortex. NEUROSCIENTIST 4:45-52, 1998

GABA-induced inactivation of functionally characterized sites in cat striate cortex: effects on orientation tuning and direction selectivity

Microiontophoresis of gamma-aminobutyric acid (GABA) was used to reversibly inactivate small sites of defined orientation/direction specificity in layers II-IV of cat area 17 while single cells were recorded in the same area at a horizontal distance of approximately 350-700 microns. We compared the effect of inactivating iso-orientation sites (where orientation preference was within 22.5 deg) and cross-orientation sites (where it differed by 45-90 deg) on orientation tuning and directionality. The influence of iso-orientation inactivation was tested in 33 cells, seven of which were subjected to alternate inactivation of two iso-orientation sites with opposite direction preference. Of the resulting 40 inactivations, only two (5%) caused significant changes in orientation tuning, whereas 26 (65%) elicited effects on directionality: namely, an increase or a decrease in response to a cell's preferred direction when its direction preference was the same as that at an inactivation site, and an increase in response to a cell's nonpreferred direction when its direction preference was opposite that at an inactivation site. It is argued that the decreases in response to the preferred direction reflected a reduction in the strength of intracortical iso-orientation excitatory connections, while the increases in response were due to the loss of iso-orientation inhibition. Of 35 cells subjected to cross-orientation inactivation, only six (17%) showed an effect on directionality, whereas 21 (60%) showed significant broadening of orientation tuning, with an increase in mean tuning width at half-height of 126%. The effects on orientation tuning were due to increases in response to nonoptimal orientations. Changes in directionality also resulted from increased responses (to preferred or nonpreferred directions) and were always accompanied by broadening of tuning. Thus, the effects of cross-orientation inactivation were presumably due to the loss of a cross-orientation inhibitory input that contributes mainly to orientation tuning by suppressing responses to nonoptimal orientations. Differential effects of iso-orientation and cross-orientation inactivation could be elicited in the same cell or in different cells from the same inactivation site. The results suggest the involvement of three different intracortical processes in the generation of orientation tuning and direction selectivity in area 17: (1) suppression of responses to nonoptimal orientations and directions as a result of cross-orientation inhibition and iso-orientation inhibition between cells with opposite direction preferences; (2) amplification of responses to optimal stimuli via iso-orientation excitatory connections; and (3) regulation of cortical amplification via iso-orientation inhibition.

Reversible deactivation of cerebral network components

Reversible deactivation techniques have shown that the cerebral network: (1) is dynamic, its functions depending on contemporaneous processing elsewhere in the network; (2) is composed of single nodes that contribute to several behaviors; (3) possesses an inherent plasticity that tends to minimize lesion-induced deficits; and (4) comprises feedforward and lateral connections that contribute in different ways to network operations. The next major advances in understanding network operations will probably be made by applying a combination of behavioral, neuron-recording and deactivation techniques. The greatest near-term gains are likely to be made in understanding the contributions that feedback projections make to cerebral network function.

Functional organization for direction of motion and its relationship to orientation maps in cat area 18

The goal of this study was to explore the functional organization of direction of motion in cat area 18. Optical imaging was used to record the activity of populations of neurons. We found a patchy distribution of cortical regions exhibiting preference for one direction over the opposite direction of motion. The degree of clustering according to preference of direction was two to four times smaller than that observed for orientation. In general, direction preference changed smoothly along the cortical surface; however, discontinuities in the direction maps were observed. These discontinuities formed lines that separated pairs of patches with preference for opposite directions. The functional maps for direction and for orientation preference were closely related; typically, an iso-orientation patch was divided into regions that exhibited preference for opposite directions, orthogonal to the orientation. In addition, the lines of discontinuity within the direction map often connected points of singularity in the orientation map. Although the organization of both domains was related, the direction and the orientation selective responses were separable; whereas the selective response according to direction of motion was nearly independent of the length of bars used for visual stimulation, the selective response to orientation decreased significantly with decreasing length of the bars. Extensive single and multiunit electrical recordings, targeted to selected domains of the functional maps, confirmed the features revealed by optical imaging. We conclude that significant processing of direction of motion is performed early in the cat visual pathway.

Functional organization for direction of motion and its relationship to orientation maps in cat area 18

The goal of this study was to explore the functional organization of direction of motion in cat area 18. Optical imaging was used to record the activity of populations of neurons. We found a patchy distribution of cortical regions exhibiting preference for one direction over the opposite direction of motion. The degree of clustering according to preference of direction was two to four times smaller than that observed for orientation. In general, direction preference changed smoothly along the cortical surface; however, discontinuities in the direction maps were observed. These discontinuities formed lines that separated pairs of patches with preference for opposite directions. The functional maps for direction and for orientation preference were closely related; typically, an iso-orientation patch was divided into regions that exhibited preference for opposite directions, orthogonal to the orientation. In addition, the lines of discontinuity within the direction map often connected points of singularity in the orientation map. Although the organization of both domains was related, the direction and the orientation selective responses were separable; whereas the selective response according to direction of motion was nearly independent of the length of bars used for visual stimulation, the selective response to orientation decreased significantly with decreasing length of the bars. Extensive single and multiunit electrical recordings, targeted to selected domains of the functional maps, confirmed the features revealed by optical imaging. We conclude that significant processing of direction of motion is performed early in the cat visual pathway.

Spatio-temporal summation of synaptic activity in visual cortical pyramidal cells in vitro

We investigated the spatio-temporal summation of excitatory postsynaptic potentials (EPSPs) in supragranular pyramidal cells of the rat extrastriate visual cortex. EPSPs were evoked orthodromically from different locations within the white matter (WM) via an 8-fold multi-electrode array. Stimuli were applied either sequentially from electrodes 1 to 8 or vice versa at defined interstimulus intervals (ISIs) or separately from each electrode. Maximum EPSP amplitudes were evoked from the WM just below the intracellularly recorded neuron. Even 800 microns lateral to this location, small EPSPs could be elicited. A sequential stimulation resulted in a large compound EPSP. In 79% (n = 34) of the cells tested, the compound response was non-directional and could be predicted from responses evoked by single stimulation electrodes. However, 21% (n = 9) of the neurons showed a non-linear spatial summation and a clear preference for the direction of the stimulation sequence. ISI-tuning curves revealed either a sharply tuned, a bandpass, a highpass or a lowpass characteristic for the non-directional as well as directional cells. This feature, together with the clear directional responses observed in some neurons, may be a correlate of the response preference to moving stimuli of cortical cells found in vivo.

Relationship between lateral inhibitory connections and the topography of the orientation map in cat visual cortex

The functional and structural topography of lateral inhibitory connections was investigated in visual cortical area 18 using a combination of optical imaging and anatomical tracing techniques in the same tissue. Orientation maps were obtained by recording intrinsic signals in regions of 8.4-19 mm2. To reveal the inhibitory connections provided by large basket cells, biocytin was iontophoretically injected at identified orientation sites guided by the pattern of surface blood vessels. The axonal and dendritic fields of two retrogradely labelled large basket cells were reconstructed in layer III. Their axonal fields extended up to 1360 microns from the parent somata. In addition to single basket cells, the population of labelled basket cell axons was also studied. For this analysis anterogradely labelled basket axons running horizontally over 460-1280 microns from the core of an injection site in layer III were taken into account. The distribution of large basket cell terminals according to orientation preferences of their target regions was quantitatively assessed. Using the same spatial resolution as the orientation map, a frequency distribution of basket cell terminals dependent on orientation specificity could be derived. For individual basket cells, the results showed that, on average, 43% of the terminals provided input to sites showing similar orientation preferences (+/- 30 degrees) to those of the parent somata. About 35% of the terminals were directed to sites representing oblique-orientation [+/- (30-60) degrees], and 22% of them terminated at cross-orientation sites [+/- (60-90) degrees]. Furthermore, the possible impact of large basket cells on target cells at different distances and orientation preferences was estimated by comparing the occurrence of orientation preferences with the occurrence of basket terminals on the distance scale. It was found that a basket cell could elicit iso-orientation inhibition with a high impact between 100-400 and 800-1200 microns, strong cross-orientation inhibition at approximately 400-800 microns, and oblique-orientation inhibition between 300-500 and 700-900 microns from the parent soma. The non-isotropic topography of large basket axons suggests a complex function for this cell class, possibly including inhibition related to orientation and direction selectivity depending on the location of the target cells and possible target selectivity.

Stimulus dependence of orientation and direction sensitivity of cat LGNd relay cells without cortical inputs: a comparison with area 17 cells

The cortical contribution to the orientation and direction sensitivity of LGNd relay cells was investigated by recording the responses of relay cells to drifting sinusoidal gratings of varying spatial frequencies, moving bars, and moving spots in cats in which the visual cortex (areas 17, 18, 19, and LS) was ablated. For comparison, the spatial-frequency dependence of orientation and direction tuning of striate cortical cells was investigated employing the same quantitative techniques used to test LGNd cells. There are no significant differences in the orientation and direction tuning to relay cells in the LGNd of normal and decorticate cats. The orientation and direction sensitivities of cortical cells are dependent on stimulus parameters in a fashion qualitatively similar to that of LGNd cells. The differences in the spatial-frequency bandwidths of LGNd cells and cortical cells may explain many of their differences in orientation and direction tuning. Although factors beyond narrowness of spatial-frequency tuning must exist to account for the much stronger orientation and direction preferences of cells in area 17 when compared to LGNd cells, the evidence suggests that the orientation and direction biases present in the afferents to the visual cortex may contribute to the orientation and direction selectivities found in cortical cells.

Spatial correlation of suppressive and excitatory receptive fields with direction selectivity of complex cells in cat striate cortex

The aim of the study was to account for direction selectivity of visual cortical neurons through systematic positional mismatch between excitatory and suppressive influences on each neuron. Direction-selective complex neurons were therefore recorded from striate cortex of cats lightly anaesthetized with halothane in nitrous oxide-oxygen. All but small residual eye movements were eradicated with intravenous gallamine triethiodide. Excitatory receptive field (ERF) dimensions and centring were quantified with optimal sine-wave grating stimuli, appropriately windowed to limit them to variable locations along and across the receptive field (RF) centre. Related suppressive receptive fields (SRFS) were similarly mapped during binocular conditioning, induced by an optimal grating applied to the other eye and drifting continuously in each neuron's preferred direction. Its purpose was to elevate ongoing levels of discharge to reveal often concealed null suppression. ERF and SRF profiles were systematically offset, especially along the line of preferred direction such that, for stimuli moving in the non-preferred direction, the SRF lay ahead of the ERF. Derivations of ERFS and SRFS during conditioning, within a single batch of trials, excluded eye movements as a source of positional mismatch. It is concluded that this mismatch may provide the basis for direction selectivity and the emergence of null suppression.

Simulation of dynamic receptive fields in primary visual cortex

A model network of spiking neurons with lateral connections was used to simulate short-term receptive field (RF) changes by removal of afferent input in the primary visual system. Several possible mechanisms for the dynamic RFs were explored and the simulation results were compared with experimental results obtained by Pettet and Gilbert [(1992) Proceedings of the National Academy of Science, U.S.A., 89, 8366-8370]. We found that appropriate input stimuli could induce a shift in the balance between modeled cortical lateral excitation and inhibition and in doing so cause RF expansion. Synaptic plasticity was neither necessary nor appropriate for short-term RF changes. An inhibition dominant network with neural adaptation successfully simulated Pettet and Gilbert's experiment of RF expansion and its reversibility induced by an "artificial scotoma". RF expansions induced by lesions were also explored with the model.

Influences of horizontal connections on visual responses in rabbit striate cortex

The goal of this study was to examine the role of horizontal connections in rabbit striate neurons. Anaesthetized rabbits were prepared in the usual fashion for single-cell recordings in area 17 of the visual cortex. We compared responses evoked by moving and stationary stimuli prior to, during and after recovery from lateral microinjection of either lidocaine (n = 61), gamma-aminobutyric acid (GABA, n = 18) or bicuculline (n = 8) 2 mm from the recording site. This procedure allows evaluation of the contribution of neighbouring neurons in visual responses. Results showed that striate neurons are dependent on the adjacent cells' excitability. Modification of responses to stationary targets suggests that lateral interactions play a role in the generation of discharges to fixed stimuli. Lateral inactivation preferentially influenced non-directional over direction-selective units. This influence usually resulted in the non-directional neuron becoming directional by attenuation of the visually driven response in one direction. Simple and complex cells tended to be influenced differently by lateral inactivation. Simple cells became less responsive, whereas complex cells became more responsive. This dichotomy among cellular types suggests that simple cells receive mainly excitatory horizontal influences, while complex cells are contacted mostly by lateral inhibitory inputs.

Lesion-induced transient suppression of inhibitory function in rat neocortex in vitro

The structural and functional consequences of a local thermolesion were examined in rat neocortex with electrophysiological in vitro techniques and immunocytochemistry. Age-matched untreated and sham-operated animals served as controls and were analysed in the same way. The lesions consisted of a core of coagulated tissue 2-3 mm in diameter and reached ventrally into the deep cortical layers. After two days reactive astrocytes and after nine days a dense gliosis were observed in the immediate vicinity. Modifications in the intrinsic membrane characteristics and the synaptic network properties were investigated with intra- and extracellular recording techniques after survival times of one to eight days. Neurons recorded in the surrounding of lesions in neocortical slices revealed a significantly more depolarized resting membrane potential and a higher neuronal input resistance. In comparison to cells in control slices, maximal discharge rates to injection of depolarizing current pulses of neurons close to a focal lesion were not significantly altered and intrinsic burst firing was never observed. However, between postlesion days 1 and 5, neurons in the surroundings of lesions showed a transient increase in synaptic excitability. This hyperactivity was most clearly pronounced at a distance of 2-3 mm from the centre of the lesion (i.e. about 1-1.5 mm away from the lesion border) and characterized by long-duration field potential responses and multiphasic long-lasting excitatory postsynaptic potentials to orthodromic stimulation of the afferent input. This lesion-induced hyperexcitability was associated with a significant reduction in the peak conductance of the Cl(-)-dependent fast inhibitory postsynaptic potential and the K(+)-dependent long-latency inhibitory postsynaptic potential, suggesting that the intracortical GABAergic system was functionally impaired. The decrease in synaptic inhibition was associated with prolonged N-methyl-D-aspartate receptor-mediated activity, which could be reversibly blocked by D-amino-phosphonovaleric acid. In addition, neurons recorded in the vicinity of the lesion responded to an orthodromic synaptic stimulus with a long-lasting burst. The lesion-induced disturbance in the balance between the excitatory and inhibitory system may not only have profound influences on the mechanisms of intracortical information processing, but may also lead to the expression of epileptiform activity and long-term functional deficits.

Quantification of directional and orientational selectivities of visual neurons to moving stimuli

Directional and orientational components usually coexist and are mixed in the cell's overall responses when moving optical stimuli are used to study the response characteristics of visual neurons. While these two properties were quantified with all the previous methods for data analysis, their effects could not be efficiently separated from each other, and thus the analyses were imperfect. In this paper, theoretical evidence and examples are provided to show the defects of the old methods. In order to separate the two components completely, we propose to apply optimal regression analysis with the sine-cosine function series as the fundamental variables. Based on this separation, we defined the orientational selectivity as variation of response strength with orientation and performed integration and averaging to quantify the two properties [cf. Eqs. (5) and (6)]. The present method has the advantages of completeness and accuracy, and can detect some details which would have been missed by other methods. An explanation of the intrinsic implications of the method and our comprehension of directional and orientational selectivities and preferred direction and orientation are also given.

Functional and structural topography of horizontal inhibitory connections in cat visual cortex

The functional organization of long-horizontal inhibitory connections was studied in cat visual cortical area 17, using a combination of electrophysiological recording and anatomical tracing in the same tissue. Orientation maps were obtained by recording multiunit activity from layer III at regular intervals (100-300 microns) in a region of approximately 1.3 mm2 of cortex at a depth corresponding to the location of the basket cell axons reconstructed later. Before the physiological mapping, the neuronal tracer biocytin had been iontophoretically injected at one functionally characterized site. On the basis of light microscopic features a total of five biocytin-labelled large basket axons, BC1-BC5, were reconstructed from series of horizontal sections of two cats. The parent somata and dendritic fields of three axons (BC1, BC4 and BC5) could also be reconstructed. The axonal field of basket cell BC1 had an overall lateral spread of 1.8 mm. The axons of basket cells BC4 and BC5 spanned a distance of 3.05 and 2.85 mm, respectively. The distribution pattern of histologically reconstructed recording sites and of five labelled basket cell axons were directly compared in the same sections. The results show that a single large basket cell provides input to regions representing the whole range of orientations, i.e. iso-orientation (+/- 30 degrees), oblique orientation (+/- [30-60] degrees) and cross-orientation (+/- [60-90] degrees) to that at the basket cell's soma. Furthermore, the differential effect mediated by the same large basket cell at sites of different orientation preference was numerically estimated for two basket cells (BC4 and BC5) whose preferred orientations could be determined on the basis of recording sites adjacent to their parent somata. We counted the number of axonal terminals of these basket cells at iso-, oblique- and cross-orientation sites and found no significant difference in the average density of terminals at sites of either orientation preference. The functional topography of large basket cell axons indicates that the same basket cell can mediate iso-, oblique- and cross-orientation inhibition at different sites. Hence, we assume that large basket cells serve a complex physiological role depending on the location of target cells in the orientation map.

Several neuronal and axonal types form long intrinsic connections in the cat primary auditory cortical field (AI)

Intrinsic connections in the cat primary auditory field (AI) as revealed by injections of Phaseolus vulgaris leucoagglutinin (PHA-L) or biocytin, had an anisotropic and patchy distribution. Neurons, labelled retrogradely with PHA-L were concentrated along a dorsoventral stripe through the injection site and rostral to it; the spread of rostrally located neurons was greater after injections into regions of low rather than high characteristic frequencies. The intensity of retrograde labelling varied from weak and granular to very strong and Golgi-like. Out of 313 Golgi like retrogradely labelled neurons 79.6% were pyramidal, 17.2% multipolar, 2.6% bipolar, and 0.6% bitufted; 13.4% were putatively inhibitory, i.e. aspiny or sparsely spiny multipolar, or bitufted. Individual anterogradely labelled intrinsic axons were reconstructed for distances of 2 to 7 mm. Five main types were distinguished on the basis of the branching pattern and the location of synaptic specialisations. Type 1 axons travelled horizontally within layers II to VI and sent collaterals at regular intervals; boutons were only present in the terminal arborizations of these collaterals. Type 2 axons also travelled horizontally within layers II to VI and had rather short and thin collateral branches; boutons or spine-like protrusions occurred in most parts of the axon. Type 3 axons travelled obliquely through the cortex and formed a single terminal arborization, the only site where boutons were found. Type 4 axons travelled for some distance in layer I; they formed a heterogeneous group as to their collaterals and synaptic specializations. Type 5 axons travelled at the interface between layer VI and the white matter; boutons en passant, spine-like protrusions, and thin short branches with boutons en passant were frequent all along their trajectory. Thus, only some axonal types sustain the patchy pattern of intrinsic connectivity, whereas others are involved in a more diffuse connectivity.

Visual responsiveness and direction selectivity of cells in area 18 during local reversible inactivation of area 17 in cats

We have investigated the effects of inactivation of localized sites in area 17 on the visual responses of cells in visuotopically corresponding regions of area 18. Experiments were performed on adult normal cats. The striate cortex was inactivated by the injection of nanoliters of lidocaine hydrochloride or of gamma-aminobutyric acid (GABA) dissolved in a staining solution. Responses of the simple and complex cells of area 18 to optimally oriented light and dark bars moving in the two directions of motion were recorded before, during, and after the drug injection. Two main effects are described. First, for a substantial number of cells, the drug injection provoked an overall reduction of the cell's visual responses. This nonspecific effect largely predominated in the complex cell family (76% of the units affected). This effect is consistent with the presence of long-range excitatory connections in the visual cortex. Second, the inactivation of area 17 could affect specific receptive-field properties of cells in area 18. The main specific effect was a loss of direction selectivity of a number of cells in area 18, mainly in the simple family (more than 53% of the units affected). The change in direction selectivity comes either from a disinhibitory effect in the nonpreferred direction or from a reduction of response in the preferred direction. It is proposed that the disinhibitory effects were mediated by inhibitory interneurones within area 18. In a very few cases, the change of directional preference was associated with a modification of the cell's response profile. These results showed that the signals from area 17 are necessary to drive a number of units in area 18, and that area 17 can contribute to, or at least modulate, the receptive-field properties of a large number of cells in the parastriate area.

Direction selectivity of cells in the cat's striate cortex: differences between bar and grating stimuli

We have investigated the notion that directional responses of cells in the visual cortex depend on the type of stimulus used to drive the cell. Specifically, we have asked if sinusoidal gratings provide a different estimate of direction selectivity than bars that are brighter or darker than the background. Using standard techniques, we recorded from 176 cells in the visual cortex of nine cats. For each cell, bright bars, dark bars, and sinusoidal gratings were presented in a randomly interleaved fashion. Complex cells exhibited around twice as many direction-selective as nondirection-selective responses. Estimates of direction selectivity were nearly identical for bright and dark bars and for gratings. For simple cells, a similar ratio of direction-selective to nondirection-selective responses was observed for gratings. However, a larger proportion of simple cells were classified as direction selective when bars were used for stimulation. A simple cell that exhibited direction selectivity to a grating behaved in a similar manner when stimulated with bright or dark bars. However, in contrast to complex cells, some simple cells classed as directionally nonselective on the basis of their responses to gratings, displayed directionally selective behavior to bars. In addition, the preferred directions for dark and bright bars sometimes differed. These results demonstrate that the classification of a simple cell as directionally selective or nonselective can depend critically on the visual stimulus used.

Influence of remote targets on directionality of striate neurons in rabbits

The described investigations study the influence of additional targets located well outside the classical receptive field on responses to motion of cortical cells in rabbits. Animals are anesthetized and prepared for acute single cells recordings in a conventional manner. The interactions between remote targets and central stimuli are abolished with microinjections of lidocaine hydrochloride or GABA at the site excited by remote stimuli. Results show that responses to motion of cortical cells are particularly sensitive to these manipulations. Although supplementary targets fail to influence spontaneous activity of all cells, they do influence responses to motion. Overall, the directionality indices (DI) declined. (53 to 45.) This decline may express itself either by a decrease of responses in the preferred direction or an enhancement of responses in the non-preferred direction or both. By contrast, responses to stationary stimuli are unaffected by additional targets in the visual field. Globally, cells whose directionality index was superior to 50% were significantly more affected then cells whose DI was less than 50%. This result suggests that similarly to cats, the directionality of cells in the striate cortex rests on a very fragile convergence of excitatory and inhibitory influences.

Temporal-frequency tuning of direction selectivity in cat visual cortex

Responses of 71 cells in areas 17 and 18 of the cat visual cortex were recorded extracellularly while stimulating with gratings drifting in each direction across the receptive field at a series of temporal frequencies. Direction selectivity was most prominent at temporal frequencies of 1-2 Hz. In about 20% of the total population, the response in the nonpreferred direction increased at temporal frequencies of around 4 Hz and direction selectivity was diminished or lost. In a few cells the preferred direction reversed. One consequence of this behavior was a tendency for the preferred direction to have lower optimal temporal frequencies than the nonpreferred direction. Across the population, the preferred direction was tuned almost an octave lower. In spite of this, temporal resolution was similar in the two directions. It appeared that responses in the nonpreferred direction were suppressed at low frequencies, then recovered at higher frequencies. This phenomenon might reflect the convergence in visual cortex of lagged and nonlagged inputs from the lateral geniculate nucleus. These afferents fire about a quarter-cycle apart (i.e. are in temporal quadrature) at low temporal frequencies, but their phase difference increases to a half-cycle by about 4 Hz. Such timing differences could underlie the prevalence of direction-selective cortical responses at 1 and 2 Hz and the loss of direction selectivity in many cells by 4 or 8 Hz.

Generation of end-inhibition in striate neurons in rabbits

Eighty-six rabbit striate neurons were tested with lateral microinjection of lidocaine, GABA or bicuculline. Seven of the neurons expressed different levels of end-inhibition. We examined these end-stopping units by injection of lidocaine or bicuculline in adjacent areas in order to determine if a lateral cortical mechanism is underlying end-inhibition in rabbits as it has been proposed in cats. Microinjection of lidocaine resulted in an attenuation of the end-inhibition strength. Application of bicuculline had the opposite effect; the end-inhibition was reinforced. We suggest that as observed in cats, in rabbits end-inhibition is mediated through horizontal cortical connections which implies a postsynaptic inhibitory input to the end-stopping cell.

Neuronal dysfunction at the border of focal lesions in cat visual cortex

Traditional concepts assume that traumatic or ischemic brain lesions are surrounded by regions with depressed neuronal function. More recently hyperactivity gained increasing attention as excitotoxic mechanisms become effective at certain stages of neuronal injury. Single cell recordings in the surrounding of small focal lesions in the cat visual cortex revealed both types of functional pathology 1-30 days after lesioning. A rim of suppressed neurons surrounded a completely silent core. Cells further away from the lesion showed bursts and long lasting hyperactivity with extremely high discharge rates. Consequently, the volume of disturbed tissue was considerably larger than the region of initial cell death. This halo of dysfunction may be important for neurological symptoms evoked by cortical lesions.

Subtraction inhibition combined with a spiking threshold accounts for cortical direction selectivity

We have modeled simple-cell direction selectivity by a nonlinearity consisting of a subtraction inhibition followed by half-wave rectification and compared the performance of this model to that of different versions of the elaborated Reichardt detector for similar inputs and parameter settings. Not only does the subtraction model fit the experimental data more closely than the elaborated Reichardt detector, but the subtraction model also is more plausible from a physiological and anatomical point of view. Moreover, the subtraction model operates optimally at plausible spatiotemporal parameter settings. Therefore, we conclude that there is no need to invoke specific synaptic interactions, such as implied in the Reichardt detector, to account for simple-cell direction selectivity.

An intracellular recording study of stimulus-specific response properties in cat area 17

Stimulus-specific response properties, such as direction or orientation selectivity, were studied intracellularly in cells recorded from area 17 of the cat. In all 5 direction selective complex cells and one orientation selective simple cell successfully studied, visually evoked excitatory postsynaptic potentials (EPSPs) were tuned to the preferred direction or orientation. Visually evoked inhibitory postsynaptic potentials (IPSPs) were also tuned to the preferred direction/orientation of stimulus. IPSPs evoked by the non-preferred stimulus when present were smaller than those evoked by the preferred stimulus. IPSPs were undetected in two of the 5 cells tested. These results suggest that directionally/orientationally tuned EPSPs make a major contribution to stimulus specificity in visual cortical neurons but IPSPs evoked by a stimulus with null-direction/orientation may sharpen the stimulus specificity.

Correlations between directional and orientational tuning of cells in cat striate cortex

Simple (N = 284) and complex cells (N = 125) in the central projection area (0-5 degrees eccentricity) of the striate cortex of cats were stimulated with moving light bars and the responses to different directions of movement were recorded and plotted as polar-plots. Fourier analysis was applied to polar plots (SDO-analysis, Wörgötter and Eysel 1987; Wörgötter et al. 1990) to determine the general sensitivity (S) of the cells to visual stimulation, the directional (D) and orientational (O) tuning strength as well as preferred direction (PD) and preferred orientation (PO). Statistical distributions of the S, D and O parameters were determined for simple and complex cells of the cortical layers II-VI. Simple cells were more strongly tuned for direction and orientation than complex cells, whereas complex cells had a greater general sensitivity to visual stimulation. Directional tuning was significantly stronger in layer VI than in layer IV simple cells, otherwise no differences were detected between these two layers. We found that cells with large D and small O components are generally rare. The D and O components were plotted against each other to determine any possible correlation between the tuning strengths. The correlations were statistically significant for simple and complex cells but the correlation coefficients were very small (r less than 0.3). It is suggested that only a very weak coupling between directional and orientational tuning exists, preferentially in the deeper layer simple cells.

Mechanisms of inhibition in cat visual cortex

1. Neurones from layers 2-6 of the cat primary visual cortex were studied using extracellular and intracellular recordings made in vivo. The aim was to identify inhibitory events and determine whether they were associated with small or large (shunting) changes in the input conductance of the neurones. 2. Visual stimulation of subfields of simple receptive fields produced depolarizing or hyperpolarizing potentials that were associated with increased or decreased firing rates respectively. Hyperpolarizing potentials were small, 5 mV or less. In the same neurones, brief electrical stimulation of cortical afferents produced a characteristic sequence of a brief depolarization followed by a long-lasting (200-400 ms) hyperpolarization. 3. During the response to a stationary flashed bar, the synaptic activation increased the input conductance of the neurone by about 5-20%. Conductance changes of similar magnitude were obtained by electrically stimulating the neurone. Neurones stimulated with non-optimal orientations or directions of motion showed little change in input conductance. 4. These data indicate that while visually or electrically induced inhibition can be readily demonstrated in visual cortex, the inhibition is not associated with large sustained conductance changes. Thus a shunting or multiplicative inhibitory mechanism is not the principal mechanism of inhibition.

An intracellular analysis of the visual responses of neurones in cat visual cortex

1. Extracellular and intracellular recordings were made from neurones in the visual cortex of the cat in order to compare the subthreshold membrane potentials, reflecting the input to the neurone, with the output from the neurone seen as action potentials. 2. Moving bars and edges, generated under computer control, were used to stimulate the neurones. The membrane potential was digitized and averaged for a number of trials after stripping the action potentials. Comparison of extracellular and intracellular discharge patterns indicated that the intracellular impalement did not alter the neurones' properties. Input resistance of the neurone altered little during stable intracellular recordings (30 min-2 h 50 min). 3. Intracellular recordings showed two distinct patterns of membrane potential changes during optimal visual stimulation. The patterns corresponded closely to the division of S-type (simple) and C-type (complex) receptive fields. Simple cells had a complex pattern of membrane potential fluctuations, involving depolarizations alternating with hyperpolarizations. Complex cells had a simple single sustained plateau of depolarization that was often followed but not preceded by a hyperpolarization. In both simple and complex cells the depolarizations led to action potential discharges. The hyperpolarizations were associated with inhibition of action potential discharge. 4. Stimulating simple cells with non-optimal directions of motion produced little or no hyperpolarization of the membrane in most cases, despite a lack of action potential output. Directional complex cells always produced a single plateau of depolarization leading to action potential discharge in both the optimal and non-optimal directions of motion. The directionality could not be predicted on the basis of the position of the hyperpolarizing inhibitory potentials found in the optimal direction. 5. Stimulation of simple cells with non-optimal orientations occasionally produced slight hyperpolarizations and inhibition of action potential discharge. Complex cells, which had broader orientation tuning than simple cells, could show marked hyperpolarization for non-optimal orientations, but this was not generally the case. 6. The data do not support models of directionality and orientation that rely solely on strong inhibitory mechanisms to produce stimulus selectivity.

Influence of GABA-induced remote inactivation on the orientation tuning of cells in area 18 of feline visual cortex: a comparison with area 17

We have investigated the effect of iontophoretically applying the inhibitory transmitter gamma-aminobutyric acid (GABA) through four pipettes, each located at a horizontal distance of some 500-600 microns from the recording site, on the orientation tuning of cells in areas 17 and 18 of the cat visual cortex for moving the stationary flash-presented bar stimuli. Forty-five of 74 cells tested in area 18 (61%) showed a significant (greater than 25%) increase in orientation tuning width (at half the maximum response) during GABA application, which reflected an increase in response to non-optimal orientations. The mean orientation tuning width of these cells increased by 79%, and the ratio of responses to the orientation orthogonal to the optimum and to the optimum increased from 0.16 to 0.46. The results were similar to those from area 17, in which 36 of 54 cells (66%) showed significant broadening of orientation tuning during GABA application, with a 90% increase in mean tuning width and an increase in the relative response to the orientation orthogonal to the optimum from 0.17 to 0.42. The distributions of cells in areas 17 and 18 with respect to the magnitude of GABA-induced effects on orientation tuning width were not significantly different (mean increase in tuning width: area 17, 102%; area 18, 87%). Although most cells were tested only with moving bars, comparable effects of remote GABA application on orientation tuning were observed when stationary flash-presented bars were used. Of 11 cells thus tested in area 18, seven showed significantly broader tuning during GABA application, with a 132% increase in mean tuning width. In some 25% of cells in each area which showed a significant effect of GABA application on orientation tuning the response to at least one non-optimal orientation exceeded, during GABA application, the response to the previous optimum. There was essentially no correlation between the changes in orientation tuning and changes in the level of spontaneous activity or in the response to the optimum orientation during GABA application. Thus, an increase in the general excitability of recorded cells or the loss of an unspecific inhibitory input cannot account for the effects of GABA application on orientation tuning. Remote GABA application presumably inactivated cells with different preferred orientations from that of the recorded cell. It is thus argued that the observed broadening of orientation tuning during GABA application reflected the loss of an inhibitory input tuned to non-optimal orientations.(ABSTRACT TRUNCATED AT 400 WORDS)

Cholinergic enhancement of direction selectivity in the visual cortex of the cat

Acetylcholine and acetyl-beta-methacholine were applied iontophoretically to single cells in the feline striate cortex. The directional bias of the visual response to an optimally oriented stimulus was assessed quantitatively, before and during drug application. For the great majority of the cells that were affected by the drugs, selectivity was either unchanged (28/60, 47%) or increased (21/60, 35%). In particular, directional bias increased for 36% (12/33) of the cells that were facilitated by acetylcholine or acetyl-beta-methacholine and 43% (nine out of 21) of those that were inhibited, as compared with 9% (three out of 33) and 24% (five out of 21) for which the bias decreased. Six additional cells, of which three showed a reduced selectivity, were apparently excited by the drugs, in that the background discharge level was greatly increased with a concomitant decrease in signal-to-noise ratio. It is known that cholinergic input has the potential to enhance cortical function, by facilitating both the excitatory and the inhibitory components of the neuronal circuit. Our data show that this combination of effects can precipitate an enhancement of selectivity as well as of response magnitude.

Directionality of cat striate cortical neurones: contribution of suppression

Direction-selective or direction-biased striate cortical neurones were assessed for absence or incidence of suppression of firing, maximal at 90 degrees or 180 degrees ("null" suppression) to the optimal direction, in 327 neurones recorded from the striate cortex of cats anaesthetized with N2O/O2/halothane. Stimuli were light or dark bars moving over uniform or stationary textured backgrounds; or square-wave gratings of optimal spatial frequency and velocity. Five identified directionality groups were correlated with neuronal class and a range of other receptive field properties. Suppression maximal at 90 degrees to optimum was common amongst direction-biased neurones, rare amongst direction-selective neurones. In the latter group, null suppression (maximal at 180 degrees to optimum) was more prevalent than at 90 degrees. Standard complex cells constituted the majority of complex neurones. They were more commonly direction-biased and less commonly showed suppression than special complex cells. The latter comprised the majority of direction-selective neurones with 180 degrees suppression. Endstopping was seen more frequently in special complex cells, but for each functional class was similarly distributed between the different directionality groups. Based on the mean and mode of partially overlapping distributions, for all neuronal classes direction-selective neurones were more broadly tuned than direction-biased neurones. Special complex neurones were appreciably more broadly tuned than standard complex neurones; those with suppression at 180 degrees were the most broadly tuned neurones in the cortex. Direction-biased neurones with suppression at 90 degrees to optimum were more sharply tuned than those lacking such suppression. Direction-selective neurones had larger receptive fields than direction-biased neurones.(ABSTRACT TRUNCATED AT 250 WORDS)

GABA-induced remote inactivation reveals cross-orientation inhibition in the cat striate cortex

We investigated the contributions of lateral intracortical connections to the orientation tuning of area 17 cells using micro-iontophoresis of the inhibitory transmitter gamma-aminobutyric acid (GABA) to inactivate small cortical sites in the vicinity of a recorded cell. GABA was ejected from an array of micropipettes each with an average horizontal distance of 500 microns from the recording site. Of 54 cells tested, 33 showed a reduction and 3 a loss of orientation selectivity due to an increase in responses to non-optimal orientations during GABA inactivation. The response to the optimal orientation remained constant in more than half of the cells and increased or decreased in others. Given that a complete cycle of orientations occupies a tangential distance of 1000 microns, the observed broadening of orientation tuning is presumably due to GABA-mediated inactivation of inhibitory interneurones with different preferred orientations from those of their target cell.

Influence of spatial frequency on tuning and bias for orientation and direction in the cat's striate cortex

Directionality, orientation and spatial frequency tuning were determined for 108 neurones recorded extracellularly from the striate cortex of anaesthetized cats. Significant sharpening of orientation selectivity with increasing spatial frequency was seen in all simple neurones and the overwhelming majority of complex neurones. Orientation selectivity sharpened in 90 and broadened in only 10 of 100 fully characterized neurones. At least four distinct classes of neurone could be characterized on the basis of their directionality at optimal spatial frequency, and the presence or absence of changes in directionality over a range of spatial frequencies: in two classes, directionality was spatial-frequency dependent; in the remaining two it was invariant. With two exceptions Type A neurones (23 cells) were direction-selective; they were narrowly tuned for orientation and spatial frequency, and their directionality was invariant with spatial-frequency. The majority of neurones (52 cells) were Type B, most of which were direction-biased; their bias for direction varied systematically with spatial frequency. Type C were direction-biased and spatial-frequency selective (5 cells), but showed a clear reversal of bias with change in spatial frequency. Type D, a subset of direction-biased cells, were bidirectional and spatial-frequency invariant (8 cells), with comparable response strengths to motion in two opposing directions at all spatial frequencies. These response types crossed traditional boundaries between categories of simple and complex neurones, assigned on the basis of spatial summation, presence or absence of end-inhibition, and receptive field size.

Pharmacological analysis of cortical circuitry

The cortical circuitry of the visual cortex has been worked out in great detail. Anatomical investigations reveal stereotyped connections within cortical columns and specific long-range connections between distant columns. Pharmacological techniques for blocking the activity in individual cortical layers or columns allow the microdissection of the cortical circuit. These studies could relate specific functional roles to particular cortical connections.