Effects of ketamine on orientation selectivity and variability of neuronal responses in primary visual cortex

The plasticity of the adult brain is one of the most highly evolving areas of recent neuroscience research. It has been acknowledged that the visual cortex in adulthood can adapt and restructure the neuronal connections in response to a sensory experience or to an imposed input such as in adaptation or ocular deprivation protocols. In order to understand the basic cellular mechanisms of plasticity in the primary visual cortex (V1), we examined the effects of ketamine, a non-competitive, glutamatergic NMDAR (N-methyl-D-aspartate receptor) antagonist, on the orientation of cortical cells by measuring their response variability and the Gaussian tuning curves in adult anesthetised mouse and cat. Neurons were recorded extracellularly using glass electrodes. The ketamine was applied locally by placing a custom-cut filter paper (1x1mm) soaked in ketamine solution (10 mg/ml) on the cortical surface next the site of the recording tip, in both species. Our results show that the local and acute exposure of ketamine on V1 changes the preferred orientation of the visual neurons established during the critical period of development. Furthermore, ketamine also leads to a decrease in the orientation selectivity (measured by orientation selectivity index, OSI) and the variability of neuronal evoked responses (measured by Fano factor), but does not affect spontaneous activity. These results suggest that ketamine induces plasticity in V1 neurons that might be operated by a different pathway than that of NMDARs.

Modulation of functional connectivity following visual adaptation: homeostasis in V1

Sensory neurons exhibit remarkable adaptability in acquiring new optimal selectivity to unfamiliar features when a new stimulus becomes prevalent in the environment. In conventionally prepared adult anesthetized cats, we used visual adaptation to change the preferred orientation selectivity in V1 neurons. Cortical circuits are dominated by complex and intricate connections between neurons. Cross-correlation of cellular spike-trains discloses the putative functional connection between two neurons. We sought to investigate changes in these links following a 12 min uninterrupted application of a specific, usually non-preferred, orientation. We report that visual adaptation, mimicking training, modulates the magnitude of crosscorrelograms suggesting that the strength of inter-neuronal relationships is modified. While individual cell-pairs exhibit changes in their response correlation strength, the average correlation of the recorded cell cluster remains unchanged. Hence, visual adaptation induces plastic changes that impact the connectivity between neurons.

Repetitive adaptation induces plasticity of spatial frequency tuning in cat primary visual cortex

Sensory neurons display transient changes in their response properties following prolonged exposure to an appropriate stimulus (adaptation). In adult cat primary visual cortex, spatial frequency-selective neurons shift their preferred spatial frequency (SF) after being adapted to a non-preferred SF. In anesthetized cats prepared for electrophysiological recordings in the visual cortex, we applied a non-preferred spatial frequency for two successive periods of adaptation (a recovery and interval of ∼90 min separated both phases of adaptation) in order to determine if a first adaptation retained an influence on a second adaptation. The first application of a non-preferred SF shifted the tuning curve of the cell mainly in the direction of the imposed SF. The results showed that attractive shifts occurred more frequently (68%) than repulsive (12%) changes in cortical cells. The increase of responsivity was band-limited and occurred around the imposed SF, while flanked responses remained unmodified in all conditions. After a recovery period allowing neurons to restore their original SF tuning curves, we carried out a second adaptation which produced four major results: (1) a higher proportion of repulsive shifts (31%) compared to attractive shifts (49%), (2) an increase of the magnitude of the attractive shifts, (3) an additional enhancement of the evoked firing rate for the newly acquired SF, and (4) for the acquired SF the variability coefficient decreased following the second adaptation. The supplementary response changes suggest that neurons in area 17 keep a "memory" trace of the previous stimulus properties. It also highlights the dynamic nature of basic neuronal properties in adult cortex since repeated adaptations modified both the spatial frequency tuning selectivity and the response strength to the preferred spatial frequency. These enhanced neuronal responses suggest that the range of adaptation-induced plasticity available to the visual system is broader than anticipated.

Adaptation changes the spatial frequency tuning of adult cat visual cortex neurons

The modular layout of striate cortex is arguably a hallmark of all cortical organization. Neurons of a given module or domain respond optimally to very few specific properties, such as orientation or direction. However, it is possible, under appropriate conditions, to compel a neuron to respond preferentially to a different optimal property. In anesthetized cats, prepared for electrophysiological recordings in the visual cortex, we applied a spatial frequency (SF) that differs (by 0.25-3.0 octaves) from the optimal one for 7-13 min without interruption. This application shifted the tuning curve of the cell mainly in the direction of the imposed SF. Indeed, results indicate an attractive push occurring more frequently (50%) than a repulsive (30%) shift in cortical cells. The increase of responsivity is band-limited and is around the imposed SF, while flanked responses remained unmodified in all conditions. We hypothesize that the observed reversible plasticity is obtained by a modulation of the balance between the strengths of the respective synaptic inputs. These changes in preferred original optimal spatial frequencies may allow a dynamic reaction of cortex to a new environment and particularly to ''zoom'' cellular activity toward persistent stimuli in spite of the tuning inherited from genetic programming of response properties and environmental conditions during critical periods in new born animals.

Responses of inferior collicular cells to species-specific vocalizations in normal and enucleated rats

The inferior colliculus (IC) is an obligatory relay for the ascending and descending auditory pathways. Cells in this brainstem structure not only analyze auditory stimuli but they also play a major role in multi-modal integration of auditory and visual information. The aim of the present study was to determine whether cells in the central nucleus of the inferior colliculus (CNIC) of normal rats respond selectively to complex auditory signals, such as species-specific vocalizations, and compare their responses to those obtained in neonatal bilateral enucleated (P2-P3) adult rats. Extra-cellular recordings were carried out in anesthetized normal and enucleated rats using auditory stimuli (pure tones, broadband noise and vocalizations) presented in free field in a semi-anechoic chamber. The results indicate that most cells in the CNIC of both groups respond selectively to species-specific vocalizations better than to the same but inverted sounds. No significant differences were found between the normal and enucleated rat groups in their responses to broadband noise and pure tones.

Effects of excitation and inactivation in area 17 on paired cells in area 18

This investigation examines how neighboring neurons of area 18 react when area 17 inputs are excited or depressed. In anesthetized cats, area 18 responses to a sine-wave grating in the receptive field were analyzed, while a second grating was positioned in its periphery and responses were recorded in area 17. This latter site was also inactivated with GABA. A waveform template process sorted out at least two individual, neighboring cells with similar orientation preferences in area 18. These cells frequently displayed opposite reactions to stimulation and inactivation in area 17. Experiments suggest that nearby neurons belonging to the same functional domain in the visual cortex may simultaneously carry disparate information.

Contextual modulation of synchronization to random dots in the cat visual cortex

Synchronization of neuronal activity has been proposed as a binding mechanism for integration of image properties into one coherent percept. In the present study, we investigated the contextual modulation of synchronization to random dot patterns. Coherent motion of random dots evoked well synchronized responses in area 17 of anaesthetized cats when the stimulus was presented in the compound receptive field of recorded sites. Gradually changing the directional coherence of random dots in the surround while maintaining fully coherent motion of the stimulus in the receptive field significantly suppressed synchronization of neuronal activity for some stimulus conditions. However, usually one or two peaks of increased synchronization were found in the surround coherence tuning curves with low (8-12%) and/or moderate (25-50%) coherence in the surround. At the population level, synchronization was significantly depressed with incoherent motion in the receptive field and when both the surround and the receptive field were jointly stimulated with 0% coherence. The intriguing finding was the discovery of two distinct groups of cells with opposite synchronization changes dependent on the presence or absence of significant synchronization in their spontaneous activity. The latter group of neurons showed peaks of increased synchronization with lower surround coherence, thus probably being more sensitive to the direction of the surround motion. Overall, our findings support the notion that binding of stimulus properties can be achieved by synchronized activity of cortical cells. However, our findings go further than the original hypothesis of feature binding by synchrony to show that synchronization of cortical activity may be directly related to the decision making processes, which in turn are related to the threshold of perception of coherent motion.

Nitric oxide and its influence on oscillations of collicular responses in developing rats

We investigated the influence of modulating NO synthesis on oscillatory components of ON and OFF evoked field potentials in developing rat superior colliculus. Nitric oxide (NO) is involved in neuronal transmission by adjusting neurotransmitter release in adults and in stabilizing synaptic connections in developing brains. NO synthesis was decreased by inhibiting nitric oxide synthase (NOS) with an acute microinjection of N-nitro-L-arginine methyl ester (L-NAME); whereas NO synthesis was augmented by an acute microinjection of L-arginine (L-ARG). The study is focused on rhythmic activity by analyzing fast Fourier transform (FFT). Collicular responses were recorded in anesthetized rats, at post-natal days (PND) 13-19 and adults. This time window was chosen because it is centered on eye opening. NO down- and upregulation resulted in a dual effect depending on age and response-type. NO synthesis inhibition decreased the magnitude of oscillations in ON responses in the youngest animals (PND13-PND14), whereas oscillations of frequencies higher than 20 Hz in OFF responses were increased in all age groups of developing rats. In adults NO downregulation increased oscillations in ON responses and decreased oscillations in OFF responses. L-arginine application produced effects opposite to those seen with L-NAME. Our data together with results reported in the literature suggest that the temporal patterns of the evoked activity are NO-dependent. This sculpting action of the evoked firing may play a role in the synchronization of action potentials in afferent axons which in turn contributes to synaptic stabilization.

Modulation of the synchronization between cells in visual cortex by contextual targets

It has been suggested that synchronization of action potentials encodes diverse features of a single image. However, properties of the synchronization, which occurs on a time scale of approximately 1-5 ms, are still poorly understood. We have tested the modulation of synchronization by manipulating the contextual targets introduced in the surround of the receptive field. Experiments were carried out on anaesthetized cats prepared for multiunit and single-cell recordings in area 17. Initially, a patch of sine-wave drifting grating was positioned over the overlapping receptive fields of several neurons. If this coherent motion produced a significant synchronization in cross-correlograms, contextual targets were added. The first contextual stimuli were two sine-wave patches placed above and below the central compound receptive field. Only the contrast of contextual targets changed. Results show that the larger the differential contrast the higher the synchronization. The second contextual stimulus was a lateral shift of a sine-wave patch. Data show that the wider the distance between the central and peripheral patches the better the synchronization. Furthermore, results suggest that the synchrony pattern computed by cross correlating multiunit recordings from two sites differs when the cross correlation is carried out between individual units belonging to each multiunit recording. Together with our previous results it appears that synchronization is stimulus dependent and its strength increases with larger disparities included in the whole stimulating image.

Comparative computations of spike synchronization in visual cortex of cats

In recent years it has been proposed that synchronous activity between neurons is a putative mechanism to bind together various trigger features of an image. Thus the measure of synchronization becomes an important issue since it may be an electrophysiological sign of visual perception. This paper describes and compares six techniques of computing synchronization strength, that is, the central peak of a cross-correlogram. Data were obtained in anesthetized cats prepared for electrophysiological recordings in a conventional fashion. Results indicate that: (1) eye fits are misleading. Visual inspection of cross-correlograms, may be interesting if one needs to estimate approximately synchronization strength and the presence of oscillations in the cross-correlograms, however it may be misleading if one wants to compare different cross-correlograms; (2) regression analysis to compare one method against the others yields a relatively poor correlation suggesting that methods are not directly comparable; (3) the sensitivity of each computational method is unequal. The results may indicate that some functional connections are either under- or over-evaluated depending upon the strategy employed to measure synchronization.

Cellular response to texture and form defined by motion in area 19 of the cat

The present study examined the neuronal sensitivity in area 19 of the cat to a motion-defined bar and to texture. Sensitivity was tested in normal, lesioned (areas 17-18) and split-chiasm cats using a kinematogram, as well as a textured bar drifting on a uniform light background and a light bar drifting on a stationary textured background. Texture density was varied. The results indicate that almost all cells of area 19 recorded in the three groups of cats responded to a motion-defined bar or to its edges. Texture density influenced the responses in that the discharge rate increased as density decreased. However, the majority of cells were sensitive to the highest texture density kinematogram. Moreover, the neural responses of all cats were either independent of the density of the textured bar or background, or were modulated by it. These results show that cells in area 19 can signal the presence of a kinetic bar and that the density of either the textured bar, the background or both can influence figure-ground detection. The results are interpreted with respect to how various inputs influence the function of area 19.

Stimuli outside the classical receptive field modulate the synchronization of action potentials between cells in visual cortex of cats

It is proposed that various attributes of an image are bound neuronally when responsive units fire in synchrony. Our investigations describe the influences of the contextual stimuli upon the occurrence of synchronization, in anaesthetized cats. Once a significant synchronization was recorded in the cross-correlogram (XCRG) between evoked action potentials of two groups of neurons in response to a drifting sine-wave grating, additional gratings were positioned outside the compound receptive field. The synchronization strength was then measured in relation to the difference between the orientations of the central and peripheral gratings. In the majority of cases results indicate that the synchronization is facilitated with larger orientation disparities. Thus, our data support the notion that contrasting features of images facilitate synchrony of activity between neurons.

Relationships between image structure and gamma oscillations and synchronization in visual cortex of cats

The relationships between visual object configurations and interneuronal spike synchronization and gamma oscillations are examined in the present investigation. Cells were initially stimulated with moving, optimally oriented, single 20 degrees -long bars of light, centred on the compound receptive field of a pool of cortical neurons. When this kind of stimulus evoked intrinsic gamma oscillations and/or synchronization, we gradually fractured the original target. In addition, colinearity was ruptured by forming L- and T-shaped configurations. All fractures and discontinuities were introduced well outside the excitatory receptive field. Multiunit activity in the visual cortex (areas 17 and 18) was recorded in anaesthetized cats. Recording sites were separated by 0.4-1.2 mm. The data analysis indicates that gamma oscillations follow a rule by which unfractured bars yielded the highest S/N ratios. Synchronization strength, as revealed by the central peak in cross-correlograms, also seemed to depend upon stimulus configuration. However, the magnitude of the central peak failed to follow a consistent trend. For instance, the greatest magnitude of the central peak occurred for both colinear and orthogonal types of target. Our results support the notion that both gamma oscillations and neuronal synchronization are stimulus-dependent.

Cross-correlated and oscillatory visual responses of superficial-layer and tecto-reticular neurones in cat superior colliculus

The present study examined, in the superior colliculus (SC) of anaesthetised cats, the functional connectivity between superficial-layer neurones (SLNs) and tectoreticular neurones (TRNs: collicular output cells). TRNs were antidromically identified by electrical stimulation of the predorsal bundle. The auto- and cross-correlation histograms of visual responses of both types of neurones were recorded and analysed. A delayed, sharp peak in cross-correlograms allowed us to verify whether SLN and TRN cells were coupled; in addition, oscillatory activities were compared to verify if rhythmic responses of SLN sites were transmitted to TRN sites. We found that oscillatory activity was rarely observed in spontaneous activity of superficial (1/74) and TRN sites (1/48). Moving light bars induced oscillation in 31% (23/74) of the superficial-layer and in 23% (11/48) of the TRN sites. The strength of the rhythmic responses was determined by specific ranges of stimulus velocity in 83% (19/23) and 64% (7/11) of oscillating SLN and TRN sites, respectively. Frequencies of oscillations ranged between 5 and 125 Hz and were confined, for 53% of the cells, to the 5-20 Hz band. Thus, the band-width of frequencies of the stimulus-related oscillations in the superior colliculus was broader than the gamma range. Analysis of cross-correlation histograms revealed a significant predominant peak with a mean delay of 2.7+/-0.9 ms in 46% (17/37) of SLN-TRN pairs. Most correlated SLN-TRN pairs (88%: 15/17) had superimposed receptive fields, suggesting they were functionally interconnected. However, individual oscillatory frequencies of correlated and oscillatory SLN and TRN cells were never the same (0/8). Together, these results suggest that the neurones in collicular superficial layer contact TRNs and, consequently, support the idea that the superficial layers contribute to collicular outputs producing eye- and head-orienting movements.

Regulation of endothelial nitric oxide synthase by PGD(2) in the developing choroid

We investigated if prostaglandins might regulate the increased choroidal endothelial (e) nitric oxide synthase (NOS) expression in the perinate. Prostaglandins, eNOS mRNA, immunoreactive protein and activity, and nitrite [stable metabolite of nitric oxide (NO)] production were markedly higher in newborn (1 day old) than juvenile (6-8 wk old) pig choroid. Treatment of isolated newborn choroids with the prostaglandin synthase inhibitor ibuprofen for 24 h reduced eNOS mRNA and nitrite production to values in juveniles. This effect was equally observed with the PGD(2) receptor (DP) blocker BW A868C and was prevented by cotreatment with PGD(2) but not other prostaglandins; similar observations were made on NOS activity in vivo. PGD(2) also increased eNOS expression on choroids of juveniles, and this effect was blocked by BW A868C. The manifestation of this upregulation of eNOS by PGD(2) on the control of choroidal vasomotor response was tested by using NO-dependent vasorelaxants, ACh, bradykinin (Bk), and substance P (SP). ACh-, Bk-, and SP-elicited choroidal vasorelaxation was greater in saline-treated newborn than juvenile pigs. Ibuprofen (24 h) decreased ACh-, Bk-, and SP-evoked vasorelaxation in newborns, whereas PGD(2) increased that in juveniles and prevented the ibuprofen-induced attenuated relaxation in newborns; infusion of N(omega)-monomethyl-L-arginine in choroids of those animals treated with PGD(2) reversed the augmented vasorelaxation to ACh, Bk, and SP. Finally, PGD(2)-induced upregulation of NOS in the perinate was also reflected by curtailed choroidal blood flow autoregulatory response to increased perfusion pressure. In conclusion, PGD(2) exhibits a major role in upregulating eNOS expression and activity in the choroid, which in turn results in greater NO-mediated vasorelaxation; a new mechanism for eNOS regulation via DP is hereby disclosed. The relationship between PGD(2) and eNOS in the developing subject provides an explanation for the interactive role of these two factors in the absent choroidal blood flow autoregulation in the perinate.

Developmental regulation of endothelial nitric oxide synthase in cerebral vessels of newborn pig by prostaglandin E(2)

We investigated whether prostaglandins regulate endothelial nitric oxide synthase (eNOS) in the pig cerebral vasculature during the neonatal period. Prostaglandins, eNOS mRNA, eNOS protein, and NO production were higher in cerebral microvessels of newborn (1 day old) than in those of adult (6- to 8-month-old) pigs. The treatment of isolated cerebral microvessels of newborn animals with ibuprofen for 24 h reduced eNOS mRNA and nitrite production to levels in the adult; this effect of ibuprofen was prevented by concurrent treatment with prostaglandin (PG)E(2) analog 16,16-dimethyl-PGE(2), nonselective PGE(2) receptor analog 11-deoxy PGE(1), and prostaglandin EP(3) receptor agonists sulprostone and M&B 28,767 but was not modified by PGI(2) analog carbaprostacyclin, PGD(2), and EP(1) receptor agonist 17-phenyl trinor PGE(2). Correspondingly, 16, 16-dimethyl-PGE(2) and M&B 28,767 increased eNOS mRNA expression of adult microvessels to values in the newborn. Data similar to those with isolated cerebral vessels were obtained through histochemical analysis (NADPH-diaphorase positivity) of brain from newborn animals treated in vivo with ibuprofen in combination or not with sulprostone. Furthermore, substance P-induced NO-mediated cerebral vasorelaxation was decreased to adult values through the treatment of newborn pigs with ibuprofen; this effect was prevented by concomitant treatment with sulprostone. It is concluded that PGE(2) regulates eNOS in newborn pig cerebral microvessels via EP(3) receptors; this may be physiologically required during normal neurovascular development.

Pulvinar participates in synchronizing neural assemblies in the visual cortex, in cats

It has been proposed that the perception of a coherent image necessitates two processes, that is, an ensemble of neurons which synchronizes discharges of individual cells and stimulus-specific gamma-band (gamma) neuronal oscillations which may serve as carrier signals for a temporal code. We tested the hypothesis that cortical gamma-oscillations and synchronization depend upon the interactions between the lateral posterior-pulvinar complex of the thalamus (LP-P) and visual cortex. Local reversible inactivation of the LP-P was achieved by pressure injections of gamma-aminobutyric acid (GABA). In the majority of cases the LP-P depression decreased the strength of the synchronization and oscillations. Also, the results demonstrate that the occurrence of stimulus-dependent oscillations and the synchronization of neuronal responses are two distinct processes and consequently they may occur or disappear independently of each other.

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.

Maturation of visual receptive field properties in the rat superior colliculus

Visually responsive neurons were recorded in the superficial layers of rat superior colliculus from postnatal day 12 to 28. Receptive field properties such as size, type (ON, OFF, ON-OFF and motion sensitive) and direction selectivity were analyzed to disclose changes during maturation. Although some aspects of sensory properties are modified during development (latency, receptive field sizes, and proportions of receptive field types), a high level of sophistication is also present in young animals even before eyelid opening. For instance, direction selective and direction biased cells, which require complex synaptic relations, are already observed when the first light evoked responses emerge in the superior colliculus (P13), strongly suggesting that this property develops without visual experience. Furthermore, direction selectivity is present in the colliculus prior to the appearance of visually evoked activity in the cortex. This indicates that direction selectivity can not be attributable to incoming cortical afferents. This study provides the first direct evidence that, unlike the cat, the rat's cortico-tectal pathway is only weakly involved in the establishment of direction selectivity in collicular neurons.

PGE2, via EP3 receptors, regulates brain nitric oxide synthase in the perinatal period

We tested the hypothesis that high prostaglandin levels during the perinatal period might regulate brain nitric oxide synthase (nNOS) expression. nNOS and cyclooxygenase (COX)-2 mRNAs were higher in brain cortex and the periventricular area of newborn rats and pigs compared with adult brain. Nitric oxide synthase activity was also 2. 5- to 4-fold higher in newborn than in adult brain. Administration of nonselective COX inhibitor ibuprofen or COX-2 inhibitor nimesulide every 8 h for 24 h to newborn rats and pigs reduced prostaglandin levels and caused comparable reductions in nNOS mRNA, protein, and activity to levels of adults; COX inhibitor-induced changes were prevented by cotreatment with PGE2 analog, 16, 16-dimethyl-PGE2, and agonist for the EP3 receptor of PGE2, sulprostone, but not by PGI2 analog carbaprostacyclin, PGD2, EP1 receptor agonist 17-phenyl trinor-PGE2, and EP2 agonist butaprost. Concordant observations were made in vitro and revealed that nNOS expression (detected by NADPH diaphorase reactivity) mostly present in neurons of the deeper cortical layers was reduced by COX inhibitor, and this effect was prevented by EP3 agonist. In conclusion, high levels of PGE2 in neonatal brain contribute to the increased expression of nNOS by acting on EP3 receptors; this positive interaction between PGE2 and nNOS might be required physiologically for normal brain development.

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.

ON and OFF field potentials in the rat superior colliculus during development

The present investigation is aimed at characterizing the development of ON and OFF visually evoked responses in the rat superior colliculus from postnatal day 13 (P13) to postnatal day 25. Depth profiles of field potentials reveal that ON and OFF long latency biphasic field potentials are already present when collicular cells are first responsive to light (P13). There is an inversion in the polarity of these responses as the electrode penetrates the collicular layers, suggesting a synaptic organization similar to the one found in adult animals. At P15, OFF field potentials begin to exhibit oscillatory activity. Local cobalt injections within the superior colliculus abolishes these OFF oscillations, suggesting a postsynaptic origin. Fast Fourier transform (FFT) analysis of the OFF field potentials demonstrates that oscillatory activity increases in frequency during development. This increase is thought to reflect the myelination and stabilization of synaptic connections that occur during this period. To our knowledge, this is the first report of OFF oscillatory responses in the superior colliculus.

Characterization and regulation of prostaglandin E2 receptor and receptor-coupled functions in the choroidal vasculature of the pig during development

Ontogenic changes in choroidal vascular prostaglandin E2 (PGE2) receptors (EP1, EP2, EP3, and EP4), changes in receptor-coupled functions, and the possible role of high perinatal prostaglandin levels in regulating expression and function of these receptors were studied. PGE2 receptors and their functions on choroidal tissues were characterized by radioligand binding; by measurements of second messengers to receptor stimulation; and by vasomotor response to EP1, EP2, EP3, and EP4 ligands on perfused choroidal vascular beds from saline- and ibuprofen-treated (40 mg/kg every 4 every 4 hours for 48 hours) newborn pigs and from adult animals. PGE2 as well as EP2- and EP4-attributed choroidal stimulation elicited greater vasorelaxation in the saline-treated newborn and was associated with higher nitrite (oxidation product of NO, N omega-nitro-L-arginine inhibitable) production than in adult tissues. In contrast, EP1 and EP3 stimulation caused significantly more constriction in the adult than in the newborn, and this was associated with increased production of inositol 1,4,5-trisphosphate (IP3) and greater reduction of cAMP synthesis in the adult. Maximum [3H]PGE2 binding was also higher (3-fold) in adult than in newborn tissues. Competition binding studies revealed that of the PGE2 receptors in the adult choroid, approximately 55% were of the EP1 subtype, 8% were EP2, 22% were EP3, and 15% were EP4. Newborn choroid contained approximately 33% each of EP1 and EP2 receptors, 20% of EP3, and 15% of EP4. Inhibition of endogenous prostaglandin synthesis for 48 hours with ibuprofen in newborns to attain levels found in the adult resulted in an upregulation of [3H]PGE2 binding, EP1- and EP3-mediated vasoconstriction, and increases and decreases in IP3 and cAMP production, respectively, in newborn tissues compared with adult tissues. On the other hand, ibuprofen treatment of newborns led to a decrease in PGE2- and EP4-mediated vasorelaxation and nitrite synthesis (associated with decreased expression of endothelial NO synthase) to levels observed in adults: EP2-elicited responses in newborns were not affected by ibuprofen. In conclusion, fewer EP1 receptors (associated with vasoconstriction), more EP2 receptors, and greater EP4-coupled NO production (coupled to vasorelaxation) seem to be responsible for the increased vasodilation to PGE2 in the newborn. The decrease in prostaglandin levels with age appears to cause, on one hand, upregulation of EP1 and EP3 receptors and receptor-coupled vasoconstriction and, on the other hand, decreased EP4-coupled NO synthesis and choroidal vasodilation. Altogether, these factors result in increased vasorelaxation to PGE2 in the newborn compared with the adult. These findings may help to explain the inability of the newborn to autoregulate choroidal blood flow.

Spatial frequency properties in area 18 during inactivation of area 17 in cats

The aim of this investigation was to understand the functions of long horizontal connections projecting from area 17 to area 18 in cats. The animals were anesthetized and prepared for recording single-cell responses to sine-wave gratings in area 18. Neuronal activity was analyzed under three conditions: prior to, during, and after inactivation of a circumscribed region of area 17. The latter was depressed with micro-injections of GABA. Cells in both areas were in close retinotopic correspondence. Cells were classified as simple and complex types. Globally, simple cells were less affected than complex units, and those which were affected shifted their optimal spatial frequency to higher values. Complex neurons were more often influenced by the interruption of area 17 input. Namely, the peaks of the tuning curves were displaced on the x-axis to a new optimal spatial frequency. This effect was obtained by a dual change: a decline in the discharge strength to the optimal spatial frequency and an enhancement to nonoptimal spatial frequency. Contrast sensitivity function disclosed similar shifts of optimal spatial frequencies. Likewise bandwith, spatial resolution, high cutoff, and low cutoff were modified to a greater extent in complex cells. It appears that there is no relationship between areas 17/18 orientation difference and the modifications observed in tuning curves to spatial frequencies. The results suggest that neurons of area 18 may carry multiple-frequency channels and that area 17 facilitates the emergence of one particular spatial frequency.

Stimulus-dependent oscillations in the cat visual cortex: differences between bar and grating stimuli

We have investigated the dependence of cortical oscillations on the type of visual stimulus. Single unit recordings were performed in areas 17 and 18 of the cat visual cortex. Among 217 cortical neurons oscillations in the frequency range of 22-102 Hz were found in 29 cells (13%). The proportion of oscillating cells was higher (16%) if both bar and grating stimuli were used to stimulate cortical neurons. It was found that gratings are more effective than bars in triggering oscillatory patterns in cortical cells. Among 21 oscillating cells which were stimulated with both bar and grating stimuli, oscillations evoked with gratings were found in 17 neurons (81%) while oscillations evoked with bar stimuli were triggered in 7 cells (33%). The distributions of oscillation frequencies were statistically different for oscillations evoked with bars and gratings. Frequencies of oscillations evoked with bars were in the lower and higher range than frequencies of oscillations evoked with gratings. In 3 cells (14%), rhythmic patterns could be evoked with both bar and grating stimuli. However, the oscillations were of different frequencies. No significant correlation was found between the strength of oscillations and firing rate of cortical neurons. Both simple and complex cells manifested the same dependence on stimulus type. However, complex cells mostly exhibited oscillations in the lower frequency range while simple cells did so when neurons were stimulated with bars. The results suggest that various classes of visual stimuli can be coded by a temporal pattern of cortical responses.

The lateral posterior-pulvinar complex modulation of stimulus-dependent oscillations in the cat visual cortex

It has been suggested that binding coherent targets depends on the capacity of excited cortical cells to fire in synchrony at approximately 40 Hz. However, the origin of stimulus-related cortical oscillations is still not clear. We hypothesized that 40 Hz oscillations might propagate to the visual cortex from the lateral posterior-pulvinar complex (LP-P) whose cells send fibers to the visual cortex and have a tendency to exhibit oscillations. To test our hypothesis, we recorded single unit activity in areas 17 and 18 of anaesthetized cats. The activity of neurons which showed oscillations evoked by optimal visual stimuli was analysed before, during and after a reversible inactivation of the LP-P with GABA. Such inactivation was found to markedly modify the strength of oscillatory activity of cortical neurons whose visual responses were affected by LP-P blockade. In contrast, the oscillation frequencies of cortical neurons were not modified by such inactivation. However, in some cells (three of nine), oscillatory activity was found to be completely abolished by injection of GABA into the LP-P. Collectively, these findings demonstrate that inputs from the LP-P play a key role in modulating the oscillatory activity of visual cortex neurons. Assuming that cortical neurons utilize oscillatory activity to encode perceptual aspects of the visual stimulus, our findings underscore the contribution of the LP-P in this process.

Visually-triggered oscillations in the cat lateral posterior-pulvinar complex

The so-called 40 Hz oscillations are found at almost all stages of visual processing are thought to play a critical role in perception. The goal of this investigation was to look at the presence of stimulus-specific oscillations in the lateral posterior-pulvinar complex of the thalampus (LP-P) for which the oscillations were still not described. Rhythmic patterns in multiunit LP-P activity of anaesthetized cats were revealed in 14% of recording sites. With the exception of one pool of LP-P cells that exhibited stimulus-dependent rhythmic activity approximately 130 Hz, 90% of autocorrelograms were modulated between 18 and 74 Hz with dominant frequencies of 20-33 Hz. Since the LP-P sends efferents to the visual cortex it seems possible that oscillations from the LP-P can propagate to cortical neurones, especially to complex cells, for which similar dominant frequencies were noted by previous investigators.

Comparison of the responses to moving texture patterns of simple and complex cells in the cat's area 17

1. Whether complex (C) cells are the only truly texture-sensitive units in the cat's primary visual cortex remains controversial. In view of the strong physiological significance of having putatively only one class of cells sensitive to visual noise in the striate cortex, we reinvestigated this issue. Sensitivities of simple (S) and C cells to noise were quantitatively studied and compared in order to clearly document the response properties of cells in the striate cortex to visual noise and to establish whether one can unequivocally segregate S from C cells on the basis of those specific properties. 2. Receptive fields were stimulated with all relevant stimuli, i.e., drifting sine-wave gratings, electronically generated noise pattern of 256 x 256 elements (ratio 1:1 of dark and light elements), and flashing and moving bars (both bright and dark). 3. A total of 60 S cells out of 85 (70.6%) and 90 C cells out of 101 (81.8%) responded to the motion of visual noise. Responses of most C cells were sustained, i.e., their discharge rate was maintained at a constant level throughout presentation of the stimulus. On the other hand, responses of the majority of S cells were characterized by several bursts of discharges. On average, optimal firing rates were greater for gratings than for noise. 4. For practically all cells, responses to noise varied as a function of direction of motion. The mean direction bandwidths were, respectively, 43 +/- 24 degrees and 48 +/- 23 degrees (mean +/- SD) for S and C cells. In both groups, neurons were more broadly tuned for the direction of noise than that of gratings (t-test, P < 0.001). We rarely observed bimodal tuning curves for noise, with each peak lying on either side of the orientation curve. These results could be expected if one considers texture stimuli not in the space domain (as dot patterns) but in the frequency domain, i.e., patterns containing all spatial frequencies and orientations. 5. In general, the direction indexes of S and C cells were similar whether they were stimulated by drifting noise or gratings. S cells had a slight tendency to be more direction selective for noise than for gratings. 6. For all S and C cells tested, responses to noise varied as a function of drift velocity. The mean optimal velocity was 12.9 and 10.2 degrees/s for S and C cells, respectively (t-test, P > 0.05). Most cells were band-pass with mean bandwidths of 2.2 and 2.7 octaves for S and C cells, respectively.(ABSTRACT TRUNCATED AT 400 WORDS)

Evolution of spontaneous activity in the developing rat superior colliculus

During the first 10 days after birth in the rat there are a succession of major developmental stages in the retinotectal pathway. During most of this time, the only recordable event in the superior colliculus is spontaneous activity. We studied and characterized this spontaneous activity, hypothesizing that it could play an important role in pathway development. The spontaneous discharges are detectable on postnatal day 5 (P5). After P5, the number of spontaneously active cells per penetration increases up to P10, after which they decrease to adult-like levels by P14-P15. Between P5 and P10, the spontaneous discharges exhibit several patterns of activity, from constant firing to intermittent bursts with periods of quiescence, without any bearing to age. We isolated the retina and superior colliculus by injecting xylocaine onto the optic nerve and found no change in collicular activity. While this suggests that the spontaneous activity in the colliculus is independent of the retina at the ages studied, the opposite experiment, i.e., electrically stimulating the optic nerve, resulted in increased firing by collicular neurons, perhaps via nonclassical synaptic transmission. Finally, we compared interval histograms for spontaneously active cells between P5 and P15. The histograms suggest that at certain ages, spontaneous firing is more regular; moreover, these ages precede major functional advances, e.g., onset of numerous spontaneously firing cells at P6, the first response to optic nerve stimulation at P10, and the first light-evoked response at P12-P13. Our results support the hypothesis that spontaneous activity in the neonatal superior colliculus has a role in development of the retinotectal pathway, but the data also indicate that classical synaptic transmission is not involved.

Connections between areas 17 and 18 depend on orientation and cell type

The functional relationships between areas 17 and 18 are still incompletely understood; many experiments indicate that cells sharing like properties are linked together, whereas other studies suggest a deviation from this principle. Here we report the effect of using GABA to block circumscribed area 17 neuronal populations with known orientation preferences. We studied the interaction of this treatment on the responses of simple and complex neurons in area 18. The data suggest that these interactions follow different rules for simple and complex cells: specifically, an iso-orientation linkage appears to hold for simple cells while a cross-orientation linkage appears to hold for complex cells.

Lateral geniculate neuron responses to drifting sine-wave gratings in rabbits

Neurons of the lateral geniculate body in rabbits were excited with drifting sine-wave gratings. Rabbits were anesthetized and paralyzed under conventional methods to record action potentials of single cells using tungsten in glass microelectrodes. All classes of geniculate cells responded in a modulatory pattern. It appears that the unmodulatory pattern typical of complex cell types of the cortex is extremely infrequent or absent. In the spatial domain most cells are low pass and bandpass. Only one unit was high pass. In the temporal domain low-pass and bandpass cells were the most frequently recorded. Four geniculate cells were high pass. It appears, therefore, that neurons of rabbits' geniculate are tuned over spatial and temporal frequencies of sine-wave gratings. The comparison with cortical recordings revealed that geniculate cells are more broadly tuned than cortical neurons. This study suggests that the rabbit's visual system is sensitive to gratings. However cells respond optimally to lower values, e.g., broader gratings, than neurons of frontalized eye animals.

Visual cortical neuron responses to drifting sine-wave gratings in rabbits

Neurons of area 17 (V1) in rabbits were stimulated with drifting sine-wave gratings. Animals were anesthetized and paralyzed under conventional methods to record action potentials of single cells using tungsten-in-glass micro-electrodes. Neurons responded in modulatory or unmodulatory patterns depending on whether cells were of the simple or complex type, respectively. In the spatial domain, most cells were Low-Pass and Band-Pass, with no recordings of High-Pass units. In the temporal domain, Low-Pass, Band-Pass and High-Pass behaviours were found. Rabbits appeared to exhibit all types of pattern responses found in other foveate mammals, including special-complex and end-stopping units. However, in the majority of cells, there was a shift towards the lower end of the spatial frequency spectrum. For instance, the optimal spatial frequency was in the vicinity of lc/d while in the temporal domain, some units responded optimally at 10 c/s. In conclusion, it appears that rabbit cortex contain neurons which are selective for spatial frequency in a manner comparable to that seen in cat and monkey.

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.

Functional development of the neonatal rat retinotectal pathway

Electrophysiological activity in the neonatal rat superior colliculus was recorded to measure neuronal and synaptic activity, and, therefore, functional development. Neonatal rat pups were studied from five days to two weeks of age. The earliest activity in the superior colliculus were spontaneous discharges at a frequency of one unit per animal on postnatal day 6 (P6). Spontaneously discharging units were more numerous at P8, and the number peaked on P10. The first clear response to optic nerve stimulation was seen on P10, with relatively long and variable latencies. By P14, electrically evoked responses had much shorter latencies. The results are in line with the first response to light flash in the superior colliculus at P12/13. The evidence suggests that functional development of the rat retinotectal pathway begins at the end of the first week after birth, and that much of the functional maturation occurs mainly during the second week after birth.

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.

Responses from outside classical receptive fields of dorsal lateral geniculate cells in rabbits

The effects of stimuli at locations remote from classic receptive fields (CRF) of lateral geniculate cells were examined in rabbits. In anesthetized rabbits, small targets positioned well outside the CRF either facilitated or decreased responses evoked by a stimulus positioned within the most active area of the CRF in 51% of the cells tested, in spite of the fact that when presented in isolation the remote target failed to modify the spontaneous activity of the recorded cell. Late components of the discharge pattern evoked by the central stimulus were mostly influenced by the peripheral target. Focal or ectopic areas surrounding the CRF are thus identified. These areas were not a direct extension of the CRF, since the normal evoked response was unchanged when the remote stimulus moved closer to the CRF. Cells whose CRF were centrally located reacted with an augmented response in the presence of the additional stimulus, whereas units whose CRF was more eccentric exhibited a weaker response when the peripheral target was introduced in the visual field. We also investigated whether superior colliculus afferents to the lateral geniculate nucleus could be associated with these ectopic areas (EA). Depressing superior colliculus activity produced two types of results: (a) often the late component of the response pattern was modified; and (b) the influence of the remote stimulus disappeared with collicular blockade in 80% of tested neurons. These results provide evidence that the CRF of geniculate cells may be surrounded by satellite zones, which modify the responses to the central target when invaded by circumscribed stimuli.(ABSTRACT TRUNCATED AT 250 WORDS)

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.

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.

Responsiveness of reorganized primary somatosensory (SI) cortex after local inactivation of normal SI cortex in chronic spinal cats

The cortical map of adult cats that sustained spinal cord transection at T12 when they were 2 weeks old is characterized by a clear duplication of the representation of the forelimb, rostral trunk, and neck. The novel representation is located in the cortical region that is, in nonoperated animals, normally devoted to the hindlimb representation. We have investigated the possibility that the reactivation of the deprived hindlimb cortex may be mediated by corticocortical projections from normal to reorganized cortex. The primary somatosensory (SI) cortex was initially mapped to determine the boundaries of the normal and reorganized cortical representations. Somatotopically corresponding regions in both normal and reorganized cortex representing the trunk, the web space, or the shoulder were more precisely mapped. Inactivation of normal cortex was achieved by the nanoinjection of a solution of lidocaine hydrochloride stained with Chicago sky blue. Two major findings are described. First, inactivation of a circumscribed region of normal cortex representing a given receptive field (RF) failed to reduce or inhibit the responsiveness of a somatotopically corresponding RF represented in reorganized cortex. Therefore, it is unlikely that intracortical connections between normal and reorganized cortex could account for the reorganizational processes observed in cats that sustained spinal cord transection at 2 weeks of age. Second, the chemical blockade of normal cortex provoked an increase of the responsiveness and of the size of the peripheral RFs represented in reorganized cortex. This finding suggests that there are corticocortical connections (possibly topographically organized) between normal and reorganized cortex, and that these connections are inhibitory.

Temporal relationship between the ERG and geniculate unit activity in rabbit: influence of background luminance

The comparative analysis of the retinal and lateral geniculate nucleus (LGN) intensity-response function revealed that the timing of the LGN unit response was highly correlated to that of one oscillatory potential (OP2). To examine if this OP2-LGN intensity-response function was retained irrespective of the state of retinal adaptation, we performed simultaneous recordings to the ERG and single-cell unit activity at the geniculate level evoked to a flash of constant energy while the level of background luminence was varied. For a stimulus of 6.7 cd m-2 sec, the shifts in latency induced by increasing the background luminance from 0 to 125 cd m-2 are of 1.17 +/- 0.61 msec for OP2, a value almost identical (P greater than 0.10; n = 36) to the one obtained at the geniculate level (1.11 +/- 0.88 msec). However, when a dimmer flash is used, the latency shifts are not so well correlated. The latter could be partly explained by the threshold nature of the resulting stimulus (i.e. high photopic background combined with a dim flash).

The effect of acute trichloroethylene exposure on electroretinogram components

The effects of acute exposure to trichloroethylene (TCE) on the electroretinogram (ERG) and oscillatory potentials (OPs) were studied on adult New Zealand rabbits anesthetized with urethane. The ERG and OPs were recorded under photopic conditions with a speculum electrode, with pupils fully dilated and the cornea anesthetized. Emulsified (Intralipid) TCE was given intravenously in three consecutive injections of equal volume for a total dose varying between 24 and 85 mg/kg. Averaged ERG and OPs (50 sweeps) were taken up to 250 minutes after the last TCE injection. Our results indicated that, irrespective of the dose used, there was a significant (p less than 0.001) reduction (average: 50%) in the amplitude of the b-wave of the ERG following TCE injection. In contrast, the OPs showed a differential effect. At low doses, OP4 showed a significant (p less than 0.001) reduction resulting in some cases to its complete extinction, while at higher dose there is a marked increase in amplitude. Our results would suggest that the OPs are a better retinal potential to assess the level of TCE toxicity than is the b-wave of the ERG.

The effect of iodoacetic acid on the electroretinogram and oscillatory potentials in rabbits

It has been shown that a single injection of iodoacetic acid selectively (but temporarily) abolishes the b-wave of the electroretinogram. We examined whether such use of this chemical further substantiate our claim that the b-wave of the electroretinogram is a composite potential resulting from the summation (or integration) of faster retinal potentials, usually referred to as the oscillatory potentials. Full-field electroretinograms were recorded from adult New Zealand rabbits before and after a single, bolus injection of 15 mg/kg of buffered iodoacetic acid. Both the 1-1000 Hz electroretinogram and the 100-1000 Hz oscillatory potentials were recorded simultaneously. The oscillatory potentials considered in this study were those normally seen on the rising phase of the b-wave. Following the intravenous injection of iodoacetic acid, there was a progressive decrease in the amplitude and peak time of the b-wave. This observation also was reflected in the oscillatory potential recordings, in which the long-latency oscillatory potentials (3 and 4) progressively disappeared while oscillatory potential 2 remained. We believe that these findings further support our contention that the oscillatory potentials are major components of the b-wave.

Influence of the superior colliculus on visual responses of cells in the rabbit's lateral posterior nucleus

The lateral posterior-pulvinar (LP-P) complex of mammals receives a major input from the superior colliculus (SC). We have studied the response properties of LP cells and investigated the effects of reversible inactivation of the colliculus on the visual responses of LP units in anesthetized and paralyzed rabbits. Cells in LP had large receptive fields responsive to either stationary or moving stimuli. One third of the motion-sensitive cells were direction selective. The size of the receptive fields increased with eccentricity and there was a retinotopic organization along the dorso-ventral axis. Comparison of the LP and superior colliculus properties revealed substantial differences in visual response characteristics of these two structures such as the size of the receptive fields and the number of direction-selective cells. Electrical stimulation of the LP evoked antidromic action potentials in tectal cells that were motion sensitive. We found a dorsoventral gradient in the projections of collicular cells. Units located more dorsally in the colliculus sent their axons to LP while cells lying more ventrally sent axons toward the region lying posterior to LP. A micropipette filled with lidocaine hydrochloride was lowered into the superficial layers of the superior colliculus in order to reversibly inactivate a small population of collicular cells. Rendering the superior colliculus inactive produced a sharp attenuation of visual responses in the majority of LP cells. Some neurons ceased all stimulus-driven activity after collicular blockade while a few cells exhibited increased excitability following collicular inactivation. These experiments also indicate that the tecto-LP path is topographically organized. An injection in the colliculus failed to influence the thalamic response when it was not in retinotopic register with the LP cells being recorded. Our results demonstrate that the superior colliculus input to LP is mainly excitatory in nature.

Susceptibility of neurons in area 18a to blockade of area 17 in rats

The present investigation is aimed at studying the influence of the striate cortex (area 17 or V1) upon responses of area 18a. Electrical activity was recorded from neurons in area 18a through glass micropipettes. At the same time lidocaine hydrochloride was injected in the same retinotopic register of area 17. Forty eight cells were tested to diffuse stroboscopic flashes and to localized stationary or moving slits in the receptive field. In addition, orientation selectivity was analyzed in 15 units. Blockade of area 17 produced the following results: discharges to stroboscopic light were unaltered while responses to stationary, localized targets were diminished in about half of the cells. Fifty percent of movement evoked responses were decreased. The orientation properties were particularly sensitive to blockade of area 17. Out of 15 units, 13 had their orientation tuning curves altered by the treatment. These results seem to suggest that the degree of trigger feature complexity depends upon lateral interactions between areas V1 and V2.

Presentation of a remote target influences visual responses of rabbit lateral geniculate cells

The described investigations analyze the influence of presenting a circumscribed target outside the limits of the classical receptive field (CRF) on the responses of lateral geniculate cells in rabbits. Animals were prepared for single cell recordings in acute experiments in the usual fashion. Cells were stimulated with a light-emitting diode (LED) positioned in the CRF and a second stimulus (S2) presented outside the field. The latter stimulus does not modify the spontaneous firing of the neuron when presented in isolation. S2 is either stationary or moving over small distances (total excursion is 3.5 degrees of arc). A moving S2 increases or decreases the responses to the LED by a greater magnitude than a stationary S2. Significantly, in 80 percent of direction-biased cells, centripetal movement of S2, that is, movement toward the receptive field, affected the neuronal discharge if it coincided with the preferred direction of the unit. These results suggest that the introduction of a remote stimulus modifies the responses of geniculate cells in rabbits.