Receptive field analysis: responses to moving visual contours by single lateral geniculate neurones in the cat

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

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

The role of the pattern edge in goldfish visual motion detection

To understand the function of edges in perception of moving objects, we defined four questions to answer. Is the focus point in visual motion detection of a moving object: (1) the body or the edge of the object, (2) the leading edge or trailing edge of the object, (3) different in scotopic, mesopic and photopic luminance levels, or (4) different for colored objects? We measured the Optomotor Response (OMR) and Edge Triggering Response (ETR) of goldfish. We used a square and sine wave patterns with black and red stripes and a square wave pattern with black and grey stripes to generate OMR's and ETR's in the goldfish. When we used black and red stripes, the black leading edges stimulated an ETR under scotopic conditions, red leading edges stimulated an ETR under photopic conditions, and both black and red leading edges stimulated an ETR under mesopic luminance levels. For black and gray stripes, only black leading edges stimulated an ETR in all three light illumination levels. We observed less OMR and ETR results using the sine wave pattern compared to using the square wave pattern. From these results, we deduced that the goldfish tend to prefer tracking the leading edge of the pattern. The goldfish can also detect the color of the moving pattern under photopic luminance conditions. We decided that ETR is an intriguing factor in OMR, and is suitable as a method of behavioral measurement in visual system research.

Cortical plasticity revealed by circumscribed retinal lesions or artificial scotomas

We review the work of others in which the effects of circumscribed, topographically corresponding binocular retinal lesions on the topographic organization of the visual cortex revealed that there is a substantial degree of topographical plasticity in the primary visual cortices of adult cats and macaque monkeys. Despite the evidence indicating that the reorganization of the topographic map in primary visual cortices of adult cats and macaques related to the input from one eye could be suppressed for a long time by inputs related to the other eye, we observed a substantial degree of topographical plasticity in the primary visual cortices of adult cats in which we have made circumscribed monocular retinal lesions. Overall, in both binocularly and monocularly lesioned adult animals, most cells recorded in the cortical projection zone of the retinal lesion (LPZ), several hours, several weeks or several months after placement of the lesions exhibited 'ectopic' excitatory visual receptive fields (RFs) which were displaced to the normal retina in the immediate vicinity of the lesion. The presence of ectopic RFs in cells recorded in the cortical LPZ, combined with the presence of normal cortical representation of the part of the retina in the vicinity of the lesion, indicate a clear expansion of the cortical representation of the part of the retina surrounding the lesion. When stimulated via the ectopic RFs, cortical cells exhibited normal orientation tuning and in the case of animals with monocular lesions, the orientation tuning of binocular cells when stimulated via ectopic RFs appeared to be very similar to that when the cells were stimulated via the RFs in the normal, unlesioned eye. In both binocularly and monocularly lesioned animals, the responses evoked by optimal visual stimuli from the ectopic RFs were substantially weaker than those evoked from their normal counterparts. Similarly, upper velocity limits were significantly lower when visual stimuli were presented via the ectopic RFs. In contrast to cats in which the retinal lesions were made in adulthood, in cats lesioned monocularly in adolescence (8-11 weeks postnatal), both the peak discharge rates and upper velocity limits of responses to photic stimuli presented via the ectopic RFs were very similar to those to stimuli presented via the normal eye. The intracortical mechanism(s) underlying the long-term cortical plasticity revealed by retinal lesions are likely to be closely linked to the mechanism(s) underlying the short-term reversible enlargement of cortical receptive fields observed with artificial scotomas. Furthermore, a similar putative intracortical mechanism(s) appears to underlie psychophysical phenomena observed in studies of retinal scotomas in humans. Overall, the research reviewed here strongly challenges the view that receptive fields of neurons in mammalian visual cortices are 'hard-wired'.

Velocity response profiles of collicular neurons: parallel and convergent visual information channels

We have recorded from single neurons in the retinorecipient layers of the superior colliculus of the cat. We distinguished several functionally distinct groups of collicular neurons on the basis of their velocity response profiles to photic stimuli. The first group was constituted by cells responding only to photic stimuli moving at slow-to-moderate velocities across their receptive fields (presumably receiving strong excitatory W-type input but not, or only subthreshold, Y-type input). These cells were recorded throughout the stratum griseum superficiale and stratum opticum and constituted 50% of our sample. The second group of cells exhibited excitatory responses only at moderate and fast velocities (presumably receiving excitatory Y-type but not W-type input). These cells constituted only about 7% of the sample and were located principally in the lower stratum griseum superficiale. The third group of cells was constituted by cells excited over the entire range of velocities tested (1-2000 /s) and presumably received substantial excitatory input from both W- and Y-channels. These cells constituted almost 26% of our sample and were located in the lower stratum griseum superficiale, stratum opticum and the upper part of the stratum griseum intermediale. Overall, cells receiving excitatory Y-type input, i.e. the sum of group two and group three cells, constituted about a third of the sample and their excitatory discharge fields were significantly larger than those of cells receiving only W-type input. A fourth distinct group of collicular neurons was also constituted by cells responding over a wide range of stimulus velocities. These cells were excited by slowly moving stimuli, while fast-moving photic stimuli evoked purely suppressive responses. The excitatory discharge fields of these cells (presumably, indicating the spatial extent of the W-input) were located within much larger inhibitory fields, the extent of which presumably indicates the spatial extent of the Y-input. These low-velocity-excitatory/high-velocity-suppressive cells were recorded from the stratum griseum superficiale, stratum opticum and stratum griseum intermediale and constituted about 17% of the sample. The existence of low-velocity-excitatory/high-velocity-suppressive cells in the mammalian colliculus has not been previously reported. Low-velocity-excitatory/high-velocity-suppressive cells might play an important role in activating "fixation/orientation" and "saccade" premotor neurons recorded by others in the intermediate and deep collicular layers. Overall, in the majority (57%) of collicular neurons in our sample there was no indication of a convergence of W- and Y-information channels. However, in a substantial minority of collicular cells (about 43% of the sample) there was clear evidence of such convergence and about 40% of these (low-velocity-excitatory/high-velocity-suppressive cells) appear to receive excitatory input from the W-channel and inhibitory input from the Y-channel.

Effects of selective pressure block of Y-type optic nerve fibers on the receptive-field properties of neurons in the striate cortex of the cat

In an aseptic operation under surgical anesthesia, one optic nerve of a cat was exposed and subjected to pressure by means of a special cuff. The conduction of impulses through the pressurized region was monitored by means of electrodes which remained in the animal after the operation. The pressure was adjusted to selectively eliminate conduction in the largest fibers (Y-type) but not in the medium-size fibers (X-type). The conduction block is probably due to a demyelination and remains complete for about 3 weeks. Within 2 weeks after the pressure-block operation, recordings were made from single neurons in the striate cortex (area 17, area V1) of the cat anesthetized with N2O/O2 mixture supplemented by continuous intravenous infusion of barbiturate. Neurons were activated visually via the normal eye and via the eye with the pressure-blocked optic nerve ("Y-blocked eye"). Several properties of the receptive fields of single neurons in area 17 such as S (simple) or C (complex) type of receptive-field organization, size of discharge fields, orientation tuning, direction-selectivity indices, and end-zone inhibition appear to be unaffected by removal of the Y-type input. On the other hand, the peak discharge rates to stimuli presented via the Y-blocked eye were significantly lower than those to stimuli presented via the normal eye. As a result, the eye-dominance histogram was shifted markedly towards the normal eye implying that there is a significant excitatory Y-type input to area 17. In a substantial proportion of area 17 neurons, this input converges onto the cells which receive also non-Y-type inputs. In one respect, velocity sensitivity, removal of the Y input had a weak but significant effect. In particular, C (but not S) cells when activated via the normal eye responded optimally at slightly higher stimulus velocities than when activated via the Y-blocked eye. These results suggest that the Y input makes a distinct contribution to velocity sensitivity in area 17 but only in C-type neurons. Overall, our results lead us to the conclusion that the Y-type input to the striate cortex of the cat makes a significant contribution to the strength of the excitatory response of many neurons in this area. However, the contributions of Y-type input to the mechanism(s) underlying many of the receptive-field properties of neurons in this area are not distinguishable from those of the non-Y-type visual inputs.

Effects of convergent strabismus on spatio-temporal response properties of neurons in cat area 18

Single-cell recording experiments were carried out to determine whether rearing kittens with surgically induced convergent strabismus (esotropia) alters the development of receptive field (RF) properties of neurons in area 18. In agreement with previous work on kittens with divergent strabismus (exotropia), there was a marked loss of binocularly driven cells in area 18 of esotropic cats. In contrast to the striate cortex of strabismic cats, the spatial properties of area 18 neurons, including receptive-field size and spatial frequency tuning, did not differ from those in normal controls. On the other hand, we found that contrast thresholds, measured at an optimal spatial frequency, were significantly elevated, and that the contrast gain in many cells was reduced in strabismic cats. These deficits were observed in both eyes, though the cells dominated by the deviating eye had a lower response amplitude at all contrasts. Furthermore, temporal frequency tuning curves were abnormal in strabismic cats in that the optimal frequencies and temporal resolutions were shifted to lower values. These effects were also bilateral. Velocity tuning, measured with a high-contrast bar stimulus, revealed that area 18 neurons in strabismic cats were unable to respond to very high velocities compared to normals. This reduced response was more severe when measured with the deviating eye in spite of the bilateral nature of the deficit. Finally, latencies to electrical stimulation of the optic chiasm or the optic radiation were significantly longer in strabismic cats. The magnitude of these effects was virtually the same for both eyes. From these observations, we conclude that the temporal properties of area 18 neurons, particularly the cells abilities to follow fast temporal modulations, are affected by raising kittens with surgically induced convergent strabismus.

Effects of patterned backgrounds on responses of lateral geniculate neurons in cat

In the lateral geniculate nucleus (LGN) of paralyzed cats under urethane anaesthesia, an extensive disinhibitory region (DIR) outside the inhibitory surround of the receptive field (RF) was found in both sustained and transient cells. Its extent is comparable to that of McIlwain's periphery effect (1964). The responses to a light spot, flashed into different parts of the RF center, were used to assess the effect of different background patterns, located over the DIR, on responsiveness and receptive field organization. A straight line background cutting across the RF center led to a marked shrinkage in RF size and to a suppression of the center response. In sustained cells, these influences were not due to the light flux of the background, but were mainly due to the spatial property of the line itself. This was demonstrated by comparing the effect of a straight line background with that of a zigzag line or of distributed dots. The light flux for the different patterns and their spatial weighting was the same, so that they differed from each other solely in their form. A straight line background elicited much stronger suppression of the center response and more marked shrinkage of the RF than if the component dots are dispersed over a wider area, but keeping the radial distances of the individual dots from the RF-center constant. The data suggest that the dispersion of the component dots in different backgrounds plays an important role as response amplitude and RF diameter increase proportional to the dispersive area of the background patterns. For transient cells, all backgrounds used showed similar effects on center responses and RF diameter, indicating that for them it was the light flux of the backgrounds rather than their spatial property that caused the effects.

A comparison of visual responses of cat lateral geniculate nucleus neurones with those of ganglion cells afferent to them

We compared visual responses of cat lateral geniculate nucleus (l.g.n.) neurones with those of retinal ganglion cells providing their afferent inputs. Quantitative studies were made on twenty such pairs; eight X on-centre, seven Y on-centre, two X off-centre and three Y off-centre pairs. Receptive field centre locations of cell pairs with correlated activities were very closely superimposed, having a mean centre displacement of 1.6 minutes of arc for X cells and 11 minutes of arc for Y cells. With flashed spots and annuli, responses of l.g.n. cells were almost always smaller than those of their retinal afferents, with peaks and troughs in ganglion cell responses being faithfully followed in the geniculate neurones. This is consistent with almost all impulses from the l.g.n. cell being triggered by the afferent feeding its centre. With spots of different sizes and contrasts, modulation of responses by l.g.n. inhibition was obvious, but effects were complex. With moving bright-bar stimuli, although response histograms were clearly reshaped to some extent in the l.g.n., peak firing rates under different stimulus conditions were often merely attenuated by a constant factor for most l.g.n. cells in comparison with their retinal inputs. For velocity tuning curves, a few cell pairs showed selective attenuation at high speeds, while others showed it at low speeds. All the latter group appeared to have more than one major excitatory afferent. These changes in velocity tuning occurred across the X/Y classification, so that differences in velocity preference of the X and Y systems is more blurred in the l.g.n. than in the retina.

Direction selectivity of simple cells in cat striate cortex to moving light bars. II. Relation to moving dark bar responses

The response properties of 84 simple striate cells in anaesthetized (N2O/O2 supplemented with sodium pentobarbital) and paralyzed cats were examined quantitatively using narrow optimally-oriented light and dark bars moving at optimal velocities. Different cells gave two to five spatially-offset response peaks, the light bar and the dark bar response peaks alternating with one another. With only 5 exceptions, the cells had the same preferred direction for movement of the dark bar as for the light bar. Static-field plots were prepared from 32 of the 84 cells using stationary flashing bars. The receptive fields of different cells had from two to four subregions responding either at light on (ON subregion) or at light off (OFF subregion) although one cell had only a single subregion. In the preferred direction of stimulus movement cells gave either the same number of response peaks to moving bars as there were subregions or one additional response peak. The additional response peak, termed a boundary response, always occurred at the end of the sequence of response peaks and was always completely direction selective. The direction selectivities of the individual response peaks in the responses from 49 of the 84 cells were analyzed. To ensure that each response peak and the corresponding peak in the opposite direction both came from the same subregion, the 49 cells were selected on the basis of having a response in the nonpreferred direction sufficient for analysis and of having a stimulus velocity less than 2.5 degrees/s so as to avoid significant spatial shifts of the peaks due to response latencies. For all but two of the 49 cells, the response peaks in any given profile always showed a progressively greater degree of direction selectivity as the stimulus advanced from one subregion to the next, the first subregion giving the least directionally-selective response peak and the last subregion the most directionally-selective peak. This observation was independent of the direction of stimulus motion and of the particular sequence in which the ON and the OFF subregions were traversed by the stimulus. The response patterns observed experimentally have been correlated with theoretical response patterns based on the responses of lateral geniculate neurons.

Functional basis for the preference for slow movement in area 17 of the cat

Cells which respond only to slowly moving bars (velocity low-pass, or VLP cells) are numerous in area 17 whereas they are lacking in the LGN. The suggestion that this preference for low stimulus velocity is linked to the presence of S cells (equivalent of simple cells) in area 17 was critically evaluated by correlating S cell characteristics (subregion overlap, receptive field width and presence of inhibitory zones) with velocity preference. It was found that VLP cells generally respond only to relatively long durations of stationary stimulation and that this need of temporal summation best explains the preference for slow movement in area 17.

Linear signal transmission from prepotentials to cells in the macaque lateral geniculate nucleus

Prepotentials preceding a neuronal action potential were recorded extracellularly in the lateral geniculate nucleus (LGN) of the macaque. Although prepotentials are found less frequently in the macaque than in the cat LGN, their electrical characteristics are similar, suggesting that they represent the arrival of impulses in a retinal afferent, as in the cat. The visual response properties of prepotentials and associated cells were similar under a variety of conditions, indicating that, apart from some response attenuation, little signal processing takes place in macaque LGN. A constant fraction of prepotentials above a threshold frequency gave rise to neuronal action potentials independent of the stimuli used, so that the frequency of cell action potentials was linearly related to the frequency of prepotentials. Since the maintained discharge rates of a cell and its prepotential always fell on the linear relation, the net responses of a cell and its prepotential to visual stimuli were approximately proportional to one another.

Responses to coloured patterns in the macaque lateral geniculate nucleus: pattern processing in single neurones

Cell responses to complex visual patterns such as compositions of broad-band (non-monochromatic) colour areas are presented. Patterns were scanned over the receptive field, and cell response at each point was recorded. "Response patterns" were constructed which display the cell transform of the stimulus pattern. Parvocellular layer (PCL) cells in the lateral geniculate nucleus, in a very sustained fashion, signal the spectral composition of areas in the pattern, cells of different classes showing different spectral responsiveness. Little or no edge enhancement was present. Magnocellular layer (MCL) cells mark luminance steps in a pattern; they are not colour-specific. Responses to monochromatic stimuli provided a reliable guide to cell responsiveness with mixed colours. However, integration by spectrally opponent mechanisms is present with broad-band colour stimulation and partly accounts for a high variability of response patterns within the same cell class. With some cells, stimulus patterns were either shown through narrow-band colour filters, or a monochromatic background was added. Response patterns to isolated wavelength components compared with those to full colour patterns revealed an almost linear additivity of individual spectral components. Adaptation to a chromatic background, on the other hand, strongly modified the effectiveness of excitatory and suppressive components in the stimulus pattern and markedly changed the structure of a response pattern compared with the non-adapted situation.

Velocity sensitivity mechanisms in cat visual cortex

To understand why some cells in the visual cortex respond to high stimulus velocities while other fail to do so, a sample of 71 of such cells were examined for their responses to stationary presented stimuli as well as to moving edges or slits of different widths. When presented with stationary stimuli it was found that cells which respond best to slowly moving stimuli generally have tonic discharges, long time to peak latencies and often long minimal durations of stimulation. In contrast, cells which respond preferentially to fast stimuli have phasic discharges, short latencies and short critical durations of stimulation when presented with stationary flashed slits. In the latter type of cells the responses to very fast stimulus movement were abolished selectively when contrast and width of the stimulus were not optimal. A few cells exhibited a velocity-response (VR) curve with a central dip indicating good responsiveness to either slow or fast movement but little to medium velocities. These cells responded both phasically and tonically to stationary slits and the latency of the tonic and phasic responses at low and high velocities, respectively. It is suggested that the ability of phasic cells to respond to high velocities is linked to their limited need for temporal summation.

A model for the temporal organization of X- and Y-type receptive fields in the primate retina

A model is proposed for the temporal characteristics of X- and Y-type responses of ganglion cells in the primate retina. The main suggestions of the model are: (I) The X-type temporal response is determined primarily by the delay between center and surround contributions. (II) The Y-type response is generated in the inner plexiform layer by a derivative-like operation on the bipolar cell's input, followed by a rectification in the convergence of these inputs onto the Y-ganglion-cell. (III) The derivative-like operation is obtained by recurrent inhibition in the dyad synaptic structure. The X- and Y-type responses predicted by the model, for a variety of stimuli, were examined and compared with available electrophysiological recordings. Finally, certain predictions derived from the model are discussed.

Directional selectivity and its use in early visual processing

The construction of directionally selective units, and their use in the processing of visual motion, are considered. The zero crossings of delta 2G (x,y) * I(x,y) are located, as in Marr & Hildreth (1980). That is, the image is filtered through centre-surround receptive fields, and the zero values in the output are found. In addition, the time derivative delta[delta 2G(x,y) * I(x,y)]/delta t is measured at the zero crossings, and serves to constrain the local direction of motion to within 180 degrees. The direction of motion can be determined in a second stage, for example by combining the local constraints. The second part of the paper suggests a specific model of the information processing by the X and Y cells of the retina and lateral geniculate nucleus, and certain classes of cortical simple cells. A number of psychophysical and neurophysiological predictions are derived from the theory.

Receptive field organization of simple cells in cat striate cortex

The receptive field organization of simple cells was studied by analyzing interaction effects between two stationary flashing light stimuli. One stimulus was positioned in the most responsive part of the receptive field to produce activity against which the effects of the other stimulus in various positions of the visual field could be determined. The receptive field was subdivided into an elongated center and elongated antagonistic flanks. The effects on the flanks were always considerably stronger on one side. Powerful flank suppression could be elicited within a region which usually was only slightly wider than the receptive field center. The suppression was just as stimulus specific as the activation of the center and occurred only by light ON or OFF. The cells were classified into ON-dominant and OFF-dominant depending on the kind of response found in the center. In ON-dominant cells the strong flank suppression occurred only by light ON, and light OFF produced enhancement. Correspondingly, the strong flank suppression occurred only by light OFF in OFF-dominant cells. This is consistent with the interpretation that simple cells have excitatory and inhibitory input from the same type of cells in the lateral geniculate nucleus (LGN), i.e., only from ON-center or OFF-center cells. The small size of the area where strong flank suppression occurred shows that inhibition comes fom a few LGN cells rather than from a large pool of cells. A model for simple cell receptive fields presuming overlapping but acentric excitatory and inhibitory fields with input to both fields frm either ON- or OFF-center LGN cells was tested by computer simulation and shown to fit the experimental data.

The afferent connexions and laminar distribution of cells in area 18 of the cat

1. The receptive field properties, laminar distribution and afferent connectivity of cells in area 18 of the cat are described. 2. Testing with both moving and stationary stimuli revealed three main receptive field types which have been termed S, C and B, respectively (cf. Henry, 1977; Henry, Lund & Harvey, 1978). All three classes may show end-zone inhibition and units exhibiting this property have been designated SH, CH and BH. 3. S cells can be divided into spatially separate lights and/or dark edge response regions when tested with moving edges and usually have separate ON and/or OFF areas when tested with stationary flashing stimuli. They are the most commonly encountered cell type in area 18 and occur most frequently in laminae IIIb, IVa and VI. 4. Both C and B cells have spatially coincident light and dark edge response regions and give mixed ON and OFF discharges when tested with stationary flashing stimuli. Compared to B cells however, C cells have large receptive fields, they are broadly tuned for stimulus orientation and generally have a relatively high rate of spontaneous activity. C cells are more common than B cells and are encountered most often in laminae IVb and V. 5. Electrical stimulation of the optic chiasm (OX) and optic radiation (OR) was used to examine the afferent connectivity of parastriate neurons. Cells driven from both OX and OR have been divided into two main groups and it is argued that group 1 cells are directly, and group 2 cells are indirectly, excited by rapidly conducting afferent fibres. Group 1 cells are found most often in laminae IIIb, IVa, IVb and VI, and their distribution closely follows the anatomically defined laminar disposition of geniculocortical afferent terminals. Group 2 neurones predominate in laminae II-IIIa, IIIA and V. 6. The majority of S and SH cells are directly driven, whereas most C and CH cells have OX and OR latencies suggestive of indirect activation by thalamic afferents. 7. The intrinsic organization and possible functional role of area 18 is discussed in the light of these results.

Receptive field properties of simple and complex striate neurons in Siamese cats

Responses of striate cortical cells in Midwestern Siamese cats to moving slits and stationary flashed spots were recorded and compared to similar data in common cat controls obtained under identical experimental conditions. Over 90% of the neurons sampled had receptive fields within 10 degrees of the area centralis. The proportion of binocularly excited cells in Siamese striate cortex was less than that found in the control cats, but more than has previously been reported, and was inversely related to the extent of convergent misalignment exhibited by each animal. Those striate neurons which could be driven by either eye had normal binocular receptive fields and demonstrated normal binocular interaction effects except facilitation. Receptive field dimensions (length, width, area) were significantly larger in Siamese than in common cats. For the simple cells, strabismic Siamese cats had larger receptive fields than orthophoric Siamese cats. Average spontaneous activity in Siamese cats was significantly higher than in the controls. However, similar encounter rates of simple, complex, and hypercomplex units were observed in both Siamese and common cats, and peak responses were not different. Velocity preference in complex Siamese units was shifted to slower velocities compared to common cat complex cells. A loss of direction selectivity was also revealed in the Siamese simple and complex neurons. Finally, sharpness of orientation tuning was dramatically reduced in Siamese complex cells, and this reduction was directly related to the extent of convergent squint exhibited by each animal. The results are discussed in terms of developmental anomalies and neurophysiological retinal abnormalities in Siamese cats.

Excitation of units in the lateral geniculate and contiguous nuclei of the cat by stretch of extrinsic ocular muscles

In cats, anaesthetized with chloralose and paralysed, the responses of units in the right lateral thalamus were recorded while the extrinsic ocular muscles (EOM) of the right eye were stretched in the dark. Phasic responses were found in all layers of the dorsal lateral geniculate nucleus (LGNd) and in the perigeniculate nucleus (PGN). A given unit usually responded to stretch of more than one EOM and thus to more than one direction of rotation of the eye in the orbit. LGNd. Of a sample of 76 units in LGNd, 55 (72%) gave visula but no muscle responses and 21 (28%) responded to EOM stretch. In all, 40 units with EOM responses were examined and 25 of the 27 tested (93%) also had visual responses. Of the 40 units, 32 could be allocated to layers, thus; layer A, 8 (25%); layer A1, 20 (63%); layer B, 3 (9%); central interlaminar nucleus, 1 (3%). It is interesting that most of the EOM responses were found in layer A1 which recieves the excitatory visual input from the eye whose EOM were stretched. Muscle responsive units occurred with ON- and OFF-centre visual responses of sustained and transient types. PGN. In PGN, 21 units gave EOM responses and most of them were also excited by visual input. The conclusion is that the LGNd and PGN receive an extraretinal proprioceptive signal which should be present during at least large saccadic eye movements. The anatomical pathways which may be involved and the significance of the signal are discussed briefly.

Hierarchical and parallel mechanisms in the organization of visual cortex

We argue that it seems fruitful to regard the retino-geniculate-cortical pathway, and perhaps the visual pathways in general, as comprising distinct neuronal channels which begin with the major groupings of ganglion cells, and subserve distinct functions within the overall operation of the visual system. One problem for future work is to determine the extent and, equally importantly, the limitations of the idea of independently functioning neuronal channels operating within the visual system. Some evidence of those limitations is already available. Kulikowski and Tolhurst have provided evidence suggesting that pattern detection is mediated by the X-like system at high spatial frequencies and by the Y-like system at low frequencies, but that at intermediate frequencies, both systems are likely to contribute to this function. Again, there is already physiological and psychophysical evidence of inhibitory interaction between X- and Y-cell systems, which may contribute to their functioning. That is, although there is little evidence of excitatory interaction between W-, X- and Y-cell systems, at least up to the first cortical synapse, the functioning of, say, the X-cell system may depend on the inhibitory influences impinging on it from Y-cell activity. Further, it may prove to be the case that one cell 'system' may be involved in several distinct functions and considerable work may be required to establish whether or not these functions can be considered constituent parts of an overall function, such as 'ambient' or 'foveal' vision. In the following section we suggest a classification and terminology for visual neurones which may provide a framework for future work on these lines.

Functional organization of lateral geniculate cells following removal of visual cortex in the newborn kitten

When the visual cortex of a newborn kitten is removed, most neurons in the dorsal lateral geniculate nucleus degenerate, but a small population of large cells is spared. Electrophysiological recording revealed that detailed visual topography in the nucleus is abnormal and that single cells have unusually large receptive fields. These results suggest that optic axons deprived of their normal synaptic targets rearrange their connections to converge on local surviving neurons.

Spatial and temporal properties of X and Y cells in the cat lateral geniculate nucleus

1. Extracellular recordings were obtained from units in the dorsal lateral geniculate nucleus of anaesthetized cats. 2. Of sixty-nine units, sixty-three could be unambiguously identified as either X (n = 33) or Y (n = 30) by testing the presence of a null response to stationary sine wave gratings presented in different spatial phases. 3. In response to stationary gratings flashed on and off, Y cells exhibited bigger, more transient responses than X cells. 4. All Y cells but few X cells exhibited a shift effect (modulated periphery effect). 5. In response to drifting sine wave gratings of different spatial frequencies, X cells preferred higher spatial frequencies and showed smaller peak contrast sensitivities and somewhat narrower tuning curves than Y cells. 6. In response to a sine wave grafting of optimal spatial frequency drifting at different velocities, X and Y cells had similar temporal tuning curves. However, Y cells, largely because they preferred lower spatial frequencies, preferred higher drift velocities than X cells. 7. Our data suggest that X and Y cells can be differentiated objectively on the basis of a number of discharge parameters. These parameters are compared with similar data collected by others from neurones in the visual cortex.

The responses of magno- and parvocellular cells of the monkey's lateral geniculate body to moving stimuli

The responses to moving stimuli of single cells in the parvo- and magnocellular layers (PCL and MCL) of the macaque lateral geniculate nucleus (LGN) have been studied. PCL cells respond with a monophasic increase or decrease in firing when a bar passes across the receptive field, according to the wavelength composition of the stimulus. MCL cells respond with a biphasic sequence of excitation and suppression or vice versa dependent on whether a cell is on-centre or off-centre and on stimulus contrast direction. With large stimuli, PCL cells respond as long as the stimulus covers the receptive field while MCL cells respond only at the contrast borders. MCL cell responses are maximal with bars just long enough to cover the field centre, while PCL cell responses show a variable relation with bar length, depending on stimulus wavelength and receptive field structure. PCL cells show broad velocity tuning while at least some MCL cells were more sharply tuned. Many cells in the macaque LGN show weak orientation or direction preference.

Visual suppression from nondominant eye in the lateral geniculate nucleus: a comparison of cat and monkey

We have studied the suppression of firing in single LGN cells of cat and monkey in response to visual stimulation of the nondominant eye. In the cat LGN most of the cells of each of the main laminae show this nondominat suppression. X cells having their dominant input from the ipsilateral eye were suppressed to a significantly greater degree than any other cell type in the cat LGN. In the monkey LGN nondominant suppression was absent in all 19 X-like cells studied, whereas 6 of 21 Y-like cells showed nondominant suppression. Thus nondominant suppression is present in the magnocellular laminae of the monkey LGN, where the Y-like cells are found, but appears to be absent from the parvocellular laminae, where the X-like cells are found.

Responses of single cells in cat's lateral geniculate nucleus and area 17 to the velocity of moving visual stimuli

Neuronal responses to moving visual stimuli were recorded in the lateral geniculate nucleus (LGN) and area 17 of cats. Response duration (DE), number of spikes (NT), and mean frequency (FM) were estimated from the response histograms and analysed for their dependence on stimulus velocity. In the LGN, for about 2/3 of cells these response parameters changed monotonically with velocity up to about 100 degrees/s. In 1/3 of the cells, the response frequency was tuned to velocity. The speed at which individual cells reached a peak or plateau firing rate was correlated with their receptive field size. In area 17, most neurones were tuned to velocity. Nine out of 59 cells were insensitive to stimulus speed in that they responded equally well at stimulus velocities up to about 10 degrees/s. The results suggest that at higher levels in the nervous system information about velocity is represented in discrete groups of neurones. It is pointed out that different response parameters may be relevant for different perceptual phenomena associated with movement. The significance of integrational properties and lateral inhibition of nerve cells for the development of complex response properties is discussed.

Properties of neurons in cat's dorsal lateral geniculate nucleus: a comparison between medial interlaminar and laminated parts of the nucleus

We studied the receptive field properties of 460 cells in the cat's dorsal lateral geniculate nucleus (LGNd), 108 cells were located in the medial interlaminar nucleus (MIN) and 352 in the laminated part of the LGNd. In both the MIN and laminated parts of the LGNd, relay cells belonging to all three functional classes (W, X and Y) have been identified. Of cells in the laminated LGNd, about 32.5% were Y cells, about 54.5% were X cells and about 8.5% were W cells. By contrast, in the MIN, about 84% were Y cells, only about 4.5% being X cells and about 7.5%, W cells. In the laminated LGNd, Y cells represented 25% of cells with receptive fields near the area centralis (0-3 degrees eccentricity group) and about 42% in the group of cells with the most peripherally located receptive fields (20-40 degrees eccentricity group). A similar but much weaker trend was observed in the MIN. In the laminated LGNd but not in the MIN the receptive field center sizes increased with increasing eccentricity of receptive field position. At any eccentricity, receptive field centers of MIN Y cells tended to be larger than those of Y cells in the laminated LGNd. Response latency ranges to orthodromic and antidromic stimulation were the same for cells located in the laminated LGNd and those in the MIN. However, the mean response latency to stimulation of the optic chiasm was significantly shorter for Y cells in MIN than for Y cells in the laminated LGNd. Our results suggest that the most numerous cells observed histologically in the MIN, class 1 cells of Guillery ('66) are morphological equivalents of Y cells.

The influence of stimulus velocity on the responses of single neurones in the striate cortex

1. Using a multi-histogram technique forty-seven response-velocity curves were prepared for a variety of visual stimuli presented to twenty-one cells in the striate cortex of the anaesthetized, paralysed cat. 2. The character of each velocity-response curve varied according to the measurement used in assessing a response. Reasons are advanced for sampling the response over a single bin of short duration at the peak of the discharge in each average response histogram. 3. The sharpness of tuning varied markedly throughout the population of cells but it was not possible to establish any definitive class differences. 4. For simple and complex cell categories there was considerable overlap in both the range of effective stimulus velocities and the distribution of the optimal velocities. An observation not emphasized in the past was that some simple cells responded to very fast stimuli while a number of complex cells were driven by very slowly moving stimuli. 5. Generally changes in stimulus parameters such as the polarity of contrast of a moving edge, its orientation or direction of movement produced only slight modifications in the profile of the velocity-response curve. 6. The abolition of the response of simple cells that failed to be driven by rapidly moving stimuli was shown to be due to the entry of the stimulus into the inhibitory flank distal to the discharge region. When the movement of the stimulus was confined to the discharge region there was little evidence of velocity dependence in the response. The duration over which the inhibition from the distal flank remained effective was evaluated for representative simple cells.

The retinal input to cells in area 17 of the cat's cortex

The activity of retinal ganglion cells and cortical cells with overlapping receptive fields was simultaneously recorded. The responses to moving stimuli of individual simple cortical cells could be accounted for on the basis of the cell receiving either on-centre or off-centre afferents; instances in which it was necessary to postulate a mixed on- and off-centre input were not found. In six instances cross correlograms of ganglion cell and cortical cell activity showed that the ganglion cell was afferent, via a relay cell in the LGN, to the cortical neurone. The receptive fields of such pairs were almost completely overlapping and concentric. In three cases a sustained ganglion cell projected to a simple cortical cell. In one case a transient ganglion cell projected to a simple cell, and in one case a sustained and a transient ganglion cell projected to the same simple cell.

Influence of movement parameters on area 18 neurones in the cat

In cats, 107 area 18 neurones with identified FR type, 10-50 degrees from the visual axis, were tested for the influence of direction, velocity and amplitude of movement. These three parameters are believed to be the primary parameters of a movement analysing system. 94% of the neurones were influenced by the direction of movement, all of them by the angular velocity and 16% by the amplitude of movement. For each of the primary parameters, tuning curves were established. Angular velocity influenced not only the response magnitude but also the response latency and the direction bias. By preparing response amplitude functions at different velocities the influence of movement duration was ruled out. The association of functional properties and RF organization suggests a model of information processing in area 18 of the cat.

Responses of single units in cat visual cortex to moving bars of light as a function of bar length

1. The responses of single units in the cat's primary visual cortex to moving bars have been examined quantitatively as a function of bar length.2. For about half the cells studied, very long bars evoked weaker responses than short bars, implying that there were inhibitory regions flanking the receptive field centre. In another quarter of the cell sample, there was evidence of flanking regions which were facilitatory in effect.3. The strength of the flanking regions was found to vary from cell to cell and there was no sudden transition between cells which were ;hyper-complex' and those which were not.4. Within the central region of the receptive field, the responses of most (but not all) cells increased with bar length. About half the cells responded to very short bars or spots of light, but about one in six would not respond at all to short bars.5. Correlations were sought between the properties of cells as simple or complex, their responsiveness to moving spots of light, the size of their receptive field centre and the polarity, strength and size of their receptive field flanks. Simple and complex cells with small receptive fields were more likely to respond well to spots, and to have strong inhibitory flanks.6. Correlations were also sought between the above properties and several other parameters of cell behaviour. Cells with strong inhibitory flanks were found to be more broadly tuned for orientation. Individual cells were also more broadly tuned for the orientation of short bars than of long bars.7. Evidence was obtained that spatial summation can be linear or non-linear for different cells.

Biases for oriented moving bars in lateral geniculate nucleus neurons of normal and stripe-reared cats

Visual receptive fields of 42 LGN cells from normal cats and 110 cells from striped cylinder-reared kittens were studied with the aid of a computer controlled optical system. In the normal cats, ten of the 42 cells were weakly biased for orientation of the visual stimulus when tested with bars swept through the receptive field. Of those ten, eight were classified as transient. The orientation preferences of the ten biased units appeared randomly distributed around the clock. Of the LGN cells from the cylinder-reared group, about half of the transient cells had weak biases for orientation; only 7% of the sustained cells had biases. The orientation preferences of the biased LGN cells in the stripe-reared animals were either parallel to or orthogonal to the stripes each animal saw during its time in the conditioning cylinder. In 16 out of 18 of the biased LGN cells it was found that increasing the velocity of the test target reduced or eliminated the bias apparent at the lower velocity. For some LGN cells special techniques, such as inhibition of activated discharge, were needed to reveal orientation biases. The results described here, considered with data from others, suggest a role for the corticofugal projection in modulating the responses of some LGN cells.

Responses of cells in the cat lateral geniculate nucleus to moving stimuli at various levels of light and dark adaptation

The responses of neurones in laminae A and A1 of the cat lateral geniculate nucleus to moving stimuli were investigated at different background luminances. Moving bright slits, dark bars and edges were employed; the contrast of stimuli against the background was held constant. Background intensities varied from 10(-3) to 10(2) td. Responses as stimuli passed across the centres of LGN receptive fields became stronger with increasing levels of light adaptation up to 10(-1)-10(1) td and then remained constant. Responses as stimuli passed through surround regions altered qualitatively with adaptation level, generally increasing in strength and complexity with background luminance. As a bright slit for on-centre cells or dark bar for off-centre cells left the surround, in almost all units a strong secondary peak could be elicited by an appropriate selection of the adaptation conditions. Many features of the responses to moving stimuli could not be predicted from the responses to stationary stimuli under different adaptation conditions described in the previous paper.

Discharges of relay cells in lateral geniculate nucleus of the cat during spontaneous eye movements in light and darkness

1. Discharges of 315 relay cells of the lateral geniculate nucleus (LGN) during spontaneous eye movements were studied in alert cats. 2. When tested in a stationary patterned field, 114 cells showed sustained discharges related to the direction of gaze (S cells) and to local differences in luminance; 109 cells showed transient response to quick shifts of retinal image during saccades (T cells); ninety-two cells showed mixed responses (M cells), i.e. transient responses to rapid shifts of retinal image and sustained firing related to local differences in luminance. 3. Following saccades occurring in the light, T and M cells showed a burst discharge, while spontaneous discharges of S cells were completely suppressed for 150-200 msec. 4. When tested in total darkness, modifications in activity which were apparent in light disappeared completely. This was true for all 315 relay cells. 5. T cells responded to optic chiasm stimulation at shorter latencies (X = 1.15 msec) than S cells (X = 1.77 msec). M cells showed a latency distribution in between those for S and T cells with a mean latency 1.40 msec. 6. When tested with moving grating stimulation, S cells responded in only one manner; with discharges to each stripe of the grating (primary response), while T and M cells showed two different responses: a primary response to a slower motion and a non-specific burst in response to a faster motion. The burst did not reflect the stimulus pattern (secondary response). 7. When tested with diffuse light switched on and off over the tangent screen, S cells showed a sustained response either to light or darkness, whereas T and M cells responded transiently either to the onset or offset of the light, or to both. M cells occasionally showed a mixture of transient and sustained responses either to light or darkness. 8. In over-all response properties, most S cells correspond to X (sustained) cells and most T cells to Y (transient) cells previously known from acute experiments. M cells had intermediate response properties between X and Y cells. 9. Functional roles of these classes of cells in relation to previously proposed functions are discussed.

The contribution of inhibitory mechanisms to the receptive field properties of neurones in the striate cortex of the cat

1. The iontophoretic application of the GABA antagonist bicuculline to simple and complex cells in the striate cortex of the cat produced extensive modifications of receptive field properties. These modifications appear to relate to a block or reduction of GABA-mediated intracortical inhibitory influences acting on the cells examined. 2. For simple cells the effects of bicuculline on receptive field properties involved a loss of the subdivision of the receptive field into antagonistic "on" and "off" regions, a reduction in orientation specificity and a reduction or elimination of directional specificity. 3. The effect on the "on" and "off" subdivisions of the simple cell receptive field was such that all stationary flashing stimuli, whether covering the whole receptive field, or located within the receptive field over a previously determined "on" or "off" region, resulted in an "on and off" response. 4. The orientation specificity of complex cells was reduced during the application of bicuculline such that in many cases the original specificity of the cell was virtually lost with the response to the orientation at 90 degrees to the optimal being of similar magnitude to the optimal. The directional specificity of complex cells was generally less affected than that of simple cells. Often when large changes in orientation specificity were observed the directional specificity was relatively unaffected. 5. For some cells apparently showing to all visual stimuli only inhibitory responses, the application of bicuculline resulted in the appearance of excitatory responses. 6. In all cases receptive field properties reverted to the original state after termination of the bicuculline application. It was not generally possible to duplicate the effects of bicuculline by raising neuronal excitability with iontophoretically applied glutamate. 7. On the basis of these results it is suggested that the normal subdivision of the simple cell receptive field into separate "on" and "off" regions and its directional specificity are dependent on intracortical inhibitory processes that are blocked by bicuculline. The orientational tuning of simple cells conversely appears to be largely determined by the excitatory input but normally enhanced by lateral type inhibitory processes acting in the orientation domain. 8. It also appears that the excitatory input to some complex cells is not orientation specific. This suggests that for these cells it is extremely unlikely that they receive an orientation specific excitatory input from simple cells.

Pattern and flicker detection analysed by subthreshold summation

1. We confirm Keesey's (1972) observation that, when a flickering line is viewed, there are distinct thresholds for detecting flicker (or movement) and for detecting a well localized line (pattern detection). Our measurements of the temporal sensitivity of these two mechanisms are similar to Keesey's. 2. The flicker and pattern detection mechanism have been analysed using subthreshold summation, i.e. by observing the effect of subthreshold flickering stimuli (lines and gratings) on the contrast threshold for a flickering test line. 3. The pattern detector shows linear spatial summation of contrast while the flicker detector is non-linear in this respect. 4. The receptive field of the (most sensitive) flicker detector is about two to four times broader than that of the pattern detector. 5. The flicker detector has relatively weak surround inhibition and so, unlike the pattern detector, it is sensitive to a uniform flickering field. 6. The spatial arrangement of the pattern detector is the same at all temporal frequencies (including steady presentation); for flicker detection, the width of the receptive field increases with temporal frequency and the strength of lateral inhibition decreases at high frequencies. 7. Flicker detectors of various widths were demonstrated by using different test stimuli (for 12 Hz modulation); surround ingibition was relatively weak for the broadest detector. 8. There is a delay of surround inhibition of about 3 ms for both flicker and pattern detection. 9. By using a broad test stimulus modulated at a high frequency, a detector can be found with no significant surround inhibition. At threshold, this stimulus produces a sensation of flicker without the appearance of lateral motion observed for finer test lines at lower frequencies. 10. The characteristics of pattern and flicker (movement) detection are compared to electrophysiological studies on X (sustained) and Y (transient) neurones respectively, and correlations are described for studies of temporal frequency response, non-linearity, width of receptive field, strength of the inhibitory surround and motion sensitivity.

The velocity tuning of single units in cat striate cortex

1. The activity of single units was recorded from the striate cortex (area 17) of anaesthetized, paralysed cats. Responses to stimuli moving at different velocities were examined. 2. Peak evoked firing frequency, rather than fotal evoked spikes, is used throughout as a measure of response. The former mea-ure gives curves of response vs. velocity that correlate well with curves of contrast sensitivity vs. velocity, wheras the latter does not. 3. Cortical receptive fields were classified according to the criteria of Hubel & Wiesel. Simple cells were found to prefer lower velocities (mean 2-2 deg sec-1) than complex cells( mean 18-8 deg sec-1). The response of simple cells to stimuli moving faster than 20 deg sec-1 is generally poor; complex cells usually discharge briskly to these speeds. 4. Cells classified as hypercomplex by the end-inhibition criterion were further chara-terized as type I or type II, according to the suggestion of Dreher (1972). Type I units are indistinguishable from simple cells in their velocity tuning, and type II units equally clearly resemble complex cells. These results are therefor consistent with Dreher's sbudivision. 5. Teh selectivity of cells for velocity is variable but can be quite marked. The average selectivities of simple and complex cells are not significantly different. There is an inverse correlation between preferred velocity and the sharpness of velocity selectivity for simple cells; no trend is apparent for other cell types. 6. No clear correlation is observed between the velocity preferances of units and their degree of direction selectivity, or receptive field arrangement. Simple cells with 'sustainef' temporal responses to flashed stimuli tend to prefer slower rates of movement than 'transient' ones, and to be less selective for velocity. 7. The results for different cortical cell-types are compared with the velocity tuning of X- and Y-cells in the lateral geniculate nucleus.