The outer disinhibitory surround of the retinal ganglion cell receptive field

1. There is an outer disinhibitory zone surrounding the classical inhibitory surround of the retinal ganglion cell receptive field.2. The disinhibitory surround is strong and narrow in ;sustained' cells but weak and laterally spread in ;transient' cells.3. The disinhibitory surround can be demonstrated using a black spot as a probing stimulus as well as by a white spot, and is therefore not an artifact of scattered light.4. Stimulation with a light spot in the disinhibitory zone gives an increase in firing to ;stimulus on' in on-centre cells and to ;stimulus off' in off-centre cells.5. The disinhibitory surround may be revealed by plotting the latency of the first spike discharge following stimulation against position in the receptive field. The disinhibitory zone shows a decrease in latency to the centre-type stimulus.6. The disinhibitory surround may be revealed by plotting the threshold intensity of a spot against position in the receptive field. It is thus a feature of the sensitivity gradients of both ;transient' and ;sustained' cells.7. Using two spots, one at the centre of the receptive field and the other at varying distances from the receptive field centre, dynamic interactions between the centre, inhibitory and disinhibitory zones are demonstrated. A spot presented in the disinhibitory zone causes an enhancement of the centre response when flashing in phase with the centre spot, while it causes inhibition of the centre response when presented 180 degrees out of phase.8. A scheme for the anatomical basis of the disinhibitory surround is proposed, and the relation of disinhibition to the spatial transfer characteristics of the visual pathways is discussed.     

Receptive field organization of `sustained' and `transient' retinal ganglion cells which subserve different functional roles

1. Post-stimulus histograms were obtained from `sustained' and `transient' retinal ganglion cells for receptive field plots using a light spot with square-wave modulation of intensity, and of variable intensity and area. Fundamental differences in their receptive field organization in time and space were revealed.

2. In `sustained' cells, excitation consists of `transient' and `sustained' components and the ratio of transient/sustained components remains constant at a given retinal locus for a wide range of intensities. The transient component becomes proportionally larger towards the periphery of the receptive field. This rule is also applicable for the inhibitory and disinhibitory surround. In `transient' cells, however, there is no true `sustained' component, but some cells produce a double peaked transient post-stimulus histogram at the R.F. centre when high flux stimuli are used, while others show a single peak transient response. The magnitude and shape of transient responses changes with intensity as well as with location in the receptive field.

3. The sensitivity gradients of `sustained' and `transient' cells show consistent differences in shape. The mean slope of the sensitivity gradients of a sample of `sustained' cells was 10 times that of a sample of `transient' cells. The sensitivity gradient of `sustained' cells shows a distinct surround region where the inhibitory mechanism is more sensitive, while that of `transient' cells usually does not, owing to an extensive `tail' on the sensitivity gradient of the centre mechanism, which overlaps the surround.

4. Ricco's Law also holds for the centre mechanism of `transient' cells. Non-linear summation occurs at supra-threshold levels, and when the surround mechanisms are involved.

5. Supra-optimal stimuli give a saturation of the response in both `transient and `sustained' cells. This saturation is associated with a decrease of latency in `transient' cells, but not in `sustained' cells.

6. The latency of retinal ganglion cells is determined by both stimulus and background flux. The effect of the background is negligible except at low values of stimulus flux, where its effect may be analysed primarily in terms of its effect on the incremental threshold.

7. The latency to stimulation with a standard small spot (25-27′) at the receptive field centre is shorter for `sustained' cells than for `transient' cells; this latency difference being related to the greater sensitivity of the `sustained' cells to stimuli of this size. Differences in conduction time along `transient' and `sustained' pathways to the lateral geniculate nucleus (LGN) and cortex were estimated, and it is concluded that despite the latency difference noted above, a response to a stimulus which is optimal for a `transient' cell reaches the cortex faster than the response to a stimulus which is optimal for a `sustained' cell.

8. The above results together with previous evidence available suggest that for most stimuli, centre and surround mechanisms are activated simultaneously and algebraically summed by a single linear stage in `sustained' cells. In `transient' cells, although the centre excitation and surround inhibition pools are also spatially co-extensive, they summate and interact in time and space with a greater complexity.

9. Differences in the receptive field organization of `sustained' and `transient' cells may reflect their different functional roles in vision: (1) analysis of spatial contrast and form recognition (`sustained' cells), and (2) fast detection of objects entering visual space to cause orientation responses (`transient' cells).

     

Quantitative aspects of sensitivity and summation in the cat retina

1. Properties of the central response mechanism of on-centre ganglion cells in the cat retina were studied by recording, from optic tract fibres, responses evoked by stimuli modulated with time in a sinusoidal or square-wave fashion.

2. The shape of averaged square-wave responses resulting from the central mechanism alone was identified. This shape was identical from one cell to another. Such an identification permits the early recognition of peripheral antagonism.

3. Threshold sensitivity for a sinusoidal stimulus was determined for fifty cells along one horizontal and vertical axis, passing through the most sensitive portion of the receptive field. These sensitivity profiles were described in terms of a central segment of constant maximum sensitivity (uniform centre) and sloping outer segments of exponentially decreasing sensitivity (exponential annulus). The dimensions of the uniform centre (horizontal axis × vertical axis) varied from 0·1° × 0·1° to 2·5° × 2·2°, the half width of the exponential annulus ranged from 0·1° to 0·63°.

4. Adapting spots of varying diameter were placed concentric with the receptive field and the (unmodulated) luminance, at each diameter, that reduced a small central (sinusoidal) stimulus to threshold, was determined. The resulting area—adaptation curve, (adapting luminance plotted against diameter) showed that within defined limits the state of adaptation is determined by the flux independent of its distribution.

5. Sinusoidal stimuli of varying diameter were placed concentric with the receptive field and the threshold luminance at each diameter was determined. Suprathreshold square-wave stimuli indicated that the central mechanism alone contributed to the response. These area-sensitivity curves did not show any decrease in sensitivity at larger diameters.

6. The shape of the area—sensitivity curve, and hence the extent of the summating area, was found to be independent of the state of adaptation.

7. For any one cell the shapes of the area—adaptation and area—sensitivity curves were shown to be identical, indicating that adapting flux and stimulus flux are independent of distribution over the same defined limits.

8. The sensitivity of combinations of small disconnected areas of the receptive field was found to be equal to the sum of their individual sensitivities.

     

Pure central responses from off-centre cells and pure surround responses from on-centre cells

1. Pure responses generated by the centre response mechanism of off-centre cells or the surround response mechanism of on-centre cells were studied in the cat by recording from single optic tract fibres while applying slow square-waves of light of varying duration but constant luminance.

2. The response, whether due to a centre or a surround mechanism, consisted of a rapid decrease in firing frequency at onset of light, followed by a gradual recovery of firing rate while the light was on. The time course of the recovery was exponential. At `off' there was a sudden increase in firing rate (off-discharge) followed by an exponential, more gradual, decay of impulse frequency. Both the gradual recovery during illumination and the off-discharge were absent for the shortest flashes.

3. Perhaps the most significant result was that the behaviour exhibited by responses generated by one only of the response mechanisms, centre or surround, is predictable and much simpler than that exhibited by mixed responses; the magnitude of the off-discharge increased, the latency decreased monotonically with increased flash duration and the magnitude of the off-discharge was linearly related to the level of firing at the moment when the light went off.

4. For two off-centre cells gain was studied by recording the response to a brief test flash superimposed upon a slow gain changing square-wave stimulus. Centre mechanism gain decreased promptly upon application of the gain changing light, stayed low while it was on, only to return promptly to its former value when the gain changing light went off.

5. A simple model which qualitatively accounts for the results (excluding latency) was suggested. The onset of light initiates both an inhibitory and excitatory process whose sum constitutes the transient input to the cell. The excitatory process grows at a slower rate so that `excitation wins more and more as the light remains on'. Similarly after `off' the excitatory process decays slower than the inhibitory one, thus leaving the cell with a transient excess of net-excitation.

     

Transmitters mediating inhibition of ganglion cells in the cat retina: iontophoretic studies in vivo

The effects of iontophoretically applied gamma-aminobutyrate (GABA) and glycine and their antagonists, bicuculline and strychnine on inhibition of retinal ganglion cells were studied in the optically intact eye of anaesthetised cats. Two kinds of inhibition were studied. One is the inhibition which occurs when a spot (a white spot for on-centre and a black spot for off-centre cells) which produces a maximal response from a cell, is removed from the receptive field centre, i.e. the central post-excitatory inhibition. The other is the inhibition which occurs when an annulus (a white annulus for on-centre and a black annulus for off-centre cells) which occupies the surround region of the receptive field, is presented, i.e. the surround inhibition. GABA enhanced and bicuculline blocked the post-excitatory inhibition at the receptive field centre and surround inhibition of on-centre but not off-centre cells regardless of whether the cell was 'sustained' or 'transient' type. On the other hand, glycine enhanced and strychnine blocked the post-excitatory inhibition at the receptive field centre and surround inhibition of off-centre but not on-centre cells, regardless of whether the cell was 'sustained' or 'transient' type. Inhibition of on-centre cells, thus, appears to be mediated by GABA, whereas that of off-centre cells, by glycine regardless of whether the cells are 'sustained' or 'transient'. Possible existence of GABAergic and glycinergic amacrine cells making postsynaptic contact with on-centre and off-centre ganglion cells, respectively, is proposed. Other possible explanations are discussed.     

Responses of single units in the monkey superior colliculus to moving stimuli

1. Single unit responses of pan-directional cells to moving and stationary flashing stimuli were studied in the superficial layers of the superior colliculus in paralysed, anaesthetized rhesus monkeys. The aim of this study was to see how far cell responses to moving stimuli fit in with what would be expected from their responses to stationary flashing stimuli. 2. Both the leading and the trailing edge of a moving stimulus evoke a transient response. If the diameter of moving light spots is increased the strength of the leading edge response increases, reaches a maximum and decreases to a constant value which is similar to the behaviour of the on response when the diameter of flashing spots is increased. The strength of the trailing edge response increases and reaches the same strength as that of the leading edge response. If the width of a long moving slit is increased, the strength of the leading edge response is the same at all slit widths, while the strength of the trailing edge response shows a course similar to that of the trailing edge response if the spot diameter is increased. If the length of a wide moving slit is increased both the leading and the trailing edge responses decrease. These results indicate that the strength of both leading and trailing edge responses is dependent on the degree the inhibitory surround is activated. 3. The leading and the trailing edge of a stimulus evoke their responses at the same position in the receptive field independent of the direction of movement. 4. Increasing the velocity of a moving stimulus shows that in general the leading edge response is present up to higher velocities than the trailing edge response independent of the sign of contrast. The burst duration to moving stimuli decreases with increasing stimulus velocity and appears to be determined by the time a moving edge is present in the receptive field centre. When this time becomes shorter than 10--20 ms, the burst duration for moving stimuli is constant and about the same as for flashing stimuli. This indicates that, although spatial receptive field properties can vary considerably, temporal receptive field properties show a strong similarity among different units. 5. The response latencies to light and dark moving edges are the same, which in turn are about equal to the response latencies to stationary flashing stimuli. 6. Stimulation experiments show that the general response characteristics to moving stimuli can be predicted by using a set of receptive field parameters derived from responses to stationary flashing stimuli. The most important variable of moving stimuli appears to be the period of time a moving contour is present within the receptive field centre, besides the degree of activation of the inhibitory surround.     

Responses to directional stimuli in retinal preganglionic units

1. Extracellular recordings were made from directionally selective ganglion cell units in the isolated frog retina and decapitated Necturus preparation.2. Intracellular recordings were made from individual photoreceptor cells in the frog and Necturus retinae while stimuli which had evoked directionally selective responses at the ganglion cell level were presented. No evidence for inhibition of photoreceptors for any direction of movement of the light stimulus was found. This appeared to rule out a mechanism for directional selectivity involving inhibition of photoreceptor potentials.3. Intracellular recordings were made from the nuclear layer between photoreceptors and ganglion cells in Necturus. The responses were of two types: either transitory or sustained.4. The sustained type responses could be divided into two classes depending on their receptive field organization. One type of sustained potential had a large receptive field without any evidence for a centre-surround antagonism and corresponded to the luminosity type S-potential recorded in fish. The other type had a smaller receptive field and showed a difference in sign of response between centre and surround if the centre was flooded with a steady light. This is very similar to what has been described for a type of on-centre, off-surround ganglion cell.5. The transitory type of responses showed some centre-surround antagonistic organization. Some of these transitory units also appeared to show some discrimination in response as a function of the distribution of light on the retina.6. No specific directional selectivity was found from units at the inner nuclear layer. This further excluded any mechanism of directional sensitivity which involves selectivity at the photoreceptor level.7. It was concluded that although inner nuclear layer units may play a role in the mechanism of directional selectivity, no specific directionality was found at the first synaptic level of the retina.     

Responses of single units in the monkey superior colliculus to stationary flashing stimuli

1. Responses of pan-directional cells in the superficial layers of the superior colliculus in paralysed anaesthetized rhesus monkeys to stationary flashing stimuli have been studied. 2. The receptive field centre response is always of the transient excitatory on-off type, while the surround response is transient inhibitory both at light-on and at light-off. The receptive field centres are circular or slightly elliptical. The average size of the receptive field centres is much larger than that of retinal ganglion cells. All units except those in the far temporal periphery receive binocular input. In each unit the on and off responses have the same latency times. With increasing stimulus area, the latency time at light-on and at light-off first decreases and then remains constant. In most units the number of spikes in the burst at light-on and at light-off first increases, reaches a maximum and then decreases with increasing stimulus area. This decrease demonstrates the presence of an inhibitory surround. 3. A model of spatial and temporal properties of centre and surround mechanisms is tested. Addition of excitatory centre input and inhibitory surround input, which have different spatial and temporal properties, determines the output of the neurone. The centre mechanism gets excitatory input from retinal ganglion cells and shows saturation. The inhibitory surround mechanism is made by an inhibitory interneurone. It could not be decided whether the excitatory input for this interneurone comes from retinal axon collaterals (forward inhibition) or from axon collaterals of "principal" cells in the superior colliculus (backward inhibition).     

Receptive field organization of bipolar and amacrine cells in the goldfish retina

1. Intracellular recordings were made from bipolar and amacrine cells in the isolated goldfish retina. Cells were identified mainly from their response patterns to a spot and an annulus in reference to the knowledge obtained from the previous work of intracellular Procion Yellow injection. Using white light and monochromatic lights receptive field organization of recorded cells were analysed.2. All bipolar cells had a centre-surround organization in their receptive fields. The field centre was estimated to be 100-200 mum in diameter, and the surround 1-1.5 mm.3. Bipolar cells were classified into two types according to the response properties to monochromatic lights. Opponent colour cells received inputs from red and green cones, responding with red on-centre, red and green off-surround or vice versa. Cells without colour coding received input from red cones both in the field centre and the surround. In these cells the centre and the surround were well balanced.4. Amacrine cells were also classified into two types, a sustained type and a transient type. The sustained type amacrine cells responded with a steady potential change and were colour coded. They were hyperpolarized by red and depolarized by green light. The transient type amacrine cells responded with transient depolarization at on and off of light flashes. They received input chiefly from red cones and were not colour coded. Both types of amacrine cells showed a large spatial summation in an area over 2.5 mm; centre-surround antagonism was not seen.5. Comparing the size of the receptive field with anatomy, especially with the size of dendritic spread, the field centre of bipolar cells agreed in size with their dendritic spread. Bipolar cell surround clearly exceeded its dendritic field. Since the response properties of the bipolar cell surround was mimicked most closely by the receptive field of external horizontal cells, the input to the bipolar cell surround is thought to be mediated by external horizontal cells.6. By comparing receptive field properties of various retinal cells it is suggested that both the opponent colour bipolar cells and the colour coded amacrine cells converge on to the double opponent ganglion cells.     

Suprathreshold spectral properties of single optic tract fibres in cat, under mesopic adaptation; cone—rod interaction

1. Responses of 122 on-centre or off-centre ganglion cells in cat to suprathreshold monochromatic stimulation have been analysed under mesopic adaptation with white light, recording from their single fibres in the optic tract at a level posterior to the chiasma. Fields described are monocularly driven, located in the right half-fields of either eye, and are all within 30 degrees of the area centralis.2. Retinal receptors are of two types, viz. 556 nm cones and 502 nm rods. At high mesopic adaptation (1 log cd/m(2)) all units receive mixed cone-rod input. Under low mesopic adaptation (0 log cd/m(2)) the great majority receive mixed input; a few receive pure rod input. These results are in agreement with the threshold data (Andrews & Hammond, 1970).3. Peaks of spectral response curves of units, to suprathreshold monochromatic stimuli of different wave-length but equal quantum flux, fall primarily between 550 and 560 nm for high mesopic adaptation, and between 500 and 520 nm for low mesopic adaptation. Peak position depends on the degree of rod or cone contamination, in units treated under high or low mesopic levels respectively.4. Units with cone-rod input to the field centre receive similar but antagonistic cone-rod input to the surround. In units with pure rod input to the field centre, only rods input to the surround.5. Cone and rod components of on-centre discharges are identifiable in terms of colour sensitivity and latency. The cone component is primarily a short-latency, high-frequency, excitatory transient; the rod component is a longer latency, lower frequency, maintained phase of excitation.6. Less direct evidence indicates that cone and rod input to the field surround give rise to inhibitory components of comparable latency, magnitude and time course.7. The identified cone and rod components of responses are used in further experiments to show that cone and rod input have different spatial organization both in the receptive field centre and in the surround.8. The boundary between the field centre and surround for rods has a diameter on average about twice as large as that for cones. This organization is such that the field centre for rods substantially overlies the cone surround.9. Changes in receptive field organization occur within the mesopic range, associated with the changeover from cone to rod vision.10. It is suggested that the difference between cone and rod input in the mesopic range may form the basis of the cat's ability behaviourally to discriminate between colours.     

Dark adaptation and receptive field organisation of cells in the cat lateral geniculate nucleus

Summary The receptive fields of LGN cells were investigated with stationary light and dark spot and annulus stimuli. Stimulus size and background intensity were varied while stimulus/background contrast was kept constant.The speed of dark adaptation varied considerably from cell to cell. Dark adaptation made responses more sustained in all neurones and eliminated the oscillatory on-responses evoked under some conditions in the light-adapted cells. Dark adaptation led also to a disappearance of early phasic inhibition in on-responses, and increased response rise time and latency.The power of surround responses to inhibit centre responses decreased slightly at low levels of light adaptation in LGN cells but much less than in retinal ganglion cells. Some other traces of changing retinal surround effects also appeared in the LGN on dark adaptation. For example, the functional size of receptive fields increased at low levels of illuminance as has been observed in retinal ganglion cells and the receptive fields as estimated from response peaks were larger than those estimated from sustained components.Trainee of the European Training Programme in Brain and Behaviour Research, 1975.     

Orientation sensitive elements in the corticofugal influence on centre-surround interactions in the dorsal lateral geniculate nucleus

In a previous study, we have shown that the corticofugal projection to the dLGN enhances inhibitory mechanisms underlying length tuning. This suggests that the inhibitory influences deriving from the corticofugal feedback should exhibit characteristics that reflect the response properties of orientation-tuned layer VI cells. Here we report data obtained from experiments using a bipartite visual stimulus, with an inner section over the dLGN cell receptive field centre and an outer section extending beyond it. For both X and Y cells there was a modulation of the strength of the surround antagonism of centre responses that was dependent on the orientation alignment of contours in the two components of the stimulus. Layer VI cells showed maximal responses when the two components were aligned to the same orientation; dLGN cells showed a minimal response. Varying the orientation alignment of the inner and outer components of the stimulus in a randomised, interleaved fashion showed that bringing the stimulus into alignment resulted in a 24.28% increase in the surround antagonism of the centre response. Blocking cortical activity showed this effect of alignment to be strongly dependent on corticofugal feedback. This effect of orientation alignment appears to apply for any absolute orientation of the alignment condition and supports the view that an entire subset of cortical orientation columns generate the feedback influencing any given dLGN cell. This mechanism makes dLGN cells sensitive to the orientation domain discontinuities in elongated contours moving across their receptive field.     

Linear and nonlinear spatial subunits in Y cat retinal ganglion cells.

1. The mechanism which makes Y cells different from X cells was investigated. 2. Spatial frequency contrast sensitivity functions for the fundamental and second harmonic responses of Y cells to alternating phase gratings were determined. 3. The fundamental spatial frequency response was predicted by the Fourier transform of the sensitivity profile of the Y cell. The high spatial frequency cut-off of a Y cell's fundamental response was in this way related to the centre of the cell's receptive field. 4. The second harmonic response of a Y cell did not cut off at such a low spatial frequency as the fundamental response. This result indicated that the source of the second harmonic was a spatial subunit of the receptive field smaller in spatial extent than the centre. 5. Contrast sensitivity vs. spatial phase for a Y cell was measured under three conditions: a full grating, a grating seen through a centrally located window, a grating partially obscured by a visual shutter. The 2nd/1st harmonic sensitivity ratio went down with the window and up with the shutter. These results implied that the centre of Y cells was linear and also that the nonlinear subunits extended into the receptive field surround. 6. Spatial localization of the nonlinear subunits was determined by means of a spatial dipole stimulus. The nonlinear subunits overlapped the centre and surround of the receptive field and extended beyond both. 7. The nature of the Y cell nonlinearity was found to be rectification, as determined from measurements of the second harmonic response as a function of contrast. 8. Spatial models for the Y cell receptive field are proposed.     

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

Summary 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.     

A simultaneous contrast effect of steady remote surrounds on responses of cells in macaque lateral geniculate nucleus

Steadily illuminated surrounds, remote from the receptive field centre, are shown to affect the responses of primate visual cells. Intensity-response curves of cells of the macaque lateral geniculate nucleus were measured using a successive contrast paradigm where chromatic or achromatic stimuli were presented in alternation with a white adaptation field of constant luminance. Adding white surround annuli around stimuli and adaptation field shifted the intensity-response curves to higher intensity ranges. Since response curves can be nonmonotonic, this remote surround effect can result in an increase or decrease in responsiveness (facilitation or suppression) dependent on stimulus intensity. Steady surrounds, remote from the receptive field centre, thus control cell sensitivity and responses by means of simultaneous contrast.     

Contrasts in spatial organization of receptive fields at geniculate and retinal levels: centre surround and outer surround

1. The organization of receptive fields of retinal ganglion cells and A-laminae cells from the dorsal lateral geniculate nucleus (LGN) of the cat are compared under identical conditions. Some aspects of the geniculate data have been given elsewhere (Hammond, 1972b).2. The receptive fields of geniculate cells consist of three zones - centre, antagonistic surround and synergistic outer surround - compared with only two for retinal cells. This result further supports the theory that the centre and surround of geniculate cell receptive fields derive from convergent, but discrete, retinal inputs.3. The surrounds of geniculate receptive fields are known to be more powerfully antagonistic on their centres than is true of retinal cells. This relationship is re-examined.4. Unlike geniculate fields, the locus of maximum sensitivity for the receptive field surround of retinal cells is not invariant either to stimulus geometry or adaptational state.5. The latter result strongly suggests that the surround mechanism for retinal cells extends through the centre of the field. It establishes unequivocally that the overlap between receptive field centre and surround mechanisms, only marginal in geniculate, is very extensive indeed in retina.     

Effets des stimulations du noyau caudé sur l'activité des terminaisons fusoriales primaires et secondaires du muscle soléaire

Receptive field organization of 135 sustained and 45 transient retinal ganglion cells was investigated in lightly pentobarbitone-anaesthetised cats. Stimuli were concentric annuli presented alone or simultaneously with a small spot centred on the receptive field, against photopic, mesopic or scotopic backgrounds. The addition of the test spot led to reduction in diameter of the centre-surround boundary of receptive fields of sustained retinal ganglion cells (assessed with annuli), and a decrease in diameter of the annulus which was most effective on the surround. In transient cells there was only marginal reduction in diameter of the centre-surround boundary, measured with annuli, and little or no decrease in diameter of the most effective annulus. Reducing background intensity from photopic to scotopic induced changes in response patterns and receptive field organization of sustained and transient retinal ganglion cells which were independent of stimulus intensity. Against photopic backgrounds, large annuli evoked surround-type responses from the majority of transient ganglion cells and from all sustained cells. In the scotopic range, surround-type responses could still be evoked from sustained cells, whereas predominantly centre-type responses were obtained throughout the receptive fields of transient cells. With transition from cone to rod vision, receptive field surrounds of sustained and transient cells became progressively less responsive than centres; in consequence the diameter of the centre-surround boundary increased. The initial, high frequency burst of impulses in discharges at annulus onset or offset became less evident and response latency increased substantially. The results are consistent with a model in which the centre and surround receptive field mechanisms are spatially co-extensive in transient retinal ganglion cells, albeit of different shape, but only partially overlapping in sustained retinal ganglion cells. It is suggested that the surround mechanism in sustained cells is spatially more extensive than the centre mechanism but does not extend entirely through the centre of the field.     

Quantitative aspects of gain and latency in the cat retina

1. The gain of the central response mechanism and the latency of the pure central response of on-centre ganglion cells were studied by recording from single optic tract fibres the responses evoked by slow square-wave stimuli applied against some steady background.2. The concept of effective flux was introduced and defined: if any portion of a stimulus extends beyond Ricco's area of complete summation, then that stimulus has an actual flux, equal to the product of its area and luminance, but it also has an effective flux which is that fraction of its actual flux which equals the actual flux of another stimulus which, when it falls entirely within Ricco's area, evokes an isobolic pure central response or has the same adaptive effect upon the central response mechanism as the first stimulus.3. The most significant finding was that when the cell responded with a pure central response to the incremental flux (the square wave) applied against a steady effective background flux, then the gain and the latency were functions exclusively of the sum of the two fluxes (the total flux), not of the incremental or background flux as such. This shows that the level of field adaptation of the central mechanism is reset within the latent period of the response to an incremental flux.4. Increment sensitivity curves based on isobolic suprathreshold responses all had the same slope of 0.9, when the log of the incremental flux was plotted against the log of the total flux. A plot of log latency against log total effective flux had a slope of -0.1.5. The stimulus-response relation derived from (3) and (4) was [Formula: see text] and [Formula: see text], where R is the response amplitude, F(et) the total flux, DeltaF(e) the incremental flux and K(1) and K(2) are constants.     

Reciprocal lateral inhibition of on- and off-center neurones in the lateral geniculate body of the cat

Summary In the lateral geniculate body (LGB), intra- and quasi-intracellular records were done. With small light stimuli shone into different parts of the receptive field, EPSPs and IPSPs could be elicited. Stimulation of the exact center of an on-center cell produced a pure excitatory response, that of an off-center neurone pure inhibition. This response lasted throughout the stimulus. At light off, inhibition was elicited in on-center cells and excitation in off-center cells. A stimulus in the field periphery produced a mixed response with a small and short excitation followed by large inhibition in on-center cells, and a short inhibition followed by postsynaptic depolarization in off-center cells. At light off, on-center cells showed depolarization after a short polarizing phase, and off-center cells a broad polarization which interrupted the initial small excitation. The latencies of both the excitatory and inhibitory center responses at light on and off characteristic for the two types of neurones, were 20–30 msec shorter than the reversed responses elicited by stimulation of the receptive field surround.The findings are compatible with a model in which each geniculate on-center cell gets its major excitatory input from one optic tract on-center fibre and inhibitory input from several off-center fibres with nearby receptive fields. An off-center LGB-cell receives its main excitation essentially from one offcenter fibre and inhibition from several on-center cells. The responses to moving stimuli also agreed with this model. The presence of recurrent inhibition within the LGB could be confirmed by electrical stimulation. But it could not be decided whether the reciprocal inhibition of on- and off-center cells was due to forward or backward inhibition. The spontaneous activity of on- and off-center cells which were simultaneously recorded with one electrode, showed a mutual inhibition 6–8 msec after one cell had fired. Anatomical data relevant to the model are discussed and some functional implications are suggested.