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

Interocular ND filter suppression: Eccentricity and luminance polarity effects

The depth and extent of interocular suppression were measured in binocularly normal observers who unilaterally adapted to neutral density (ND) filters (0, 1.5, 2, and 3 ND). Suppression was measured by dichoptically matching sectors of a ring presented to the adapted eye to a fixed contrast contiguous ring presented to the non-adapted eye. Other rings of alternating polarity were viewed binocularly. Rings were defined by luminance (L), luminance with added dynamic binary luminance noise (LM), and contrast modulating the same noise (CM). Interocular suppression depth increased with increasing ND, nearing significance (p = 0.058) for 1.5 ND. For L and LM stimuli, suppression depth across eccentricity (±12° visual field) differed for luminance increment (white) versus luminance decrement (black) stimuli, potentially confounding eccentricity results. Suppression for increment-only (white) luminance stimuli was steeper centrally and extended across the visual field, but was deeper for L than for LM stimuli. Suppression for decrement-only (black) luminance stimuli revealed only central suppression. Suppression was deeper with CM than LM stimuli, suggesting that CM stimuli are extracted in areas receiving predominantly binocular input which may be more sensitive to binocular disruption. Increment (white) luminance stimuli demonstrate deeper interocular suppression in the periphery than decrement (black) stimuli, so they are more sensitive to changes in peripheral suppression. Asymmetry of suppression in the periphery for opposite polarity luminance stimuli may be due to interocular receptive field size mismatch as a result of dark adaptation separately affecting ON and OFF pathways. Clinically, measurement of suppression with CM stimuli may provide the best information about post-combination binocularity.

Contour interaction under photopic and scotopic conditions

In the present study, we asked whether contour interaction undergoes significant changes for different luminance levels in the central and peripheral visual field. This study included nine normal observers at two laboratories (five at Palacky University Olomouc, Czech Republic and four at the University of Houston, USA). Observers viewed a randomly selected Sloan letter surrounded by four equally spaced bars for several separations measured edge-to-edge in min arc. Stimuli were viewed foveally under photopic and mesopic luminances and between 5° and 12° peripherally for four different background luminances of the display monitors, corresponding to photopic, mesopic, scotopic, and dim scotopic levels. The extent of the contour interaction in the fovea is approximately 20 times smaller than in the periphery. Whereas the magnitude of foveal contour interaction markedly decreases with decreasing luminance, no consistent luminance-induced change occurs in peripheral contour interaction. The extent of contour interaction does not scale with the size of the target letter, either in the fovea or peripherally. The results support a neural origin of contour interaction consistent with the properties of center-surround antagonism.

The divisive normalization model of V1 neurons: a comprehensive comparison of physiological data and model predictions

The physiological responses of simple and complex cells in the primary visual cortex (V1) have been studied extensively and modeled at different levels. At the functional level, the divisive normalization model (DNM; Heeger DJ. Vis Neurosci 9: 181-197, 1992) has accounted for a wide range of single-cell recordings in terms of a combination of linear filtering, nonlinear rectification, and divisive normalization. We propose standardizing the formulation of the DNM and implementing it in software that takes static grayscale images as inputs and produces firing rate responses as outputs. We also review a comprehensive suite of 30 empirical phenomena and report a series of simulation experiments that qualitatively replicate dozens of key experiments with a standard parameter set consistent with physiological measurements. This systematic approach identifies novel falsifiable predictions of the DNM. We show how the model simultaneously satisfies the conflicting desiderata of flexibility and falsifiability. Our key idea is that, while adjustable parameters are needed to accommodate the diversity across neurons, they must be fixed for a given individual neuron. This requirement introduces falsifiable constraints when this single neuron is probed with multiple stimuli. We also present mathematical analyses and simulation experiments that explicate some of these constraints.

Photosensitivity in mild traumatic brain injury (mTBI): a retrospective analysis

PRIMARY OBJECTIVE

To determine whether photosensitivity (PS) changes over time and, if so, what factors may be related to the change; furthermore, to determine whether tint density changes over time, all in mild traumatic brain injury (mTBI).

DESIGN AND METHODS

A retrospective analysis of 62 patient records (aged 18-40 years) with mTBI and PS was conducted. All charts were obtained from the SUNY/College of Optometry clinics from 2004-2011.

RESULTS

Fifty per cent demonstrated reduced PS over time, with most occurring after year 1 post-injury (40%). Promotion of PS reduction appears to be associated with the lack of spectacle tint usage (p = 0.01) and the use of contact lenses (p = 0.03). Inhibition of PS reduction appears to be associated with tinted lenses (p = 0.06), hyperacusis (p = 0.03), dry eye (p = 0.04), migraines (p = 0.03) and loss of consciousness at the time of injury (p = 0.05). Concerning tint density changes over time, 71% (p = 0.002) maintained the same degree over time, while 27% (p = 0.002) reduced and 2% waxed and waned.

CONCLUSION

Neural adaptation to PS appears to be a long-term process. Tint usage may act to inhibit this adaptive process, while the use of contact lenses may act to promote it. These findings may provide guidance in the clinical management of photosensitivity in the mTBI population.

Ambient illumination toggles a neuronal circuit switch in the retina and visual perception at cone threshold

VIDEO ABSTRACT

Gradual changes in the sensory environment can lead to abrupt changes in brain computations and perception. However, mechanistic understanding of the mediating microcircuits is missing. By sliding through light levels from starlight to daylight, we identify retinal ganglion cell types in the mouse that abruptly and reversibly switch the weighting of center and surround interactions in their receptive field around cone threshold. Two-photon-targeted recordings and genetic and viral tracing experiments revealed that the circuit element responsible for the switch is a large inhibitory neuron that provides direct inhibition to ganglion cells. Our experiments suggest that weak excitatory input via electrical synapses together with the spiking threshold in inhibitory cells act as a switch. We also reveal a switch-like component in the spatial integration properties of human vision at cone threshold. This work demonstrates that circuits in the retina can quickly and reversibly switch between two distinct states, implementing distinct perceptual regimes at different light levels.

Mesopic rod and S-cone interactions revealed by modulation thresholds

We analyzed mesopic rod and S-cone interactions in terms of their contributions to the blue-yellow opponent pathway. Stimuli were generated using a four-primary colorimeter. Mixed rod and S-cone modulation thresholds (constant L-, M-cone excitation) were measured as a function of their phase difference. Modulation amplitude was equated using threshold units and contrast ratios. This study identified three interaction types: (1) a linear and antagonistic rod:S-cone interaction, (2) probability summation, and (3) a previously unidentified mutual nonlinear reinforcement. Linear rod:S-cone interactions occur within the blue-yellow opponent pathway. Probability summation involves signaling by different postreceptoral pathways. The origin of the nonlinear reinforcement is possibly at the photoreceptors.

Receptive field properties of neurons in the primary visual cortex under photopic and scotopic lighting conditions

Knowledge of the physiology of the primate visual cortex (area V-1) comes mostly from studies done in photopic conditions, in which retinal cones are active and rods play little or no part. Conflicting results have come from research into the effects of dark adaptation on receptive field organization of cells in the retina and the lateral geniculate nucleus. These studies claim either that the effect of the surround disappears with dark adaptation or that it does not. The current study has as its objective a comparison of responses of V-1 cells in awake-alert macaque monkeys under conditions of light and dark adaptation. We reasoned that basic receptive field properties of V-1 cells such as orientation selectivity, direction selectivity, and end-stopping should be preserved in scotopic conditions if the receptive field organization of antecedent cells is maintained in dim light. Our results indicate that dark adaptation does not alter basic V-1 receptive field characteristics such as selectivity for orientation, direction, and bar length.

Motion-induced blindness does not affect the formation of negative afterimages

Aftereffects induced by invisible stimuli constitute a powerful tool to investigate what type of neural information processing can occur in the absence of visual awareness. This approach has been successfully used to demonstrate that awareness of oriented gratings or translating stimuli is not necessary to obtain a robust orientation-specific or motion-specific aftereffect. We exploit motion-induced blindness (MIB, Bonneh, Cooperman, & Sagi, 2001) to investigate the related question of the influence of visual awareness on the formation of negative afterimages. Our results show that MIB does not affect the persistence and intensity of afterimages. Thus, there is no significant contribution to the formation of afterimages beyond the sites mediating MIB.

Correlation-based development of ocularly matched orientation and ocular dominance maps: determination of required input activities

We extend previous models for separate development of ocular dominance and orientation selectivity in cortical layer 4 by exploring conditions permitting combined organization of both properties. These conditions are expressed in terms of functions describing the degree of correlation in the firing of two inputs from the lateral geniculate nucleus (LGN), as a function of their retinotopic separation and their "type" (ON center or OFF center and left eye or right eye). The development of ocular dominance requires that the correlations of an input with other inputs of the same eye be stronger than or equal to its correlations with inputs of the opposite eye and strictly stronger at small retinotopic separations. This must be true after summing correlations with inputs of both center types. The development of orientation-selective simple cells requires that (1) an input's correlations with other inputs of the same center type be stronger than its correlations with inputs of the opposite center type at small retinotopic separation; and (2) this relationship reverse at larger retinotopic separations within an arbor radius (the radius over which LGN cells can project to a common cortical point). This must be true after summing correlations with inputs serving both eyes. For orientations to become matched in the two eyes, correlated activity within the receptive fields must be maximized by specific between-eye alignments of ON and OFF subregions. Thus the correlations between the eyes must differ depending on center type, and this difference must vary with retinotopic separation within an arbor radius. These principles are satisfied by a wide class of correlation functions. Combined development of ocularly matched orientation maps and ocular dominance maps can be achieved either simultaneously or sequentially. In the latter case, the model can produce a correlation between the locations of orientation map singularities and local ocular dominance peaks similar to that observed physiologically. The model's main prediction is that the above correlations should exist among inputs to cortical layer 4 simple cells before vision. In addition, mature simple cells are predicted to have certain relationships between the locations of the ON and OFF subregions of the left and right eyes' receptive fields.

Spatial and temporal properties of cat horizontal cells after prolonged dark adaptation

We studied the change of spatial and temporal response properties for cat horizontal (H-) cells during prolonged dark adaptation. H-cell responses were recorded intracellularly in the optically intact, in vivo eye. Spatial and temporal properties were first measured for light-adapted H-cells, followed by a period of dark adaptation, after which the same measurements were repeated. During dark adaptation threshold sensitivity was measured at regular intervals. Stable, long lasting recordings allowed us to measure changes of sensitivity and receptive field characteristics for adaptation periods up to 45 min. Although cat H-cells showed no signs of dark suppression or light sensitization, they remained insensitive in the scotopic range, even after prolonged dark adaptation. Absolute thresholds were in the low mesopic range. The sensitization was brought about by a shift from cone to rod input, and by substantial increases of both spatial and temporal integration upon dark adaptation. The length constant in the light-adapted state was on average about 4 deg. After dark adaptation it was up to a factor of three larger, with a median ratio of 1.85. Response delays, latencies and durations for (equal amplitude) threshold flash responses substantially increased during dark adaptation.

The AII amacrine network: coupling can increase correlated activity

Retinal ganglion cells in the cat respond to single rhodopsin isomerizations with one to three spikes. This quantal signal is transmitted in the retina by the rod bipolar pathway: rod-->rod bipolar-->AII-->cone bipolar-->ganglion cell. The two-dimensional circuit underlying this pathway includes extensive convergence from rods to an AII amacrine cell, divergence from a rod to several AII and ganglion cells, and coupling between the AII amacrine cells. In this study we explored the function of coupling by reconstructing several AII amacrine cells and the gap junctions between them from electron micrographs; and simulating the AII network with and without coupling. The simulation showed that coupling in the AII network can: (1) improve the signal/noise ratio in the AII network; (2) improve the signal/noise ratio for a single rhodopsin isomerization striking in the periphery of the ganglion cell receptive field center, and therefore in most ganglion cells responding to a single isomerization; (3) expand the AII and ganglion cells' receptive field center; and (4) expand the "correlation field". All of these effects have one major outcome: an increase in correlation between ganglion cell activity. Well correlated activity between the ganglion cells could improve the brain's ability to discriminate few absorbed external photons from the high background of spontaneous thermal isomerizations. Based on the possible benefits of coupling in the AII network, we suggest that coupling occurs at low scotopic luminances.

Effect of ambient illumination on the spatial properties of the center and surround of Y-cell receptive fields

The primary goal of this study was to expand the description of the filtering properties of the Y-cell receptive field, by quantitatively characterizing the spatial filtering properties of the receptive field's center-and-surround components as a function of adapting light level. A range of more than five orders of magnitude in retinal illuminance were covered, including the vast majority of the cat's functional range of vision. Recordings were taken from optic tract fibers of Y cells in cats under general anesthesia. Sinusoidal gratings and a stimulus designed to selectively probe the properties of the surround mechanism were used. The cells' responses to these stimuli were fit to a Gaussian center-surround receptive-field model, in which six parameters define the properties of the center and surround. Fits were made independently to data collected at each light level and changes in the values of the model's parameters with illuminance are reported. A set of equations that summarize the changes in parameter values is given. From these summary equations, reasonable estimates of the parameters' values can be determined across a wide range of illuminances. Hence, a quantitative model of the spatial properties of the center and surround of the Y-cell receptive field can now be derived from these equations for most of the levels of retinal illuminance experienced by a Y cell. The consistency between the description provided by our equations and results from earlier work is considered.

Impairment of visual perception and visual short term memory scanning by transcranial magnetic stimulation of occipital cortex

Transcranial magnetic stimulation (TMS) of occipital cortex was performed using a magneto-electric stimulator with a maximum output of 2 Tesla in 24 normal volunteers. The identification of trigrams, presented for 14 ms in horizontal or vertical arrays was significantly impaired when the visual stimulus preceded the occipital magnetic shock by 40 to 120 ms. The extent of impairment was related to TMS intensity. The latency of perceptual impairment was shorter for more intense TMS. No perceptual impairment was obtained by "sham" stimulation when TMS shocks were applied to the upper cervical region rather than the occipital region to rule out unspecific startle reactions affecting attention possibly responsible for the observed reduction in performance. Occipital TMS did not evoke systematic eye movements except for blink responses at latencies beyond 40 ms which were too late to interfere with visual input. Depending on the required serial order of readout of the trigram perceptual impairment was more marked for the second and third part of the trigram. This demonstrates that TMS interferes with the internal serial processing of visual input. To elucidate this further, TMS was used in a Sternberg short term visual memory scanning task. TMS caused a marked decrease in memory scanning rates whereas visual stimulus encoding and storage remained unaffected when tested at various TMS delays. TMS appears to be a useful method to study processes of visual perception and short term memory handling in the occipital cortex. Advantages over classical visual masking techniques especially regarding topical localisation are discussed.

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

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

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.

Mesopic spectral responses and the Purkinje shift of macaque lateral geniculate nucleus cells

Spectral responsiveness of different classes of macaque LGN cells at eccentricities smaller than 12 degrees was studied at low light intensities. Cone thresholds of cells varied from 1 to 10 td. Rod inputs were found in all classes of cells, including inhibitory inputs in some cells. Rod inputs were not apparent above 10-40 td, giving a total mesopic range of about 1-40 td. Strong rod-mediated responses could be evoked in broadband phasic cells and in spectrally opponent cells excited by short wavelengths. Only weak if any excitatory responses could be evoked by short wavelengths at scotopic levels in spectrally opponent long-wavelength excited cells. Hence, rod inputs do not confound the spectral responsiveness of cells because no spectrally opponent cell excited by long-wavelength stimuli at photopic levels became significantly responsive to short wavelength stimuli at mesopic or scotopic intensities. The so-called "rod color" may be blue. An increase in the dominance of wide-band cell responses that may explain the Bezold-Brücke hue shift was observed at higher stimulus radiances at wavelengths near 450 and 650 nm.

Synaptic relationships of a type of GABA-immunoreactive neuron (clutch cell), spiny stellate cells and lateral geniculate nucleus afferents in layer IVC of the monkey striate cortex

The precise stimulus specificity of striate cortical neurons is strongly influenced by processes involving gamma-aminobutyric acid (GABA). In the visual cortex of the monkey most afferents from the lateral geniculate nucleus terminate in layer IVC. We identified a type of smooth dendritic neuron (clutch cell) that was immunoreactive for GABA, and whose Golgi-impregnated dendrites and axon were largely restricted to layer IVC beta. The slightly ovoid somata were 8-12 micron by 12-15 micron and the dendritic field was often elongated, extending 80-200 micron in one dimension. The axonal field was 100-150 micron in diameter and densely packed with large bulbous boutons. Although mainly located in IVC beta both the dendritic and axonal processes entered IVC alpha. Fine structural features of GABA-immunoreactive and-impregnated clutch cells and impregnated spiny stellate cells were compared. Clutch cells had more cytoplasm, more densely packed mitochondria and endoplasmic reticulum, and made type II as opposed to type I synapses. A random sample of 159 elements postsynaptic to three clutch cells showed that they mainly terminated on dendritic shafts (43.8-58.5%) and spines (20.8-46.3%), rather than somata (10-17%). The majority of the postsynaptic targets were characteristic of spiny stellate cells. This was directly demonstrated by studying synaptic contacts between an identified GABA positive clutch cell and the dendrites and soma of an identified spiny stellate cell. The termination of clutch cells mainly on dendrites and spines of spiny stellate cells suggests that they interact with other inputs to the same cells. Following an electrolytic lesion in the ipsilateral lateral geniculate nucleus we examined the distribution of degenerating terminals on three identified spiny stellate neurons in layer IVC beta. Out of eight synapses from the lateral geniculate nucleus one was on a dendritic shaft, the rest on spines. Only a small fraction of all synapses on the cells were from degenerating boutons. A clutch cell within the area of dense terminal degeneration was not contacted by terminals from the lateral geniculate nucleus. The results show that layer IVC in the monkey has a specialized GABAergic neuron that terminates on spiny stellate cells monosynaptically innervated from the lateral geniculate nucleus. Possible functions of clutch cells may include inhibitory gating of geniculate input to cortex; maintenance of the antagonistic subregions in the receptive fields; and the creation from single opponent of double colour opponent receptive fields.

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.

The role of gamma-aminobutyric acid mediated inhibition in the response properties of cat lateral geniculate nucleus neurones

We studied the effect of local ionophoretic application of bicuculline on the response of cat lateral geniculate nucleus (laminae A) cells to stimulation by sinusoidal gratings and spots of light. Application of bicuculline produced an increase both of spontaneous and visually driven discharge of both X and Y cells. On stimulation by drifting sinusoidal gratings, the average discharge of both X and Y cells remained constant with increasing contrast under normal conditions. Application of bicuculline caused the average discharge to increase with contrast, indicating that the constancy of the average discharge was maintained by gamma-aminobutyric acid mediated inhibition. Under normal conditions, the amplitude of response modulation of both X and Y cells to sinusoidal grating stimulation increased monotonically with stimulus contrast. During bicuculline application, the slope of the contrast-response curve for X cells but not for Y cells increased, indicating that the inhibition which dampened the modulation of X cells (but not Y cells) was contrast dependent. Application of acetylcholine also increased the average discharge and the amplitude of modulation of the cell responses, but this increase did not depend on stimulus contrast. Under normal conditions, X but not Y cells showed an attenuation of response and an increase in contrast threshold to low spatial frequencies. This attenuation vanished during bicuculline application. The shape of Y-cell response curves was unaffected by bicuculline. Bicuculline had the same effect on the non-linear component of Y-cell response as on the linear component. Although bicuculline had a different effect on the response of X and Y cells to stimulation by gratings, it reduced the antagonistic surround of both X and Y cells to a similar extent (revealed by plotting the cell receptive fields with flashed spots of light).

Spatio-temporal interaction in neurones of the cat's dorsal lateral geniculate nucleus

Temporal contrast sensitivities in the range 0.33-42 Hz for optimum spatial frequency grating stimuli were measured for large populations of XG and YG neurones. For fewer cells, complete spatial contrast sensitivity profiles were measured at a series of temporal frequencies and, in some cases, at a range of levels of retinal illumination too. Contrast sensitivities were measured from responses of cells reliably different from their maintained discharges. The cells' discharges were recorded extracellularly from the binocular segment of the A laminae of the cat's dorsal lateral geniculate nucleus. At their respective optimum spatial frequencies, YG cell were more sensitive on average than XG cells for most temporal frequencies, though the average temporal contrast sensitivity profiles of both cell classes had similar shapes. The optimum temporal frequency for both cell types was around 5 Hz. XG and YG cells seem to be relatively less sensitive to low temporal frequencies than their ganglion cell counterparts. At a retinal illumination of 230 cd/m2 (pupil, 3 mm2), increasing temporal frequency in the range 0.65-21 Hz produced a relative improvement in low spatial frequency contrast sensitivity in most XG and all YG cells studied. There were some XG cells, though, which showed little or no effect of temporal frequency on their spatial contrast sensitivity curves. At all temporal frequencies, the shapes of spatial contrast sensitivity curves and the cells' temporal contrast sensitivity profiles were not markedly dependent on the criterion level set to measure 'threshold' contrast. Reducing the level of retinal illumination in the range 230-0.007 cd/m2 (pupil, 3 mm2) produced a fall in contrast sensitivities for both XG and YG cells. The loss in sensitivity was more marked at high spatial and high temporal frequencies. The similar shapes of the temporal contrast sensitivity curves of XG and YG cells weakens the suggestion that the human counterparts of these cells would provide a suitable physiological substrate for the psychophysical sustained and transient channels. Although the behaviour of XG and YG cells parallels quite closely changes in cat and human psychophysical spatial contrast sensitivities with temporal frequency and retinal illumination, many problems remain for equating results from the two fields.

The influence of temporal frequency and adaptation level on receptive field organization of retinal ganglion cells in cat

1. The discharges of single X and Y ganglion cells (distinguished by a test of linearity of spatial summation) were recorded in the optic tract of anaesthetized, paralysed cats.2. Fourier techniques were used to analyse the distribution of amplitudes of several component temporal frequencies in the maintained discharge. X and Y cells were distinguished by their mean rates, but not by the amplitude or variability of other component frequencies.3. Sensitivities to moving sinusoidal gratings were measured by an automatic procedure in which stimulus contrast was adjusted to give the smallest modulation of discharge that reliably exceeded that of the relevant component frequency in the maintained discharge.4. Spatial contrast sensitivity curves of X cells and of on-centre Y cells could be described by a model of the receptive field as two concentric Gaussian sensitivity profiles representing the centre and the antagonistic surround.5. Changes in temporal frequency altered the shapes of the spatial contrast sensitivity curves of most units. For X cells sensitivity at the optimum spatial frequency was greater at a temporal frequency of 10.4 Hz than at lower or higher temporal frequencies. The relative sensitivity to low spatial frequencies improved as temporal frequency was raised from 0.16 to 20.8 Hz. The shapes of the contrast sensitivity functions of Y cells were less affected by changes in temporal frequency: at all spatial frequencies sensitivity was greater at 2.6 Hz than at lower or higher frequencies.6. The effect of temporal frequency upon the shape of the spatial contrast sensitivity curve could be explained by assuming that the centre and surround changed their sensitivities without changing their characteristic radii. A simple model, using a temporal R-C filter in the surround pathway, predicted qualitatively similar changes in the shape of contrast sensitivity curves but failed to provide acceptable fits to the observations. A second model, which assumed that surround signals are delayed by a fixed amount before being combined with those from the centre, fitted the observations of most, but not all, X cells.7. Dark adaptation produced changes in the shape of the spatial contrast sensitivity curve consistent with a reduction in the relative sensitivity of the surround, but did not bring about systematic changes in the space constants of the best-fitting theoretical curves.8. The effects of adaptation level upon contrast sensitivity were expressed as plots of increment-threshold against mean illumination. The shallowest of these curves, obtained for the optimum spatial stimulus moving at about 10 Hz, had slopes averaging 0.77. Decreases in spatial or temporal frequency increased the slopes of the curves.

Stimulus-response function at several levels of background luminance, in the cat visual areas 17 and 18

Stimulus-response curves of simple cells of the visual cortex were obtained by using 500-msec stationary stimuli. Background influence on single unit responses was studied. The contrast sensitivity of simple cells increases as a function of background luminance. The resolution power of these cortical cells for detecting differences in stimulus contrast decreases at background levels above 0.09 cd/m2.

Lateral geniculate cell responses to electrical stimulation of the retina

Electrical stimulation of the retina evokes at the optic tract level rhythmic bursts of activity whose temporal structure is predictable from the polarity of the stimulation and the receptive field type. The reaction of lateral geniculate units to this input was studied in fast and slow relay cells as well as in interneurons. The results revealed that fast relaty cells presented a response whose temporal structure remained essentially unmodified in comparison to that observed at the optic tract level: that both anodal and cathodal polarities produced rythmic pattern of excitation the latency of which depended upon receptive field type and polarity applied. In slow relay cells and interneurons responses with equal latencies could be evoked for both polarities. Following cortical depression with 3 M KCl the latency of first bursts was unaffected in relay cells, while about one third of interneurons showed a temporal pattern which was similar to that recorded at the optic tract level after the treatment. This suggests that both ON and OFF retinal networks converge upon one geniculate slow P cell and interneuron, whereas fast relay cells are mostly driven by one of the two systems. Furthermore this convergence may be achieved through visual cortex in some units.