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

The influence of anesthesia upon binocular processes controlling the recordability of X- and Y-cells in the lateral geniculate nucleus of the cat

The interaction between anesthesia and binocular physiology was explored using chronic monocular paralysis. Monocular paralysis allows analysis and classification of lateral geniculate nucleus (LGN) cells without systemic paralysis and anesthesia and also produces a tonic bias in binocular mechanisms which control the relative recordability of X- and Y-cells (i.e. the LGN X/Y ratio). This effect appears to be reversed by the induction of anesthesia. In this study we (1) assessed the effects of anesthesia induction and withdrawal upon the X/Y ratio in a large number of chronic monocularly paralyzed cats, and (2) evaluated the degree to which a change in excitability versus a change in functional identity in individual LGN cells may contribute to these anesthesia-induced shifts in the X/Y ratio. Although anesthesia induction invariably increased the X/Y ratio (which is typically quite low in chronic monocular paralysis), it never caused a reliable shift between X- and Y-categories in any cell. Congruent with its effects upon the X/Y ratio, however, anesthesia induction increased excitability in 73% of X-cells and decreased excitability in 55% of Y-cells. Control experiments indicated that these systematic effects of anesthesia are not characteristic of normal animals but are specific to those with chronic monocular paralysis. Thus, the induction of anesthesia does reverse the effects of chronic monocular paralysis upon the LGN X/Y ratio apparently by inducing reciprocal changes in X- and Y-excitability. Further, while we find no evidence that anesthesia produces a qualitative distortion in the monocular properties of LGN cells, the induction and withdrawal of anesthesia does appear to modulate the operation of binocular processes controlling the recordability of LGN X- and Y-cells.

Function of the Y optic nerve fibres in the cat: do they contribute to acuity and ability to discriminate fast motion?

1. A controlled pressure block has been applied to the optic nerve of the cat, sufficient to bring about degeneration of the axons of the large (Y) nerve fibres caudal to the block site. This degeneration has been monitored by means of implanted electrodes in optic nerve and tract which have shown a loss of the short-latency (t1) response 4-6 days after the block, and also by histological examination of the optic nerve. 2. Cats with one optic nerve blocked in this way have been used in behavioural experiments, one or other eye being covered during the tests. Tested via the blocked nerve, all cats with loss of only Y fibres could perform certain tests: the visual placing reaction, the blink reflex, the pupillary (light) reflex and simple manoeuvres such as walking a plank and jumping from table to floor. 3. When acuity was tested by means of the Mitchell jumping apparatus, cats with loss of only Y fibres showed the same acuity using either eye. This was true also of one cat in which many X fibres had also degenerated, as evidenced by a 55% loss of the medium-latency (t2) response, but in another cat with 90% loss of the t2 response acuity was reduced to about half-normal. 4. Ability to discriminate fast motion was tested by a modification of the Mitchell apparatus. All cats were able to discriminate the motion of an 11.5 deg spot up to a velocity of 6260 deg/s, whether using their normal eye or their affected eye. However, the loss of the Y fibres reduced the ability to discriminate fast motion, so that for any given level of contrast the velocity which could be discriminated was about two-thirds of the velocity discriminated using the normal eye. The ability of the cat to discriminate fast motion seems to be similar to that of the human. 5. These results suggest that there is no sharp restriction of function between the Y and X systems but instead considerable overlap. However, each system possesses specialized features giving it superiority in certain conditions.

Morphology of retinogeniculate X and Y axon arbors in monocularly enucleated cats

We examined the terminal arbors of single, physiologically identified retinogeniculate X and Y axons from the remaining retinas of adult cats raised from birth with monocular enucleation. These were compared with arbors of X and Y axons in normally reared cats. We used intra-axonal injections of horseradish peroxidase to label each axon after recording its response properties. While the axons in monocularly enucleated cats exhibited normal response properties, both X and Y axons in these cats had abnormally large terminal arbors. Each of the hypertrophied X arbors appeared to be completely confined to the single geniculate lamina A or A1 appropriate to its eye of origin (i.e., lamina A for the contralateral retina and lamina A1 for the ipsilateral retina). In contrast, in addition to their normal terminations, most of the Y arbors seemed to extend well into laminae normally innervated only by the retina that was removed. Thus most or all of the translaminar sprouting previously reported for monocularly enucleated cats appears to reflect extensions of Y axon arbors. These data, in addition to earlier, analogous data from young kittens and cats reared with monocular lid suture, suggest the following sequelae during postnatal development: the retinogeniculate X arbors mature first and develop exuberant arbors that are later competitively pruned as the Y axons expand their innervation of the lateral geniculate nucleus; monocular lid suture prevents the Y axons from succeeding in this competition, so they fail to establish normal arbors and cannot reduce the exuberant X arbors; monocular enucleation offers a less resistant path in the denervated laminae for the rapidly growing Y arbors from the remaining eye, and the expansion of these arbors there reduces the competitive pressure on the exuberant X arbors. Thus, in monocularly enucleated cats, sprouting is limited to Y axons, either because only they possess the capacity to sprout or because they are in the midst of a period of relatively rapid growth at the time of the neonatal enucleation. The X axon arbors are also abnormally large within their appropriate laminae. This occurs presumably because they are able to maintain their immature exuberance, although we cannot rule out the possibility that they are pruned and later regrow to the final size seen in our experiments.

Responsiveness of the visual system in childhood migraine studied by means of VEPs

We have tried to ascertain whether the increased visual evoked potential (VEP) amplitude found in adult migraineurs is present also in children with migraine. We investigated 43 children, 26 male and 17 female, with a mean age of 11.4 years, 24 with common and 19 with classic migraine, and compared them with a control group of 20 children, 11 male and 9 female, with a mean age of 9.7 years. Flash and pattern reversal VEPs were recorded in both groups, and the study was carried out in the pain-free interval between attacks. The children with migraine showed a significant (p less than 0.01) increase in VEP amplitude on flash stimulation but not on pattern reversal. There were no differences between classic and common migraine. The abnormal responsiveness of the visual system seems to be related to variations in light intensity rather than to spatial contrasts.

The relationship between axonal and perisynaptic conduction times in the retinogeniculate pathway of cats

We examined the degree to which conduction velocity differences between X- and Y-cells are preserved during transmission through the retinogeniculate synaptic zone. This analysis focussed upon two measures: axon time; and perisynaptic time. The first is the time required for an action potential to travel from the optic chiasm to the point of major branching in the optic tract. The second is the sum of terminal invasion time, synaptic delay, and the time required for postsynaptic processes which precede the generation of an action potential. The X- and Y-cell distributions of axonal conduction times differed markedly. In contrast, the X- and Y-cell distributions of perisynaptic conduction times overlapped extensively. Therefore, the overlap in optic chiasm latency distributions between geniculate X- and Y-cells arises primarily from the perisynaptic zone. Further, on a unit-by-unit basis, we observed small but significant negative correlations between axon conduction times and perisynaptic times within both the X- and Y-cell classes. That is, units innervated by faster conducting afferents tended to have longer perisynaptic processing times, and vice versa. This sort of relationship may act to enhance within-class synchrony in postsynaptic activity.

The control of retinogeniculate transmission in the mammalian lateral geniculate nucleus

In the mammalian visual system, the lateral geniculate nucleus is commonly thought to act merely as a relay for the transmission of visual information from the retina to the visual cortex, a relay without significant elaboration in receptive field properties or signal strength. However, many morphological and electrophysiological observations are at odds with this view. Only 10-20% of the synapses found on geniculate relay neurons are retinal in origin. Roughly half of all synapses derive from cells in layer VI of visual cortex; roughly one third are inhibitory and GABAergic, derived either from interneurons or from cells of the nearby perigeniculate nucleus. Most of the remaining synapses probably derive from cholinergic, noradrenergic, and serotonergic sites within the brainstem reticular formation. Moreover, recent biophysical studies have revealed several ionic currents present in virtually all thalamic neurons. One is a Ca2+-dependent K+ current underlying the afterhyperpolarization (or the IAHP), which may last up to 100-200 ms following an action potential. Activation of the IAHP leads to spike frequency adaptation in response to a sustained, suprathreshold input. Intracellular recordings from other neuronal preparations have shown that the IAHP can be blocked by noradrenaline or acetylcholine, leading to an increased cellular excitability. Another ionic current results from a voltage- and time-dependent Ca2+ conductance that produces a low threshold spike. Activation of this conductance transforms a geniculate neuron from a state of faithful relay of information to one of bursting behavior that bears little relationship to the activity of its retinal afferents. We propose that state-dependent gating of geniculate relay cells, which may represent part of the neuronal substrate involved in certain forms of selective visual attention, can be effected through at least three different mechanisms: conventional GABAergic inhibition, which is largely controlled via brainstem and cortical afferents through interneurons and perigeniculate cells; the IAHP, which is controlled via noradrenergic and cholinergic afferents from the brainstem reticular formation; and the low threshold spike, which may be controlled by GABAergic inputs, cholinergic inputs, and/or the corticogeniculate input, although other possibilities also exist. Furthermore, it seems likely that gating functions involving the corticogeniculate pathway are suited to attentional processes within the visual domain (e.g., saccadic suppression), whereas brainstem inputs seem more likely to have more global effects that switch attention between sensory systems.(ABSTRACT TRUNCATED AT 400 WORDS)

Psychophysics of lateral tachistoscopic presentation

Human visual performance depends upon the retinal position to which a target is delivered. A general finding is that performance measured in a variety of psychophysical tasks deteriorates as a target is presented to more eccentric retinal regions. One purpose of this paper is to describe differences between foveal and peripheral vision in a number of psychophysical tasks. A second purpose is to review studies which have attempted to account for the fall off in visual performance between central and peripheral target presentations. A third purpose is to consider the contribution of the periphery to perception since targets which are sufficiently large project not only on receptors in the fovea but also on those in the periphery. In addition, stimuli presented to the peripheral retina can influence the processing of a target presented to the central retinal region. A fourth purpose is to review studies which have attempted to compensate for foveal and peripheral differences by scaling the target in size or some other attribute in proportion to the cortical magnification factor. A final purpose of this paper is to consider whether the fovea and the periphery are specialized for different functions.

Factors influencing the temporal phase of response to bar and grating stimuli for simple cells in the cat striate cortex

We have characterized the speed of response of simple cells in cat striate cortex by the temporal phase of the response to bar and grating stimuli. Stimulation of the most responsive subregion (either ON or OFF) in the receptive field with a 1 Hz temporally modulated bar elicited responses whose phase led the excitatory phase of the stimulus by about 25 degrees. The response to stationary gratings whose contrast was sinusoidally modulated at 2 Hz also showed a phase lead. The differences in the phase of response of ON and OFF sub-regions exhibited a marked scatter about the expected value of 180 degrees. The phase of response to both temporally modulated bars and laterally moving gratings advanced by 20-35 degrees as the stimulus contrast was raised by a factor of 5.

Gain control mechanisms in X- and Y-type retinal ganglion cells of the cat

A change in responsiveness caused by a spot of light (conditioning spot, CS; 3 sec in duration) presented within a central region of the receptive field of X- and Y-type retinal ganglion cells of the cat was investigated by measuring the magnitude of responses to another spot of light (test spot, TS; 50 msec in duration) which was juxtaposed with the CS within the same receptive field's central region. Responses to the TS were suppressed steadily during the on-phase of the CS as if it were divided by a certain value. This fact indicates that the gain of the center mechanism was changed by the CS presentation. The setting of the gain to a new level was rapid (within 100 msec after the onset or the cessation of the CS), and the magnitude of a gain change was not affected by the surround antagonism. These characteristics of the gain control were common to X- and Y-cells under both mesopic and scotopic levels of light adaptation.

Characteristics of rod driven off-responses in cat ganglion cells

The off-response of dark adapted cat ganglion cells shows a tripartite response-intensity function in the optic nerve response (ONR) as well as in extracellular recordings of single cells. While responses increase when stimuli of low or high intensities are increased, the rod driven off-response shows a strong decrease (dip) for intermediate intensities before the cone part of the function starts to rise. In contrast, on-responses increase monotonically or stay at a maximum. The dip in the response-intensity function of the off-response has a constant shape with test lights of increasing as well as of decreasing irradiance. The action spectrum of the descending part of the function peaks at 500 nm, indicating that a rod driven mechanism is responsible for the response reduction reflected by the dip. Changing the stimulus diameter from 24 deg to a 1 deg test field centred on a ganglion cell's receptive field has minimal effect on the response reduction. This points to a temporal rather than a spatial mechanism being responsible for the dip.

Spatial displacement sensitivity of X- and Y-cells in the dorsal lateral geniculate nucleus of the cat

The sensitivity of X- and Y-cells in the dorsal lateral geniculate nucleus of the cat to small, temporally modulated displacements of grating stimuli was measured at 0.175, 0.25, 0.50, 1.00, and 2.00 c/deg. For every cell, two threshold measures were determined: first, a contrast threshold with a counterphase grating and then a displacement threshold with a grating matched in spatial frequency, but whose contrast was 2.5 times the threshold value. The results showed that displacement thresholds of both X- and Y-cells decreased with increasing spatial frequency. At low spatial frequencies, mean displacement thresholds of X- and Y-cells were similar, but at intermediate spatial frequencies, Y-cell thresholds were lower than X. X-cell displacement thresholds were lower than Y only at the highest spatial frequency tested. Consistent with previous reports, contrast thresholds also varied with spatial frequency for both X- and Y-cells. The local luminance differences produced by the contrast threshold and displacement threshold stimuli for the two classes of cells were compared. Across all spatial frequencies, the change in position of the gratings at displacement threshold produced smaller luminance differences than the counterphase gratings at contrast threshold. This enhanced sensitivity of X- and Y-cells to a local luminance changes produced by grating displacement was related to the high spatial contrast of the grating and not to the displacement per se.

Visual response latency of X- and Y-cells in the dorsal lateral geniculate nucleus of the cat

Visual response latencies and rise times of X- and Y-cells in the dorsal lateral geniculate nucleus (dLGN) of anaesthetized, paralyzed cats were measured during repeated stimulation with sinusoidal grating patterns. Measurements were restricted to individual stimulus trials on which the instantaneous discharge rate exceeded a criterion amplitude defined in terms of the baseline activity of each cell. The latencies of response onsets and response peaks were systematically related to the spatial frequency and contrast of the grating stimuli. Response latencies of Y-cells were shortest for gratings of low spatial frequency (0.17 c/deg) and increased monotonically with increases in spatial frequency. Response latencies of X-cells were shortest for gratings of intermediate spatial frequency (0.75 c/deg) and longer for lower and higher spatial frequencies. Latencies decreased monotonically with increases in stimulus contrast from 5 to 40% for both X- and Y-cells. In general, short-latency responses were less variable than long-latency responses. This was true for absolute as well as relative measures of variability. The mean onset and peak latencies of Y-cell responses were 10-15 msec shorter than the corresponding latencies of X-cell responses to stimuli of optimal spatial frequency and contrast. The rise times (latency of response peak minus latency of response onset) of Y-cell responses were consistently shorter than those of X-cells in spite of the higher peak responses of Y-cells. The results of this study are consistent with the idea that low spatial frequency information is passed through the lateral geniculate nucleus more quickly than is high spatial frequency information. These data provide support for models of visual processing wherein a coarse, global analysis of the visual scene by Y-cells precedes a finer, local analysis by X-cells.

Morphological correlates of Y, X and W type ganglion cells in the cat's retina

After physiological recordings with microelectrodes containing horseradish peroxidase (HRP), morphological properties of Y, X and W type ganglion cells were studied on flat-mounted cat's retinas. While all the Y cells (N = 9) and the X cells (N = 8) revealed alpha and beta cell morphologies of Boycott and Wässle [J. Physiol. 240, 397-419 (1974)], respectively, W cells (N = 4) revealed various morphological types including their gamma and delta cells. The Y cells were larger than X cells in soma diameter, but the W cells were of the same range as X cells. Electron microscopic observations of the cross-sectioned nerve fiber bundles provided evidence for the segregation of axon diameters into the three groups corresponding to Y, X and W axons. It was discussed that functional trichotomy of retinal ganglion cells is related to the differentiation more in axon diameter rather than in soma size.

The action of inhibitory neurotransmitters, gamma-aminobutyric acid and glycine may distinguish between the area centralis and the peripheral retina in cats

The effects of iontophoretically applied gamma-aminobutyric acid (GABA) and glycine, and of their antagonists, bicuculline and strychnine, were compared between ganglion cells from the central and peripheral retinae of optically intact eyes in barbiturate-anaesthetised cats. The visual response of on-cells was inhibited by GABA and enhanced by bicuculline. The visual response of off-cells was inhibited by glycine and enhanced by strychnine. The sensitivity of cells to the transmitters was lower in the peripheral retina than in the area centralis, whilst the sensitivity to the antagonists was similar in both regions of the retina. Cells from the area centralis were inhibited by either GABA or glycine, but never both. Cells from the periphery were less selective and were inhibited by both transmitters.

Independent processing of visual form and motion

Rotating or stationary targets were presented simultaneously with a random-dot mask which was itself either rotating or stationary. Observers were required to judge both the form of the target and its motion; these target attributes were manipulated orthogonally in the experiment. The results support the hypothesis that under these conditions figural information is encoded independently from motion information. On a trial-by-trial basis within conditions of target and mask motion combinations, performance in the two tasks was independent. Across conditions, different effects were observed in the two dependent measures. Form judgments were generally more accurate for rotating than for stationary targets, particularly when the mask was stationary. Motion judgments showed frequent intrusions from the motion of the mask, so that most errors occurred when target and mask were dissimilar in terms of their motion. The results show differential interference of the random-dot mask on the encoding of the two independent stimulus attributes, and support the possibility of separate processing of figural and motion information.

The position sensitivity of retinal X- and Y-cells in cats

We have devised a measure of a retinal ganglion cell's sensitivity to changes in the spatial position of a grating stimulus. At maximum, this relative position sensitivity is a scaled product of the stimulus spatial frequency and the cell's fundamental component of response to that spatial frequency. We obtained the relative position sensitivity as a function of spatial frequency for 13 X-cells and 14 Y-cells. X-cell functions peak at significantly higher spatial frequencies than do those of Y-cells. At their peaks, X-cells display significantly higher values of relative position sensitivity than do Y-cells. However, Y-cells have higher position sensitivity at lower spatial frequencies, but exhibit less of a range of variation from maximum to minimum than do X-cells. These results are consistent with a hypothesis that Y-cells provide the crucial substrate for form vision at lower spatial frequencies, while X-cells are important for details carried by the higher spatial frequencies.

An electrophysiological assessment of X and Y cells as pattern and flicker detectors in the dorsal lateral geniculate nucleus of the cat

We tested the hypothesis that geniculate X cells are the neural substrate of psychophysically identified pattern channels and that geniculate Y cells are the neural substrate of psychophysically identified flicker channels. The hypothesis was tested by measuring the relative sensitivity of isolated X and Y cells in the dorsal lateral geniculate nucleus of the cat to counterphase and on-off grating presentations. The fundamental and second harmonic responses of X and Y cells to sinusoidal counterphase and on-off temporal modulation were measured at a number of spatial frequencies using two contrasts, 0.1 and 0.4. The fundamental responses of both X and Y cells to sinusoidal counterphase were greater relative to on-off responses. The second harmonic responses of Y cells to counterphase were larger at high spatial frequencies. Contrast sensitivity also was measured. At all spatial frequencies, both X and Y cells were slightly more sensitive to counterphase than to on-off presentations. Since flicker sensitivity in humans is twice as high for counterphase as for on-off presentations across all spatial frequencies, whereas pattern sensitivity is equal for the two presentations, we conclude that X and Y cells do not subserve uniquely pattern and flicker sensitivity, respectively. This conclusion is based on the result that differences between X and Y cells to counterphase and on-off presentations were inconsistent with the differences observed for pattern and flicker sensitivity. We suggest then that a spatial/temporal dichotomy does not seem to accurately characterize the functional roles of X and Y cells.

Morphology of physiologically identified X-, Y-, and W-type retinal ganglion cells of the cat

Retinal ganglion cells of the cat have been classified physiologically into X-, Y-, and W-cells on the basis of the receptive field properties, and morphologically into alpha-, beta-, and gamma-cells. In order to study directly the correspondence between these classifications, intracellular recordings from the ganglion cells in superfused eye-cup preparations were made with the aid of microelectrodes filled with Lucifer yellow CH. The cells were stained after their photic responses were studied under mesopic adaptation. X-cells, showing sustained depolarization (on-center cells) or hyperpolarization (off-center cells) in response to a spot of light had medium-sized round somata and spread bushy dendrites within a narrow retinal area. On the other hand, on-center and off-center Y-cells, showing transient responses to the spot stimulus, had large somata and widely expanded thick dendrites which were sparsely branched. W-cells which showed weak sustained responses had widely extended thin and winding dendrites, despite a small somal size. These morphological features of Y-, X-, and sustained W-cells correspond well to those of alpha-, beta-, and delta-cells (a subtype of gamma-cells), respectively. The hypothesis of "morphology reflecting function" is strongly supported.

Response to the length of moving visual stimuli of the brisk classes of ganglion cells in the cat retina

Response histograms were collected for brisk-sustained and brisk-transient ganglion cells in the cat retina as narrow bars were moved backwards and forwards across their receptive fields. When a bar of fixed length was moved across the centre of the receptive field with contrast proportional to velocity, a constant response was obtained as long as the centre of the receptive field was crossed within the summation time. However, if the length of the bar was such that it extended beyond the centre, then there was a small but steady increase in surround antagonism for an increase in velocity. The same response was produced by a brief whole-field flash as by an extended bar moving across the receptive field at high velocity if both stimulus conditions delivered the same energy uniformly across the receptive field. With brisk-sustained cells it was observed, for small bar lengths, that bar length and contrast could be exchanged to give a constant response, even when there was considerable non-linearity in the over-all stimulus-response relationship. Thus conditions that resulted in constant stimulus flux produced a constant response. This property was seen at both high and low velocities for the majority of brisk-sustained units. The stimulus-response relationship had a greater range of linearity at high velocities than at low velocities. From similar experiments with brisk-transient cells it was observed that bar length and contrast could only be exchanged to give a constant response at high velocities. At low velocities there was considerable non-linearity: there appeared to be saturation of the response from local regions and it was necessary to extend the bar outside such a region to obtain an increase in response. At lower velocities, despite the changes seen in length-response curves under different conditions of contrast and velocity, the degree of surround antagonism remained constant for a given cell. Further, both brisk-sustained and brisk-transient cells showed the same degree of surround antagonism.

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.

Pharmacological and morphological differences between X- and Y-type ganglion cells in the cat's retina

Pharmacological and morphological differences between X- and Y-cells of the cat's retina were studied using extracellular as well as intracellular recordings of the ganglion cells in the perfused eye-cup preparations. First, the effects of strychnine and bicuculline on the center and the surround responses were investigated. Strychnine blocked the surround inhibition of on-center X-cells, whereas bicuculline blocked that of on-center Y-cells, suggesting that these two-types of cells have different inhibitory interneurons which employ different neurotransmitters. In contrast, the center and the surround responses of off-center cells were reduced by bicuculline, leaving brief transient excitations, irrespective of whether the cells were X- or Y-type. Second, cells whose responses were studied intracellularly and classified as X- or Y-type, were stained with Lucifer yellow CH and observed in whole-mount preparations. It was found that X-cells have morphological characteristics of beta-cells, and Y-cells those of alpha-cells.

Spatiotemporal contrast sensitivity and visual field locus

Contrast sensitivity, measured as a function of retinal eccentricity for stimuli differing in temporal and spatial frequency (0.25-9 c/deg; 0-16 Hz, 0-12 degrees eccentricity), was maximum at the fovea and declined linearly with eccentricity. The slope of the decrease depended upon spatial but not temporal frequency. Contrast sensitivity for drifting gratings was approximately twice that for sinusoidal counterphase gratings at all eccentricities. For central viewing log contrast sensitivity increased with grating length. The shape of this function was systematically related to spatial frequency but independent of temporal frequency, indicating that the visual field is homogeneous in sensitivity for change in contrast over time. The implications of these findings for mechanisms of threshold vision in fovea and periphery are discussed.

The oblique effects of pattern and flicker sensitivity: implications for mixed physiological input

The inferior contrast sensitivity for oblique gratings has been previously demonstrated at high spatial frequencies with an absolute criterion. In this study contrast sensitivity at oblique and main axis orientations was obtained under separate pattern and flicker threshold criteria across a range of both spatial and temporal frequencies. The anisotropy of contrast sensitivity was not observed at low spatial frequencies under any stimulus or criterion conditions. At high spatial frequencies the anisotropy was observed for both pattern and flicker thresholds, although the flicker anisotropy was less pronounced. If, as previously suggested, this anisotropy is mediated by X cells, then these psychophysical findings suggest a mixed model in which both pattern and flicker systems receive input from both X and Y-cell pathways.

Acetylcholine may be an excitatory transmitter mediating visual excitation of 'transient' cells with the periphery effect in the cat retina: iontophoretic studies in vivo

The effects of iontophoretically-applied acetylcholine and its antagonists on the response of retinal ganglion cells at the receptive field centre were studied in the optically intact eye of anaesthetised cats. Acetylcholine enhanced visually-driven excitation of all 'transient' cells with the property of a periphery effect, but failed to excite those weak 'transient' cells without a periphery effect. It also mimicked visual stimulation when applied iontophoretically on spontaneously firing 'transient' cells with a periphery effect and a pulse of acetylcholine produced a transient response. Visually-driven excitation at the receptive field centre and the periphery effect, as well as the acetylcholine-induced excitation in these 'transient' cells, were blocked by dihydro-beta-erythroidine but not by atropine applied by iontophoresis. Neither acetylcholine nor dihydro-beta-erythroidine, on the other hand, produced any significant effect on the visually-driven firing of 'sustained' cells. Acetylcholine, however, suppressed the spontaneous firing of 'sustained-on' cells with a relatively high rate of background discharge, whereas it enhanced that of 'sustained-off' cells with relatively low rates of background discharge, and such effects were again blocked by dihydro-beta-erythroidine. It is suggested that (1), visual excitation of 'transient' cells with the periphery effect is mediated by a cholinergic system and the receptor is nicotinic rather than muscarinic and (2), the spontaneous firing rate of 'sustained' cells may be regulated also by a cholinergic system.

Foveal inhibition and facilitation caused by remote grating jerks: interaction between long-range and short-range effects

Periodic oscillation of a luminance grating imaged upon the peripheral retina reduces the threshold visibility of a foveally presented test spot. This new effect has been named the "jerk effect". The present investigation is concerned with the effect of a single jerk of the remote grating on threshold sensitivity. Foveal sensitivity changes were measured for different delays between grating jerk and test spot presentation. For 0.38 degrees, 100 ms test spot, long-range transient inhibition was found for all delays, with a maximal effect between 0 and 30 ms delay. By combining the jerk effect with the Westheimer paradigm, both facilitatory and inhibitory long-range effects could be demonstrated. For facilitation to occur, it was necessary that the steady background extended into the sensitization zone of the Westheimer area. Inhibition was the only result for smaller backgrounds. Reduced visibility is consistent with the hypothesis that peripheral transient mechanisms inhibit foveal sustained mechanisms. Enhanced visibility indicates that thresholds depend on an interaction between foveal-sustained and foveal-transient units. Transient peripheral stimulation and steady backgrounds of increasing diameter change the balance of inhibitory and facilitatory processes between these units.

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.

Response latency of brisk-sustained (X) and brisk-transient (Y) cells in the cat retina

1. Several methods for evaluating light-evoked response latency and its variability in brisk-sustained (X) and brisk-transient (Y) retinal ganglion cells were tested. The most accurate procedure proved to be that described by Levick (1973), in which the time of the occurrence of the fourth impulse after stimulus onset is taken as an estimate of the latency.2. The shortest response latencies are obtained when the stimuli are the same size as the receptive field centre. At medium and high response amplitudes (> 150 impulses/sec) the response of brisk-transient (Y) cells to these optimal stimuli is 10-15 msec faster than that of adjacent brisk-sustained (X) cells.3. The response latency of brisk-sustained (X) cells for stimuli larger than the receptive field centre increases, whereas that of brisk-transient (Y) cells remains constant. Brisk-sustained (X) cells respond faster than do brisk-transient (Y) cells to stimuli smaller than the receptive field centre.4. No systematic difference exists between brisk-sustained (X) and brisk-transient (Y) cells in regard to the temporal variability of the response. The standard deviation of the latency for stimuli of optimal size decreases from 2.0-8.0 msec at medium stimulus contrast to 0.6-2.0 msec at high stimulus contrast.5. The response of OFF-centre cells to the disappearance of a light spot is always slower than that of an ON-centre cell of the same class to the onset of this stimulus. However, when OFF-centre cells are stimulated with dark spots, their response latency does not differ from that of ON-centre cells of the same class.6. No simple relationship exists between the response latency and the response amplitude. At medium and high discharge rates, most brisk-transient (Y) cells respond faster than an adjacent brisk-sustained (X) cell with equal response. At the same response amplitude, the latencies become shorter as the background illumination is raised. The same discharge rate can be obtained with stimuli of sub-optimal and supra-optimal size, but the latency for the larger stimulus is shorter than that for the smaller one. Latency, therefore, is an additional parameter characterizing the light-evoked response.

Aspartate may be an excitatory transmitter mediating visual excitation of "sustained" but not "transient" cells in the cat retina: iontophoretic studies in vivo

Although the excitatory amino acids, aspartate and glutamate are present in large quantities in the layers of the mammalian retina where the bipolar and amacrine cells make contact with the retinal ganglion cells, it was not known whether these amino acids are the actual neurotransmitters which excite the retinal ganglion cells. To answer this L-aspartate, L-glutamate and the recently discovered powerful and selective antagonist for the N-methyl-D-aspartate receptor, 2-amino-5-phosphonovalerate, were applied iontophoretically to the "sustained" and the "transient" classes of retinal ganglion cells in the optically intact eye of anaesthetised cats. The visually-driven excitation of all "sustained" cells was significantly suppressed by 2-amino-5-phosphonovalerate, whereas that of "transient" cells was not. L-aspartate enhanced the visually-driven excitation and increased the spontaneous firing rare of all "sustained" cells but not of "transient" cells and these effects were blocked by 2-amino-5-phosphonovalerate. The results with L-glutamate were inconclusive. It is suggested that L-aspartate may be an excitatory transmitter mediating the visual response at the receptor field centre of "sustained" retinal ganglion cells, but that excitation of "transient" retinal ganglion cells is mediated by a different transmitter.

Temporal contrast sensitivity and cortical magnification

We measured temporal and spatial contrast sensitivity functions of foveal and peripheral photopic vision at various locations in the nasal visual field. Sensitivity decreased monotonically with increasing eccentricity when it was measured by using the same test gratings at different eccentricities. When the gratings were normalized in area, spatial frequency, and translation velocity by means of the cortical magnification factor M so that the calculated cortical representations of the gratings became equivalent at different eccentricities, the temporal contrast sensitivity functions became similar at all eccentricities. The normalization was effective under all experimental conditions that included various kinds of temporal modulation from 0 to 25 Hz (movement, counterphase flicker and on-off flicker) and different threshold tasks (detection, orientation discrimination, and discrimination of movement direction), independently of the subjective appearances of the gratings at threshold. We conclude that central and peripheral vision are qualitatively similar in spatiotemporal visual performance. The quantitative differences observed without normalization seem to be caused by the spatial sampling properties of retinal ganglion cells that are directly related to the values of M used in the normalization.

Critical flicker fusion in Siamese cats

Critical flicker fusion was determined over a wide luminance range for six Siamese and four normal cats. The Siamese cats had the lowest CFF at all luminance levels when compared to normal and binocularly deprived (BD) cats. The Siamese cat's inferior temporal resolution is most likely due to their profound y-cell loss because (1) CFF is proportional to y-cell population across normal, BD, and Siamese cats; and (2) unlike BD cats, the visual cortex of Siamese cats is comparatively normal. The implication of this finding for the CFF of dark-reared cats and normal cats viewing a stimulus which does not stimulate y-cell is discussed.

Critical bands in cat spatial vision

1. The ability of cats to detect sinusoidal grating patterns superimposed on one-dimensional visual noise was assessed using behavioural methods. 2. The magnitude of elevation in contrast threshold due to noise increased monotonically within limits with increasing noise contrast. 3. Visual noise was filtered using various techniques (band-reject, low-pass, high-pass and band-pass noise); filtered noise resulted in threshold elevation only when it contained frequencies similar to the test frequency. 4. In all cases the masking functions indicated that the band widths of the channels mediating detection ranged from +/- 0.50 to +/- 0.75 octaves across three spatial frequencies and that the channel sensitive to low spatial frequencies was asymmetrical in its tuning. 5. The spatial properties of these psychophysical detecting channels closely resemble the spatial frequency selectivity exhibited by some cat cortical neurones, both in the general narrowness of tuning and the asymmetry in tuning at lower, but not higher, spatial frequencies.

Signal detection analysis of visual flicker in deaf and hearing individuals

A comparison of deaf and hearing subjects on temporal visual resolving power was conducted within a signal-detection paradigm. Subjects were required to make forced-choice judgments of a visual-flicker task under three stimulus probability conditions (0.25, 0.50, 0.75). A total of 600 trials were given each subject from which d' and Beta, indices for sensory sensitivity and response bias respectively, were computed. No significant differences existed on sensory sensitivity or response bias which questions some traditional assumptions about sensory compensation.

The shift-effect enhances X- and suppresses Y-type response characteristics of cat retinal ganglion cells

Responses to a number of stimuli have been studied during the continuous movement of a "global" pattern covering a large part of the retina but excluding the receptive field of the ganglion cell under investigation. With remarkable consistency, the motion of the pattern induced a reinforcement of response properties usually associated with X-cells. In particular, the following responses characteristic of Y-cells were abolished or strongly reduced: (1) the response to simultaneous increment and decrement switching in a bipartite field in the receptive field center; (2) the "discrete" shift-effect, elicited by a jerk of the global pattern; and (3) the relative elevation of the mean discharge rate as a response to a fine drifting grating. Futhermore, responses to center illumination became sustained, and the ongoing discharge rose ("continuous shift-effect"). Y-type responses were most strongly affected, except for the sustained components of center responses which increased in a rather unpredictable way. The results strengthen the view that the shift-effect accounts for most of the functional differences between X- and Y-cells. Saturating the shift-effect mechanism by continuous stimulation is a tool by which the shift-effect components in Y-type responses can be largely removed so that essentially X-type responses are left. Possible neuronal pathways involved in the transmission of the resposes are discussed.

Response of X and Y cat retinal ganglion cells to moving stimuli

Single optic tract fibers in the cat were classified as X or Y cells by a contrast reversal stimulus. A slit of light was then moved across the receptive field at velocities from 10--1000 degrees/s. The preferred velocity was that velocity which elicited the strongest response from the cell. The maximum velocity was the highest velocity target to which a cell could respond. Y cells as a group both preferred and could follow faster targets than X cells, and were more broadly tuned to the preferrred velocity.