Selectivity for relative motion in the monkey superior colliculus

1. Cells in the superficial layers of the colliculus were studied in immobilized monkeys anesthetized with nitrous oxide. We examined sensitivity to the relative motion between two stimuli: a small target in a cell's receptive field and a large random-dot background pattern that filled most of the visual field outside the receptive field. 2. Most cells were nonselective for either target direction or speed when the background pattern was stationary but were selective for both direction and speed relative to a moving background. Selectivity for relative motion was independent of the absolute direction and speed of both target and background. When both moved at the same speed in the same direction, the response evoked by the target was strongly suppressed. Changing the background direction relative to the target reduced the suppression; suppression was minimal when the two moved in opposite directions. Selectivity for relative direction was broad: the average tuning width at half-amplitude was 136 degrees. When target and background moved in the same direction, increasing or decreasing background speed relative to the target likewise reduced suppression. Average tuning width for relative speed was 1.4 log units. 3. Selectivity for relative motion was a global phenomenon. Suppression was present even when the background pattern was excluded from a region 10 times the receptive-field diameter. However, suppression gradually diminished with increasing distance between the receptive field and the background pattern. 4. Relative motion selectivity was most common in the deeper part of the superficial layers. Ninety percent of the cells below the middle of the stratum griseum superficiale were selective for relative direction, whereas above this level only 45% of the cells were. 5. Cells in the magnocellular and parvocellular layers of the lateral geniculate nucleus did not show selectivity for relative direction. 6. We suggest that the lower one-half of the superficial grey layer and the stratum opticum together constitute a subdivision of the superior colliculus that is specialized to detect strong discontinuities in relative motion. Descending input by way of the corticotectal tract is probably essential for the detection process. the projections from this tectal motion zone to the pulvinar, and from there to prestriate cortex, may provide a feedback pathway through which motion discontinuities such as occur at dynamic occlusion boundaries can influence local feature detection by cortical neurons.     

Loss of relative-motion sensitivity in the monkey superior colliculus after lesions of cortical area MT

 Many cells in the superficial layers of the monkey superior colliculus are sensitive to relative motion. The response to a small stimulus moving through a cell’s receptive field is strongly modulated by the relative motion between the stimulus and a textured pattern moving through the surrounding visual field; modulation is independent of absolute direction and speed of the stimulus. To determine whether cortical visual area MT is essential for this type of relative-motion sensitivity, colliculus cells were studied in the anesthetized, immobilized preparation after ablation of area MT. Unilateral MT lesions were made by either aspiration, kainic acid injection, or a combination of both methods. Data from the lesioned animals were compared with those from intact animals. Ipsilateral to the lesions, colliculus cells showed an almost total loss of sensitivity to relative motion. This loss was related neither to inadvertent injury of cortical areas neighboring MT nor to incidental optic radiation damage. Two other forms of motion-dependent, center-surround interactions were still present in the colliculus after the cortical lesions. These were a rudimentary sensitivity to differential motion between stimulus and background, which occurs for only one direction of stimulus movement, and a nonselective center-surround suppression, which is induced by movement of a background stimulus in any direction. Visual responsiveness, ocular dominance, and flash-evoked responses were also unaffected by the cortical lesions. We conclude that input from area MT is crucial for relative-motion sensitivity, but not for other response properties, in the superficial layers of the monkey colliculus. Received: 27 September 1996 / Accepted: 25 April 1997     

The structural and functional characteristics of striate cortical neurons that innervate the superior colliculus and lateral posterior nucleus in hamster

Intracellular recording and horseradish peroxidase injection techniques were used to structurally and functionally characterize the striate cortical neurons in hamster that projected to the superior colliculus and/or lateral posterior nucleus of the thalamus. With two exceptions, the receptive field properties and morphological characteristics of the neurons antidromically activated from the colliculus and lateral posterior nucleus were quite similar. Striate corticotectal and striate cortico-lateral posterior neurons generally had non-oriented receptive fields which gave either "on-off' or no responses to flashed stimuli. Only a small number (less than 5%) were orientation selective, but about one-third were directionally selective. Most of the cells preferred movement with an upward component. Most striate corticotectal and cortico-lateral posterior cells responded to a wide range of stimulus velocities and exhibited little spatial summation. With the possible exception of two cells, all the projection neurons we recovered were large lamina V pyramidal cells whose apical dendrites extended to and branched extensively in layer I. All had extensive (in some cases over 1 mm) tangential axon collaterals, primarily in layers V and/or VI. The electrophysiological experiments also demonstrated that some (50% of a sample of 20 cells) corticotectal neurons also sent an axon collateral to the lateral posterior nucleus. Finally, our recordings showed that many (56% of a sample of 27 neurons) cells which could be antidromically activated from the lateral posterior nucleus, but not the superior colliculus had response latencies which exceeded those of almost all the cells which could be antidromically activated from the tectum. Retrograde transport of diamidino yellow and true blue confirmed the electrophysiological result that individual cortical neurons projected to both the superior colliculus and lateral posterior nucleus. These experiments showed that 20% of the striate cortical cells that projected into colliculus also sent an axon collateral to the lateral posterior nucleus.     

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

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

Topographic organization of somatosensory corticotectal influences in cat

Using electrophysiological techniques, the present study demonstrated that substantial direct somatosensory cortical influences on the superior colliculus (SC) originate from three areas: a) SIV, b) para-SIV (the cortex adjacent to SIV but deeper in the anterior ectosylvian sulcus (AES) and for which no topography has yet been described), and c) the rostral suprasylvian sulcus. Influences also appeared to originate from SI and SII, but these may have been indirect. Detailed examination of the AES revealed that these corticotectal projections are topographically organized, and stimulation of a given cortical locus was observed to affect only those cells in the SC whose receptive fields overlapped those of cells at the stimulation site. A similar receptive-field register was found between the suprasylvian sulcus and the SC. Within this topographic pattern, considerable convergence was evident and an individual SC cell could be influenced from a surprisingly large cortical area. This was particularly evident within the representation of the forelimb. Thus, an SC cell with a receptive field covering the forelimb and paw could receive convergent input from many cortical cells with receptive fields covering all or restricted portions of this body region. Considerable corticotectal divergence also was observed within this general topographic scheme. For example, a given corticotectal site representing the digits sent projections to many different SC cells that included the digits within their receptive fields. These data are more consistent with a block-to-block than a point-to-point corticotectal projection. Somatosensory corticotectal projections excited only those SC cells that could also be activated by peripheral somatosensory stimuli. Similarly, the caudal AES, which contains auditory cells, excited only those SC cells activated also by peripheral auditory stimuli. Yet convergent influences from both auditory and somatosensory regions of the AES were observed in the SC cells that could be activated by both auditory and somatosensory stimuli. These data indicate that the AES is a major source of excitatory input to cells of the deep laminae of the SC. Since it is these deep laminae cells that project to premotor regions of the brain stem and the spinal cord, it is reasonable to suppose that the AES has a significant impact on the output signals of the SC that initiate the orientation responses to peripheral sensory stimulation.     

Modulatory influences of a moving visual noise background on bar-evoked responses of cells in area 18 of the feline visual cortex

Summary The modulatory influence of a synchronously moving visual noise background on responsiveness to an optimally-oriented moving bar stimulus was investigated in visual cortical area 18 of the lightly-anaesthetized cat. The bar and noise background were swept along the axis orthogonal to bar orientation, with the same phase, velocity and amplitude of motion. Cells which were insensitive to motion of visual noise per se or weakly responsive to individual grains in the noise sample showed suppression of bar-evoked responses by simultaneous motion of the noise background. Percent suppression declined with increase in bar length, over a range which could exceed the maximum estimate of receptive field length. The decline in percent suppression was non-linear, becoming progressively flatter in slope as bar length was increased until an asymptotic value was reached; observations on end-stopped cells and on end-free cells with restricted length summation verified that percent suppression was related specifically to the length of the comparison bar and not to the strength of response it evoked. Percent suppression and the extent over which it declined with increase in bar length were comparable for preferred and opposite directions of bar motion even in cells with radically different length-response functions in the two directions, including end-stopped cells with direction-selective end-zones. In contrast to end-inhibition, which was maximal at or near the preferred velocity for a bar of optimal length, percent suppression by motion of the noise background was essentially velocity-invariant; in velocity tuned and velocity high-pass cells, background motion reduced the slope(s) of the velocity-response function, implying that the suppressive action of moving noise backgrounds is divisive rather than subtractive. It is argued that the suppression derives predominantly from an axo-somatic noise-sensitive inhibitory input from superficial- and deep-layer, large basket cells in orientation columns at some distance from those of their target cells.     

Indirect, across-the-midline retinotectal projections and representation of ipsilateral visual field in superior colliculus of the cat

1. In agreement with previous work, we have found that the ipsilateral visual field is represented in an extensive rostral portion--from one-third to one-half--of the superior colliculus (SC) of the cat. This representation is binocular. The SC representation of the ipsilateral visual field can be mediated both directly, by crossed retinotectal connections originating from temporal hemiretina, and indirectly, by across-the-midline connections relaying visual information from one-half of the brain to contralateral SC. 2. In order to study the indirect, across-the-midline visual input to the SC, we have recorded responses of SC neurons to visual stimuli presented to either the ipsilateral or the contralateral eye of cats with a midsagittal splitting of the optic chiasm. Units driven by the ipsilateral eye, presumably through the direct retinotectal input and/or corticotectal connections from ipsilateral visual cortex, were found throughout the SC, except at its caudal pole, which normally receives fibers from the extreme periphery of the contralateral nasal hemiretina. Units driven by the contralateral eye, undoubtedly through an indirect across-the-midline connection, were found only in the anterior portion of the SC, in which is normally represented the ipsilateral visual field. Receptive fields in both ipsilateral and contralateral eye had properties typical of SC receptive fields in cats with intact optic pathways. 3. All units having a receptive field in the contralateral eye had also a receptive field in the ipsilateral eye; for each of these units, the receptive fields in both eyes invariably abutted the vertical meridian of the visual field. The receptive field in one eye had about the same elevation relative to the horizontal meridian and the same vertical extension as the receptive field in the other eye; the two receptive fields of each binocular unit matched each other at the vertical meridian and formed a combined receptive field straddling the vertical midline of the horopter...     

Response to motion in extrastriate area MSTl: disparity sensitivity

Many neurons in the lateral-ventral region of the medial superior temporal area (MSTl) have a clear center surround separation in their receptive fields. Either moving or stationary stimuli in the surround modulates the response to moving stimuli in the center, and this modulation could facilitate the perceptual segmentation of a moving object from its background. Another mechanism that could facilitate such segmentation would be sensitivity to binocular disparity in the center and surround regions of the receptive fields of these neurons. We therefore investigated the sensitivity of these MSTl neurons to disparity ranging from three degrees crossed disparity (near) to three degrees uncrossed disparity (far) applied to both the center and the surround regions. Many neurons showed clear disparity sensitivity to stimulus motion in the center of the receptive field. About (1)/(3) of 104 neurons had a clear peak in their response, whereas another (1)/(3) had broader tuning. Monocular stimulation abolished the tuning. The prevalence of cells broadly tuned to near and far disparity and the reversal of preferred directions at different disparities observed in MSTd were not found in MSTl. A stationary surround at zero disparity simply modulated up or down the response to moving stimuli at different disparities in the receptive field (RF) center but did not alter the disparity tuning curve. When the RF center motion was held at zero disparity and the disparity of the stationary surround was varied, some surround disparities produced greater modulation of MSTl neuron response than did others. Some neurons with different disparity preferences in center and surround responded best to the relative disparity differences between center and surround, whereas others were related to the absolute difference between center and surround. The combination of modulatory surrounds and the sensitivity to relative difference between center and surround disparity make these MSTl neurons particularly well suited for the segmentation of a moving object from the background.     

Retrograde transneuronal transport of wheat-germ agglutinin to the retina from visual cortex in the cat

Summary The stability of visual perception despite eye movements suggests the existence, in the visual system, of neural elements able to recognize whether a movement of an image occurring in a particular part of the retina is the consequence of an actual movement that occurred in the visual field, or self-induced by an ocular movement while the object was still in the field of view. Recordings from single neurons in area V3A of awake macaque monkeys were made to check the existence of such a type of neurons (called real-motion cells; see Galletti et al. 1984, 1988) in this prestriate area of the visual cortex. A total of 119 neurons were recorded from area V3A. They were highly sensitive to the orientation of the visual stimuli, being on average more sensitive than V1 and V2 neurons. Almost all of them were sensitive to a large range of velocities of stimulus movement and about one half to the direction of it. In order to assess whether they gave different responses to the movement of a stimulus and to that of its retinal image alone (self-induced by an eye movement while the stimulus was still), a comparison was made between neuronal responses obtained when a moving stimulus swept a stationary receptive field (during steady fixation) and when a moving receptive field swept a stationary stimulus (during tracking eye movement). The receptive field stimulation at retinal level was physically the same in both cases, but only in the first was there actual movement of the visual stimulus. Control trials, where the monkeys performed tracking eye movements without any intentional receptive field stimulation, were also carried out. For a number of neurons, the test was repeated in darkness and against a textured visual background. Eighty-seven neurons were fully studied to assess whether they were real-motion cells. About 48% of them (42/87) showed significant differences between responses to stimulus versus eye movement. The great majority of these cells (36/42) were real-motion cells, in that they showed a weaker response to visual stimulation during tracking than to the actual stimulus movement during steady fixation. On average, the reduction in visual response during eye movement was 64.0 ± 15.7% (SD). Data obtained with a uniform visual background, together with those obtained in darkness and with textured background, indicate that real-motion cells receive an eye-motion input, either retinal or extraretinal in nature, probably acting presynaptically on the cell's visual input. In some cases, both retinal and extraretinal eye-motion inputs converge on the same real-motion cell. No correlation was observed between the real-motion behaviour and the sensitivity to either orientation or direction of movement of the visual stimulus used to activate the receptive field, nor with the retinotopic location of the receptive field. We suggest that the visual system uses real-motion cells in order to distinguish real from self-induced movements of retinal images, hence to recognize the actual movement in the visual field. Based on psychophysical data, the hypothesis has been advanced of an internal representation of the field of view, stable despite eye movement (cf. MacKay 1973). The real-motion cells may be neural elements of this network and we suggest that the visual system uses the output of this network to properly interpret the large number of sensory changes resulting from exploratory eye movements in a stable visual world.     

Differential responsiveness of simple and complex cells in cat striate cortex to visual texture

Summary The responsiveness of 254 simple and complex striate cortical cells to various forms of static and dynamic textured visual stimuli was studied in cats, lightly anaesthetised with N₂O/O₂ mixtures supplemented with pentobarbitone.Simple cells were unresponsive to all forms of visual noise presented alone, although about 70% showed a change in responsiveness to conventional bar stimuli when these were presented on moving, rather than stationary, static-noise backgrounds. Bar responses were depressed by background texture motion in a majority of cells (54%), but were actually enhanced in a few instances (16%).In contrast, all complex cells were to some extent responsive to bars of static visual noise moving over stationary backgrounds of similar texture, or to motion of a whole field of static noise. The optimal velocity for noise was generally lower than for bar stimuli.Since moving noise backgrounds were excitatory for complex cells, they tended to reduce specific responses to bar stimulation; in addition, directional bias could be modified by direction and velocity of background motion.Complex cells fell into two overlapping groups as regards their relative sensitivity to light or dark bars and visual noise. Extreme examples were insensitive to conventional bar or edge stimuli while responding briskly to moving noise.In many complex cells, the preferred directions for motion of noise and of an optimally oriented black/white bar were dissimilar.The ocular dominance and the degree of binocular facilitation of some complex cells differed for bar stimuli and visual texture.Preliminary evidence suggests that the deep-layer complex cells (those tolerant of misalignment of line elements; Hammond and MacKay, 1976) were most sensitive to visual noise. Superficial-layer complex cells (those preferring alignment) were less responsive to noise.Only complex-type hypercomplex cells showed any response to visual noise.We conclude that, since simple cells are unresponsive to noise, they cannot provide the sole input to complex cells. The differences in the response of some complex cells to rectilinear and textured stimuli throw a new light on their rôle in cortical information-processing. In particular, it tells against the hypothesis that they act as a second stage in the abstraction of edge-orientation.     

Cat area 17. III. Response properties and orientation anisotropies of corticotectal cells

The receptive field properties of antidromically identified corticotectal (CT) cells in area 17 were explored in the paralyzed, anesthetized cat. To compare these with another population of infragranular cells, we also examined the receptive field properties of cells in layer 6. Sixty percent of our sample of CT cells showed increased response to increased stimulus length (length summation) and were classified as standard complex cells. The other 40% showed little or no length summation, were generally end stopped, and were classified as special complex cells. Standard and special complex CT cells have complementary orientation anisotropies: the distribution of orientation preferences of standard complex cells is biased toward obliquely oriented stimuli, whereas special complex cells are biased toward horizontally and vertically oriented stimuli. The receptive fields of the cells in our sample were primarily along the horizontal meridian so we cannot determine if these anisotropies are defined relative to the vertical meridian or relative to the meridian passing through the receptive field. The effects of these anisotropies in preferred orientation are minimized by the broad orientation tuning of CT cells. There was no simple relationship between the direction bias of CT cells and the reported direction bias of tectal cells. In contrast to the heterogeneity of corticotectal cells, layer 6 cells uniformly showed strong length summation, tight orientation tuning, and little spontaneous activity.     

Both striate cortex and superior colliculus contribute to visual properties of neurons in superior temporal polysensory area of macaque monkey

Although the tectofugal system projects to the primate cerebral cortex by way of the pulvinar, previous studies have failed to find any physiological evidence that the superior colliculus influences visual activity in the cortex. We studied the relative contributions of the tectofugal and geniculostriate systems to the visual properties of neurons in the superior temporal polysensory area (STP) by comparing the effects of unilateral removal of striate cortex, the superior colliculus, or of both structures. In the intact monkey, STP neurons have large, bilateral receptive fields. Complete unilateral removal of striate cortex did not eliminate visual responses of STP neurons in the contralateral visual hemifield; rather, nearly half the cells still responded to visual stimuli in the hemifield contralateral to the lesion. Thus the visual properties of STP neurons are not completely dependent on the geniculostriate system. Unilateral striate lesions did affect the response properties of STP neurons in three ways. Whereas most STP neurons in the intact monkey respond similarly to stimuli in the two visual hemifields, responses to stimuli in the hemifield contralateral to the striate lesion were usually weaker than responses in the ipsilateral hemifield. Whereas the responses of many STP neurons in the intact monkey were selective for the direction of stimulus motion or for stimulus form, responses in the hemifield contralateral to the striate lesion were not selective for either motion or form. Whereas the median receptive field in the intact monkey extended 80 degrees into the contralateral visual field, the receptive fields of cells with responses in the contralateral field that survived the striate lesions had a median border that extended only 50 degrees into the contralateral visual field. Removal of both striate cortex and the superior colliculus in the same hemisphere abolished the responses of STP neurons to visual stimuli in the hemifield contralateral to the combined lesion. Nearly 80% of the cells still responded to visual stimuli in the hemifield ipsilateral to the lesion. Unilateral removal of the superior colliculus alone had only small effects on visual responses in STP. Receptive-field size and visual response strength were slightly reduced in the hemifield contralateral to the collicular lesion. As in the intact monkey, selectivity for stimulus motion or form were similar in the two visual hemifields. We conclude that both striate cortex and the superior colliculus contribute to the visual responses of STP neurons. Striate cortex is crucial for the movement and stimulus specificity of neurons in STP.(ABSTRACT TRUNCATED AT 400 WORDS)     

Response properties of neurons in area 17 projecting to the striate-recipient zone of the cat's lateralis posterior-pulvinar complex: comparison with cortico-tectal cells

The main input of the lateral part of the cat's lateralis posterior-pulvinar complex (LP-P) comes from the primary visual cortex. We investigated the response properties of cells in area 17 projecting to the striate-recipient zone (LPl) of the cat's LP-P complex. The cells' receptive fields were stimulated with drifting sine-wave gratings. Cells whose fibres terminate in the superior colliculus were also recorded, to determine how their properties compare with those of cortico-LPl cells and to investigate the possibility that LPl is innervated by collaterals of cortico-tectal units. A total of 26 cells in the striate cortex were identified by antidromic activation from the LPl (mean latency 2.2 ms) and 22 from the colliculus (mean latency 2.5 ms). Only six cortical cells could be activated from the LPl and the colliculus. All cortico-LPl cells except for two responded to drifting sinusoidal gratings with unmodulated discharges (AC/DC ratios <1). On the basis of their modulation index, these units were classified as complex cells. All cortico-LPl cells were selective for the orientation of gratings (mean bandwidth of 28°). There was a tendency for cortico-LPl cells to prefer vertical and horizontal orientations. More than half of these cells (57%) were direction selective. Strong orientation anisotropies were also found in the receptive fields of cortico-tectal cells, since almost all units responded preferentially to horizontally oriented gratings. The mean preferred spatial and temporal frequencies of cortico-LPl cells were 0.74 c/deg (bandwidth 2.03 octaves) and 2.7 Hz (bandwidth 2.5 octaves), respectively. These properties did not differ significantly from those of cortico-tectal cells. Most cortico-LPl cells (72%) exhibited contrast-response curves with saturation at low contrast (mean half-saturation 0.2). For the remaining units, the responses increased linearly with contrast without clear saturation. For more than half of cortico-tectal cells (60%), the contrast function was also characterised by a response saturation. Almost all cortico-LPl cells responded to moving random dot patterns with mean tuning functions of 43.6°. Standard as well as special complex cells were found to be equally responsive to the motion of visual noise. Similar properties were recorded for cortico-tectal cells (mean bandwidth of 44.2°). Cortico-LPl and cortico-tectal cells were either binocularly or monocularly driven by the contralateral eye and their mean spontaneous firing rates were 11.7 and 10.9 spikes/s, respectively. These cells were presumably located in layer V. Stimulation of LPl and colliculus also evoked trans-synaptic responses in area 17. The average latency of the orthodromic responses from LPl was much shorter than that from the colliculus (medians 3.5 and 50 ms, respectively). The findings indicate that almost all cortico-LPl units have complex receptive fields and that their overall properties differ from those of recipient cells in LPl. These results also indicate that LPl is not likely to be innervated by collaterals of fibres of cortico-tectal cells. While cortico-LPl and cortico-tectal cells appear to form two distinct populations, there is no significant difference between the overall properties of these two cell groups.     

On the sensitivity of complex cells in feline striate cortex to relative motion

Summary Responses of superficial-layer, texturesensitive complex cells in cat striate cortex to relative motion between an oriented bar stimulus and its textured background were recorded. Some cells responded best to motion in one particular direction across the receptive field of the cell, irrespective of whether the bar and background moved simultaneously in the same (in-phase) or opposite (antiphase) directions. Others showed a clear preference for either in-phase or antiphase relative motion, irrespective of direction of motion across the receptive field.Supported by the Medical Research Council (grant no. G 80/ 0833/4/N to P. Hammond)On leave from: Newcastle-upon-Tyne Polytechnic     

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).     

Influence of a moving textured background on direction selectivity of cat striate neurons

The influence of a moving textured background on direction selectivity for a moving bar was tested in 118 striate neurons and in 19 dorsal lateral geniculate neurons of anesthetized and paralyzed cats. In the standard conditions the background was a two-dimensional noise pattern, the bar moved at optimal speed, and its contrast was adjusted to the level producing 50% of the maximum response. These experiments revealed a new typology of cortical cells based on relative direction selectivity. Six different relative-direction-selectivity types are described. Two types of cells were found to have opposite kinds of relative direction selectivity: antiphase direction-selective cells (5% of the cortical sample) preferred the direction of the bar opposite to the direction of background motion, and absolutely direction-selective cells (20% of the cortical sample) kept their direction selectivity for bar motion independently of the background motion. Three types of cortical cells were direction selective for bar motion only in restricted background motion conditions: conditionally direction-selective cells (20% of cortical sample) only expressed their direction selectivity when the bar and the background moved in antiphase, differencing direction-selective cells (5% of the cortical sample) only expressed their direction selectivity when the bar and the background differed in speed, and limited direction-selective cells (20% of the cortical sample) only expressed their direction selectivity for near zero background speeds. The sixth type, relative nondirection-selective cells (30% of the cortical sample and all of the geniculate cells) were direction selective for none of the background motion conditions. These different relative-direction-selectivity types differed in RF organization, in ocular dominance, velocity sensitivity, in laminar distribution, and in distribution in the visual field. The relative-direction-selectivity types were invariant for changes in the contrast and bar speed. The construction of these relative-direction-selectivity types from the geniculate input requires some inhibitory, but mainly facilitatory, intracortical interactions. These experimental findings suggest that area 17 in the cat has the neuronal machinery to extract depth from motion (limited direction-selective cells) and to segregate visual scenes by motion cues (antiphase, conditionally and differencing direction-selective cells).     

Modulations of collicular visual responses by acoustic stimuli in rabbits

This study analyzes the influences of an acoustic stimulus upon neuronal light responses of superficial layers of the superior colliculus in anesthetized and paralyzed rabbits. The results have revealed that even if visually-responsive cells fail to be excited by sound, the latter is still capable of modifying light-evoked discharge. The influence may be "short-term" (the discharge rate recovers within 500 ms) or it may be "long-term" (the firing rate remains modified for several seconds). This audio-visual interaction depends upon several factors: the time of occurrence of both stimuli, the physical aspects of the visual target, the relative positions of the speaker and the visual receptive field, and finally, the sensitivity of the unit to movement direction. Data indicates that cells of the most dorsal (hence visual) layers of the superior colliculus are influenced by sound. It is concluded that the colliculus may use the sound as an additional cue to orientate the animal. Also, collicular cells could "memorize" for several seconds various features present in the environment.     

Effects of cooling somatosensory cortex on response properties of tactile cells in the superior colliculus

The corticotectal influences of somatosensory cortex were investigated by using reversible deactivation of cortex by cooling. More than half of the somatosensory superior colliculus (SC) cells studied exhibited a response depression (often not apparent qualitatively) or an elimination of responses to somatosensory stimuli during the period in which cortex was rendered inactive. Responses were restored to their initial levels by cortical rewarming. Hyperresponsiveness was never observed as a consequence of cortical cooling. Susceptibility to cooling-induced depression was not invariably linked to a specific cell type, location in the SC, or receptive-field size. Yet cells that had small receptive fields and were activated by hair displacement had the highest probability of being affected by this procedure. In some cells a contraction of the receptive field was induced by cortical cooling. This observation is consistent with previous experiments that showed that SC somatosensory receptive fields are constructed by the convergence of ascending and descending inputs and indicates that the responsiveness of specific receptive-field regions may depend on the functional integrity of cortex. Two cortical regions were found to produce cooling-induced effects in somatosensory SC cells: 1) SIV (and para-SIV), located in the anterior ectosylvian sulcus, and 2) the cortex within the rostral suprasylvian sulcus. These results indicate that somatosensory cortex, like visual cortex, plays a critical role in modulating the responses of SC cells. Apparently, the ability of both somatosensory and visual SC cells to code the presence of peripheral stimuli depends largely on the functional influences of their respective cortices. However, in contrast to previous observations on visual corticotectal influences, no specific receptive-field properties could be shown to be impressed on SC cells by somatosensory cortex.     

The effects of contrast on the linearity of spatial summation of simple cells in the cat's striate cortex

Summary Non-linearities of spatial summation were examined in simple cells in the cat's striate cortex. The degree of non-linearity was assessed from an examination of the waveforms of the responses to moving sinusoidal gratings and was quantified by a measure called relative modulation. Relative modulation was affected little by changes in contrast at either optimal or non-optimal spatial frequencies. The non-linearities of spatial summation exhibited by some simple cells are, therefore, essential. Those simple cells which exhibit linear spatial summation are no less linear at high stimulus contrasts. These results support a push-pull model of simple cell receptive field organization in which ON and OFF centre l.g.n. input is combined both additively and subtractively.     

Cat colour vision: one cone process or several?

1. Peripheral mechanisms that might contribute to colour vision in the cat have been investigated by recording from single units in the lateral geniculate and optic tract. Evidence is presented that the input to these cells comes from a single class of cones with a single spectral sensitivity.2. In cats with pupils dilated a background level of 10-30 cd/m(2) was sufficient to saturate the rod system for all units. When the rods were saturated, the spectral sensitivity of all units peaked at 556 nm; this was true both for centre and periphery of the receptive field. The spectral sensitivity curve was slightly narrower than the Dartnall nomogram. It could not be shifted by chromatic adaptation with red, green, blue or yellow backgrounds.3. In the mesopic range (0.1-10 cd/m(2)), the threshold could be predicted in terms of two mechanisms, a cone mechanism with spectral sensitivity peaking at 556 nm, and a rod mechanism with spectral sensitivity at 500 nm. The mechanisms were separated and their increment threshold curves measured by testing with one colour against a background of another colour. All units had input from both rods and cones. The changeover from rods to cones occurred at the same level of adaptation in both centre and periphery of the receptive field. Threshold for the cones was between 0.04 and 0.25 cd/m(2) with the pupil dilated, for a spot covering the centre of the receptive field.4. None of the results was found to vary between lateral geniculate and optic tract, with layer in the lateral geniculate, or with distance from area centralis in the visual field.5. The lack of evidence for more than one cone type suggests that colour discrimination in the cat may be a phenomenon of mesopic vision, based on differences in spectral sensitivity of the rods and a single class of cones.