Visual FMRI responses in human superior colliculus show a temporal-nasal asymmetry that is absent in lateral geniculate and visual cortex

Eye patching has revealed enhanced saccadic latencies or attention effects when orienting toward visual stimuli presented in the temporal versus nasal hemifields of humans. Such behavioral advantages have been tentatively proposed to reflect possible temporal-nasal differences in the retinotectal pathway to the superior colliculus, rather than in the retinogeniculate pathway or visual cortex. However, this has not been directly tested with physiological measures in humans. Here, we examined responses of the human superior colliculus (SC) to contralateral visual field stimulation, using high spatial resolution fMRI, while manipulating which hemifield was stimulated and orthogonally which eye was patched. The SC responded more strongly to visual stimulation when eye-patching made this stimulation temporal rather than nasal. In contrast, the lateral geniculate nucleus (LGN) plus retinotopic cortical areas V1-V3 did not show any temporal-nasal differences and differed from the SC in this respect. These results provide the first direct physiological demonstration in humans that SC shows temporal-nasal differences that LGN and early visual cortex apparently do not. This may represent a temporal hemifield bias in the strength of the retinotectal pathway, leading to a preference for the contralateral hemifield in the contralateral eye.     

Central core control of developmental plasticity in the kitten visual cortex: I. Diencephalic lesions

Summary In five, dark-reared, 4-week-old kittens the posterior two thirds of the corpus callosum were split, and a lesion comprising the intralaminar nuclei was made of the left medial thalamic complex. In addition, the right eye was closed by suture. Postoperatively, the kittens showed abnormal orienting responses, neglecting visual stimuli presented in the hemifield contralateral to the side of the lesion. Sudden changes in light, sound, or somatosensory stimulation elicited orienting responses that all tended toward the side of the lesion. These massive symptoms faded within a few weeks but the kittens continued to neglect visual stimuli in the hemifield contralateral to the lesion when a second stimulus was presented simultaneously in the other hemifield. Electrophysiologic analysis of the visual cortex, performed after the end of the critical period, revealed marked interhemispheric differences. In the visual cortex of the normal hemisphere most neurons were monocular and responded exclusively to stimulation of the open eye, but otherwise had normal receptive field properties. In the visual cortex of the hemisphere containing the thalamic lesion, the majority of the neurons remained binocular. In addition, the selectivity for stimulus orientation and the vigor of responses to optimally aligned stimuli were subnormal on this side. Thus, the same retinal signals, which in the control hemisphere suppressed the pathways from the deprived eye and supported the development of normal receptive fields, failed to do either in the hemisphere containing the thalamic lesion. Apparently, experience-dependent changes in the visual cortex require both retinal stimulation and the functioning of diencephalic structures which modulate cortical excitability and control selective attention.     

Restoration of acoustic orienting into a cortically deaf hemifield by reversible deactivation of the contralesional superior colliculus: the acoustic "Sprague Effect"

Removal of all contiguous visual cortical areas of one hemisphere results in a contralateral hemianopia. Subsequent deactivation of the contralesional superior colliculus (SC) nullifies the effects of the visual cortex ablation and restores visual orienting responses into the cortically blind hemifield. This deficit nullification has become known as the "Sprague Effect." Similarly, in the auditory system, unilateral ablation of auditory cortex results in severe sound localization deficits, as assessed by acoustic orienting, to stimuli in the contralateral hemifield. The purpose of this study was to examine whether auditory orienting responses can be restored into the impaired hemifield during deactivation of the contralesional SC. Three mature cats were trained to orient toward and approach an acoustic stimulus (broadband, white noise burst) that was presented centrally, or at one of 12 peripheral loci, spaced at 15 degrees intervals. After training, a cryoloop was chronically implanted over the dorsal surface of the right SC. During cooling of the cooling loop to temperatures sufficient to deactivate the superficial and intermediate layers (SZ, SGS, SO, SGI), auditory orienting responses were eliminated into the left (contracooled) hemifield while leaving acoustic orienting into the right (ipsicooled) hemifield unimpaired. This deficit was temperature-dependently graded from periphery to center. After the effectiveness of the SC cooling loop was verified, auditory cortex of the middle and posterior ectosylvian and anterior and posterior sylvian gyri was removed from the left hemisphere. As expected, the auditory cortex ablation resulted in a profound deficit in orienting to acoustic stimuli presented at any position in the right (contralesional) hemifield, while leaving acoustic orienting into the left (ipsilesional) hemifield unimpaired. The ablations of auditory cortex did not have any impact on a visual detection and orienting task. The additional deactivation of the contralesional SC to temperatures sufficient to cool the superficial and intermediate layers nullified the deficit caused by the auditory cortex ablation and acoustic orienting responses were restored into the right hemifield. This restoration was temperature-dependently graded from center to periphery. The deactivations were localized and confirmed with reduced uptake of radiolabeled 2-deoxyglucose. Therefore deactivation of the right superior colliculus after the ablation of the left auditory cortex yields a fundamentally different result from that identified during deactivation of the right superior colliculus before the removal of left auditory cortex in the same animal. Thus the "Sprague Effect" is not unique to a particular sensory system and deactivation of the contralesional SC can restore either visual or acoustic orienting responses into an impaired hemifield after cortical damage.     

Effect of corticotectal tract lesions on relative motion selectivity in the monkey superior colliculus

Summary Many cells in the superficial layers of the monkey superior colliculus are sensitive to the relative motion between a small target moving through the classic receptive field and a textured, moving background pattern that fills the visual field beyond the classic receptive field. The cells respond well when motion of the target differs from that of the background, but their responses are suppressed when the target moves in phase with the background. To determine whether this relative motion sensitivity depends on input to the colliculus from visual cortex, we studied colliculus cells in immobilized, anesthetized monkeys after unilateral thermocoagulation, or anesthetic blockade, of the corticotectal tract at the level of the pulvinar. In the colliculus ipsilateral to the corticotectal tract lesions, relative motion sensitivity was significantly reduced when compared either with the colliculus in intact animals or with the colliculus contralateral to the lesion. However, a moving-background stimulus still had a modest suppressive effect compared with a stationary background (background motion sensitivity), as is the case for intact animals. Anesthetic blockade of the corticotectal tract had similar effects; relative motion sensitivity, but not background motion sensitivity, was lost following injection of mepivacaine or bupivacaine. Pulvinar cell loss alone, induced by kainic acid injection, had no effect on relative motion sensitivity in the colliculus. The corticotectal tract lesions, but not the anesthetic injections, also had minor effects on flash-evoked responses and spontaneous discharge rates; these effects may reflect a retrograde response of some tectopulvinar cells to injury of their axons by the corticotectal tract lesions. In the colliculus opposite the corticotectal tract lesion, relative motion sensitivity was similar to that in normal animals. However, responses in the presence of a moving background were enhanced, suggesting that removal of cortical input to one colliculus may disinhibit the contralateral colliculus, a phenomenon reminiscent of the Sprague effect in the cat. We conclude that while cortical input to the colliculus may contribute little to the classic receptive field properties of superficial-layer cells, it clearly does contribute to relative motion sensitivity.     

Functional properties of monkey caudate neurons. II. Visual and auditory responses

1. Visual responses of caudate neurons were studied in monkeys trained to fixate on a small spot of light. A visual stimulus (another spot of light) was presented in various contexts of behavior using different behavioral paradigms. Visual receptive fields were usually large and centered on the contralateral hemifield. Among 217 neurons with visual responses, 184 were further classified into subtypes. 2. Visual responses in 64 neurons were not modulated by changing the paradigms (unconditional visual responses). In the other neurons, visual responses were dependent on the behavioral contexts in which the stimulus was presented. Three types of behavioral modulation were found. 3. A saccade-enhanced visual response (n = 37) was the one that was enhanced if the monkey made a saccade to the stimulus on its appearance. The enhancement was spatially selective: the response was depressed if the saccade was directed away from the stimulus. 4. Memory-contingent visual responses (n = 36) were present preferentially when the monkey remembered the location of the stimulus and a few seconds later made a saccade to the remembered location. Responses were greater when the location of the stimulus was randomized between trials. 5. Expectation-contingent visual responses (n = 46) were present preferentially when the stimulus came on while the monkey was not fixating another spot, and the stimulus was related directly to a reward. Unlike the other types, its receptive field included both contralateral and ipsilateral hemifields without a particular preference. 6. A small number of neurons (n = 16) showed a visual response that easily habituated. 7. Latencies of visual responses were usually between 100 and 200 ms. The latencies of the memory-contingent, expectation-contingent, and habituated visual responses tended to be longer than the others and tended to be more variable between trials. 8. Among auditory responsive neurons only a small proportion were related to the tasks. The response was greater to a contralateral sound. It was enhanced if the monkey used the sound as the cue for the future target location. 9. The results suggest that sensory responses of caudate neurons could be used to guide a subsequent sequence of learned behaviors by confirming predicted environmental states, renewing memory, or establishing a motor set.     

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.     

Distinct early and late subcomponents of the photic blink reflex: Response characteristics in patients with retrogeniculate lesions

To assess cortical contributions to the photic blink reflex, signal averaged electromyograms (EMG) were compared for responses to strobe flashes presented within the blind and sighted hemifields of 13 patients with occipital lobe lesions. Reflexes evoked by flashes within the scotoma were virtually identical to those evoked by flashes within the intact visual field. This suggests that both the early and late components of this reflex (R50 and R80, respectively) are mediated by subcortical structures that do not require, or benefit from, conscious visual processing. Additional findings included larger R80s at the eyelid contralateral to the lesion, regardless of stimulated hemifield. This presumably reflects the loss of a tonic descending influence of visual cortex onto the motor limb of the reflex arc. The R80 was also larger for stimuli activating the crossed (temporal hemifield) rather than the uncrossed (nasal hemifield) afferent pathway.     

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...     

Cortex controls multisensory depression in superior colliculus

Multisensory depression is a fundamental index of multisensory integration in superior colliculus (SC) neurons. It is initiated when one sensory stimulus (auditory) located outside its modality-specific receptive field degrades or eliminates the neuron's responses to another sensory stimulus (visual) presented within its modality-specific receptive field. The present experiments demonstrate that the capacity of SC neurons to engage in multisensory depression is strongly dependent on influences from two cortical areas (the anterior ectosylvian and rostral lateral suprasylvian sulci). When these cortices are deactivated, the ability of SC neurons to synthesize visual-auditory inputs in this way is compromised; multisensory responses are disinhibited, becoming more vigorous and in some cases indistinguishable from responses to the visual stimulus alone. Although obtaining a more robust multisensory SC response when cortex is nonfunctional than when it is functional may seem paradoxical, these data may help explain previous observations that the loss of these cortical influences permits visual orientation behavior in the presence of a normally disruptive auditory stimulus.     

Comparison of receptive-field organization of the superior colliculus in Siamese and normal cats

1. The superior colliculus has been studied in Siamese and normal cats by recording the responses of single tectal units to visual stimuli.

2. The retinotopic organization of the superior colliculus has been compared in the two breeds. In the normal cat, the contralateral half-field is represented in the central and caudal part of the colliculus, and a vertical strip of the ipsilateral half-field, 15-20° wide, is represented at the anterior tip. The Siamese cat superior colliculus receives an abnormally large projection from the ipsilateral half-field so that units with visual receptive fields which extend as far as 40° into the ipsilateral half-field can be found. The area of the tectal surface devoted to the representation of the ipsilateral half-field is about twice as large in Siamese cats as in normal cats. The enhanced representation of the ipsilateral half-field in Siamese cats is reflected in a displacement of the vertical meridian and the area centralis on the tectal surface.

3. The area centralis in the Siamese cat is located at about the same point on the tectal surface as would be occupied by a point in the visual field about 6-7° contralateral to the area centralis in the normal cat. The smallest receptive fields in both breeds are located near the area centralis. The size of the receptive field for a tectal unit seems to be determined by the retinal location of the receptive field and not by the absolute position of the unit on the tectal surface.

4. The receptive-field characteristics of tectal units show many similarities in the two breeds. The receptive fields of individual units consist of activating regions flanked by suppressive surrounds. Units respond well to stimuli of different shapes and orientation provided they are moving. The optimum stimulus for a given unit can be much smaller than the size of the activating region. About two thirds of the units studied in both breeds show directional selectivity. Most of the units studied in normal cats can be activated by stimulation of either eye, while in Siamese cats, 80% of the units studied can be driven only by the contralateral eye. A few monocularly driven units with two separated receptive fields have been observed in Siamese cats.

5. In the left tectum of both breeds, units respond well to left-to-right stimulus movement. The reverse situation obtains in the right tectum. In Siamese cats, units located at the anterior tip of the tectum with their receptive fields located in the visual half-field ipsilateral to the tectum under study respond better to stimulus movement toward the area centralis than away from it. The preferred direction for a tectal unit seems to be determined by its tectal location rather than by the location of its receptive field in the retina.

6. Visual cortex lesions in both breeds increase the responsiveness of tectal units to flashing spots and almost entirely remove the directional selectivity exhibited by tectal units, although units with asymmetric surrounds are still found. In normal cats, the lesions change the ocular dominance distribution, skewing it more strongly toward the contralateral eye. In Siamese cats, the ocular dominance distribution remains unchanged after a visual cortex lesion.

7. The squint commonly exhibited by Siamese cats is regarded as a compensation for the anomalous retinotectal topography. It is suggested that, in the absence of an adaptive modification, the anomalous retinotectal projection would lead to mislocalization in Siamese cats just as it does in frogs and hamsters whose retinotectal projection has been experimentally altered. The convergent strabismus which Siamese cats commonly exhibit may be a cure for the abnormal retinal projections rather than a disease.

     

Bilateral impact of unilateral visual cortex lesions on the superior colliculus

We examined the functional impact of a long-standing, unilateral primary visual cortex lesion on the superior colliculus (SC) using radiolabeled 2-deoxyglucose (2DG) as a marker of neural activity. In accord with known corticotectal connectivity and functional influence, 2DG uptake in the superficial layers of the ipsilesional SC was decreased. We also found a decrease in the superficial layers of the contralesional SC. These data suggest that modifications in activity in one SC can have a substantial influence on activity in its contralateral partner, and that processing in one visual hemifield does not occur independently of processing of signals in the opposite hemifield. The effects are not mediated by the contralateral hemisphere but are probably mediated by intercollicular circuitry.     

Neuronal organization underlying visually elicited prey orienting in the frog--I. Effects of various unilateral lesions

We have studied the effects on frog orienting behavior of three lesions: unilateral optic nerve section, unilateral tectal lobe ablation, and unilateral transverse hemisection of the neuraxis at a level just caudal to the optic tectum. Unilateral optic nerve section and unilateral tectal lobe ablation produce very similar deficits in visually elicited responses to prey items, an absence of responses for stimuli at locations within the monocular field of one eye. Unilateral hemisection, in contrast, results in abnormalities in visually elicited responses over a wider area, encompassing the entire ipsilateral visual hemifield. The hemisection deficit also differs in character from that following optic nerve section or tectal lesion. Within the affected hemifield, frogs do not fail to respond to stimuli but rather respond with abnormally directed movements. The movements, regardless of stimulus eccentricity on the horizontal, are always forwardly directed. While not varying with horizontal eccentricity, the movements do vary with stimulus elevation and distance. The variation with stimulus distance in the affected hemifield is somewhat different from that in the opposite hemifield. We conclude from the behavior that remains after hemisection lesions that there must exist bilateral descending tectofugal paths capable of triggering movements which vary with stimulus elevation and distance, and a crossed descending tectofugal path capable of triggering turns into one visual hemifield. That the deficit area is larger following a hemisection than following tectal lobe ablation indicates that the hemisection has affected the ability of both tectal lobes to trigger turns in one direction. A possible interpretation of this finding is that the lesion has interrupted not only the crossed descending tectofugal path from one tectal lobe but an uncrossed descending tectofugal path from the other. This hypothetical pathway as well as the others mentioned is incorporated in a model of the organization of the post-tectal circuitry involved in orienting.     

Position selectivity in scene- and object-responsive occipitotemporal regions

Complex visual scenes preferentially activate several areas of the human brain, including the parahippocampal place area (PPA), the retrosplenial complex (RSC), and the transverse occipital sulcus (TOS). The sensitivity of neurons in these regions to the retinal position of stimuli is unknown, but could provide insight into their roles in scene perception and navigation. To address this issue, we used functional magnetic resonance imaging (fMRI) to measure neural responses evoked by sequences of scenes and objects confined to either the left or right visual hemifields. We also measured the level of adaptation produced when stimuli were either presented first in one hemifield and then repeated in the opposite hemifield or repeated in the same hemifield. Although overall responses in the PPA, RSC, and TOS tended to be higher for contralateral stimuli than for ipsilateral stimuli, all three regions exhibited position-invariant adaptation, insofar as the magnitude of adaptation did not depend on whether stimuli were repeated in the same or opposite hemifields. In contrast, object-selective regions showed significantly greater adaptation when objects were repeated in the same hemifield. These results suggest that neuronal receptive fields (RFs) in scene-selective regions span the vertical meridian, whereas RFs in object-selective regions do not. The PPA, RSC, and TOS may support scene perception and navigation by maintaining stable representations of large-scale features of the visual environment that are insensitive to the shifts in retinal stimulation that occur frequently during natural vision.     

The influence of visual and auditory receptive field organization on multisensory integration in the superior colliculus

The spatial register of the different receptive fields of multisensory neurons in the superior colliculus (SC) plays a significant role in determining the responses of these neurons to cross-modal stimulus combinations. Spatially coincident visual-auditory stimuli fall within these overlapping receptive fields and generally produce response enhancements that exceed the individual modality-specific responses and can exceed their sum. Yet, in this context, it has not been clear how "spatial coincidence" is operationally defined. Given the large size of SC receptive fields, visual and auditory stimuli could be within their respective receptive fields even when there are substantial spatial disparities between them. Indeed, previous observations have raised the possibility that there may be a second level of determinism in how SC neurons deal with the relative spatial locations of within-field cross-modal stimuli; specifically, that multisensory response enhancements become progressively weaker as the within-field visual and auditory stimuli become increasingly disparate. While the present experiments demonstrated that SC multisensory neurons have heterogeneous receptive fields, and that the greatest number of impulses evoked were by stimuli that fell within the area of cross-modal receptive field overlap, they also indicate that there is no systematic relationship between cross-modal stimulus disparity and the magnitude of multisensory response enhancement. Thus, two within-field cross-modal stimuli produced the same proportionate change (i.e., multisensory response enhancement) when they were widely disparate as they did when they overlapped one another in space. These observations indicate that cross-modal spatial coincidence can be defined operationally by the borders of an SC neuron's receptive fields regardless of the size of those receptive fields and/or the absolute spatial disparity between within-field cross-modal stimuli. Electronic Publication     

Influence of the presentation of remote visual stimuli on visual responses of cat area 17 and lateral suprasylvian area

Summary Single units were recorded extracellularly from area 17 and lateral suprasylvian area (LSSA) in curarized cats. Visual stimuli, usually a 10 ° black spot, were introduced abruptly in the visual field remote from the discharge area of a neuron's receptive field and moved at a speed of about 30 °/sec. The effect of these remote stimuli (S₂) on the response to a restricted visual stimulus (S₁) crossing the discharge area was studied.It was found that most units in area 17 were not affected by the presentation of remote stimuli, the remainder being either slightly facilitated or slightly inhibited. In contrast the LSSA neurons were usually inhibited by the presentation of S₂: this effect was strong, was present in all classes of LSSA neurons and was independent of the relative directions of movement of S₁ and S₂.On the basis of these data and those previously obtained from the superior colliculus it is concluded that the way the extrageniculate centres respond to a stimulus abruptly introduced in the visual field is substantially different from that of the striate cortex. Only in the extrageniculate centres a new stimulus, besides exciting the neurons which correspond to the position of the stimulus in the field, concomitantly decreases the responses of neurons located in positions of the visual field remote from that stimulus. Possible behavioral implications of the findings are discussed.     

Spatial response properties of acoustically responsive neurons in the superior colliculus of the ferret: a map of auditory space

Extracellular single-unit recordings were made from auditory neurons in the superior colliculus of ferrets anesthetized with either a neuroleptic or a combination of barbiturate with paralysis. The response properties of these neurons were studied using white-noise bursts presented under free-field conditions in an anechoic chamber. Auditory neurons were found throughout the intermediate and deep layers of the superior colliculus. All neurons were spontaneously active, the rates of discharge varying from 0.1 to 61.1 spikes X s-1. Although the spontaneous discharge interspike-interval histograms for many units approximated to exponential distributions, the histograms of 44% had clear secondary peaks, indicating more than one preferred interval, and could not be modeled by a simple process. Most neurons (50%) responded only at stimulus onset, whereas 12% exhibited sustained discharges and 38% gave onset responses followed by a period of silence or reduced activity and then a period of elevated discharge, which was not apparently related to stimulus offset. Neurons with multipeaked response patterns were concentrated in the stratum griseum profundum. The latencies from arrival of the stimulus at the ear to the onset of neural activity ranged from 6 to 49 ms and decreased with increasing stimulus intensity. Although responsive to sounds over a large region of space, most neurons had clearly defined best positions at which the strongest response was obtained. The response declined as the speaker was moved away from this position, and nearly all units had peaked response profiles. The spatial tuning varied between different neurons, but most were more sharply tuned in elevation than in azimuth. Increasing the stimulus intensity did not, in general, alter the best positions of these neurons, but usually resulted in a broadening of the receptive fields, although other units became more sharply tuned. The best positions of auditory neurons varied systematically in azimuth from 20 degrees into the ipsilateral hemifield to 130 degrees into the contralateral hemifield as the electrode was moved from the rostrolateral to the caudomedial end of the superior colliculus. The best positions shifted in elevation along a rostromedial to caudolateral axis from 60 degrees above to 50 degrees below the visuoaural plane.(ABSTRACT TRUNCATED AT 400 WORDS)     

Assessment of brightness and color differences by neurons in the superior colliculus of the rabbit

The responses of 83 neurons in the rabbit superior colliculus to substitution of color stimuli of different brightnesses and black-and-white stimuli of different intensities were studied. Superior colliculus neurons were found to respond with initial and late phasic discharges (over the periods 50–90 msec and 120–300 msec from the moment of stimulus substitution respectively), along with prolonged tonic discharges whose spike frequencies depended on the intensity of the stimulus. Analysis of the phasic responses of the neurons allowed three groups of cells to be identified. One group of cells (25 of the cells studied, 30%), identified on the basis of early neuron responses, were specialized for detecting brightness differences between black-and-white and color stimuli of different intensities. The sensory spaces reconstructed on the basis of spike discharge frequencies in the early discharges of these neurons were achromatic and two-dimensional. Another group of neurons (16 of the cells studied, 19%) were mainly identified on analysis of late phasic discharges and had four-dimensional spaces with two color and two achromatic axes. The third group of cells (four neurons, 5%) had early discharges with two-dimensional achromatic sensory spaces and late discharges with four-dimensional spaces. It is suggested that reconstruction of the four-dimensional space requires processing of information from the visual cortex on color and intensity differences between stimuli. The sensory spaces of superior colliculus neurons reconstructed on the basis of phasic discharges essentially coincided with the sensory spaces of neurons in the visual cortex and lateral geniculate body and spaces obtained by analysis of the N85 component of visual evoked potentials in rabbits recorded using similar stimulation. This may support the vector coding principle in the visual analyzer. __________ Translated from Zhurnal Vysshei Nervnoi Deyatel'nosti imeni I. P. Pavlova, Vol. 57, No. 5, pp. 576–590, September–October, 2007.     

Primate pupillary light reflex: receptive field characteristics of pretectal luminance neurons

This study examined the response properties of luminance neurons found within the pretectal olivary nucleus (PON), which is the pretectal nucleus that mediates the primate pupillary light reflex. We recorded the activity of 121 single units in alert, behaving rhesus monkeys trained to fixate a back-projected laser spot while a luminance stimulus was presented. The change in the firing rate of luminance neurons was measured as a function of changes in the size, retinal illuminance, and position of the stimulus. We found that these neurons possessed large receptive fields, which were sufficiently distinct that they could be placed into three classes. Approximately 40% of the PON luminance neurons responded well to stimuli presented in either the contralateral or ipsilateral hemifield. These neurons were classified as "bilateral" neurons. In the primate, retinal projections to the pretectum and other retinorecipient nuclei are organized such that direct retinal input can only account for the contralateral hemifield responses of these neurons. Thus the representation of the ipsilateral hemifield in "bilateral" PON cells must result from input from a nonretinal source. Approximately 30% of PON neurons responded only to stimuli presented in the contralateral hemifield. These neurons were classified as "contralateral" neurons. Finally, approximately 30% of PON neurons responded to stimuli presented at or near the animal's fixation point. These neurons were classified as "macular" neurons. The mean firing rates of all classes of neurons increased with increases in stimulus size and luminance within their receptive fields. The thresholds and magnitude of these responses closely matched those that would be appropriate for mediating the pupillary light reflex. In summary, these results suggest that all three classes of PON neurons contribute to the behaviorally observed pupillomotor field characteristics in which stimuli at the macular produce substantially larger pupillary responses than more peripheral stimuli. The contributions of "bilateral" and "contralateral" cells account for pupillary responses evoked by peripheral changes in luminance, whereas the contributions of all three cell classes account for the larger pupillary responses evoked by stimuli in the central visual field.     

Excitotoxic lesions of the superior colliculus preferentially impact multisensory neurons and multisensory integration

The superior colliculus (SC) plays an important role in integrating visual, auditory and somatosensory information, and in guiding the orientation of the eyes, ears and head. Previously we have shown that cats with unilateral SC lesions showed a preferential loss of multisensory orientation behaviors for stimuli contralateral to the lesion. Surprisingly, this behavioral loss was seen even under circumstances where the SC lesion was far from complete. To assess the physiological changes induced by these lesions, we employed single unit electrophysiological methods to record from individual neurons in both the intact and damaged SC following behavioral testing in two animals. In the damaged SC of these animals, multisensory neurons were preferentially reduced in incidence, comprising less than 25% of the sensory-responsive population (as compared with 49% on the control side). In those multisensory neurons that remained following the lesion, receptive fields were nearly twofold larger, and less than 25% showed normal patterns of multisensory integration, with those that did being found in areas outside of the lesion. These results strongly suggest that the multisensory behavioral deficits seen following SC lesions are the combined result of a loss of multisensory neurons and a loss of multisensory integration in those neurons that remain.     

Deficits of visual attention and saccadic eye movements after lesions of parietooccipital cortex in monkeys

1. Visual attention is often profoundly disturbed in humans after damage to the cortex of the posterior parietal lobe, particularly of the minor hemisphere, with some patients being totally unaware of visual stimuli in the hemifield of extrapersonal space contralateral to the cortical damage. This severe form of visual inattention is usually called contralateral neglect and has occasionally been reported following posterior parietal lesions in monkeys. However, in monkeys, only qualitative observations have been published and those reports are not in agreement concerning the severity of the deficit. The present experiments were designed to measure quantitatively the amount of disruption of selective visual attention which is produced by lesions of posterior parietal and parietooccipital cortical lesions in monkeys. 2. Five monkeys were trained to visually fixate and follow with their gaze a small visual stimulus as it suddenly moved varying distances (8, 16, or 24 degrees) from the midline into the left or right visual hemifields. Two animals then received a unilateral cortical lesion limited to the inferior parietal lobule (IPL). Three animals received unilateral lesions which included both the inferior parietal lobule and a portion of adjacent dorsal prestriate cortex (IPL-PS). 3. Visual inattention is commonly divided into two levels of severity. The more severe form, contralateral neglect, is the complete absence of behavioral response to a stimulus in the visual field contralateral to hemisphere damage. The less severe deficit, usually called visual extinction, is a tendency to ignore the contralateral of two visual stimuli when they appear simultaneously and symmetrically placed with respect to the center of the subject's surroundings. The five monkeys in this study were tested on a stimulus paradigm which simultaneously measured the severity of visual neglect and also the amount and duration of visual extinction which were produced by the cortical lesions. 4. All monkeys displayed contralateral visual extinction after unilateral posterior parietal or parietooccipital lesions. Three of the five monkeys showed a reversal of the visual extinction after a second, symmetrical lesion was placed in the opposite hemisphere. No monkey showed evidence of full-blown contralateral neglect after lesions limited to the parietooccipital cortex, either in the formal testing situation or during informal neurological examinations. The severity of the observed inattention did not appear to be related to the size of the cortical lesions.(ABSTRACT TRUNCATED AT 400 WORDS)