Striate cortex-superior colliculus projection in squirrel monkey

Visual Functions of the Primate Superior Colliculus

The superior colliculus (SC) is a subcortical brain structure that is relevant for sensation, cognition, and action. In nonhuman primates, a rich history of studies has provided unprecedented detail about this structure's role in controlling orienting behaviors; as a result, the primate SC has become primarily regarded as a motor control structure. However, as in other species, the primate SC is also a highly visual structure: A fraction of its inputs is retinal and complemented by inputs from visual cortical areas, including the primary visual cortex. Motivated by this, recent investigations are revealing the rich visual pattern analysis capabilities of the primate SC, placing this structure in an ideal position to guide orienting movements. The anatomical proximity of the primate SC to both early visual inputs and final motor control apparatuses, as well as its ascending feedback projections to the cortex, affirms an important role for this structure in active perception.

Active vision at the foveal scale in the primate superior colliculus

The primate superior colliculus (SC) has recently been shown to possess both a large foveal representation as well as a varied visual processing repertoire. This structure is also known to contribute to eye movement generation. Here, we describe our current understanding of how SC visual and movement-related signals interact within the realm of small eye movements associated with the foveal scale of visuomotor behavior. Within the SC's foveal representation, there is a full spectrum of visual, visual-motor, and motor-related discharge for fixational eye movements. Moreover, a substantial number of neurons only emit movement-related discharge when microsaccades are visually guided, but not when similar movements are generated toward a blank. This represents a particularly striking example of integrating vision and action at the foveal scale. Beyond that, SC visual responses themselves are strongly modulated, and in multiple ways, by the occurrence of small eye movements. Intriguingly, this impact can extend to eccentricities well beyond the fovea, causing both sensitivity enhancement and suppression in the periphery. Because of large foveal magnification of neural tissue, such long-range eccentricity effects are neurally warped into smaller differences in anatomical space, providing a structural means for linking peripheral and foveal visual modulations around fixational eye movements. Finally, even the retinal-image visual flows associated with tiny fixational eye movements are signaled fairly faithfully by peripheral SC neurons with relatively large receptive fields. These results demonstrate how studying active vision at the foveal scale represents an opportunity for understanding primate vision during natural behaviors involving ever-present foveating eye movements.NEW & NOTEWORTHY The primate superior colliculus (SC) is ideally suited for active vision at the foveal scale: it enables detailed foveal visual analysis by accurately driving small eye movements, and it also possesses a visual processing machinery that is sensitive to active eye movement behavior. Studying active vision at the foveal scale in the primate SC is informative for broader aspects of active perception, including the overt and covert processing of peripheral extra-foveal visual scene locations.

Does the visual system of the flying fox resemble that of primates? The distribution of calcium-binding proteins in the primary visual pathway of Pteropus poliocephalus

It has been proposed that flying foxes and echolocating bats evolved independently from early mammalian ancestors in such a way that flying foxes form one of the suborders most closely related to primates. A major piece of evidence offered in support of a flying fox-primate link is the highly developed visual system of flying foxes, which is theorized to be primate-like in several different ways. Because the calcium-binding proteins parvalbumin (PV) and calbindin (CB) show distinct and consistent distributions in the primate visual system, the distribution of these same proteins was examined in the flying fox (Pteropus poliocephalus) visual system. Standard immunocytochemical techniques reveal that PV labeling within the lateral geniculate nucleus (LGN) of the flying fox is sparse, with clearly labeled cells located only within layer 1, adjacent to the optic tract. CB labeling in the LGN is profuse, with cells labeled in all layers throughout the nucleus. Double labeling reveals that all PV+ cells also contain CB, and that these cells are among the largest in the LGN. In primary visual cortex (V1) PV and CB label different classes of non-pyramidal neurons. PV+ cells are found in all cortical layers, although labeled cells are found only rarely in layer I. CB+ cells are found primarily in layers II and III. The density of PV+ neuropil correlates with the density of cytochrome oxidase staining; however, no CO+ or PV+ or CB+ patches or blobs are found in V1. These results show that the distribution of calcium-binding proteins in the flying fox LGN is unlike that found in primates, in which antibodies for PV and CB label specific separate populations of relay cells that exist in different layers. Indeed, the pattern of calcium-binding protein distribution in the flying fox LGN is different from that reported in any other terrestrial mammal. Within V1 no PV+ patches, CO blobs, or patchy distribution of CB+ neuropil that might reveal interblobs characteristic of primate V1 are found; however, PV and CB are found in separate populations of non-pyramidal neurons. The types of V1 cells labeled with antibodies to PV and CB in all mammals examined including the flying fox suggest that the similarities in the cellular distribution of these proteins in cortex reflect the fact that this feature is common to all mammals.

Somatostatin (SRIF)-like immunoreactivity in subcortical and cortical visual centers of the rat

The distribution of neuronal elements containing immunoreactive somatostatin (I-SRIF) in the rat central visual pathway was examined by light-microscopic immunocytochemistry. These studies were concerned with the location and morphology of neurons and innervated cells and the distribution of fiber and terminal plexuses in the primary visual cortex (area 17), visual association areas 18 and 18a, the superior colliculus, the lateral geniculate nucleus, and the pretectum. In the superior colliculus, I-SRIF-containing fibers and perikarya were distributed predominantly in the superficial, or visual, layers; these elements were moderately dense and occupied the entire mediolateral extent of these layers. In the intermediate and deep layers, immunoreactive neurons were widely scattered, and fibers were located mainly in the medial third. Immunoreactive cell populations in the superior colliculus included small bipolar neurons with fusiform perikarya and multipolar neurons with round to ovoid perikarya. In the pretectum, the peptide was demonstrable in large and small multipolar neurons of the nucleus of the optic tract and in the posterior and olivary pretectal nuclei. I-SRIF-containing neurons were also present in the nucleus of the posterior commissure, the nucleus of Edinger-Westphal, and the ventral division of the lateral geniculate nucleus. In the visual cortex, the peptide was present in all layers and in a variety of morphologically defined cell populations, including some which are presumed excitatory (pyramidal and bipolar cells) and others which are presumed inhibitory (bitufted and stellate cells). Our data suggest that somatostatin is involved in visual and visuomotor reflex pathways and in the horizontal optokinetic nystagmus reflex pathway. These results provide a foundation for further studies to evaluate the role of this peptide in visual processes.

Interactions between cortical and subcortical visual areas: evidence from human commissurotomy patients

Two commissurotomy patients performed an oculomotor task that required integrating visual information from the two hemifields. In marked contrast to previous "split-brain" findings, visual information from both hemifields was available for oculomotor control and perceptual function, provided that accurate performance depended on relatively crude visual discrimination. When finer spatial resolution was required, interfield performance declined to chance levels. The failure of a subject with occipital damage to accurately localize comparable stimuli in her blind field implies that cross-integration of the visual fields in commissurotomy patients requires interactions between cortical and subcortical visual areas.

Indirect visual cortical input to the deep layers of the hamster's superior colliculus via the basal ganglia

Anterograde and retrograde tracing techniques were employed to delineate the organization of a visual cortical input to the deep layers of the hamster's superior colliculus which may be mediated by links in the striatum and substantia nigra. Autoradiographic experiments showed that areas 17, 18a, and the cortex medial to area 17 (areas 18b and 29) all projected to the dorsocaudal part of the ipsilateral striatum. This projection was organized so that the rostrocaudal axis of the visual cortex was represented along the antero posterior axis of the striatum. Large posterior neocortical injections which included all of these areas also revealed a weak, crossed corticostriatal pathway. Such injections also demonstrated clear discontinuities in the terminal distribution of the visual corticostriatal projection, similar to those which have been noted after injections of tracers into the motor and premotor cortices. Retrograde tracing experiments showed that the cells of origin of the visual cortical projections to the striatum were medium-sized pyramidal neurons located primarily in the upper portion of lamina V. Anterograde transport of [3H]-leucine and HRP showed that the portion of the striatum heavily innervated by the visual cortex projected to the part of substantial nigra, pars reticulata immediately adjacent to the cerebral peduncle. Injections in the rostral striatum labeled more medial portions of this nucleus. The cells of origin of the striatonigral pathway measured between 13 and 20 micrometers in diameter and they were located primarily in the dorsal and lateral parts of the striatum. Anterograde tracing after substantia nigra, pars reticulata injections revealed a projection to both superior colliculi. The uncrossed pathway terminated primarily as a series of patches throughout the mediolateral and rostrocaudal extents of the lower stratum griseum intermediale and stratum album intermedium. Labeling was also visible in the lateral portion of the stratum griseum profundum. The crossed nigrotectal pathway terminated primarily in the rostrolateral stratum griseum profundum. The cells of origin of the nigrocollicular pathway were fusiform or multipolar cells and were located primarily adjacent to the cerebral peduncle throughout the rostral half of the substantias nigra, par reticulata.

Functional influences of the visuocortical projection on superior colliculus neurons in the rabbit

The effects of electrical stimulation of the visual cortex on superior colliculus neurons were investigated in adult Dutch-belted rabbits. Single units were recorded in the superior colliculus and classified as to receptive field type. Stimulation of the ipsilateral visual cortex activated 29% of the recorded superior colliculus units. No units were driven by stimulation of the contralateral visual cortex. Comparison of the relative proportional distributions of cortically driven and not driven cells having various receptive field types revealed an over-representation of driven motion type cells. The excitatory influence of the visuocortical projection to the superior colliculus in the rabbit shows a preference for neurons responsive to moving visual stimuli.

Organization of somatosensory input to the deep collicular laminae in hamster

Retrograde transport of horseradish peroxidase (HRP) was used to delineate the sources of somatosensory input to the hamster's superior colliculus. Cells in the ipsilateral somatosensory cortex and contralateral dorsal horn of the spinal cord, dorsal column nuclei, lateral cervical nucleus, internal basilar nucleus, nucleus of the spinal trigeminal tract and deep layers of the superior colliculus were labeled following HRP injections centered in the deep tectal laminae. The response characteristics of somatosensory corticotectal, spinotectal and intertectal neurons were investigated with extracellular single unit recording methods and, with the exception of the fact that the receptive fields of corticotectal and spinotectal neurons were consistently smaller than those of cells recorded in the colliculus, the response characteristics of these neurons were quite similar to those of somatosensory neurons in the deep layers of the tectum. Lesions of the somatosensory cortex or dorsal half of the spinal cord were also combined with single unit recording in the colliculus to determine whether or not such damage altered the incidence and/or response characteristics of deep layer somatosensory cells. These lesions had no appreciable effect upon the functional organization of the deep tectal laminae. The implications of these results with regard to the convergence of visual and somatosensory information in the tectum are discussed.

Functional properties of the corticotectal projection in the golden hamster

Approximately 31% of the cells recorded in the hamster's superior colliculus could be activated by stimulation of the ipsilateral primary visual cortex. While cortically activated cells were encountered in all laminae of the colliculus where visual cells were isolated, the highest probability of driving visual cells was observed in the deeper laminae, that is, those ventral to the stratum opticum. Response latency, jitter (latency variability), latency shifts as a function of shock intensity, thresholds, and spike numbers did not vary as a function of depth in the colliculus. There was a clear correspondence between the visual fields of the best cortical stimulus points and the receptive fields of cortically activated cells recorded in the superficial laminae of the colliculus. However, there was considerably less retinotopic fidelity for the cortical areas from which cells isolated in the deeper laminae could be driven. This suggests a greater degree of convergence from relatively widespread cortical regions upon visual cells of the deeper laminae. The visal organization) of the cortically activated cells did not differ appreciably from the overall sample of visual cells recorded in the colliculus. Only 3 of the 159 cells tested were driven by stimulation of the contralateral visual cortex and two of these were responsive only at very long latencies.

Rod and cone sensitivity in destriate monkeys

Photopic and scotopic spectral sensitivity of rhesus monkeys was determined before and after complete removal of the striate cortex. The monkeys were required to choose between a white and a series of monochromatic stimuli distributed throughout the visible spectrum. A modified method of limits was used to determine the psychophysical point of subjective equality at which the colored and white lights were perceived as being equally bright. The preoperative results indicated that the method of testing was appropriate to determine spectral sensitivity since the curves obtained compared favorably to the theoretical sensitivity curves. Postoperatively, the scotopic sensitivity curve was normal whereas the photopic curve was completely displaced towards the scotopic curve. The results as indicating that cone information is processed by the geniculo-striate visual system whereas the extra-striate structures receive their input mainly from the rod receptors of the retina.