No abstract

No abstract

The primary visual cortex (V1) receives its driving input from the eyes via the lateral geniculate nucleus (LGN) of the thalamus. The lateral pulvinar nucleus of the thalamus also projects to V1 but this input is little understood. We manipulated lateral pulvinar neural activity and assessed the effect on supra-granular layers of V1 that project to higher visual cortex. Reversibly inactivating lateral pulvinar prevented supra-granular V1 neurons from responding to visual stimulation. Reversible, focal excitation of lateral pulvinar receptive fields increased 4-fold the visual responses in coincident V1 receptive fields and shifted partially overlapping V1 receptive fields towards the center of excitation. V1 responses to regions surrounding the excited lateral pulvinar receptive fields were suppressed. LGN responses were unaffected by these lateral pulvinar manipulations. Excitation of lateral pulvinar after LGN lesion activated supra-granular layer V1 neurons. Thus, lateral pulvinar is able to powerfully control and gate information outflow from V1.

Rosa, Marcello G. P., Vivien A. Casagrande, Todd Preuss, and Jon H. Kaas. Visual field representation in striate and prestriate cortices of a prosimian primate ( Galago garnetti). J. Neurophysiol. 77: 3193–3217, 1997. Microelectrode mapping techniques were used to study the visuotopic organization of the first and second visual areas (V1 and V2, respectively) in anesthetized Galago garnetti, a lorisiform prosimian primate. 1) V1 occupies ∼200 mm2 of cortex, and is pear shaped, rather than elliptical as in simian primates. Neurons in V1 form a continuous (1st-order) representation of the visual field, with the vertical meridian forming most of its perimeter. The representation of the horizontal meridian divides V1 into nearly equal sectors representing the upper quadrant ventrally, and the lower quadrant dorsally. 2) The emphasis on representation of central vision is less marked in Galago than in simian primates, both diurnal and nocturnal. The decay of cortical magnification factor with increasing eccentricity is almost exactly counterbalanced by an increase in average receptive field size, such that a point anywhere in the visual field is represented by a compartment of similar diameter in V1. 3) Although most of the cortex surrounding V1 corresponds to V2, one-quarter of the perimeter of V1 is formed by agranular cortex within the rostral calcarine sulcus, including area prostriata. Although under our recording conditions virtually every recording site in V2 yielded visually responsive cells, only a minority of those in area prostriata revealed such responses. 4) V2 forms a cortical belt of variable width, being narrowest (∼1 mm) in the representation of the area centralis and widest (2.5–3 mm) in the representation of the midperiphery (>20° eccentricity) of the visual field. V2 forms a second-order representation of the visual field, with the area centralis being represented laterally and the visual field periphery medially, near the calcarine sulcus. Unlike in simians, the line of field discontinuity in Galago V2 does not exactly coincide with the horizontal meridian: a portion of the lower quadrant immediately adjacent to the horizontal meridian is represented at the rostral border of ventral V2, instead of in dorsal V2. Despite the absence of cytochrome oxidase stripes, the visual field map in Galago V2 resembles the ones described in simians in that the magnification factor is anisotropic. 5) Receptive field progressions in cortex rostral to dorsal V2 suggest the presence of a homologue of the dorsomedial area, including representations of both quadrants of the visual field. These results indicate that many aspects of organization of V1 and V2 in simian primates are shared with lorisiform prosimians, and are therefore likely to have been present in the last common ancestor of living primates. However, some aspects of organization of the caudal visual areas in Galago are intermediate between nonprimates and simian primates, reflecting either an intermediate stage of differentiation or adaptations to a nocturnal niche. These include the shape and the small size of V1 and V2, the modest degree of emphasis on central visual field representation, and the relatively large area prostriata.

Visual information reaching striate cortex comes from parallel pathways, and the information is organized, or processed, by the layers and columns of striate cortex. To better understand how this is accomplished anatomically, we asked whether parallel pathways originating in the lateral geniculate nucleus (LGN), and terminating separately in layer IV, remain separate in layer Im of macaque monkeys. Layer m is of interest since it may play a special role in color and form vision but not in analysis ofvisual motion. The chieffinding was that cells in "blobs" of layer m that stain densely for cytochrome oxidase receive indirect input, via layer IVC, from both LGN magnocellular (M) and parvocellular (P) cells. This is important because the P and M pathways may represent color/form and motion-processing channels, respectively. Interblob cells receive indirect input, via layers IVC and IVA, from the LGN P cells. Also, as suggested by others, our data demonstrate that layer HI can be subdivided. The bottom tier, layer ITB, receives direct projections from all cortical layers.Output from layer HMB appears to remain intrinsic to striate cortex. In contrast, the top tier, layer IlA, receives projections from layer ITB as well as from layers IVA, IVB (blobs only), and V, but it receives no direct projections from LGN recipient layers IVC and VI. Unlike layer hUB, the output of layer HIA reaches extrastriate areas. Thus, impulses arriving from parallel LGN pathways may be recombined through serial stages in striate cortex to produce a set of parallel pathways that are qualitatively different from the original LGN set.The early stages of the primate visual system are characterized by the creation and segregation of functionally distinct pathways that work independently of, but parallel to, one another to provide a uniform perception of visual space (see refs. 1-6). In macaque monkey, three visual pathways have been described that arise from morphologically distinct cells located in the retina. These cells project to different layers of the lateral geniculate nucleus (LGN): the magnocellular (M), the parvocellular (P), and the interlaminar or S layers (7). It has been proposed that segregation of visual pathways in the LGN is preserved at higher levels in the visual system in separate layers and/or columns of primary visual cortex (striate cortex) and in separate hierarchies of extrastriate visual areas. According to one view (3), visual signals from the M LGN cells project via striate cortical layer IVCa to layer IVB and then out of the striate cortex. This pathway eventually terminates in areas within superior temporal and parietal cortex that are concerned with visual motion and orientation in space, respectively. Visual signals from the P pathway project to layer IVCp where they are split and conveyed to layer II/III, either to the cytochrome oxidaserich patches known as blobs, or to the lighter-staining spaces between these zones, the interblobs (8). These P pathways eventually contribute to color and form vision...