Aglai P. B. Sousa

Visual receptive fields of units in the pulvinar of cebus monkey.

Visually driven units, isolated in the ventrolateral group -- Pv1g (109) and in subnucleus Pmu (33) of the pulvinar of the cebus monkey, were studied in acute and chronic preparations under nitrous oxide N2O/O2 anesthesia during periods of EEG arousal. Taking into consideration the response properties to static or moving stimuli as well as the organization of the receptive fields, units isolated in the pulvinar were subdivided into 8 groups. Units displaying dynamic properties predominate over static ones. Static units were classified in 3 groups; of these, one showed uniform receptive fields; the remaining two groups, with non-uniform RFs, were further subdivided in terms of orientation selectivity. By testing for directional sensitivity, organization of the RFs and orientation selectivity, the dynamic units were divided in 5 groups. Among these there was a predominance of directional units, displaying uniform RFs and showing orientation selectivity. Although the receptive fields would extend into the ipsilateral hemifield (up to 10 degrees), their centers were always located in the contralateral visual hemifield. Binocularly driven units predominate in both static and dynamic categories.

Visuotopic organization of the Cebus pulvinar: A double representation of the contralateral hemifield

The projection of the visual field in the pulvinar nucleus was studied in 17 Cebus monkeys using electrophysiological techniques. Visual space is represented in two regions of the pulvinar; (1) the ventrolateral group, Pvlg, comprising nuclei P delta, P delta, P gamma, P eta and P mu 1; and (2) P mu. In the first group, which corresponds to the pulvinar inferior and ventral part of the pulvinar lateralis, we observed a greater respresentation of the central part of the visual field. Approximately 58% of the volume of the ventrolateral group is concerned with the visual space within 10 degrees of the fovea. This portion of the visual field is represented at its lateral aspects, mainly close to the level of the caudal pole of the lateral geniculate nucleus (LGN). Projection of the vertical meridian runs along its lateral border while that of the horizontal one is found running from the dorsal third of the LGNs hilus to the medial border of the ventro-lateral group. The lower quadrant is represented at its dorsal portion while the upper quadrant is represented at the ventral one. In Pmu the representation is rotated 90 degrees clockwise around the rostrocaudal axis: the vertical meridian is found at the ventromedial border of this nucleus. Thus, the lower quadrant is represented at the later portion of Pmu and the upper at its medial portion. Both projections are restricted to the contralateral hemifield.

Single unit response types in the pulvinar of the Cebus monkey to multisensory stimulation.

Response to sensory stimulation was studied in 162 neurons in the pulvinar of Cebus monkeys in 4 acute and 5 chronic preparations. Two basic response patterns were observed: type I responses, similar to those obtained in primary relay centers, were only observed after visual stimulation. Type II responses were obtained after stimulation of more than one sensory modality. Characteristically these responses presented fatigue and habituation. Temporal relationship between stimulus and response was not as clear as in type I responses, afterdischarge frequently occurred. Taking these response types into consideration two groups of units were identified in the pulvinar. Units of group A (91 neurons) showed type I response to visual stimulation. For these units receptive fields similar to those found in other regions of visual projection could be defined. As a rule units of group A displayed type II responses to other sensory modalities. Units of group B (71) did not display type I responses; they always responded to visual, somatic, auditory and olfactory stimuli with type II responses. They could be activated by a single sensory modality (B, unimodal) or by more than one sensory modality (B, multimodal).

Visual Responses Outside the Classical Receptive Field in Primate Striate Cortex: A Possible Correlate of Perceptual Completion

In this chapter we address the dynamic properties of single neurons in primary visual cortex (V1) that can be related to the completion phenomenon. This phenomenon accounts for the perceptual filling in of blind regions in the visual field, such as the optic disk, small retinal lesions (Bender and Teuber, 1946; Sergent, 1988), or lesions in the geniculo-striatal projection system (Poppel, 1985, 1986). We propose that completion is achieved by dynamic changes in receptive field size of cells in V1.

Three streams of visual information processing in V2 of Cebus monkey.

Gattass and collaborators (Gattass R, Rosa MGP, Souza APB, Piñon MCG, Neuenschwander S [1990a] Braz J Med Biol Res 23:375–393) proposed that the dorsal stream of visual processing, as defined by Ungerleider and Mishkin (Ungerleider LG, Mishkin M [1982] In: Ingle DJ, Goodale MA, Mansfield RJW, editors. Analysis of visual behavior. Cambridge: Massachusetts Institute of Technology. p 549–586), can be subdivided into dorsolateral and dorsomedial streams, and suggested that they may be involved in different aspects of the processing of motion and spatial perception, respectively. The goal of the present study was to provide additional evidence for this hypothesis by using cytochrome oxidase immunohistochemistry combined with retrograde tracing techniques. In Old World monkeys, the locations of visual area 4 (V4; ventral stream) and middle temporal area (MT; dorsal stream) projecting neurons in V2 supports the hypothesis that the cytochrome oxidase (CytOx)–rich thin stripes and the CytOx‐poor interstripes are associated with the ventral stream, and that the CytOx‐rich thick stripes belong to the dorsal stream. In this study we describe, in the New World monkey Cebus, the distribution of retrogradely labeled cells in V2 relative to the CytOx compartments after fluorescent tracers were placed in areas V4, MT, and the parietooccipital area (PO). We found PO‐projecting neurons in CytOx‐rich thick stripes and CytOx‐poor interstripes in V2, whereas MT‐projecting neurons appeared almost exclusively in thick stripes. In contrast, V4‐projecting neurons were located mostly in CytOx‐poor interstripes and CytOx‐rich thin stripes. In addition, V4‐ and MT‐projecting neurons were located mainly in supragranular layers, whereas PO‐projecting neurons were located in supragranular and infragranular layers. These results support the hypothesis for the existence of three distinct streams of visual processing: ventral (including V4), dorsolateral (including MT), and dorsomedial (including PO). J. Comp. Neurol. 466:104–118, 2003.