Visual area MT in the Cebus monkey: Location, visuotopic organization, and variability

Abstract

The representation of the visual field in the dorsal portion of the superior temporal sulcus (ST) was studied by multiunit recordings in eight Cebus apella, anesthetized with N2O and immobilized with pancuronium bromide, in repeated recording sessions. On the basis of visuotopic organization, myeloarchitecture, and receptive field size, area MT was distinguished from its neighboring areas. MT is an oval area of about 70 mm2 located mainly in the posterior bank of the superior temporal sulcus. It contains a visuotopically organized representation of at least the binocular visual field. The representation of the vertical meridian forms the dorsolateral, lateral, and ventrolateral borders of MT and that of the horizontal meridian runs across the posterior bank of ST. The fovea is represented at the lateralmost portion of MT, while the retinal periphery is represented medially. The representation of the central visual field is magnified relative to that of the periphery in MT. The cortical magnification factor in MT decreases with increasing eccentricity following a negative power function. Receptive field size increases with increasing eccentricity. A method to evaluate the scatter of receptive field position in multiunit recordings based on the inverse of the magnification factor is described. In MT, multiunit receptive field scatter increases with increasing eccentricity. As shown by the Heidenhain-Woelcke method, MT is coextensive with two myeloarchitectonically distinct zones: one heavily myelinated, located in the posterior bank of ST, and another, less myelinated, located at the junction of the posterior bank with the anterior bank of ST. At least three additional visual zones surround MT: DZ, MST, and FST. The areas of the dorsal portion of the superior temporal sulcus in the diurnal New World monkey Cebus are comparable to those described for the diurnal Old World monkey, Macaca. This observation suggests that these areas are ancestral characters of the simian lineage and that the differences observed in the owl monkey may be secondary adaptations to a nocturnal ecological niche.

Electrophysiological Imaging of Functional Architecture in the Cortical Middle Temporal Visual Area ofCebus apellaMonkey

“…Alternate sections were stained for myelin by the Gallyas (1979) method or by using a Nissl stain for cell bodies. The boundaries of area MT were determined on the basis of characteristic myeloarchitecture (Ungerleider and Mishkin, 1979;Gattass and Gross, 1981;Van Essen et al, 1981;Fiorani et al, 1989). Microelectrode tracks were reconstructed from the positions of electrolytic lesions, which were made at known points along each penetration (generally at the end), and using stereotaxic coordinates.…”

Two-dimensional map of direction selectivity in cortical visual area MT of Cebus monkey

“…The definitions of receptive field size, cortical magnification factor and point image size (Gattass et al 1990 for review) are useful to define the maps of visual topography of an area. MT in Cebus has point images of about 1 mm in diameter, virtually invariant with eccentricity (Fiorani et al 1989). Figure 6 shows a representation of the visual topography of MT in Cebus and its relation to circular arrangements (pinwheels) of direction selectivity and to point image sizes.…”

“…Area MT in Cebus is homologous to area MT of Old World macaque monkey (Fiorani et al 1989, Gattass andGross 1981). In Cebus, MT is an oval area mainly located in the posterior bank of the dorsal portion of the superior temporal sulcus.…”

Analysis of Retinotopic Maps in Extrastriate Cortex

“…Double contour maps have often been presented in analyses of retinotopy (Allman and Kaas, 1971;Wagor et al, 1980;Desimone and Ungerleider, 1986;Fiorani et al, 1989;Rosa et al, 1993) Yet these have rarely been explicit!)' derived by quantitatively interpolating and contouring the data from an individual case.…”

“…The region of cortex involved, however, is large, and there is substantial variability among species and among individual animals of a species. Even well-defined areas like primary visual cortex (VI) and the middle temporal area (MT) show variability in their location, shape, size, and appearance (on MT in New World monkeys, see, e.g., Tootell et al, 1985;Fiorani et al, 1989). Since the majority of visual areas are not graced with the convenient suite of easily distinguishable features that characterize VI and MT (e.g., prominent myeloarchitectonic borders, nearly complete, not-too-distorted, topological maps of the hemiretina), it has proved quite difficult to define their boundaries convincingly.…”