Optical imaging of visually evoked responses in prosimian primates reveals conserved features of the middle temporal visual area

Abstract

Optical imaging of intrinsic cortical responses to visual stimuli was used to characterize the organization of the middle temporal visual area (MT) of a prosimian primate, the bush baby (Otolemur garnetti). Stimulation with moving gratings revealed a patchwork of oval-like domains in MT. These orientation domains could, in turn, be subdivided into zones selective to directional movements that were mainly orthogonal to the preferred orientation. Similar, but not identical, zones were activated by movements of random dots in the preferred direction. Orientation domains shifted in preference systematically either around a center to form pinwheels or as gradual linear shifts. Stimuli presented in different portions of the visual field demonstrated a global representation of visual space in MT. As optical imaging has revealed similar features in MT of New World monkeys, MT appears to have retained these basic features of organization for at least the 60 million years since the divergence of prosimian and simian primates.

Unequal representation of cardinal vs. oblique orientations in the middle temporal visual area

“…Moreover, the MT isoorientation domains consisted of subdomains preferring motion in one direction and the opposite direction (17, 18). These results are similar to those described in previous studies of bush baby and owl monkey MT (17,18,24).…”

“…Moreover, the MT isoorientation domains consisted of subdomains preferring motion in one direction and the opposite direction (17, 18). These results are similar to those described in previous studies of bush baby and owl monkey MT (17,18,24).To examine whether MT neurons proportionately or disproportionately represent cardinal and oblique orientations, we mapped the amount of MT cortical space devoted to representing different orientations. Specifically, in the orientation preference maps, we examined the distributions of pixels signifying different orientations and assessed the overall relationship between preferred orientations and the amount of cortical space that represented these orientations.…”

“…The average isoorientation domain sizes in MT [0.136 Ϯ 0.075 (mean Ϯ SD) mm 2 , n ϭ 48, measured from three cases], however, are much larger than those in V1 (0.076 Ϯ 0.046 mm 2 ) of owl monkeys (23). Moreover, the MT isoorientation domains consisted of subdomains preferring motion in one direction and the opposite direction (17,18). These results are similar to those described in previous studies of bush baby and owl monkey MT (17,18,24).…”

“…Because of a preponderance of direction-selective neurons, MT is thought to be important to the processing of visual motion (14). Given the columnar arrangement of orientation-and direction-of-motionselective cells in MT (17,18), a possible neural mechanism for the oblique effect in motion perception and discrimination could lie in having more cortical space devoted to direction-of-motion-sensitive neurons representing horizontal and vertical orientations rather than oblique orientations in MT.…”

“…Yet there are reasons for believing that orientation anisotropies might be found in other visual cortical areas, especially the middle temporal visual area (MT; also known as V5), because MT receives direct and indirect inputs from V1, and there is a very high proportion of MT neurons that are selective for the direction of motion and the orientation of moving gratings (14). Neurons of similar orientation or direction-of-motion selectivity are clustered into functional columns in MT (15)(16)(17)(18). In addition, an oblique effect has been observed in humans for the perception of objects moving in cardinal vs. oblique directions (7,(19)(20)(21)(22).…”