Visuotopic organization of bush baby primary visual cortex (V1) revealed by optical imaging

Abstract

 

The bush baby (Galago garnetti) is a small nocturnal prosimian primate with a smooth cortex. In this study, we used optical imaging to examine the visuotopic organization of V1, taking advantage of the fact that a significant portion of V1 is available to imaging on the dorsal and lateral surface in this species. For visual mapping, topographically limited horizontal and vertical grating stimuli were presented monocularly either within 2 rectangular windows or 2 patches at eccentricities ranging from 0 to 15 . Shifting the mapping stimuli by as little as 1 produced discrete shifts in activation foci in cortex. The visuotopic organization of bush baby V1 was similar to that previously reported based upon microelectrode mapping (Rosa et al., 1997). V1 shows a continuous representation of the visual field with the vertical meridian marking the V1/V2 border and the horizontal meridian bisecting V1. The cortical magnification factor (CMF) was calculated as the average mm distance in cortex for 1 deg of visual space. CMF systematically decreased with increasing eccentricity: CMF = −0.11 * eccentricity + 1.78 (R= 0.67, P< 0.0001), but was less steep than reported in macaque monkeys and did not show the vertical versus horizontal meridian anisotropy reported for macaque monkeys (Van Essen et al., 1984). The relationship was comparable to that described by Rosa et al. (1997). Taken together, these results indicate that: 1) the organization of bush baby V1 is analogous to that of other primates and 2) optical imaging can usefully be applied as an efficient and high resolution visual mapping tool.