Using noninvasive functional magnetic resonance imaging (fMRI) techniques, we analyzed the responses in human area MT with regard to visual motion, color, and luminance contrast sensitivity, and retinotopy. As in previous PET studies, we found that area MT responded selectively to moving (compared to stationary) stimuli. The location of human MT in the present fMRl results is consistent with that of MT in earlier PET and anatomical studies. In addition we found that area MT has a much higher contrast sensitivity than that in several other areas, includ-ing primary visual cortex (Vl). Functional MRI half-ampli-tudes in Vl and MT occurred at approximately 15 % and 1% luminance contrast, respectively. High sensitivity to con-trast and motion in MT have been closely associated with magnocellular stream specialization in nonhuman pri-mates. Human psychophysics indicates that visual motion ap-pears to diminish when moving color-varying stimuli are equated in luminance. Electrophysiological results from macaque MT suggest that the human percept could be due to decreases in firing of area MT cells at equiluminance. We show here that fMRl activity in human MT does in fact decrease at and near individually measured equilumi-nance. Tests with visuotopically restricted stimuli in each hem-ifield produced spatial variations in fMRl activity consistent with retinotopy in human homologs of macaque areas Vl, V2, V3, and VP. Such activity in area MT appeared much less retinotopic, as in macaque. However, it was possible to measure the interhemispheric spread of fMRl activity in human MT (half amplitude activation across the vertical meridian =-15’).

Using functional magnetic resonance imaging (fMRI) and cortical unfolding techniques, we analyzed the retinotopy, motion sensitivity, and functional organization of human area V3A. These data were compared with data from additional human cortical visual areas, including V1, V2, V3/VP, V4v, and MT (V5). Human V3A has a retinotopy that is similar to that reported previously in macaque: (1) it has a distinctive, continuous map of the contralateral hemifield immediately anterior to area V3, including a unique retinotopic representation of the upper visual field in superior occipital cortex; (2) in some cases the V3A foveal representation is displaced from and superior to the confluent foveal representations of V1, V2, V3, and VP; and (3) inferred receptive fields are significantly larger in human V3A, compared with those in more posterior areas such as V1. However, in other aspects human V3A appears quite different from its macaque counterpart: human V3A is relatively motionselective, whereas human V3 is less so. In macaque, the situation is qualitatively reversed: V3 is reported to be prominently motion-selective, whereas V3A is less so. As in human and macaque MT, the contrast sensitivity appears quite high in human areas V3 and V3A. Key words: fMRI; V3A; retinotopy; motion selectivity; visual cortex; MT/V5; human; primate After cortical visual areas V3 and V4 were identified and named in macaque monkeys, another region was discovered between them and named “V3 accessory ” (V3A) (Van Essen and Zeki, 1978; Zeki, 1978a,b). V3A is now regarded as a cortical area that is entirely independent and distinct from its similarly named neighbor, V3, in terms of its retinotopy (Van Essen and Zeki, 1978; Zeki, 1978a,b; Gattass et al., 1988), its histology (Burkhalter et al., 1986; Felleman and Van Essen, 1987; DeYoe et al.,

form by Buckley et al. (1998).

The chromatic properties of an image yield strong cues for object boundaries and thus hold the potential to facilitate the detection of object motion. The extent to which cortical motion detectors exploit chromatic information, however, remains a matter of debate. To address this further, we quantified the strength of chromatic input to directionally selective neurons in the middle temporal area (MT) of macaque cerebral cortex using an equivalent luminance contrast (EqLC) paradigm. This paradigm, in which two sinusoidal gratings, one heterochromatic and the other achromatic, are superimposed and moved in opposite directions, allows the sensitivity of motion detectors to heterochromatic stimuli to be quantified and expressed relative to the benchmark of sensitivity for a luminance-defined stimulus. The results of these experiments demonstrate that the chromatic