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Retinotopy in the Human MT Complex

R.F. Dougherty, R.M. Khan, W.A. Press, A.R. Wade, H.A. Baseler, D.J. Heeger & B.A Wandell.
Department of Psychology, Stanford University

The human MT complex (MT+) is located bilaterally on the lateral aspect of the occipital lobes. This cortical region responds preferentially to moving compared to static stimuli and is easily identified in functional MRI (fMRI) studies. Region MT+ is thought to comprise several distinct visual areas, including homologues to monkey MT, MSTd, and MSTl. Here we report that it is possible to subdivide the MT+ region into two parts based on retinotopic mapping.

Methods

MT+ was first identified in four human observers using white dots on a black background that alternated between stationary and radial motion displays. Retinotopic organization was measured using several different stimuli, including rotating wedges and expanding rings, composed of either moving dots or high-contrast flickering checkerboards. FMRI measurements were made using a 1.5T GE Signa (TR=2-3s, TE=40ms, spiral acquisition). Eight to sixteen planes were acquired at various spatial resolutions (ranging from 1x1 mm to 3x3 mm inplane) and orientations.

Results and Conclusions

Retinotopic organization was present within a 2.5 cm2 area that covered roughly half of the MT+ region. Retinotopic organization was apparent in all four subjects, and all subjects showed the clearest retinotopy in the right hemisphere.

The retinotopic region within human MT+ is likely to be the homologue of visual area MT (V5) in monkey because this area exhibits the clearest retinotopy of the various motion-selective areas in the monkey. In addition, the functional specialization of this retinotopic area, as shown by R.M. Khan, et al (SFN99), is consistent with this claim. The discovery of this sub-division of MT+ makes it possible to study stimulus selectivity in separate portions of the human MT complex, removing a confound present in previous studies of this region.

Supported by NEI RO1 EY03164, EY11794 and the McKnight Foundation.

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