L T Likova, C W Tyler, A R Wade
Smith-Kettlewell Institute, 2318 Fillmore Street
San Francisco, CA 94115, USA
Stereomotion (SM) is motion-in-depth produced by disparity changes. We have found a complex of brain areas involved in long-range stereomotion processing. How distinct is this SM-complex from that for lateral motion? In this study, we used fMRI to focus on a non-retinotopic area in the intra-occipital sulcus (IOS), which we have found to give one of the strongest responses to SM. In experiment 1, stereomotion, seen as backward/forward motion in depth, was generated by dynamic autostereograms. It was contrasted with stationary stimuli. In experiment 2, the same stimuli were used, but observed non-stereoscopically, thus producing lateral motion instead of stereomotion. In this way the monocular-motion images in experiments 1 and 2 were identical. In experiment 3, stereomotion was contrasted directly with lateral motion at the horopter. fMRI responses were obtained on a GE Signa 3T scanner with spiral acquisition in 26 axial slices, 4 mm thick, at 3 s TR through occipital, temporal, and most of posterior parietal cortex. Test and null stimuli alternated for 9 s each in 36 blocks per scan. The target IOS area was strongly activated in both motion/static contrasts (experiments 1 and 2). Does this area represent (a) a generic motion module, processing any retinal motion regardless of its depth interpretation, or (b) an area coding locally for azimuthal direction of motion, and hence specifically activated by every motion direction? Experiment 3 demonstrated surprisingly high activity in the same region, thus suggesting a distinct SM-selective neuronal subpopulation within the non-retinotopic portion of the IOS. The results revealed a stereomotion-specific activation in the IOS in superior occipital cortex, and they may also contribute to the understanding of the organisation of overlapping functionally distinct neuronal subpopulations.
[Supported by NEI 7890.