Attention to speed of motion, speed discrimination, and task difficulty: An fMRI study

We studied the functional neuroanatomy of attention to speed of motion usin g functional magnetic resonance imaging in eight healthy subjects, who perf ormed a speed discrimination (SID) task using a random textured pattern mov ing at a reference speed of 6 deg/s. During the control condition (DIM), wi th retinal stimulation identical to that during SID, subjects detected the dimming of the central fixation point. Attention to speed (SID compared to DIM) activated mainly ventral V3 and V4, dorsal V3 and V3A. Compared to a f ixation control condition, speed discrimination recruited a large visuomoto r network, including hMT/V5+. However, hMT/V5+ was only marginally more act ive during speed discrimination than during dimming detection. Thus hMT/V5 is involved in speed discrimination, in line with the speed discrimination impairments following hMT/V5+ lesions, but our results suggest that this a ctivity simply reflects the processing of motion rather than attention to s peed. Manipulating the difficulty of the speed discrimination task over a l arge range of the psychometric curve revealed that increasing difficulty li nearly increases activity in right frontal regions, as well as in lateral o ccipital and dorsal parietal regions. A weak effect of difficulty was also observed in dorsal V3. (C) 2000 Academic Press.