Timing in the visual hierarchy

Abstract

Vision scientists assume a hierarchy of visual areas in which `earlier' or `lower' areas respond faster than `later' or `higher' ones. A recent physiological study by Schmolesky et al. provides an extensive analysis of neural response latencies and their variability[ 1 Schmolesky M.T et al. Signal timing across the macaque visual system. J. Neurophysiol. 1998; 79: 3272-3278 PubMed Google Scholar ]. They presented four anesthetized macaque monkeys with visual stimuli and recorded visual-evoked response latencies in nine different areas of the visual system. By including a wide range of visual areas tested under identical procedures, their method enables comparisons of timing between various areas. The data revealed a cascade of activity, beginning in the magnocellular (M), then parvocellular (P), layers of the lateral geniculate nucleus (LGN), followed by cortical area V1. Next, activation was observed concurrently in areas V3, MT, MST, and the frontal eye fields (FEF). The latest activation was observed in V2 and V4. Response latencies ranged between 33 ms in the magnocellular LGN to 104 ms in V4. Strikingly, regions that fall in the ventral or `P' stream showed much slower and more variable responses than those in the dorsal or `M' stream, suggesting that M-stream cells might modulate the responses of P-stream cells. This demonstrates that visual hierarchies can be examined using timing properties in addition to the more established methods of neural connectivity.