Speed encoding in human visual cortex revealed by fMRI adaptation.

In macaque visual cortex, the conventional view is that image motion is initially detected by direction-sensitive neurons that are tuned in terms of local spatial and temporal frequency (TF), from which speed is encoded later. We used functional magnetic resonance imaging (fMRI) adaptation to seek evidence for speed or TF tuning in human visual cortex. Drifting sine-wave gratings were presented in pairs (S1: adapter, 100% contrast; S2: probe, 15, 40 or 80% contrast). In each trial, either speed or TF was the same for S1 and S2, whereas the other dimension changed. We investigated whether the response was weaker (adapted) for repetitions of the same speed, indicating speed coding, or for repetitions of TF, indicating TF coding. For high-contrast (80%) probes, we observed clear speed coding in MT and MST with similar but weaker trends in several earlier visual areas. For medium- and low contrast probes, our data indicated a trend towards temporal frequency coding in most visual areas studied. In a second experiment, we adjusted stimuli in terms of perceived rather than physical speed and found a trend for speed coding even for low-contrast probes. Our results suggest that speed coding dominates in MT/MST for high contrast stimuli, and possibly also in other visual areas and/or at lower contrasts.