The generation of a behaviorally relevant cue to the speed of objects around us is critical to our ability to navigate safely within our environment.
However, our perception of speed is often distorted by prevailing
conditions. For instance, as luminance is reduced, our perception of
the speed of fast-moving patterns can be increased by as much as
30%. To investigate how the cortical representation of speed may vary
under such conditions, we have measured the functional MRI blood
oxygen level-dependent (BOLD) response of visual cortex to drifting
sine gratings at two very different luminances. The average BOLD
response in all areas was band-pass with respect to speed (or equivalently, temporal frequency) and thus contained no unambiguous
speed information. However, a multivariate classifier was able to
predict grating speed successfully in all cortical areas measured.
Similarly, we find that a multivariate classifier can predict stimulus
luminance. No differences in either the mean BOLD response or the
multivariate classifier response with respect to speed were found as
luminance changed. However, examination of the spatial distribution
of speed preferences in the primary visual cortex revealed that
perifoveal locations preferred slower speeds than peripheral locations
at low but not high luminance. We conclude that although an explicit
representation of perceived speed has yet to be demonstrated in the
human brain, multiple visual regions encode both the temporal structure of moving stimuli and luminance implicitly.