The cortical pathway responsible for motion processing is relatively well defined (see e.g. Britten, 2003 for review). However, understanding the precise mechanisms involved in encoding the speed of a moving image has proven elusive. A variety of models have been proposed, including labeled line, ratio, and Bayesian models (e.g., Priebe & Lisberger, 2004; Smith & Edgar, 1994; Thompson, Brooks, & Hammett, 2006; Hammett, Champion, Thompson, & Morland, 2007; Stocker & Simoncelli, 2006; Langley & Anderson, 2007), but there is still no clear and shared picture of exactly where in the path speed regulation occurs, nor how it is achieved. The location of an unambiguous velocity signal is not only of anatomical interest, but likely constrains models of how spatiotemporally separable signals generated in the retina are transformed to provide behaviorally relevant velocity signals. Unfortunately, previous attempts to determine the location of velocity coding using both electrophysiological and imaging techniques have produced inconclusive results. There is considerable electrophysiological evidence to suggest that the early stages of visual processing are mediated by neurons whose responses are spatiotemporally separable (e.g. Tolhurst & Movshon, 1975; Foster, Gaska, Nagler & Pollen, 1985). Such neurons are tuned to limited spatial and temporal frequency ranges and therefore do not provide a unique code for speed. More recent evidence for speed regulation of many neurons in MT (e.g. Perrone & Thiele, 2001) and for a direct link between their activity and speed perception (e.g. Rudolph & Pasternak, 1999; Liu & Newsome, 2005) raise the possibility that an explicit code for speed can be extracted from the beginning spatially and temporally... middle of paper... answer. The expectation of such coupling, at least in MT, seems reasonable in light of the evidence (Liu & Newsome, 2005) that individual neurons in MT play a direct role in speed perception. Therefore, examining the effect of a stimulus attribute known to influence perceived speed on BOLD responses may provide a clearer picture of how and where cortical speed encoding occurs. Recently, Hammett et al. (2007) demonstrated that perceived speed is modulated by mean luminance such that low luminance stimuli appear significantly faster at high speeds. Any coupling of the BOLD response to perceived speed should therefore be manifested by systematic differences in areas that encode speed. We then measured the BOLD response to drifting sinusoidal gratings at one speed range and at two luminance levels above the scotopic range, in areas between the LGN and MST.