Behavioral/Systems/Cognitive Interactions between Speed and Contrast Tuning in the Middle Temporal Area: Implications for the Neural Code

A car driving through the fog appears tomovemore slowly than one driving on a clear and sunny day. In the laboratory, this observation has been confirmed as a pronounced reduction of perceived speed caused by a reduction in contrast. We measured the influence of contrast on cells in the middle temporal area (MT) of the macaque, which has been hypothesized to underlie the perception of speed. The influence of contrast on the responsiveness and speed tuning of these cellswas pervasive andhighly regular.As expected,most cells responded less at lowcontrast.More importantly, thepreferred speedofmost cells shifted to lower speeds at lower contrasts.Moreover, approximately one-third of cells surprisingly respondedmore strongly to slow low-contrast stimuli than to slow high-contrast stimuli. Currentmodels of speedperception suggest that eachMTcell votes for its preferred speed,with a vote determinedby its firing rate.We tested anumber of these labeled-linemodels by entering theneural responseswe recorded fromMTand comparing the predictions of the modelswith theperceptual reports of humansubjects andmonkeys. Contrary to theperceptual reports, the labeled-linemodelspredicted that perceived speed should increase when contrast is decreased. We therefore conclude that perceived speed is not based on a labeled-line interpretation of MT cells. Key words:motion perception; speed coding; macaque monkey; middle temporal area; vector average model; labeled-line model; ratio mode