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The Journal of Neuroscience, August 30, 2006, 26(35):8988-8998; doi:10.1523/JNEUROSCI.1983-06.2006

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

Bart Krekelberg,^1,2 Richard J. A. van Wezel,³ and Thomas D. Albright¹

¹The Salk Institute for Biological Studies, Howard Hughes Medical Institute, La Jolla, California 92037, ²Center for Molecular and Behavioral Neuroscience, Rutgers University, Newark, New Jersey 07102, and ³Helmholtz Institute, Functional Neurobiology, 3508 TC Utrecht University, Utrecht, The Netherlands

Correspondence should be addressed to Bart Krekelberg, Center for Molecular and Behavioral Neuroscience, Rutgers University, 197 University Avenue, Newark, NJ 07102. Email: bart{at}rutgers.edu

A car driving through the fog appears to move more 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 cells was pervasive and highly regular. As expected, most cells responded less at low contrast. More importantly, the preferred speed of most cells shifted to lower speeds at lower contrasts. Moreover, approximately one-third of cells surprisingly responded more strongly to slow low-contrast stimuli than to slow high-contrast stimuli.

Current models of speed perception suggest that each MT cell votes for its preferred speed, with a vote determined by its firing rate. We tested a number of these labeled-line models by entering the neural responses we recorded from MT and comparing the predictions of the models with the perceptual reports of human subjects and monkeys. Contrary to the perceptual reports, the labeled-line models predicted 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 model

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Received Oct. 27, 2005; revised July 21, 2006; accepted July 24, 2006.

Correspondence should be addressed to Bart Krekelberg, Center for Molecular and Behavioral Neuroscience, Rutgers University, 197 University Avenue, Newark, NJ 07102. Email: bart{at}rutgers.edu

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