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Behavioral/Systems/Cognitive Interactions between Speed and Contrast Tuning in the
 

Summary: 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
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
underlietheperceptionofspeed.Theinfluenceofcontrastontheresponsivenessandspeedtuningofthesecellswaspervasiveandhighly
regular.Asexpected,mostcellsrespondedlessatlowcontrast.Moreimportantly,thepreferredspeedofmostcellsshiftedtolowerspeeds
atlowercontrasts.Moreover,approximatelyone-thirdofcellssurprisinglyrespondedmorestronglytoslowlow-contraststimulithanto
slow high-contrast stimuli.
CurrentmodelsofspeedperceptionsuggestthateachMTcellvotesforitspreferredspeed,withavotedeterminedbyitsfiringrate.We
testedanumberoftheselabeled-linemodelsbyenteringtheneuralresponseswerecordedfromMTandcomparingthepredictionsofthe
modelswiththeperceptualreportsofhumansubjectsandmonkeys.Contrarytotheperceptualreports,thelabeled-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

  

Source: Albright, Tom - Vision Center Laboratory, Salk Institute for Biological Studies

 

Collections: Biology and Medicine