Interactions between speed and contrast tuning in the middle temporal area: implications for the neural code for...

Area of Science:

• Neuroscience
• Computational Neuroscience
• Vision Science

Background:

• Perceived speed is reduced in low-contrast conditions, such as fog.
• This phenomenon is linked to contrast reduction affecting neural responses.
• The middle temporal area (MT) is a key region for processing visual speed perception.

Purpose of the Study:

• To investigate the influence of contrast on the responsiveness and speed tuning of middle temporal area (MT) neurons.
• To test whether current labeled-line models of speed perception accurately predict human and monkey perception under varying contrast levels.

Main Methods:

• Recorded neural responses from MT cells in macaques while varying stimulus contrast.
• Analyzed how contrast affects cell responsiveness and preferred speed.
• Compared predictions from labeled-line models using recorded neural data against perceptual reports from humans and monkeys.

Main Results:

• Decreased contrast significantly reduced MT cell responsiveness.
• Lower contrast caused a shift in the preferred speed of most MT cells towards lower speeds.
• A subset of MT cells showed enhanced responses to slow, low-contrast stimuli compared to slow, high-contrast stimuli.
• Labeled-line models, based on MT cell activity, incorrectly predicted an increase in perceived speed with decreased contrast.

Conclusions:

• Perceived speed is not determined by a simple labeled-line interpretation of activity in MT neurons.
• The findings challenge existing computational models of speed perception.
• Neural mechanisms underlying contrast-dependent speed perception are more complex than previously modeled.