April 12, 2021

Encoding of motion by spatial and temporal frequency in the human middle temporal cortex measured by 7T fMRI

Perception of environmental dynamic scenes results from the evaluation of visual features such as the fundamental spatial and temporal frequencies components of a moving object. The ratio between these two components represents its speed of motion. The human middle temporal cortex hMT+ has a crucial biological role in the direct encoding of object speed. However, the link between hMT+ speed encoding and the spatiotemporal frequency components of a moving object may be more complex than we thought. Both animal studies and recent human electrocorticography data showed that recorded neuronal populations within MT+/V5 change their speed preferences in accordance with the stimulus fundamental spatial frequency. We disentangle whether such mechanism holds for the entire human MT+. We recorded high resolution 7T blood oxygen level-dependent BOLD responses to different visual motion stimuli as a function of their fundamental spatial and temporal frequency components. We fitted each hMT+ BOLD response with a 2D Gaussian model allowing for distinct and independent selectivity for spatial and temporal frequencies of the visual stimuli or tuning for the speed of motion. We show that: 1) hMT+ encodes the speed of motion via independent tuning of the fundamental spatial frequency component of the visual stimuli, 2) the optimal spatial frequency selectivity of hMT+ is tuned for the low spatial frequency of the visual stimuli and is highly reproducible within subjects. Our results show that hMT+ speed preference changes according to the fundamental spatial frequency presented, demonstrating a primary role of the entire hMT+ in the evaluation of the spatial features of the moving visual input. These findings confirm a more complex mechanism involved in the direct perception of speed than initially thought.

 bioRxiv Subject Collection: Neuroscience

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