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