Speed tuning to real-world- and retinal motion in cortical motion regions

Motion signals can arise for two reasons in the retina: due to self-motion or due to real motion in the environment. Prior studies on speed tuning always measured joint responses to real and retinal motion, and for some of the more recently identified human motion processing regions, speed tuning has not been examined in at all. We localized motion regions V3A, V6, V5/MT, MST and cingulate sulcus visual area (CSv) in 20 human participants, and then measured their responses to motion velocities from 1-24 degrees per second. Importantly, we used a pursuit paradigm that allowed us to quantify responses to objective and retinal motion separately. In order to provide optimal stimulation, we used stimuli with natural image statistics derived from Fourier scrambles of natural images. The results show that all regions increased responses with higher speeds for both, retinal and objective motion. V3A stood out in that it was the only region whose slope of the speed-response function for objective motion was higher than that for retinal motion. V6, V5/MT, MST and CSv did not differ in objective and retinal speed slopes, even though V5/MT and MST tended to respond more to objective motion at all speeds. These results reveal highly similar speed tuning functions for early and high-level motion regions, and support the view that human V3A encodes primarily objective rather than retinal motion signals.