Modulation of local motion signals by the global structure of optic flows: evidence for feedback from high-density EEG recordings

Abstract

 

Movement through the environment creates characteristic patterns of optic flow on the retinae. The form that these patterns take can be determined only after integration of a large number of local measurement s. Here we show using high-density EEG recordings that signals time-locked to the appearance of global flow modulate the amplitude of local motion signals. Frequency tagging was used to label separately the global motion updates (0.67–2 Hz) and the local update rate of limited lifetime dots (10–12 Hz). Spectral analysis was used to recover responses to the global and local updates at harmonics of their respective update rates. Robust global responses were recorded to rotation, translation and dilation flows. We also recorded responses at frequencies equal to low order sums and differences of the frequencies of the local and global update rates. These interaction signals indicate that the global and local signals are combined non-linearly. This interaction appears to be a simple multiplication of the global and local signals. Topographic analysis of the local, global and interaction signals indicates that the site of global signal action on the local signals is distant from the site where the global signal is generated. This is consistent with feedback enhancement of the local signal by the global signal. Global flow activations were found in hMT+ , V3a and sometimes on the ventral surfaces using fMRI. The interaction signal was maximal on the occipital mid-line, dorsal to the maximum of the dot update response, suggesting the interaction occurs in V3a.

 

EY06579 and The Smith-Kettlewell Eye Research Instititute