Brain activity evoked by motion direction changes and by global motion coherence shows different spatial distributions

Abstract

Visual evoked potentials (VEPs) can be generated by the transitions between coherent and incoherent optic flow patterns. These must arise from neural systems that detect global properties of visual motion. The reversal of motion direction of a coherent pattern could activate both global and local motion mechanisms. Here we ask whether VEPs for such motion reversal events have a different source to those for changes in motion coherence. High-density steady-state VEPs were recorded with a 128-channel geodesic sensor net for dot patterns moving in either concentric circular paths or in uniform translation. Coherence VEPs were elicited by alternation at 2 Hz between patterns of 100% coherent motion and similar dots with random movement directions. Response at the first harmonic frequency was taken as an indicator of global coherence processing (Braddick et al, VSS 2006). Direction-reversal VEPs were elicited by reversal of motion direction in the same patterns at 2 Hz. Amplitudes of the VEP signals were compared across each of five regions of the posterior scalp. As in previous work (Braddick et al VSS 2006) coherence VEPs were greatest close to the posterior midline of the scalp. Direction-reversal VEPs, in contrast, showed a more lateral focus, with a distribution significantly different on ANOVA from the coherence VEPs. We conclude that the neural generators of the direction response, local or global, are at least in part different from those for the global coherence response. We will discuss how these results relate to the multiple areas shown to respond to local and global motion in neuroimaging studies, particularly V5/MT and V3A, and to the changes in the location of global coherence responses seen in development.

Footnotes

 Supported by Research Grant G0601007 from the Medical Research Council.