Visual regions V2, V3, and MT can discriminate between visual motion trajectories even when you can't.

摘要

Radial frequency (RF) motion trajectories are a class of visual stimuli that consist of a target moving along a closed trajectory defined by a sinusoidal variation of the radius relative to a circular path. The results of a previous fMRI multivoxel pattern analysis study demonstrated that trajectory shape can be distinguished in regions V2 and V3 for radial frequency patterns ranging from RF2 to RF5 (Gorbet, Wilkinson, and Wilson, 2012). These low frequency trajectories formed recognizable shapes that were oval-, triangle-, diamond-, and star-like. These results indicate that V2 and V3 have a role in processing closed-circuit visual motion but do not reveal whether discrimination involves encoding global categories of shape or more local differences in trajectory curvatures. If a region is involved in recognizing the overall shape of a trajectory, multivoxel pattern discrimination should disappear for high frequency trajectories that form non-discriminable shapes. In the current study, we used a multivoxel pattern analysis fMRI approach to test this prediction. In particular, we tested whether patterns of voxel activity in independently localized visual regions could distinguish both between recognizable RF4 and RF5 trajectories and between unrecognizable RF9 and RF10 trajectories. As expected, RF4 and RF5 trajectories could be reliably distinguished in regions V2, V3, and additionally, in region MT. However, the data revealed that these same regions can also discriminate between RF9 and RF10 trajectories even though separate psychophysical testing indicates that observers cannot tell these motion trajectories apart any better than chance. These results suggest that distinguishing between different RF motion trajectories in regions V2, V3, and MT relies on local properties of trajectory curvature. Preliminary further examination of the data using a whole-brain recursive feature elimination approach suggests that perception of global trajectory shape may occur in higher level parietal and frontal cortical regions.