Neural correlates of 3D rotation
Neural correlates of 3D rotation
Tuesday, 3 July 2012
Mean switch-related signal-change within LOC and hMT+/V5 for contralateral stimulation. LOC is disproportionately more activated with subjective reversals of Cylinder rotation, compared to Flat stimuli, where dots just appear to reverse their direction and depth.
Results of whole-brain-corrected random-effects SPM analyses. Red and blue areas show increased and decreased activation during switch events respectively. Green areas show where block-related activity was greater for Cylinder than Flat stimuli.
Ambiguous stimuli change their appearance while we look at them, even if they physically remain the same. In the example movies shown above, the apparent depth and direction of moving dots appears to change randomly. There has been much recent interest in how brain activity correlates with such changes in our subjective perception. Many previous brain imaging studies have just examined just one stimulus type at a time, and consequently identified the brain regions that change their activity whenever there is some change in appearance. This can leave it uncertain which aspects of brain activity relate to which specific aspects of the changing conscious percept. For example, if subjects only viewed ambiguous rotating cylinders (see movie) , we would not know which aspects of the neural response relate to subjective changes in perceived dot depth, dot motion direction, or rotation direction.
Here, in the first study of its kind, our aim was to identify the distinct neural correlates specific to two qualitatively different kinds of subjective switches. To this end, we contrasted two random-dot kinematograms which could appear either as two flat transparently overlapping sheets of dots, or a transparent cylinder rotating in depth. Both stimuli were bistable, periodically appearing to switch their direction of translation or rotation, respectively. We measured the neural correlates of these subjective switches, reported by observers as they viewed the flat and cylinder stimuli in separate scanning blocks.
Our methodology allowed us to first partial out the brain response to the physical differences between the two stimulus types. This left just the neural correlates of purely subjective switches in translation versus rotation.
We found that the BOLD signal within the Lateral Occipital Complex (LOC) of the visual cortex was specifically associated with changes in perceived 3D rotation. In contrast, motion sensitive area hMT/V5+ was activated by both rotational and translational switches. These results provide the first specific neural correlates of different types of subjective switches in motion perception, while also ruling out potential contributing factors such as eye-movements or blinks that were not carefully considered in much prior work.
