MEG study of temporal parameters and localization of brain responses during the detection of transient changes in the direction of moving stimuli

Abstract

It is well known that the human visual cortex possesses areas specialized for the processing of visual motion attributes (i.e. direction, speed). One issue that remains less investigated is whether the same areas are also specialized in the detection of transient changes in such attributes. Here we used MEG-recordings to identify and localize human cortical areas acting as detectors of abrupt changes in direction of translational motion. Subjects (n=8) were presented with linearly moving random dot patterns (RDP) on a computer monitor positioned at 7.8 deg eccentricity from a central fixation point and moving in one of the four cardinal directions. An abrupt change in direction (+/− 40 deg) was introduced 500 ms after motion onset for a period of 50 ms followed by a return to the original direction of motion. The subjects were asked to identify whether the direction change was clockwise or counterclockwise by pressing one of two buttons with their right index and middle finger. Magnetic responses were collected using a whole-head MEG system (CTF Systems). Analysis of the collected data showed a mean evoked response peaking at 94 ms after the onset of the direction change. Synthetic Aperture Magnetometry (SAM) analysis was used to measure changes in the power of the evoked response. The analysis showed activation in the vicinity of MT+/V5 complex in both hemispheres in 5 subjects with the right hemisphere being predominant. No correlation between stimulus location (i.e. left or right hemifield) and predominant activation of the right or left hemisphere was found. Our results suggest that areas in the vicinity of human MT/V5+ in which neurons possess a bilateral representation of the visual field are very likely to provide the critical sensory signal for detecting the occurrence of direction changes in moving stimuli.