Activation of MTV 5 and Inferior Parietal Lobe

Activation of MT/V 5 and Inferior Parietal Lobe During the Detection of Changes in the Direction of Coherent Motion Stimuli D. Cheyne 1, J. Martinez-Trujillo 2, E. Simine 2, W. Gaetz 1, J. Tsotsos 2 1 Neuromagnetic Imaging Laboratory, Hospital for Sick Children Research Institute, Toronto, Ontario, Canada 2 Centre for Vision Research, York University, Toronto, Ontario, Canada Introduction Several cortical regions of the human brain are known to participate in the processing of visual motion, including human middle temporal complex (h. MT/V 5+) and regions of the parietal lobe [1, 2]. However, little is known about the contribution of these areas to the detection of sudden changes in direction of moving visual objects. The present experiment was designed to measure the time varying cortical activity accompanying transient changes in direction in coherently moving random dot patterns. Conclusions Methods MEG was recorded (CTF OMEGA-151) from nine right-handed subjects. Subjects fixated on the center of a LCD display and were instructed to detect a transient change in the direction of motion of a random dot pattern (RDP) presented in either the left or right visual hemifield (7. 8 deg) moving in one of four cardinal directions. A sudden change in the direction of motion of the pattern (+/- 40 deg) was introduced 500 ms after motion onset for a period of 50 ms, followed by a return to the original direction (Fig. 1). The subjects indicated whether the direction change was clockwise or counterclockwise by a button press with their right hand. Data Analysis Synthetic Aperture Magnetometry We used the Synthetic Aperture Magnetometry (SAM) spatial filtering algorithm [3, 4] to create 3 -dimensional images of source activity as an increase or decrease in power over a 200 ms time window (W 2) encompassing the cortical response (Fig. 1). By subtracting source power over a baseline period (W 1) that included linear motion only, we produced a differential (pseudo-t) SAM image showing power changes specific to the direction change response. The time course of activation in these brain areas was assessed by computing the averaged spatial filter output at peak locations (SAM virtual sensors). Fig. 2. Group averaged SAM images superimposed on a 3 D MRI template brain showing significant power changes (p<0. 05) to direction change for left hemifield (top) and right hemifield (bottom) stimuli. The virtual sensor waveforms corresponding to different peaks in the SAM images, averaged across all subjects, are shown on the left (direction change occurs at t = 0 ms). Results MEG responses showed a averaged evoked response to changes in motion direction in all subjects with peak amplitude at a mean latency of 143 ms. The group averaged SAM images of source power during the period of direction change (relative to motion only) showed activation predominantly in the vicinity of the contralateral h. MT/V 5+ and in the right inferior parietal lobe (IPL), independently of the side of stimulation (Fig. 2, center). Weaker activations were also noted in the ipsilateral MT/V 5+ region in individual subjects but were not statistically significant in the group results. Group Averaging The SAM images of source power change were spatially normalized, averaged across subjects, and thresholded using a statistical nonparametric permutation test of the mean using Sn. PM-99 [5 -7] for left and right hemifield stimuli separately. Fig. 1. (a) Random dot patterns presented to the left visual hemfield showing responses in contralateral visual and parietal areas in a single subject. (b) Time windows used to compute differential SAM images are denoted by gray outlines. The topographic map shows surface field pattern for the peak response in the MEG waveforms detected at 143 ms following direction change. Averaged virtual sensor analysis (Fig 2, left) shows that peak responses in contralateral h. MT/V 5+ were similar for both RHS and LHS conditions, occurring approximately 134 ms after direction change. These were followed by responses in the right IPL 15 to 25 ms later. Note that responses in the right IPL to stimuli presented to the ipsilateral (right) hemifield were significantly delayed compared to when the stimuli were presented to the contralateral (left) hemifield. Overall performance was 93% correct detections with no significant differences in performance for left and right hemifield stimuli. • Using differential SAM imaging we can measure the location and time course of brain responses to changes in the direction of coherent motion stimuli (relative to motion only). • Direction changes produce an early response in contralateral MT/V 5+ consistent with a role of this brain area in monitoring direction changes in the contralateral visual field. • MT/V 5+ activation is followed by activity in the right inferior parietal lobe, independently of the side of presentation, suggesting that this brain area may serve a more global role in the processing of changes in visual motion (8). Acknowledgements This work was supported by the Natural Sciences and Engineering Research Council of Canada (No. 184018 -02), Canadian Institutes of Health Research (CIHR) and NCE IRIS. We would also like to thank Dr. Krish Singh for the mri 3 d. 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