Transcranial Magnetic Stimulation Differentially Affects Speed & Direction Judgments #4674 Nestor Matthews1, Bruce Luber 2,3, Ning Qian1, Sarah H. Lisanby2,3 Center for Neurobiology & Behavior, Columbia University1; Department of Psychiatry, Columbia University2; Department of Biological Psychiatry, New York State Psychiatric Institute3. Results Conclusions Purpose: To determine the extent to which humans’ speed and direction judgments could be differentially affected by Transcranial Magnetic Stimulation (TMS). Spontaneous Reports - Each of the five subjects spontaneously reported that the motion appeared to slow when TMS was applied. There was no corresponding change in perceived direction. Data Analysis - The data were analyzed with respect to four dependent variables: Criterion (b); discriminability (d’); reaction time; ‘efficiency’ ( d’ / reaction time ). For each subject and task, the data from the TMS conditions were normalized to the data from the no-TMS condition (dotted, horizontal line on each graph). 1. TMS significantly reduced perceived speed without affecting perceived direction. This was true at both stimulation sites. 2. Speed discrimination was significantly more impaired than direction discrimination, but only when TMS was applied medially. This was also true for our ‘efficiency’ measure, which controls for tradeoffs between the speed and accuracy of responding. 3. Averaging across subjects, stimulation sites, and SOAs, TMS significantly reduced the efficiency of speed discrimination but did not alter the efficiency of direction discrimination. This suggests a single dissociation between the mechanisms responsible for these two tasks. Methods: Subjects viewed two successively presented moving stimuli that differed from each other both in the speed and direction of motion, as schematically shown here. Discriminability (d’) Reaction Time Efficiency Criterion (b) 1st Stimulus 200 msec 2nd Stimulus 200 msec In separate trial blocks, subjects judged the speed (faster / slower) or the direction (clockwise / anti-clockwise) of the 2nd stimulus relative to the first. The Bottom Line TMS significantly affected speed judgments, but not direction judgments. This dissociation is the converse of the earlier finding that axis-of-motion affects direction discrimination but not speed discrimination (Matthews & Qian, Vision Res, 39, 2205-11). Taken together, the two studies constitute evidence for a double dissociation between the mechanisms responsible for speed and direction discrimination. Single pulse TMS was applied with a round coil either medially (2 cm above the inion) or laterally (5 cm lateral to and 4 cm above the inion). The pulse occurred either 0, 40, 80, 120, 160, or 200 msec after the onset of the 2nd stimulus, or not at all (control condition). The physical stimulation (visual and TMS) was identical on the two tasks, as was discriminability (d’) when TMS was not applied. This poster can be viewed at http://brahms.cpmc.columbia.edu/~nestor/arvo.htm