Event-related Fmri of Tactile Stimulus Detection: Gain Relations and Functional Distinctions Between Anterior and Posterior Pocg Representations

Abstract:
We investigated the effects of frequency (20- or 100-Hz) and amplitude (4 levels) of tactile stimulation with event-related fMRI at 3T. Subjects were visually cued 1s prior to a 1s-duration vibrotactile stimulus of the third-digit fingertip. On 1/3 of trials, subjects performed a detection task with the contralateral hand 2s after tactile onset. Subjects also received a 100-Hz stimulus without visual cue. Selected stimulation amplitudes were psychophysically matched for loudness across frequencies. The 12 conditions (520 trials) were interleaved across 10 6-min runs, with 0-14s between trials. Our primary findings are: 1) In all subjects, event-related tactile stimulation evoked activation in the postcentral gyrus (PoCG); 2) In 5/6 subjects, distinct activation foci were observed in the anterior- and posterior-PoCG; 3) In the posterior-PoCG, a longer-latency response correlated to ipsilateral motor activity was observed that was absent in the anterior-PoCG; 4) Responses in both foci were generally scaled to perceived amplitude; 5) However, a thresholded non-linearity was observed in a subset of subjects, in which high-amplitude stimuli evoked a post-response undershoot, leading to a lower percent signal increase than evoked by medium-amplitude stimuli. We are currently analyzing activation during correct/incorrect detection of low-amplitude stimuli (mean detection rate = 65%).