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Abstract:
Introduction:
Despite sustained attention, weak sensory event often evade our perceptual awareness. The neural mechanisms that determine whether a stimulus is consciously perceived remain poorly understood. Conscious visual perception is thought to rely on a widespread neural system encompassing primary and higher order visual areas, frontoparietal areas and subcortical regions such as the thalamus. This concurrent TMS-fMRI study applied TMS to the right anterior intraparietal sulcus (IPS) and in a sham control to investigate how perturbations to IPS influence the neural systems underlying visual perception of weak sensory events.
Methods:
7 subjects took part in the concurrent TMS-fMRI experiment (3T Siemens Magnetom Tim Trio System, GE-EPI, TR = 3290ms, TE = 35ms, 40 axial slices, size = 3mm x 3mm x 3.3mm). The 2x2x2 factorial design manipulated: (i) visual target (present, absent), (ii) visual percept (yes, no) and (ii) TMS condition (IPS, Sham). In a visual target detection task, subjects fixated a cross in the centre of the screen. On 50 of the trials a weak visual target was presented in their left lower visual field. Subjects were instructed to answer 'yes' only when completely sure. Visual stimuli were individually tailored to yield a detection threshold of 70 in visual present trials. Bursts of 4 TMS pulses (10Hz) were applied in image acquisition gaps at 100ms after each trial onset over the right IPS (x=42.3, y=-50.3, z=64.4) and during a sham condition using a MagPro X100 stimulator (MagVenture, Denmark) and a MR-compatible figure of eight TMS coil (MRi-B88). Stimulation intensity was 69 for IPS and was adjusted during Sham stimulation to evoke similar side effects. Trials were presented in blocks of 12 that were interleaved with baseline periods of 13s. Each run consisted of 7 blocks with 4 runs per TMS condition, giving a total of 168 trials per condition. Each TMS condition was performed in different sessions and all conditions were counterbalanced across subjects.
Behavioral responses were categorized in hit, miss, false alarm and correct rejection (CR). Performance measures for each category were computed separately for IPS- and Sham-TMS and averaged across subjects. While each condition was modelled at the 1st level (using SPM8), 2nd level random effects analyses (one-sample t-tests) were restricted to target present trials (i.e. hits, misses). We tested for the main effects of TMS, visual percept and their interaction. Results are reported at p<0.05 at cluster level corrected for the whole brain using an auxiliary uncorrected voxel threshold of p=0.01.
Conclusions:
Visual detection involves perceptual decisions based on uncertain sensory representations. As participants set a high criterion for determining whether they are aware of targets, missed trials were associated with more uncertainty as indexed by long response times and thereby placed more demands on decisional processes. TMS to IPS perturbed this neural system involved in perceptual decisions and awareness. Critically, while the right precentral/middle frontal gyrus associated with the frontal eye field usually discriminates between hits and misses, TMS-IPS abolishes this difference in activation indicating that IPS-FEF closely interact in perceptual awareness and decisions.
