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Abstract

If a stimulus is followed by a mask performance often strongly deteriorates. However, detection tasks are usually less affected than discrimination tasks. For example, existence versus non-existence of a masked vernier can more easily be judged than disrimination between the left and right offsets. Magnetic pulses of transcranial magnetic stimulation(TMS) applied at the occipital cortex are known to act like visual masks. Here, we investigate whether or not detection and discrimination tasks show identical time courses under TMS stimulation using vernier stimuli.
We presented vertical verniers, 1200" long with a vertical gap of 60", to one of the lower visual quadrants. A fixation dot was permanently present during a stimulus presentation. Distance between vernier and fixation dot was 1272''. Stimuli were displayed for 20ms on an analog monitor controlled by a Power Macintosh computer via fast 16 bit D/A converters (1MHz pixel rate). Luminance of the stimuli was 2 cd/m2 and luminance of the background smaller than 0.01 cd/m2. In a temporal 2AFC detection task a vernier was flashed in either one of two subsequent intervals which always began with the presentation of the fixation dot. Subjects were asked to indicate the interval containing the vernier and correct reponses were determined. In the discrimination task subjects had to indicate the direction of vernier offset (left or right). Thresholds were determined using an adaptive strategy (PEST). TMS pulses were applied to the occipital cortex. Maintenance of the coil position was monitored using a stereotactic positioning device. In the experiments the independent variable is the SOA between vernier onset and TMS pulse.
Performance for all three subjects was well in the hyperacuity range, i.e. between 11'' and 50'' if no TMS was applied. TMS shifted discrimination thresholds to higher values in dependence of the SOA in the typical U-shaped manner. Depending on observer, the maximal modulation was reached between 100 ms and 130 ms SOA. With these SOAs discrimination was strongly deteriorated and sometimes even impossible. Similarly, detection of the vernier was modulated by TMS in a U-shaped manner. Chance level (50 correct responses) was reached between 100 ms and 130 ms, too. In both tasks only one narrow peak of deterioration was found.
TMS applied over the occipital lobe strongly modulates visual detection as well as visual discrimination. Since the time courses of modulation of detection and discrimination tasks do not differ, we suggest that in the early cortical visual system both processes have an identical temporal structure. Discrimination processes do not require a longer processing time. The data suggest that both perceptual qualities are based on the same signal process in the early visual system.