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Yoshiaki Tsushima, Yuka Sasaki, Takeo Watanabe; Greater disruption by sub-threshold task-irrelevant signals. Journal of Vision 2007;7(9):539. doi: 10.1167/7.9.539.
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© ARVO (1962-2015); The Authors (2016-present)
While considerable studies have shown that a sub-threshold (undetectable) stimulus influences brain activity and performance, it is still unclear how sub-threshold stimuli are processed in the brain. Tsushima and Watanabe (VSS04 and 05) showed that task performance is more greatly disrupted by sub-threshold, task-irrelevant signals than supra-threshold signals. Here we examined the neural mechanism for this effect by an event-related fMRI paradigm. Subjects (n=6) viewed a sequence of 2digits/6letters at the center of the display and numerous moving dots in the periphery. The subjects were asked to report two digits by pressing buttons on the box held by themselves. In the background, some dots contracted toward the center of the display while the others moved randomly. The ratio of the number of contracting dots to the total number of dots was varied from trial to trial in 4 steps (0, 5, 10, 20%). The results showed that with sub-threshold 5% coherent motion, MT+ was significantly more highly activated than 0% and also supra-threshold 10% and 20% coherent motion. On the other hand, in the lateral prefrontal cortex (LPFC) that provides inhibitory control of irrelevant signals, fMRI activity with 5% coherent motion showed no significant difference from with 0% coherent motion, but was significantly lower than supra-threshold 10% and 20% coherent motion.These results indicate the following model: LPFC has a higher detection threshold for incoming signals than the visual cortex. Sub-threshold task-irrelevant signals may be sufficiently strong to be processed in the visual system but not sufficiently strong for LPFC to “notice” and, therefore, to provide effective inhibitory control on the signals. Meanwhile, supra-threshold coherent motion may be “noticed”, given successful inhibitory control by LPFC, and leave more resources for a task. Such neural mechanisms may have caused performance with sub-threshold distractors to be lower than with supra-threshold distractors.
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