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Andrei Gorea, Florian Waszak; Two modus operandi of the motor system in relation to perceptual behavior. Journal of Vision 2004;4(8):153. doi: 10.1167/4.8.153.
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© ARVO (1962-2015); The Authors (2016-present)
Many studies suggest that visual stimulation can modulate a motor response in the absence of conscious experience. None of them has assessed perceptual and motor behavior within a Yes/No paradigm allowing the joint appraisal of observers' sensitivity (d′) and response criterion (c), on the one hand, and of reaction time (RT), on the other. Here we use such a paradigm in conjunction with a metacontrast stimulus setup in order to answer the question of whether a motor response is triggered by the perceptual-related internal response exceeding a (perceptual) response criterion, or by some perceptual-unrelated activity exceeding a threshold proper to the motor system. On any given trial the RT to the occurrence of any of the two, masked and masking stimuli was recorded together with the observer's subsequent response on the presence/absence of the masked stimulus. Independent randomization over time of the onsets of the masked and masking stimuli permitted the independent assessment of their correlation with RT. RTs were analyzed as a function of (1) the presence/absence of the masked stimulus, (2) the internal state of the observer (stimulus reported or not), (3) the strength of the masked stimulus, hence d′, and (4) its occurrence probability (.2, .5 and .8), hence c. The masked stimuli had relatively low and high energies while manipulation of the masking energy insured a constant d′. Data show that, at any given d′, the motor system operates under one of two distinct modes determined by the physical energy of the visual stimulus. Low energy stimuli trigger a motor response only on Hit trials (hence only for perceptual-related internal responses exceeding observer's perceptual response criterion) implying that in this regime the motor system is under conscious perceptual control. High energy stimuli trigger a motor response on both Hit and Miss trials, a reflex, non-conscious modus operandi revealing the existence of a threshold proper to the motor system.
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