Twenty-ninth PPLS Interdisciplinary Seminar

Speaker: Professor Aaron Schurger (Chapman University)

Title: The threshold of conscious sensory perception

Abstract: Experimental manipulations of conscious sensory perception ordinarily rely on behavioral metrics that can be used to infer the presence or absence of consciousness. Such metrics imply that there is a threshold intensity above which subjects are conscious of stimuli and below which they are not. Wherever the “true” threshold of conscious perception might lie, any behaviorally-based metric of conscious perception makes a commitment to where to place that threshold. However, no metric has become universally agreed upon. Importantly, placement of this threshold determines how to group the data in experiments aimed at identifying neural correlates of consciousness. Here we argue that if a given purported threshold is a threshold of subjective experience, then there is one test that, at a minimum, this threshold must pass: If identical stimuli are presented at two different levels of intensity that straddle the threshold, then subjects should tend to report that the two stimuli are subjectively different. Furthermore, the probability that both stimuli are reported as different should be lower if both stimuli are below or both are above the putative threshold. Any threshold that does not pass this test, though it might be a threshold of something, is not a threshold of subjective experience. We estimated thresholds using this approach and compared them to thresholds obtained using d-prime, accuracy, meta d-prime, and subjective report. Our data offer a means to test the subjective validity of perceptual thresholds and also provide evidence for an abrupt non-linear threshold of conscious perception.

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Dr Tillmann Vierkant