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Relative vs. absolute orientation judgment: A psychophysical evaluation of neural decoding models
Dr. Ning Qian
Monday, March 02, 2015, 12:00pm - 07:00pm
Columbia University Medical Center, Department of Neuroscience
Neural decoding models relate neuronal activities to perception. However, these models typically decode neuronal responses for a single stimulus at a time to arrive at a value or a distribution of values (an absolute judgment). To explain discrimination of two stimuli (a relative judgment), most models assume that each stimulus is decoded separately and the two resulting absolute judgments are then compared, via, e.g., signal detection theory. Surprisingly, this widely used absolute-to-relative assumption has never been rigorously tested. The assumption predicts that the absolute-judgment distributions fully determine the corresponding relative-judgment distribution and the discrimination performance. We tested and refuted this prediction. Distributions of absolute orientation judgments were biased and much broader than orientation discrimination thresholds. The two orientations presented sequentially for comparison were perceptually correlated, and this inter-stimulus correlation reduced the variance of the relative distribution but was not reflected in the absolute distributions. Additionally, the two orientations biased each other’s perception, leading to both forward and backward aftereffects not well explained by conventional adaptation or its optimal coding theory. These bi-directional interactions, rather than sharp absolute and relative distributions assumed by decoding models, produced good discrimination performance. We conclude that the brain does not compare absolute judgments to reach relative judgments, and that for discrimination of two sequentially presented stimuli, the neuronal working memory of the first stimulus must be altered by the second stimulus retrospectively. Since ordinal relationships among stimuli are more invariant and informative than absolute stimulus values, the brain might have evolved liquid neural dynamics that emphasizes relative discrimination at the expense of absolute judgments.