Sequential Effects: Irrational Superstition or Adaptive Behavior?
Tuesday 17th of March 2009 at 12:00pm
Just as visual illusions reveal principles and mechanisms underlying natural visual processing, "cognitive illusions" can provide similar insight into the computations underlying decision-making. In a variety of behavioral tasks involving randomized sequences of stimuli, subjects exhibit an odd "sequential effect": their responses are facilitated by chance runs of repetitions and alternations in stimulus identity, and
are hampered by violations of such transient local patterns. Such behavior is sub-optimal in the context of the task, since these transient patterns are spurious and have no true predictive power of future stimuli. We hypothesize that this apparently superstitious behavior may in fact reflect the engagement of adaptive mechanisms that
are critical for responding to environments containing statistics that change over time. We use a normative Bayesian framework to show that a prior belief in changing statistics is sufficient to produce the type of sequential effects observed in subjects' behavior. The Bayesian algorithm is shown to have an approximately linear dependence on the history of past observations, where the dependence decays exponentially into the past; this exponential decay is also apparent in human and
monkey behavior, as we found by re-analyzing data from published studies. We show that the necessary computations can be implemented by neurons with dynamics previously thought to underlie a variety of sensory processing and decision making computations. Our work provides a unifying account of how decision-making SHOULD adapt to a changing
environment, a functional theory for WHY subjects exponentially discount the past, as well as a principled hypothesis of HOW neurons can implement the necessary computations.
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