Visual decision making is most influenced by past experience of weak signals

Visual decision making is regarded as a process in which sensory signals are integrated toward an appropriate action. Decisions are based not only on current sensory signals, but also on statistical knowledge about past incidences. It is naturally thought that the statistical knowledge formed by stronger visual signals more greatly influences decision makings. However, here we have found that very weak signals on previous trials have a greater influence on current decisions than do stronger signals on current decisions. On each trial, subjects were presented with a noisy, oriented stimulus, and asked to report which one of two alternative orientations was presented. The signal-to-noise ratio (0%, 5%, 15%, or 20%) was varied from trial to trial. While 5% signal was too weak to perceive, 15% and 20% signals were conspicuous. One session consisted of 6 runs, with 144 trials per run. In every block of 48 trials, one of three pairs of incidence probabilities (33%/66%, 50%/50%, or 66%/33%) was assigned to the two orientations. Two groups of 10 subjects were employed In the first group, incidence probability was manipulated for the 5%, 15%, and 20% signal trials. In the second group, incidence probability was manipulated for the 15% and 20% signal trials only and the incidence probability for the 5% signal trials was constantly 50%. While the first group showed a significant degree of positive correlation between the past stimulus sequence and current response sequence, the second group showed no correlation. Given that the only difference between two groups was the manipulation of trails with imperceptible 5% signal, the present results indicate that on the contrary to the general thought visual decision making is more influenced by past experience of weak signals than that of stronger signals.