Setting precedent: initial feature variability affects the subsequent precision of regularly varying sound contexts

The task of making sense of the world around us is supported by brain processes that simplify the environment. For example, repetitive patterns of sensory input help us to predict future events. This study builds on work, suggesting that sensory predictions are heavily influenced by first impressions. We presented healthy adults with a sequence comprising three sounds each differing from the other two on three dimensions; for simplicity A, B and C. These three sounds were arranged in blocks where two were equally common and one was rare, and the probabilities rotated creating three different block types (i.e., probabilities, A < B = C, B < A = C, C < A = B). Sequences included two of each block type with three versions–one starting with A < B = C, one with B < A = C and one with C < A = B. The common tone evoked responses in any given block were highly suppressed consistent with the auditory system predicting regular events, while the rare tone in each block elicited a larger response signaling a prediction error. However, results indicated that the auditory system assessed the configurations in which the two common tones were adjacent in space (within the three locations used) as less volatile compared to when they were highly separate. When the more volatile environment was encountered at the beginning of the sequence, all deviance-related responses were significantly lower in amplitude. Results suggest that the representation of a stimulus configuration is affected by the estimate drawn from the initial context, expanding our notion of the nature of primacy bias to include powerful effects of initial feature variance.