Prior Expectations Primes The Brain

The paper linked here shows that prior expectation of an external stimulus “tunes” the brain to expect that stimulus and thus affects the ability for that stimulus to be processed. When the expected “feature” was “task-oriented,” expecting the stimulus in advance improved participant performance. The authors also claim that: “We observed a representation of expected stimuli in the neural signal shortly before they were presented, showing that expectations indeed induce a preactivation of stimulus templates.”  In other words, the nervous system actually physically prepares for the expected stimulus so that evidence of the stimulus in a neural signal can be discerned even before the stimulus is actually presented.  This sheds light on how expectation can affect perception and, indeed, improve performance.  Abstract:

Perception can be described as a process of inference, integrating bottom-up sensory inputs and top-down expectations. However, it is unclear how this process is neurally implemented. It has been proposed that expectations lead to prestimulus baseline increases in sensory neurons tuned to the expected stimulus, which in turn, affect the processing of subsequent stimuli. Recent fMRI studies have revealed stimulus-specific patterns of activation in sensory cortex as a result of expectation, but this method lacks the temporal resolution necessary to distinguish pre- from poststimulus processes. Here, we combined human magnetoencephalography (MEG) with multivariate decoding techniques to probe the representational content of neural signals in a time-resolved manner. We observed a representation of expected stimuli in the neural signal shortly before they were presented, showing that expectations indeed induce a preactivation of stimulus templates. The strength of these prestimulus expectation templates correlated with participants' behavioral improvement when the expected feature was task-relevant. These results suggest a mechanism for how predictive perception can be neurally implemented.