History bias and its perturbation of the stimulus representation in the macaque prefrontal cortex.

Multiple history biases affect our representation of magnitudes, such as time, distance and size. It is not clear whether the previous stimuli interfere with the discrimination process from the moment of stimulus presentation, during working memory retention or even later during the decision-making phase. We used a spatial discrimination task involving two stimuli of different magnitudes, presented sequentially at various distances from the centre. The monkey's task was to select the farthest of them. We showed that the previous stimulus magnitude produced an attractive effect on the current stimulus magnitude and that this effect was stronger when their stimulus features differed. In this case at the neural level we also observed that decoding of the stimulus magnitude achieved the highest accuracy when it matched the magnitude of the preceding stimulus for which the decoder was trained. This indicates that past stimuli can affect magnitude processing already during the stimulus presentation, even before the decision-making process. Interestingly this effect coincided with an 'activity-silent' period, followed by the reactivation of the decoding of the previous stimulus magnitude. KEY POINTS: Previous experience alters how we perceive the world, including the magnitudes of stimuli. We show that the magnitude of the previous stimulus exerts an attractive effect on the perceived magnitude of the current stimulus, and unexpectedly, this effect is enhanced when their visual features mismatch. It is still debated whether the history effect results from interference with stimulus processing, from its persistence in memory or during the decision-making phase. By recording from the monkey's prefrontal cortex, we found that decoding of the first stimulus is facilitated when its magnitude is similar to that of the recent past stimulus, indicating that the influence of the past stimulus begins during stimulus processing. The effect of the previous stimulus magnitude on the representation of the first current stimulus was stronger during the period in which the past stimulus was not explicitly decoded (the activity-silent phase) and preceded its reactivation.