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.