Ultrafast initiation of a neural race by impending errors.

key pointsThe brain makes decisions by means of races between neural units representing alternative choices. In the present study, we record the eyemovements made in the Wheeless task, when a visual stimulus is followed after a short delay by another stimulus demanding a different response. The behaviour can be very precisely described as a race between three independent decision processes: one Go process for each of the responses, and a Stop process that tries to cancel the first, now erroneous, response. To explain the high success rate for cancellation that we observe, the onset time for the Stop process must be some 10-20 ms shorter than for Go. As well as extending our understanding of the dynamics of complex decision-making, this task provides a rapid, non-invasive method for quantifying disorders of higher neural function. ABSTRACT: The brain makes decisions by means of races between neural units representing alternative choices, and such models can predict behaviour in decision tasks in a precisely quantitative way. But what is less clear is how soon after the stimulus the race actually starts. In the present study, we re-visit a complex decision experiment: the Wheeless task, in which a saccadic stimulus is followed after a short delay by a second stimulus, with the subject sometimes making a saccade to the first, now inappropriate, stimulus, and sometimes going straight to the correct one. We demonstrate that a simple model with three accumulator units, two 'Go' and one 'Stop', can then account in detail for the individual responses made, as well as their timing. This complex decision-making behaviour is predicted directly for each individual subject by their performance in a simple step saccadic task, which identifies the two free parameters that are specific for each subject. By contrast to previous assumptions, we find that it is necessary for the onset time of the Stop unit to be shorter than for Go by 10-20 ms. This suggests a specifically fast mechanism for altering responses in situations where urgent action is needed to prevent an impending error.