Nonstationarity and the measurement of psychophysical response in a visual inspection-time task

Inspection time (IT) is an index of speed of early visual processing that correlates significantly with measures of higher cognitive ability. An obstacle to the understanding of this important association is the lack of an agreed mechanistic model of the processes involved in IT performance. It is argued here that, whereas in current attempts to model IT processing, stationarity in IT performance is assumed, it is necessary to address this key issue empirically. Two practised subjects undertook 38,400 trials on a standard two-choice visual IT discrimination task. Testing was spread evenly over 60 days. Isotonic regression analysis revealed that both subjects showed nonstationarity in performance over very long periods. The results were not merely due to long-term practice effects; one subject improved in some durations but worsened at others. Therefore, current attempts at modelling in which stationarity in IT performance is assumed must be altered to incorporate the temporal dynamics of IT performance, which might have individual differences.