Easier tasks can have higher processing loads: task difficulty and cognitive resource limitations in schizophrenia

A differential deficit on the more difficult of 2 cognitive tasks has been found in numerous studies of schizophrenia. Resource limitations, rather than impairments in particular cognitive abilities, can explain these findings. Researchers often match tasks for difficulty level (healthy participant performance) to rule out resource limitations. This assumes processing resource load can be estimated from task difficulty, which is not always the case. A direct measure of resource allocation is needed. In this study, pupillary responses were recorded to index resource allocation (greater dilation indicates greater allocation) during performance of the digit span distractibility task in participants with schizophrenia (n=35) and nonpsychiatric participants (n=35). Nonpsychiatric participants recalled significantly more digits in the distraction, relative to the neutral, condition but also showed significantly greater pupil dilation in the distraction condition. The psychometrically "easier" condition, therefore, had a higher processing load. Participants with schizophrenia showed greater impairment relative to nonpsychiatric participants in the distraction condition. This finding cannot be attributed to a deficit in distractibility, however, because poorer performance was found in the higher-load condition, so resource limitations cannot be ruled out.