Response inhibition and monitoring in schizophrenia: evidence from countermanding saccades
Thakkar, Katharine Natasha
The goal of this work was to investigate response inhibition and response monitoring in patients with schizophrenia using a saccade stop signal task, which assesses the ability to stop a planned action. Medicated patients and controls performed the task during eye tracking. On no-stop signal (NSS) trials, a peripheral target appeared, and subjects were instructed to look towards it. On stop signal (SS) trials, the fixation point was re-illuminated after a variable delay (stop signal delay; SSD) following target onset, and subjects were instructed to withhold the saccade. Successfully or unsuccessfully inhibited trials were labeled cancelled and noncancelled, respectively. The probability of failing to cancel at each SSD describes the inhibition function. A flatter slope reflects impaired control over the inhibitory process. The duration of the inhibitory process (stop signal reaction time; SSRT) was determined according to a race model. Response monitoring was indexed by the magnitude and direction of RT adjustments as a function of performance in the prior trial. SSRT did not differ across groups, but the slope of the inhibition function tended to be reduced in patients. Patients evidenced increased RT adjustments based on trial history, and increased positive symptoms were associated with decreased slowing following an error. The data suggest that response inhibition impairments in schizophrenia may be due to a failure to trigger the inhibitory response, rather than an increase in the time needed to inhibit. This deficit exists despite greater RT adjustments, which were associated with decreased symptom severity.