Frontal and striatal brain lesions increase susceptibility to masking in perceptual decisions

Recent data suggest that perceptual decisions on masked stimuli involve neural activity in frontal cortex. We examined the effect of damage to frontal and striatal brain regions in man on the susceptibility to backward masking in rapid stimulus streams. Patients with unilateral frontal excisions and patients with early Huntington's disease were compared to controls in the identification of a brief white letter embedded in short streams of black letters at two presentation rates: (a) 9 letters/s; (b) 12.5 letters/s and also in a control condition in which the first post-target masking letter was absent. Patients could identify the target when the post-target mask was absent, but reducing the delay between stimuli significantly increased the error rates in patients. Intrusion errors often involved reporting post-target or pre-target distractors instead of the target. These results suggest that fronto-striatal lesions increase the period during which perceptual decisions are susceptible to perturbation. This deficit is compatible with a functional role of frontal systems in the cognitive control of brief perceptual decisions.