Prefrontal cortical involvement in visual working memory

Studies of human amnesia provide evidence for a short-term memory store wit h information transfer to long term memory occurring within 60 a of sensory encoding. Human and nonhuman primate research has shown that maintenance o f this short-term or working memory store is dependent upon frontal cortica l activation, although the critical temporal parameters of frontal involvem ent throughout this 60-s window are undetermined. We examined prefrontal co ntributions to rapid (under 2 s) and sustained (over 4 s) visual working me mory by recording behavioral performance and event-related potentials (ERPs ) in patients with lesions in dorsolateral frontal cortex and age-matched c ontrol subjects. Prefrontal lesioned patients generated a reduced sustained frontal positivity at all delays. At short delays, patients generated redu ced performance to stimuli presented in the contralesional field. Patients generated a negative potential (N400), greatest to contralesionally present ed stimuli, that was observed in the control subjects only at long delays. The results indicate that prefrontal lesions impair the frontal component o f an anterior-posterior working memory network activated during rapid and s ustained visual memory processing. Frontal patients may require activation of limbic cortex, indexed by N400, for maintenance of both rapid and sustai ned working memory. (C) 1999 Elsevier Science B.V. All rights reserved.