Functional Stability and Learning in the Dorsolateral Prefrontal Cortex

Abstract

"Stable multi-day recordings from chronically implanted microelectrodes within the dorsolateral prefrontal cortex of two monkeys performing three Go/NoGo visual discrimination tasks (one requiring well-learned responses, two requiring learning) demonstrated that the majority of prefrontal neurons were 'functionally stable'. Action potentials of 94 neurons were stable over 2-9 days; 66/94 (70%) of these cells responded each day, 22/94 (23%) never responded significantly, and 6/94 (6%) responded one day but not the next. Of 66 responsive neurons, 55 were selective for either Go or NoGo trials, individual stimuli, or eye movements." (Greenberg and Wilson, 2004) Selectivity was maintained, for 46/55 neurons across all recording days. Response strength (baseline vs. post-stimulation firing rates) and event-related response timing also displayed stability. Stability generalized across neuronal response type suggesting that functional stability is a general property. Long-term recordings from other studies supported similar conclusions suggesting that neurons throughout the brain are functionally stable. Single-day recordings from different neurons within the same cortical regions demonstrated neuronal response flexibility while monkeys learned associations among visual cues, and Go/NoGo behavioral responses. Of 116 neurons, 57 (49%) displayed significant change points in firing rates during novel learning (n=18), reversal learning (n=12), or both tasks (n=27). Six of 57(10.5%) neurons had firing rates changes prior to learning and might have been causally related to the monkeys' behavioral changes. However, only 18/152 (12%) of the total number of firing rate changes occurred prior to the monkeys' learning meaning that most appeared to be the consequences of learning rather than the causes.