Cortical steady potential shifts and anodal polarization during delayed response performance

Two experiments are described in which monkeys with chronically implanted non-polarizable electrodes in prefrontal, precentral, and occipital cortex were trained on delayed response (DR) tasks for food rewards. In the first experiment, involving recordings of cortical steady potentials (SPs), variations in DR task parameters enabled us to delineate three surface negative SP shifts during the DR trial: (i) before cue presentation, (ii) following the response when reward is eaten, and (iii) at the start of the delay period. The first two shifts are interpreted as indicative of diffuse neuronal processes related to "expectancy" and "reinforcement", respectively, since similar shifts were recorded from other cortical areas. The third shift, recorded only from prefrontal cortex, is interpreted as a correlate of short-term spatial memory formation. Its magnitude was unaffected by variation in duration of either the cue or the delay period, but was significantly correlated with the level of correct perfomance (r's from 0.74 to 0.90 for four monkeys). In the second experiment, surface anodal polarization was applied to prefrontal cortex in three monkeys during acquisition sessions of 4-sec or 8-sec DR. In all monkeys higher acquisition rates were obtained under polarization than under comparable non-polarization conditions. The most dramatic facilitation was obtained with 40 pa current applied throughout several successive sessions. These findings support the concept of excitability changes of prefrontal neurons during DR performance.