Cognitive N140 electrogenesis and concomitant 40 Hz synchronization in mid-dorsolateral prefrontal cortex (area 46) identified in non-averaged human brain potentials

The mid-dorsolateral prefrontal area 46 has working memory functions for putting current cognitive processing into context and for updating relevant information on a trial-by-trial basis. Using non-averaged human brain responses to a target finger stimulus attended by the subject, we identified the cognitive prefrontal N140 electrogenesis with the Z method which numerically assesses the detailed consistency between scalp topographies of any single response and a grand average template. The cognitive N140 was present between 100 and 180 ms at the scalp contralateral to the attended target finger stimulus. Control responses to physically identical finger stimuli that were ignored by the subject did not elicit a prefrontal N140. In brain mapping the N140 focus was located 30-80 mm from scalp midline, and 50-60 mm in front of the vertex (Cz) coronal plane of the head. The data were consistent with recently disclosed anatomical locations of area 46 and they further document its interindividual variations in brain-to-scalp relationship. N140, is thought to manifest cortico-cortical activation from the posterior parietal area 7b which generates the cognitive P100 electrogenesis that precedes N140 in the target response. At the prefrontal site, the non-averaged EEG showed desynchronized 40 Hz oscillations, but these became transiently phase-locked in conjunction with the cognitive N140 excitatory effect. This seems in line with the Traub model suggesting that excitation of certain cortical inhibitory interneurons generates doublet firings which will pace the spiking of pyramidal neurons so as to promote synchronization at about 40 Hz. These results complement our recent finding that P300 inhibition conversely disrupts on-going 40 Hz phase-locking ('binding'). Considering the marked trial-to-trial variations in timing of the cortical cognitive processes, such physiological relationships between target response components and 40 Hz dynamics could only be uncovered by studying non-averaged single responses.

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