Sequential effects of increasing propofol sedation on frontal and temporal cortices as indexed by auditory event-related potentials

Background: It is an open question whether cognitive processes of auditory perception that are mediated by functionally different cortices exhibit the same sensitivity to sedation. The auditory event-related potentials P1, mismatch negativity (MMN), and early right anterior negativity (ERAN) originate from different cortical areas and reflect different stages of auditory processing. The P1 originates mainly from the primary auditory cortex. The MAIN is generated in or in the close vicinity of the primary auditory cortex but is also dependent on frontal sources. The ERAN mainly originates from frontal generators. The purpose of the study was to investigate the effects of increasing propofol sedation on different stages of auditory processing as reflected in P1, MMN, and ERAN. Methods. The P1, the MMN, and the ERAN were recorded preoperatively in 18 patients during four levels of anesthesia adjusted with target-controlled infusion: awake state (target concentration of propofol 0.0 mug/ml), fight sedation (0.5 mug/ml), deep sedation (1.5 mug/ml), and unconsciousness (2.5-3.0 mug/ml). Simultaneously, propofol anesthesia was assessed using the Bispectral Index. Results: Propofol sedation resulted in a progressive decrease in amplitudes and an increase of latencies with a similar pattern for MMN and ERAN. MAIN and ERAN were elicited during sedation but were abolished during unconsciousness. In contrast, the amplitude of the P1 was unchanged by sedation but markedly decreased during unconsciousness. Conclusion: The results indicate differential effects of propofol sedation on cognitive functions that involve mainly the auditory cortices and cognitive functions that involve the frontal cortices.