Time of day and semantic category effects on late components of the visual ERP

The N100, P200, N400 and P600 components of the visual event-related potential were recorded from 11 female subjects every 2 h from 09:00 to 21:00 hours using a semantic categorization task. All subjects scored as "Intermediate" or marginal "Evening" types on a Morningness-Eveningness questionnaire. Amplitude and latency of components, tympanic temperature, and performance measures for positive and negative category instances were assessed. Amplitude of P200 increased across the day. Amplitude of N400 was larger, and latency of P600 was longer, for negative category instances, but neither component varied with time of day. N100 was unaffected by time of day. The results suggest that previous reports of diurnal variations in visual N100-P200 were due to variations in P200 alone, and that diurnal variations in P200 may reflect diurnal variations in underlying arousal levels. In addition, overlap with P600 may have obscured time of day effects for N400.