Changes in the Primary Sensory Cortex during Cognitive Tasks

To study early cognitive processes during selective attention tasks, we recorded auditory evoked potentials (AEPs) to 1000 Hz and 2000 Hz tone bursts randomly presented at right or left ear. Twelve normal paid volunteers with normal hearing participated in the study. Cognitive tasks included pitch (high or low) or laterality (left or right) discrimination. In task conditions, the subjects were requested to attend to the stimuli and extend the right index finger as a response to the target stimuli that appeared in 20% of the probabilities of sequences; for example, when the target is high pitch, the probability of 2000 Hz tone was 10% each at right and left ears: and, that of 1000 Hz tone as non-target was 40% each at right and left ears. Reaction times to target stimuli were measured as an onset time of right finger extension movement. In control sessions, 1000 Hz or 2000 Hz tone alone was presented at left or right ear and no task was required. We compared AEPs elicited by target, non-target and control stimuli. We found that the N100 response is enlarged for target and non-target condition in both pitch and laterality discriminating tasks. However, a large P300 response was noticed for only target conditions in the cognitive tasks. Furthermore, the mean or median latency of reaction times in many trials was shorter than the peak latency of the P300 response. We therefore speculate that marked enlargement of N100 response to target stimuli in task conditions supports early selection model and that the P300 response reflects neural processes in relation to awareness of the target stimuli rather than differentiation of the target from non-target stimuli.