Abstract
Contingent negative variations were recorded while 8 adult subjects performed either letter or drawing task. The letter task required subjects to pay attention to a letter in a letter-drawing embedded figure and classify it into Hiragana and digit ; the drawing task was to classify a drawing in an embedded figure into animal and plant. In a blocked condition, subjects performed one of the two tasks over all 32 trials within a block, whereas in a mixed condition the task to be required was informed by different pitches of a warning tone and hence varied across trials. The subjects received 4 blocks per condition. Irrespective of these conditions, the CNV during the foreperiod for the drawing task was larger at the right temporo-parietal site than its left hemisphere homologue, while no such difference was found in the letter task. The CNV asymmetry, therefore, is interpreted as to reflect the cognitive set that determines which hemisphere engages in the task more predominantly. The lack of blocked/ mixed effect indicates that the cognitive set can be switched from trial to trial during such a short foreperiod of 2.45s. The correct classification score, however, was lower for the mixed than the blocked condition, suggesting that a longer period would be necessary to make up the cognitive set for the higher level of task-performance.
