Cognitive Neuroscience and Neuropsychology

The control of action and thought plays an important role in the organization of human behavior. Cognitive and neuropsychological theories have therefore proposed a cognitive system which guides behavior in novel or challenging situations. Theories of attention1 and of working memory2,3 have stressed the significance of an ‘executive control system’ in cognitive processing. According to Norman and Shallice,4 executive control is exerted by a supervisory attention system which is engaged in situations requiring decision making, conflict resolution, error correction and response inhibition. In short, executive control is involved in cognitive processing when learned routines are not available or ineffective for task performance. Executive function can be studied in go/no go tasks which require inhibition of a prepared response. In such tasks, subjects usually have to respond to a given target stimulus (a tone or a letter) overtly or covertly (go condition). In a second condition, however, they have to withhold a response to a given stimulus (no go condition). The go/no go paradigm has been applied in several electrophysiological studies to investigate the effects of response inhibition on eventrelated potentials (ERP). Two major ERP events related to response inhibition have been frequently described. First, a negative peak between 200 and 300 ms over the fronto-central scalp, termed the N2 component, is enlarged in the no go condition compared to the go condition.5,6 This augmented N2 has also been found in variants of the go/no go task when a covert response such as counting of tones (but no overt motor response) had to be suppressed.7 It is generally assumed that the N2 effect reflects higher level inhibitory processes in an executive control system. Second, a positive wave usually peaking between 300 and 60 0 ms (the P3 component) is increased in the no go condition over fronto-central regions,7,8 particularly over the left hemisphere of subjects responding with their right hands.9 This augmented P3 is often referred to as ‘no go P3’ because of its peculiar topography (maximum over fronto-central sites) which differs from the centroparietal ‘go P3’. Both the N2 and the P3 effect are usually interpreted as electrophysiological signatures of response inhibition. This interpretation has been questioned for the following reasons. First, it has been a matter of debate whether the enlarged fronto-central positivity is related to the inhibition of a motor response or just Cognitive Neuroscience and Neuropsychology