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Background: Lesion studies in human and non-human primates have linked several different regions of prefrontal cortex (PFC) with the ability to inhibit inappropriate motor responses. However, recent functional neuroimaging studies have specifically implicated right inferior PFC in response inhibition. Right frontal dominance for inhibitory motor control has become a commonly accepted view, although support for this position has not been consistent. Particularly conspicuous is the lack of data on the importance of the homologous region in the left hemisphere. To investigate whether the left inferior frontal gyrus (IFG) is critical for response inhibition, we used neuropsychological methodology with carefully characterized brain lesions in neurological patients.

What are the neural bases of semantic memory? Traditional beliefs that the temporal lobes subserve the retrieval of semantic knowledge, arising from lesion studies, have been recently called into question by functional neuroimaging studies finding correlations between semantic retrieval and activity in left prefrontal cortex. Has neuroimaging taught us something new about the neural bases of cognition that older methods could not reveal or has it merely identified brain activity that is correlated with but not causally related to the process of semantic retrieval? We examined the ability of patients with focal frontal lesions to perform a task commonly used in neuroimaging experiments, the generation of semantically appropriate action words for concrete nouns, and found evidence of the necessity of the left inferior frontal gyrus for certain components of the verb generation task. Notably, these components did not include semantic retrieval per se.One of the earliest findings in cognitive neuroimaging was that the left inferior frontal gyrus (IFG) is activated when subjects are shown a concrete noun and must generate a semantically appropriate verb (1-5). This finding, which now has been replicated many times, has been interpreted as evidence that the left IFG plays a role in the retrieval of semantic knowledge. This interpretation is supported by other neuroimaging studies finding left IFG activation during different tasks requiring semantic retrieval, such as living͞nonliving classification (6-8). In contrast, the literature on cognitive impairments after focal brain lesions reveals no particular association between semantic retrieval and left prefrontal cortex (9). Patients with prefrontal lesions have normal language comprehension; although lesions to either left or right prefrontal cortex do impair the ability to generate semantically related words on a category fluency task, they also impair performance in nonsemantic fluency tasks (10), consistent with an underlying impairment that is not semantic per se (11). Impairments of semantic knowledge are most associated with temporal lobe, not frontal lobe, pathology (12)(13)(14).In this paper, we consider two possible explanations for the discrepancy between the findings from neuroimaging and neuropsychological studies of the role of prefrontal cortex in semantic retrieval. First, differences in the outcomes from these two types of studies may reflect simply the different types of inferences to which neuroimaging studies and lesion studies lend themselves. Neuroimaging studies are limited to inferences about brain regions that are engaged by a cognitive process as revealed by correlated changes in activity related to processing demands. In contrast, a neuropsychological approach allows one to make inferences about brain regions that are necessary for a cognitive process, when that process can be shown to depend on the integrity of a given brain region (15). Thus, one interpretation of the neuroimaging findings that can be tested in patients with frontal...

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The relative importance of the anterior cingulate cortex (ACC) for the detection and resolution of response conflicts versus its role in error monitoring remains under debate. One disputed issue is whether conflict detection and error monitoring can be viewed as unitary functions performed by the same region of the ACC, or whether these processes can be dissociated functionally and anatomically. We used a combination of electrophysiological and neuropsychological methods to assess these competing hypotheses. A neurological patient with a rare focal lesion of rostral-tomiddorsal ACC was tested in an event-related potential study designed to track the time course of neural activity during conflicts and erroneous responses. Compared with controls, the errorrelated negativity component after incorrect responses was attenuated in the patient, accompanied by lower error-correction rates. Conversely, the stimulus-locked component on correct conflict trials, the N450, was enhanced, and behavioral performance was impaired. We hypothesize that intact regions of lateral prefrontal cortex were able to detect response conflict, but damage to the dorsal ACC impaired response inhibition, which may be due to disconnection from cingulate and supplementary motor areas. The results implicate rostral-dorsal ACC in error monitoring and suggest this function can be dissociated from conflict-detection processes.

To clarify the time course of neural responses to faces with different emotional expressions, we used event-related potential (ERP) and reaction time measures. Faces expressing four different emotions (happy, neutral, fearful, disgusted) and houses were shown in both upright and inverted orientations while subjects performed an immediate-repeats task. Results indicated that upright fearful expressions enhanced the frontocentral P200. However, emotional effects on the N170 and late positive component interacted with face orientation and were not selective for any specific expression. A unique negative component for upright disgust faces was observed at approximately 300 ms at occipital regions. These results provide evidence for emotion-specific ERPs associated with fear and disgust, distinct from other non-specific configurational and attentional effects.

The anterior cingulate cortex (ACC) has been proposed as part of the brain's attentional control network, but the exact nature of its involvement in cognitive and motor operations is under debate. Assessing effects of human ACC damage directly addresses the problem of ACC function. We report that executive control processes of a patient with a focal right hemisphere anterior cingulate lesion were not compromised. However, her performance level depended on the response modality used. Under the same task requirements, she was impaired when giving manual responses, but not vocal responses. Thus, we provide neuropsychological evidence for functional specialization within the human ACC.

To compare neural activity produced by visual events that escape or reach conscious awareness, we used event-related MRI and evoked potentials in a patient who had neglect and extinction after focal right parietal damage, but intact visual fields. This neurological disorder entails a loss of awareness for stimuli in the field contralateral to a brain lesion when stimuli are simultaneously presented on the ipsilateral side, even though early visual areas may be intact, and single contralateral stimuli may still be perceived. Functional MRI and event-related potential study were performed during a task where faces or shapes appeared in the right, left, or both fields. Unilateral stimuli produced normal responses in V1 and extrastriate areas. In bilateral events, left faces that were not perceived still activated right V1 and inferior temporal cortex and evoked nonsignificantly reduced N1 potentials, with preserved face-specific negative potentials at 170 ms. When left faces were perceived, the same stimuli produced greater activity in a distributed network of areas including right V1 and cuneus, bilateral fusiform gyri, and left parietal cortex. Also, effective connectivity between visual, parietal, and frontal areas increased during perception of faces. These results suggest that activity can occur in V1 and ventral temporal cortex without awareness, whereas coupling with dorsal parietal and frontal areas may be critical for such activity to afford conscious perception. R ight parietal damage may cause a loss of awareness for contralateral (left) sensory inputs, such as hemispatial neglect and extinction (1-3). Visual extinction is the failure to perceive a stimulus in the contralesional field when presented together with an ipsilesional stimulus (bilateral simultaneous stimulation, BSS), even though occipital visual areas are intact and unilateral contralesional stimuli can be perceived when presented alone. It reflects a deficit of spatial attention toward the contralesional side, excluding left inputs from awareness in the presence of competing stimuli (2, 3). Spatial attention involves a complex neural network centered on the right parietal lobe (4, 5), but how parietal and related areas interact with sensory processing in distant cortices is largely unknown.Here we combined event-related functional MRI (fMRI) and event-related potentials (ERPs) to study the regional pattern and temporal course of brain activity produced by seen and unseen stimuli in a patient with chronic neglect and extinction caused by parietal damage. In keeping with intact early visual areas in such patients, behavioral studies suggest that some residual processing may still occur for contralesional stimuli without attention, or without awareness, including ''preattentive'' grouping (e.g., refs. 6 and 7) and semantic priming (e.g., ref. 8). It has been speculated (3, 9) that such effects might relate to separate cortical visual streams, with temporal areas extracting object features for identification, and parietal areas encoding spatial...