Mapping task-switching in frontal cortex through neuropsychological group studies

This paper considers evidence provided by large neuropsychological group studies and meta-analyses of functional imaging experiments on the location in frontal cortex of the subprocesses involved in the carrying out of task-switching paradigms. The function of the individual subprocesses is also considered in the light of analyses of the performance of normal subjects

MMN and Differential Waveform

A mismatch negativity response (MMN) and a new differential waveform were derived in an effort to evaluate a neural refractory or recovery effect in adult listeners. The MMN was elicited using oddball test runs in which the standard and deviant stimuli differed in frequency. To derive the differential waveform, the same standard and deviant stimuli were presented alone. MMN responses were obtained by subtracting the averaged responses to standards from the deviants. The differential waveforms were obtained by subtracting the averaged responses to standards presented alone from deviants presented alone. Scalp topography for the MMN and differential waveforms were similar. A significant (p < .05) positive and negative correlation was found between the earlier and later components of the bimodal MMN and the N1 and P2 component of the differential waveform, respectively. Further, N1 and P2 of the differential waveform were significant (p < .05) predictor variables of early and late peak amplitudes of the MMN. These results suggest that refractory effects may overlay/modify the morphology of the MMN waveform

Regional frontal injuries cause distinct impairments in cognitive control

Background: Lesions of the frontal lobes may impair the capacity of patients to control otherwise intact cognitive operations in the face of ambiguous sensory input or conflicting possible responses. Objective: To address the question of whether focal lesions in different regions of the frontal lobes produced specific impairments in cognitive control. Methods: We evaluated 42 patients with chronic frontal lesions and 38 control subjects on a modified Stroop test that allowed measurement of reaction times and errors. Planned, stratified analyses permitted identification of discrete frontal lesions that are critical for impaired performance. Results: Lesions of the left ventrolateral region produced an increased number of incorrect responses to distractors. Lesions of a large portion of the right superior medial region, including anterior cingulate, supplementary motor area (SMA), pre-SMA, and dorsolateral areas, caused a slow reaction time and a decreased number of correct responses to targets. Conclusion: Lesions in two distinct frontal regions impair cognitive control for a Stroop task, and the mechanisms of impairment are specific to the region of injury. This is support for a general proposal that the supervisory system is constructed of distinct subsystems

Effects of focal lesions on response inhibition

This study examined the performance of 38 normal subjects and 43 patients with focal lesions of the frontal lobes on a simple go\u2013nogo task where the probability of the nogo stimulus was either 75% or 25%. Patients with lesions to the superior medial parts of the frontal lobes, in particular to the left superior portion of Brodmann area 6 (which includes the supplementary motor areas and the premotor areas for the right hand) had an increased number of false alarms (incorrect responses to the nogo stimulus). These results indicate that area 6 is specifically involved in the inhibition of response. Patients with lesions to the right anterior cingulate (areas 24 and 32) were slower and more variable in their reaction time. These findings could be explained by an inability to sustain stimulus-response contingencies. Lesions to the right ventrolateral prefrontal cortex (Brodmann areas 44, 45, 47) also increased the variability of response, perhaps by disrupting monitoring performance