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Flexibly switching between tasks is one of the paradigmatic functions of so-called "executive control" processes. Neuroimaging studies have implicated both prefrontal and parietal cortical regions in the processing necessary to effectively switch task. Beyond their general involvement in this critical function, however, little is known about the dynamics of processing across frontal and parietal regions. For instance, it remains to be determined to what extent these areas play a role in preparing to switch task before arrival of the stimulus to be acted upon and to what extent they play a role in any switching processes that occur after the stimulus is presented. Here, we used the excellent temporal resolution afforded by high-density mapping of brain potentials to explore the time course of the processes underlying (1) the performance of and (2) the preparation for a switch of task. We detail the contributions of both frontal and parietal processes to these two aspects of the task-switching process. Our data revealed a complex pattern of effects. Most striking was a period of sustained activity over bilateral parietal regions preceding the switch trial. Over frontal regions, activity actually decreased during this same period. Strongest sustained frontal activity was in fact seen for trials on which no switch was required. Further, we find that the first differential activity associated with switching task was over posterior parietal areas (220 ms), whereas over frontal scalp, the first differential activity is found more than 200 ms later. These and other effects are interpreted in terms of a "competition" model in which preparing to switch task is understood as the beginning of a competition between the potentially relevant tasks that is resolved during the switch trial. Our findings are difficult to account for with models that posit a strong role for frontal cortical regions in "reconfiguring" the system during switches of task.

The aim of this study was to analyze the timing and topography of brain activity in relation to the cognitive processing of different types of auditory information. We specifically investigated the effects of familiarity on environmental sound identification, an issue which has been little studied with respect to cognitive processes, neural substrates, and time course of brain activity. To address this issue, we implemented and applied an electroencephalographic mapping method named event-related desynchronization, which allows one to assess the dynamics of neuronal activity with high temporal resolution (here, 125 ms); we used 19 recording electrodes with standard positioning. We designed an activation paradigm in which healthy subjects were asked to discriminate binaurally heard sounds belonging to one of two distinct categories, “familiar” (i.e., natural environmental sounds) or “unfamiliar” (i.e., altered environmental sounds). The sounds were selected according to strict preexperimental tests so that the former should engage greater semantic, and the latter greater structural, analysis, which we predicted to preferentially implicate left posterior and right brain regions, respectively. During the stimulations, significant desynchronizations (thought to reflect neuronal activations) were recorded over left hemisphere regions for familiar sounds and right temporofrontal regions for unfamiliar sounds, but with only few significant differences between the two sound categories and a common bilateral activation in the frontal regions. However, strongly significant differences between familiar and unfamiliar sounds occurred near the end of and following the stimulations, due to synchronizations (thought to reflect deactivations) which appeared over the left posterior regions, as well as the vertex and bilateral frontal cortex, only after unfamiliar sounds. These unexpected synchronizations after the unfamiliar stimuli may reflect an awareness of the unfamiliarity of such sounds, which may have induced an inhibition of semantic and episodic representations because the latter could not be associated with meaningless sounds.

The ability to dynamically modulate the intensity of sustained attention (i.e., alertness) is an essential component of the human executive control system, allowing us to function purposefully in accordance with our goals. In this study we examine high-density ERP markers of alert responding during the fixed sequence sustained attention to response task (SART(fixed)). This paradigm has proven to be a sensitive clinical metric in patient populations with deficits in their ability to sustain attention (e.g., attention deficit hyperactivity disorder). In this task subjects withhold a button press to an infrequent no-go target ('3') embedded within a predictable sequence of numbers ('1' to '9'). Our data reveal a complex pattern of effects across the trial sequence of the SART, with clear contributions from frontal and parietal cortices to sustained attentional performance. Over occipito-parietal regions, early visual attention processes were increased during trial 2 (i.e., trial in which the digit '2' was presented) and trial 3, giving rise to the so-called selection negativity (SN). Two prominent late components were manifest during trial 2: LP1 (550-800 ms) and LP2 (850-1150 ms) over occipito-parietal and central sites. We interpret the LP1 component on trial 2 as reflecting retrieval of the task goal and the subsequent LP2 as reflecting competition between the currently relevant go response and the subsequent no-go response. On trial 3, an enhanced "no-go N2" (250-450 ms) was seen fronto-centrally in the absence of the "no-go P3" that typically follows. Fronto-polar activity was also seen across all trials and may be indicative of subgoal processes to integrate the association between stimulus and goal. Prior to a lapse of attention (i.e., failure to inhibit a response to "3") the LP1 was significantly attenuated on the preceding trial 2 indicating a failure of anticipatory goal-directed processing. The results are discussed in terms of models of sustained attention involving frontal and parietal cortices.

Recent studies have revealed a distinct cortical network activated during the analysis of sounds' spatial properties. Whether common brain regions in this auditory where pathway are involved in both auditory motion and location processing is unresolved. We investigated this question with multichannel auditory evoked potentials (AEPs) in 11 subjects. Stimuli were binaural 500-ms white noise bursts. Interaural time differences (ITD) created the sensation of moving or stationary sounds within each auditory hemifield, and subjects discriminated either their position or direction of motion in a blocked design. Scalp potential distributions (AEP maps) differentiated electric field configurations across stimulus classes. The initial approximately 250-ms poststimulus yielded common topographies for both stimulus classes and hemifields. After approximately 250-ms, moving and stationary sounds engaged distinct cortical networks at two time periods, again with no differences observed between hemifields. The first ( approximately 250- to 350-ms poststimulus onset) was during stimulus presentation, and the second ( approximately 550- to 900-ms poststimulus onset) occurred after stimulus offset. Distributed linear inverse solutions of the maps over the 250- to 350-ms time period revealed not only bilateral inferior frontal activation for both types of auditory spatial processing, but also strong right inferior parietal activation in the case of auditory motion discrimination. During the later 550-to 900-ms time period, right inferior parietal and bilateral inferior frontal activity was again observed for moving sounds, whereas strong bilateral superior frontal activity was seen in the case of stationary sounds. Collectively, the evidence supports the existence of partly segregated networks within the auditory where pathway for auditory location and auditory motion processing.