A carry-over task rule in task switching: an ERP investigation using a Go/Nogo paradigm.

Investigations of executive control using a task-switching paradigm have consistently found longer reaction times for task-switch trials than task-repetition trials. This switch cost may result from interference by a stimulus-response (SR) rule carried over from the preceding alternative task. We examined event-related brain potential (ERP) evidence for such carry-over effects using a combined paradigm of task switching with Go/Nogo; Nogo trials, which require no response execution, should expose carry-over effects from preceding trials. On Go trials, twelve participants performed a button-pressing task in compatible (hand and signal direction consistent) and incompatible conditions, which switched predictably every three trials. Reaction times were longer on switch than on repetition trials. On compatible switch trials, a stimulus-locked lateralized readiness potential (sLRP) for Nogo stimuli revealed a positive dip, suggesting incorrect response activation in the early automatic process that was induced by a SR rule carried over from the preceding task.

[Asymmetric switch cost: an investigation using lateralized readiness potentials].

Switching to a dominant task incurs larger costs than switching to a non-dominant task. This study investigated whether this cost asymmetry derives from the inhibition of the dominant task rule that occurred during the previous trial. Participants were presented with a five-letter array consisting of (left) and (right), and asked to respond to the central target letter after being informed about the task rule with a pre-cue. The rule was switched between dominant (left-hand to, right-hand to) and non-dominant tasks (right-hand to, left-hand to) every two trials. Reaction times revealed the asymmetrical switch cost and the effect of target-flanker congruency. Stimulus-locked lateralized readiness potentials showed that the dominant task rule was inhibited during the non-dominant task, whereas this inhibition was not carried over to the dominant task trial.

[Hemispheric difference in the time course of semantic activation: evidence from event-related potentials].

To investigate hemispheric differences of semantic activation, event-related potentials were recorded when two pairs of words were successively presented with a SOA of 200 ms or 800 ms. Each word pair was simultaneously exposed to the left (LVF) and right (RVF) visual fields. Participants were required to attend one visual field and make a judgment whether the words (prime-target) presented at the attended visual field were semantically related or not. A priming effect on reaction time was observed for RVF targets with SOA 200 ms, and for both LVF and RVF targets with SOA 800 ms, consistent with the idea that semantic activation is faster in the left than the right hemisphere. In contrast, the priming effect on N400 amplitude was not affected by the SOA and visual field, and the onset latency was shorter for RVF than for LVF targets, irrespective of SOA. The N400 priming effects were interpreted to be associated with task-induced semantic processing.

An ERP investigation of task switching using a flanker paradigm.

To investigate whether a task rule carried over from the preceding trial modifies an early portion of the stimulus-locked lateralized readiness potential (sLRP), participants were asked to perform two different flanker tasks that were switched. One task required a left-hand response for a central letter "H" in a five-letter array, and a right-hand response for "S." The stimulus-response rule was reversed for the other task. Reaction times (RTs) were prolonged for switch trials as compared to repeat trials, irrespective of whether stimuli in the five-letter array were congruent or incongruent flankers. For the incongruent stimuli, the onset latency of the sLRP was shorter on switch trials compared to repeat trials although the onset for congruent stimuli was delayed on the switch trials. These different sLRP effects as a function of congruent versus incongruent stimuli were interpreted as a manifestation that automatic response activation, dependent on a task rule carried over from a preceding trial, was induced by the early transmission of flanker-related information from stimulus evaluation processes. The finding that the earlier sLRP onset for incongruent stimuli on switch trials did not shorten RTs suggests that RT switch costs are not attributable simply to passive processes.

[Semantic priming within and between the visual fields: an event-related brain potential study].

To investigate interhemispheric transfer of language information, event-related brain potentials (ERPs) were recorded during performance of a semantic matching task in which prime and target word pairs were sequentially presented to either the left (LVF) or the right (RVF) visual field. A posterior slow negative shift prior to the target presentation developed over both hemispheres for LVF primes, whereas for RVF primes the negative shift declined over the right hemisphere. An N400 attenuation was observed for targets semantically related to the primes and was predominant over the left parietal site. The N400 priming effect was significantly reduced for LVF targets preceded by RVF primes, compared to other patterns of presentation. These findings indicate that semantic priming is attenuated in the right hemisphere when information is transferred from the left hemisphere.

[Stimulus set and response set in task switching: a comparison using ERP].

Event-related potential (ERP) and reaction time (RT) were recorded to investigate the time course of processes involved in set switching. The cued set-switching paradigm required participants to switch stimulus task sets between male and female face-images memorized as targets prior to a trial block and response task sets between two stimulus-response mappings for each stimulus task. Replicating previous findings, an RT switch-cost was found when compared with set-repeat trials. The RT was also prolonged for a stimulus task requirement of memory comparison with two-face targets rather than one face. A similar prolongation with memory comparison was observed in P3b latency, which showed no switch effect. The switch effect was observed for the onset latency of stimulus-locked lateralized readiness potential (LRP), measured as an index of commencement of motor processes after response selection. The response-locked LRP indicated that the final process of motor execution itself was not modified by set switching. The processes producing the stimulus-locked LRP switch cost, associated with response task set, were discussed in terms of two hypotheses, exogenous reconfiguration and carryover.

Recognition memory for unfamiliar faces in Parkinson's disease: behavioral and electrophysiologic measures.

We analyzed event-related potentials (ERPs) and behavioral measurements during a recognition memory task in 15 normal elderly subjects and 15 patients with Parkinson's disease (PD). To elicit ERPs unfamiliar faces were repeated immediately after initial presentation (at lag 0), after one intervening face (at lag 1) or at lag 3. Compared to normal controls, PD patients showed decreased accuracy in recognizing new unfamiliar faces. P170 latency and amplitude were similar between both groups. ERP amplitude between 300 and 500 ms after the stimulus in control subjects showed a positive shift (ERP repetition effect) for lag 0 at all sites and for lag 1 and 3 repetitions at the Fz site, while effects in the PD group were not noted at any site, even for the lag 0 repetition. ERP waveforms for the first presentation of faces in PD patients showed a significant positive shift compared to normal controls. These data suggest intact perception but impaired recognition memory for unfamiliar faces in PD. In addition, recognition memory deficits in PD may result from impairment of comparison of structural representations of presented faces with stored representations of faces known to the observer.

[Interaural attention switching suppresses N400 attenuation with spoken word repetition].

Event-related brain potentials were recorded from participants who listened to two pairs of words (and occasionally nonwords) simultaneously presented to both ears with stimulus onset asynchrony (SOA) of 1 s on a trial. When participants were instructed to fixate their attention to one ear and detect nonwords, the N400 attenuation was observed for a word repeated in the attended ear. The requirement of interaural attention switching between the first and second presentations of word pairs, however, suppressed the repetition effect on N400, even when the first and repeated presentations of a word were attended. The suppression of N400 attenuation was interpreted to indicate that attention switching interferes with the immediate availability of the decision for the first presentation in working memory.