Reconsidering electrophysiological markers of response inhibition in light of trigger failures in the stop-signal task.

This study investigates the neural correlates underpinning response inhibition using a parametric ex-Gaussian model of stop-signal task performance, fit with hierarchical Bayesian methods, in a large healthy sample (N = 156). The parametric model accounted for both stop-signal reaction time (SSRT) and trigger failure (i.e., failures to initiate the inhibition process). The returned SSRT estimate (SSRTEXG3 ) was attenuated by ≈65 ms compared to traditional nonparametric SSRT estimates (SSRTint ). The amplitude and latency of the N1 and P3 event-related potential components were derived for both stop-success and stop-failure trials and compared to behavioral estimates derived from traditional (SSRTint ) and parametric (SSRTEXG3 , trigger failure) models. Both the fronto-central N1 and P3 peaked earlier and were larger for stop-success than stop-failure trials. For stop-failure trials only, N1 peak latency correlated with both SSRT estimates as well as trigger failure and temporally coincided with SSRTEXG3 , but not SSRTint . In contrast, P3 peak and onset latency were not associated with any behavioral estimates of inhibition for either trial type. While the N1 peaked earlier for stop-success than stop-failure trials, this effect was not found in poor task performers (i.e., high trigger failure/slow SSRT). These findings are consistent with attentional modulation of both the speed and reliability of the inhibition process, but not for poor performers. Together with the absence of any P3 onset latency effect, our findings suggest that attentional mechanisms are important in supporting speeded and reliable inhibition processes required in the stop-signal task.

Late deviance detection in rats is reduced, while early deviance detection is augmented by the NMDA receptor antagonist MK-801.

One of the most robust electrophysiological features of schizophrenia is reduced mismatch negativity, a component of the event related potential (ERP) induced by rare and unexpected stimuli in an otherwise regular pattern. Emerging evidence suggests that mismatch negativity (MMN) is not the only ERP index of deviance detection in the mammalian brain and that sensitivity to deviant sounds in a regular background can be observed at earlier latencies in both the human and rodent brain. Pharmacological studies in humans and rodents have previously found that MMN reductions similar to those seen in schizophrenia can be elicited by N-methyl-d-aspartate (NMDA) receptor antagonism, an observation in agreement with the hypothesised role of NMDA receptor hypofunction in schizophrenia pathogenesis. However, it is not known how NMDA receptor antagonism affects early deviance detection responses. Here, we show that NMDA antagonism impacts both early and late deviance detection responses. By recording EEG in awake, freely-moving rats in a drug-free condition and after varying doses of NMDA receptor antagonist MK-801, we found the hypothesised reduction of deviance detection for a late, negative potential (N55). However, the amplitude of an early component, P13, as well as deviance detection evident in the same component, were increased by NMDA receptor antagonism. These findings indicate that late deviance detection in rats is similar to human MMN, but the surprising effect of MK-801 in increasing ERP amplitudes as well as deviance detection at earlier latencies suggests that future studies in humans should examine ERPs over early latencies in schizophrenia and after NMDA antagonism.

Corrigendum: No Effect of Anodal Transcranial Direct Current Stimulation Over the Motor Cortex on Response-Related ERPs during a Conflict Task.

[This corrects the article on p. 384 in vol. 10, PMID: 27547180.].

No Effect of Anodal Transcranial Direct Current Stimulation Over the Motor Cortex on Response-Related ERPs during a Conflict Task.

Anodal transcranial direct current stimulation (tDCS) over the motor cortex is considered a potential treatment for motor rehabilitation following stroke and other neurological pathologies. However, both the context under which this stimulation is effective and the underlying mechanisms remain to be determined. In this study, we examined the mechanisms by which anodal tDCS may affect motor performance by recording event-related potentials (ERPs) during a cued go/nogo task after anodal tDCS over dominant primary motor cortex (M1) in young adults (Experiment 1) and both dominant and non-dominant M1 in older adults (Experiment 2). In both experiments, anodal tDCS had no effect on either response time (RT) or response-related ERPs, including the cue-locked contingent negative variation (CNV) and both target-locked and response-locked lateralized readiness potentials (LRP). Bayesian model selection analyses showed that, for all measures, the null effects model was stronger than a model including anodal tDCS vs. sham. We conclude that anodal tDCS has no effect on RT or response-related ERPs during a cued go/nogo task in either young or older adults.

Sustained brain activation supporting stop-signal task performance.

Stop-signal paradigms operationalize a basic test of goal-directed behaviour whereby an overarching stop goal that is performed intermittently must be maintained throughout ongoing performance of a reaction time go task (go goal). Previous studies of sustained brain activation during stop-signal task performance in humans did not observe activation of the dorsolateral prefrontal cortex (DLPFC) that, in concert with the parietal cortex, is known to subserve goal maintenance. Here we explored the hypothesis that a DLPFC and parietal network has a key role in supporting ongoing stop-signal task performance. We used a blocked functional magnetic resonance imaging design that included blocks of trials containing typical stop-signal paradigm stimuli that were performed under three conditions: Stop condition, which required reaction time responding to go stimuli and inhibition of cued responses upon presentation of a stop signal; Go condition, identical except that the tone was ignored; and Passive condition, which required only quiescent attention to stimuli. We found that, whereas a distributed corticothalamic network was more active in Stop compared with Go, only the right DLPFC and bilateral parietal cortex survived after masking that contrast with Stop compared with Passive. These findings indicate that sustained activation of a right dominant frontoparietal network supports stop goal processes during ongoing performance of the stop-signal task.

The spatial and temporal dynamics of anticipatory preparation and response inhibition in task-switching.

We investigated ERP and fMRI correlates of anticipatory preparation and response inhibition in a cued task-switching paradigm with informatively cued, non-informatively cued and no-go trials. Cue-locked ERPs showed evidence for a multicomponent preparation process. An early cue-locked differential positivity was larger for informative vs. non-informative cues and its amplitude correlated with differential activity for informatively vs. non-informatively cued trials in the dorsolateral prefrontal cortex (DLPFC), consistent with a goal activation process. A later differential positivity was larger for informatively cued switch vs. repeat trials and its amplitude correlated with informatively cued switch vs. repeat activity in the posterior parietal cortex (PPC), compatible with a category-response (C-R) rule activation process. No-go trials elicited a frontal P3, whose amplitude was negatively correlated with activity in the ventrolateral prefrontal cortex (VLPFC) and basal ganglia motor network, suggesting that a network responsible for response execution was inhibited in the course of a no-go trial. These findings indicate that anticipatory preparation in task-switching is comprised of at least two processes: goal activation and C-R rule activation. They also support a functional dissociation between DLPFC and VLPFC, with the former involved in top-down biasing and the latter involved in response inhibition.

Event-related potentials to Stroop and reverse Stroop stimuli.

In the Stroop task, the latency of response to a colour is either faster or slower in the presence of a congruent or incongruent colour-word (J. Exp. Psychol. 18 (1935) 643). Debate remains as to whether this effect occurs during early stimulus processing or late response competition. The present study examined the task using reaction time (RT) and event-related potentials to determine temporal differences in this processing. The 'reverse Stroop' effect (where colour interferes with processing of a colour-word) which is much less well established, was also examined. Standard Stroop interference was found as well as reverse Stroop interference. A late lateralised negativity at frontal sites was greater for Incongruent trials and also for the word-response (reverse Stroop) task, and was interpreted as semantic selection and word-rechecking effects. Late positive component latency effects generally mirrored the speed of processing of the different conditions found in RT data. Stroop effects were also found in early temporal N100 and parietal P100 components, which differentiated Congruent from Incongruent trials in the reverse Stroop but not the standard Stroop, and were interpreted as early perception of physical mismatch between the colour and word. It was concluded that Stroop stimuli are processed in parallel in a network of brain areas rather than a particular structure and that Stroop interference arises at the output stage.

Visual signal detection measured by event-related potentials.

Signal Detection tasks typically involve within-subject signal changes. Such a procedure does not lend itself to event-related potential (ERP) experiments where the need for averaging necessitates the maintenance of consistent stimulus parameters. In the present ERP study we adopt a novel approach to thresholding that allows within-subject signal manipulation. The Signal Detection task required the identification of letter targets, formed from dots, in a random dot field. ERP waveforms were segmented into three windows corresponding to N1, N2, and P300 components. Analysis shows that ERP variations are dependent on both task demands and response characteristics for N1, N2, and P300 components.

A study of three cases of familial related agenesis of the corpus callosum.

This paper provides data on a family in which three members, all female, have been diagnosed as having agenesis of the corpus callosum. That all three acallosal individuals came from the same family and showed relatively uniform neuropsychological impairment and could be compared in similar terms with their "callosal" siblings, also female, provides a unique sample. Inter-hemispheric transfer, psychometric measures, and motor and cognitive function were examined in the acallosal individuals, all of whom had borderline to low-average intelligence, with results compared to their non-acallosal siblings. The data indicated that all acallosal individuals exhibited deficits with the cognitive tests indicating difficulties of inter-hemispheric transfer of tactile information, difficulties in some areas of memory and, at least as far as the children are concerned, a marked difference in Verbal IQ and Performance IQ.

Event-related potential response to attended and unattended locations in an interference task.

Event-related potentials were measured in response to an interference task in which unattended stimulus items were compatible, incompatible a neutral with regard to the attended stimulus items. Two stimulus items were presented simultaneously and bilaterally--one in each visual field. This allowed examination of the event-related potential waveform according to whether recording sites were contralateral to the attended or unattended location. The first experiment used sustained cueing with 3.5 degrees separation between attended and unattended locations. Attentional modulation of the N1 was observed but not for the P1. In the second experiment, separation between attended and unattended locations was increased to 11.5 degrees. In both experiments, the hemisphere contralateral to the unattended material (unattended hemisphere) showed a greater negativity in the N2 latency range in the temporal regions to compatible and incompatible conditions compared to a neutral condition. These data are inconsistent with findings suggesting the filtering of material early in visual processing.

Time-till-breakdown and scalp electrical potential maps of long-range apparent motion.

A series of psychophysical and electrophysiological experiments is reported using the apparent motion (AM) breakdown effect. Breakdown describes an effect in AM in which, during continuous viewing, the percept of smooth of a single stimulus alternates with the percept of two discrete alternating stimuli. Visual evoked potentials (VEPs) were recorded during periods of motion or breakdown ("nonmotion") in horizontal and vertical displays. VEPs were compared with synthetic VEPs ("composite-flash") produced by adding VEPs to each element of the display recorded in isolation. Subtraction of VEPs was used in an attempt to compare the electrical responses with the processing of information relating to the form of the stimulus, subthreshold motion processing, and suprathreshold motion processing. The results, presented as scalp electrical potential distribution maps, were interpreted as consistent with a central adaptation process underlying the breakdown effect. The results also indicated that the hemispheric asymmetries in AM VEPs described by Manning, Finlay, and Fenelon (1988) were most likely due to the position of the stimuli in the visual field, rather than as a lateralization of motion processes per se. The results also provided evidence that the subthreshold and suprathreshold motion responses to the display were the product of different populations of motion units.

An event-related potential examination of attended and unattended stimuli in visual selection using bilateral stimulus presentation.

The experiment reported here examined an interference paradigm using a bilateral stimulus presentation in which stimuli were presented simultaneously in the left and right visual fields. The lateralization of the early visual components allowed an ERP examination of material presented in each field. Attention was directed to one field or the other on each trial by a 100% valid cue. Two letters were nominated as targets and the simultaneous presentation allowed presentation of material compatible, incompatible or neutral with reference to the target. A negative peak was observed at 230 ms post stimulus at occipital and temporal sites. There was a variation in this peak for unattended stimuli, with compatible and incompatible target letters being significantly different to non-target letters. Contrary to previous research, this finding suggests that material is not filtered out at an early stage as proposed by early selection. It was found that the response to unattended target material also varied according to the type of item presented at the attended location. These findings were discussed in relation to previous studies which found no difference in the processing of unattended target and non-target material, and also in relation to suggestions that automatic processing of unattended material occurs only when there is controlled processing occurring simultaneously.

Differential motion thresholds to sinusoidal gratings at two eccentricities.

Differential motion thresholds were measured at eccentricities of 9 degrees and 16.6 degrees using computer-generated sinusoidal gratings. Three spatial frequencies (0.51, 0.25, and 0.13 cycles/deg) were examined at reference velocities of 2, 4, 8, 16, 32, and 48 deg/sec. Minimum differential velocity thresholds were between 20 and 30% of the reference velocities for the three spatial frequencies at both eccentricities. Increasing eccentricity produced an increase in the velocity at which minimum velocity discrimination occurred. Temporal frequency tuning was between 4 and 8 Hz, regardless of eccentricity.

Time-till-breakdown and VEP measures of short-range apparent motion.

Varying short-range apparent motion (AM) stimulus displacements from 2.7 to 21.6 min it was found that VEP amplitudes varied as a function of the limits for short-range AM described using time-till-breakdown as a behavioural measure of AM strength. This VEP amplitude difference was, however, in the reverse direction to that predicted as the "motion" condition elicited lower VEP amplitude responses than the "non-motion" conditions (which did not significantly differ from each other). This direction of VEP amplitude difference was supported by an intensive study of a single subject. The "breakdown effect" enabled VEPs to be gathered during periods in which the subjective experience was of either coherent lateral motion, or breakdown (incoherent motion) without changing any stimulus parameter. The VEP component identified in expt 2, as predicted, was of lower amplitude during motion with respect to periods of breakdown. The results of these experiments are discussed in terms of describing motion and breakdown in short-range AM displays as "coherent" and "incoherent" motion, rather than as "motion" and "non-motion".