Non-target Trials Research Articles

Rapid categorization of foveal and extrafoveal natural images: Associated ERPs and effects of lateralization

Humans are fast and accurate at performing an animal categorization task with natural photographs briefly flashed centrally. Here, this central categorization task is compared to a three position task in which photographs could appear randomly either centrally, or at 3.6° eccentricity (right or left) of the fixation point. A mild behavioral impairment was found with peripheral stimuli with no evidence in support of hemispheric superiority; but enlarging the window of spatial attention to three possible stimuli locations had no behavioral cost on the processing of central images. Performance in the central categorization task has been associated with a large difference between the potentials evoked to target and non-target correct trials, starting about 150 ms after stimulus onset on frontal sites. Present results show that this activity originates within extrastriate visual cortices and probably reflects perceptual stimuli differences processed within areas involved in object recognition. Latencies, slopes, and peak amplitudes of this differential activity were invariant to stimulus position and attentional load. Stimulus location uncertainty and lateralization did not affect speed of visual processing.

Behavioural state affects saccadic eye movements evoked by microstimulation of striate cortex

We examine how behavioural conditions affect the manner in which electrical stimulation of striate cortex (V1) influences the generation of saccadic eye movements. Monkeys were trained (i) to acquire a fixation spot and remain fixated for juice reward and (ii) to acquire a fixation spot and generate a saccade to a visual target for reward. Electrical stimulation was delivered at various times during the execution of these tasks. For stimulation trials, pulses were delivered at 200 Hz using a 100- or 200-ms train duration. Currents as high as 1500 micro A were not sufficient to evoke saccades from V1 when monkeys were actively fixating a visual target, whereas current < 100 micro A was sufficient to evoke saccades when monkeys were not actively fixating. By interleaving trials in which a visual target was presented in the receptive field of stimulated neurons with nontarget stimulation trials, saccades could be evoked from V1 during the nontarget stimulation trials with currents as low as 2 micro A. The position of the visual target on the interleaved trials affected the probability of saccade evocation on the nontarget stimulation trials. Additional factors that affected the evoked saccades were time of reward delivery, ratio of stimulation to nonstimulation trials, and whether stimulation was delivered on the interleaved trials in which a target was positioned in the receptive field of the stimulated neurons. We argue that the behavioural state of an animal acts on the nigra-collicular pathway to lower the current threshold for the elicitation of saccades from V1.

A two‐process model of strategic monitoring in event‐based prospective memory: Activation/retrieval mode and checking

Theorists have suggested that individuals may remember to execute event‐based intended actions by deploying executive or attentional resources to monitor for the markers or target events that indicate that it is appropriate to execute the intended actions (e.g., McDaniel &amp; Einstein, 2000; Shallice &amp; Burgess, 1991), but these strategic monitoring views are not specific about the processes that strategic monitoring entails. A more specific idea is outlined here (see also Guynn, 2001) and an experiment with results consistent with this view is reported. According to this two‐process view, strategic monitoring entails maintaining the cognitive system in a prospective memory retrieval mode, which may be mediated by increased activation of the prospective memory representation, plus checking whether the circumstances to execute the intended action are present. In the current experiment, concurrent task impairment on nontarget trials, on which participants were instructed to press a key if they saw a target event (i.e., experimental trials), relative to trials on which participants were not instructed to press a key if they saw a target event (i.e., control trials), provided a footprint of strategic monitoring. An interaction of trial type and whether the experimental and control trials alternated or were blocked revealed greater impairment on experimental trials relative to control trials when the trials were blocked than when the trials alternated. Performance on experimental trials did not vary whether the trials alternated or were blocked, while performance on control trials was significantly worse when the trials alternated than when the trials were blocked. The results are consistent with the two‐process view and the idea that participants maintained a retrieval mode/activation and checked on experimental trials, neither maintained a retrieval mode/activation nor checked on blocked control trials, and maintained a retrieval mode/activation but did not check on alternating control trials.

Mapping brain functions of ADHD children

Brain functions underlying inattention and impulsivity can be measured with event-related potentials (ERPs) in the cued continuous performance test (CPT A-X). Children with attention deficit hyperactivity disorder (ADHD) exhibit attenuation of the attentional cue P300 microstate, which reflects relatively pure attention and precedes target (go) and nontarget (no-go) trials. Multicenter studies indicate that this attentional deficit is also present in children with AD/HD of the combined subtype (DSM-IV) and in children with hyperkinetic disorders (HD). Tomographic source solutions converge to posterior cue P300 sources that are attenuated in ADHD or HD children, contrasting with the predominance of frontal deficits in metabolic studies of ADHD. A reanalysis of our data indicated that these posterior deficits remain the most robust ERP marker of ADHD. The posterior tomography of the cue P300 proved robust across different LORETA versions but sensitive to spatial undersampling. After more detailed time segmentation, reduced frontal N2 and central no-go P300, considered to reflect frontal deficits, also became evident in ADHD children. A wide range of ADHD subtypes, thus, suffers from activation deficits of both posterior and anterior attention systems. Resolving them with tomographic ERP imaging requires high-density ERPs.

Top-down modulation of early selective attention processes in children

It has been demonstrated in adults that attention can modulate very early stages of perceptual/cognitive processing, but it has not been determined if this capacity for modulation develops with age. We investigated early attentional processes in parallel and serial visual search tasks in 40 children 7–12 years old, using event-related potentials (ERPs). Two single-feature pop-out tasks were used to assess parallel processing; a conjunction of features task was used to study serial processing. There were significant decreases in latencies with age for the peaks measured [posterior P1, N1 and anterior P2 (P2a)]. P1 and N1 latency effects also varied with trial type and were consistent with top-down modulation of processing, which differed between the parallel tasks. P2a amplitude for the hit trials decreased with increasing age for the colour and serial tasks. Increasing R>L hemispheric asymmetries with age in P2a amplitude were seen on non-target trials for the size and serial tasks only, reflecting serial processing. These data demonstrate that developmental changes in visual selective attention for early stages of processing are continuing through childhood, and that there is top-down modulation at these latencies in children.

Catégorisation visuelle rapide et vitesse de traitement chez le migraineux

It has been suggested that the visual cortex of migraine sufferers is hypersensitive. To test this hypothesis, we compared the speed of visual processing in eight migraine patients (including three with aura) and nine controls performing two categorisation tasks of different complexity. In the complex task developed by Thorpe et al. (1996), subjects had to detect the presence of animals in briefly presented photographs (20 ms). The speed of visual processing was measured by the latency of the differential cerebral activity that appears around 150 ms between target and non-target trials. In the simple task, subjects performed a square/circle categorisation. All subjects were faster (445 ms/497 ms) with a higher rate of success (97%/91%) in the simple task. But in both tasks, the behavioural performance and the associated evoked potentials did not show any difference suggesting a cortical hypersensitivity in migraine sufferers.

Time course of human 40 Hz EEG activity accompanying P3 responses in an auditory oddball paradigm

In order to quantify the time course of auditory P3-related gamma activity, root mean square (RMS) values were calculated from band-filtered (30–45 Hz) target and non-target responses in an auditory oddball experiment. Evoked (phase locked) gamma activity was evaluated from the time domain averages, whereas induced (not necessarily phase locked) activity was analyzed on the basis of single trials. Gamma RMS values were integrated across different time windows, namely the prestimulus, N50/P50, N100, pre P3, P3 and post P3 window. The single trial P3 window hereby was defined by a maximum amplitude criterion. In accordance with other studies, we found a pronounced increase of evoked gamma activity in the time window up to 80 ms after stimulus onset. In contrast, induced gamma activity as revealed by single trial analysis decreased markedly after stimulus presentation. Starting with the P3 window, induced activity recovered to baseline level for the non-target trials, whereas it remained significantly suppressed for the target responses.

Blink activity in a discrimination task as a function of stimulus modality and schedule of presentation.

ABSTRACTEyeblink performance parameters were investigated in subjects engaged in a series of duration discrimination tasks differing in modality (visual vs. auditory) and presentation schedule (fixed vs. variable). Visual tasks were associated with slower blink rates and shorter blink durations than auditory tasks. Sensitivity measures suggested that this difference might be due, in part, to the greater difficulty of the visual tasks. Blink latency declined within and across tasks and was longer for target stimuli which were followed by responses. Since the target stimuli were the short duration stimuli, the latter effect could be a compound of two opposing effects. The first is related to the response, which tends to delay the blink on target trials, while the second, related to decision processes, would tend to increase latencies on nontarget trials. Schedule of stimulus presentation did not affect dependent measures as predicted. RT was unaffected by either of the experimental variables.