Christophe Le Dantec

Making decisions about time: event-related potentials and judgements about the equality of durations.

Participants were exposed to a temporal generalization task where the duration of a small visual stimulus was judged. People received a 600ms standard duration, then had to judge whether other durations (longer than, shorter than, or equal to the standard) were or were not the standard (making a YES or NO response). In different experimental conditions, the spacing of non-standard durations around the standard was 150ms (Easy condition), or 75ms (Difficult condition), so the two conditions involved some judgements made with the same stimuli (450, 600, and 750ms). The experiment thus compared judgements of the same physical stimuli, when the basis of the judgement was the same, thus avoiding some problems of control that have been present in earlier electrophysiological studies of time judgements. As in previous work, fewer YES responses occurred in the Difficult condition and the 450ms duration was less confused with the 600ms standard than the 750ms one was. Computer modelling suggested that this (fewer YES responses) was due to a decrease in the decision threshold for the YES judgement. The electrophysiological results showed a distinction between the Easy and Difficult conditions observable by a change in the LCPt (Late Positive Component of Timing) measured after the stimulus presentations and by a change in the P1, the CNV (Contingent Negative Variation), and its positive counterpart during the presentation of the stimulus, which were larger when the discrimination was difficult. Our results therefore suggest that the increase in the difficulty of the generalization task not only changes decision processes but also alters attentional mechanisms. They also reveal that the decision does not seem to involve a unitary mechanism but depends on a group of sub-processes, notably attentional mechanisms which are altered from the moment of presentation of the stimulus.

ERPs in anterior and posterior regions associated with duration and size discriminations.

The main objective of the present study was to determine whether event-related potentials (ERPs) predominant in prefrontal cortex (PFC) respond in a similar fashion to ERPs predominant in posterior parietal cortex (PPC) in duration and size discrimination tasks. The results indicate that contingent negative variation (CNV) and P300 components changed according to task demands. In the time-related task, amplitudes and duration of both components increased as a function of stimulus duration and easier discriminability. This was not the case in the size discrimination task. These results are in accordance with the hypothesis of a functional link between PFC and PPC in timing behavior. The later-appearing LPCt component was observed in both tasks, but its amplitude increased only as a function of stimulus duration.

Frontal and parietal ERPs associated with duration discriminations with or without task interference.

The main objective of this study was to examine fronto-parietal networks underlying visual duration discriminations. Two types of interference tasks were used to augment cognitive load: line orientation associated with the right hemisphere and multiplication with the left. Both subtasks deteriorated duration discriminations, more severely for line orientation. Relative to the condition without interference, the dual task paradigm decreased amplitudes of the contingent negative variation (CNV) wave, predominant at frontal sites, and the P300 wave, predominant at parietal sites. Inversely, amplitudes of a later appearing positive component (LPC) and its parietal counterpart of opposite polarity (LNC) increased with spatial or numeric task interference. These results are concordant with the view that fronto-parietal networks underlying duration discriminations act in a concerted fashion, with the LPC/LNC waves acting as a warning signal to mitigate errors during high cognitive load.

Event-related potentials in the frontal lobe during performance of a visual duration discrimination task

Summary An event-related potential (ERP) study was conducted to elucidate the role of the prefrontal cortex (PFC) in time estimation. Subjects discriminated between three pairs of visual stimuli lasting from 100 ms and 2 seconds by determining whether the second stimulus was longer or briefer than the first. Event-related potentials were recorded in frontal and prefrontal regions after offset of the second stimulus (S2). The results indicated that the accuracy of the performances depended on stimulus duration and presentation order. In the brief-long order, the number of successful responses was higher as a function of stimulus duration. A time-related late positive component (LPCt) was revealed at prefrontal and frontal electrodes whose latency and amplitude differed depending on stimulus duration and order. The amplitude of this positive wave was higher when performance levels increased in the brief-long but not the reverse order. These results indicate that the LPCt may reflect successful decision-making or retrieval during time estimation as a result of neuronal activity in the PFC.

A triple dissociation between learning of target, distractors, and spatial contexts.

When we perform any task, we engage a diverse set of processes. These processes can be optimized with learning. While there exists substantial research that probes specific aspects of learning, there is a scarcity of research regarding interactions between different types of learning. Here, we investigate possible interactions between Perceptual Learning (PL) and Contextual Learning (CL), two types of implicit learning that have garnered much attention in the psychological sciences and that often co-occur in natural settings. PL increases sensitivity to features of task targets and distractors and is thought to involve improvements in low-level perceptual processing. CL regards learning of regularities in the environment (such as spatial relations between objects) and is consistent with improvements in higher level perceptual processes. Surprisingly, we found CL, PL for target features, and PL for distractor features to be independent. This triple dissociation demonstrates how different learning processes may operate in parallel as tasks are mastered.

A prefrontal ERP involved in decision making during visual duration and size discrimination tasks.

Recently, a late positive component (LPCt) with prefrontal dominance was identified in a duration discrimination task as a marker of decision-making processes (Paul et al., 2003). In the present study, LPCt amplitudes and latencies were measured in visual and size discrimination tasks for the purpose of determining the selectivity of this phenomenon. LPCt amplitudes were larger and latencies shorter for longer stimulus pairs, at a time of maximal behavioral performances. Wave amplitudes were also larger for smaller stimuli, but were not directly related to behavioral performances. These results indicate that the LPCt is not specific to temporal discrimination but can reflect more general decision-making processes.

Alpha-band EEG activity in perceptual learning.

In studies of perceptual learning (PL), subjects are typically highly trained across many sessions to achieve perceptual benefits on the stimuli in those tasks. There is currently significant debate regarding what sources of brain plasticity underlie these PL-based learning improvements. Here we investigate the hypothesis that PL, among other mechanisms, leads to task automaticity, especially in the presence of the trained stimuli. To investigate this hypothesis, we trained participants for eight sessions to find an oriented target in a field of near-oriented distractors and examined alpha-band activity, which modulates with attention to visual stimuli, as a possible measure of automaticity. Alpha-band activity was acquired via electroencephalogram (EEG), before and after training, as participants performed the task with trained and untrained stimuli. Results show that participants underwent significant learning in this task (as assessed by threshold, accuracy, and reaction time improvements) and that alpha power increased during the pre-stimulus period and then underwent greater desynchronization at the time of stimulus presentation following training. However, these changes in alpha-band activity were not specific to the trained stimuli, with similar patterns of posttraining alpha power for trained and untrained stimuli. These data are consistent with the view that participants were more efficient at focusing resources at the time of stimulus presentation and are consistent with a greater automaticity of task performance. These findings have implications for PL, as transfer effects from trained to untrained stimuli may partially depend on differential effort of the individual at the time of stimulus processing.

High resolution, high capacity, spatial specificity in perceptual learning.

Research of perceptual learning has received significant interest due to findings that training on perceptual tasks can yield learning effects that are specific to the stimulus features of that task. However, recent studies have demonstrated that while training a single stimulus at a single location can yield a high-degree of stimulus specificity, training multiple features, or at multiple locations can reveal a broad transfer of learning to untrained features or stimulus locations. We devised a high resolution, high capacity, perceptual learning procedure with the goal of testing whether spatial specificity can be found in cases where observers are highly trained to discriminate stimuli in many different locations in the visual field. We found a surprising degree of location specific learning, where performance was significantly better when target stimuli were presented at 1 of the 24 trained locations compared to when they were placed in 1 of the 12 untrained locations. This result is particularly impressive given that untrained locations were within a couple degrees of visual angle of those that were trained. Given the large number of trained locations, the fact that the trained and untrained locations were interspersed, and the high-degree of spatial precision of the learning, we suggest that these results are difficult to account for using attention or decision strategies and instead suggest that learning may have taken place for each location separately in retinotopically organized visual cortex.

Ventriloquism effect and aftereffect in the distance dimension

When an auditory target is presented simultaneously with a spatially displaced visual target, the perceived auditory target location shifts toward the visual target. This effect, known as the ventriloquism effect or visual capture, has been extensively studied in the horizontal dimension, but not in distance. Here, we measured distance localization performance in a reverberant room. Stimuli were either audio-visual (AV) 300-ms broadband noise bursts presented synchronously with spatially congruent or incongurent visual stimuli/LEDs, or auditory-only (A-only) noise bursts. One of eight speakers (distance 70 cm to 203 cm directly ahead of the listener) presented a stimulus on each trial. During adaptation runs, the AV stimuli were presented with the V-component closer or further by 30% than the A-component (displacement direction fixed within session). The ventriloquism effect was observed for both V-closer and V-further AV stimuli, with slightly stronger shifts induced by the V-closer stimuli. Ventriloquis...