Franck Vidal

Is the ‘error negativity’ specific to errors?

When subjects make an erroneous response in a choice reaction time task, an error negativity, or error-related negativity (N(E)/ERN), peaking at about 100 ms after EMG onset, has been described. This wave is often considered to be absent on correct response trials. We report a small N(E)/ERN wave on correct response trials during a choice reaction time task in which surface Laplacians were estimated by the source derivation method. This wave is well focused at FCz, and its time course is the same for correct responses trials, incorrect sub-threshold EMG activation trials, and error trials. Current source density maps, also indicate a focus at FCz. A second experiment showed the existence of a N(E) at FCz on correct trials during a simple RT task. Rather than an error detection process per se, we propose that the N(E)/ERN reflects either a comparison process leading secondarily to error detection, or an emotional reaction.

Brain Activation Induced by Estimation of Duration: A PET Study

Duration information about a visual stimulus requires processing as do other visual features such as size or intensity. Using positron emission tomography, iterative H215O infusions, and statistical parametric mapping, we investigated the neural correlates of time processing. Nine normal subjects underwent six serial rCBF. Three tasks were studied: (a) A temporal generalization task (D task) in which the subjects had to judge (by pressing one of two keys) whether the duration of the illumination of a green LED was equal to or different from that of a previously presented standard; (b) An intensity generalization task (I task) in which the judgment concerned the intensity of the LED; and (c) A control task (C task) in which the subjects had to press one of the two keys at random in response to LED illumination. A significant increase in rCBF during the D task, compared to that during the C task, was observed in right prefontal cortex, right inferior parietal lobule, anterior cingulate cortex, vermis, and a region corresponding to the left fusiform gyrus. A significant increase in rCBF during the I task, compared to that during the C task, was observed in right prefontal cortex, right inferior parietal lobule, right extrastriate cortex, anterior cingulate cortex, left inferior parietal lobule, vermis, and two symmetrical regions corresponding to the fusiform gyri. No significant activation was observed in the D task when compared to that in the I task. We propose that these cortical maps are best explained by the recruitment of visual attention and memory structures, which play a major role in prospective time judgements as indicated by behavioral studies. The data also suggest that the temporal dimension of a visual stimulus is processed in the same areas as other visual attributes.

The supplementary motor area in motor and sensory timing: evidence from slow brain potential changes

Abstract The present study investigated the processing of durations on the order of seconds with slow cortical potential changes. The question is whether trial-to-trial fluctuations in temporal productions or judgments correspond to variations in the amplitude of surface Laplacians computed over particular scalp regions. Topographical analyses were done using the source derivation method. Subjects performed three successive tasks: (1) time production, in which they produced a 2.5-s interval separated by two brief trigger presses; (2) time discrimination, in which they detected small differences in intervals delimited by two brief clicks in comparison with a memorized standard interval; and (3) intensity discrimination (control task, devoid of time judgments), in which they detected small differences between the intensity of clicks, in comparison with standard clicks initially memorized. In order to focus on subjective differences, in the two discrimination tasks most comparison stimuli were identical to the standard, without the subjects being aware of it. At FCz, reflecting activity from the mesial frontocentral cortex that mainly includes the supplementary motor area (SMA), larger negativities were found during the longer target intervals, whether these were produced (task 1) or judged so (task 2). Those performance-dependent trends were restricted to the target intervals of the temporal tasks; they appeared neither during the 2 s preceding the target, nor during the control task. The data therefore suggest that the SMA subserves important functions in timing both sensory and motor tasks. We propose that the SMA either provides the ”pulse accumulation” process commonly postulated in models of time processing or that it receives output from this process through striatal efferent pathways.

Physiological evidence for response inhibition in choice reaction time tasks

Inhibition is a widely used notion proposed to account for data obtained in choice reaction time (RT) tasks. However, this concept is weakly supported by empirical facts. In this paper, we review a series of experiments using Hoffman reflex, transcranial magnetic stimulation and electroencephalography to study inhibition in choice RT tasks. We provide empirical support for the idea that inhibition does occur during choice RT, and the implications of those findings for various classes of choice RT models are discussed.

Sum-frequency generation spectroscopy of interfaces

This paper reviews aspects of nonlinear optical spectroscopy of interfaces. The emphasis is put on second-order nonlinear optical techniques, such as sum-frequency generation (SFG), which possess intrinsic surface or interface selectivity and can therefore be used to probe buried interfaces accessible by light. The basic concepts of the second-order nonlinear response of surfaces and interfaces are given. While SFG in the ultraviolet–visible range allows one to achieve surface-specific electronic spectroscopy, infrared–visible SFG spectroscopy allows one to have access to absolute vibrational spectra of adsorbates at an interface. The main experimental schemes commonly employed are described. Selected experimental examples are given for studies of liquid surfaces and interfaces, polymer surfaces and interfaces, solid surfaces under ultra-high vacuum conditions or in inert atmospheres, solid–gas interfaces, solid–liquid interfaces and solid–solid interfaces. Both frequency-resolved studies and time-domain measurements are addressed.

Error negativity on correct trials: a reexamination of available data

The error negativity, an EEG wave observed when subjects commit an error in a choice reaction time (RT) task, is often considered as a sign of error detection. Recently, reports of Ne-like waves on correct responses did challenge this interpretation. It has been proposed, however, that these Ne-like waves result either from an artifactual contamination of response-locked activities by stimulus-locked ones, or from an implicit monitoring of the time elapsing during the RT. Our aim was to reprocess published data: (1) to compare the shape and amplitude of EMG-locked and stimulus-locked ERPs on correct trials, and (2) to compare the size of the EMG-locked Ne-like waves obtained on fast and slow trials. The results neither support the artifact hypothesis nor the RT monitoring one. Therefore, it seems that the Ne-like waves observed on correct trials do correspond to a Ne, which suggests that the Ne has a broader significance than just error detection.

Error negativity does not reflect conflict: A reappraisal of conflict monitoring and anterior cingulate cortex activity

Our ability to detect and correct errors is essential for our adaptive behavior. The conflict-loop theory states that the anterior cingulate cortex (ACC) plays a key role in detecting the need to increase control through conflict monitoring. Such monitoring is assumed to manifest itself in an electroencephalographic (EEG) component, the error negativity (Ne or error-related negativity [ERN]). We have directly tested the hypothesis that the ACC monitors conflict through simulation and experimental studies. Both the simulated and EEG traces were sorted, on a trial-by-trial basis, as a function of the degree of conflict, measured as the temporal overlap between incorrect and correct response activations. The simulations clearly show that conflict increases as temporal overlap between response activation increases, whereas the experimental results demonstrate that the amplitude of the Ne decreases as temporal overlap increases, suggesting that the ACC does not monitor conflict. At a functional level, the results show that the duration of the Ne depends on the time needed to correct (partial) errors, revealing an on-line modulation of control on a very short time scale.

Timing, storage, and comparison of stimulus duration engage discrete anatomical components of a perceptual timing network

The temporal discrimination paradigm requires subjects to compare the duration of a probe stimulus to that of a sample previously stored in working or long-term memory, thus providing an index of timing that is independent of a motor response. However, the estimation process itself comprises several component cognitive processes, including timing, storage, retrieval, and comparison of durations. Previous imaging studies have attempted to disentangle these components by simply measuring brain activity during early versus late scanning epochs. We aim to improve the temporal resolution and precision of this approach by using rapid event-related functional magnetic resonance imaging to time-lock the hemodynamic response to presentation of the sample and probe stimuli themselves. Compared to a control (color-estimation) task, which was matched in terms of difficulty, sustained attention, and motor preparation requirements, we found selective activation of the left putamen for the storage (encoding) of stimulus duration into working memory (WM). Moreover, increased putamen activity was linked to enhanced timing performance, suggesting that the level of putamen activity may modulate the depth of temporal encoding. Retrieval and comparison of stimulus duration in WM selectively activated the right superior temporal gyrus. Finally, the supplementary motor area was equally active during both sample and probe stages of the task, suggesting a fundamental role in timing the duration of a stimulus that is currently unfolding in time.

The dual nature of time preparation: neural activation and suppression revealed by transcranial magnetic stimulation of the motor cortex

Single‐pulse transcranial magnetic stimulations (TMSs) of the motor cortex (M1) were performed in order to decipher the neural mechanisms of time preparation. We varied the degree to which it was possible to prepare for the response signal in a choice reaction time (RT) task by employing either a short (500 ms) or a long (2500 ms) foreperiod in separate blocks of trials. Transcranial magnetic stimulations were delivered during these foreperiods in order to study modulations in both the size of the motor evoked potential (MEP) and the duration of the silent period (SP) in tonically activated response agonists. Motor evoked potential area and silent period duration were assumed to reflect, respectively, the excitability of the cortico‐spinal pathway and the recruitment of inhibitory cortical interneurons. Shorter reaction times were observed with the shorter foreperiod, indicating that a better level of preparation was attained for the short foreperiod. Silent period duration decreased as time elapsed during the foreperiod and this decrement was more pronounced for the short foreperiod. This result suggests that time preparation is accompanied by a removal of intracortical inhibition, resulting in an activation. Motor evoked potential area decreased over the course of the short foreperiod, but not over the long foreperiod, revealing that time preparation involves the inhibition of the cortico‐spinal pathway. We propose that cortico‐spinal inhibition secures the development of cortical activation, preventing erroneous premature responding.

The supplementary motor area in motor and perceptual time processing: fMRI studies

The neural bases of timing mechanisms in the second-to-minute range are currently investigated using multidisciplinary approaches. This paper documents the involvement of the supplementary motor area (SMA) in the encoding of target durations by reporting convergent fMRI data from motor and perceptual timing tasks. Event-related fMRI was used in two temporal procedures, involving (1) the production of an accurate interval as compared to an accurate force, and (2) a dual-task of time and colour discrimination with parametric manipulation of the level of attention attributed to each parameter. The first study revealed greater activation of the SMA proper in skilful control of time compared to force. The second showed that increasing attentional allocation to time increased activity in a cortico-striatal network including the pre-SMA (in contrast with the occipital cortex for increasing attention to colour). Further, the SMA proper was sensitive to the attentional modulation cued prior to the time processing period. Taken together, these data and related literature suggest that the SMA plays a key role in time processing as part of the striato-cortical pathway previously identified by animal studies, human neuropsychology and neuroimaging.