Richard Ragot

P300, as a function of S-R compatibility and motor programming.

Abstract A paradigm with lateralized stimuli and responses was used to investigate the relationship between P300 latency of the evoked brain potential and reaction time. The P300, like the RT, was found to be dependent on the S—R spatial compatibility, but not on the motor programming itself.

Relationship between CNV and timing of an upcoming event

This study reports an analysis of the Contingent Negative Variation (CNV) recorded on the human scalp during the comparison of a test duration with a previously memorized duration. Results show that CNV activity peaks at the end of the memorized duration, and that its slope varies inversely with the length of this duration. These features of CNV activity are similar to those of climbing neuronal activity observed through intracerebral recordings in animals, and suggest that both activities reflect how the brain encodes the timing of an upcoming event. These results also show that the time-course of the CNV in timing tasks is compatible with a pacemaker-accumulator model of temporal processing.

Role of frontal cortex in memory for duration: an event-related potential study in humans.

Event-related potentials were recorded in order to determine how brain activity is lateralized during the encoding and the recognition of visual durations (700 and 2500 ms ranges). It is assumed in the Hemispheric Encoding Retrieval Asymmetry model that the encoding of words, faces and odours involves left frontal areas whereas their recognition involves right frontal areas. The present results indicate that, for temporal information, the hemispheric bias is different: a negativity developed over right frontal electrodes for both encoding and recognition, and for both duration ranges. Thus, the involvement of right frontal areas appears critical for time perception. Conversely to what was expected, contingent negative variation during recognition was large over both left and right frontal electrodes. These results suggest that the involvement of both hemispheres is necessary for recognition of temporal information.

Heterogeneity of information-processing alterations according to dimensions of depression: An event-related potentials study

To identify alterations in elementary cognitive operations according to dimensions of depression, two stages of information processing, namely the response choice and the motor preparation stages, were explored using an event-related potential paradigm in two subgroups of depressed patients (retarded and blunted affect versus anxious-agitated and impulsive) compared to controls. Two results are common to all depressed patients: a slow encoding of stimuli (P1 wave) and a prolonged processing of stimulus-response compatibility (after P3b). This is compensated by a global velocity increase in stimulus evaluation or decision making (P3b) in anxious-agitated patients or, on the contrary, cumulated with its velocity decrease in retarded-blunted-affect patients. Such results could provide an explanation for the massive retardation observed in blunted-affect patients, contrary to anxious-agitated patients, whose normal reaction times may come from a very high energetical involvement at the P3b level. Results as a whole suggest that impairments in blunted-affect patients concern effort mechanisms, whereas those in anxious-agitated patients concern perceptual processes.

Automatic attentional shifts induced by a noradrenergic drug in Alzheimer's disease: evidence from evoked potentials.

Prior research showed that attentional deficits are observed in Alzheimers disease (AD). These deficits can further impair other cognitive processes. The present experiment was designed to study the shifts in attention induced by a noradrenergic drug (S 12024-2) through their electrophysiological correlates in 12 outpatients with mild AD, using an auditory oddball paradigm. The P3a component, known to be related to automatic attentional processing, was increased by the drug, whereas no changes occurred either in PN or in P3b, which are considered to reflect conscious processing. These results point to an involvement of the noradrenergic system in the modulation of automatic attentional processing, and provide evidence for weakening of the orienting reflex in AD, due to a possible noradrenergic deficit in patients with mild AD.

Differential effects of the stimulus sequence on CNV and P300

During the presentation of stimulus sequences in oddball paradigms, participants tend to implicitly evaluate the conditional probability of target occurrence. It is not sure, however, if subjective estimation of conditional probabilities modulates target expectancy and target processing in the same manner. In the present experiment, the amplitudes of CNV and P300 were studied separately to compare preparatory and decision mechanisms and their sensitivity to variations in target probability. Amplitudes of both components were measured at different positions in the stimulus sequence, which modulates target probability as a function of distance from the preceding target. Results showed shorter RTs with increased probability of target occurrence (that is, with longer distance to the previous target). CNV amplitude was low when target probability was zero and maximal when the occurrence of a target was possible, regardless of its probability. Conversely, a gradual increase with augmenting probability was observed for P300 amplitude. Thus, preparatory activity as reflected by the CNV showed an all or nothing response to variations in probability, whereas action closure mechanisms, indexed by P300 components, were found to be more sensitive to subtle differences in stimulus expectancy.

Correct and incorrect responses in a choice reaction time task and the endogenous components of the evoked potential.

Publisher Summary This chapter discusses correct and incorrect responses in a choice reaction time task and the endogenous components of the evoked potential. The results clearly demonstrate that, when the subject has to deal successively with auditory and visual stimuli within one same run, an endogenous negative wave develops and peaks at the same location as that of the modality-specific potential obtained in response to relevant stimuli. These modality-specific negative potentials (MSNPs) are the only ones to be highly correlated with RTs. Some more points must be emphasized that may contribute to the interpretation of the vertex non-specific negative wave observed for visual stimuli: (1) this wave is followed by a P300 and looks like the N2-P3a complex in response to novel irrelevant stimuli, (2) its amplitude increases for unexpected stimuli whatever the modality (P= 0.01 across modalities, sign test), and (3) in the case of incorrect responses, some preliminary results obtained from a trial-by-trial analysis showed two kinds of vertex activity in response to auditory stimuli: the first one looked like the usual AEmP and the second one, of much higher amplitude, is similar to the K-complex described long ago as representing a surprise effect.

Tonotopic cortical representation of periodic complex sounds

Most of the sounds that are biologically relevant are complex periodic sounds, i.e., they are made up of harmonics, whose frequencies are integer multiples of a fundamental frequency (Fo). The Fo of a complex sound can be varied by modifying its periodicity frequency; these variations are perceived as the pitch of the voice or as the note of a musical instrument. The center frequency (CF) of peaks occurring in the audio spectrum also carries information, which is essential, for instance, in vowel recognition. The aim of the present study was to establish whether the generators underlying the 100m are tonotopically organized based on the Fo or CF of complex sounds. Auditory evoked neuromagnetic fields were recorded with a whole‐head magnetoencephalography (MEG) system while 14 subjects listened to 9 different sounds (3 Fo × 3 CF) presented in random order. Equivalent current dipole (ECD) sources for the 100m component show an orderly progression along the y‐axis for both hemispheres, with higher CFs represented more medially. In the right hemisphere, sources for higher CFs were more posterior, while in the left hemisphere they were more inferior. ECD orientation also varied as a function of the sound CF. These results show that the spectral content CF of the complex sounds employed here predominates, at the latency of the 100m component, over a concurrent mapping of their periodic frequency Fo. The effect was observed both on dipole placement and dipole orientation. Hum. Brain Mapping 20:71–81, 2003.

Cerebral Evoked Potentials: Early Indexes of Compatibility Effects

Publisher Summary This chapter discusses the early indexes of compatibility effects of cerebral evoked potentials. Stimulus-response compatibility effects stand at a crucial node between stimulus identification and response initiation. An evaluation of the global performance is classically given by two behavioral indexes, reaction time and error rate. Psychophysiological indexes of performance also can be used. Among these are cerebral electrophysiological potential variations that reflect brain operations as they proceed from the earliest stages of stimulus evaluation to motor programming, and thus are liable to give a more precise insight into the mechanisms involved. It is suggested that when the compatibility effects are not because of two conflicting aspects of the stimulus, then no P300 latency variations should be observed. The peak amplitude of each event-related potentials (ERP) component is a relevant parameter, usually interpreted as reflecting the amount of processing that goes on or the level of activation or motor preparation, depending on the component that is being studied. The scalp distribution of the various ERP components can help to identify and separate components that occur at very close time intervals.

Stimulus congruence and stimulus-response compatibility: Two variables disentangled in an auditory reaction time task

Abstract Stimulus—response (S-R) compatibility was a tem fmt used by Wtts and Seeger (1953) to describe effcts observed on reaction time (RT) when the stimulus—response relationship was varied, as in the following instructions: “move to tbe right when the stimulus appears on the right” (compatible) or “move to the left when the stimulus appears on the right” (incompatible). This term was later employed in a broader sense (Simon & Rudell, 1967) with paradigms involving elaborate stimuli, such as the verbal command ‘RIGHT’ delivered to the right tar (compatible) or to the left ear (incompatible). In such paradigms. subjects respond faster when the response is delivered on the same side as the stimulus (the so-called “Simon effect”). It has been shown, however, that this effect could be reversed under some circumstanas in the visual domain. In this paper, we report data showing that it can also be reversed in the auditory domain when using the above-mentioned verbal commands. This brings further evidence tha...