Brain activity during interval timing depends on sensory structure.

Precise timing is crucial for accurate perception and action in the range of hundreds of milliseconds. One still unresolved question concerns the influence of sensory information content on timing mechanisms. Numerous studies have converged to suggest that the CNV (Contingent Negative Variation), a slow negative wave that develops between two events, notably reflects temporal processing of the interval between these two events. The present study aimed at investigating CNV activity in duration discrimination tasks using either filled (continuous tones) or empty intervals (silent periods bounded by two brief tones). Participants had to compare a test duration with a 600-ms standard. Time perception was markedly better in the 'empty' than in the 'filled' condition. Electrophysiological analyses performed on the longest test duration (794 ms) of the comparison phase revealed an effect of the sensory structure on both the CNV amplitude and CNV time-course. The CNV amplitude was larger for filled than for empty intervals, suggesting a superimposition of timing-dependent activity and sensory sustained activity. Furthermore, the CNV time-course paralleled the temporal structure of the memorized sensory event: for filled intervals, the CNV amplitude stopped increasing at 600 ms, i.e. the expected end of the continuous tone; for empty intervals, in contrast, the CNV amplitude precisely increased at 600 ms, i.e. the expected onset of the second brief tone. These results suggest that the CNV reflects the mental rehearsal of the memorized sensory event, in line with the idea that temporal processing in the sub-second range is based on sensory information.

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.

The effect of encoding manipulation on word-stem cued recall: an event-related potential study.

The purpose of the present study was to find out whether the neural correlates of explicit retrieval from episodic memory would vary according to conditions at encoding when the words were presented in separate study/test blocks. Event-related potentials (ERPs) were recorded while participants performed a word-stem cued-recall task. Deeply (semantically) studied words were associated with higher levels of recall and faster response times than shallowly (lexically) studied words. Robust ERP old/new effects were observed for each encoding condition. They varied in magnitude, being largest in the semantic condition. As expected, scalp distributions also differed: for deeply studied words, the old/new effect resembled that found in previous ERP studies of word-stem cued-recall tasks (parietal and right frontal effects, between 400-800 and 800-1100 ms post-stimulus), whereas for shallowly studied words, the parietal old/new effect was absent in the latter latency window. These results can be interpreted as reflecting access to different kinds of memory representation depending on the nature of the processing engaged during encoding. Furthermore, differences in the ERPs elicited by new items indicate that subjects adopted different processing strategies in the test blocks following each encoding condition.

Neural correlates of word-stem priming.

The purpose of this study was to identify the neural correlates of implicit memory in a word-stem completion task. Given that both explicit and implicit retrieval tend to occur in this type of memory task, conventional analyses of old/new event-related potential effects are equivocal. To overcome this problem, depth of processing was manipulated and subjective awareness measured. From 400 ms poststimulus, event-related potentials evoked by stems completed with studied words were more positive than those evoked by stems completed with unstudied items. This difference was maximal at parietooccipital electrode sites. Event-related potentials were not modulated by either depth of processing or awareness. Behavioral and event-related potential data converged to indicate that the old/new effect reflects processes either contributing to, or contingent upon, implicit memory retrieval.

Auditory/visual duration bisection in patients with left or right medial-temporal lobe resection.

Patients with unilateral (left or right) medial temporal lobe lesions and normal control (NC) volunteers participated in two experiments, both using a duration bisection procedure. Experiment 1 assessed discrimination of auditory and visual signal durations ranging from 2 to 8 s, in the same test session. Patients and NC participants judged auditory signals as longer than equivalent duration visual signals. The difference between auditory and visual time discrimination was equivalent for the three groups, suggesting that a unilateral temporal lobe resection does not modulate the modality effect. To document interval-timing abilities after temporal lobe resection for different duration ranges, Experiment 2 investigated the discrimination of brief, 50-200 ms, auditory durations in the same patients. Overall, patients with right temporal lobe resection were found to have more variable duration judgments across both signal modality and duration range. These findings suggest the involvement of the right temporal lobe at the level of the decision process in temporal discriminations.

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.

What is common to brain activity evoked by the perception of visual and auditory filled durations? A study with MEG and EEG co-recordings.

EEG and MEG scalp data were simultaneously recorded while human participants were performing a duration discrimination task in visual and auditory modality, separately. Short durations were used ranging from 500 to 900 ms, among which participants had to discriminate a previously memorized 700-ms "standard" duration. Behavioral results show accurate but variable performance within and between participants with expected modality effects: the percentage of responses was greater and the mean response time was shorter for auditory than for visual signals. Sustained electric and magnetic activities were obtained correlatively to duration estimation, but with distinct spatiotemporal properties. Electric CNV-like potentials showed fronto-central negativity in both modalities, whereas magnetic sustained fields were distributed with respect to the modality of the interval to be timed. Time courses of these slow brain activities were found to be dependent on stimulus duration but not on its modality nor on the recording signal (EEG or MEG). Source reconstruction demonstrated that these sustained potentials/fields were generated by superimposed contributions from visual and auditory cortices (sustained sensory responses, SSR) and from prefrontal and parietal regions. By using these two complementary techniques, we thus demonstrated the involvement of frontal and parietal cerebral cortex in human timing.

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 100 m 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 x 3 CF) presented in random order. Equivalent current dipole (ECD) sources for the 100 m 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 100 m component, over a concurrent mapping of their periodic frequency Fo. The effect was observed both on dipole placement and dipole orientation.

When time is up: CNV time course differentiates the roles of the hemispheres in the discrimination of short tone durations.

Numerous studies have suggested that the CNV (contingent negative variation), a negative slow wave developing between a warning and an imperative stimulus, reflects, among other things, temporal processing of the interval between these two stimuli. One aim of the present work was to specify the relationship between CNV activity and the perceived duration. A second aim was to establish if this relationship is the same over the left and right hemispheres. Event-related potentials (ERPs) were recorded for 12 subjects performing a matching-to-sample task in which they had to determine if the duration of a tone (490 ms, 595 ms, 700 ms, 805 ms, and 910 ms) matched that of a previously presented standard (700 ms). CNV activity measured at the FCZ electrode was shown to increase until the standard duration had elapsed. By contrast, right frontal activity increased until the end of the current test duration, even when the standard duration had elapsed. Moreover, for long test durations (805 ms and 910 ms), correlations were observed between CNV peak latency and subjective standard, over left and medial frontal sites. We propose that left and medial frontal activity reflects an accumulation of temporal information that stops once the memorized standard duration is over, while right frontal activity subserves anticipatory attention near the end of the stimulus.

The "ticktock" of our internal clock: direct brain evidence of subjective accents in isochronous sequences.

The phenomenon commonly known as subjective accenting refers to the fact that identical sound events within purely isochronous sequences are perceived as unequal. Although subjective accenting has been extensively explored using behavioral methods, no physiological evidence has ever been provided for it. In the present study, we tested the notion that these perceived irregularities are related to the dynamic deployment of attention. We disrupted listeners' expectancies in different positions of auditory equitone sequences and measured their responses through brain event-related potentials (ERPs). Significant differences in a late parietal (P3-like) ERP component were found between the responses elicited on odd-numbered versus even-numbered positions, suggesting that a default binary metric structure was perceived. Our findings indicate that this phenomenon has a rather cognitive, attention-dependent origin, partly affected by musical expertise.

Processes involved in tempo perception: a CNV analysis.

The purpose of this research was to study the mechanisms underlying tempo perception, by looking at their electrophysiological brain correlates. The subjects' task consisted of comparing the tempos of two isochronous tone sequences made up of either three (condition 13) or six (condition 16) 600-ms intervals. Contingent negative variation (CNV), known to be linked to the judgment of a single interval, kept increasing in amplitude for three intervals during tempo encoding, thereby providing evidence of the occurrence of CNVs also for several intervals in succession. This CNV increase could reflect the use of interval-based processes in the building of the interval memory trace. During the comparison phase, a CNV decrease was observed in condition 16, suggesting that subjects did not build a new memory trace, but used beat-based processes to check whether the beats of the new tempo occurred at the times they anticipated.