Research report
Sub-second “temporal attention” modulates alpha rhythms. A high-resolution EEG study

https://doi.org/10.1016/j.cogbrainres.2003.12.010Get rights and content

Abstract

In the present high-resolution electroencephalographic (EEG) study, event-related desynchronization/synchronization (ERD/ERS) of alpha rhythms was computed during an S1–S2 paradigm, in which a visual cue (S1) predicted a SHORT (600 ms) or LONG (1400 ms) foreperiod, preceding a visual go stimulus (S2) triggering right or left finger movement. Could orienting attention to a selective point in time influence the alpha rhythms as a function of the SHORT vs. LONG foreperiod? Stronger selective attentional modulations were predicted for the SHORT than LONG condition. EEG data from 54 channels were “depurated” from phase-locked visual evoked potentials and spatially enhanced by surface Laplacian estimation (i.e., final data analysis was conducted on 16 subjects having a sufficient number of artifact-free EEG single trials). Low-band alpha rhythms (about 6–10 Hz) were supposed to be related to anticipatory attentional processes, whereas high-band alpha rhythms (10–12 Hz) would indicate task-specific visuo-motor processes.

Compared to the LONG condition (foreperiod), the SHORT condition induced a quicker and stronger ERS at low-band alpha rhythm (about 6–8 Hz) over midline and bilateral prefrontal, sensorimotor, and posterior parietal areas. In contrast, the concomitant high-band alpha (about 10–12 Hz) ERD/ERS showed no significant difference between the two conditions. In conclusion, temporal attention for a sub-second delay (800 ms) did modulate low-band alpha rhythm over large regions of both cortical hemispheres.

Introduction

“Temporal attention” can be defined as a process aimed at allocating brain resources on the predicted onset of an incoming event. Previous studies on temporal aspects have mainly concerned subjective time estimation [10], [29], [32], [44], [48]. Typically, they used S1–S2 paradigms in which a warning stimulus (S1) provided information on the duration of the time interval (foreperiod) preceding an imperative stimulus (S2) triggering a motor response. It has been shown that reaction time improves as a function of the temporal certainty of the warning and imperative stimuli [10], [11], [12], [48], possibly due to an increase of alertness/vigilance [33], [42].

Cortical regions involved in tasks where estimation of time where requested showed involvement of prefrontal, posterior parietal, supplementary motor, and temporal cortices [14], [15], [18], [28], [34], [36], which are also engaged by “spatial attention” tasks [31], [47], [49]. With respect to the “spatial attention”, the “temporal attention” tasks could induce a stronger activation of left hemisphere [13], skilled for serial processes.

In a recent S1–S2 study [35], event-related potentials (ERPs) have modeled the spatiotemporal evolution of the cortical activation during “temporal attention” towards incoming events. A warning cue stimulus (S1) predicted with 80% of validity an imperative stimulus (S2) after 600 or 1400 ms. The results have shown that the reaction time was faster when the visual cues correctly predicted the S1–S2 interval, thus suggesting a key role of the “temporal attention” on the performance. Furthermore, slow negative potentials (contingent negative variation, CNV) increased between S1 and S2 stimuli and were faster in shape when the S1 stimulus predicted a short (600 ms) rather than long (1400 ms) foreperiod. This was explained as a faster increase of the attentional resources with the short compared to long foreperiod. However, it should be remarked that CNV could not disentangle attentional respect to concomitant task-specific visuomotor processes. Theoretically, this could be do by the analysis of event-related desynchronization/synchronization (ERD/ERS) of “idling” electroencephalographic (EEG) alpha rhythms (6–12 Hz). The alpha ERD (i.e., percentage reduction in power) is proportional to the cortical information processing, whereas the ERS reflects the “recovery” of the neuronal activity after that processing [40]. Noteworthy, the low-band alpha ERD (about 6–10 Hz) would reveal unspecific “alertness” and/or “expectancy” ([25], Klimesch et al. 1996). In parallel, the concomitant high-band alpha ERD (about 10–12 Hz) would depend on task-specific sensory, motor, and/or semantic processes [22], [24].

Here, EEG data of the mentioned study [35] were re-analyzed in terms of the ERD/ERS of the alpha sub-bands, which are differently sensitive to concomitant attentional (low-band alpha) and task-specific visuo-motor (high-band alpha) processes. Could temporal attention modulate the alpha rhythms as a function of short vs. long foreperiod (i.e., just difference of 800 ms)? Throughout the foreperiod, stronger selective attentional modulation were expected for the short than long condition.

Section snippets

Methods

Full details on the experimental procedures were previously reported [35]. In the following, we just summarized them.

Spatio-temporal distribution of ERD/ERS peaks

Fig. 3 illustrates the topographical maps of alpha ERD/ERS at fixed latencies such as T1 (+200 ms), T2 (+400 ms), and T3 (+600 ms) for SHORT and LONG conditions (grand average). For the SHORT condition, a largely diffused alpha ERD was observed at T1. Afterwards (T2 and T3), amplitude of the ERD values reduced especially at alpha 1 band. Notably, a sustained ERD was seen over left central area (C3) at alpha 3 band. Compared to the SHORT condition, the LONG condition was characterized by a

Methodological remarks

In the present study, EEG data were spatially enhanced by the surface Laplacian estimation, which minimized potentials originated from sub-cortical sources and removed the effects of reference electrode [38]. However, the present surface Laplacian estimates should be interpreted with caution. Surface Laplacian maxima could not overlie cortical sources of EEG potentials, due to the influence of tangentially oriented cortical generators [1], [2]. As a consequence of the intrinsic limitations in

Conclusions

In this EEG study, the parallel “temporal attention” and task-specific visuo-motor processes, as revealed by alpha ERD/ERS, were modeled. The experimental task provided a cue stimulus predicting short or long foreperiod between the warning and imperative stimuli. The time difference between short and long foreperiods was of only 800 ms. Nevertheless, compared to the LONG condition, the SHORT condition induced a quicker and stronger ERS at the slowest alpha sub-band (about 6–8 Hz) over midline

Acknowledgements

The authors thank Prof. Fabrizio Eusebi (Chairman of the Biophysics Group of Interest, University of Rome “La Sapienza”) for his continuous support and Anling Rao for his technical assistance. The research was granted by the Wellcorne Trust and Fatebenefratelli Association for Research (AFaR).

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