Research report
EEG spectral dynamics during discrimination of auditory and visual targets

https://doi.org/10.1016/j.cogbrainres.2004.12.013Get rights and content

Abstract

This study measured the changes in the spectrum of the EEG (electroencephalogram) and in the event-related potentials (ERPs) as subjects detected an improbable target in a train of standard stimuli. The intent was to determine how these measurements are related, and to what extent the ERPs might represent phase-locked changes in EEG rhythms. The experimental manipulations were the stimulus modality (auditory or visual), the discriminability of the target, and the presence or absence of distraction. The ERPs showed sensory-evoked potentials that were specific to the modality and a target-evoked P300 wave that was later in the visual modality than in the auditory, and later and smaller when the discrimination was more difficult. The averaged EEG spectrograms showed that targets increased the frontal theta activity, decreased posterior and central alpha and beta activity, and decreased the central gamma activity. The scalp topography of the changes in the alpha and beta activity indicated a posterior desynchronization specific for the visual task and occurring with both targets and standards and a more widespread desynchronization for targets in either modality. Increased phase synchronization occurred during the event-related potentials, but modeling demonstrated that this can be seen when an evoked potential waveform is simply added to the background EEG. However, subtracting the spectrogram of the average ERP from the average spectrogram of the single trials indicated that phase-resetting of the background EEG rhythms can occur during the ERP. The idea that the ERPs and the EEG rhythms “share generators” can explain these findings.

Introduction

The neuronal processes underlying target discrimination are commonly studied using the oddball paradigm, wherein subjects are asked to distinguish infrequent (target) stimuli from frequent (standard) stimuli. The main ERP correlate of target discrimination is the P300, a large positive potential occurring over the parietal electrode sites in response to the target stimuli. The P300 for a target that is easy to distinguish from the standard has a peak latency between 300 and 350 ms in the auditory modality and about 50 ms later in the visual modality [61]. The scalp distribution of the P300 differs between the modalities, indicating that it derives, in part, from modality-specific generators [34], [35]. However, the P300 can also be independent of the physical characteristics of the stimulus since it occurs following the detected omission of a stimulus from a regular train [78]. In all likelihood, several different cerebral processes contribute to what is recorded from the scalp as the P300 wave [44].

When the brain perceives a stimulus, two types of changes in the EEG may occur: “evoked” activities which are exactly time-locked to the stimulus, and “induced” activities which are changes in the EEG that are not phase-locked to the stimulus [23]. Evoked activity is caused by direct neuronal activation whereas induced activities are caused by changes in functional connectivity within the cortex [57]. Evoked activity can be extracted from the ongoing EEG by averaging the EEG voltage–time waveforms following multiple repetitions of a stimulus. Induced activities are studied by averaging the EEG power spectrograms (power–frequency–time plots) following the same stimuli.

Changes in the EEG spectrogram are of two types: a power decrease within a frequency band referred to as an event-related desynchronization (ERD), and a power increase referred to as an event-related synchronization (ERS) [52], [53]. An occipital ERD in the alpha frequencies (8–13 Hz) typically occurs during visual stimulation [3], [52], [54], [55], [59]. A more central alpha rhythm referred to as the mu rhythm becomes desynchronized during motor movement or somatosensory stimulation [14]. The ERD of the alpha (and beta rhythms) preceding and during a voluntary movement is usually followed by a transient rebound ERS [54], [67]. A “tau” rhythm may occur in the temporal lobe but is only clearly visible in intracortical EEG or magnetoencephalography (MEG) [29], [51]. As well as reflecting sensory and motor processing, ERD of the alpha band correlates with cortical activation during perception and memory [9], [36], [37], [38], [56]. Since activation of the cortex is associated with ERD, these rhythms are often considered ‘idling rhythms’.

Several studies have looked specifically at EEG spectral dynamics during target detection. Probably the most consistent finding is a theta ERS with a peak amplitude about 300 ms after target onset [6], [89]. This EEG theta response is influenced by the same task variables that affect the P300 component, such as stimulus probability and task difficulty [6], [11], [75]. In addition, targets induce an ERD of the alpha activity [75], sometimes preceded by a brief ERS [88]. Cacace and McFarland [11] reported an ERD of the beta frequencies for attended targets. The amount of this beta ERD was greatest over the left hemisphere, contralateral to the response finger, suggesting that it was mainly related to the motor response. The relationship between the gamma band response and the target stimuli is less clear with different studies reporting an increase [27], [28], [85] or a decrease in gamma power [8], [20], [48].

The present study compared the spectral dynamics of the EEG during the detection of auditory and visual targets. We manipulated the difficulty of the task by making the target and standard stimuli very similar in the difficult condition and easily discriminable in the easy condition. A final manipulation was to add a distracting stimulus in the form of speech babble in the opposite ear during the auditory task, or a movie in the right visual field during the visual task. The rationale behind this manipulation derived from auditory ERP studies in which speech babble played in one ear attenuated the ERPs to stimuli in the opposite ear [15], [22], [30].

Section snippets

Participants

Ten normal young adults (5 females) with a mean age of 25 (range 20–29) years and a mean of 18 (range 20–29) years of education participated in the experiment. All participants were right-handed and all had normal or corrected-to-normal (better than 6/8) vision, normal hearing (<20 dB HL) at 1000 and 2000 Hz, and no history of neurological disease.

Experimental procedure

The stimuli were presented in 12 blocks of 250 trials. Six of the blocks presented the auditory task, while the other six presented the visual task.

Behavioral results

Table 1 presents the averaged RTs to targets. The mean RT was significantly faster in the auditory (403 ms) than visual modality (462 ms) [F(1,9) = 17.9, P < 0.002]. There was no significant difference in the accuracy between the two modalities. The RTs were significantly longer in the difficult conditions than the easy conditions [F (2,18) = 55.6, P < 0.001]. However, there was no significant difference between the difficult and distraction conditions.

ERPs

Fig. 3 shows some of the ERP waveforms.

Behavioral data

The manipulation of task difficulty resulted in longer RTs in the difficult and distraction conditions, indicating that these two tasks were harder to perform than the easy condition. The lack of any effect of distraction may be related to the youth of our subjects. For the auditory stimuli, contralateral competition affects the early physiological responses in elderly but not young subjects [30].

N1 waves

In the visual modality, the N1 response was larger for target stimuli than standard, and larger for

Acknowledgments

This research was supported by the Canadian Institutes for Health Research. Patricia van Roon assisted with the preparation of the manuscript. The data were presented as part of a Master's thesis at the University of Toronto, and the authors appreciate the comments of David L. Woods, who was the external examiner.

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    Current address: F.C Donders Centre for Cognitive Neuroimaging, Post Office 9101, NL-6500 HB Nijmegen, The Netherlands.

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