Research reportEEG spectral dynamics during discrimination of auditory and visual targets
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.
References (90)
- et al.
Investigating the contamination of electroencephalograms by facial muscle electromyographic activity using matching pursuit
Brain Lang.
(1999) - et al.
On and off effects in the background EEG activity during one-second photic stimulation
Electroencephalogr. Clin. Neurophysiol.
(1978) - et al.
Alpha oscillations in brain functioning: an integrative theory
Int. J. Psychophysiol.
(1997) - et al.
P300-response: possible psychophysiological correlates in delta and theta frequency channels. A review
Int. J. Psychophysiol.
(1992) - et al.
Dipole models of eye movements and blinks
Electroencephalogr. Clin. Neurophysiol.
(1991) - et al.
Auditory-induced 40-Hz activity during a frequency discrimination task
NeuroImage
(1998) - et al.
Event-related desynchronization: the effects of energetic and computational demands
Electroencephalogr. Clin. Neurophysiol.
(1992) - et al.
Spectral dynamics of electroencephalographic activity during auditory information processing
Hear. Res.
(2003) - et al.
The blocking of the rolandic wicket rhythm and some central changes related to movement
Electroencephalogr. Clin. Neurophysiol., Suppl.
(1959) P300 in serial tasks: an essential post-decision closure mechanism
Prog. Brain Res.
(1980)
Localization of the event-related potential novelty response as defined by principal components analysis
Cogn. Brain Res.
Time course of human 40 Hz EEG activity accompanying P3 responses in an auditory oddball paradigm
Neurosci. Lett.
EMG contamination of EEG: spectral and topographical characteristics
Clin. Neurophysiol.
Gamma band activity in an auditory oddball paradigm studied with the wavelet transform
Clin. Neurophysiol.
Peak gamma latency correlated with reaction time in a conventional oddball paradigm
Clin. Neurophysiol.
Magnetoencephalographic cortical rhythms
Int. J. Psychophysiol.
Event-related desynchronization and synchronization during an auditory lexical matching task
Electroencephalogr. Clin. Neurophysiol.
EEG-alpha rhythms and memory processes
Int. J. Psychophysiol.
Episodic and semantic memory: an analysis in the EEG theta and alpha band
Electroencephalogr. Clin. Neurophysiol.
Phase-locked alpha and theta oscillations generate the P1–N1 complex and are related to memory performance
Cogn. Brain Res.
Frontal midline theta and the error-related negativity: neurophysiological mechanisms of action regulation
Clin. Neurophysiol.
Auditory event-related dynamics of the EEG spectrum and effects of exposure to tones
Electroencephalogr. Clin. Neurophysiol.
Mining event-related brain dynamics
Trends Cogn. Sci.
Alpha rhythms as physiological and abnormal phenomena
Int. J. Psychophysiol.
Graphical display and statistical evaluation of event-related desynchronization (ERD)
Electroencephalogr. Clin. Neurophysiol.
Event-related synchronization (ERS): an electrophysiological correlate of cortical areas at rest
Electroencephalogr. Clin. Neurophysiol.
Patterns of cortical activation during planning of voluntary movement
Electroencephalogr. Clin. Neurophysiol.
Event-related EEG/MEG synchronization and desynchronization: basic principles
Clin. Neurophysiol.
Event-related desynchronization (ERD) during visual processing
Int. J. Psychophysiol.
The correction of ocular artifacts: a topographic perspective
Clin. Neurophysiol.
High-frequency brain activity: its possible role in attention, perception and language processing
Prog. Neurobiol.
Human cortical 40 Hz rhythm is closely related to EMG rhythmicity
Neurosci. Lett.
Functional segregation of movement-related rhythmic activity in the human brain
NeuroImage
Neuronal synchrony: a versatile code for the definition of relations?
Neuron
Tempo dependence of middle and long-latency auditory responses: power and phase modulation of the EEG at multiple time-scales
Clin. Neurophysiol.
Basic mechanisms of cerebral rhythmic activities
Electroencephalogr. Clin. Neurophysiol.
Human perceptual processing: inhibition of transient prefrontal–parietal 40 Hz binding at P300 onset documented in non-averaged cognitive brain potentials
Neurosci. Lett.
Trial-to-trial variability of cortical evoked responses: implications for the analysis of functional connectivity
Clin. Neurophysiol.
The relationship between the visually evoked P300 event-related potential and gamma band oscillation in the human medial and basal temporal lobes: an electrocorticographic study
Neurosci. Res.
A transient dominance of theta event-related brain potential component characterizes stimulus processing in an auditory oddball task
Clin. Neurophysiol.
Guidelines for standard electrode position nomenclature
J. Clin. Neurophysiol.
Functional aspects of evoked alpha and theta responses in humans and cats. Occipital recordings in “cross modality” experiments
Biol. Cybern.
Short duration synchronization of human theta rhythm during recognition memory
NeuroReport
Human theta oscillations related to sensorimotor integration and spatial learning
J. Neurosci.
The Scree test for the number of factors
Multivar. Brain Res.
Cited by (0)
- 1
Current address: F.C Donders Centre for Cognitive Neuroimaging, Post Office 9101, NL-6500 HB Nijmegen, The Netherlands.