European Journal of Neuroscience, Vol. 25, pp. 900–907, 2007 doi:10.1111/j.1460-9568.2007.05324.x Optimal sustained attention is linked to the spectral content of background EEG activity: greater ongoing tonic alpha (10 Hz) power supports successful phasic goal activation Paul M. Dockree,1,5 Simon P. Kelly,2,3,5 John J. Foxe,1,3,4,5 Richard B. Reilly2,5 and Ian H. Robertson1,5 1Department of Psychology & Trinity College Institute of Neuroscience, Trinity College Dublin, Dublin 2, Ireland 2Department of Electronic and Electrical Engineering, University College Dublin, Dublin, Ireland 3The Cognitive Neurophysiology Laboratory, Nathan S. Kline Institute for Psychiatric Research, 140 Old Orangeburg Road, Orangeburg, NY, USA 4Program in Cognitive Neuroscience, Department of Psychology, The City College of the City University of New York, 138th St. & Convent Avenue, New York, USA 5The Cognitive Neurophysiology Laboratory, St Vincent’s Hospital, Fairview, Dublin, Ireland Abstract Efficient executive control frequently requires the timely activation or re-activation of a task-goal to enable purposeful behaviour. Additionally, more generalized factors such as alertness or neurological health will influence the efficiency with which control can be implemented. Goal-directed processes have been investigated by examining event-related potentials (ERPs), but much less is known about the involvement of background or ‘tonic’ processes reflected in the ongoing electroencephalogram (EEG), and how these affect the phasic processes expressed in the broad-band ERP. Here, we investigate the relationship between a key attention- sensitive tonic process – the alpha rhythm – and relevant phasic processes observed during a sustained attention paradigm in neurologically healthy subjects. We report that subjects with relatively higher tonic alpha power (10 Hz) show a larger-amplitude late positive ERP component that is thought to index goal activation and has been found to predict good sustained attention performance as defined by correct response patterns. Source localization results suggest that the neural generators responsible for oscillatory alpha activity, which are found primarily in the parietal and occipital lobes, are distinct from those giving rise to the late positive component. The results are discussed in terms of increased alpha synchrony facilitating goal-directed behaviour. Introduction Goal-directed behaviour depends on the efficiency of executive and reflect characteristics or traits of normal individuals that change control processes that select and manage goals in working memory more slowly over time. (Posner & Peterson, 1990). Additionally, background factors such as Event-related potentials (ERPs) have been informative in under- circadian rhythms (Manly et al., 2002), age (Grigsby et al., 1995) or standing phasic activity during goal-directed behaviour. We have neurological health (Robertson et al., 1997) influence the degree to previously charted the ERP components elicited during the fixed which behaviour can be controlled and purposeful. The consequences sequence sustained attention to response task (SARTfixed; Dockree of breakdown in goal-directed behaviour can be dramatic, as seen in et al., 2005). In this task a predictable series of single digits are patients with frontal lobe damage or psychiatric illnesses such as presented (1–9) and subjects are required to make a response to each schizophrenia. In support of goal-directed behaviour is an alertness or number (go-trials) with the exception of the number 3 (no-go trial). sustained attention system involving a right lateralised fronto-parietal This task requires the activation and maintenance of a task goal, which network (Posner & Peterson, 1990; Sturm et al., 1999). This system is is manifest in the ERP as a broadly distributed positivity over parieto- essential for maintaining an intrinsic goal-directed focus in otherwise central areas. This goal activation component, termed late positive 1 unarousing contexts where exogenous stimuli are not present to (LP1) is observed 550–800 ms following stimulus onset on trial 2 (i.e. increase alertness through novelty, demand or perceived difficulty the digit ‘2’) and exhibits divergence between the conditions of correct (Robertson & Garavan, 2004). The aim of the current study is to and incorrect responding on the following target (no-go) trial 3. examine the relationship between short-term ‘phasic’ neurophysio- Specifically, it was found that preceding an error, the LP1 was logical processes at play during a test of sustained attention and attenuated. Thus on a grand average level this component is most longer-term ‘tonic’ neurophysiological signals. The former reflect clearly indicative of successful performance on the SART and so moment-to-moment cognitive events whereas the latter are more stable stands out as a component of interest. As exemplified by the aforementioned ERP component, neuro- physiological processes are most commonly examined with respect to Correspondence: Dr Paul M. Dockree, 1Department of Psychology & Trinity College event-related, phasic changes in electro-cortical signals that occur at a Institute of Neuroscience, as above. rapid rate. By contrast, tonic EEG processes vary at a much slower E-mail: [email protected]. rate and can be influenced by daily changes in circadian rhythms Received 9 August 2006, revised 26 November 2006, accepted 28 November 2006 (Aeschbach et al., 1999), fatigue (Borbely et al., 1981) and mood ª The Authors (2007). Journal Compilation ª Federation of European Neuroscience Societies and Blackwell Publishing Ltd Optimal sustained attention and EEG activity 901 states (Papousek & Schulter, 2002). Furthermore, tonic EEG signals perceptual feature of the target trial (‘3¢). Digit font sizes were 100, are also associated with particular traits of individuals such as their age 120, 140, 160 and 180 in Arial text. The five allocated digit sizes (Duffy et al., 1984), intelligence (Klimesch et al., 1998) or subtended vertical angles of 1.39°, 1.66°, 1.92°, 2.18° and 2.45°, neurological health (Besthorn et al., 1997). Tonic activity in the alpha respectively, at a viewing distance of 152 cm. Digits were presented frequency range (10 Hz) has been informative regarding the 0.25° above a central yellow fixation cross on a grey background. The cognitive resources available to an individual (Klimesch, 1999) and task specifications were programmed and stimuli were delivered using the level of alertness of an individual following the sedative effects of the PresentationÒ software package (Version 0.75, http://www. benzodiazepine and barbiturate administration (Feshchenko et al., neurobs.com). Figure 1 shows a schematic of the fixed sequence SART. 1997). This evidence suggests that high tonic alpha power may be indicative of alert wakefulness and enhanced cognitive performance in healthy individuals. Measurements Here, we examine how tonic alpha relates both to behavioural High-density EEG recordings were acquired from 128 scalp elec- response patterns [in terms of response time (RT)] and to specific ERP trodes (interelectrode spacing 2.4 cm) referenced to the nasion. components (late positive components) that augment goal-activation Electrophysiological data were recorded in AC mode with a gain of during the SARTfixed. In light of the abovementioned evidence, we 1000 and a band pass of 0.15–100 Hz and the A ⁄ D conversion rate hypothesize that subjects with high tonic alpha power will exhibit late was 500 Hz. Recording began when electrical impedance had been positive ERP components of larger amplitude than subjects with lower reduced to less than 5 kOhms at all scalp sites. Vertical eye alpha power. To help elucidate the potential functional relationships movements were recorded with two VEOG electrodes placed above between tonic and phasic activity we conducted source dipole analysis and below the left eye, while HEOG electrodes at the outer canthus of of both the late positive ERP component and tonic alpha to determine each eye recorded horizontal movements. whether the neural generators of the phasic and tonic processes under investigation were common or separate. The current study, in neurologically healthy subjects, will provide a baseline for further Broad-band ERP analysis studies of clinical groups and their documented sustained attention deficits (Robertson et al., 1997; Dockree et al., 2004; O’Connell et al., The late positive ERP component LP1 was measured from scalp site 2004; Mullins et al., 2005). POZ over parietal cortex, selected by visual inspection of overall data (Figs 3, 4, 5, 6, 7, 8, 9 and 10) as the area under the average waveform (compared to the 0 lV baseline) in the poststimulus interval between Materials and methods 550 and 800 ms on trial 2. Statistical tests of response-related differences (correct withhold vs. commission error) were conducted Subjects using a randomly selected set of precorrect withhold sweeps for Further analysis was conducted on 14 (six female) right-handed trial 2, to equate with the lower number of pre-error sweeps. The LP1 neurologically normal volunteers whose data was originally collected area measures were the dependent variables for these tests. and analysed as part of a broad-band ERP study published in NeuroImage (Dockree et al., 2005). They were paid $100 for one day of participation. Subjects were aged between 18 and 32 years (mean, Alpha power 23.86, SD ± 4.24). All subjects gave written informed consent, and The average power spectrum over the entire