Gratton G, Coles MG, Donchin E. A new method for off-line removal of ocular artifact
ABSTRACT A new off-line procedure for dealing with ocular artifacts in ERP recording is described. The procedure (EMCP) uses EOG and EEG records for individual trials in an experimental session to estimate a propagation factor which describes the relationship between the EOG and EEG traces. The propagation factor is computed after stimulus-linked variability in both traces has been removed. Different propagation factors are computed for blinks and eye movements. Tests are presented which demonstrate the validity and reliability of the procedure. ERPs derived from trials corrected by EMCP are more similar to a 'true' ERP than are ERPs derived from either uncorrected or randomly corrected trials. The procedure also reduces the difference between ERPs which are based on trials with different degrees of EOG variance. Furthermore, variability at each time point, across trials, is reduced following correction. The propagation factor decreases from frontal to parietal electrodes, and is larger for saccades than blinks. It is more consistent within experimental sessions than between sessions. The major advantage of the procedure is that it permits retention of all trials in an ERP experiment, irrespective of ocular artifact. Thus, studies of populations characterized by a high degree of artifact, and those requiring eye movements as part of the experimental task, are made possible. Furthermore, there is no need to require subjects to restrict eye movement activity. In comparison to procedures suggested by others, EMCP also has the advantage that separate correction factors are computed for blinks and movements and that these factors are based on data from the experimental session itself rather than from a separate calibration session.
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- "Subsequent to this, 800 ms epochs of data (starting from 200 ms prior to fixation cross colour-change) were extracted from the continuous EEG signal for each trial and experimental condition (see above). Following the creation of epochs for all conditions, ocular artefacts were corrected via the algorithm described by . Next, the final 200 ms of the task-irrelevant white fixation cross immediately preceding the conditional colour-change were used to baseline the EEG epochs. "
ABSTRACT: The execution of an antisaccade selectively increases the reaction time (RT) of a subsequent prosaccade (the unidirectional prosaccade switch-cost). To explain this finding, the task-set inertia hypothesis asserts that an antisaccade requires a cognitively mediated non-standard task-set that persists inertially and delays the planning of a subsequent prosaccade. The present study sought to directly test the theoretical tenets of the task-set inertia hypothesis by examining the concurrent behavioural and the event-related brain potential (ERP) data associated with the unidirectional prosaccade switch-cost. Participants pseudo-randomly alternated between pro- and antisaccades while electroencephalography (EEG) data were recorded. As expected, the completion of an antisaccade selectively increased the RT of a subsequent prosaccade, whereas the converse switch did not influence RTs. Thus, the behavioural results demonstrated the unidirectional prosaccade switch-cost. In terms of the ERP findings, we observed a reliable change in the amplitude of the P3 – time-locked to task-instructions – when trials were switched from a prosaccade to an antisaccade; however, no reliable change was observed when switching from an antisaccade to a prosaccade. This is a salient finding because extensive work has shown that the P3 provides a neural index of the task-set required to execute a to-be-completed response. As such, results showing that prosaccades completed after antisaccades exhibited increased RTs in combination with a P3 amplitude comparable to antisaccades provides convergent evidence that the unidirectional prosaccade switch-cost is attributed to the persistent activation of a non-standard antisaccade task-set.Behavioural Brain Research 10/2015; DOI:10.1016/j.bbr.2014.10.012 · 3.03 Impact Factor
- "EEG data were re-referenced to the numeric mean of the mastoids and band-pass filtered with cutoffs of 0.1 and 30 Hz (12 dB/oct rolloff). All trials were also corrected for eye movements and blinks using the method developed by Gratton et al. (1983). A computerbased algorithm was used to detect physiological artifacts such that individual trials were rejected if there was a voltage step greater than 50 V between sampling points, a voltage difference of more than 200 V within a trial, or a maximum voltage difference less than 0.5 V within a trial. "
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- "EEGs were epoched from À200 before to 800 ms following presentation of the stimulus. The Gratton and Coles algorithm (Gratton et al., 1983) was used to correct eye movement and blink artifacts. Specific intervals for individual channels were rejected in each epoched trial using a semi-automated procedure, with physiological and motor artifacts identified by the following criteria: a voltage step of more than 50.0 μV between sample points, a voltage difference of 200.0 μV within a segment, a maximum of 100 mV and a minimum of À 100 mV within a segment, and a maximum voltage difference of less than 0.50 μV within 100 ms intervals. "
ABSTRACT: Previous studies examining sustained attention ability in older adults have yielded inconsistent results: age-related decline in studies using traditionally formatted tasks (TFT), in which subjects have to respond to rare targets, and preservation in studies using Go/No-Go tasks, in which subjects have to withhold response to rare targets. The purpose of this study was to examine whether these discrepancies could be explained by a differential use of automatic and controlled processes according to age. To that end, we used two versions of the same task differing in response mode (TFT, Go/No-Go), and the event-related potential (ERP) technique. The within-task comparison first revealed that older adults exhibited a vigilance decrement in the TFT SART, while their performance actually improved in the Go/No-Go SART. Secondly, in both tasks, ERP results notably evidenced increased P2s and non-target P3s in older adults, components related to the allocation of attentional resources. Altogether, our results suggest that in both tasks older adults adopted a controlled processing mode, which resulted in opposite effects on performance according to the nature of the task. Copyright © 2015. Published by Elsevier Ltd.Neuropsychologia 07/2015; 75. DOI:10.1016/j.neuropsychologia.2015.07.021 · 3.30 Impact Factor