ASCS09: Proceedings of the 9th Conference of the Australasian Society for Cognitive Science
Multiple Sources Underlie ERP Indices of Task-Switching Sharna Jamadar (
[email protected]) Centre for Brain & Mental Health Research, University of Newcastle Callaghan, NSW, Australia
Alexander Provost (
[email protected]) W. Ross Fulham (
[email protected]) Patricia T. Michie (
[email protected]) & Frini Karayanidis (
[email protected]) Centre for Brain & Mental Health Research, University of Newcastle Callaghan, NSW, Australia Abstract Previous investigations of task-switching have reported that cue-related processes are indexed by a differential switch positivity (D-Pos) and that stimulus-related processes are indexed by a differential switch-negativity (D-Neg). The aim of the current study was to use low resolution electromagnetic tomography to localize the sources of DPos and D-Neg. Participants switched randomly between simple tasks and showed an increase in reaction time (RT) for switch relative to repeat trials, i.e., an RT switch cost. ERP waveforms showed a D-Pos in the cue-related interval and a D-Neg in the stimulus-related interval. D-Pos was localized to the superior parietal cortex, supporting arguments that D-Pos is associated with activating task rules during anticipatory reconfiguration. D-Neg was localized to the dorsolateral prefrontal cortex, implicating D-Neg in post-stimulus control. Keywords: Task-switching; source analysis
event-related
potentials;
Task-switching paradigms require rapid alternation between simple tasks and typically show increased reaction time (RT) on trials that follow a switch as compared to a repeat in task. The task active on any particular trial is defined either by a cue presented prior to each trial (cued trials paradigm; Meiran, 1996) or by a predictable repeating task sequence (ie., AABB; alternating runs paradigm; Rogers & Monsell, 1995). Increasing the interval between the informative cue and the stimulus (i.e., cue-to-stimulus interval, CSI) in a cuedtrials paradigm, or the interval between the response to the preceding stimulus and onset of the next stimulus (i.e., response-to-stimulus interval, RSI) in an alternating runs paradigm significantly reduces RT switch cost (e.g. Rogers & Monsell, 1995). This reduction in switch cost with increasing preparation interval has been attributed to anticipatory preparation for an impending switch trial occurring in advance of stimulus onset (Rogers & Monsell, 1995; Rubinstein, Meyer & Evans., 2001). However, a significant ‘residual’ RT switch cost remains even at preparation intervals exceeding 1000ms (Rogers & Monsell, 1995) suggesting either that advance preparation is not sufficient to overcome the cost of switching or that post-stimulus interference is greater on switch than repeat trials.
Article DOI: 10.5096/ASCS200924
Rogers and Monsell (1995) proposed that the process of shifting from a state of readiness to complete one task to readiness to complete a different task, which they coined task-set reconfiguration, is comprised of at least two separable components. Given adequate time and information, anticipatory task-set reconfiguration may occur in the preparation interval and facilitate performance by biasing the system towards the currently relevant task-set, thereby bringing the system into a state of task readiness prior to stimulus onset. Stimulustriggered reconfiguration is invoked to account for the residual switch cost which remains even at long preparation intervals. Alternatively, residual switch cost may result from increased between-task interference on switch relative to repeat trials (e.g., Allport, Styles & Hseih, 1994). ERP studies have consistently shown differential electrical activity for switch relative to repeat trials both in the preparatory interval and after stimulus onset. ERP waveforms time-locked to the beginning of the preparatory interval usually show a larger posterior positivity for switch as compared to repeat trials, an effect that has been observed in many paradigms and across stimulus modalities (e.g., Karayanidis, Coltheart, Michie & Murphy, 2003; Nicholson, Karayanidis, Poboka, Heathcote & Michie, 2005). This cue-locked differential switch-positivity (labeled ‘D-Pos’) emerges as early as 150 ms after cue onset and varies in duration depending on cue and task parameters. With long preparation intervals, D-Pos fully resolves prior to stimulus onset, whereas with short preparation intervals, it continues after stimulus onset (Karayanidis et al., 2003). The amplitude of D-Pos has been found to be related to behavioral indices of preparation (Kieffaber & Hetrick, 1995; Lavric, Mizon, & Monsell, 2008). There is continuing disagreement regarding the electrophysiological origins of D-Pos. Some researchers argue that D-Pos reflects enhancement of the amplitude of the centroparietal P3 for switch relative to repeat trials (Kieffaber & Hetrick, 2005). Others argue that D-Pos represents a distinct ERP component (or set of components) that temporally overlaps the centroparietal P3 as well as other earlier and later ERPs in the preparation interval (Karayanidis et al., 2003). Within this framework, D-Pos is conceptualized as an index of one or more processes that contribute to anticipatory task-set reconfiguration. This framework is
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supported by findings that a number of temporally distinct cue-locked differential switch-positivities can be differentiated within the preparation interval depending on the duration of the RCI and CSI (Nicholson et al., 2005) and the informativeness of the cue (Jamadar, Michie & Karayanidis, in press; Karayanidis, Mansfield, Galloway, Smith, Provost & Heathcote, 2009). Stimulus-locked ERP waveforms have consistently shown that the large centroparietal positivity is reduced in amplitude for switch trials as compared to repeat trials (Karayanidis et al., 2003; Nicholson et al., 2005). In order to maintain consistency with our previous work, we will refer to this as the stimulus-locked differential switchnegativity (‘D-Neg’). However, note that, other researchers argue that it reflects reduced P3 for the more difficult switch trials (Kieffaber & Hetrick, 2005). With long preparation intervals, D-Neg emerges as early as 150 ms and peaks around 400-800 ms after stimulus onset, whereas with short preparation intervals, it is delayed by more than 300 ms and is often preceded by a D-Pos (Karayanidis et al., 2003). It has been argued that since the processes indexed by D-Neg are not initiated until the processes indexed by the preceding D-Pos are initiated or completed, it is likely to be associated with residual switch cost (Nicholson et al., 2005). D-Neg may reflect either stimulus-triggered reconfiguration (Rogers & Monsell, 1995) or between-task interference (Allport et al., 1994). Recent ERP studies have attempted to localize the sources of the switch-differential ERP effects. Using a cued-trials task-switching paradigm with task-specific cues, Lavric et al. (2008) recorded a D-Pos emerging around 500ms post-cue. LORETA analyses indicated sources in the left superior prefrontal cortex (PFC), left mid-temporal and posterior parietal cortices (PPC) over 500-600 ms, followed by sources in left sensorimotor cortex, and superior and inferior temporal gyri over 600800 ms. Brass, Ullsperger, Knoesche, von Cramon & Phillips (2005) used fMRI results from an earlier study with the same paradigm (Brass & von Cramon 2004) to constrain ERP dipole placement so as to model differential switch activity in a 400-520 ms post-cue window. Differential switch ERP activity was associated with early activation in bilateral inferior frontal junction (400-460 ms) followed by activation in right PPC (464520 ms); a pattern of activation consistent with hierarchical organization of frontal and posterior circuitry. However, in this study, the differential ERP activity was a relative negativity for switch compared to repeat trials, unlike previous ERP studies that have reported a D-Pos (see Jamadar et al., in press for a review). A few studies have examined electrical or magnetic brain sources using tasks that involve transition cues – that is cues that signal trial transition (switch/repeat) but are not task-specific. Rushworth and colleagues identified dipole sources for ERP components associated with intentional-set (also referred to response-sets; Rushworth Passingham & Nobre, 2002) and attentional-set (stimulussets; Rushworth Passingham & Nobre, 2005) switching using occasional (8-17 trials) transition cues. Cue-locked
Article DOI: 10.5096/ASCS200924
waveforms showed that intentional set switching resulted in an early frontally-distributed switch positivity (360-520 ms) that was modeled by dipoles in dorsomedial and ventromedial frontal cortex, whereas attentional-set switching was associated with a parietally-distributed switch positivity around the same latency range that was modeled by a left fronto-temporal source. Both intentional and attentional set switching showed a later centroparietally-distributed D-Pos (520-1000 ms), that was modeled by a dipole in ventromedial occipitotemporal region. Perianez, Maestu, Barcelo, Fernandez, Amo & Alonso (2004) also used transition cues, and using magneto-encephalography (MEG) recording identified three sources of significantly greater activity for switch as compared to repeat rule cues. Switch-related activity in the inferior frontal gyrus emerged first over 100-200 ms post-cue and was followed by anterior cingulate (ACC) activity emerging 200 ms post-cue and inferior PPC activity at 300 ms post-cue. Note that transition cues require the retrieval of the previously active task-set from memory in order to activate the correct task-set. Furthermore, in Rushworth et al. (2002; 2005) and Perianez et al. (2004), the switch transition cues were less frequent than stay cues. Therefore, there are a number of other processes that differed between switch and repeat cues that may have at least partly affected the resulting ERP differences and their underlying brain sources. In summary, despite substantial differences in experimental design and methodology, there appears to be a consistent pattern of frontal activity followed by temporo-parietal cortical network activation in preparation in anticipation of a switch in task-set. However, there is also substantial variation in the location and timing of these effects that may result partly from differences in paradigm and partly from differences in methodology. In addition, most of the above studies have focused on cue-locked effects only. Lavric et al. (2008) computed LORETA for stimulus-locked waveforms, but focused primarily on the carryover of the switch-positivity in the short-CSI condition. Rushworth et al. (2002) was unable to model a post-target dipole source for intentional set-shifting whereas Rushworth et al. (2005) modelled a single dipole in the medial occipital cortex for the later part of the stimulus-locked epoch only. The aim of the current study was to examine areas of activation associated with D-Pos and D-Neg using a cuedtrials task-switching paradigm. We used a simplified version of the cued-trials paradigm with equal probability of switch and repeat trials and task rather than transition cues so as to be able to directly relate any cue-related differences between switch and repeat trials to differential preparation for an upcoming change in relevant task-set. We encouraged engagement in anticipatory task-set reconfiguration by removing the cue just prior to stimulus onset and using bivalent stimuli presented in a single spatial location so that the stimulus itself included attributes relevant to both task-sets and could not be used to identify the relevant task. A fixed long cue-stimulus interval of 600 ms was used as this has been shown to
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result in optimal engagement of anticipatory task-set reconfiguration processes (Rogers & Monsell, 1995; Nicholson et al., 2005) and to provide good temporal dissociation between cue- and stimulus-related ERPs (Nicholson et al., 2005). LORETA was calculated separately for switch and repeat trials within the cue-locked and stimulus-locked epochs. We selected EEG tomography analysis rather than dipole fitting because the former does not require predetermined assumptions about the number or location of sources. Low-resolution electromagnetic tomography (LORETA; Pascual-Marqui, Michel, Lehmann, 1994) was chosen over other tomography analysis methods (e.g., Minimum Norm: Hämäläinen & Illmonemi, 1994) because it has a low localization error rate, provides a single solution to the inverse problem of source localization (i.e., how to use surface data to identify sources) by searching for the ‘smoothest’ possible solution using only the assumption that neighboring voxels have maximally similar electrical activity (Pascual-Marqui, 1999). It has also been successfully used to localize brain regions involved in cognitive tasks (e.g., Herrmann & Fallgatter, 2004; among others). We expected that LORETA would reveal both frontal and parietal regions underlying D-Pos, supporting the notion that this component reflects processes involved in anticipatory task set reconfiguration (Karayanidis et al., 2003; Nicholson et al., 2005). In addition, based on the earlier findings by Brass et al. (2005), Rushworth (2002; 2005) and Perianez et al. (2004), we expected earlier sources in frontal areas compared to parietal areas. However, if the cue-related activations and sources reported in previous studies are related to other processes triggered by the cue in these other paradigms as noted earlier, we may find no or more limited differences between switch and repeat cues. The results pertaining to the generators of D-Neg are of particular interest, as the nature of this component is less well-defined than D-Pos. Major models of task-switching implicate an interference-related mechanism in poststimulus processes (Allport et al., 1994; Allport & Wylie, 2000), or a post-stimulus control process (Rogers & Monsell, 1995; Rubinstein et al., 2001), and it as yet remains unclear which of these processes is indexed by DNeg.
Methods
Stimuli and Tasks A square box outlined in gray (120 by 120 pixels) was continuously displayed in the center of a computer monitor (viewed from approximately 90 cm). The stimulus was a single digit (1-4 and 6-9, 60 x 60 pixels) presented in the center of the box. In the parity task, participants responded whether the digit was odd or even. In the magnitude task, participants responded whether the digit was less than or greater than 5. Participants responded with either their left or right index finger (e.g., left hand = even or less than 5; right hand = odd or greater than 5). Prior to stimulus onset, the outline of the box changed from gray to one of eight colors and provided a valid cue as to the task to be performed on the subsequent stimulus. Four ‘cold’ colors (dark blue, green, light blue, turquoise) were mapped to one task and four ‘hot’ colors (red, pink, orange, burgundy) were mapped to the other task. Cue color was never repeated on successive trials in order to avoid the effects of cue repetition benefit (Logan & Bundesen, 2003). The color cue remained on for the duration of the cue-stimulus interval, but was removed immediately prior to stimulus onset (i.e., the outline color of the box returned to gray). A task switch occurred on 50% of trials. Stimulusresponse mapping was congruent for the two tasks (parity or magnitude) on 20% of trials (e.g., ‘7’: odd and >5 mapped to the same hand) and incongruent on all remaining trials (e.g., ‘3’: odd and
