Caffeine improves anticipatory processes in task switching

We studied the effects of moderate amounts of caffeine on task switching and task maintenance using mixed-task (AABB) blocks, in which participants alternated predictably between two tasks, and single-task (AAAA, BBBB) blocks. Switch costs refer to longer reaction times (RT) on task switch trials (e.g. AB) compared to task-repeat trials (e.g. BB); mixing costs refer to longer RTs in task-repeat trials compared to single-task trials. In a double-blind, within-subjects experiment, two caffeine doses (3 and 5mg/kg body weight) and a placebo were administered to 18 coffee drinkers. Both caffeine doses reduced switch costs compared to placebo. Event-related brain potentials revealed a negative deflection developing within the preparatory interval, which was larger for switch than for repeat trials. Caffeine increased this switch-related difference. These results suggest that coffee consumption improves task-switching performance by enhancing anticipatory processing such as task set updating, presumably through the neurochemical effects of caffeine on the dopamine system.     

Sequence effects in cued task switching modulate response preparedness and repetition priming processes

In task‐switching paradigms, reaction time (RT) switch cost is eliminated on trials after a no‐go trial (no‐go/go sequence effect). We examined the locus of no‐go interference on task‐switching performance by comparing the event‐related potential (ERP) time course of go/go and no‐go/go sequences from cue onset to response execution. We also examined whether noninformative trials (i.e., delayed reconfiguration, no response inhibition) produce similar sequence effects. Participants switched using informative and noninformative cues ( Experiment 2 ) intermixed with no‐go trials ( Experiment 1 ). Repeat RT was slower for both no‐go/informative (pNG/I) and noninformative/informative (pNI/I) than informative/informative sequences. ERPs linked to anticipatory preparation showed no effect of trial sequence. ERPs indicated that pNG/I sequences reduce response readiness whereas pNI/I sequences reduce repetition benefit for repeat trials. Implications for task‐switching models are discussed.     

Pre-stimulus EEG effects related to response speed, task switching and upcoming response hand

The task-switching paradigm provides an opportunity to study whether oscillatory relations in neuronal activity are involved in switching between and maintaining task sets. The EEG of subjects performing an alternating runs [Rogers, R.D., Monsell, S., 1995. Costs of a predictable switch between simple cognitive tasks. Journal of Experimental Psychology: General 124, 207-231] task-switching task was analyzed using event-related potentials, the lateralized readiness potential, instantaneous amplitude and the phase-locking value [Lachaux, J.P., Rodriguez, E., Martinirie, J., Varela, F.J., 1999. Measuring phase synchrony in brain signals. Human Brain Mapping 8, 194-208]. The two tasks differed in the relevant modality (visual versus auditory) and the hand with which responses were to be given. The mixture model [de Jong, R., 2000. An intention driven account of residual switch costs. In: Monsell, S., Driver, J. (Eds.), Attention and Performance XVII: Cognitive Control. MIT Press, Cambridge] was used to assign pre-stimulus switch probabilities to switch trials based on reaction time; these probabilities were used to create a fast-slow distinction between trials on both switch and hold trials. Results showed both time- and time-frequency-domain effects, during the intervals preceding stimuli, of switching versus maintenance, response speed of the upcoming stimulus, and response hand. Of potential importance for task-switching theory were interactions between reaction time by switch-hold trial type that were found for a frontal slow negative potential and the lateralized readiness potential during the response-stimulus interval, indicating that effective preparation for switch trials involves different anticipatory activity than for hold trials. Theta-band oscillatory activity during the pre-stimulus period was found to be higher when subsequent reaction times were shorter, but this response speed effect did not interact with trial type. The response hand of the upcoming task was associated with lateralization of pre-stimulus mu- and beta-band amplitude and, specifically for switch trials, beta-band phase locking.     

Teasing apart the anticipatory and consummatory processing of monetary incentives: An event‐related potential study of reward dynamics

The monetary incentive delay (MID) task has been widely used in fMRI studies to investigate the neural networks involved in anticipatory and consummatory reward processing. Previous efforts to adapt the MID task for use with ERPs, however, have had limited success. Here, we sought to further decompose reward dynamics using a comprehensive set of anticipatory (cue‐N2, cue‐P3, contingent negative variation [CNV]) and consummatory ERPs (feedback negativity [FN], feedback P3 [fb‐P3]). ERP data was recorded during adapted versions of the MID task across two experiments. Unlike previous studies, monetary incentive cues modulated the cue‐N2, cue‐P3, and CNV; however, cue‐related ERPs and the CNV were uncorrelated with one another, indicating distinct anticipatory subprocesses. With regard to consummatory processing, FN amplitude primarily tracked outcome valence (reward vs. nonreward), whereas fb‐P3 amplitude primarily tracked outcome salience (uncertain vs. certain). Independent modulation of the cue‐P3 and fb‐P3 was observed, indicating that these two P3 responses may uniquely capture the allocation of attention during anticipatory and consummatory reward processing, respectively. Overall, across two samples, consistent evidence of both anticipatory and consummatory ERP activity was observed on an adapted version of the MID paradigm, demonstrating for the first time how these ERP components may be integrated with one another to more fully characterize the time course of reward processing. This ERP‐MID paradigm is well suited to parsing reward dynamics, and can be applied to both healthy and clinical populations.     

Behavioural and electrophysiological measures of task switching during single and mixed-task conditions

In order to understand how the brain prepares for and executes a switch in task demand, we measured reaction time (RT), accuracy, and event-related brain potentials associated with performance in single and mixed-task blocks using a cued design. Our results show that trials which repeat in a mixed-task block (repeat trials) were more demanding than trials which repeated in a single-task block, as reflected by the presence of a RT mixing cost and by the presence of a smaller target-locked positivity (P3b) on repeat trials. Within a mixed-task block, repeat and switch trials also differed, where repeat trials showed evidence of greater preparation (larger cue-locked negativity), more efficient target processing (larger target-locked P3b), and shorter RTs. In addition, the cue-locked negativity difference remained despite equating repeat and switch trials on RT, suggesting that this negativity difference is specific to the switching process. Our results are discussed in light of existing models of task switching.     

The time course of the asymmetrical "local" switch cost: evidence from event-related potentials

The goal of the study was to explore the time-course of the asymmetrical “local” switch cost observed in task switching. We investigated event-related potentials induced by cue and target processing when participants were engaged in a card-sorting switching task. Participants were instructed to match each card (target) following one of two possible task rules, the color or the form. The correct task rule changed unpredictably after a variable number of trials, and was signalled by cues indicating to switch or repeat the previous task rule. We observed that transition type (switch versus repeat) and task rule (color versus form) influenced both cue and target processing. Interestingly, the interaction between transition type and task rule, indicating an asymmetry in the local switch cost, affected the brain responses during target processing but not during cue processing. These results suggest that the asymmetry in the local switch cost relates to task execution processes.     

Reward processing in gain versus loss context: An ERP study

Previous research has shown that consummatory ERP components are sensitive to contextual valence. The present study investigated the contextual valence effect across anticipatory and consummatory phases by requiring participants to play a simple gambling task during a gain context and a loss context. During the anticipatory phase, the cue‐P3 was more positive in the gain context compared to the loss context, whereas the stimulus‐preceding negativity (SPN) was comparable across the two contexts. With respect to the consummatory phase, the feedback‐related negativity (FRN) in response to the zero‐value outcome was more negative in the gain versus loss context, whereas the feedback P3 (fb‐P3) in response to the zero‐value outcome was insensitive to contextual valence. These findings suggest that contextual valence effect occurs at a relative early stage of both the reward anticipation and consumption. Moreover, across the gain and loss contexts, the SPN was selectively correlated with the FRN, whereas the cue‐P3 was selectively associated with the fb‐P3, pointing to a close association between the anticipatory and consummatory phases in reward dynamics.     

Object switching within working memory is reflected in the human event-related brain potential

In two experiments applying a memory updating task subjects are asked to perform several arithmetic operations on stored numbers. From a trial-to-trial perspective these operations could be either performed on a previously processed item or on a new item which requires an object switch in working memory. Object switching results in prolonged operation times; these operation time costs reflect the switch of the focus of attention to the relevant information. Event-related brain potentials obtained in object switch trials show an increased P3a around 300 ms and a late, central negative component between 400 ms and 500 ms. The data suggest that the P3a may reflect the unhitching of the focus of attention from the current information or task set through the inhibition of the prepared response while the late negative component may reflect the allocation of the focus of attention to another object in working memory. The present results are best explained within a controlled attention view of working memory supporting the idea that object switching relies on the ability to orient the focus of attention within working memory.     

Task-switching costs have distinct phase-locked and nonphase-locked EEG power effects

Event-related potentials (ERPs) and total time–frequency power analyses have shown that performance costs during task switching are related to differential preparation to switch tasks (switch cost) and repeat the same task (mixing cost) during both proactive control (cue-to-target interval; CTI) and reactive control (post-target). The time–frequency EEG signal is comprised of both phase-locked activity (associated with stimulus-specific processes) and nonphase-locked activity (represents processes thought to persist over longer timeframes and do not contribute to the average ERP). In the present study, we used a cued task-switching paradigm to examine whether phase-locked and nonphase-locked power are differentially modulated by switch and mixing effects in intervals associated with the need for proactive control (CTI) and reactive control (post-target interval). Phase-locked activity was observed in the theta and alpha bands, closely resembled that seen for total power, and was consistent with switch and mixing ERP positivities. Nonphase-locked analyses showed theta and alpha power effects for both switch and mixing effects early in the CTI and as well as more sustained alpha and beta activity around cue onset, and extending from mid-CTI into the post-target interval. Nonphase-locked activity in pretarget alpha and posttarget theta power were both correlated with response time mixing cost. These findings provide novel insight into phase-locked and nonphase-locked activity associated with switch and mixing costs that are not evident with ERP or total time–frequency analyses.     

Age differences in attentional control: an event-related potential approach

We examined age differences in event-related potentials (ERPs) associated with attentional control of task-set selection and response interference by means of a cue-based switching paradigm in which participants performed the color or word Stroop task. The results of ERPs in the cue interval indicated that P3 latencies were slowed for older adults, suggesting age-related slowing in updating currently relevant task sets. Older adults also showed a larger CNV under switching than nonswitching conditions, indicating age differences in maintaining task sets over longer periods of time. The results of target-locked ERPs revealed a negativity to incompatible Stroop trials (Ni) that was prolonged for older adults, suggesting age differences in early conflict processing. Response-locked ERPs showed a negative deflection to incompatible Stroop trials (CRN) only for younger adults, suggesting age differences also in response-related conflict processing.     

Switch‐specific and general preparation map onto different ERP components in a task‐switching paradigm

We examined whether the cue‐locked centroparietal positivity is associated with switch‐specific or general preparation processes. If this positivity (300–400 ms) indexes switch‐specific preparation, faster switch trials associated with smaller RT switch cost should have a larger positivity as compared to slower switch trials, but no such association should be evident for repeat trials. We extracted ERP waveforms corresponding to semi‐deciles of each participant's RT distribution (i.e., fastest to slowest 5% of trials) for switch and repeat conditions. Consistent with a switch‐specific preparation process, centroparietal positivity amplitude was linked to slower RT and larger RT switch cost for switch but not repeat trials. A later pre‐target negativity (500–600 ms) was inversely correlated with RT for both switch and repeat trials, consistent with a general anticipatory preparation processes.     

The development of anticipatory cognitive control processes in task-switching: an ERP study in children, adolescents, and young adults

To investigate the development of advance task-set updating and reconfiguration, behavioral and event-related potential (ERP) data were recorded in children (9-10 years), adolescents (13-14 years), and young adults (20-27 years) in a cued task-switching paradigm. In pure blocks, the same task was repeated. In mixed blocks, comprised of stay and switch trials, two tasks were intermixed. Age differences were found for stay-pure performance (mixing costs) in the 600-ms but not in the 1200-ms cue-target interval (CTI). Children showed larger reaction time mixing costs than adults. The ERPs suggested that the larger costs were due to delayed anticipatory task-set updating in children. Switch-stay performance decrements (switch costs) were age-invariant in both CTIs. However, ERP data suggested that children reconfigured the task-set on some stay trials, rather than only on switch trials, suggesting the continued maturation of task-set reconfiguration processes.     

Sleep continuity and total sleep time are associated with task‐switching and preparation in young and older adults

Ageing is associated with changes in sleep and decline executive functions, such as task‐switching and task preparation. Given that sleep affects executive function, age‐related changes in executive function may be attributable to changes in sleep. The present study used a sleep detection device to examine whether or not wake time after sleep onset and total sleep time moderated age differences in task‐switching performance and participants' ability to reduce switch costs when given time to prepare. Participants were cognitively healthy [Mini Mental State Examination > 26] younger (n = 54; mean age = 22.9; 67.8% female) and older (n = 45; mean age 62.8; 71.1% female) adults. Using a task‐switching paradigm, which manipulated preparation time, we found that smaller global switch costs were associated with lower wake time after sleep onset and longer total sleep time. Greater preparation effects on local switch costs and adoption of a task‐set were associated with lower wake time after sleep onset, although this effect was significant only in older adults when stratified by age group. This association was independent of inhibition and working memory abilities. The lack of interactions between sleep and age group indicated that age differences in switch costs were not moderated by better sleep. Our results suggest that young and older adults may benefit similarly from lower wake time after sleep onset and longer total sleep time in overall performance, and individuals with less wake time after sleep onset are more likely to engage preparatory strategies to reduce switch costs and boost task‐switching performance.     

EEG delta oscillations index inhibitory control of contextual novelty to both irrelevant distracters and relevant task‐switch cues

Delta oscillations contribute to the human P300 event‐related potential evoked by oddball targets, although it is unclear whether they index contextual novelty (event oddballness, novelty P3, nP3), or target‐related processes (event targetness, target P3b). To examine this question, the electroencephalogram (EEG) was recorded during a cued task‐switching version of the Wisconsin card‐sorting test. Each target card was announced by a tone cueing either to switch or repeat the task. Novel sound distracters were interspersed among trials. Time‐frequency EEG analyses revealed bursts of delta (2–4 Hz) power associated with enhanced nP3 amplitudes to both task‐switch cues and novel distracters—but no association with target P3b. These findings indicate that the P300‐delta response indexes contextual novelty regardless of whether novelty emanates from endogenous (new task rules) or exogenous (novel distracters) sources of information.     

Effects of caffeine on anticipatory control processes: evidence from a cued task-switch paradigm

Effects of caffeine on task switching were studied using ERPs in a cued task-switch paradigm. The need for advance preparation was manipulated by varying the number of task-set aspects that required switching. In a double-blind, within-subjects experiment, caffeine reduced shift costs compared to placebo. ERPs revealed a negative deflection developing within the preparatory interval, which was larger for shift than for repeat trials. Caffeine increased this shift-induced difference. Furthermore, shift costs increased as a function of the number of task-set features to be switched, but this pattern was not modulated by caffeine. The results suggest that caffeine improves task-switching performance by increasing general effects on task switching, related to task-nonspecific (rather than task-specific) anticipatory processes. Caffeine's actions may be mediated by dopaminergic changes in the striatum or anterior cingulate cortex.     

The prefrontal cortex and the executive control of attention

We review two studies aimed at understanding the role of prefrontal cortex (PFC) in the control of attention. The first study examined which attentional functions are critically dependent on PFC by removing PFC unilaterally and transecting the forebrain commissures in two macaques. The monkeys fixated a central cue and discriminated the orientation of a colored target grating presented among colored distracter gratings in either the hemifield affected by the PFC lesion or the normal control hemifield. When the cue was held constant for many trials, task performance in the affected hemifield was nearly normal. However, performance was severely impaired when the cue was switched frequently across trials. The monkeys were unimpaired in a pop-out task with changing targets that did not require top-down attentional control. Thus, the PFC lesion resulted in selective impairment in the monkeys’ ability to switch top-down control. In the second study, we used fMRI to investigate the neural correlates of top-down control in humans performing tasks identical to those used in the monkey experiments. Several fronto-parietal and posterior visual areas showed enhanced activation when attention was switched, which was greater on color cueing (top-down) trials relative to pop-out trials. Taken together, our findings indicate that both frontal and parietal cortices are involved in generating top-down control signals for attentive switching, which may then be fed back to visual processing areas. The PFC in particular plays a critical role in the ability to switch attentional control on the basis of changing task demands.     

Dimensional weighting and task switching following frontal lobe damage: Fractionating the task switching deficit

Deficits in task switching can be found after frontal lobe damage. Here we demonstrate an impairment in task switching specifically linked to when perceptual weights have to be moved between different dimensions of the same stimulus. A patient (DS) with left frontal lobe damage showed normal performance when he responded to the meaning (a word task) or location (a location task) of a word presented to the left or right of fixation when there was no switching between the tasks. However, when the two tasks were switched every 16 trials in a block, DS showed severe difficulty in performing both tasks (Experiment 1). There were then abnormally large switch costs and effects of stimulus-response congruency. The difficulty was not simply due to switching tasks per se: There were no costs of switching when one of the tasks was modified to have different stimulus displays from the other (Experiment 2). The deficit was also not greater when the switch had to be made from a well-practised task to an unpractised task with more arbitrary stimulus-response mappings, indicating no particular problem in disengaging from a learned task or in configuring new stimulus-response links (Experiment 4). We suggest instead that DS was impaired at shifting attentional weights across different dimensions of the same stimulus, a process required with practised and unpractised tasks alike. The results link this process of shifting attention across stimulus dimensions to the left frontal lobe.     

Dimensional weighting and task switching following frontal lobe damage: Fractionating the task switching deficit

Deficits in task switching can be found after frontal lobe damage. Here we demonstrate an impairment in task switching specifically linked to when perceptual weights have to be moved between different dimensions of the same stimulus. A patient (DS) with left frontal lobe damage showed normal performance when he responded to the meaning (a word task) or location (a location task) of a word presented to the left or right of fixation when there was no switching between the tasks. However, when the two tasks were switched every 16 trials in a block, DS showed severe difficulty in performing both tasks (Experiment 1). There were then abnormally large switch costs and effects of stimulus–response congruency. The difficulty was not simply due to switching tasks per se: There were no costs of switching when one of the tasks was modified to have different stimulus displays from the other (Experiment 2). The deficit was also not greater when the switch had to be made from a well-practised task to an unpractised task with more arbitrary stimulus–response mappings, indicating no particular problem in disengaging from a learned task or in configuring new stimulus–response links (Experiment 4). We suggest instead that DS was impaired at shifting attentional weights across different dimensions of the same stimulus, a process required with practised and unpractised tasks alike. The results link this process of shifting attention across stimulus dimensions to the left frontal lobe.     

Task set switching in schizophrenia

The authors used a task-switching paradigm to investigate set shifting ability in schizophrenia. This paradigm included 2 choice reaction time (RT) tasks: up-down and right-left. Switching tasks were associated with costs (i.e., longer RT in task-switch trials than in task-repetition trials); patients responded more slowly than controls and suffered greater switching costs, were as efficient as controls in engaging in an upcoming task set, and were faster than controls in disengaging from the previous task set. There were indications that patients quickly forgot what each keypress indicated, making it necessary for them to acquire response meaning information anew in each trial. To test this notion, the authors subsequently tested normal participants in conditions in which response meaning information needed to be acquired anew in each trial. These participants produced a pattern of switching costs resembling that of patients. Results suggest that set switching difficulties in schizophrenia, as exhibited in the present paradigm, reflect poor memory for task context information.     

The lateralized readiness potential and P300 of stimulus-set switching

The present study aimed at investigating whether stimulus-set switching involves the same stages of information processing as response-set switching. A pair-wise task switching paradigm in which the trial sequences comprised only two tasks was used. The effect of preparation was manipulated so that the participants performed only the repeat trials in some blocks and only the switch trials in other blocks, or both the repeat and switch trials were randomly mixed within a single block. P300 peak latency and stimulus- and response-locked lateralized readiness potential intervals were used to indicate the processing stage of stimulus identification, response selection and motor execution, respectively. The results demonstrated that the stimulus-set switching involves stages of information processing following stimulus identification and before motor execution.