Nitzan Shahar

Selective influence of working memory load on exceptionally slow reaction times.

The rate of exceptionally slow reaction times (RTs), described by the long tail of the RT distribution, was found to be amplified in a variety of special populations with cognitive deficits (e.g., early-stage Alzheimers disease, attention-deficit/hyperactivity disorder, low intelligence, elderly). Previous individual differences studies found high correlations between working memory (WM) and parameters that characterize the magnitude of the long-RT tail. However, the causal direction remains unknown. In 3 choice-reaction task experiments, we examined this relationship by directly manipulating WM availability. In Experiment 1, the stimulus-response rules were either arbitrary (WM demanding) or nonarbitrary. In Experiment 2, the arbitrary rules were either novel (demanding) or practiced. In Experiment 3, WM was loaded with either declarative (stimulus-stimulus) or procedural (stimulus-response) arbitrary rules. Using an ex-Gaussian model fitting, we found across all experiments that WM demands uniquely influenced the τ parameter, mostly responsible for the long-RT distribution tail. Evidence accumulation modeling of the choice process indicated that WM load had little influence on the decision process itself and primarily affected the duration of an exponentially distributed nondecision component, assumed to reflect the process of rule retrieval. Theoretical interpretations and implications are discussed.

Exploring relations between task conflict and informational conflict in the Stroop task.

In this study, we tested the proposal that the Stroop task involves two conflicts—task conflict and informational conflict. Task conflict was defined as the latency difference between color words and non-letter neutrals, and manipulated by varying the proportion of color words versus non-letter neutrals. Informational conflict was defined as the latency difference between incongruent and congruent trials and manipulated by varying the congruent-to-incongruent trial ratio. We replicated previous findings showing that increasing the ratio of incongruent-to-congruent trials reduces the latency difference between the incongruent and congruent condition (i.e., informational conflict), as does increasing the proportion of color words (i.e., task conflict). A significant under-additive interaction between the two proportion manipulations (congruent vs. incongruent and color words vs. neutrals) indicated that the effects of task conflict and informational conflict were not additive. By assessing task conflict as the contrast between color words and neutrals, we found that task conflict existed in all of our experimental conditions. Under specific conditions, when task conflict dominated behavior by explaining most of the variability between congruency conditions, we also found negative facilitation, thus demonstrating that this effect is a special case of task conflict.

The role of working memory in rapid instructed task learning and intention-based reflexivity: An individual differences examination☆

The ability to efficiently follow novel task instructions (Rapid Instructed Task Learning, RITL) appears late in evolution, is required for successful collaborative teamwork, and appears to involve maintaining instructions in working-memory (WM). RITL is indexed by the efficiency in which the instructions are performed (RITL success) and by whether the instructions operate automatically (intention-based reflexivity). Based on prior normative work employing WM-load manipulations, we predicted that individual differences in WM would positively correlate with these RITL indices. Participants (N=175) performed the NEXT paradigm, which is used to assess RITL, and tests of choice reaction time, intelligence, and WM. Confirmatory factor analyses showed that, contrary to our predictions, successful performance in WM tasks did not predict RITL performance. Tests tapping general-fluid intelligence and reaction time positively correlated with RITL success. However, contrary to our predictions, RITL success positively correlated with little intention-based reflexivity. We suggest that for a RITL paradigm to produce intention-based reflexivity, its WM demand must be low, and, thus, performance does not reflect individual differences in WM.

Memory for Action Rules and Reaction Time Variability in Attention-Deficit/Hyperactivity Disorder

BACKGROUND Patients with attention-deficit/hyperactivity disorder (ADHD) exhibit increased reaction time (RT) variability. This finding is consistent across various choice RT tasks and is considered a core ADHD phenotype, often interpreted as expressing occasional attention lapses. This study explores the selective contribution of perceptual and working memory (WM) processes to increased RT variability in ADHD. METHODS Low and high WM demands were manipulated in a battery of choice RT tasks administered to two groups of college students (subjects with ADHD vs. healthy control subjects). RESULTS Ex-Gaussian distribution fitting revealed an increased rate of exceptionally slow RTs (i.e., higher τ values) in subjects with ADHD under all conditions. These group differences interacted with WM demands, showing the largest group differences when WM processing was most demanding (ηp2 = .32). Under demanding WM conditions, evidence accumulation modeling demonstrated that increased RT variability in ADHD is not associated with either momentary or constant deficits in perceptual processing of the target. Rather, results favored a model associating increased RT variability in ADHD with reduced rate of WM retrieval. CONCLUSIONS These results suggest a pivotal contribution for the retrieval of action rules from WM to increased RT variability in ADHD.

Differential contribution of task conflicts to task switch cost and task mixing cost in alternating runs and cued task-switching: evidence from ex-Gaussian modeling of reaction time distributions

Task switching involves switch cost (poorer performance switch trials than in task-repetition trials) and mixing cost (poorer performance in task-repetition trials than in trials from blocks without task switching). These are mainly studied with the alternating runs task-switching (ARTS) paradigm (in which the task changes every constant number of trials) or the cued task-switching (CTS) paradigm, in which the tasks change randomly. The authors tested the hypothesis that dealing with actual or potential conflicts regarding which task is currently required contribute to mixing cost in CTS more than in ARTS and contribute to switch costs more in ARTS than in CTS. This hypothesis was tested using ex-Gaussian modeling of reaction time (RT) distributions, in which the heaviness of the right tail marks task conflicts (Steinhauser and Hübner in J Exp Psychol Human Percept Perform 35:1398–1412 2009). As predicted, a heavier RT-distribution tail marked switch cost more strongly in ARTS than in CTS and marked mixing costs more strongly in CTS than in ARTS. These results indicate that switch cost and mixing cost reflect different processes in different task-switching paradigms.

Learning to control actions: transfer effects following a procedural cognitive control computerized training.

Few studies have addressed action control training. In the current study, participants were trained over 19 days in an adaptive training task that demanded constant switching, maintenance and updating of novel action rules. Participants completed an executive functions battery before and after training that estimated processing speed, working memory updating, set-shifting, response inhibition and fluid intelligence. Participants in the training group showed greater improvement than a no-contact control group in processing speed, indicated by reduced reaction times in speeded classification tasks. No other systematic group differences were found across the different pre-post measurements. Ex-Gaussian fitting of the reaction-time distribution revealed that the reaction time reduction observed among trained participants was restricted to the right tail of the distribution, previously shown to be related to working memory. Furthermore, training effects were only found in classification tasks that required participants to maintain novel stimulus-response rules in mind, supporting the notion that the training improved working memory abilities. Training benefits were maintained in a 10-month follow-up, indicating relatively long-lasting effects. The authors conclude that training improved action-related working memory abilities.

Examining procedural working memory processing in obsessive-compulsive disorder

Previous research has suggested that a deficit in working memory might underlie the difficulty of obsessive-compulsive disorder (OCD) patients to control their thoughts and actions. However, a recent meta-analyses found only small effect sizes for working memory deficits in OCD. Recently, a distinction has been made between declarative and procedural working memory. Working memory in OCD was tested mostly using declarative measurements. However, OCD symptoms typically concerns actions, making procedural working-memory more relevant. Here, we tested the operation of procedural working memory in OCD. Participants with OCD and healthy controls performed a battery of choice reaction tasks under high and low procedural working memory demands. Reaction-times (RT) were estimated using ex-Gaussian distribution fitting, revealing no group differences in the size of the RT distribution tail (i.e., τ parameter), known to be sensitive to procedural working memory manipulations. Group differences, unrelated to working memory manipulations, were found in the leading-edge of the RT distribution and analyzed using a two-stage evidence accumulation model. Modeling results suggested that perceptual difficulties might underlie the current group differences. In conclusion, our results suggest that procedural working-memory processing is most likely intact in OCD, and raise a novel, yet untested assumption regarding perceptual deficits in OCD.

Formation of abstract task representations: Exploring dosage and mechanisms of working memory training effects

Working memory is strongly involved in human reasoning, abstract thinking and decision making. Past studies have shown that working memory training generalizes to untrained working memory tasks with similar structure (near-transfer effect). Here, we focused on two questions: First, we ask how much training might be required in order to find a reliable near-transfer effect? Second, we ask which choice- mechanism might underlie training benefits? Participants were allocated to one of three groups: working-memory training (combined set-shifting and N-back task), active-control (visual search) and no-contact control. During pre/post testing, all participants completed tests tapping procedural and declarative working memory as well as reasoning. We found improved performance only in the procedural working-memory transfer tasks, a transfer task that shared a similar structure to that of the training task. Intermediate testing throughout the training period suggest that this effect emerged as soon as after 2 training sessions. We applied evidence accumulation modeling to investigate the choice process responsible for this near-transfer effect and found that trained participants, compared with active-controls had quicker retrieval of the action rules, and more efficient classification of the target. We conclude that participants were able to form abstract representations of the task procedure (i.e., stimulus-response rules) that was then ~applied to novel stimuli and responses.