Pupillometric contributions to deciphering Stroop conflicts

Distributional analyses reveal the polymorphic nature of the Stroop interference effect: It's about (response) time

The study addressed the still-open issue of whether semantic (in addition to response) conflict does indeed contribute to Stroop interference (which along with facilitation contributes to the overall Stroop effect also known as Congruency effect). To this end, semantic conflict was examined across the entire response time (RT) distribution (as opposed to mean RTs). Three (out of four) reported experiments, along with cross-experimental analyses, revealed that semantic conflict was absent in the participants' faster responses. This result characterizes Stroop interference as a unitary phenomenon (i.e., driven uniquely by response conflict). When the same participants' responses were slower, Stroop interference became a composite phenomenon with an additional contribution of semantic conflict that was statistically independent of both response conflict and facilitation. While the present findings allow us to account for the fact that semantic conflict has not been consistently found in past studies, further empirical and theoretical efforts are still needed to explain why exactly it is restricted to longer responses. Indeed, since neither unitary nor composite models can account for this polymorphic nature of Stroop interference on their own, the implications for the current state of theory are outlined.

The contribution of difficulty of an irrelevant task to task conflict

In the standard colour-word Stroop task, participants are presented with colour words and required to respond to their colour while ignoring their meaning. Two types of conflict might occur in such experiments: information conflict and task conflict. Information conflict reflects the processing of two contradicting pieces of information and is indicated by shorter reaction times (RTs) in congruent than in incongruent trials. Task conflict reflects the additional effort associated with performing two tasks, as opposed to one, and is indicated by shorter RTs in neutral trials than in congruent trials (termed reverse facilitation). While information conflict is commonly seen in Stroop and Stroop-like tasks, task conflict is rarely observed. In the present study, participants were presented with coloured segments that, by applying Gestalt principles, could be perceived as colour words. We found that incongruent trials were slower than congruent trials, suggesting that participants successfully perceived the colour words, which led to involuntary reading. In addition, reversed facilitation was found so that neutral trials (i.e., trials that only consist of one task) were faster than congruent trials (as well as incongruent trials; both consist of two tasks). The presence of both interference from the incongruent trials and reverse facilitation suggests that involuntary reading could also occur in scenarios requiring cognitive effort.

Pupillary correlates of preparatory control in the Stroop task

In three experiments, individual differences in preparatory control in the Stroop task were examined. Participants performed variants of the Stroop task while pupillary responses were examined during the preparatory interval. Variation in working memory capacity was also examined. High Stroop performers tended to demonstrate larger preparatory pupillary responses than low Stroop performers. In Experiment 2, when participants were given pre-cues indicating the congruency of the upcoming trial (MATCHING vs. CONFLICTING), high Stroop performers had larger preparatory pupillary responses for incongruent trials compared to congruent trials, whereas low Stroop performers demonstrated similar preparatory pupillary responses on both incongruent and congruent trials. These results suggest that variation in Stroop performance is partially due to individual differences in the ability to ramp up and regulate the intensity of attention allocated to preparatory control processes. Additionally, there was limited evidence that preparatory control processes partially account for the relation between working memory capacity and performance on the Stroop. Overall, these results provide evidence that individual differences in Stroop performance are partialy due to variation in preparatory control.

The contribution of temporal analysis of pupillometry measurements to cognitive research

Reaction time (RT) is one of the most frequently used measures to detect cognitive processes. When tasks require more cognitive processes/resources, reaction is slower. However, RTs may provide only restricted information regarding the temporal characteristics of cognitive processes. Pupils respond reflexively to light but also to cognitive activation. The more cognitive resources a task requires, the more the pupil dilates. However, despite being able to use temporal changes in pupil size (advanced devices measure changes in pupil diameter with sampling rates of above 1000 samples per second), most past studies using pupil dilation have not investigated temporal changes in pupil response. In the current paper, we discuss the advantage of the temporal approach to analyze pupil changes compared to a more traditional perspective, specifically, singular value methods such as mean value and peak amplitude value. Using data from two recent studies conducted in our laboratory, we demonstrate the differences in findings arising from the various analyses. In particular, we focus on the advantage of temporal analysis in detecting hidden effects, investigating temporal characterizations of the effects, and validating the experimental manipulation.

Do Task Sets Compete in the Stroop Task and Other Selective Attention Paradigms?

Task sets have been argued to play an important role in cognition, giving rise to the notions of needing to switch between active task sets and to control competing task sets in selective attention tasks. For example, it has been argued that Stroop interference results from two categories of conflict: informational and task (set) conflict. Informational conflict arises from processing the word and is resolved by a late selection mechanism; task conflict arises when two task sets (i.e., word reading and colour identification) compete for activation and can be construed as involving an early selection mechanism. However, recent work has argued that task set control might not be needed to explain all of the switching cost in task switching studies. Here we consider whether task conflict plays a role in selective attention tasks. In particular, we consider whether S-R associations, which lead to informational conflict, are enough on their own to explain findings attributed to task set conflict. We review and critically evaluate both the findings that provided the original impetus for proposing task conflict in selective attention tasks and more recent findings reporting negative facilitation (longer RTs to congruent than to neutral stimuli) - a unique marker of task conflict. We then provide a tentative alternative account of negative facilitation based on poor control over informational conflict and apply it to a number of paradigms including the Colour-Object interference and Affordances tasks. It is argued that invoking competition between task sets in selective attention tasks might not be necessary.

A pupillometric study of developmental and individual differences in cognitive effort in visual word recognition

Throughout the history of modern psychology, the neural basis of cognitive performance, and particularly its efficiency, has been assumed to be an essential determinant of developmental and individual differences in a wide range of human behaviors. Here, we examine one aspect of cognitive efficiency—cognitive effort, using pupillometry to examine differences in word reading among adults (N = 34) and children (N = 34). The developmental analyses confirmed that children invested more effort in reading than adults, as indicated by larger and sustained pupillary responses. The within-age (individual difference) analyses comparing faster (N = 10) and slower (N = 10) performers revealed that in both age groups, the faster readers demonstrated accelerated pupillary responses compared to slower readers, although both groups invested a similar overall degree of cognitive effort. These findings have the potential to open up new avenues of research in the study of skill growth in word recognition and many other domains of skill learning.

Is There Semantic Conflict in the Stroop Task?

This research addressed current controversies concerning the contribution of semantic conflict to the Stroop interference effect and its reduction by a single-letter coloring and cueing procedure. On the first issue, it provides, for the first time, unambiguous evidence for a contribution of semantic conflict to the (overall) Stroop interference effect. The reported data remained inconclusive on the second issue, despite being collected in a considerable sample and analyzed with both classical (frequentist) and Bayesian inferential approaches. Given that in all past Stroop studies, semantic conflict was possibly confounded with either response conflict (e.g., when semantic-associative items [SKY_blue] are used to induce semantic conflict) or with facilitation (when color-congruent items [BLUE_blue] are used as baseline to derive a magnitude for semantic conflict), its genuine contribution to the Stroop interference effect is the most critical result reported in the present study. Indeed, it leaves no doubt - in complete contrast to dominant single-stage response competition models (e.g., Roelofs, 2003) - that selection occurs at the semantic level in the Stroop task. The immediate implications for the composite (as opposed to unitary) nature of the Stroop interference effect and other still unresolved issues in the Stroop literature are outlined further.

Some further clarifications on age-related differences in the Stroop task: New evidence from the two-to-one Stroop paradigm

Previous studies (Augustinova et al., Psychonomic Bulletin & Review, 25(2), 767-774, 2018; Li & Bosman, Aging, Neuropsychology, and Cognition, 3(4), 272-284, 1996) have shown that the larger Stroop effects reported in older adults is specifically due to age-related differences in the magnitude of response - and not semantic - conflict, both of which are thought to contribute to overall Stroop interference. However, the most recent contribution to the issue of the unitary versus composite nature of the Stroop effect argues that semantic conflict has not been clearly dissociated from response conflict in these or any other past Stroop studies, meaning that the very existence of semantic conflict is at present uncertain. To distinguish clearly between the two types of conflict, the present study employed the two-to-one Stroop paradigm with a color-neutral word baseline. This addition made it possible to isolate a contribution of semantic conflict that was independent of both response conflict and Stroop facilitation. Therefore, this study provides the first unambiguous empirical demonstration of the composite nature of Stroop interference - as originally claimed by multi-stage models of Stroop interference. This permitted the further observation of significantly higher levels of semantic conflict in older adults, whereas the level of response conflict in the present study remained unaffected by healthy aging - a finding that directly contrasts with previous studies employing alternative measures of response and semantic conflict. Two qualitatively different explanations of this apparent divergence across studies are discussed.

The loci of Stroop effects: a critical review of methods and evidence for levels of processing contributing to color-word Stroop effects and the implications for the loci of attentional selection

Despite instructions to ignore the irrelevant word in the Stroop task, it robustly influences the time it takes to identify the color, leading to performance decrements (interference) or enhancements (facilitation). The present review addresses two questions: (1) What levels of processing contribute to Stroop effects; and (2) Where does attentional selection occur? The methods that are used in the Stroop literature to measure the candidate varieties of interference and facilitation are critically evaluated and the processing levels that contribute to Stroop effects are discussed. It is concluded that the literature does not provide clear evidence for a distinction between conflicting and facilitating representations at phonological, semantic and response levels (together referred to as informational conflict), because the methods do not currently permit their isolated measurement. In contrast, it is argued that the evidence for task conflict as being distinct from informational conflict is strong and, thus, that there are at least two loci of attentional selection in the Stroop task. Evidence suggests that task conflict occurs earlier, has a different developmental trajectory and is independently controlled which supports the notion of a separate mechanism of attentional selection. The modifying effects of response modes and evidence for Stroop effects at the level of response execution are also discussed. It is argued that multiple studies claiming to have distinguished response and semantic conflict have not done so unambiguously and that models of Stroop task performance need to be modified to more effectively account for the loci of Stroop effects.

The contribution of meaning to the detection of task conflict

The colour-word Stroop task produces both information conflict (detection of the ink colour vs word meaning) and task conflict (respond to the ink colour vs read the word). In this study, we measured both reaction time and pupil dilation, and the neutral stimuli in our study were non-readable letter strings as well as meaningless non-readable stimuli (i.e., coloured patches and abstract character strings). Our results showed slowest responses in the incongruent trials and fastest responses in the congruent trials. However, no differences were found between the investigated neutrals. In contrast, pupil dilation was largest in the incongruent trials and smallest in the neutral trials. Moreover, the more the neutral stimuli were meaningless, the less the pupil dilation that was observed. Our results suggest that non-word meaningless stimuli reduced task conflict (compared with all the investigated conditions). Neutral equivalence should be taken into consideration in Stroop and Stroop-like tasks.

Look into my eyes: Pupillometry reveals that a post-hypnotic suggestion for word blindness reduces Stroop interference by marshalling greater effortful control

The mechanisms underpinning the apparently remarkable levels of cognitive and behavioural control following hypnosis and hypnotic suggestion are poorly understood. Numerous independent studies have reported that Stroop interference can be reduced following a post-hypnotic suggestion that asks participants to perceive words as if made up of characters from a foreign language. This effect indicates that frontal executive functions can be more potent than is generally accepted and has been described as resulting from top-down control not normally voluntarily available. We employed eye tracking and pupillometry to investigate whether the effect results from voluntary visuo-attentional strategies (subtly looking away from the word to prevent optimal word processing), reduced response conflict but not overall conflict, Stroop effects being pushed from response selection to response execution (response durations) or increased proactive effortful control given enhanced contextual motivation (as indexed via pupil dilation). We replicated the reduction in Stroop interference following the suggestion despite removing any trials on which eye movements were not consistent with optimal word processing. Our data were inconclusive with regards to conflict type affected by the suggestion in the latency data, although preserved semantic conflict was evident in the pupil data. There was also no evidence of Stroop effects on response durations. However, we show that baseline-corrected pupil sizes were larger following the suggestion indicating the socio-cognitive context and experimental demands motivate participants to marshal greater effortful control.

Neutral stimuli and pupillometric task conflict

It has been suggested that the interference in the Stroop task arises due to both information conflict (color vs. word meaning) and task conflict (respond to the color vs. read the word). Interestingly, the task conflict (faster responses in the congruent condition compared to the neutral condition) is influenced by the readability of the stimulus. However, behavioral results indicate the influence of the contribution of readability to the task conflict appears only when vocal responses are required. We conducted a Stroop experiment with four readability levels of neutrals. Participants were required to respond manually, and both their response times and pupil dilation were measured. Our results showed the slowest responses for incongruent trials and the fastest responses for congruent trials. However, no differences in responses were found between the readability levels of the neutrals. In contrast, pupil dilation showed the largest dilation in the incongruent trials and smallest dilation in the neutral trials. In addition, the differences between congruent and neutral trials were influenced by the readability of the stimulus. Specifically, unreadable neutral stimuli showed meaningful differences early on, which also remained for longer time compared with readable neutral stimuli. These results are important and can help in specifying new requirements regarding the theory and modeling of the Stroop task; in particular, studies that want to control the neutrality of their neutral trials should take the readability into account.