The effect of bipolar bihemispheric tDCS on executive function and working memory abilities

Introduction

Cognitive functioning is central to the ability to learn, problem solve, remember, and use information in a rapid and accurate manner and cognitive abilities are fundamental for communication, autonomy, and quality of life. Transcranial electric stimulation (tES) is a very promising tool shown to improve various motor and cognitive functions. When applied as a direct current stimulus (transcranial direct current stimulation; tDCS) over the dorsolateral pre-frontal cortex (DLPFC), this form of neurostimulation has mixed results regarding its ability to slow cognitive deterioration and potentially enhance cognitive functioning, requiring further investigation. This study set out to comprehensively investigate the effect that anodal and cathodal bipolar bihemispheric tDCS have on executive function and working memory abilities.

Methods

72 healthy young adults were recruited, and each participant was randomly allocated to either a control group (CON), a placebo group (SHAM) or one of two neurostimulation groups (Anodal; A-STIM and Cathodal; C-STIM). All participants undertook cognitive tests (Stroop & N Back) before and after a 30-minute stimulation/ sham/ control protocol.

Results

Overall, our results add further evidence that tDCS may not be as efficacious for enhancing cognitive functioning as it has been shown to be for enhancing motor learning when applied over M1. We also provide evidence that the effect of neurostimulation on cognitive functioning may be moderated by sex, with males demonstrating a benefit from both anodal and cathodal stimulation when considering performance on simple attention trial types within the Stroop task.

Discussion

Considering this finding, we propose a new avenue for tDCS research, that the potential that sex may moderate the efficacy of neurostimulation on cognitive functioning.

Interlinguistic conflict: Word-word Stroop with first and second language colour words

The congruency (or Stroop) effect is a standard observation of slower and less accurate colour identification to incongruent trials (e.g. "red" in green) relative to congruent trials (e.g. "red" in red). This effect has been observed in a word-word variant of the task, when both the distracter (e.g. "red") and target (e.g. "green") are colour words. The Stroop task has also been used to study the congruency effect between two languages in bilinguals. The typical finding is that the congruency effect for L1 words is larger than that for L2 words. For the first time, the present report aims to extend this finding to a word-word variant of the bilingual Stroop task. In two experiments, French monolinguals performed a bilingual word-word Stroop task in which target word language, language match, and congruency between the distracter and target were manipulated. The critical manipulation across two experiments concerned the target language. In Experiment 1, target language was manipulated between groups, with either French (L1) or English (L2) target colour words. In Experiment 2, target words from both languages were intermixed. In both experiments, the congruency effect was larger when the distracter and target were from the same language (language match) than when they were from different languages (language mismatch). Our findings suggested that this congruency effect mostly depends on the language match between the distracter and target, rather than on a target language. It also did not seem to matter whether the language-mismatching distracter was or was not a potential response alternative. Semantic activation of languages in bilinguals and its implications on target identification are discussed.

Picture-word interference is a Stroop effect: A theoretical analysis and new empirical findings

The picture-word interference (PWI) paradigm and the Stroop color-word interference task are often assumed to reflect the same underlying processes. On the basis of a PRP study, Dell’Acqua et al. (Psychonomic Bulletin & Review, 14: 717-722, 2007) argued that this assumption is incorrect. In this article, we first discuss the definitions of Stroop- and picture-word interference. Next, we argue that both effects consist of at least four components that correspond to four characteristics of the distractor word: (1) response-set membership, (2) task relevance, (3) semantic relatedness, and (4) lexicality. On the basis of this theoretical analysis, we conclude that the typical Stroop effect and the typical PWI effect mainly differ in the relative contributions of these four components. Finally, the results of an interference task are reported in which only the nature of the target – color or picture – was manipulated and all other distractor task characteristics were kept constant. The results showed no difference between color and picture targets with respect to all behavioral measures examined. We conclude that the assumption that the same processes underlie verbal interference in color and picture naming is warranted.

Word production and the picture-word interference paradigm: the role of learning

Psycholinguistic experiments conducted with the picture-word interference paradigm are typically preceded by a phase during which participants learn the words they will have to produce in the experiment. In Experiment 1, the pictures (e.g., a frog) were to be named and were presented with a categorically related (e.g., cat) or unrelated distracter (e.g., pen). In the related condition responses were slower relative to the unrelated condition for the participants who had gone through the learning phase. In contrast, participants who had not been previously familiarized with the materials showed facilitation. In Experiment 2 one group of participants, as usual, learned to produce the targets upon presentation of the corresponding pictures (e.g., a frog). The other group learned to produce the same targets upon presentation of unrelated pictures (e.g., a clock). They showed very similar semantic effects. The implications of the findings in the study of word production are discussed.

Predictive Inference Activation

Predictive inferences are anticipations of what could happen next in the text we are reading. These inferences seem to be activated during reading, but a delay is necessary for their construction. To determine the length of this delay, we first used a classical word-naming task. In the second experiment, we used a Stroop-like task to verify that inference activation was not due to strategies applied during the naming task. The results show that predictive inferences are naturally activated during text reading, after approximately 1 s.

Selective attention and response set in the Stroop task

Response set membership contributes much to the interference in the color-word Stroop task. This may be due to selective allocation of attention to eligible responses or, alternatively, to greater inhibition of distractors that are not responses. In the present article, we report two experiments that were designed to adjudicate between these accounts. In Experiment 1, membership was manipulated on a trial-by-trial basis by cuing the possible responses for each trial. Response time (RT) was longer for distractors that corresponded to a cued, eligible response than to an ineligible one. This cuing effect was independent of the number of different responses. In Experiment 2, the distractor was cued on half the trials. Cuing the distractor decreased RTs on both incongruent and congruent trials. Vincentile analyses in both experiments revealed that the effects were constant throughout the entire RT distributions. These results suggest that response set effects arise because of selective allocation of attention to eligible responses.

Differential components of the manual and vocal Stroop tasks

In this study, four components of the Stroop effect were examined for manual word and vocal responses. The components were lexical, semantic relatedness, semantic relevance, and response set membership. The results showed that all four components were present in the vocal response task. However, in the manual word response task, the only component that produced significant interference on its own was response set membership. These results do not support predictions made by recent translation models (see W. R. Glaser & M. O. Glaser [1989] and Sugg & McDonald [1994]). A possible solution was suggested that located two sites for Stroop interference. The lexical, semantic relatedness, and semantic relevance effects were located in the lexical system, whereas the response set membership effect was located at a response selection stage.

Inhibition in picture naming and word reading

Some previous researchers assumed that the underlying process of inhibition is similar in the modally pure target-distractor task, e.g., picture-picture task, color-color task, and in the modally mixed target-distractor task, such as the picture-word task or the color-word task, but some did not. The present purpose was to compare the inhibition in the modally pure tasks (word-word task and picture-picture task) with that in the modally mixed task (picture-word task) through the effects of the two factors, that is, response set and the semantic relation between target and distractor. These factors were assumed to affect inhibition in some kinds of interference tasks. Analysis showed the effect of response set was evident on the picture-picture task but not on the word-word and the picture-word tasks. On the picture-word task, only when the distracting word was in the same set of the target picture, was the effect of the semantic relation exhibited. The differential inhibition for the three tasks was discussed.

Controlling Stroop effects by manipulating expectations for color words

An important characteristic of automatic processing is its uncontrollability. The Stroop phenomenon is regarded as a prototypical example of this characteristic of automatic processing, hence, the Stroop effect should not change when the percentages of color words versus neutral stimuli are manipulated to induce controlled processing. We found that Stroop interference decreased as the percentage of color words increased. Furthermore, the magnitude of the inhibitory component of the Stroop effect was negatively correlated with the percentage of color words; the facilitatory component was insensitive to the manipulation. These results suggest that the Stroop effect is controllable (see Logan, 1980) and that the locus of control is postlexical. The results also suggest that facilitation and inhibition are produced by different mechanisms and challenge those models of the Stroop phenomenon (e.g., Cohen, Dunbar, & McClelland, 1990; Phaf, Van der Heijden, & Hudson, 1990) that assume that a single processing mechanism causes facilitation and inhibition and that control affects facilitation and inhibition alike (Logan, 1980).

Picture naming

Picture naming has become an important experimental paradigm in cognitive psychology. To name a picture can be considered an elementary process in the use of language. Thus, its chronometric analysis elucidates cognitive structures and processes that underlie speaking. Essentially, these analyses compare picture naming with reading, picture categorizing, and word categorizing. Furthermore, techniques of double stimulation such as the paradigms of priming and of Stroop-like interference are used. In this article, recent results obtained with these methods are reviewed and discussed with regard to five hypotheses about the cognitive structures that are involved in picture naming. Beside the older hypotheses of internal coding systems with only verbal or only pictorial format, the hypotheses of an internal dual code with a pictorial and a verbal component, of a common abstract code with logogen and pictogen subsystems, and the so-called lexical hypothesis are discussed. The latter postulates two main components: an abstract semantic memory which, nevertheless, also subserves picture processing, and a lexicon that carries out the huge amount of word processing without semantic interpretation that is necessary in hearing, reading, speaking and writing.

Abnormal processing of irrelevant information in chronic schizophrenia: selective enhancement of Stroop facilitation

Thirteen medication-free chronic schizophrenic patients and 11 normal control subjects were administered a trial by trial version of the Stroop Color Naming Task which evaluated separately the processes of interference and facilitation. There was no difference between the groups in the amount of interference to naming the colors of color-incongruent words. However, patients with schizophrenia showed significantly greater speed in naming the colors of color-congruent words when compared with control subjects. Thus, facilitation on the Stroop Task appears to be selectively enhanced in schizophrenia. Similar findings have been recently observed in patients with Parkinson's disease. This result may indicate a selective disruption of an automatic inhibitory process in this patient group and is consistent with the hypothesis that a deficit in mesocortical dopamine projections to the frontal cortex underlies some of the cognitive deficits of chronic schizophrenia.

Cuing efficiency in a Stroop-like task with visual half-field presentation

Stroop-like stimuli were presented to either the left or the right visual half-field. Subjects responded to the identity of the words above and below (the target dimension), which appeared above or below a reference point (the cuing dimension). Automatic Stroop-like effects were assessed as the difference in reaction times between congruent trials (e.g., above the reference point) and incongruent trials (e.g., above below the reference point) when both trial types were equally frequent. In blocks in which most trials were of one type (e.g., 80% congruent trials), controlled Stroop-like effects could be assessed. Automatic Stroop-like effects remained unchanged under different task manipulations. In contrast, controlled Stroop-like effects were reduced by lowering cue-response compatibility and by increasing the response alternatives from two to four. Thus, similar to other cuing effects, controlled Stroop-like effects are susceptible to manipulations that affect the response-decision stage and appear to involve response-selection processes. The resources supporting these response-selection decisions were not hemisphere-specific, and were sufficiently nonspecific that interference from a memory-load task was found. When resources were scarce, a consistent bias to attend to stimuli presented or responded to on the right was evident.