Philipp A. Schroeder

Targeting the biased brain: non-invasive brain stimulation to ameliorate cognitive control

Non-invasive brain stimulation has become important for the investigation of healthy and impaired neuronal functioning. Moreover, non-invasive brain stimulation has emerged as a new means of psychiatric treatment, although the mechanisms of action are still not understood and the optimal mode of application is still under development. Dysfunctional cognitive control is a central characteristic of various psychiatric disorders and is associated with dysregulations of prefrontal cortex activity and biased information processing. With non-invasive brain stimulation, enhancement and reduction of prefrontal cortex activity were shown to ameliorate and impair cognitive control, respectively. These findings suggest a neurocognitive mechanism of therapeutic effects and that non-invasive brain stimulation can be combined with training to target dysfunctional cognitive control and related clinical symptomatology. Nevertheless, the intra-individual and inter-individual diversity of neurocognitive processes, the multiplicity of possible stimulation parameters, and the complexity of interactions between those factors pose considerable challenges for interpretation of these findings and their clinical application.

Keep Calm and Carry On: Improved Frustration Tolerance and Processing Speed by Transcranial Direct Current Stimulation (tDCS)

Cognitive control (CC) of attention is a major prerequisite for effective information processing. Emotional distractors can bias and impair goal-directed deployment of attentional resources. Frustration-induced negative affect and cognition can act as internal distractors with negative impact on task performance. Consolidation of CC may thus support task-oriented behavior under challenging conditions. Recently, transcranial direct current stimulation (tDCS) has been put forward as an effective tool to modulate CC. Particularly, anodal, activity enhancing tDCS to the left dorsolateral prefrontal cortex (dlPFC) can increase insufficient CC in depression as indicated by a reduction of attentional biases induced by emotionally salient stimuli. With this study, we provide first evidence that, compared to sham stimulation, tDCS to the left dlPFC enhances processing speed measured by an adaptive version of the Paced Auditory Serial Addition Task (PASAT) that is typically thwarted by frustration. Notably, despite an even larger amount of error-related negative feedback, the task-induced upset was suppressed in the group receiving anodal tDCS. Moreover, inhibition of task-related negative affect was correlated with performance gains, suggesting a close link between enhanced processing speed and consolidation of CC by tDCS. Together, these data provide first evidence that activity enhancing anodal tDCS to the left dlPFC can support focused cognitive processing particularly when challenged by frustration-induced negative affect.

Counteracting Implicit Conflicts by Electrical Inhibition of the Prefrontal Cortex

Cognitive conflicts and distractions by task-irrelevant information often counteract effective and goal-directed behaviors. In some cases, conflicting information can even emerge implicitly, without an overt distractor, by the automatic activation of mental representations. For instance, during number processing, magnitude information automatically elicits spatial associations resembling a mental number line. This spatial–numerical association of response codes (SNARC) effect can modulate cognitive-behavioral performance but is also highly flexible and context-dependent, which points toward a critical involvement of working memory functions. Transcranial direct current stimulation to the PFC, in turn, has been effective in modulating working memory-related cognitive performance. In a series of experiments, we here demonstrate that decreasing activity of the left PFC by cathodal transcranial direct current stimulation consistently and specifically eliminates implicit cognitive conflicts based on the SNARC effect, but explicit conflicts based on visuospatial distraction remain unaffected. This dissociation is polarity-specific and appears unrelated to functional magnitude processing as classified by regular numerical distance effects. These data demonstrate a causal involvement of the left PFC in implicit cognitive conflicts based on the automatic activation of spatial–numerical processing. Corroborating the critical interaction of brain stimulation and neurocognitive functions, our findings suggest that distraction from goal-directed behavior by automatic activation of implicit, task-irrelevant information can be blocked by the inhibition of prefrontal activity.

Behavioral Bias for Food Reflected in Hand Movements: A Preliminary Study with Healthy Subjects

Palatable food induces general approach tendencies when compared to nonfood stimuli. For eating disorders, the modification of an attention bias toward food was proposed as a treatment option. Similar approaches have been efficient for other psychiatric conditions and, recently, successfully incorporated approach motivation. The direct impact of attentional biases on spontaneous natural behavior has hardly been investigated so far, although actions may serve as an intervention target, especially seeing the recent advances in the field of embodied cognition. In this study, we addressed the interplay of motor action execution and cognition when interacting with food objects. In a Virtual Reality (VR) setting, healthy participants repeatedly grasped or warded high-calorie food or hand-affordant ball objects using their own dominant hand. This novel experimental paradigm revealed an attention-like bias in hand-based actions: 3D objects of food were collected faster than ball objects, and this difference correlated positively with both individual body mass index and diet-related attitudes. The behavioral bias for food in hand movements complements several recent experimental and neurophysiological findings. Implications for the use of VR in the treatment of eating-related health problems are discussed.

Emotional Distraction and Bodily Reaction: Modulation of Autonomous Responses by Anodal tDCS to the Prefrontal Cortex.

Prefrontal electric stimulation has been demonstrated to effectively modulate cognitive processing. Specifically, the amelioration of cognitive control (CC) over emotional distraction by transcranial direct current stimulation (tDCS) points toward targeted therapeutic applications in various psychiatric disorders. In addition to behavioral measures, autonomous nervous system (ANS) responses are fundamental bodily signatures of emotional information processing. However, interactions between the modulation of CC by tDCS and ANS responses have received limited attention. We here report on ANS data gathered in healthy subjects that performed an emotional CC task parallel to the modulation of left prefrontal cortical activity by 1 mA anodal or sham tDCS. Skin conductance responses (SCRs) to negative and neutral pictures of human scenes were reduced by anodal as compared to sham tDCS. Individual SCR amplitude variations were associated with the amount of distraction. Moreover, the stimulation-driven performance- and SCR-modulations were related in form of a quadratic, inverse-U function. Thus, our results indicate that non-invasive brain stimulation (i.e., anodal tDCS) can modulate autonomous responses synchronous to behavioral improvements, but the range of possible concurrent improvements from prefrontal stimulation is limited. Interactions between cognitive, affective, neurophysiological, and vegetative responses to emotional content can shape brain stimulation effectiveness and require theory-driven integration in potential treatment protocols.

Arbitrary numbers counter fair decisions: trails of markedness in card distribution

Converging evidence from controlled experiments suggests that the mere processing of a number and its attributes such as value or parity might affect free choice decisions between different actions. For example the spatial numerical associations of response codes (SNARC) effect indicates the magnitude of a digit to be associated with a spatial representation and might therefore affect spatial response choices (i.e., decisions between a “left” and a “right” option). At the same time, other (linguistic) features of a number such as parity are embedded into space and might likewise prime left or right responses through feature words [odd or even, respectively; markedness association of response codes (MARC) effect]. In this experiment we aimed at documenting such influences in a natural setting. We therefore assessed number-space and parity-space association effects by exposing participants to a fair distribution task in a card playing scenario. Participants drew cards, read out loud their number values, and announced their response choice, i.e., dealing it to a left vs. right player, indicated by Playmobil characters. Not only did participants prefer to deal more cards to the right player, the card’s digits also affected response choices and led to a slightly but systematically unfair distribution, supported by a regular SNARC effect and counteracted by a reversed MARC effect. The experiment demonstrates the impact of SNARC- and MARC-like biases in free choice behavior through verbal and visual numerical information processing even in a setting with high external validity.

Space in Numerical and Ordinal Information: A Common Construct?

Space is markedly involved in numerical processing, both explicitly in instrumental learning and implicitly in mental operations on numbers. Besides action decisions, action generations, and attention, the response-related effect of numerical magnitude or ordinality on space is well documented in the Spatial-Numerical Associations of Response Codes (SNARC) effect. Here, right- over left-hand responses become relatively faster with increasing magnitude positions. However, SNARC-like behavioral signatures in non-numerical tasks with ordinal information were also observed and inspired new models integrating seemingly spatial effects of ordinal and numerical metrics. To examine this issue further, we report a comparison between numerical SNARC and ordinal SNARC-like effects to investigate group-level characteristics and individual-level deductions from generalized views, i.e., convergent validity. Participants solved order-relevant (before/after classification) and order-irrelevant tasks (font color classification) with numerical stimuli 1-5, comprising both magnitude and order information, and with weekday stimuli, comprising only ordinal information. A small correlation between magnitude- and order-related SNARCs was observed, but effects are not pronounced in order-irrelevant color judgments. On the group level, order-relevant spatial-numerical associations were best accounted for by a linear magnitude predictor, whereas the SNARC effect for weekdays was categorical. Limited by the representativeness of these tasks and analyses, results are inconsistent with a single amodal cognitive mechanism that activates space in mental processing of cardinal and ordinal information alike. A possible resolution to maintain a generalized view is proposed by discriminating different spatial activations, possibly mediated by visuospatial and verbal working memory, and by relating results to findings from embodied numerical cognition.

Switching between Multiple Codes of SNARC-Like Associations: Two Conceptual Replication Attempts with Anodal tDCS in Sham-Controlled Cross-Over Design

In societies with left-to-right reading direction, left-side vs. right-side behavioral decisions are faster for relatively small vs. large number magnitudes, and vice versa, a phenomenon termed Spatial-Numerical Associations of Response Codes (SNARC) effect. But also for non-numerical sequential items, SNARC-like effects were observed, suggesting a common neurocognitive mechanism based on the ordinal structures of both numbers and sequences. Modulation of prefrontal networks that are involved in providing spatial associations during cognitive behavior can contribute to elaborate their neuropsychological theoretical foundations. With transcranial direct current stimulation (tDCS) directed to the left prefrontal cortex, we recently showed that (i) cathodal tDCS can block the emergence of spatial-numerical associations and that (ii) anodal tDCS can reverse spatial associations of sequential order, most likely based on markedness correspondence. Two conceptual replication attempts of the latter reversal of space-order associations are presented in the current sham-controlled experiment, using either weekdays (Monday-Friday) or month names (January-December) as stimuli in the temporal order classification task. In addition, to control for possible influences of notation, number stimuli were presented as written German names (One-Five). We report on a successful modulation of spatial-numerical associations of response codes (SNARC) effects with month stimuli induced by anodal tDCS, but failed to observe the same reversal of SNARC effects for weekday stimuli. The former stimulation effect was orthogonal to the small anodal tDCS effect on written number words, which replicates the dissociation of SNARC effects for numbers vs. non-numerical sequences. Moreover, this result reinforces the hypothesis that the ordinal item and task structure was the source of dissociation (as opposed to verbal presentation). We suggest that the diverging results can be explained by the markedness correspondence account of spatial associations in a multiple coding framework. Left-hemispheric prefrontal excitation from anodal tDCS renders verbal markedness relatively more dominant, but this effect is not absolute. We discuss task contagion, study design, and individual differences in performance measures or tDCS response as possible contributors to systematic variation of the weights of multiple coding parameters for spatial-numerical associations.

Prefrontal neuromodulation reverses spatial associations of non-numerical sequences, but not numbers

Numerical and non-numerical sequence items interact with spatial responding, pointing towards mental representations that are grounded in space and referred to as SNARC effects (spatial-numerical association of response codes). An ongoing controversy pertains to the universal origin of different SNARC effects and whether their underpinning is a spatial arrangement of cardinal magnitude (mental number line) or a sequential arrangement of ordinal elements in working memory. Recent results from prefrontal neuromodulation with transcranial direct current stimulation (tDCS) were supportive of the unified working memory account. The current tDCS experiment was designed to empirically test the generalizability of the prefrontal modulation effects previously found for numbers in a non-numerical sequence (weekdays) and to examine predictions from the universal account. Participants performed a series of classification tasks with numerical and non-numerical sequences (1-5, Monday-Friday) before and concurrent to a prefrontal stimulation with either anodal (N=24) or cathodal polarity (N=24). Results show a dissociation of SNARC effects for numbers and weekdays by anodal tDCS: Spatial associations of weekdays were reversed by stimulation, when order was relevant for the task, but SNARC effects with number symbols were emphasized in the regular left-to-right direction, corroborating previous results. A control experiment showed that the polarity-dependent neuromodulation effects were absent in order-irrelevant font color classification, supporting the tDCS principle of activity-dependence. We discuss differences in linguistic markedness between temporal and magnitude-related classifications in an integrative account explaining the full pattern. We suggest that stimulation-enhanced psycholinguistic processing can evoke space-number associations whose direction is opposite to cultural visuospatial experience.

EP 126. Virtual Reality in the assessment of a manual bias towards food

Biased attentional and cognitive processes are documented for various psychiatric conditions, mostly linked with illness-specific stimuli. Prominent cognitive bias modification paradigms aim at addressing specific cognitive-behavioral deviations and corroborate therapy outcomes. Interestingly, because bodily and motivational tendencies might contribute to biased behavior, actual movements are starting to be included in bias modification paradigms in the form of approach-avoidance trainings ( Wiers et al., 2011 ). Here, we introduce a novel experimental setup in Virtual Reality to include actual hand ward and grasp movements in manual interactions with critical stimuli ( Fig. 1 ). The aim of this study was to identify motor contributions to the attentional bias towards food, a well-established finding. First, an attentional bias towards food was replicated during the course of grasping 3D objects of palatable objects as compared to ball objects. Food objects were collected faster than ball objects and the difference increased with larger body-mass indices of the healthy subjects ( Schroeder et al., 2015 ). This finding was replicated in a larger sample, but the behavioral bias towards food was driven by high-calorie food objects exclusively, as compared to 3D objects of balls, office objects, and low-calorie fruits, crucially. Exploratory analyses of the hand movement trajectories are presented and we discuss the implications for and limitations of the use of Virtual Reality in the assessment of biased behavior. An embodied perspective on the development and maintenance of biased cognitive processing may opt further therapeutic applications of (full-) body tracking technologies and may suggest the involvement of motor regions in addition to a fronto-limbic network.