Electrophysiological correlates of attentional selection in tactile search tasks: The impact of singleton distractors on target selection

Biophysical Modeling of Frontocentral ERP Generation Links Circuit-Level Mechanisms of Action-Stopping to a Behavioral Race Model

Human frontocentral event-related potentials (FC-ERPs) are ubiquitous neural correlates of cognition and control, but their generating multiscale mechanisms remain mostly unknown. We used the Human Neocortical Neurosolver's biophysical model of a canonical neocortical circuit under exogenous thalamic and cortical drive to simulate the cell and circuit mechanisms underpinning the P2, N2, and P3 features of the FC-ERP observed after Stop-Signals in the Stop-Signal task (SST; N = 234 humans, 137 female). We demonstrate that a sequence of simulated external thalamocortical and corticocortical drives can produce the FC-ERP, similar to what has been shown for primary sensory cortices. We used this model of the FC-ERP to examine likely circuit-mechanisms underlying FC-ERP features that distinguish between successful and failed action-stopping. We also tested their adherence to the predictions of the horse-race model of the SST, with specific hypotheses motivated by theoretical links between the P3 and Stop process. These simulations revealed that a difference in P3 onset between successful and failed Stops is most likely due to a later arrival of thalamocortical drive in failed Stops, rather than, for example, a difference in the effective strength of the input. In contrast, the same model predicted that early thalamocortical drives underpinning the P2 and N2 differed in both strength and timing across stopping accuracy conditions. Overall, this model generates novel testable predictions of the thalamocortical dynamics underlying FC-ERP generation during action-stopping. Moreover, it provides a detailed cellular and circuit-level interpretation that supports links between these macroscale signatures and predictions of the behavioral race model.

A role for retro-splenial cortex in the task-related P3 network

Objective

The P3 is an event-related response observed in relation to task-relevant sensory events. Despite its ubiquitous presence, the neural generators of the P3 are controversial and not well identified.

Methods

We compared source analysis of combined magneto- and electroencephalography (M/EEG) data with functional magnetic resonance imaging (fMRI) and simulation studies to better understand the sources of the P3 in an auditory oddball paradigm.

Results

Our results suggest that the dominant source of the classical, postero-central P3 lies in the retro-splenial cortex of the ventral cingulate gyrus. A second P3 source in the anterior insular cortex contributes little to the postero-central maximum. Multiple other sources in the auditory, somatosensory, and anterior midcingulate cortex are active in an overlapping time window but can be functionally dissociated based on their activation time courses.

Conclusion

The retro-splenial cortex is a dominant source of the parietal P3 maximum in EEG.

Significance

These results provide a new perspective for the interpretation of the extensive research based on the P3 response.

Searching for a tactile target: the impact of set-size on the N140cc

The time needed to find a visual target amongst distractors (search task) can increase as a function of the distractors' number (set-size) in the search-array (inefficient search). While the allocation of attention in search tasks has been extensively investigated and debated in the visual domain, little is known about these mechanisms in touch. Initial behavioral evidence shows inefficient search behavior when participants have to distinguish between target and distractors defined by their vibro-tactile frequencies. In the present study, to investigate the allocation of attention to items of the search-array we measured the N140cc during a tactile task in which the set-size was manipulated. The N140cc is a lateralized component of event-related brain potentials recently described as a psychophysiological marker of attentional allocation in tactile search tasks. Participants localized the target, a singleton frequency, while ignoring one, three or five homogeneous distractors. Results showed that error rates linearly increased as a function of set-size, while response times were not affected. Reliable N140cc components were observed for all set-sizes. Crucially, the N140cc amplitude decreased as the number of distractors increased. We argue that the presence of additional distractors hindered the preattentive analysis of the search array resulting in increased uncertainty about the target location (inefficient preattentive stage). This, in turn, increased the variability of the deployment of attention to the target, resulting in reduced N140cc amplitudes. Consistent with existing behavioral evidence, these findings highlight systematic differences between the visual and the tactile attentional systems.

Searching on the Back: Attentional Selectivity in the Periphery of the Tactile Field

Recent evidence has identified the N140cc lateralized component of event-related potentials as a reliable index of the deployment of attention to task-relevant items in touch. However, existing ERP studies have presented the tactile search array to participants' limbs, most often to the hands. Here, we investigated distractor interference effects when the tactile search array was presented to a portion of the body that is less lateralized and peripheral compared to the hands. Participants were asked to localize a tactile target presented among distractors in a circular arrangement to their back. The N140cc was elicited contralateral to the target when the singleton distractor was absent. Its amplitude was reduced when the singleton distractor was present and contralateral to the target, suggesting that attention was directed at least in part to the distractor when the singletons are on opposite sides. However, similar N140cc were observed when the singleton distractor was ipsilateral to the target compared to distractor absent trials. We suggest that when target and singleton distractor are ipsilateral, the exact localization of the target requires the attentional processing of all items on the same side of the array, similar to distractor absent trials. Together, these observations replicate the distractor interference effects previously observed for the hands, suggesting that analogous mechanisms guide attentional selectivity across different body parts.

Dissociating Perceptual Awareness and Postperceptual Processing: The P300 Is Not a Reliable Marker of Somatosensory Target Detection

A central challenge in the study of conscious perception lies in dissociating the neural correlates of perceptual awareness from those reflecting its precursors and consequences. No-report paradigms have been instrumental in this endeavor, demonstrating that the event-related potential P300, recorded from the human scalp, reflects reports rather than awareness. However, these paradigms cannot probe the degree to which stimuli are consciously processed from trial to trial and, thus, leave open the possibility that the P300 is a genuine correlate of conscious access enabling reports. Here, instead of removing report requirements, we took the opposite approach and equated postperceptual task demands across conscious and unconscious trials by orthogonalizing target detection and overt reports in a somatosensory detection task. We used Bayesian model selection to track the transformation from physical to perceptual processing stages in the EEG data of 24 male and female participants and show that the early P50 component scaled with physical stimulus intensity, whereas the N140 component was the first correlate of target detection. The late P300 component was elicited for both perceived and unperceived stimuli and was not substantially modulated by target detection. This was in stark contrast to a control experiment using a classical direct report task, which replicated the P50 and N140 effects but additionally showed a strong effect of target detection in the P300 time range. Our results demonstrate the task dependence of the P300 in the somatosensory modality and show that late cortical potentials dissociate from perceptual awareness even when stimuli are always reported.SIGNIFICANCE STATEMENT The time it takes for sensory information to enter our conscious experience can be an indicator of the neural processing stages that lead to perceptual awareness. However, because many cognitive processes routinely correlate with perception, isolating those signals that uniquely reflect perceptual awareness is not a trivial task. Here, we show that late electroencephalography signals cease to correlate with somatosensory awareness when common task confounds are controlled. Importantly, by balancing report requirements instead of abolishing them, we show that the lack of late effects cannot be explained by a lack of conscious access. Instead, we propose that conscious access occurs earlier, at ∼150 ms, supporting the view that early activity in sensory cortices is a neural correlate of conscious perception.

Behavioural and electrophysiological evidence for the effect of target-distractor separation in a tactile search task

Evidence suggests that the N140cc component of event-related potentials (ERP) observed in tactile search tasks reflects the attentional selection of the target. Here, we investigated whether the target selection processes are affected by the separation between the target and an ipsilateral singleton distractor (singletons delivered to contiguous or non-contiguous fingers of the same hand). In addition, the external distance between search items was varied through posture (splayed or touching fingers). Accuracy improved when target and distractor were delivered to contiguous fingers that were also touching. Regardless of target-distractor separation, the N140cc was larger when the external distance between search-array stimuli decreased (touching fingers). Importantly, a smaller N140cc was observed at reduced target-distractor separations, suggesting a narrower attentional focus for contiguous singletons. These findings reveal that the mechanisms responsible for tactile target selection in the presence of an ipsilateral singleton distractor are fundamentally different from those emerged in vision.