The Role of the Parietal Cortex in Multisensory and Response Integration: Evidence from Transcranial Direct Current Stimulation (tDCS)

Abstract

The question of how the brain forms unified representations from multisensory data that are processed in distinct cortical regions is known in the literature as 'the binding problem'. In the last decade, several studies have suggested possible neural mechanisms and brain regions that might be involved in integration processes. One of the brain regions that is implicated with multisensory perception is the posterior parietal cortex (PPC). Evidence from patients with parietal lesions suggests the involvement of the PPC in coherent perception. Here, we investigated the role of the PPC in multisensory feature integration through experimental manipulation of non-invasive brain stimulation with healthy participants using transcranial direct current stimulation (tDCS). In different sessions, healthy participants received anodal, cathodal, or sham stimulation (2 mA, 20 min) over the right PPC while performing an audio-visual event-file task. The results underscore two interesting observations. Firstly, there was a significant difference in integration effects between features from different modalities in the anodal stimulation compared to sham, suggesting interference of the multisensory integration processes during the brain stimulation. And secondly, after anodal stimulation, the unattended feature became more likely to be integrated with the response feature compared to the other conditions, presumably through an interference of attentional processes. Hence, these findings emphasize the role of the right PPC in multisensory integration. Furthermore, from a methodological perspective, tDCS can be used as an experimental tool by creating a temporary, reversible disruption in cognitive processes in order to explore the mechanisms underlying cognitive functions.

Neural oscillations in the temporal pole for a temporally congruent audio-visual speech detection task

“…Similarly, anatomical connectivity indicates that this brain area is a convergent hub of auditory, somatosensory, and visual motion systems59. Indeed, applying anodal transcranial direct current stimulation over the right PPC prevented the multisensory integration of visual and auditory features6061. Furthermore, clinical reports have demonstrated that PPC lesions cause feature-binding deficits62.…”

Transcranial direct current stimulation (tDCS) of frontal cortex decreases performance on the WAIS-IV intelligence test

“…Other work has applied anodal tDCS to visual areas and improved perceptual learning [62], while parietal tDCS has improved reaction time on contralateral search tasks [63]. However, another body of research has found that both right anodal and cathodal tDCS on DLPFC impair the efficiency of managing stimulus-response feature bindings, which taxes perceptual abilities; this study proposed that tDCS could create reversible ‘frontal lesions’, for at least specific cognitive tasks [64, 65]. Another study found that anodal and cathodal tDCS over medial-frontal cortex did not change perceptual processing, but only subsequent error- and feedback-related negativities [66].…”

Zapping the gap: Reducing the multisensory temporal binding window by means of transcranial direct current stimulation (tDCS)

“…The role of the PPC in multisensory integration of other modalities was also manifest in a study by Pasalar, Ro, and Beauchamp (2010), where TMS over the PPC interrupted visual-tactile integration. In addition, Zmigrod (2014) has found that tDCS delivered to the right PPC disrupts audio-visual integration, and Bolognini et al, 2010 have illustrated that anodal tDCS delivered to the right, but not left, PPC helps to facilitate performance on an audio-visual exploration training task. Thus, the current findings offer additional evidence for the causal relationship between the right PPC and the temporal binding window.…”

“…The active electrode was placed over either P4, P3, or F3 (depending on the participant's stimulation group), a location atop the right PPC, left PPC, and the left DLPFC respectively, according to the international 10-20 system for EEG electrode placement. The reference electrode was placed over the contralateral supraorbital area as this montage has been proven to be effective in neurostimulation studies involving multisensory perception (Bolognini, Fregni, Casati, Olgiati, & Vallar, 2010;Bolognini & Maravita, 2011;Zmigrod, 2014;Zmigrod et al, 2014) and other cognitive functions (Cerruti & Schlaug, 2009;Javadi & Walsh, 2012;Metuki, Sela, & Lavidor, 2012; for review see Nitsche et al, 2008). The stimulation lasted 15 min with a constant current of 2 mA and with a 15-s fade-in and fade-out.…”

“…In particular, we investigated the posterior parietal cortex (PPC) and the dorsolateral prefrontal cortex (DLPFC), which have been previously associated with the detection of temporal asynchrony in auditory-visual stimulus onset (Bushara, Grafman, & Hallett, 2001; for review see Calvert, 2001). In addition, neurostimulation studies have linked the DLPFC to cognitive control processes related to feature binding (Zmigrod, Colzato, & Hommel, 2014), and have demonstrated the role of the PPC in multisensory integration (Bolognini & Maravita, 2011;Zmigrod, 2014). Thus, using a non-invasive brain stimulation technique, we sought to examine the nature of this crucial yet elusive multisensory temporal binding window.…”