Binding Mechanisms in Visual Perception and Their Link With Neural Oscillations: A Review of Evidence From tACS

Abstract

Neurophysiological studies in humans employing magneto- (MEG) and electro- (EEG) encephalography increasingly suggest that oscillatory rhythmic activity of the brain may be a core mechanism for binding sensory information across space, time, and object features to generate a unified perceptual representation. To distinguish whether oscillatory activity is causally related to binding processes or whether, on the contrary, it is a mere epiphenomenon, one possibility is to employ neuromodulatory techniques such as transcranial alternating current stimulation (tACS). tACS has seen a rising interest due to its ability to modulate brain oscillations in a frequency-dependent manner. In the present review, we critically summarize current tACS evidence for a causal role of oscillatory activity in spatial, temporal, and feature binding in the context of visual perception. For temporal binding, the emerging picture supports a causal link with the power and the frequency of occipital alpha rhythms (8–12 Hz); however, there is no consistent evidence on the causal role of the phase of occipital tACS. For feature binding, the only study available showed a modulation by occipital alpha tACS. The majority of studies that successfully modulated oscillatory activity and behavioral performance in spatial binding targeted parietal areas, with the main rhythms causally linked being the theta (~7 Hz) and beta (~18 Hz) frequency bands. On the other hand, spatio-temporal binding has been directly modulated by parieto-occipital gamma (~40–60 Hz) and alpha (10 Hz) tACS, suggesting a potential role of cross-frequency coupling when binding across space and time. Nonetheless, negative or partial results have also been observed, suggesting methodological limitations that should be addressed in future research. Overall, the emerging picture seems to support a causal role of brain oscillations in binding processes and, consequently, a certain degree of plasticity for shaping binding mechanisms in visual perception, which, if proved to have long lasting effects, can find applications in different clinical populations.

Effects of Rhythmic Transcranial Magnetic Stimulation in the Alpha-Band on Visual Perception Depend on Deviation From Alpha-Peak Frequency: Faster Relative Transcranial Magnetic Stimulation Alpha-Pace Improves Performance

“…This is in line with alphafrequency playing a role in visual and/or attentional sampling (VanRullen and Koch, 2003;VanRullen, 2016;Kienitz et al, 2021). It shows that pre-stimulus alpha-pace may not only impact tasks relying on temporal resolution of visual perception such as temporal integration and segregation (Samaha and Postle, 2015;Wutz et al, 2018;Migliorati et al, 2020; for causal, mostly tACS-evidence see Cecere et al, 2015;Minami and Amano, 2017;Ronconi et al, 2018;Battaglini et al, 2020;Ghiani et al, 2021) but may also impact on perceptual accuracy more generally (Di Gregorio et al, 2022).…”

Auditory cortex modelled as a dynamical network of oscillators: understanding event-related fields and their adaptation

“…Sensory binding refers to this process of melding, and it seems to involve the long-distance synchronisation of the spatially disparate neuronal populations representing the individual features (Bertrand and Tallon-Baudry 2000 ; Ghiani et al. 2021 ). Selective attention is likewise associated with coherence: the cortical neurons representing the attended stimulus produce enhanced, synchronised gamma-band oscillations (Fries 2015 ).…”

Distinct cortical networks subserve spatio-temporal sampling in vision through different oscillatory rhythms

“…The definition of these different networks at the source-level might stimulate future transcranial electrical stimulation (tES) studies, in an attempt to bring causal evidence about the role of oscillatory activity within these networks in determining spatio-temporal aspects of perception. In particular, transcranial alternating current stimulation (tACS) could be of central relevance due to its ability to modulate brain oscillations in a frequency-dependent manner, with initial evidence linking tACS at specific frequencies with some basics binding processes in the visual system (for a review see Ghiani et al 2021). Using the two-flash fusion tasks, (Battaglini et al, 2020) showed recently that participants tend to integrate two subsequent flashes more often (i.e., they tend to report just one flash) when 10 Hz tACS (i.e., alpha tACS) was applied over V5/MT of the right hemisphere and surrounding extrastriate visual regions.…”