Synchronisation of Neural Oscillations and Cross-modal Influences
Authors: Anna-Katharina R. Bauer, Stefan Debener, Anna C. Nobre
Journal: Trends in Cognitive Sciences (2020)

Electrophysiological studies suggest that cross-modal influences in sensory cortices are mediated by the synchronisation of neural oscillations through phase-resetting and neural entrainment mechanisms.Low-frequency neural oscillations in the delta, theta, and lower alpha ranges are suggested to provide temporal windows for cross-modal influences.Top-down factors, such as task goals and expectations, may modulate multisensory processing as well as neural oscillations.Bayesian computational modelling provides a new approach to probe cross-modal influences. At any given moment, we receive multiple signals from our different senses. Prior research has shown that signals in one sensory modality can influence neural activity and behavioural performance associated with another sensory modality. Recent human and nonhuman primate studies suggest that such cross-modal influences in sensory cortices are mediated by the synchronisation of ongoing neural oscillations. In this review, we consider two mechanisms proposed to facilitate cross-modal influences on sensory processing, namely cross-modal phase resetting and neural entrainment. We consider how top-down processes may further influence cross-modal processing in a flexible manner, and we highlight fruitful directions for further research. At any given moment, we receive multiple signals from our different senses. Prior research has shown that signals in one sensory modality can influence neural activity and behavioural performance associated with another sensory modality. Recent human and nonhuman primate studies suggest that such cross-modal influences in sensory cortices are mediated by the synchronisation of ongoing neural oscillations. In this review, we consider two mechanisms proposed to facilitate cross-modal influences on sensory processing, namely cross-modal phase resetting and neural entrainment. We consider how top-down processes may further influence cross-modal processing in a flexible manner, and we highlight fruitful directions for further research. In our daily life, we continuously receive information from different sensory modalities, such as sight, sound, and touch. Think of a glass falling and breaking on the floor or footsteps of a person walking into a room. Incoming sensory signals are often interrelated and provide complementary evidence about our environment. To form a rich and adaptive understanding of our environment, signals from different modalities can influence one another. When originating from common sources, spatial proximity and temporal correlation may lead to integration into multisensory representations. To shed light on the mechanisms of cross-modal influences (see Glossary) and integration, in this review we consider whether and how oscillatory activity in cortical areas may contribute. Specifically, we use the term 'cross-modal influences' to express how the processing of sensory stimulation in one modality affects the neural processing or behaviour associated with another sensory modality [1.Stein B.E. et al.Semantic confusion regarding the development of multisensory integration: a practical solution.Eur. J. Neurosci. 2010; 31: 1713-1720Crossref PubMed Scopus (0) Google Scholar,2.Keil J. Senkowski D. Neural oscillations orchestrate multisensory processing.Neuroscientist. 2018; 24: 609-626Crossref PubMed Scopus (19) Google Scholar]. The first cortical regions that process incoming visual, auditory, and somatosensory information are the primary visual (V1), auditory (A1), and somatosensory (S1) cortices. According to the standard understanding of perceptual systems, processing of incoming sensory information evolves from the extraction of simple features in these highly specialised primary cortical structures through progressively more integrated representations in unimodal and multimodal associative regions [3.Mesulam M.M. From sensation to cognition.Brain. 1998; 121: 1013-1052Crossref PubMed Scopus (1834) Google Scholar]. Outputs from these loosely hierarchical sensory networks then converge in multisensory and higher order cortical regions, in particular the superior temporal sulcus (STS), intraparietal sulcus (IPS), and prefrontal cortical regions (PFC) [4.Ghazanfar A.A. Schroeder C.E. Is neocortex essentially multisensory?.Trends Cogn. Sci. 2006; 10: 278-285Abstract Full Text Full Text PDF PubMed Scopus (883) Google Scholar,5.Van Atteveldt N. et al.Multisensory integration: flexible use of general operations.Neuron. 2014; 81: 1240-1253Abstract Full Text Full Text PDF PubMed Scopus (137) Google Scholar]. Traditionally, it has been believed that the merging of sensory information from different modalities in cortex occurred exclusively in these multisensory and higher order regions. However, several human and animal studies have provided convincing evidence that cross-modal cortical influences can occur much earlier, even at the level of the primary sensory cortices [2.Keil J. Senkowski D. Neural oscillations orchestrate multisensory processing.Neuroscientist. 2018; 24: 609-626Crossref PubMed Scopus (19) Google Scholar,4.Ghazanfar A.A. Schroeder C.E. Is neocortex essentially multisensory?.Trends Cogn. Sci. 2006; 10: 278-285Abstract Full Text Full Text PDF PubMed Scopus (883) Google Scholar,6.Kayser C. Logothetis N.K. Do early sensory cortices integrate cross-modal information?.Brain Struct. Funct. 2007; 212: 121-132Crossref PubMed Scopus (192) Google Scholar, 7.Driver J. Noesselt T. Multisensory interplay reveals crossmodal influences on 'sensory-specific' brain regions, neural responses, and judgments.Neuron. 2008; 57: 11-23Abstract Full Text Full Text PDF PubMed Scopus (609) Google Scholar, 8.Senkowski D. et al.Crossmodal binding through neural coherence: implications for multisensory processing.Trends Neurosci. 2008; 31: 401-409Abstract Full Text Full Text PDF PubMed Scopus (251) Google Scholar, 9.Schroeder C.E. Foxe J. Multisensory contributions to low-level, 'unisensory' processing.Curr. Opin. Neurobiol. 2005; 15: 454-458Crossref PubMed Scopus (342) Google Scholar]. These early cross-modal influences in primary sensory cortices are modulatory in nature. Rather than driving neuronal activity, sensory signals from another modality change the cortical excitability to the signals in the dominant modality [7.Driver J. Noesselt T. Multisensory interplay reveals crossmodal influences on 'sensory-specific' brain regions, neural responses, and judgments.Neuron. 2008; 57: 11-23Abstract Full Text Full Text PDF PubMed Scopus (609) Google Scholar,10.Lakatos P. et al.Neuronal oscillations and multisensory interaction in primary auditory cortex.Neuron. 2007; 53: 279-292Abstract Full Text Full Text PDF PubMed Scopus (653) Google Scholar]. These findings have prompted a revision of our understanding of unimodal cortical regions and of the pathways that enable cross-modal influences and the integration of sensory information in cortex. In addition to indirect cross-modal influences through higher order multimodal cortical regions (STS, IPS, and PFC) [4.Ghazanfar A.A. Schroeder C.E. Is neocortex essentially multisensory?.Trends Cogn. Sci. 2006; 10: 278-285Abstract Full Text Full Text PDF PubMed Scopus (883) Google Scholar,5.Van Atteveldt N. et al.Multisensory integration: flexible use of general operations.Neuron. 2014; 81: 1240-1253Abstract Full Text Full Text PDF PubMed Scopus (137) Google Scholar,7.Driver J. Noesselt T. Multisensory interplay reveals crossmodal influences on 'sensory-specific' brain regions, neural responses, and judgments.Neuron. 2008; 57: 11-23Abstract Full Text Full Text PDF PubMed Scopus (609) Google Scholar], there are pathways through multimodal subcortical regions (e.g., superior colliculus and the pulvinar nucleus of the thalamus) [10.Lakatos P. et al.Neuronal oscillations and multisensory interaction in primary auditory cortex.Neuron. 2007; 53: 279-292Abstract Full Text Full Text PDF PubMed Scopus (653) Google Scholar, 11.Hackett T.A. et al.Multisensory convergence in auditory cortex, II. Thalamocortical connections of the caudal superior temporal plane.J. Comp. 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Recently, several studies promoted the notion that the synchronisation of neural oscillations may be an important mechanism for enabling cross-modal influences by facilitating the transfer of information across sensory modalities [5.Van Atteveldt N. et al.Multisensory integration: flexible use of general operations.Neuron. 2014; 81: 1240-1253Abstract Full Text Full Text PDF PubMed Scopus (137) Google Scholar,8.Senkowski D. et al.Crossmodal binding through neural coherence: implications for multisensory processing.Trends Neurosci. 2008; 31: 401-409Abstract Full Text Full Text PDF PubMed Scopus (251) Google Scholar] (for a recent review, see [2.Keil J. Senkowski D. Neural oscillations orchestrate multisensory processing.Neuroscientist. 2018; 24: 609-626Crossref PubMed Scopus (19) Google Scholar]). Neural oscillations reflect the rhythmic fluctuations of excitability in neuronal ensembles related to the dynamics of the circuits in which ensembles are embedded as well as the kinetics of their ionic channels [14.Buzsáki G. Rhythms of the Brain. Oxford University Press, 2009Google Scholar]. Rhythmic transitions between states of relatively high and low excitability can be characterised in terms of their frequency, amplitude, and phase [14.Buzsáki G. Rhythms of the Brain. Oxford University Press, 2009Google Scholar]. The phase indicates the particular point along the oscillatory cycle between 0 and 2 pi, corresponding to the peak, trough, or somewhere in between. Sensory inputs coinciding with the high-excitability state elicit stronger neural responses, whereas inputs coinciding with the low-excitability phase are attenuated (e.g., [15.Lakatos P. et al.Entrainment of neuronal oscillations as a mechanism of attentional selection.Science. 2008; 320: 110-113Crossref PubMed Scopus (960) Google Scholar]). This suggests that there are phases at which the processing of sensory information is optimised. Indeed, several studies have shown that behavioural performance across various tasks and in different sensory modalities fluctuates according to the phase of ongoing neural oscillations (for a review, see [16.VanRullen R. Perceptual cycles.Trends Cogn. Sci. 2016; 20: 723-735Abstract Full Text Full Text PDF PubMed Scopus (181) Google Scholar]). While amplitude and frequency can also impact neural excitability and behavioural performance [17.Schalk G. et al.Instantaneous voltage as an alternative to power- and phase-based interpretation of oscillatory brain activity.Neuroimage. 2017; 157: 545-554Crossref PubMed Scopus (9) Google Scholar], this review mainly focuses on phase-dependent effects. Neural oscillations have been repeatedly suggested to facilitate cross-modal influences between primary visual, auditory, and/or somatosensory areas (e.g., [10.Lakatos P. et al.Neuronal oscillations and multisensory interaction in primary auditory cortex.Neuron. 2007; 53: 279-292Abstract Full Text Full Text PDF PubMed Scopus (653) Google Scholar,18.Schroeder C.E. Lakatos P. Low-frequency neuronal oscillations as instruments of sensory selection.Trends Neurosci. 2009; 32: 9-18Abstract Full Text Full Text PDF PubMed Scopus (865) Google Scholar]). In general terms, two brain regions are considered to be synchronised or 'phase coherent' when there is a constant phase relationship between the two modality-specific activations over time [19.Fries P. A mechanism for cognitive dynamics: neuronal communication through neuronal coherence.Trends Cogn. Sci. 2005; 9: 474-480Abstract Full Text Full Text PDF PubMed Scopus (2258) Google Scholar,20.Fries P. Rhythms for cognition: communication through coherence.Neuron. 2015; 88: 220-235Abstract Full Text Full Text PDF PubMed Scopus (750) Google Scholar]. Previous theoretical and empirical work suggests that the synchronisation of ongoing neural oscillations is essential for determining the selection and routing of information both within and between cortical areas [2.Keil J. Senkowski D. Neural oscillations orchestrate multisensory processing.Neuroscientist. 2018; 24: 609-626Crossref PubMed Scopus (19) Google Scholar,5.Van Atteveldt N. et al.Multisensory integration: flexible use of general operations.Neuron. 2014; 81: 1240-1253Abstract Full Text Full Text PDF PubMed Scopus (137) Google Scholar,8.Senkowski D. et al.Crossmodal binding through neural coherence: implications for multisensory processing.Trends Neurosci. 2008; 31: 401-409Abstract Full Text Full Text PDF PubMed Scopus (251) Google Scholar,19.Fries P. A mechanism for cognitive dynamics: neuronal communication through neuronal coherence.Trends Cogn. Sci. 2005; 9: 474-480Abstract Full Text Full Text PDF PubMed Scopus (2258) Google Scholar,20.Fries P. Rhythms for cognition: communication through coherence.Neuron. 2015; 88: 220-235Abstract Full Text Full Text PDF PubMed Scopus (750) Google Scholar]. Whereas signals occurring in synchrony with high-excitability phases are effectively exchanged, asynchronous signals or signals linked to low-excitability phases are likely impeded. Synchronisation of oscillatory activity is usually considered to come about through one of two different mechanisms: cross-modal phase resetting ([10.Lakatos P. et al.Neuronal oscillations and multisensory interaction in primary auditory cortex.Neuron. 2007; 53: 279-292Abstract Full Text Full Text PDF PubMed Scopus (653) Google Scholar]; for a review, see [21.Thorne J.D. Debener S. Look now and hear what's coming: on the functional role of cross-modal phase reset.Hear. Res. 2014; 307: 144-152Crossref PubMed Scopus (36) Google Scholar]) or neural entrainment [15.Lakatos P. et al.Entrainment of neuronal oscillations as a mechanism of attentional selection.Science. 2008; 320: 110-113Crossref PubMed Scopus (960) Google Scholar] (for a schematic representation, see Figure 1). The concept of phase resetting was first introduced to intramodal processing [22.Makeig S. et al.Dynamic brain sources of visual evoked responses.Science (80-. ). 2002; 295: 690-694Crossref PubMed Scopus (0) Google Scholar] and has sparked interest in the non-invasive study of event-related brain dynamics [23.Makeig S. et al.Mining event-related brain dynamics.Trends Cogn. Sci. 2004; 8: 204-210Abstract Full Text Full Text PDF PubMed Scopus (911) Google Scholar] (Box 1). Cross-modal phase reset refers to the process by which the phase of ongoing neural oscillations in one sensory modality can be 'reset' by a transient event in another sensory modality (Figure 1A). The benefits of synchronising neural processing by transient events may bring similar benefits to cross-modal influences. In this case, a single salient or attended external (or internal) stimulus can 'set' the phase of a neural oscillation to a particular state of excitability within another sensory modality [24.Lakatos P. et al.The leading sense: supramodal control of neurophysiological context by attention.Neuron. 2009; 64: 419-430Abstract Full Text Full Text PDF PubMed Scopus (242) Google Scholar].Box 1Phase Resetting MechanismThe phase of an ongoing neural oscillation can be reset by a salient external (or internal) event. In visual tasks, salient visual events result in fluctuations of behavioural accuracy and reaction times in the theta and alpha frequency bands [37.Landau A.N. Fries P. Attention samples stimuli rhythmically.Curr. Biol. 2012; 22: 1000-1004Abstract Full Text Full Text PDF PubMed Scopus (252) Google Scholar,106.Fiebelkorn I.C. et al.Rhythmic sampling within and between objects despite sustained attention at a cued location.Curr. Biol. 2013; 23: 2553-2558Abstract Full Text Full Text PDF PubMed Scopus (160) Google Scholar, 107.Helfrich R.F. et al.Neural mechanisms of sustained attention are rhythmic.Neuron. 2018; 99: 854-865Abstract Full Text Full Text PDF PubMed Scopus (79) Google Scholar, 108.Fiebelkorn I.C. et al.A dynamic interplay within the frontoparietal network underlies rhythmic spatial attention.Neuron. 2018; 99: 842-853Abstract Full Text Full Text PDF PubMed Scopus (67) Google Scholar, 109.de Graaf T.A. et al.Alpha-band rhythms in visual task performance: phase-locking by rhythmic sensory stimulation.PLoS One. 2013; 8e60035Crossref PubMed Scopus (94) Google Scholar]. In audition a recent study further demonstrated behavioural fluctuations of auditory target detection performance in the theta frequency range in response to a salient auditory tone [110.Ho H.T. et al.Auditory sensitivity and decision criteria oscillate at different frequencies separately for the two ears.Curr. Biol. 2017; 27: 3643-3649Abstract Full Text Full Text PDF PubMed Scopus (21) Google Scholar]. Phase resets have also been observed across sensory modalities at different frequency bands in response to salient visual cues (e.g., red disk) [41.Mercier M.R. et al.Neuro-oscillatory phase alignment drives speeded multisensory response times: an electro-corticographic investigation.J. Neurosci. 2015; 35: 8546-8557Crossref PubMed Scopus (48) Google Scholar], brief auditory tones (e.g., white noise burst [28.Naue N. et al.Auditory event-related response in visual cortex modulates subsequent visual responses in humans.J. Neurosci. 2011; 31: 7729-7736Crossref PubMed Scopus (0) Google Scholar]), or in response to somatosensory stimulation [10.Lakatos P. et al.Neuronal oscillations and multisensory interaction in primary auditory cortex.Neuron. 2007; 53: 279-292Abstract Full Text Full Text PDF PubMed Scopus (653) Google Scholar] (see also Table 1 in the main text). Internal events can also reset the phase of periodic behavioural performance. Recent studies have shown that internally generated motor events [111.Benedetto A. et al.Rhythmic modulation of visual contrast discrimination triggered by action.Proc. R. Soc. B Biol. Sci. 2016; 283: 20160692Crossref PubMed Google Scholar,112.Tomassini A. et al.Rhythmic oscillations of visual contrast sensitivity synchronized with action.J. Neurosci. 2015; 35: 7019-7029Crossref PubMed Scopus (50) Google Scholar] can reset performance fluctuations on visual tasks (for review, see [113.Benedetto A. et al.The common rhythm of action and perception.J. Cogn. Neurosci. 2019; 32: 187-200Crossref PubMed Scopus (1) Google Scholar]). Overall, the emergence of behavioural periodicities time-locked to a reset event is a prime indicator of the involvement of neural oscillations [21.Thorne J.D. Debener S. Look now and hear what's coming: on the functional role of cross-modal phase reset.Hear. Res. 2014; 307: 144-152Crossref PubMed Scopus (36) Google Scholar,27.Fiebelkorn I.C. et al.Ready, set, reset: stimulus-locked periodicity in behavioral performance demonstrates the consequences of cross-sensory phase reset.J. Neurosci. 2011; 31: 9971-9981Crossref PubMed Scopus (78) Google Scholar,38.Henry M.J. Obleser J. Frequency modulation entrains slow neural oscillations and optimizes human listening behavior.Proc. Natl. Acad. Sci. U. S. A. 2012; 109: 20095-20100Crossref PubMed Scopus (190) Google Scholar].On a physiological level, difficulties may arise in determining a genuine phase reset. A pure phase reset is characterised by a stimulus-induced phase realignment of oscillatory activity without any concomitant change in power [23.Makeig S. et al.Mining event-related brain dynamics.Trends Cogn. Sci. 2004; 8: 204-210Abstract Full Text Full Text PDF PubMed Scopus (911) Google Scholar]. In the brain, phase resets by transient events are unlikely to be pure. Stimuli evoke a time-locked neural response characterised by an increase in power across a range of frequencies, thus also leading to an increase in phase concentration measures [23.Makeig S. et al.Mining event-related brain dynamics.Trends Cogn. Sci. 2004; 8: 204-210Abstract Full Text Full Text PDF PubMed Scopus (911) Google Scholar,114.Shah A.S. et al.Neural dynamics and the fundamental mechanisms of event-related brain potentials.Cereb. Cortex. 2004; 14: 476-483Crossref PubMed Scopus (173) Google Scholar]. Cross-modal influences by phase reset involve the evoked response to a salient unimodal stimulus in one modality resetting the phase of oscillatory activity in another modality. Separating evoked responses from phase resets can be problematic. The high spatial and temporal resolutions of intracranial recordings make it easier to distinguish evoked responses from phase reset (e.g., [32.Mercier M.R. et al.Auditory-driven phase reset in visual cortex: human electrocorticography reveals mechanisms of early multisensory integration.Neuroimage. 2013; 79: 19-29Crossref PubMed Scopus (71) Google Scholar,41.Mercier M.R. et al.Neuro-oscillatory phase alignment drives speeded multisensory response times: an electro-corticographic investigation.J. Neurosci. 2015; 35: 8546-8557Crossref PubMed Scopus (48) Google Scholar]) than is possible using standard scalp EEG data [21.Thorne J.D. Debener S. Look now and hear what's coming: on the functional role of cross-modal phase reset.Hear. Res. 2014; 307: 144-152Crossref PubMed Scopus (36) Google Scholar]. To overcome limitations in non-invasive studies, it is necessary both to increase the spatial resolution of the recordings by using dense sampling and computing the sources of the cortical oscillations (e.g., [28.Naue N. et al.Auditory event-related response in visual cortex modulates subsequent visual responses in humans.J. Neurosci. 2011; 31: 7729-7736Crossref PubMed Scopus (0) Google Scholar,40.Thorne J.D. et al.Cross-modal phase reset predicts auditory task performance in humans.J. Neurosci. 2011; 31: 3853-3861Crossref PubMed Scopus (0) Google Scholar]) and to increase the temporal resolution of analysis methods to investigate the instantaneous phase of oscillatory activity before, during, and after a transient stimulus. The phase of an ongoing neural oscillation can be reset by a salient external (or internal) event. In visual tasks, salient visual events result in fluctuations of behavioural accuracy and reaction times in the theta and alpha frequency bands [37.Landau A.N. Fries P. Attention samples stimuli rhythmically.Curr. Biol. 2012; 22: 1000-1004Abstract Full Text Full Text PDF PubMed Scopus (252) Google Scholar,106.Fiebelkorn I.C. et al.Rhythmic sampling within and between objects despite sustained attention at a cued location.Curr. Biol. 2013; 23: 2553-2558Abstract Full Text Full Text PDF PubMed Scopus (160) Google Scholar, 107.Helfrich R.F. et al.Neural mechanisms of sustained attention are rhythmic.Neuron. 2018; 99: 854-865Abstract Full Text Full Text PDF PubMed Scopus (79) Google Scholar, 108.Fiebelkorn I.C. et al.A dynamic interplay within the frontoparietal network underlies rhythmic spatial attention.Neuron. 2018; 99: 842-853Abstract Full Text Full Text PDF PubMed Scopus (67) Google Scholar, 109.de Graaf T.A. et al.Alpha-band rhythms in visual task performance: phase-locking by rhythmic sensory stimulation.PLoS One. 2013; 8e60035Crossref PubMed Scopus (94) Google Scholar]. In audition a recent study further demonstrated behavioural fluctuations of auditory target detection performance in the theta frequency range in response to a salient auditory tone [110.Ho H.T. et al.Auditory sensitivity and decision criteria oscillate at different frequencies separately for the two ears.Curr. Biol. 2017; 27: 3643-3649Abstract Full Text Full Text PDF PubMed Scopus (21) Google Scholar]. Phase resets have also been observed across sensory modalities at different frequency bands in response to salient visual cues (e.g., red disk) [41.Mercier M.R. et al.Neuro-oscillatory phase alignment drives speeded multisensory response times: an electro-corticographic investigation.J. Neurosci. 2015; 35: 8546-8557Crossref PubMed Scopus (48) Google Scholar], brief auditory tones (e.g., white noise burst [28.Naue N. et al.Auditory event-related response in visual cortex modulates subsequent visual responses in humans.J. Neurosci. 2011; 31: 7729-7736Crossref PubMed Scopus (0) Google Scholar]), or in response to somatosensory stimulation [10.Lakatos P. et al.Neuronal oscillations and multisensory interaction in primary auditory cortex.Neuron. 2007; 53: 279-292Abstract Full Text Full Text PDF PubMed Scopus (653) Google Scholar] (see also Table 1 in the main text). Internal events can also reset the phase of periodic behavioural performance. Recent studies have shown that internally generated motor events [111.Benedetto A. et al.Rhythmic modulation of visual contrast discrimination triggered by action.Proc. R. Soc. B Biol. Sci. 2016; 283: 20160692Crossref PubMed Google Scholar,112.Tomassini A. et al.Rhythmic oscillations of visual contrast sensitivity synchronized with action.J. Neurosci. 2015; 35: 7019-7029Crossref PubMed Scopus (50) Google Scholar] can reset performance fluctuations on visual tasks (for review, see [113.Benedetto A. et al.The common rhythm of action and perception.J. Cogn. Neurosci. 2019; 32: 187-200Crossref PubMed Scopus (1) Google Scholar]). Overall, the emergence of behavioural periodicities time-locked to a reset event is a prime indicator of the involvement of neural oscillations [21.Thorne J.D. Debener S. Look now and hear what's coming: on the functional role of cross-modal phase reset.Hear. Res. 2014; 307: 144-152Crossref PubMed Scopus (36) Google Scholar,27.Fiebelkorn I.C. et al.Ready, set, reset: stimulus-locked periodicity in behavioral performance demonstrates the consequences of cross-sensory phase reset.J. Neurosci. 2011; 31: 9971-9981Crossref PubMed Scopus (78) Google Scholar,38.Henry M.J. Obleser J. Frequency modulation entrains slow neural oscillations and optimizes human listening behavior.Proc. Natl. Acad. Sci. U. S. A. 2012; 109: 20095-20100Crossref PubMed Scopus (190) Google Scholar]. On a physiological level, difficulties may arise in determining a genuine phase reset. A pure phase reset is characterised by a stimulus-induced phase realignment of oscillatory activity without any concomitant change in power [23.Makeig S. et al.Mining event-related brain dynamics.Trends Cogn. Sci. 2004; 8: 204-210Abstract Full Text Full Text PDF PubMed Scopus (911) Google Scholar]. In the brain, phase resets by transient events are unlikely to be pure. Stimuli evoke a time-locked neural response characterised by an increase in power across a range of frequencies, thus also leading to an increase in phase concentration measures [23.Makeig S. et al.Mining event-related brain dynamics.Trends Cogn. Sci. 2004; 8: 204-210Abstract Full Text Full Text PDF PubMed Scopus (911) Google Scholar,114.Shah A.S. et al.Neural dynamics and the fundamental mechanisms of event-related brain potentials.Cereb. Cortex. 2004; 14: 476-483Crossref PubMed Scopus (173) Google Scholar]. Cross-modal influences by phase reset involve the evoked response to a salient unimodal stimulus in one modality resetting the phase of oscillatory activity in another modality. Separating evoked responses from phase resets can be problematic. The high spatial and temporal resolutions of intracranial recordings make it easier to distinguish evoked responses from phase reset (e.g., [32.Mercier M.R. et al.Auditory-driven phase reset in visual cortex: human electrocorticography reveals mechanisms of early multisensory integration.Neuroimage. 2013; 79: 19-29Crossref PubMed Scopus (71) Google Scholar,41.Mercier M.R. et al.Neuro-oscillatory phase alignment drives speeded multisensory response times: an electro-corticographic investigation.J. Neurosci. 2015; 35: 8546-8557Crossref PubMed Scopus (48) Google Scholar]) than is possible using standard scalp EEG data [21.Thorne J.D. Debener S. Look now and hear what's coming: on the functional role of cross-modal phase reset.Hear. Res. 2014; 307: 144-152Crossref PubMed Scopus (36) Google Scholar]. To overcome limitations in non-invasive studies, it is necessary both to increase the spatial resolution of the recordings by using dense sampling and computing the sources of the cortical oscillations (e.g., [28.Naue N. et al.Auditory event-related response in visual cortex modulates subsequent visual responses in humans.J. Neurosci. 2011; 31: 7729-7736Crossref PubMed Scopus (0) Google Scholar,40.Thorne J.D. et al.Cross-modal phase reset predicts auditory task performance in humans.J. Neurosci. 2011; 31: 3853-3861Crossref PubMed Scopus (0) Google Scholar]) and to increase the temporal resolution of analysis methods to investigate the instantaneous phase of oscillatory activity before, during, and after a transient stimulus. Cross-modal phase reset was first described in nonhuman primate studies investigating auditory cortical responses to auditory and nonauditory stimuli [10.Lakatos P. et al.Neuronal oscillations and multisensory interaction in primary auditory cortex.Neuron. 2007; 53: 279-292Abstract Full Text Full Text PDF PubMed Scopus (653) Google Scholar,25.Kayser C. et al.Visual modulation of neurons in auditory cortex.Cereb. Cortex. 2008; 18: 1560-1574Crossref PubMed Scopus (320) Google Scholar]. For instance, somatosensory stimulation of the median nerve preceding a brief auditory tone changed the phase of ongoing oscillations in A1. Moreover, the response to the subsequent auditory tone was modulated by the reset ph