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Top-down control and early multisensory processes: chicken vs. egg

Traditional views contend that behaviorally-relevant multisensory interactions occur relatively late during stimulus processing and subsequently to influences of (top-down) attentional control. In contrast, work from the last 15 years shows that information from different senses is integrated in the brain also during the initial 100 ms after stimulus onset and within low-level cortices. Critically, many of these early-latency multisensory interactions (hereafter eMSI) directly impact behavior. The prevalence of eMSI substantially advances our understanding of how unified perception and goal-related behavior emerge. However, it also raises important questions about the dependency of the eMSI on top-down, goal-based attentional control mechanisms that bias information processing toward task-relevant objects (hereafter top-down control). To date, this dependency remains controversial, because eMSI can occur independently of top-down control, making it plausible for (some) multisensory processes to directly shape perception and behavior. In other words, the former is not necessary for these early effects to occur and to link them with perception (see Figure ​Figure1A).1A). This issue epitomizes the fundamental question regarding direct links between sensation, perception, and behavior (direct perception), and also extends it in a crucial way to incorporate the multisensory nature of everyday experience. At the same time, the emerging framework must strive to also incorporate the variety of higher-order control mechanisms that likely influence multisensory stimulus responses but which are not based on task-relevance. This article presents a critical perspective about the importance of top-down control for eMSI: In other words, who is controlling whom? 
 
 
 
Figure 1 
 
(A) Depiction of manners in which top-down attentional control and bottom-up multisensory processes may influence direct perception in multisensory contexts. In this model, the bottom-up multisensory processes that occur early in time (eMSI; beige box) ...


Traditional
views contend that behaviorally-relevant multisensory interactions occur relatively late during stimulus processing and subsequently to influences of (top-down) attentional control. In contrast, work from the last 15 years shows that information from different senses is integrated in the brain also during the initial 100 ms after stimulus onset and within low-level cortices. Critically, many of these early-latency multisensory interactions (hereafter eMSI) directly impact behavior. The prevalence of eMSI substantially advances our understanding of how unified perception and goal-related behavior emerge. However, it also raises important questions about the dependency of the eMSI on top-down, goal-based attentional control mechanisms that bias information processing toward task-relevant objects (hereafter top-down control). To date, this dependency remains controversial, because eMSI can occur independently of top-down control, making it plausible for (some) multisensory processes to directly shape perception and behavior. In other words, the former is not necessary for these early effects to occur and to link them with perception (see Figure 1A). This issue epitomizes the fundamental question regarding direct links between sensation, perception, and behavior (direct perception), and also extends it in a crucial way to incorporate the multisensory nature of everyday experience. At the same time, the emerging framework must strive to also incorporate the variety of higher-order control mechanisms that likely influence multisensory stimulus responses but which are not based on taskrelevance. This article presents a critical perspective about the importance of topdown control for eMSI: In other words, who is controlling whom?

THE UBIQUITY OF eMSI
For the purposes of this article we focus exclusively on auditory-visual interactions and define eMSI as those multisensory processes that occur within the first 100 ms post-stimulus onset (but see (Giard and Peronnet, 1999); Giard and Peronnet, who qualified effects <200 ms as early-latency). This definition is in keeping with influential models of visual perception and attentional selection, positing that topdown and recursive inputs manifest after the initial 100 ms of stimulus-driven brain activity, which is believed to be sensoryperceptual and bottom-up in nature (e.g., Luck et al., 1997;Lamme and Roelfsema, 2000). It is likewise important to distinguish between integration effects, which are responses elicited by a combination of inputs to different senses, and cross-modal effects, which refer to influences of inputs to one sense on activity associated with another sense (e.g., Stein et al., 2010).
The typical perceptual outcome of multisensory integration is that stimulus processing is facilitated (as shown by faster and/or more accurate responses) in contexts where inputs to different senses are carrying similar (redundant) information and are presented close in time. This behavioral facilitation is typically accompanied by brain responses to multisensory stimuli that diverge from the summed brain responses to the constituent unisensory signals (nonlinear responses; Figure 1B). Given the growing evidence for links between the brain and the behavioral responses (reviewed in Murray et al., 2012), one mechanism may be that the temporal cooccurrence of multisensory information lowers the threshold for neural activity that in turn drives perception and action (e.g., Rowland and Stein, 2007).
Based on the extant literature, we argue that these particular multisensory processes, which are reflected by eMSI, are stimulus-driven, bottom-up in nature and affect perception and behavior in a direct manner and largely independently of topdown control ( Figure 1A). The idea of a variety, or even a range of multisensory processes, where some are "automatic" while others dependent on one's current behavioral goals, has until now been systematically investigated mainly in the context of attentional selection of objects in space, rather than their perception per se (e.g., Eimer, 2011, 2013;Matusz et al., 2015;Talsma and Woldorff, 2005; but see Murray et al., 2004;Soto-Faraco et al., 2004;Tiippana et al., 2004;Alsius et al., 2005Alsius et al., , 2007Thelen et al., 2012Thelen et al., , 2014Matusz et al., in press). However, control processes are likely to be important for both cognitive functions (e.g., Gunseli et al., 2014); this should hold for both unisensory and multisensory processes, and bottom-up and top-down processes alike (i.e., multisensory processes are not mechanistically "special"; van Atteveldt et al., 2014).
It is difficult to argue with the idea that early responses are a hallmark of bottom-up multisensory processes in the service of perception, if one considers how ubiquitous and context-independent they are in both humans and in the animal models (see Figure 1B; reviewed in Murray et al., 2012;Kajikawa et al., 2012). The eMSI in local field potentials as well as spiking activity have been measured in the primary and secondary auditory fields of fixating monkeys (Ghazanfar et al., 2005;Kayser et al., 2008; see also Lakatos et al., 2008;Wang et al., 2008 for cross-modal effects). Importantly, these eMSI occurred for both ethological objects (conspecific communication signals) and simple audiovisual stimuli, though modulated according to bottomup stimulus salience and neural efficacy. Moreover, non-linear interactions mirroring the behavioral gains in stimulus detection have been recorded in single neurons in the area 4 of the monkey motor cortex within 100-150 ms post-stimulus (Miller et al., 2001).
The interpretability of the eMSI in terms of bottom-up vs. top-down mechanisms critically depends on their localization. Despite the ubiquity of the eMSI in extant EEG/MEG studies, only few have applied the requisite signal analysis and source reconstruction methods. Localization results support the predominant role of low-level cortices in the eMSI (Cappe et al., 2010;Raij et al., 2010). While the localization of the eMSI to low-level cortices could be taken as evidence for their strictly bottom-up nature, their latency at ∼50-100 ms is sufficiently "late" to provide ample opportunity for recursive processing (Musacchia and Schroeder, 2009; also Moran and Reilly, 2006 for modeling results). This may involve top-down modulation or the extraction and disambiguation of stimulus features (Lamme and Roelfsema, 2000).

THE CHICKEN: TOP-DOWN CONTROL AND ITS LIMITED ROLE IN eMSI
The strongest evidence for the dependence of eMSI on top-down control comes from studies where attended and unattended multisensory stimuli were directly compared (e.g., Alsius et al., 2005Alsius et al., , 2007Talsma and Woldorff, 2005;Talsma et al., 2007). However, the literature seems prone to misconstruing the full breadth of the results. In one study participants detected infrequent targets in one of two central streams of rapidly presented alphanumeric symbols or combinations of beeps and flashes (Talsma et al., 2007). When attended, audiovisual stimuli triggered early enhanced (super-additive) nonlinear responses. 1 To our knowledge semantic congruence does not modulate eMSI (Fort et al., 2002b;Molholm et al., 2004;Yuval-Greenberg and Deouell, 2007). But, when the competing stream was attended, these nonlinear interactions changed polarity, becoming suppressed (sub-additive). One interpretation of these results is that top-down control regulates multisensory integration, from its magnitude and quality to its very presence (Koelewijn et al., 2010). We believe this viewpoint should perhaps be more nuanced. The top-down control manipulations modulated the eMSI, but did not eliminate them. Additionally, the eMSI were observed despite the paradigm manipulating in fact multiple top-down mechanisms (inter-modal, but also spatial, feature-based, and object-based). While further research is required to fully characterize the mechanistic underpinnings of super-vs. sub-additive interactions, the results of this study are in line with the importance of top-down control processes revealed by unisensory studies, wherein responses to stimuli are enhanced according to the task-relevance of their location, features or identity (reviewed in Nobre and Kastner, 2013). Talsma et al. (2007) was the first to demonstrate the pivotal role of the task-relevance of multisensory pairings for the quality of the eMSI they trigger. However, the presence of the eMSI in this study was independent of taskrelevance, though some evidence would suggest that the eMSI are preferentially observed in unattended contexts ( Table 2 in Talsma and Woldorff, 2005). This latter evidence is in line with the eMSI being a hallmark of stimulus-driven processing.
It is difficult to ignore that in these few studies, where top-down control mechanisms were directly manipulated, the eMSI were sub-additive in nature. What is striking is that this is precisely the direction of effects reported in the literature irrespective of whether responses to targets, non-targets or passively presented stimuli are considered ( Figure 1B). Historically, sub-additive effects were dismissed as confounds related to common activity across both unisensory and multisensory conditions. More recently, they have been increasingly recognized as a canonical mechanism that can convey information particularly efficiently (Kayser et al., 2009;Altieri et al., in press;reviewed in Stevenson et al., 2014). The issue of the quantification of the eMSI is further complicated by the fact that the overwhelming majority of the human EEG studies have used relative, reference-dependent measures of amplitude (cf., Murray et al., 2008).

THE EGG: eMSI AS A BOTTOM-UP PHENOMENON
Several independent lines of research across various species provide converging evidence for the bottom-up nature of the eMSI. On the one hand, there are reports of eMSI in anesthetized animals (e.g., rats, Barth et al., 1995;cats, Rowland and Stein, 2007; see also reviews in Sarko et al., 2012;Rowland and Stein, 2014), where top-down modulations are blocked 2 . On the other hand, sounds have been shown to enhance the excitability of low-level visual cortices, as measured via phosphene perception. Several aspects of this effect demonstrated by TMS studies in humans support the bottom-up nature of the eMSI and the causal links between eMSI and behavior (Romei et al., 2007(Romei et al., , 2009(Romei et al., , 2013Spierer et al., 2013).
First, it is modulated by low-level sound features, with greater excitability increases observed for narrowband and higher pitch sounds. Visual cortex excitability is furthermore enhanced selectively by structured approaching (looming) sounds versus stationary or receding sounds as well as non-structured white-noise versions of these sounds. Second, the effect is delimited in time, occurring when sounds precede the TMS by 30-150 ms, in correspondence with the eMSI identified using EEG/MEG. Third, the soundinduced enhancements of visual cortex excitability transpire before subjects can explicitly differentiate between the sounds, i.e., at pre-perceptual processing stages. Relatedly, increases in the occipital excitability occur with sounds that themselves fail to elicit startle responses, arguing against an alerting explanation. Fourth, evidence against a top-down account of these effects comes from studies 2 We would hasten to remind the reader that convergent anatomical input is necessary but in and of itself insufficient for eMSI as defined it in this opinion piece. It is true that the anatomic pathways/connectivities as well as their shaping by experiences are prerequisites for multisensory processes. However, the activation of these physical substrates in relation to the cascade of sensory-evoked responses must be sufficiently early so as to influence perception and behavior directly and thus be qualified as eMSI.
demonstrating that individuals' attentional preference (as independently measured in an auditory-visual divided attention task) affect late, but not early, stages of the excitability changes.
Finally, the TMS-driven visual cortex activity is behaviorally relevant. Occipital TMS delivered 60-90 ms post-stimulus has opposing effects of roughly equal magnitude (∼15 ms) on reaction times to unisensory auditory and visual stimuli (speeding and slowing, respectively) and has no measurable effect on reaction times to simultaneous auditory-visual multisensory stimuli. Critically, the response speed facilitation obtained from the combination of occipital TMS and an external auditory stimulus was as great as and correlated with that obtained from presenting participants with genuine multisensory stimuli. The TMS-induced cross-modal effects seem to emulate those observed with multisensory stimuli.

CONCLUSIONS AND FUTURE DIRECTIONS
We demonstrated that the eMSI are robust phenomena, observable across species, experimental paradigms and measures of neural activity (Figure 1B). To refer more explicitly to the Research Topic of this issue, we subscribe to a view of multiple multisensory processes: The eMSI are a hallmark of bottom-up multisensory processes that facilitate perception and behavior directly, independently of top-down control ( Figure 1A).
We focused here exclusively on stimulus-locked brain activity. Thus, temporal dynamics complement the understanding of the interplay between bottom-up and top-down mental processes as hitherto provided from the vantage-point of brain oscillations, which assay both intra-population excitability as well as inter-population communication van Atteveldt et al., 2014).
A critical next step will be the detailed mechanistic characterization of the eMSI. The sub-additive archetype of the eMSI goes together with the evidence from unisensory research linking reduced responses with more efficient and information-rich processing akin to the repetition suppression phenomena and the predictive coding accounts (e.g., Grill-Spector et al., 2006;Summerfield and Egner, 2009). When and why do top-down control processes flip the sub-additive eMSI to become super-additive? If topdown control affects the nature, rather than the presence, of multisensory processes, then what are the consequences for our understanding of perception? Paradoxically, while the eMSI are on the one hand upturning somewhat dogmatic views on the brain functional organization, they simultaneously are entrenching a classic model of perceptual processing positing direct links between sensation, perception, and behavior. An accurate picture of the nature of perceptual processes is thus provided by studying them in naturalistic, multisensory contexts and where the task demands dynamically vary.

AUTHOR CONTRIBUTIONS
All authors have contributed to all aspects of this work. All authors have approved the final version of the manuscript and agreed to be held accountable for all aspects of the work.