Introduction

Our attention is often captured by the sudden and unpredictable sensory events that frequent the environments in which we live. For instance, we will normally turn our heads if someone suddenly calls our name at a crowded cocktail party. Similarly, if a mosquito lands on our arm, our eyes will be drawn immediately to the source of the unexpected tactile event. In these and many other such situations, objects that are initially processed in one sensory modality “grab” our attention in such a way as to enhance the sensory processing of stimuli presented in other modalities at the same spatial location. The cross-modal consequences of the involuntary orienting of our spatial attention is an area that has been extensively researched in recent years and is the subject of this review.

Research has demonstrated that the reflexive overt orienting of our attention conveys immediate cross-modal benefits: Not only do we see visual events more accurately at the fovea than in the periphery of our visual fields, but perhaps more surprisingly, we also hear and feel more acutely if we look—or even if we simply prepare to look—in the direction of nonvisual sensory stimulation (e.g., Driver & Grossenbacher, 1996; Gopher, 1973; Honoré, 1982; Honoré, Bourdeaud'hui, & Sparrow, 1989; Kato & Kashino, 2001; Rorden & Driver, 1999). While many researchers have focused their attention on the nature and consequences of these cross-modal shifts in specifically overt attention (i.e., involving shifts of the eyes, head, or body to better inspect an event of interest; e.g., Amlôt, Walker, Driver, & Spence, 2003; Jay & Sparks, 1990; Perrott, Saberi, Brown, & Strybel, 1990; Whittington, Hepp-Reymond, & Flood, 1981; Zambarbieri, Beltrami, & Versino, 1995; Zambarbieri, Schmid, Prablanc, & Magenes, 1981; Zambarbieri, Schmid, Magenes, & Prablanc, 1982), others have investigated the consequences of the covert shifts of attention that may occur prior to, or in the absence of, any overt orienting.

Covert shifts of attention take place very rapidly following the presentation of a peripheral sensory event, occurring prior to any shift of the sensory receptors themselves. Under normal circumstances, there is a close coupling between overt and covert orienting (e.g., Jonides, 1981a; Posner, 1978), with covert shifts of attention typically preceding any overt orienting response (e.g., Klein, Kingstone, & Pontefract, 1992; Rafal, Henik, & Smith, 1991; Rizzolatti, Riggio, Dascola, & Umiltà, 1987; Shepherd, Findlay, & Hockey, 1986), and both being controlled, at least in part, by the same neural structures (such as the superior colliculus; see Desimone, Wessinger, Thomas, & Schneider, 1992; Groh & Sparks, 1996a, 1996b; Robinson & Kertzman, 1995; Stein & Meredith, 1993; Stein, Wallace, & Meredith, 1995; Thompson & Masterton, 1978).

Psychologists have known for many years that the presentation of a spatially nonpredictive visual stimulus, or cue, can lead to a rapid but short-lasting facilitation of responses to visual targets subsequently presented at the cued location (or elsewhere on the cued side), even in the absence of any overt orienting toward the cue itself (e.g., Jonides, 1981b; Posner & Cohen, 1984). Similar intramodal cuing effects following the presentation of a nonpredictive cue have also been reported in subsequent years between auditory cue and target stimuli (e.g., McDonald & Ward, 1999; Spence & Driver, 1994), and more recently, between tactile cue and target stimuli as well (Spence & McGlone, 2001).

In most situations, these cuing effects appear to reflect the consequences of a transient shift of attention to the cued location rather than a passive sensory effect per se (see Posner & Cohen, 1984; but see also Tassinari, Aglioti, Chelazzi, Peru, & Berlucchi, 1994, for a sensory explanation of some facilitatory cuing effects reported in earlier visual cuing studies). Because the cues were spatially nonpredictive with regard to the likely target location in these early studies, researchers concluded that spatial attention can be oriented involuntarily, and at least somewhat automatically, to the location of a cuing event. In line with the majority of previous research on this topic, we will refer to this as exogenous attentional orienting (as compared to the endogenous orienting that occurs following the presentation of a spatially predictive peripheral, or central symbolic, cue; e.g., see Driver & Spence, 2004; Klein & Shore, 2000; Spence & Driver, 1994; Wright & Ward, 1994).

Having provided evidence that exogenous shifts of attention to visual, auditory, and tactile cues can facilitate responses to targets presented subsequently in the same modality, the obvious question arises as to whether such shifts of covert attention can also facilitate responses to targets presented in sensory modalities other than that of the cue. Would, for example, an exogenous shift of attention to a sound on the left facilitate responses to subsequent visual targets appearing on that side? Any such cross-modal cuing effect might reflect the existence of a supramodal attentional mechanism, a finding that would have important implications at both the theoretical and applied levels (see, e.g., Spence, 2001). The evidence suggesting that the presentation of auditory or tactile cues can trigger an overt shift of visual attention in the cued direction provides at least prima facia evidence that this might be the case. However, as is often the case in the field of experimental psychology, proving (to the satisfaction of all) what is intuitively obvious to the majority of people has taken rather longer than one might have expected! Part of the problem with research in this area has often been the adoption of inappropriate experimental designs that have either used response measures that are relatively insensitive to the manipulation of attention or else do not satisfactorily rule out nonattentional explanations (such as simple detection latencies; see below). Meanwhile, other studies have incorporated experimental setups that failed to maximize the possibility of detecting any cuing effect present because the cue and target stimuli on ipsilaterally cued trials were presented from different spatial locations.1

A further problem has been an overreliance on particular experimental paradigms with relatively few attempts to understand why different groups of researchers have found different patterns of cross-modal cuing effects in their different experimental paradigms (see Ward, Prime, & McDonald, 2002, on this point). Over the past decade, contradictory and often seemingly incompatible findings have emerged from the laboratories of Lawrence Ward, John McDonald, and their colleagues, on the one hand, and Charles Spence, Jon Driver, and their colleagues, on the other (see, e.g., Spence & Driver, 1997a; Ward, 1994; Ward, McDonald, & Golestani, 1998; Ward, McDonald, & Lin, 2000). Although these differences were originally attributed to methodological problems with the particular studies involved, recent work has confirmed the validity and robustness of each pattern of results, at least within the particular experimental paradigms in which they were tested. Fortunately, as the various different research groups have argued over the “true” nature of the cross-modal links in covert spatial attention that exist between the different sensory modalities, a number of important methodological and theoretical advances have emerged from the debate in this area. What is more, and as we hope to show in this chapter, there is now convincing empirical evidence that the covert orienting of exogenous attention that is triggered by the presentation of auditory, visual, or tactile cue stimuli can facilitate the perception of target stimuli presented subsequently at the cued location, no matter what their sensory modality. In fact, cross-modal cuing effects have now been demonstrated behaviorally between all possible combinations of auditory, visual, and tactile cue and target stimuli under a subset of experimental testing conditions.

The way is now open, therefore, for researchers to start investigating a number of theoretically more interesting issues in this area, among them the following: (1) the precise nature of the relationship between mechanisms underlying exogenous shifts of attention to auditory, visual, and tactile stimuli (McDonald & Ward, 2003a); (2) the possible modulation of cross-modal cuing effects by top-down, or endogenous, attentional factors (e.g., McDonald & Ward, 1999; Spence, 2001); (3) the effects of posture change on cross-modal covert attentional orienting (e.g., Driver & Spence, 1998; Kennett, Spence, & Driver, 2002); and (4) the underlying reasons behind the different patterns of cross-modal cuing effects reported in different experimental paradigms (i.e., over and above any simple methodological limitations inherent in particular studies; Prime, McDonald, & Ward, 2003; Ward et al., 2002). Cognitive neuroscientists have also begun to investigate some of the neural underpinnings (and consequences) of the cross-modal orienting of covert exogenous spatial attention (e.g., Kennett, Eimer, Spence, & Driver, 2001; Macaluso, Frith, & Driver, 2000; McDonald, Teder-Sälejärvi, Heraldez, & Hillyard, 2001; McDonald, Teder-Sälejärvi, Di Russo, & Hillyard, 2003; McDonald & Ward, 2000), an area that has led to the posing of some challenging questions concerning what exactly the difference is between cross-modal exogenous attentional orienting and multisensory integration (see Macaluso, Frith, & Driver, 2001; McDonald, Teder-Sälejärvi, & Ward, 2001; Spence, McDonald, & Driver, 2004). In this chapter we highlight the current state of understanding regarding the nature and consequences of the exogenous cross-modal covert orienting of spatial attention at both the behavioral and neural levels (see Driver & Spence, 2004, for a related discussion of this cross-modal question specifically for the case of endogenous spatial orienting).