Multisensory Integration and Crossmodal Attention Effects in the Human Brain Macaluso et al. (1) provided functional mag- netic resonance imaging (f MRI) evidence for multisensory processing in the human brain. In their study, a light presented in the right visual field produced a larger neural response in the left fusiform gyrus, a modality-specific area of visual cortex, when a concurrent tactile stimu- lus was delivered to the right hand (seen next to the right light) than when the light was present- ed alone or when a concurrent tactile stimulus was delivered to the left hand. These findings dovetail nicely with evidence for spatially spe- cific multisensory effects at the behavioral and neural levels (2, 3) and for bimodal-stimulation effects on activity in auditory cortex (4, 5 ). Macaluso et al. stated [p. 1206 in (1)] that their finding “provides a neural explanation for crossmodal links in spatial attention.” We view that conclusion as premature, however, because of the stimulus parameters that were chosen. Attributing modulations in behavior or neural activity to a spatial attention mech- anism is a nontrivial task in intramodal situ- ations as well as in multimodal ones. A non- predictive visual cue presented at one spatial location might facilitate responses to a sub- sequent visual target at that location either because it elicited an involuntary shift of attention or simply because the sensory re- sponses to the cue and target were temporally integrated (6, 7 ). In general, converging evi- dence must be obtained to rule out the latter, sensory-based explanation for behavioral or neural facilitation. A minimum condition that is usually im- posed to reduce sensory integration is to present the target stimulus some time after the initial cue stimulus has disappeared (7 ). One might assume that sensory integration would also be reduced by presenting the cue and target in different modalities; however, presenting different-modality stimuli within 100 to 150 ms actually produces a superad- ditive sensory response from specialized neu- rons that are capable of responding to both stimuli (3). As a result of this superadditive effect, stimuli that appear at about the same time and place are integrated to form a uni- fied perceptual object, rather than being left as a collection of unrelated sensations. This phenomenon is called multisensory integra- tion, and it provides a neural explanation for several dramatic multisensory perceptual ef- fects such as the ventriloquist’s illusion (8). Relatively long-lasting visual and tactile stimuli were presented simultaneously in the experiments of Macaluso et al. (1); thus, the neural interactions they observed could have been those involved in multisensory integra- tion of vision and touch. Interestingly, in the Perspectives article that accompanied the Macaluso et al. study, de Gelder (9) interpret- ed their finding in terms of multisensory in- tegration rather than spatial attention and speculated that simultaneous presentation was crucial for the f MRI findings. Clearly, the presentation of visual and tactile stimuli at the same time and place is sufficient to produce multisensory integration at both neu- ral and behavioral levels. However, the ques- tions remain as to whether and how multisen- sory integration is related to the crossmodal consequences of involuntary spatial attention. If multisensory integration constituted the neural mechanism that causes involuntary shifts of attention to modulate processing of objects in different modalities, then the con- clusions drawn by Macaluso et al. about the neurophysiological basis of crossmodal spa- tial attention effects would be justified. To our knowledge, however, there is no empiri- cal evidence supporting the hypothesis that multisensory integration plays any role in generating crossmodal attention effects. On the contrary, there are convincing lines of evidence that multisensory integration and shifts of spatial attention are independent. (i) Multisensory integration occurs in anaesthe- tized animals (i.e., without any intent). (ii) The ventriloquism effect, a well-known per- ceptual consequence of multisensory integra- tion, has been shown to occur preattentively and independently of both voluntary and in- voluntary spatial attention shifts (10, 11), al- though it can aid voluntary attentional focus- sing (12). (iii) Whereas multisensory integra- tion occurs preattentively and without intent, involuntary shifts of attention depend on the attentional goals of the observer (13, 14 ). (iv) Approximate temporal synchrony is required for multisensory integration but not for invol- untary spatial attention effects to occur. Im- portantly, involuntary spatial attention effects occur even when stimuli are brief (100 ms) and are separated by 100 to 500 ms (15, 16 ). Under such conditions, multisensory integra- tion and many of its perceptual consequences (e.g., ventriloquism) are greatly reduced (17, 18). These findings provide evidence that in- voluntary shifts of spatial attention arise from stimulus-driven processes that are separate from those involved in multisensory integra- tion. Of course, in some experimental para- digms, the two effects might co-occur and produce additive facilitation of responses to targets. The f MRI study by Macaluso et al. is valuable because it firmly demonstrated an effect of multimodal stimulation on modality- specific cortical processing. Because of the specific experimental procedures used, how- ever, their demonstration did not provide a clear explanation for the neural basis of crossmodal spatial attention effects. Involun- tary shifts of spatial attention caused by the appearance of nonvisual stimuli do seem to produce similar enhancements of neural re- sponses to visual stimuli within visual cortex (19, 20), but the details of how this occurs remain to be discovered. John J. McDonald Department of Psychology Simon Fraser University 8888 University Drive Burnaby, BC V5A 1S6, Canada E-mail: jmcd@sfu.ca Wolfgang A. Teder-Sa ¨leja ¨rvi Department of Neurosciences University of California, San Diego 9500 Gilman Drive MC 0608 La Jolla, CA 92093– 0608, USA E-mail: wat@sdepl.ucsd.edu Lawrence M. Ward Department of Psychology University of British Columbia 2136 West Mall Vancouver, BC V6T 1Z4, Canada E-mail: lward@cortex.psych.ubc.ca References 1. E. Macaluso, C. D. Frith, J. Driver, Science 289, 1206 (2000). 2. R. B. Welch, D. H. Warren, in Handbook of Perception and Human Performance, K. R. Boff, L. Kaufman, J. P. Thomas, Eds. (Wiley, New York, 1986), vol. 1, chap. 25. 3. B. E. Stein, M. A. Meredith, The Merging of the Senses (MIT Press, Cambridge, MA, 1993). 4. G. A. Calvert et al., Science 276, 593 (1997). 5. M. Sams et al., Neurosci. Lett. 127, 141 (1991). 6. C. Richard, thesis, University of British Columbia (1999). 7. G. Tassinari, S. Aglioti, L. Chelazzi, A. Peru, G. Berluc- chi, Vision Res. 34, 179 (1994). 8. P. Bertelson, in Cognitive Contributions to the Percep- tion of Spatial and Temporal Events, G. Aschersleben, T. Bachmann, J. Mu ¨sseler, Eds. (Elsevier, Amsterdam, 1999), pp. 347–362. 9. B. de Gelder, Science 289, 1148 (2000). 10. P. Bertelson, J. 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