Effective processing of multisensory stimuli relies on both the peripheral sensory organs and central processing in subcortical and cortical structures. As we age, there are significant changes in all sensory systems and a variety of cognitive functions. Visual acuity tends to decrease and hearing thresholds generally increase (Kalina 1997; Liu and Yan 2007), whereas performance levels on tasks of motor speed, executive function, and memory typically decline (Rapp and Heindel 1994; Birren and Fisher 1995; Rhodes 2004). There are also widespread changes in the aging brain, including reductions in gray and white matter volume (Good et al. 2001; Salat et al. 2009), alterations in neurotransmitter systems (Muir 1997; Backman et al. 2006), regional hypoperfusion (Martin et al. 1991; Bertsch et al. 2009), and altered patterns of functional activity during cognitive tasks (Cabeza et al. 2004; Grady 2008). Given the extent of age-related alterations in sensation, perception, and cognition, as well as in the anatomy and physiology of the brain, it is not surprising that multisensory integration also changes with age.
Several early studies provided mixed results on the differences between multisensory processing in older and younger adults (Stine et al. 1990; Helfer 1998; Strupp et al. 1999; Cienkowski and Carney 2002; Sommers et al. 2005). For example, Stine and colleagues (1990) reported that although younger adults’ memory for news events was better after audiovisual presentation than after auditory information alone, older adults did not show improvement during the multisensory conditions. In contrast, Cienkowski and Carney (2002) demonstrated that audiovisual integration on the McGurk illusion was similar for older and younger adults, and that in some conditions, older adults were even more likely to report the fusion of visual and auditory information than their young counterparts. Similarly, in a study examining the contribution of somatosensory input to participants’ perception of visuospatial orientation, Strupp et al. (1999) reported an age-related increase in the integration of somatosensory information into the multisensory representation of body orientation.
Despite providing a good indication that multisensory processing is somehow altered in aging, the results of these studies are somewhat difficult to interpret due to their use of complex cognitive tasks and illusions, and to the variability in analysis methods. Several newer studies that have attempted to address these factors more clearly demonstrate that multisensory integration is enhanced in older adults (Laurienti et al. 2006; Peiffer et al. 2007; Diederich et al. 2008).
On a two-choice audiovisual discrimination task, Laurienti and colleagues (2006) showed that response time (RT) benefits for multisensory versus unisensory targets were larger for older adults than for younger adults (Figure 20.1). That is, older adults’ responses during audiovisual conditions were speeded more than younger adults’, when compared with their respective responses during unisensory conditions. Multisensory gains in older adults remained significantly larger than those observed in younger adults, even after controlling for the presence of two targets in the multisensory condition (redundant target effect; Miller 1982, 1986; Laurienti et al. 2006).
Using similar analysis methods, Peiffer et al. (2007) also reported increased multisensory gains in older adults. On a simple RT task, where average unisensory RTs were equivalent in younger and older adults, older adults actually responded faster than younger adults on multisensory trials because of their enhanced multisensory integration (Peiffer et al. 2007). Diederich and colleagues (2008) have also shown that older adults exhibit greater speeding of responses to multisensory targets than younger adults on a saccadic RT task. The analysis methods used in this experiment indicate a slowing of peripheral sensory processing, as well as a wider time window over which integration of auditory and visual stimuli can occur (Diederich et al. 2008).
These experiments highlight several possible explanations that could help answer a critical question about multisensory processing in aging: Why do older adults exhibit greater integration of multisensory stimuli than younger adults? Potential sources of enhanced integration in older adults include age-related cognitive slowing not specific to multisensory processing, inverse effectiveness associated with sensory deficits, alterations in the temporal parameters of integration, and inefficient top–down modulation of sensory processing. In the following sections we will investigate each of these possible explanations in greater detail and offer some alternative hypotheses for the basis of enhanced multisensory integration in older adults.
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