Regular articleSpatio-temporal patterns of event-related potentials related to audiovisual synchrony judgments in older adults
Introduction
Our environment often contains visual and auditory information originating from separate events. To accurately interpret our environment, the sensory system needs to be able to combine related sensory inputs, yet separate unrelated information. Given that related auditory and visual stimulus pairs generally occur at or around the same time, temporal synchrony is one factor that identifies sensory information as related or unrelated (Calvert et al., 2004). One common method for examining human audiovisual temporal perception is the synchrony judgment task in which observers are presented with audiovisual stimulus pairs at various temporal offsets and are required to respond to whether they perceive the pairs as synchronous or asynchronous (Alm and Behne, 2013, Fujisaki et al., 2004, Hay-McCutcheon et al., 2009, Love et al., 2013, Navarra et al., 2010, Silva et al., 2013, Zampini et al., 2005). The temporal range over which observers perceive the audiovisual pairs as synchronous at least 50% of the time is termed the synchrony window width. For simple audiovisual stimuli like a visual flash and an auditory tone pip, younger adults have an average synchrony window width of around 200 ms (Zampini et al., 2005). Older adults have been reported to have extended window widths (Alm and Behne, 2013, Hay-McCutcheon et al., 2009), even when the stimuli are scaled for changes to detection thresholds due to age (Chan et al., 2014a). The neural mechanisms underlying such functional differences in audiovisual processing in older adults are not well understood, hence the aim of this study was to compare between younger and older adults the neural correlates for audiovisual synchrony judgments using electroencephalography (EEG).
Audiovisual information is processed across many brain regions, including sensory-specific cortices (Giard and Peronnet, 1999, Noesselt et al., 2007) and multisensory areas (Bushara et al., 2001, Calvert et al., 2004, Noesselt et al., 2007, Noesselt et al., 2012). With older age, the neural system undergoes anatomical and physiological changes that have been well studied in the sensory-specific visual (Spear, 1993) and auditory (Mazelová et al., 2003) pathways. These changes include, but are not exclusive to, delayed latencies of peak waveform components (Anderer et al., 1996, Beck et al., 1980, Mullis et al., 1985, Pekkonen et al., 1996, Schiff et al., 2008). Consequently, age-related changes to unisensory processing are potentially important when examining audiovisual neural processing in older adults.
The EEG waveforms for synchrony judgment tasks have not been previously compared between older and younger adults. However, the time course of several alternative measures of audiovisual processing has been investigated. These tasks include a speeded syllable identification task (Winneke and Phillips, 2011) and a temporal order judgment task using simple flash-pip stimuli (Setti et al., 2011). These studies found that within the first 200 ms after stimulus onset, older observers showed reduced amplitudes for all the major waveform peaks of the audiovisual waveform (N1, P1, and P2). Note, however, that the neural processes required to judge the temporal order of 2 stimuli (which of 2 stimuli came first) or the synchrony of 2 stimuli (did 2 stimuli occur at the same time) are different. Functional magnetic resonance imaging revealed additional activation of the prefrontal cortex, parietal, and occipito-temporal regions for temporal order judgments relative to synchrony judgments (Binder, 2015). Therefore, it is unclear from the current literature if older and younger adults use the same or different time course of neural processes to judge the synchrony of audiovisual stimuli.
The aim of this study was to determine whether there are differences in the neural mechanisms used by older adults to encode audiovisual synchrony judgments, accounting for age differences in unisensory processing. We directly related behavioral performance of younger and older adults for audiovisual synchrony judgments to differences in amplitudes of several event-related potential (ERP) components, and furthermore by using multivariate measures to investigate when successful judgments were predictable from distributed patterns of ERPs (Bode and Stahl, 2014, Bode et al., 2012). More specifically, using 64-channel EEG, 2 analysis approaches were taken: (1) the overall spatio-temporal profile of neural activity for audiovisual synchrony judgments in older adults was compared with a group of younger controls and (2) the earliest time point that allowed the prediction of behavioral audiovisual synchrony judgments from patterns of ERPs was determined and compared between the age groups. Hence, this study addressed whether the magnitude of neural activity as well as the relative timing of sensory processing differed between age groups.
Section snippets
Participants
Fourteen younger (18–32 years old; mean = 26 years; 6 males) and 16 older (61–74 years old; mean = 65 years; 9 males) adults participated in this study. Younger observers were students at the University of Melbourne, whereas the older observers were recruited from the community. An abbreviated clinical ophthalmic examination and a hearing test were conducted on their first visit. Visual inclusion criteria were normal or corrected-to-normal visual acuity (equal to or better than 6/7.5),
Unisensory stimulus identification thresholds were not different between age groups
Each of the older and younger observers had unisensory visual and auditory identification thresholds above 96.9% (i.e., more than 155 correct responses out of a total of 160 trials), except for 1 older observer who had a visual identification accuracy of 92.5% (i.e., 148 correct responses out of 160 trials).
Older adults perceived fewer sound-lead pairs as synchronous
Fig. 2A shows the audiovisual synchrony judgment results with the proportion of synchronous responses as a function of temporal asynchrony, and Fig. 2B shows the 95% confidence intervals for
Discussion
This study determined the onset of predictive neural activity for audiovisual synchrony judgments in older adults and examined the spatio-temporal profile of audiovisual processing in older adults by comparing them to a group of younger adults. The behavioral data showed that older observers perceived fewer sound-lead pairs as synchronous but perceived synchrony of physically synchronous pairs, and of sound-lag pairs were similar to the younger observers, on average. Although the earliest onset
Conclusions
This study reports novel findings for the age-related changes to the early processing of audiovisual stimuli. Our results showed that the older adults recruited more spatially distributed neural resources to maintain performance in the audiovisual synchrony judgment task. The onset of predictive sensory information to enable audiovisual synchrony judgments was approximately 40–70 ms after stimulus onset and did not differ between younger and older adults.
Disclosure statement
The authors have no conflicts of interest to disclose.
Acknowledgements
This research was funded by the Australian Research Council FT0990930 to author Allison M. McKendrick and the Australian Research Council DECRA140100350 to Stefan Bode.
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