Greater BOLD variability in older compared with younger adults during audiovisual speech perception

Abstract

Older adults exhibit decreased performance and increased trial-to-trial variability on a range of cognitive tasks, including speech perception. We used blood oxygen level dependent functional magnetic resonance imaging (BOLD fMRI) to search for neural correlates of these behavioral phenomena. We compared brain responses to simple speech stimuli (audiovisual syllables) in 24 healthy older adults (53 to 70 years old) and 14 younger adults (23 to 39 years old) using two independent analysis strategies: region-of-interest (ROI) and voxel-wise whole-brain analysis. While mean response amplitudes were moderately greater in younger adults, older adults had much greater within-subject variability. The greatly increased variability in older adults was observed for both individual voxels in the whole-brain analysis and for ROIs in the left superior temporal sulcus, the left auditory cortex, and the left visual cortex. Increased variability in older adults could not be attributed to differences in head movements between the groups. Increased neural variability may be related to the performance declines and increased behavioral variability that occur with aging.

Figure 1. Mean and standard deviation of BOLD responses to audiovisual speech within subjects.

A: Hemodynamic response in the left STS of a single older adult (subject JI). Error bars indicate standard deviation of the response within that subject (intrasubject variability) at each time point. The variability at the 4-second and 6-second time points (bold error bars) was used for group analysis. Representative single subject chosen as the subject whose standard deviation was closest to the mean standard deviation for all older subjects. B: Hemodynamic response in the left STS of a single younger adult (subject HU). Representative single subject chosen as the subject whose standard deviation was closest to the mean standard deviation for all younger subjects. C: Scatter plot of age vs. within-subject standard deviation of the STS response. Blue symbols represent younger adults (n = 14), red symbols represent older adults (n = 19). The lines show the mean of the within-subject standard deviation across each group. The brackets show the results of an unpaired t-test between the within-subject standard deviation in each group. D: Scatter plot of age vs. within-subject standard deviation of the left auditory cortex response. E: Scatter plot of age vs. within-subject standard deviation of the left visual cortex response.

Figure 2. Mean and standard deviation of BOLD responses to audiovisual speech across subjects.

A: Average hemodynamic response to audiovisual syllables in the left STS for older adults (red) and younger adults (blue). Shaded region indicates standard deviation of the group response (intersubject variability). B: Response amplitudes in the left STS (STS), left auditory cortex (Aud), and left visual cortex (Vis) across all older adults (red) and younger adults (blue). Error bars show the complete range of data (subjects with maximum and minimum response); middle bar shows median subject.

Figure 3. Whole-brain analysis of differences in intrasubject variability, response amplitude, and intersubject variability in older and younger adults.

A: Regions that show a significant positive response (t >2 for all audiovisual syllables vs. baseline) to audiovisual speech in both older and younger adults. L: left hemisphere, R: right hemisphere. B: Differences in intrasubject variability (variability of the amplitude of the BOLD response to audiovisual speech within each subject) between older and younger subjects, masked by active regions in (A). Orange regions indicate areas with greater intrasubject variability in older adults. C: Differences in response amplitude. Green regions indicate no difference in response amplitude; blue regions indicate areas of greater response amplitude in younger adults. D: Differences in intersubject variability (variability of the amplitude of the BOLD response across subjects). Orange regions indicate areas with greater response variability in older adults.