Short-term neurovascular and electrophysiological responses to combined visual and vibration stimulation in older adults with mild cognitive impairment

Figures

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  Figure 1. Experimental paradigm and conceptual framework of rhythmic stimulation. (A) Illustration of an older adult participant seated on a vibration platform delivering rhythmic mechanical stimulation, synchronized with visual light flicker. The intervention provided simultaneous gamma-frequency visual stimulation and theta-frequency mechanical stimulation in a seated, non-volitional manner. (B) Conceptual schematic illustrating the theoretical relationship between theta- and gamma-band neural oscillations, in which faster gamma activity is temporally organized within slower theta rhythms. This schematic represents a conceptual framework motivating the present study and does not imply direct measurement of phase-based cross-frequency coupling.

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  Figure 2. Validation of theta-frequency mechanical vibration during PTMD stimulation. Time-series signals of seat vibration are shown for the X-axis (purple), Y-axis (blue), and Z-axis (orange). The oscillatory motion exhibited a dominant frequency of approximately 8 Hz, confirmed by cycle counts and timestamps (cyan: seconds; magenta: cycles). The X-axis component showed the largest initial amplitude, which gradually decreased over time, whereas the Y-axis component increased. A minor Z-axis component was also observed, indicating multidirectional but predominantly X–Y–oriented vibration.

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  Figure 3. Baseline hemispheric comparison of total hemoglobin (HbT) in frontal regions. Mean total hemoglobin (HbT) values prior to PTMD intervention were significantly higher in the left frontal region (0.308 ± 0.508 μM) than in the right frontal region (0.225 ± 0.584 μM; Wilcoxon signed-rank test: Z = −26.166, p < 0.001). Error bars indicate standard deviation.

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  Figure 4. Pre- to post-intervention changes in left frontal HbT. HbT in the left frontal region decreased significantly following the PTMD intervention, from 0.308 ± 0.508 μM at baseline to 0.209 ± 0.543 μM post-intervention (Wilcoxon signed-rank test: Z = −21.278, p < 0.001). Error bars indicate standard deviation.

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  Figure 5. Pre- to post-intervention changes in right frontal HbT. Right frontal HbT decreased from 0.225 ± 0.584 μM at baseline to −0.004 ± 0.871 μM following the PTMD intervention (Wilcoxon signed-rank test: Z = −25.671, p < 0.001). The magnitude of reduction was larger than that observed in the left frontal region. Error bars indicate standard deviation.

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  Figure 6. Time-window analysis of post-intervention HbT in the left frontal region (eyes open). Post-intervention HbT values were segmented into six consecutive 30 s windows. HbT reached a relative maximum during the fourth window (90–120 s; 0.259 ± 0.791 μM) and declined in the subsequent window (120–150 s; 0.171 ± 0.599 μM). Bonferroni-corrected comparisons indicated that the fourth window differed significantly from the other windows (p < 0.001). Error bars indicate standard deviation.

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  Figure 7. Time-window analysis of post-intervention HbT in the right frontal region (eyes open). A transient HbT peak was observed in the third window (60–90 s; 0.305 ± 1.923 μM), followed by a marked decrease in the fifth window (−0.123 ± 0.802 μM). Temporal variability across windows was greater in the right frontal region than in the left. Error bars indicate standard deviation.

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  Figure 8. Post-intervention changes in gamma-band power at frontal electrodes (eyes open). Gamma-band (27–45 Hz) power decreased following the intervention across frontal electrodes (AF3, AF4, F3, F4, F7, F8), with the largest reductions observed at AF4 (−48.1%) and F4 (−54%). Repeated-measures ANOVA revealed a significant main effect of frequency band (F = 3247.811, p < 0.001). In contrast, theta-band (4–8 Hz) power showed a modest increase.

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  Figure 9. Post-intervention theta- and gamma-band power changes at temporoparietal electrodes (eyes closed). Following the PTMD intervention, theta-band power increased significantly at the right temporal electrode T8 (+23%, p < 0.001), whereas gamma-band power decreased at the right parietal electrode P8 (−12.306 dB, Cohen’s d = 1.47). These findings indicate concurrent, frequency-specific modulation of theta and gamma activity at temporoparietal sites.