Aitchison, Ross Thomas

Denham, Sue

Shahani, Uma

Ward, Laura

Purpose : As we age, our oxyhaemoglobin ([HbO]) and deoxyhaemoglobin [HbR] concentrations decline during visual cortical activation. The purpose of this study was to determine whether the combined effect of auditory and visual stimulation would enhance or diminish the hemodynamic response (HDR) in V1in normal ageing adults.Methods : We recruited 34 healthy participants: 15 oldparticipants (mean: 59 years) &amp; 19 young participants (mean: 23 years), all of whom had normal hearing and a BCVA of 0.3 logMAR or better per eye. The study was approved by the local Ethics Committee &amp; in accordance with the Declaration of Helsinki, consent was obtained from all participants prior to testing. We used a two-channel oximeter (OxiplexTS) to record an HDR comprising [HbO] &amp; [HbR] to stimulus on or off. HDRs were recorded over V1 to either a reversing checkerboard (unimodal visual), a tone cloud (unimodal auditory) or a combination of both (multimodal). The 4 conditions, each using a block design were: V= visual stimulation 30s on / 30s off; A=auditory stimulation 30s on/ 30s off; VA=visual stimulation 15s on, multimodal stimulation 15s on, followed by 30s stimulus off; AV=auditory stimulation 15s on, multimodal stimulation 15s on, followed by 30s stimulus off. Prior to each condition, participants were presented with a baseline stimulus, which consisted of an isoluminant grey screen and absence of tone cloud (silence).Results : All participants demonstrated the characteristic increase in [HbO] and decrease in [HbR] during visual stimulation in condition V. As expected, old participants had lower [HbO] than their young counterparts. During condition A, HDRs of old participants remained at noise level. Young participants showed an initial increase in [HbO] at the onset of the tone cloud. [HbR] in both groups remained at noise level. However, in condition VA, HDRs in both young and old participants were enhanced. During condition AV, [HbO] increased in the characteristic manner described in condition V, but showed a time lag commensurate with stimulus onset.Conclusions : Our results indicate that when auditory stimulation followed the checkerboard (VA), the visual HDR was enhanced. Although preliminary, these findings are suggestive of multisensory integration in V1. However, this was not the case when the order of stimulation was reversed (AV)

Hemodynamic responses recorded in V1 to multimodal stimulation using functional near infrared spectroscopy

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