Reduced haemodynamic response in the ageing visual cortex measured by absolute fNIRS

The effect of healthy ageing on visual cortical activation is still to be fully explored. This study aimed to elucidate whether the haemodynamic response (HDR) of the visual cortex altered as a result of ageing. Visually normal (healthy) participants were presented with a simple visual stimulus (reversing checkerboard). Full optometric screening was implemented to identify two age groups: younger adults (n = 12, mean age 21) and older adults (n = 13, mean age 71). Frequency-domain Multi-distance (FD-MD) functional Near-Infrared Spectroscopy (fNIRS) was used to measure absolute changes in oxygenated [HbO] and deoxygenated [HbR] haemoglobin concentrations in the occipital cortices. Utilising a slow event-related design, subjects viewed a full field reversing checkerboard with contrast and check size manipulations (15 and 30 minutes of arc, 50% and 100% contrast). Both groups showed the characteristic response of increased [HbO] and decreased [HbR] during stimulus presentation. However, older adults produced a more varied HDR and often had comparable levels of [HbO] and [HbR] during both stimulus presentation and baseline resting state. Younger adults had significantly greater concentrations of both [HbO] and [HbR] in every investigation regardless of the type of stimulus displayed (p<0.05). The average variance associated with this age-related effect for [HbO] was 88% and [HbR] 91%. Passive viewing of a visual stimulus, without any cognitive input, showed a marked age-related decline in the cortical HDR. Moreover, regardless of stimulus parameters such as check size, the HDR was characterised by age. In concurrence with present neuroimaging literature, we conclude that the visual HDR decreases as healthy ageing proceeds

Haemodynamic Responses in the Primary Visual Cortex to Multisensory Stimulation: a Pilot Study using fNIRS

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

Mean quick Contrast Sensitivity Function (qCSF) per age group with young: n = 12, mean age 21, and old: n = 10, mean age 72.

<p>Black markers represent the old group average, white indicate young. Red lines demonstrate the sine-wave spatial frequencies that are analogous to our square wave checkerboard spatial frequencies.</p

Experimental protocol showing baseline recording and cycles of visual stimulation, repeated 10x per condition.

<p>Experimental protocol showing baseline recording and cycles of visual stimulation, repeated 10x per condition.</p

Diagram of fNIRS set-up with the sensor placed over O2 (right hemisphere, 10–20 EEG system).

<p>All recordings were taken from over both right (O2) and left (O1) primary visual cortices (blue region).</p

Average response cycle to visual stimulation (grey) comparing younger adults (bright) and older adults’ (dark) group HDR of an increase in [HbO] (red) and decrease in [HbR] (blue).

<p>Check sizes are averaged for each 100% contrast stimuli (left) and 50% (right), means are plotted with standard error, error bars.</p

Individual grand average haemodynamic response to reversing checkerboards (average check size used).

<p>[HbO] is plotted in red, with correlation for 100% at r = -0.55, 50% at r = -0.53. [HbR] is plotted in blue, with correlation for 100% at r = 0.59, 50% at r = 0.49, all correlations are significant at p<0.01.</p

Individual grand average haemodynamic response at rest (grey screen with mean luminance to checkerboard stimulus).

<p>[HbO] is plotted in red, with correlation at R = -0.517. [HbR] is plotted in blue, with correlation at r = 0.391, p<0.05.</p