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

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

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

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

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

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 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