Shedding infrared light on sight: Age and vision

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  • Published: May 1, 2015
  • Author: David Bradley
  • Channels: Infrared Spectroscopy
thumbnail image: Shedding infrared light on sight: Age and vision

Checkered vision

Diagram of fNIRS set-up with the sensor placed over O2 (right hemisphere, 10–20 EEG system). All recordings were taken from over both right (O2) and left (O1) primary visual cortices (blue region). (Credit: PLOS One/Ward et al)

Functional near-infrared spectroscopy (fNIRS) can be used as a non-invasive optical neuroimaging technique to look at changes in blood flow in the visual cortex as people age.

As medicine progresses, so populations age as youngster avoid the infectious ravages of youth, they enter their senior years with the prospect of what we might call the diseases of old age. But cancer, heart failure, stroke, Alzheimer's and Parkinson's aside, even the healthy body succumbs to problems with the advancing years. But, there is only limited understanding of the healthy ageing process, maturation without disease. One area of interest to Laura McKernan Ward, Ross Thomas Aitchison, Melisa Tawse, Anita Jane Simmers and Uma Shahani of the Department of Vision Sciences, at Glasgow Caledonian University, UK, is the effect of healthy ageing on activation of the brain's visual cortex.

Ageing site

The Glasgow team wanted to know how ageing affects the haemodynamic response (HDR) of the visual cortex. As such, they presented healthy patients - both old and young - with a simple visual stimulus, a reversing chessboard pattern of black and white squares. All volunteers were given a full eye test so that results could be weighted for visual acuity (VA) and contrast sensitivity function (CSF). The team then used frequency-domain multi-distance (FD-MD) functional near-infrared spectroscopy (fNIRS) to measure absolute changes in oxygenated and deoxygenated haemoglobin concentrations in the occipital cortices. The technique is non-invasive but can nevertheless capture the slow HDR response associated with firing of neurons in the region of the brain of interest.

Old and young

The tests were carried out on two groups of volunteers: 12 volunteers with a mean age 21 years and a group of 13 of average age 71 years. "Both groups showed the characteristic response of increased [HbO] and decreased [HbR] during stimulus presentation," the team reports in the journal Plos One. "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." Moreover, the young adults had higher levels of HbO and HbR irrespective of the visual stimulus they were watching during the scan. Things were markedly different for the older group. "Passive viewing of a visual stimulus, without any cognitive input, showed a marked age-related decline in the cortical HDR," the team says. The results corroborate other neuroimaging studies undertaken with functional magnetic resonance imaging (fMRI) and transcranial Doppler ultrasonography (TCD) and lead to the general conclusion that visual HDR falls in healthy individuals as they get older.

"This study provides compelling physiological evidence of a reduced cortical response in elderly adults, regardless of conventional measures such as visual acuity, contrast sensitivity or visual fields," the team adds. This finding concurs with an earlier functional NIRS study that examined both cardiorespiratory fitness in the ageing visual cortex (Fabiani et al., 2014).

"The PLoS One paper reveals a marked decline in the visual cortical haemodynamic response (HDR) as we age, a result that has previously been reported using different neuroimaging techniques, but never with fNIRS and absolute measures of [HbO] and [HbR]," McKernan Ward told SpectroscopyNOW. "We are currently conducting research to investigate exactly what happens to our HDR over the lifespan and have used fNIRS to study the visual cortical response from participants in their 30s, 40s and 50s."

The work could have broader implications for middle-aged adults. "The current studies I am undertaking will potentially feed into research investigating a possible intervention for adults aged 50+ to increase their [HbO] based on perceptual learning," she adds, "but the basic science needs research first." McKernan Ward also points out that fNIRS, while a relatively new neuroimaging technique, offers potential for studying ageing populations and has distinct advantages over fMRI. "[It is] increased patient tolerance to environment, increased inclusion (e.g. pacemaker and diabetic implant patients), and reduced sensitivity to motion," she explains. "There is a growing number of researchers utilising fNIRS, however there are only a handful of labs investigating the ageing brain, therefore, it is a very exciting place to be."

Related Links

PLOS One2015, online: "Reduced Haemodynamic Response in the Ageing Visual Cortex Measured by Absolute fNIRS"

Article by David Bradley

The views represented in this article are solely those of the author and do not necessarily represent those of John Wiley and Sons, Ltd.

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