Abstract
In this thesis, the visual cortex's reaction to perceptual stimulation as reflected by the changing haemodynamic response (HDR) over the lifespan is presented. HDRs were recorded by functional Near Infrared Spectroscopy (fNIRS) which measures changes in oxy- ([HbO]) and deoxygenated haemoglobin concentrations ([HbR]). Healthy participants (visually normal) were presented with a simple visual stimulus(reversing checker board), with contrast and check size manipulations (15 and 30 minutes of arc, 50% and 100% contrast). Both younger adults (n=12, mean age 21)and older adults (n=13, mean age 71) showed a characteristic HDR from the primary visual cortex (VI). That is, an increase in [HbO] and decrease [HbR] during checkerboard presentation compared to a luminance matched grey screen (p<0.01).However, older adults had an attenuated HDR in comparison to their younger counterparts, regardless of the type of stimulus displayed (p<0.05).To examine the response to a more complex visual stimulus, dynamic Random-Dot Stereograms (RDS) were used. Binocular disparity was manipulated so that the test image contained a 'pop-out' sinusoidal wave, whilst the control image appeared fused and 'flat'. HDRs were recorded from young participants' (n=13, mean age 24)V I and the parieto-occipital cortex (PO). The V I HDR was bimodal showing a response to both test and control RDS. This was in contrast to the PO region,specifically, the right PO showed an increase in [HbO] in response to the test image(p<0.001), confirming the strong association of this area with processing depth.
To elucidate the specific impact of healthy ageing on perceptual processing, global motion perception was studied using a comprehensive approach. This study was the first to record the HDR using fNIRS from V5 in response to a real motion stimulus(Random-Dot-Kinematogram, RDK). A large cohort of healthy participants (n=73,range 18 - 85 years old) were divided into groups of younger (n=28, <40), middle aged (n=22, 41 - 59) and older adults (n=23, >50). During fNIRS recording,participants discriminated global motion direction of the RDKs at 3 different speeds,providing a behavioural measure of motion perception. For the very first time, an increase in [HbO] and decrease in [HbR] in response to a real motion stimulus is reported (p<0.05). Moreover, there was a linear age-related increase in [HbO] and decrease in [HbR] during the fastest speed of stimuli (p<0.05). This was mirrored in the behavioural responses with older adults presenting with the poorest motion perception (for all three speeds of stimuli). Other measures collected (blood pressure, heart rate, cardio respiratory fitness and personality), failed to correlate either to V5's HDR or the behavioural responses.
To research whether the altered HDR was due to a vascular or mechanical change in older adults, we explored its clinical manifestations. The relationship between glaucoma patients (n=8, mean age 72) and habitual snorers (n=6, mean age 51) was studied, with approximate age-matched controls (n=10, mean age 53). HDRs were recorded in response to two tasks: firstly over VI, using a reversing checkerboard (30 minutes of arc, 100% contrast), secondly over the frontal cortex during a breath hold task designed to induce mild hypercapnia. Results indicated that the V I [HbO] was attenuated regardless of whether participants suffered from glaucoma or snoring, in comparison to healthy controls (p<0.01). The mild hypercapnia task provided further evidence for a compromised vascular function in these two groups. Furthermore,visual fields and acuity of glaucoma patients significantly correlated with V I [HbO](p<0.05) indicating that those patients with higher field defects and poorer acuity presented with a reduced V I [HbO] response to checkerboard stimulation.These results show the efficacy of fNIRS to reliably record the HDR elicited to visual stimuli, demonstrating the potential of NIRS to characterise the changes that occur across the lifespan in the visual cortex of healthy and clinical populations.
| Date of Award | 2016 |
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| Original language | English |
| Awarding Institution |
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| Supervisor | Gordon Morison (Supervisor), Anita Simmers (Supervisor) & Uma Shahani (Supervisor) |