Perceptual centre-surround processing in older adults

© 2012 Dr. Renee KarasThe aim of this thesis was to investigate the effects of perceptual centre-surround suppression in older adults. It has previously been shown that older adults show increased contrast-contrast surround suppression for textured centre-surround stimuli. As the amount of centre-surround suppression is known to depend on stimulus parameters such as contrast, orientation and size, the purpose of the current experiments was to use a variety of stimulus parameters in order to assess under which conditions older observers demonstrate increased surround suppression compared to younger adults. Two groups of adult observers one young and one old participated in centre-surround contrast matching tasks. Experiment 1 aimed to investigate border cues between centre and surround stimuli. Surround suppression was measured when centre and surround were presented in-phase and when they were presented out-of-phase. Older observers produced greater amounts of suppression for both conditions when compared to younger observers indicating that the phase information at the border is not responsible for the increases in surround suppression. Additionally, Experiment 1 revealed that increases in surround suppression cannot be attributed to decreased contrast sensitivity of the older groups. Experiment 2 aimed to investigate surround suppression in older observers for drifting stimuli. Observers performed the contrast-contrast task as well as a motion discrimination task which has also been used to measure perceptual centre-surround suppression. Consistent with Experiment 1, older observers showed increased surround suppression for the contrast-contrast task, however performed similarly to younger observers for the motion discrimination task implying that the two tasks involve different mechanisms. Finally, Experiment 3 investigated the contrast ratios between the centre and surround. Results revealed that older adults showed increased surround suppression when contrasts were low, more specifically when centre contrast was low. When centre-surround contrasts were high (80/80%), younger and older observers performed similarly. The findings of this thesis demonstrate that perceptual contrast surround suppression is strengthened in older adults when compared to younger observers for a variety of stimulus conditions. The findings of Experiment 3 provide a possible explanation for the differences found between analogous perceptual tasks (contrast vs. motion tasks), with results enabling some inferences regarding neurophysiological mechanisms responsible for the age-related differences. Additionally, the results presented herein suggest that a series of perceptual tasks are needed in order to measure the balance of excitation and inhibition within the human visual system