| Summary: | The world's population is ageing, which represents challenges and opportunities concerning different aspects of life. Some older people exhibit various perceptual and cognitive declines, which are related to their inability to construct a stable representation of the external world, it being usually ambiguous and degraded. These declines require thorough research in order to understand their cortical mechanisms. Changes in perceptual stability as a function of ageing were explored in this PhD work using methods of psychophysics, visual evoked potentials, and eye- movement recordings. Specifically studied were attentional control in perceptual rivalry, the ability to construct a stable and efficient internal representation of degraded words and finally, saccadic suppression as an important contributory factor to the underlying perceptual stability of the visual world during saccadic eye movements. Perceptual rivalry, produced by an ambiguous Rubin vase-faces figure, was slower in older people during passive viewing than in young participants. Additionally, older adults, in contrast to young adults, were unable to hold the dominant percept longer than in passive viewing. Using current models of perceptual rivalry and studies of age-related changes in cortical activity, the increased response gain in older adults might be a possible factor leading to prolonged dominance durations in older people during passive viewing as well as impaired ability to hold the dominant percept longer. Visual evoked potentials, elicited by frequency-tagged stimulation, showed that perceptual rivalry under passive viewing caused in both age group suppression of visual evoked potentials to a contrast-modulated pattern. This finding is only true for the vase percept, when the vase percept was not dominant. Thus, the mechanism, underlying perceptual rivalry under passive viewing in early visual cortical areas, could be inhibition of neuronal populations associated with the non-dominant percept, rather than excitation of neuronal populations, corresponding to the dominant percept. Attentional control of holding the dominant percept caused an enhancement of the visual evoked potentials, representing the dominant percept, in young subjects but not in older people. Holding and switching the dominant percept also produced enhancement of gamma activity, recorded with electrodes over the occipital and parietal cortices, only in young adults. These results could be related to age-related deficits in top-down modulated attentional and figure-ground segregation processes. A novel paradigm for recognising words embedded in positional letter noise and a model for estimating internal noise components were developed. Older people showed noise-exclusion deficits when reading normal words and reversed words, but not when reading nonwords. These deficits suggest that older people may experience perceptual instability of words due to inefficient noisy perceptual representations of words. This method has the potential for use as a tool in the clinical testing of noise- exclusion deficits in normal and pathological ageing and in children with reading dysfunctions. Investigation of age-related changes in saccadic suppression revealed for the first time that older adults experienced reduced and delayed saccadic suppression compared to young adults. This could result in perceiving instability of the external world and thus affect their mobility and performance in everyday environments. The results of this thesis provide evidence of age-related deficits in the ability of older people to voluntarily perceive a stable dominant percept during perceptual rivalry, to efficiently exclude positional noise during reading degraded words and to suppress visual information during saccadic eye movements. Although these deficits involve different cortical mechanisms, the underlying cause for them could be related to impairments in inhibitory mechanisms at various cortical processing stages.
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