Advancing age is associated with a decline in physical and cognitive abilities. Multiple theories have been proposed to account for the psychological impairments seen in older age. These include changes in sensory acuity, general slowing of the nervous system, declines in working memory capacity, and a loss of inhibitory control. Although not inherently incompatible, each of the theories provides a unique interpretation of the observed pattern of psychological and neuronal data.To test the veracity of the inhibitory theory a series of experiments was performed. The first experiment used functional magnetic resonance imaging (fMRI) to assess a classical Sternberg paradigm with five increasing levels of cognitive demand ranging from a set size of two letters up to six letters. Comparisons of mean levels of recruitment and inhibition were done between younger and older adults revealing that older adults greatly over recruit but under inhibit areas of the brain compared to their younger counterparts. A region of interest analysis revealed that older adults recruit tissue in a linear fashion from almost all areas in the task evoked visual and attentional networks while younger adults modulate the activity in only a subset of regions. Conversely the younger adults showed a graded level of deactivation across numerous areas of the default network while the older adults did not modulate any regions. A follow-up analysis of this data was performed to assess questions of anatomical variability and homogeneity. Comparisons of peak location revealed that older adults peak location of activation and deactivation were significantly more spread out than those of younger adults. A multiple sphere analysis was done to assess the homogeneity or spread of activation and inhibition. Spheres ranging from 3mm to 13 mm were placed around peak locations of activation and inhibition. The values were normalized by the values of the initial 3mm sphere to account for baseline amplitude differences. The remaining values represent the relative spread of activation and inhibition as indexed by the decline in normalized value with increasing sphere size. Older and younger adults had similar levels of recruitment but the older adults had an impaired extend of inhibition. This indicates that the older adults have a lower peak level of inhibition coupled with a reduced spread of this deactivation. A second study was completed that required subjects to make a male or female judgment to frequently presented names while ignoring rare nouns. Electrophysiological measures of subsequent memory as well as inhibitory control were examined. Both age groups possessed a late positive going event-related potential (ERP) component that was larger for subsequently remembered names. Young adults demonstrated suppressed amplitude of both the N2 and N4 complexes to the more frequent names. The older adults did not show this effect. This was interpreted as an inability of older adults to recognize and utilize the global probability of names in a predictive manner. Coupled with work from the literature, this was thought to reflect an inability to successfully segregate the stimuli into two categorical streams due to a breakdown in inhibitory control. The combination of these analyses depicts older adults as having problems with inhibitory control. The fMRI analyses indicate a reduction in the amplitude and extent of functional deactivation in the brain. The electrophysiological experiment suggests this failed inhibition results from an inability in older adults to segregate incoming stimuli and maintain separate processing streams.
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