Comparing the effects of normal ageing on dorsolateral and orbitofrontal functioning using event-related potentials : further clarification of the frontal lobe hypothesis of cognitive ageing / Pia Van Beek

2009

[Truncated abstract] The prefrontal cortex is considered vulnerable to early and rapid age-related decline, however research has yet to clarify how normal ageing differentially impacts upon the sub-regions of the prefrontal cortex. The purpose of the current research was therefore to investigate age-related change within two prefrontal sub-regions, namely the dorsolateral and orbitofrontal cortices, in an attempt to further refine the frontal lobe hypothesis of cognitive ageing. Previous research claims that reversal learning is a common function associated with the orbitofrontal cortex, while increasing memory load relies heavily upon dorsolateral functioning. A delayed-response paradigm was therefore modified to independently assess purported functioning within the orbitofrontal and dorsolateral regions, by manipulating the frequency of rule reversal and the memory load demand, respectively. The basic task procedure involved the presentation of a rule ("match" or "non-match") followed by a memory set to be maintained across a retention interval. A pair of items was then presented and participants decided which item matched (or did not match) one of the items in the memory set. Three task conditions were created: a baseline condition (low memory load/low reversal); a high memory load condition (high memory load/low reversal); and a high reversal condition (low memory load/high reversal). The aim of Study 1 was to compare the difficulty of the high memory load and high reversal conditions, relative to the baseline task, in a group of younger participants (N = 24). Compared to the baseline condition, performance was significantly less accurate and slower for the high memory load and high reversal conditions. ..... Within the older group, individuals were identified as scoring above (Non-Declining group) or below (Declining group) expected on a measure of fluid

intelligence, with reference to their scores on a measure of crystallised intelligence. Accuracy was

similar between age groups during the high reversal condition, and similar reversal-related positivity (300-600 ms following rule presentation) was observed for the two age groups. For both groups, reversal-related positivity was localised within the orbitofrontal cortex. On the other hand, agerelated behavioural impairment emerged during the high memory load condition, limited to the Declining group. In both the older and younger groups, greater positivity (1024-2124 ms following memory item presentation) was elicited for the high, relative to the low, memory load condition. This memory-related positivity was larger for the older, relative to the younger, group at parietal sites only, and did not differentiate between the Declining and Non-Declining groups. Older and younger participants recruited the dorsolateral and parietal cortices during the high memory load condition. During this condition, the Non-Declining group relied upon more widespread scalp-recorded activity, possibly reflecting successful compensatory reorganisation. Those in the Declining group failed to recruit this broader network, suggesting that processing inefficiencies may have contributed to the observed performance decrements. Overall, the results further clarify the frontal lobe hypothesis of cognitive ageing, suggesting that tasks associated with dorsolateral functioning are more sensitive to normal ageing, relative to those associated with the orbitofrontal cortex.

Thesis (Ph.D.)--University of Western Australia, 2009

http://repository.uwa.edu.au:80/R/-?func=dbin-jump-full&object_id=13192&silo_library=GEN01