Abstract
In the Behavioural Inhibition System (BIS) theory, Gray and McNaughton (2000) propose that goal-directed behaviour in response to an event can be effectively classed as “approach” or “avoidance”. When opposing approach and avoidance tendencies are equal, neither tendency can easily direct the motor system and we experience goal conflict. The hippocampus is a key structure in the BIS. It operates to resolve goal conflict by amplifying the subjective repulsive value of conflicting tendencies through recursive loops, thus increasing behavioural avoidance. Coordination of these loops is maintained by resonating at “theta” (4-12Hz) frequency in rodents. Thus the BIS contains both a behavioural and neural component of goal conflict. It is primarily a rodent model, but recent efforts have been made to extend this to humans. Theta activity directly produced in the hippocampus cannot be recorded from the scalp. During goal conflict resolution however, human theta (4-7Hz) superficial in the cortex could be modulated by hippocampal theta. During these times it could be possible to measure a component of human hippocampal theta at the scalp. Thus, in the current work theta spectra power in the human range of 4-7Hz was recorded at the scalp.
Neo and McNaughton (2011) assessed goal conflict theta in humans using a simple choice task which manipulated monetary gain and loss. They found goal conflict theta localised to the right frontal cortex. They also found a distinct separation between theta specific to goal conflict and net avoidance. However, money as an incentive value may differ largely between individuals. One objective of the present thesis was to find a stimulus that would have equal incentive value between individuals. It was hoped that manipulation of this stimulus could be used to elicit right frontal goal conflict theta.
In the present thesis, goal conflict was assessed in a similar choice experiment termed “The Bubble Task”. Two variations of the task were tested. In one, bubble rewards were representative of money and in the other, they were not. Three key conditions were created. Goal conflict was maximal in the intermediate condition when gain and loss value were equal. Net gain and net loss were maximal in adjacent conditions when gain and loss value were unequal. Goal conflict processing was calculated as theta in the intermediate condition minus the average of theta in adjacent conditions. Defensive personality and behavioural aversion was measured by questionnaire for assessment of correlation with theta changes.
Changes in goal conflict theta in the human range of 4-7Hz, were distinct from net loss theta. Increases in both goal conflict and net loss theta were found only with monetary motivation, and were observed at the right frontal cortex at F8 but not solely localised to this site. Increases in goal conflict and net loss theta showed no correlation with defensive personality or behavioural aversion. Overall, the present experiment partly supports the existence of a neural component (but not the behavioural component) of a human BIS similar to that in rodents, but only when monetary incentive is used.