Effect of γ parameter
Parameter γ is the recency weight included in the equation describing the update of the activation level of the experienced activity profiles (Eq. (9.13)). In this equation, the logarithmic form is used to account for saturation. Specifically, when an activity profile is newly experienced, its activation level rapidly increases, until reaching a saturation point, where the activation level increase slows down. Thus, the higher the value of 7, the higher the impact of the experience and the faster the increase of the activation level of this activity profile. As 7 increases, the activation values become stronger and memory fading takes more time, which actually implies that the agents 'stick' more to their previous choices. On the other hand, as 7 decreases, the activation value decreases faster (during the interval time of experiencing again the same combination of activity type and context-condition), resulting in a greater flexibility and more exploitations of the choice set (Figure 9.6).
Regarding the overall expected utilities of the choice modes (Figure 9.7), the habitual and exploitation choice mode seem to be slightly affected (as 7 increases, there is a tendency to 'stick' more to previous choices, which seems to result in a slight decrease of the overall expected utility of the habitual and exploitation choices). Similarly, the mean overall expected utility of the choice set also seems to slightly decrease, as 7 increases. This is plausible because the agents exploit the choice set better when 7 is lower and, therefore, the alternatives that perform well have a lower chance to be discarded from memory. The size of the choice set, as
Figure 9.6: Effect of γ parameter on frequency of choice-modes (exploitation, exploration) and lowering aspirations incidents.
Figure 9.7: Effect of 7 parameter on overall expected utility of choice-modes (habit, exploitation and exploration).
Figure 9.8: Effect of 7 parameter on aspiration values.
well as the aspiration values are not affected by 7 (Figure 9.8), as this parameter only influences the impact of an experience event on activation memory.