Modeling Hydra Behavior Using Methods Founded in Behavior-Based Robotics

38 Chapter 7. Results and discussionTransfer functionThe TF of B3 share the same task as the one of B2, namely to evoke a CP or an LP,whichever is applicable. Eqs. 7.9 and 7.10 were applied to determine response type alsoin this case. Similar to B2, the time for the applicable response was set to t + L l , wheret is the time for activation of the AC, and L l is the latency as determined above. Also inthis case, the duration of a CP was set to 10 s, and the duration of an LP to 60 s.7.1.4 B4: FeedingThe feeding behavior of Hydra is activated by the presence of GSH and, as discussedin Section 5.1.3, both activation threshold and duration of the response depend on thenutritional state of the animal. Throughout the feeding response, there is a total inhibitionof other behaviors [52].Applicability clauseAlso for B4, an event-driven AC was used. The activation threshold and duration ofresponse were determined with respect to measured data from experimental studies of theanimal.The threshold for activation of the feeding behavior in Hydra depends on time ofstarvation. In [31], a quantitative study of this relationship was conducted. The resultsindicates an exponential decrease in the feeding threshold, T f , for an increase in starvationtime. In a least square function fitting to the measured data, Eq. 7.12 was obtained. Theresults are shown in Fig. 7.8.T f =1.29 · 104, (7.12)Mh1.95where M h is increased by 1 per second while the animal is not feeding, see Eq. 6.5.The duration of B4 have been studied in [33], and indicates a gradual recovery ofthe ability to respond following a prior feeding response. The following relation wasdetermined for the ability of feeding durationD f = 1800 ( 1 − e −0.2x) , (7.13)where x = M h3600. Fig. 7.9 shows the gradual recovery.Activation of the AC occurs if M c > T f , while the AC is reset if t B4 > D f , or if M c = 0,where t B4 is the time for which B4 has been active and D f is the ability of duration of thefeeding response at the time of activation of B4. Thus, feeding occurs until repletion oruntil the presence of food is no longer detected by the sensory system.