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Supplementary material for the paper:

From Solitary to Collective Behaviours:
Decision Making and Cooperation

Vito Trianni, Christos Ampatzis, Anders Lyhne Christensen,
Elio Tuci, Marco Dorigo, and Stefano Nolfi

This page contains support material of the paper "From Solitary to Collective Behaviours: Decision Making and Cooperation" submitted to the 9th European Conference on Artificial Life (ECAL 2007). In particular, we provide a detailed description of the experimental setup, which was left out due to space limit. We also provide an extensive behavioural analysis of the obtained results, supported by videos of the evolved behaviours. Finally, we describe the behaviours produced by all controllers that make use of communication.

Communicative Behaviours

Communication characterises also the behaviour produced by some controllers that do not belong to class C: C4,C7,C16,C19. The signalling behaviour and the modalities with which perceived signals are exploited significantly differ among these controllers, and will be analysed individually.


Controller C4

Controller C4 belongs to class U, but it presents an interesting behaviour that exploits communication. A continuous signal is emitted when s-bots are positioned over a white floor, while no signalling is performed over the circular band. Whenever a signal is perceived, s-bots either move straight or they loop over the circular band. S-bots leave the circular band as soon as no signal is perceived, and they move towards the opposite side. Finally, s-bots avoid each other by changing the direction of motion.

In environment B, the above rules produce an oscillatory behaviour of the s-bots, which travel back and forth from the circular band. However, s-bots never remain close to each other for a long time, and generally obtain low fitness.
The communication strategy employed here has the side effect of synchronising the movements of the s-bot, in a similar way to what described in another study on synchronisation (see Trianni and Nolfi, 2007). In fact, s-bots leave the circular band only when no signal is perceived. Given that signalling ceases only when all s-bots are on the band, it results that they leave the band synchronously.

When placed in environment A, s-bots perform the same oscillatory movements. However, in this case s-bots have the chance to find the way out and to move away from the arena centre. When this happens, s-bots emit a continuous signal, which is perceived by the other s-bots and is exploited to continue looping over the circular band and eventually exit. Similarly to what happens with class C controllers, communication is exploited here to share the information belonging by the individual s-bots: if an s-bot finds the way out, it emits a signal that is successfully exploited by the rest of the group.


Controller C16

ControllerC16 belongs to class B, therefore it produces a systematic search behaviour when s-bots are in state S, and a "bouncing" aggregation when s-bots switch to state C. However, the switch from state S to state C is performed exploiting sound signals, as described below.

The robots do not signal while in state S. However, the first s-bot that switches from state S to state C emits a signal, which triggers the behavioural switch also in the other s-bots (see the video: s-bots shortly signal before leaving the circular band). In this way, the aggregation process is more precise and faster, and results in a high performance of the group.


Controller C7

Robots controlled byC7 also make use of communication. However, in this case they exploit it to allocate different roles within the group.

While robots are looping on the circular band in search of the way out, they emit short sound signals in order to decide who takes the leader role. If the way out is not encountered, eventually one s-bot starts emitting a continuous tone and stops on the circular band, therefore becoming the "leader". The other s-bots react to the perception of a continuous signal by taking a "follower" role: they continue to travel on the circular band without signalling, until they aggregate with the leader. The main drawback of this strategy is that the role allocation may take a long time, sometimes exceeding the duration of a trial. Aggregation may also take a long time, because s-bots may need to travel across the whole circular band before meeting the leader. The variability of the role allocation process deeply influences the success rate in environment B.

When placed in environment A, the s-bots that find the way out stop any signalling, therefore dropping out from the role allocation process.


Controller C19

ControllerC19 belongs to the class M. Apparently, the group dynamics produced by this controller are identical to the other controllers of the same class, but they present a much smaller coverage of the circular band, which contrasts with the individual decision making mechanism described above. In this case, s-bots exploit communication to collectively switch from state S to state C, as described below

s-bots integrate over time the intermittent sound signals they produce while searching the circular band for the way out. The more s-bots are contemporary searching, the more signals are produced and integrated over time, the sooner s-bots switch from state S to state C. In order to confirm this hypothesis, we performed some test varying the group size from 1 to 4, and we observed that larger groups are faster in taking a decision than smaller ones.

In environment A, robots stop signalling when they find the way out, and therefore they slow down the integration over time performed by those robots that are still searching, which remain in the state S for a longer time. This communicative behaviour is very interesting not only because it exploits the signals emitted by the group to take a collective decision, but also because it takes into account the amount of signals perceived and the time at which those signals are emitted.


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