Scalable Rules for Coherent Group Motion in a Gregarious Vertebrate

Individuals of gregarious species that initiate collective movement require mechanisms of cohesion in order to maintain advantages of group living. One fundamental question in the study of collective movement is what individual rules are employed when making movement decisions. Previous studies have revealed that group movements often depend on social interactions among individual members and specifically that collective decisions to move often follow a quorum-like response. However, these studies either did not quantify the response function at the individual scale (but rather tested hypotheses based on group-level behaviours), or they used a single group size and did not demonstrate which social stimuli influence the individual decision-making process. One challenge in the study of collective movement has been to discriminate between a common response to an external stimulus and the synchronization of behaviours resulting from social interactions. Here we discriminate between these two mechanisms by triggering the departure of one trained Merino sheep (Ovis aries) from groups containing one, three, five and seven naïve individuals. Each individual was thus exposed to various combinations of already-departed and non-departed individuals, depending on its rank of departure. To investigate which individual mechanisms are involved in maintaining group cohesion under conditions of leadership, we quantified the temporal dynamic of response at the individual scale. We found that individuals' decisions to move do not follow a quorum response but rather follow a rule based on a double mimetic effect: attraction to already-departed individuals and attraction to non-departed individuals. This rule is shown to be in agreement with an adaptive strategy that is inherently scalable as a function of group size

Low complexity transform competition for HEVC

International audienceThe use of multiple transforms in video coding can lead to substantial bit-rate savings. However, these savings come at the expense of increased coding complexity and storage requirements, which challenge the usability of this approach. In this paper, a systematic procedure is proposed to design low complexity systems making use of transform competition. Multiple trade-offs accommodating the complexity are unveiled and it is demonstrated that they can keep a certain level of performance. Compared to the HEVC standard, some of them provide bit-rate savings around 2% with a 50% increase in the encoding time, using less than 4 kB of extra ROM and no added decoding complexity. © 2016 IEEE