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1 1 2 Visuo-motor transformations involved in the escape response to 3 looming stimuli in the crab Neohelice (=Chasmagnathus) granulata. 2 SUMMARY 29 30 Escape responses to directly approaching predators represent one instance of the animals' 31 ability for collision avoidance. Usually, such responses can be easily evoked in the laboratory using 32 two dimensional computer simulations of approaching objects, known as looming stimuli. Therefore, 33 escape behaviors are considered useful models for the study of computations performed by the brain 34 to efficiently transform visual information into organized motor patterns. The escape response of the 35 crab Neohelice (previously Chasmagnathus) granulata offers an opportunity to investigate the 36 processing of looming stimuli and its transformation into complex motor patterns. Here we studied 37 the escape performance of this crab to a variety of different looming stimuli. The response always 38 consisted of a vigorous run away from the stimulus. However, the moment at which it was initiated, 39 as well as the developed speed, closely matched the expansion dynamics of each particular stimulus. 40 Thus, we analyzed the response events as a function of several variables that could theoretically be 41 used by the crab (angular size, angular velocity, etc.). Our main findings were: a) the decision to 42 initiate the escape run is made when the stimulus angular size increases by 7˚. b) The escape run is 43 not a ballistic kind of response, as its speed is adjusted concurrently with changes in the optical 44 stimulus variables. c) The speed of the escape run can be faithfully described by a phenomenological 45 input-output relation based on the stimulus angular increment and angular velocity of the stimulus. 3 INTRODUCTION 50 51 Collision avoidance behaviors are given particular interest in view of their biological 52 importance. In effect, most visual animals are highly efficient in detecting and avoiding collisions, 53 which may occur either by encounters with obstacles while they move, or by moving objects that 54 directly approach them. Natural instances of objects approaching in collision course are the sudden 55 attacks of predators. The maneuvers executed to evade predatory assaults are paramount behaviors 56 that must be controlled by rather straightforward neural circuits to generate quick and reliable 57 avoidance responses. To be effective, those responses need to be executed in a timely manner, which 58 implies that the approaching object must be monitored in real time …