Visuo-motor transformations involved in the escape response to looming stimuli in the crab Neohelice (=Chasmagnathus) granulata

Abstract

SUMMARYEscape responses to directly approaching predators represent one instance of an animalʼs ability to avoid collision. Usually, such responses can be easily evoked in the laboratory using two-dimensional computer simulations of approaching objects, known as looming stimuli. Therefore, escape behaviors are considered useful models for the study of computations performed by the brain to efficiently transform visual information into organized motor patterns. The escape response of the crab Neohelice (previously Chasmagnathus) granulata offers an opportunity to investigate the processing of looming stimuli and its transformation into complex motor patterns. Here we studied the escape performance of this crab to a variety of different looming stimuli. The response always consisted of a vigorous run away from the stimulus. However, the moment at which it was initiated, as well as the developed speed, closely matched the expansion dynamics of each particular stimulus. Thus, we analyzed the response events as a function of several variables that could theoretically be used by the crab (angular size, angular velocity, etc.). Our main findings were that: (1) the decision to initiate the escape run is made when the stimulus angular size increases by 7deg; (2) the escape run is not a ballistic kind of response, as its speed is adjusted concurrently with changes in the optical stimulus variables; and (3) the speed of the escape run can be faithfully described by a phenomenological input-output relationship based on the stimulus angular increment and the angular velocity of the stimulus.

Differences in the escape response of a grapsid crab in the field and in the laboratory

“…The decision to escape underground, however, does depend on angular size or the increase in angular size. The median angular size increase at the time the crabs retreated underground was 4.0 deg, which is substantially smaller than the 7 deg measured in the laboratory for the initiation of the escape response (Oliva and Tomsic, 2012).…”

“…In comparison to the field results for U. vomeris, N. granulata escape responses are triggered much later, when the stimulus has reached a much larger angular size (Hemmi and Tomsic, 2012;Oliva and Tomsic, 2012). The response criterion that initiates escape is based on a fixed increase in angular size of the stimulus (Oliva and Tomsic, 2012).…”

“…5C, Table 2; N=606, 54 crabs, P=0.40). This indicates that angular size, or a fixed increase in angular size (Oliva and Tomsic, 2012), played a major role in the crabs' decision to retreat underground. Predictably, if we recalculate the angular size of the large dummy at the time of the response as if it was the same size as the small dummy, the two survival curves, plotted against angular size, clearly separate again (Fig.…”

“…Crabs on a treadmill run directly away from the stimulus, and the speed (Oliva and Tomsic, 2012) and direction (Medan et al, 2015) of their escape are continuously adjusted according to visual information. In comparison to the field results for U. vomeris, N. granulata escape responses are triggered much later, when the stimulus has reached a much larger angular size (Hemmi and Tomsic, 2012;Oliva and Tomsic, 2012). The response criterion that initiates escape is based on a fixed increase in angular size of the stimulus (Oliva and Tomsic, 2012).…”

Object approach computation by a giant neuron and its relation with the speed of escape in the crab Neohelice

“…Such behaviors have been investigated in connection with animal navigation (e.g. Srinivasan and Zhang, 2004), and less often in connection with predator avoidance (Land and Layne, 1995;Oliva and Tomsic, 2012;Medan et al, 2015).…”