Collision avoidance
responses to looming stimuli: behavioral and neuronal analises in the
crab Chasmagnathus.
Daniel
Tomsic,
Damian Oliva, Violeta Medan
Laboratorio
de Neurobiología de la Memoria. Dpto. Fisiología
Biología Molecular y Celular. Universidad de Buenos Aires.
IFIBYNE-CONICET. Argentina
An
object that approaches on a collision course (looming stimulus)
triggers in the crab Chasmagnathus
a strong and directional escape response. Recently, we have shown
that this response can be reliably elicited and precisely measured in
the laboratory. Moreover, by performing in
vivo intracellular
recordings from the optic lobe, we found two classes of lobula giant
neurons that appear to play a key role in the behavioral response to
looming stimuli (Oliva et al. J. Exp. Biol. 2007). To investigate
what features of the image expansion generated during object
approaches are taken into account by the crab to initiate the escape
and to determine the speed of its run, we challenged the animals with
a set of different looming stimuli that varied either in their size
or speed of approach. Our results indicate that regardless the
dynamic of the image expansion, the escape always initiates when the
stimulus angular size increases by 10 degrees. Following launch, the
speed of the run goes along with the increase in the borders velocity
of the expanding image. Thus, the maximum speed of run is reached at
the end of the expansions, after which the crab immediately
decelerates. Then, we evaluated the response of two classes of lobula
giant neurons named MLG1 and MLG2 to the same set of looming stimuli
and found a remarkable correspondence with the behavioral
performance. In fact, the image borders velocity appears to be
greatly encoded by the rate of firing of MLG1 and MLG2, which in turn
correlates with the running speed of the animal. Our behavioral
experiments clearly show that the escape performance of the crab is
closely and continuously guided by the visual imput. On the other
hand, the electrophysiological experiments show that the activity of
MLG1 and MLG2 closely match the dynamic of the expanding images as
well as the speed of run. Because MLG1 and MLG2 project centripetally
to the midbrain, these elements are good candidates to convey
information on object approaches that can be used downstream to
organize the corresponding motor pattern.