Visual parameters of looming stimuli that trigger the escape of a crab and neuronal correlates

DAMIAN OLIVA, VIOLETA MEDAN, DANIEL TOMSIC

Vancouver, Canada

Congreso; Eighth International Congress of Neuroethology; 2007

International Sosciety of Neuroethology

An object that approaches on a collision course (looming stimuli) provokes in Chasmagnathus a strong and highly 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 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 the object approach are the ones considered by the crab to initiate the escape, we challenged the animals with a set of 12 different looming stimuli that varied either in their size or speed of approach. The different stimuli triggered escapes at times that were significantly different from each other. We then analyzed the values reached by the image expansions (angular size, retinal speed, etc) at the time the escapes were launched. Results strongly suggest that the decision for initiating the escape is highly determined by a fixed increase in angular size. The temporal course of the running response also varied significantly with the different looming stimuli. But for every stimuli the speed of run closely matched the dynamic of the image expansion. The results demonstrate that the escape is accurately controlled by visual feedback. In addition, we show that a group of previously identified neurons, called M1 neurons, respond to the different stimuli by increasing their rate of firing in a way that closely matches the dynamic of each particular image expansion. We hypothesize that M1 neurons encode information about object approach and convey such information downstream to determine the speed of escape run.