Directional escape to visual stimuli: retinotopic organization and response properties of lobula giant neurons in the crab.

MEDAN, VIOLETA; TOMSIC, DANIEL

Washngton, DC

Congreso; 38th Annual Meeting of the Society for Neuroscience; 2008

Society for Neuroscience

While running away from a predator, Chasmagnathus crabs continuouslyadjust direction and velocity of escape according to variations in thetrajectory and speed of the visual danger stimulus. These adjustments requirea continuous visual feedback. We have previously identified a group oflobula giant (LG) neurons of the crab that are thought to play a central role inthe organization of behavioral responses to visual stimuli. By intracellularrecordings in the intact animal we investigated LG responses to a variety ofvisual stimuli. Recordings followed by intracellular dye injections allowedestablishing unequivocal relations between the morphology and responsesensitivity for each neuron. These studies revealed that the group of LG´s iscomposed of at least 4 different classes of neurons termed MLG1, MLG2,BLG1, and BLG2, all of which project to the midbrain. Each one of the lastthree classes is composed of few or even a single element, whereas MLG1class contains 14 elements retinotopically arranged throughout the lobula.Here we analyze the correspondence between receptive field (RF) andanatomical position in the lobula of MLG1 neurons. Results show thatneurons from the medial, central and lateral part of the lobula have their RFrespectively in the lateral, frontal and contralateral visual field of the animal.Thus, the 14 MLG1 cells, with their overlapping dendritic arborizationslying serially, portrait the entire panorama. We hypothesize that theinformation on the spatial location of motion stimuli that elicit directionalescape responses in crabs is encoded in the concerted activity of the MLG1ensemble.The columnar retinotopic area covered by the dendrites of each MLG1 is~10% of the lobula length. Considering that the monocular visual field ofcrabs span 360º and that the 14 MLG1 cells are uniformly distributed acrossthe lobula, the receptive field of each MLG1 should amount ~36º. However,the measured RF of MLG1´s is about three times wider (between 90-120º).Thus, either inputs from unknown wide field multicolumnar elements areinvolved or the RF of the MLG1 is built by adding interactions amongseveral MLG1 cells. The later possibility is supported by the fact thatintracellular neurobiotin injections of an MLG1 cell often result in thestaining of neighboring cognate cells. MLG1 neurons present intimatecontact both at the dendritic and axonal level, where electrical coupling maytake place. We hypothesize that lateral electrical connections among LGM1cells would form the basis of their unexpectedly wide RF, whichsubstantially exceeds the proportion of the lobula covered by the dendrites ofa single MLG1.