INTRACELLULAR CHARACTERIZATION OF NEURONS IN THE LOCUST BRAIN SIGNALING IMPENDING COLLISION

1. In response to a rapidly approaching object, intracellular recordin gs show that excitation in the locust lobula giant movement detecting (LGMD) neuron builds up exponentially, particularly during the final s tages of object approach. After the cessation of object motion, inhibi tory potentials in the LGMD then help to terminate this excitation. Ex citation in the LGMD follows object recession with a short, constant l atency but is cut back rapidly by hyperpolarizing potentials. The timi ng of these hyperpolarizing, potentials in the LGMD is variable, and t heir latency following object recession is shortest with the highest v elocities of motion simulated. The hyperpolarizing potentials last fro m 50-300 ms and are often followed by re-excitation. The observed hype rpolarizations of the LGMD can occur without any preceding excitation and are accompanied by a measurable conductance increase. The hyperpol arizations are likely to be inhibitory postsynaptic potentials (PSPs). The behavior of the intracellularly recorded inhibitory PSPs (IPSPs) closely parallels that of the feed forward inhibitory loop in the neur al network described by Rind and Bramwell. 2. The preference of the LG MD for approaching versus receding objects remains over a wide range o f starting and finishing distances. The response to object approach, m easured both as membrane potential and spike rate, remains single peak ed with starting distances of between 200 and 2,100 mm, and approach s peeds of 0.5-2 m/s. These results confirm the behavior predicted by th e neural network described by Rind and Bramwell but contradicts the fi ndings of Rind and Simmons, forcing a re-evaluation of the suitability of some of the mechanical visual stimuli used in that study. 3. For d epolarization of the LGMD neuron to be maintained or increased through out the motion of image edges, the edges must move with increasing vel ocity over the eye. Membrane potential declines before the end of edge motion with constant velocities of edge motion. 4. A second identifie d neuron, the LGMD2 also is shown to respond directionally to approach ing objects. In both the LGMD and LGMD2 neurons, postsynaptic inhibiti on shapes the directional response to object motion.