Adaptation adjusts a neuron’s operating range to its input. When sensory systems use population codes, selective adaptation of a subset of neurons could corrupt coding. To see how adaptation influences the coding of optic flow we recorded from motion-sensitive Tangential Neurons (TNs) in the lobula plate of the blowfly Calliphora vicina.

Each TN’s receptive field is matched to the pattern of optic flow induced by a certain rotation or translation. A population of about 50 TNs codes information on ego-motion the fly uses for flight and gaze stabilisation. The matched receptive field of a TN is formed by combining inputs from Elementary Motion Detectors (EMDs) aligned along the 3 axes of the hexagonal sampling lattice of the compound eye (Krapp et al. 1998). To monitor the horizontal motion induced by yaw the spiking Tangential Neuron H1 combines inputs from two sets of EMDs, aligned at +30 degrees and -30 degrees to the horizontal. We investigated whether adaptation of H1 to motion in one direction alters the directional tuning of its receptive field.Extracellular recordings were made from the lobula plate of 4 restrained and un-anaesthetised blowflies. We used the rotating dot method (Krapp and Hengstenberg, 1997) to measure the directional tuning of a small region of H1’s receptive field immediately before and after a 5 s presentation of an adapting grating. The determination of directional sensitivity took less than 1s. The adapting grating (contrast>90% contrast frequency 12 Hz, spatial wavelength 5 degrees) reduced H1’s response by 50 %. The grating was oriented at 45 to the horizontal to selectively stimulate one set of EMDs. A linear model of directionally selective adaptation of EMDs predicted that this grating should shift H1’s preferred direction of motion by approximately 10°. However, local mapping of H1’s receptive field failed to find a statistically significant shift in directional sensitivity (Student’s unpaired t test, n = 40, P < 0.1). Changing the direction and contrast frequency of the adapting stimulus did not change this finding.

Our results show that strong adaptation to motion in one direction does not distort the receptive field of H1. This suggests that adaptation in H1 is dominated by the non-directional mechanism (Harris et al. 2000). The use of non-directional adaptation to preserve receptive field structure is an important prerequisite for the coding of self-motion by a population of neurons.

This work was supported by the CT Taylor Fund (GC), the Royal Soc (HGK), and the Rank Price Fund (SBL)