Mechanisms for Neural Signal Enhancement in the Blowfly Compound Eye

In the blowfly Calliphora vicina visual signals are enhanced by amplification and antagonism as they pass from the site of phototransduction in the retina to secondorder neurones (LMCs) in the first optic neuropile, the lamina. The mechanisms responsible for amplification and antagonism were investigated, using currentclamp techniques, to examine the conductance mechanisms generating LMC responses. LMCs responded Ohmically to injected current. Voltage-sensitive conductances and feedback mechanisms driven by the potential of a single LMC played a minor role in shaping responses. The LMCs response to an increment in illumination, a transient hyperpolarization, was generated by a large and transient conductance increase with a reversal potential close to the maximum response amplitude (30-40mV below dark resting potential). The depolarization of the LMC in response to a decrement in light intensity was partially generated by a reduction in direct synaptic input from the photoreceptors. Changes in depolarizing conductances with positive reversal potentials played a secondary role, contributing to large-amplitude responses to dimming or light-off, and to the slow decay of the LMC response to steady illumination. Antagonism, including lateral antagonism, operated principally by shutting down the direct photoreceptor input, presumably by presynaptic regulation. The results of dye injection suggested that the identified large monopolar cell L2 is more strongly affected by lateral antagonism than the similar cells LI and L3. We conclude that LMCs are essentially passive integrators of a well-regulated direct input from the photoreceptors. This suggests that the intrinsic properties of photoreceptor-LMC synapses and presynaptic interactions are primarily responsible for amplification and antagonism.