Abstract
The responses of the lamina ganglionaris in the compound eye of the fly Sarcophaga bullata to light stimulation were investigated. Measurements made at known depths from the corneal surface and upon visual placement of extracellular electrodes in the lamina of decapitated animals showed that the response to back‐ground illumination consisted of slow potentials which were mainly monophasic, 10–15 mv positive, and arose from a negative DC bias of 30–90 mv existing between the lamina and a neutral electrode placed in the thorax. These responses and DC characteristics differed markedly from those observed in adjacent regions of the visual system and allowed accurate placement of microelectrodes in the lamina from the rear in intact animals. The amplitude of the monophasic positive portion of the laminar response was used as a guide to adjust the position and angle of incidence of a focused light beam to yield a maximum response. Following a 10 to 15 minute period in the dark, flashes of light evoked responses of up to 65 mv, with usual values of 30 to 45 mv. These dark‐adapted responses were characterized by an initial dynamic phase followed by a post dynamic repolarization and finally a static phase that was maintained for the duration of the stimulus. The static phase was attained within 50 ms of the onset of stimulation. The latency of the rising limb of the dynamic phase had a minimum value of 4.5 ms. The laminar action potential is nearly identical to the receptor potential (described by others) in form, latency, and dependence on light intensity. It is concluded that the laminar response is dominated by activity in a DC, low resistance, electronic pathway between retinular units and a site or sites in the lamina ganglionaris.