Do weak adapting backgrounds uncover multiple components in the electroretinogram of the horseshoe crab?

The lateral eye of the horseshoe crab, Limulus polyphemus, has been used as a model system for over a century to study visual and circadian processes. One advantage of this system is the relative simplicity of the retina. The input pathway of the retina consists of photoreceptor cells that are electrically coupled to the dendrite of a second-order cell, which sends action potentials to the brain. Electroretinograms (ERGs) recorded from the lateral eye show a biphasic shape, with a leading negative wave and a later positive peak. The purpose of these experiments was to determine whether adapting backgrounds could be used to uncover multiple adaptation mechanisms within the ERG. To test this idea, ERGs were elicited using variable intensity flashes presented under dark-adapted conditions, as well as in the presence of weak adapting backgrounds. Flashes and backgrounds were generated using green LEDs (lambda max = 525 nm) under software control. ERGs were recorded using a corneal wick electrode placed on the lateral eye of the horseshoe crab. Preliminary results suggest that ERGs recorded in the presence of adapting backgrounds are linearly scaled versions of dark-adapted FRGs. This suggests that there is a single adaptation stage in the Limulus retina. This is in contrast with analogous results from mammals, including mouse, cat and monkey, which show multiple stages of adaptation within their more complex retinas.