Processing of visual information in dim light : functional variability, matched filtering and spike coding in cockroach (<em>Periplaneta americana</em>) photoreceptors

Abstract

<div lang="en" class="abs"><p class="abs_header">Abstract</p><p>Sensory systems are considered to be optimized for their ecological niche. In vision this means highly organised regular structure and function, where nearly identical photoreceptors have graded light responses in order to be able to handle as much information as possible. Instead, cockroach compound eyes show large amounts of irregularities in their optics and structure, and unusually long axons.</p><p>In this thesis photoreceptors of the cockroach were studied with intracellular recordings of their light responses, biophysical systems analysis, and modelling of the relations between the light stimuli and responses. Cockroaches prefer living in dark or extremely dim environments. However, they have large and complex compound eyes. The aim of this study was to find out the functional properties by which the visual system and especially photoreceptors have adapted to cope with, i.e. to see in, dim light conditions.</p><p>The function of photoreceptors was found to vary randomly in many respects, and the long axons seemed to utilise action potential coding of visual signals. Through model simulations it was shown that signals of a group of these functionally variable and spiking photoreceptors, when pooled, could provide more reliable coding than signals of identical cells of any experimentally characterised type. This naturally sacrifices spatial resolution. The filtering dynamics of the photoreceptors is matched to low light intensities and their temporal resolution does not markedly improve with increasing light adaptation. Adaptation processes in the photoreceptors saturated near an intensity of about 1000 effective photons/s. These are all both unexpected and novel features of photoreceptor function. Spatial summation of functionally different photoreceptors and reduced temporal resolution and contrast coding abilities can be considered to be permanent optimizations to a dim environment.</p></div>