Normal Tritan Discrimination in Amblyopia Suggests Preservation of Koniocellular Function

Abstract

Background: Human stereoscopic vision is a psychological abstraction emerging from sufficiently grouped monocular signals within a broader unity of simultaneous binocular perception. Incongruent central signals inevitably arise when moving through a visual landscape, due to changing asymmetries of perception, producing signal rivalry. Humans have an evolved capacity to dynamically inhibit incongruous or ambiguous signals in order to maintain a coherent unity of stereoscopic perception, a facility sometimes exploited by optical illusions. Unlike the beneficial mental construct that arises from physiological inhibition, the amblyopic inhibition is permanently unilaterally destructive, has a large field size, is associated with irreversible signal inhibition at the dorsal lateral geniculate nucleus (dLGN), and leads to structural alterations in dLGN that can be demonstrated histologically. Recent reports suggest amblyopic signal inhibition at the dLGN may be selective, preferentially involving the parvocellular system and sparing the koniocellular system. It is not yet known whether physiological inhibition is also pathway specific, but it would be surprising if it were not. The two chromatic opponent systems display differing vulnerabilities in a number of disease states, including optic neuritis and toxic neuropathies, and they followed independent evolutionary paths during phylogenetic development from small primates into apes and humans with larger eyes. Tritan contrast is conducted exclusively by the koniocellular system as far as the striate cortex, so clinical tests of tritan contrast may provide information about koniocellular function in humans. If koniocellular function is not inhibited in amblyopia then tritan discrimination tests may be normal in amblyopic eyes. Objective: In this study the C test for tritan discrimination was employed as a proxy for koniocellular function to discover whether tritan discrimination was affected in proportion with achromatic contrast in the amblyopic eye of 18 amblyopic adults. Method: The tritan contrast and logmar visual acuity results were compared between eyes, using two non parametric statistical tests. Results: The C test scores for tritan discrimination were statistically similar in the normal and amblyopic eyes, but mean logmar acuity scores were significantly different, with a mean of 0.76 in the amblyopic eyes compared to 0.18 in the non-amblyopic eyes. Conclusion: This finding is consistent with preservation of koniocellular signal perception despite parvocellular signal inhibition in amblyopic eyes.