Can invertebrates see the e-vector of polarization as a separate modality of light?

Thomas Labhart

doi:10.1242/jeb.139899

Abstract

ABSTRACTThe visual world is rich in linearly polarized light stimuli, which are hidden from the human eye. But many invertebrate species make use of polarized light as a source of valuable visual information. However, exploiting light polarization does not necessarily imply that the electric (e)-vector orientation of polarized light can be perceived as a separate modality of light. In this Review, I address the question of whether invertebrates can detect specific e-vector orientations in a manner similar to that of humans perceiving spectral stimuli as specific hues. To analyze e-vector orientation, the signals of at least three polarization-sensitive sensors (analyzer channels) with different e-vector tuning axes must be compared. The object-based, imaging polarization vision systems of cephalopods and crustaceans, as well as the water-surface detectors of flying backswimmers, use just two analyzer channels. Although this excludes the perception of specific e-vector orientations, a two-channel system does provide a coarse, categoric analysis of polarized light stimuli, comparable to the limited color sense of dichromatic, ‘color-blind’ humans. The celestial compass of insects employs three or more analyzer channels. However, that compass is multimodal, i.e. e-vector information merges with directional information from other celestial cues, such as the solar azimuth and the spectral gradient in the sky, masking e-vector information. It seems that invertebrate organisms take no interest in the polarization details of visual stimuli, but polarization vision grants more practical benefits, such as improved object detection and visual communication for cephalopods and crustaceans, compass readings to traveling insects, or the alert ‘water below!’ to water-seeking bugs.

Highlights

The visual world provides an abundance of linearly polarized light stimuli hidden from the human eye, but many invertebrate organisms exploit polarized light as a source of useful visual information
The role of polop neurons in the celestial compass In polarization vision systems used for navigation purposes, where rough categorizing of e-vector orientations does not suffice and where gaining exact directional information is crucial, the dipolatic ommatidia of the dorsal rim area (DRA) represent just the first level of analysis
It cannot be excluded that polarization and luminance images are combined at some level in the brain (How et al, 2014), confounding true polarization vision again. In both object-oriented polarization vision and water-surface detection, the orthogonally dipolatic ommatidia show a constant orientation in the retina such that the microvilli are directed horizontally or vertically (Fig. 5A)

Summary

Introduction

The visual world provides an abundance of linearly polarized ( plane-polarized; see Glossary) light stimuli hidden from the human eye, but many invertebrate organisms exploit polarized light as a source of useful visual information. Polop neurons Neurons receiving opponent input from two polarization-sensitive analyzer channels with different e-vector tuning axes; input from two photoreceptor populations with orthogonal microvilli orientations in individual ommatidia.

Results

Conclusion