A Clearer View from Fuzzy Images

Rather than having a single layer of photoreceptor cells, these eyes have a multitiered retina with four distinct photoreceptor layers. Nagata et al. investigated the spectral sensitivity of these retinal layers using a combination of molecular biology and electrophysiology. Opsins, a group of genes coding for specific color sensitivity (8, 9), were identified in the four retinal layers by gene sequencing, expression analysis, and in situ hybridization. Electrophysiology revealed that layers 1 and 2 are maximally sensitive to green light, whereas layers 3 and 4 are maximally sensitive in the ultraviolet. These sensitivities matched the identified opsin genes in the retinal layers. Curiously, however, even though layers 1 and 2 contain primarily green-sensitive photoreceptors, the effect of chromatic aberration means that incoming green light is only clearly focused on layer 1. This means that the second main green-sensitive retinal layer (layer 2) receives a defocused or “fuzzy” image. Nagata et al. suggest that the spiders obtain depth cues from the amount of defocus in this layer, which is proportional to the distance of the object to the lens.

Science 27 January 2012:
Vol. 335 no. 6067 pp. 409-410 

Depth Perception from Image Defocus in a Jumping Spider

Abstract

The principal eyes of jumping spiders have a unique retina with four tiered photoreceptor layers, on each of which light of different wavelengths is focused by a lens with appreciable chromatic aberration. We found that all photoreceptors in both the deepest and second-deepest layers contain a green-sensitive visual pigment, although green light is only focused on the deepest layer. This mismatch indicates that the second-deepest layer always receives defocused images, which contain depth information of the scene in optical theory. Behavioral experiments revealed that depth perception in the spider was affected by the wavelength of the illuminating light, which affects the amount of defocus in the images resulting from chromatic aberration. Therefore, we propose a depth perception mechanism based on how much the retinal image is defocused.  

Science 27 January 2012:
Vol. 335 no. 6067 pp. 469-471