Null point of discrimination in crustacean polarisation vision

Abstract

The polarisation of light is used by many species of cephalopods and crustaceans to discriminate objects or to communicate. Most visual systems with this ability, such as that of the fiddler crab, include receptors with photopigments that are oriented horizontally and vertically relative to the outside world. Photoreceptors in such an orthogonal array are maximally sensitive to polarised light with the same fixed e-vector orientation. Using opponent neural connections, this two-channel system may produce a single value of polarisation contrast and, consequently, it may suffer from null points of discrimination. Stomatopod crustaceans use a different system for polarisation vision, comprising at least four types of polarisationsensitive photoreceptor arranged at 0, 45, 90 and 135 deg relative to each other, in conjunction with extensive rotational eye movements. This anatomical arrangement should not suffer from equivalent null points of discrimination. To test whether these two systems were vulnerable to null points, we presented the fiddler crab Uca heteropleura and the stomatopod Haptosquilla trispinosa with polarised looming stimuli on a modified LCD monitor. The fiddler crab was less sensitive to differences in the degree of polarised light when the e-vector was at −45 deg than when the e-vector was horizontal. In comparison, stomatopods showed no difference in sensitivity between the two stimulus types. The results suggest that fiddler crabs suffer from a null point of sensitivity, while stomatopods do not.

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

“…Stimulus detection was strongly impaired when the e-vectors of stimulus and background were at +45 deg and −45 deg to the horizon, respectively, which agrees with an expected null point of discrimination of a horizontal/vertical 2D system (Bernard and Wehner, 1977; How et al, 2014). The stomatopods also tested in that study for comparison (How et al, 2014) did not exhibit a null point, which is not unexpected considering their various separate and differently oriented 2D systems (Box 1). Alternatively, they could have used a successive approach based on eye stalk rotation (Daly et al, 2016; Land et al, 1990).…”

“…But most behavioral observations can be explained by the mere presence of polarization-sensitive photoreceptors without opponent signal interactions (Fig. 4C), at least qualitatively (crabs: How et al, 2012, 2014; cephalopods: Moody and Parriss, 1961; Pignatelli et al, 2011; Shashar and Cronin, 1996; Shashar et al, 1996; Temple et al, 2012). This is because even 1D polarization vision with a retina containing just one polarization-sensitive e-vector type of photoreceptor (e.g.…”

“…Like cuttlefish, fiddler crabs respond to looming polarized stimuli with e-vector contrasts of just 3.2 deg to the background, in the absence of luminance contrast (How et al, 2012). Looming polarized stimuli were also used to test whether the responses of fiddler crabs were compatible with a 2D system of polarization vision (How et al, 2014). In that study, stimulus and background had the same e-vector but differed in degree.…”

“…The general occurrence of 2D input stages suggests that true polarization vision may be quite frequent in object-based polarization vision. However, 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.…”

Colour vision in mantis shrimps: understanding one of the most complex visual systems in the world

“…Behavioural studies carried out by How et al (2014aHow et al ( , 2014b on the stomatopods ability to discriminate different angles of polarised light indicate that their polarization vision may also function on similar simplified principles.…”