How an Octopus Sees the World and the World Sees an Octopus: Aspects of Octopus Vision and Camouflage

Octopuses, along with other coleoid cephalopods (cuttlefish, & squid) have a unique ability to change their body colour and texture thanks to neurally controlled elements in their skin, allowing them to camouflage in a fraction of a second in virtually any environment, from a kelp forest to a sandy estuary, a seagrass bed, or a coral reef. Octopuses have a highly developed camera-type eye which is polarisation sensitive. However, they are colourblind, and therefore it is not understood how they obtain colour information from their surroundings in order to produce an eective camouflage. By studying octopus vision, insight into which visual cues are used and how they influence their camouflage can be obtained, and aspects of their camouflage can be better understood. This thesis uses a novel method based on a reflex that allows to obtain large amounts of behavioural data on psychophysics experiments using sinusoidal gratings of dierent spatial frequencies presented on an LCD screen. For the first time, the octopus contrast sensitivity function is obtained both for luminance and polarisation vision, as well as the angular dependence of polarisation sensitivity. It is demonstrated that both luminance and polarisation contrast sensitivity decrease at low spatial frequencies and peak at the same spatial frequency, suggesting that, in octopus, luminance and polarisation signals are processed via similar pathways. The shape of the contrast sensitivity function provides evidence that lateral inhibition is present in the octopus visual system as indicated by the decrease of sensitivity at low spatial frequencies. It is shown that maximum polarisation sensitivity is achieved at vertical and horizontal orientations of polarisation, parallel to the orientation of their photoreceptors. Two models are introduced: one of polarisation and luminance contrast sensitivity based on the properties of vertebrate retinal ganglion cells with centre-surround receptor fields, and one describing the angular dependence of polarisation sensitivity with only one parameter – the maximum deviation of photoreceptor orientation from its mean value. In this thesis the reflectance spectra of octopus skin are presented, and octopus colour change is demonstrated using quantitative methods for the first time. It is shown that, even though octopus reflectance spectra dier considerably from those of colourful background objects, the variability of octopus colours allows them to be cryptic in the eyes of their predators.