Comparison of functional and anatomical estimations of visual acuity in two species of coral reef fish.

The high-contrast, complex patterns typical of many reef fish serve several purposes, including providing disruptive camouflage and a basis for vision-based communication. In trying to understand the role of a specific pattern, it is important to first assess the extent to which an observer can resolve the pattern, itself determined, at least in part, by the observer's visual acuity. Here, we studied the visual acuity of two species of reef fish - Pomacentrus amboinensis and Pseudochromis fuscus - using both anatomical and behavioural estimates. The two species share a common habitat but are members of different trophic levels (predator versus herbivore/omnivore) and perform different visual tasks. On the basis of the anatomical study, we estimated visual acuity to lie between 4.1 and 4.6 cycles deg-1 for P. amboinensis and 3.2 and 3.6 cycles deg-1 for P. fuscus Behavioural acuity estimates were considerably lower, ranging between 1.29 and 1.36 cycles deg-1 for P. amboinensis and 1.61 and 1.71 cycles deg-1 for P. fuscus Our results show that two species from the same habitat have only moderately divergent visual capabilities, despite differences in their general life histories. The difference between anatomical and behavioural estimates is an important finding as the majority of our current knowledge on the resolution capabilities of reef fish comes from anatomical measurements. Our findings suggest that anatomical estimates may represent the highest potential acuity of fish but are not indicative of actual performance, and that there is unlikely to be a simple scaling factor to link the two measures across all fish species.

Categorical face perception in fish: How a fish brain warps reality to dissociate "same" from "different".

Categorical perception (CP) is the phenomenon by which a smoothly varying stimulus property undergoes a nonlinear transformation during processing in the brain. Consequently, the stimuli are perceived as belonging to distinct categories separated by a sharp boundary. Originally thought to be largely innate, the discovery of CP in tasks such as novel image discrimination has piqued the interest of cognitive scientists because it provides compelling evidence that learning can shape a category's perceptual boundaries. CP has been particularly closely studied in human face perception. In nonprimates, there is evidence for CP for sound and color discrimination, but not for image or face discrimination. Here, we investigate the potential for learned CP in a lower vertebrate, the damselfish Pomacentrus amboinensis. Specifically, we tested whether the ability of these fish to discriminate complex facial patterns tracked categorical rather than metric differences in the stimuli. We first trained the fish to discriminate sets of two facial patterns. Next, we morphed between these patterns and determined the just noticeable difference (JND) between a morph and original image. Finally, we tested for CP by analyzing the discrimination ability of the fish for pairs of JND stimuli along the spectrum of morphs between two original images. Discrimination performance was significant for the image pair straddling the boundary between categories, and chance for equivalent stimulus pairs on either side, thus producing the classic "category boundary" effect. Our results reveal how perception can be influenced in a top-down manner even in the absence of a visual cortex.

A species of reef fish that uses ultraviolet patterns for covert face recognition.

The evolutionary and behavioral significance of an animal's color patterns remains poorly understood [1-4], not least, patterns that reflect ultraviolet (UV) light [5]. The current belief is that UV signals must be broad and bold to be detected because (1) they are prone to scattering in air and water, (2) when present, UV-sensitive cones are generally found in low numbers, and (3) long-wavelength-sensitive cones predominate in form vision in those species tested to date [6]. We report a study of two species of damselfish whose appearance differs only in the fine detail of UV-reflective facial patterns. We show that, contrary to expectations, the Ambon damselfish (Pomacentrus amboinensis) is able to use these patterns for species discrimination. We also reveal that the essential features of the patterns are contained in their shape rather than color. The results provide support for the hypothesis that UV is used by some fish as a high-fidelity "secret communication channel" hidden from predators [7, 8]. In more general terms, the findings help unravel the details of a language of color and pattern long since lost to our primate forebears, but which has been part of the world of many seeing organisms for millions of years.