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

Visual shape discrimination in goldfish, modelled with the neural circuitry of optic tectum and torus longitudinalis

There is no satisfactory neurally-based theory as to how vertebrates that lack a neocortex discriminate even simple geometric shapes. In fishes, an intact optic tectum is necessary for such discriminations, but physiological studies of it have found nothing like the hierarchically arranged feature detecting neurons of mammalian visual cortex. Here, a neural model attempts a solution by basing shape discrimination upon the responses of only those elementary detectors (e.g. of size) that are within a focus of attention, formed by a winner-take-all arrangement of retinotopically mapped units representing tectal pyramidal cells. While this relatively primitive mechanism could recognize an object irrespective of position in space, it fails to distinguish patterns that differ only in their features' spatial relationships. The model's solution - imitating goldfish that naturally attend to the top of shapes - is to shift attention to the edges of a shape by spatially offsetting inputs to the pyramidal neurons, effected by the torus longitudinalis and its prolific synapses on pyramidal dendrites. The model's shape discrimination was compared to an extensive behavioral study using shapes with points and projections. In one test series fish were sensitive to the relative number of points on the tops of shapes. In another, fish were trained to discriminate points on the sides. By using different offset connections and only one elementary feature detector for small dark spots, the model successfully emulated the two sets of goldfish data, as judged by significant correlations between model response and fish discrimination.

African jewel fish (Hemichromis bimaculatus) distinguish individual faces based on their unique iridophore patterns

Previous research has shown that African jewel fish (Hemichromis bimaculatus) recognize pair-bonded mates during their exchanges of egg-guarding duties. The current research examined the perceptual cues for face recognition by comparing two face models displaying anatomically realistic arrangements of blue iridophores derived from discriminant function analysis of distinct sibling groups. Four groups each consisting of 9 subadults were examined using a narrow compartment restraining lateral movement where face models were presented at eye level for eight trials. Because respiratory movement of the operculum can mechanically displace the eye thereby shifting the retinal image, jewel fish reduce their respiration rate during increased attention. When two experimental groups were presented with the same face models on four trials following initial model presentations, both groups exhibited stable respiration rates indicative of model habituation. When the habituated face models were switched to novel face models on the fifth trial, the rates of respiration decreased as measured by reliable increases in the elapsed times of opercular beats. Switching the models back to the habituated models on the sixth trial caused reliable decreases in the elapsed times of opercular beats, resembling the earlier trials for the habituated models. Switching the face models again to the formerly novel models on the seventh trial produced respiration rates that resembled those of the habituated models. The two control groups viewing the same models for all eight trials exhibited no substantial change in respiration rates. Together, these findings indicate that jewel fish can learn to recognize novel faces displaying unique arrangements of iridorphores after one trial of exposure.

Recognition of natural objects in the archerfish

Recognition of individual objects and their categorization is a complex computational task. Nevertheless, visual systems can perform this task in a rapid and accurate manner. Humans and other animals can efficiently recognize objects despite countless variations in their projection on the retina due to different viewing angles, distance, illumination conditions and other parameters. To gain a better understanding of the recognition process in teleosts, we explored it in archerfish, a species that hunts by shooting a jet of water at aerial targets and thus can benefit from ecologically relevant recognition of natural objects. We found that archerfish not only can categorize objects into relevant classes but also can do so for novel objects, and additionally they can recognize an individual object presented under different conditions. To understand the mechanisms underlying this capability, we developed a computational model based on object features and a machine learning classifier. The analysis of the model revealed that a small number of features was sufficient for categorization, and the fish were more sensitive to object contours than textures. We tested these predictions in additional behavioral experiments and validated them. Our findings suggest the existence of a complex visual process in the archerfish visual system that enables object recognition and categorization.

Highlighted Article: Archerfish are capable of natural object recognition and categorization based on a small number of visual features.