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

Abstract

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.

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

“…Since then, some fish species have been shown to recognize complex patterns such as the faces of both fish and humans (Parker et al, 2020;Coss & Tyler, 2023;Newport et al, 2016) that would implicate sophisticated discriminative and memory mechanisms. Moreover, goldfish and red-tail splitfins can interpret shape from subjective contours, and in the case of the latter species, recognize a stimulus shape even when partially occluded (amodal completion) (Wyzisk & Neumeyer, 2007;Sovrano & Bisazza, 2009).…”

Recognition of natural objects in the archerfish

“…A number of studies have indicated that fish also have the capacity to differentiate between different shapes ( Lucon-Xiccato et al, 2019 ; Mackintosh and Sutherland, 1963 ; May et al, 2016 ; Oliveira et al, 2015 ; Siebeck et al, 2009 ) and intensities ( Agrillo et al, 2016 ), use hierarchy of visual processing in discriminating shapes ( Truppa et al, 2010 ) and fish faces ( Parker et al, 2020 ), and the archerfish can even be trained to discriminate between human faces ( Newport et al, 2016 , 2018 ). For simple abstract stimuli, fish are capable of overcoming changes in orientation ( DeLong et al, 2018 ), size consistency ( Douglas et al, 1988 ) and the occlusion of objects ( Sovrano and Bisazza, 2008 ).…”

Recognition of natural objects in the archerfish

“…Similarly, bees (Avargues-Weber et al, 2010; Giurfa et al, 1997; Werner et al, 2016), wasps (Oliveira et al, 2015; Sheehan and Tibbetts, 2011), and adult zebrafish (May et al, 2016; Oliveira et al, 2015) have all been shown to be capable of conspecific visual identification. A number of studies have indicated that fish also have the capacity to differentiate between different shapes (Mackintosh and Sutherland, 1963; Siebeck et al, 2009), fish faces (Parker et al, 2020), and the archerfish can even be trained to discriminate between human faces (Newport et al, 2016; Newport et al, 2018). Clearly, some of these stimuli, such as human faces, are not ecologically relevant to birds, insects, or fish, nor do we expect fish to possess specific brains areas dedicated to face processing, as is the case for humans.…”