Categorical colour perception occurs in both signalling and non-signalling colour ranges in a songbird

Abstract

Although perception begins when a stimulus is transduced by a sensory neuron, numerous perceptual mechanisms can modify sensory information as it is processed by an animal's nervous system. One such mechanism is categorical perception, in which (1) continuously varying stimuli are labelled as belonging to a discrete number of categories and (2) there is enhanced discrimination between stimuli from different categories as compared with equally different stimuli from within the same category. We have shown previously that female zebra finches ( Taeniopygia guttata ) categorically perceive colours along an orange–red continuum that aligns with the carotenoid-based coloration of male beaks, a trait that serves as an assessment signal in female mate choice. Here, we demonstrate that categorical perception occurs along a blue–green continuum as well, suggesting that categorical colour perception may be a general feature of zebra finch vision. Although we identified two categories in both the blue–green and the orange–red ranges, we also found that individuals could better differentiate colours from within the same category in the blue–green as compared with the orange–red range, indicative of less clear categorization in the blue–green range. We discuss reasons why categorical perception may vary across the visible spectrum, including the possibility that such differences are linked to the behavioural or ecological function of different colour ranges.

Birds Perceive More Intraspecific Color Variation in Bird-Pollinated Than Bee-Pollinated Flowers

“…First, recent research raises the intriguing possibility that categorical color perception may be common among birds both in signaling and non-signaling contexts ( Caves et al, 2018 ; Zipple et al, 2019 ), and so may shape the functional relevance of apparent signal variation. Categorical perception suggests that (at least some) birds group color stimuli into categories, most likely during post-retinal processing, and canalize a consistent response to those stimuli which share a category despite possessing the low-level sensory apparatus to allow for discrimination ( Caves et al, 2018 ; Zipple et al, 2019 ). A parallel in human vision is humans’ categorical perception of discrete bands in a rainbow, despite the continuously varying wavelengths involved.…”

“…A parallel in human vision is humans’ categorical perception of discrete bands in a rainbow, despite the continuously varying wavelengths involved. In examining zebra finch ( Taeniopygia guttata ) vision in the context of mate choice and foraging, boundaries within the orange-red ( Caves et al, 2018 ) and blue-green ( Zipple et al, 2019 ) regions of the spectrum have been documented, such that color pairs on one side of the boundary were less readily distinguished than pairs which spanned it, despite approximately equal color distances between all pairs. For example, in a foraging experiment where food was available under bicolored (but not unicolored) disks, finches were less able to increase foraging success by targeting bicolored disks when the two colors were on one side of the orange-red boundary, than when the two colors spanned the boundary, despite equal discriminability ( Caves et al, 2018 ).…”

“…Improving our understanding of the relationship between pollinator-mediated selection and floral color variation will require progress on the mechanics of categorical vision ( Caves et al, 2018 ; Zipple et al, 2019 ). Unfortunately, at this time the categorical boundaries are not mapped with enough precision to apply them to datasets such as ours; for example, the UV region of the spectrum has not been explored for possible boundaries in birds.…”

“…They share the common theme that birds may not generate as strong selection against color variants as do bees, and thus 'tolerate' higher flower color variation than bees within the plant species that they pollinate. First, recent research raises the intriguing possibility that categorical color perception may be common among birds both in signaling and non-signaling contexts (Caves et al, 2018;Zipple et al, 2019), and so may shape the functional relevance of apparent signal variation. Categorical perception suggests that (at least some) birds group color stimuli into categories, most likely during post-retinal processing, and canalize a consistent response to those stimuli which share a category despite possessing the low-level sensory apparatus to allow for discrimination (Caves et al, 2018;Zipple et al, 2019).…”

Threshold assessment, categorical perception, and the evolution of reliable signaling

“…2018; Zipple et al. 2019; Caves et al. 2020) as well as context dependence of categorical boundaries (Lachlan and Nowicki 2015).…”