Behavioral and Neuronal Category Representations in the Carrion Crow

Abstract

Categorization is the key to simplification of the numerous stimuli which an animal encounters in a complex environment. It enables an animal to react fast and appropriately to all sorts of stimuli, even to novel ones. Behavioral studies conducted during the last decades show that birds master even most complex levels of categorizations. However, knowledge about the neuronal mechanisms that underlie this ability are scarce. My thesis includes a series of behavioral and neurophysiological experiments on carrion crows (Corvus corone corone) aimed at elucidating categorization capabilities in this bird species. Addressing auditory categorization abilities, I trained carrion crows to categorize auditory stimuli based on the direction of frequency modulation. The results show that crows possess flexible categorical working memory to maintain highlevel auditory category information and that they formed open-ended auditory categories which allowed them to immediately group novel stimuli correctly. Furthermore, we showed that crows use active working memory to protect visual information against interference and that they can discriminate images of crow and human faces but do not seem to represent faces as special categories. To explore how neurons in the nidopallium caudolaterale (NCL) represent category-related information, I conducted single unit recordings while crows performed controlled behavioral protocols. NCL neurons represented spontaneously present as well as learned complex magnitude categories. We found that neurons in the NCL of numerically naïve crows responded to discrete numerosities even though they were not relevant to the task, suggesting that crows possess an innate ‘sense of number’. Furthermore, NCL neurons encoded learned arbitrary categories of continuous spatial quantity (i.e. line length) and adapted flexibly to changed behavioral demands. Lastly, I trained carrion crows to report their subjective percept about presence or absence of a visual stimulus and showed that discrete populations of NCL neurons actively encoded the two perceptual states, thereby constituting the neural correlate of conscious subjective perceptions. Taken together, the results of the included studies add further insights on the categorization abilities of crows and show striking similarities of category processing between NCL and the primate prefrontal cortex.