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.
neurophysiology