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Sensory systems provide organisms with information on the current status of the environment, thus enabling adaptive behavior. The neural mechanisms by which sensory information is exploited for action selection are typically studied with mammalian subjects performing perceptual decision-making tasks, and most of what is known about these mechanisms at the single-neuron level is derived from cortical recordings in behaving monkeys. To explore the generality of neural mechanisms underlying perceptual decision making across species, we recorded single-neuron activity in the pigeon nidopallium caudolaterale (NCL), a non-laminated associative forebrain structure thought to be functionally equivalent to mammalian prefrontal cortex, while subjects performed a visual categorisation task. We found that, whereas the majority of NCL neurons unspecifically upregulated or downregulated activity during stimulus presentation, ~20% of neurons exhibited differential activity for the sample stimuli and predicted upcoming choices. Moreover, neural activity in these neurons was ramping up during stimulus presentation and remained elevated until a choice was initiated, a response pattern similar to that found in monkey prefrontal and parietal cortices in saccadic choice tasks. In addition, many NCL neurons coded for movement direction during choice execution and differentiated between choice outcomes (reward and punishment). Taken together, our results implicate the NCL in the selection and execution of operant responses, an interpretation resonating well with the results of previous lesion studies. The resemblance of the response patterns of NCL neurons to those observed in mammalian cortex suggests that, despite differing neural architectures, mechanisms for perceptual decision making are similar across classes of vertebrates.

Extinction learning provides the ability to flexibly adapt to new contingencies by learning to inhibit previously acquired associations in a context-dependent manner. The neural networks underlying extinction learning were mostly studied in rodents using fear extinction paradigms. To uncover invariant properties of the neural basis of extinction learning, we employ pigeons as a model system. Since the prefrontal cortex (PFC) of mammals is a key structure for extinction learning, we assessed the role of N-methyl-D-aspartate receptors (NMDARs) in the nidopallium caudolaterale (NCL), the avian functional equivalent of mammalian PFC. Since NMDARs in PFC have been shown to be relevant for extinction learning, we locally antagonized NMDARs through 2-Amino-5-phosphonovalerianacid (APV) during extinction learning in a within-subject sign-tracking ABA-renewal paradigm. APV-injection slowed down extinction learning and in addition also caused a disinhibition of responding to a continuously reinforced control stimulus. In subsequent retrieval sessions, spontaneous recovery was increased while ABA renewal was unaffected. The effect of APV resembles that observed in studies of fear extinction with rodents, suggesting common neural substrates of extinction under both appetitive and aversive conditions and highlighting the similarity of mammalian PFC and the avian caudal nidopallium despite 300 million years of independent evolution.

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