However, when the same behavioral discrimination functions were plotted on a logarithmic axis, the shapes were roughly Gaussian, suggesting a logarithmic representation of numerosities. When plotted on a linear number scale, the shapes of the behavioral performance functions were asymmetric with a steeper slope toward smaller numerosities. We investigated the coding scheme by plotting the performance data on different number scales. (The values of sigma are related to the specific compression scheme.) Dotted lines indicate linear fits (error bars ± SEM). ( H) The SD (sigma) of the Gauss fits for nonlinear scaling plotted against the center of the Gauss function (which is identical to the numerosity of the match stimulus). The goodness-of-fit was significantly better for the three nonlinear scaling schemes (error bars ± SEM). ( G) Goodness-of-fit of Gauss functions fitted to the performance curves plotted on different scales. ( G and H) Quantification of Gauss fits to the behavioral data. Behavioral performance functions are plotted on a linear ( E) and logarithmic number scale ( F) the latter resulted in more symmetric functions. Colors represent performance curves for a given sample numerosity. ( E and F) The behavioral performance functions (both crows and average) indicated whether they judged the first test stimulus (after the delay) as containing the same number of items as the sample display (“% same as sample”). ( D) Average performance (error bars represent SEM) of both crows to the standard and the control conditions during the recording sessions. ( C) Behavioral performance (percent correct) for both crows in the DMS task over all recording sessions (chance level = 50%). The physical appearance of the displays varied widely for the same quantities (see details in Table 1). ( B) A small subset of the stimulus displays are shown as examples. All numerosities were used as nonmatch stimuli (probability = 0.125) for the respective sample numerosities. The crow had to peck the test1 display if it contained the same number of items as the sample and had to refrain from pecking if it did not (probability = 0.5). After a 600-ms presample period, a sample stimulus was presented for 800 ms, followed by a 1,000-ms delay. The crow initiated a trial by keeping its head still in front of the monitor (automatically detected) to activate a go stimulus. Task protocol, stimuli, and behavioral performance. Here, we show that neurons in the telencephalic NCL of corvid songbirds respond to numerosity and show a specific code for numerical information. Where and how numerosity is encoded in vertebrates lacking a neocortex is unknown. Instead, birds developed different pallial parts as dominant endbrain structures ( 20, 21) based on convergent evolution, with the nidopallium caudolaterale (NCL) as a high-level association area ( 22– 26). The birds’ lineage diverged from mammals 300 Mya ( 19), at a time when the neocortex had not yet developed from the pallium of the endbrain. In contrast to primates, birds lack a six-layered neocortex. The neuronal correlates of numerosity representations have only been explored in humans ( 7– 9) and primates ( 10– 18), and they have been found to reside in the prefrontal and posterior parietal neocortices. Birds show elaborate quantification skills ( 1– 3) that are of adaptive value in naturalistic situations like nest parasitism ( 4), food caching ( 5), or communication ( 6).
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