To better understand the brain activity behind these behaviors, the team implanted tiny, glass-coated wires into the birds' brains to record electrical activity while the crows repeated the behavioral tests. "This is an indication that they treat the empty set, not just as 'nothing' versus 'something,' but really as a numerical quantity," in that they perceive zero dots as proximal to one dot. That means that the birds mixed up the zero-dot image with the one-dot image more often than with two-, three- or four-dot images, Nieder explained. However, importantly, the birds still demonstrated the numerical distance effect in trials that included the empty screen. In the more recent study, which included a blank screen, "what we found is that the crows, after this training, could discriminate zero from the other countable numerosities," Nieder said. This phenomenon is known as the "numerical distance effect," which can also be observed in monkeys and humans during similar tests, Nieder told Live Science. The greater the difference between the two sets of dots, the more accurately the birds responded in other words, the birds mixed up closer quantities, such as two and three, more often than more divergent quantities, such as one and four. This previous study did not include an empty screen, standing in for zero, but it did demonstrate that the crows could differentiate an image containing three dots from a screen containing five, for instance. In a previous study using the same setup, the group showed that crows could successfully identify the matched and unmatched pairs of images about 75% of the time after undergoing extensive training for the experiment, according to a report published in 2015 in the journal Proceedings of the National Academy of Sciences. The crows were trained to peck at the screen or move their heads if the two images matched one another, and to remain still if they did not match. In each trial, a grey screen containing zero to four black dots popped up in front of the crows this "sample" image was followed by a "test" image containing either the same or a different number of dots. In the new study, published June 2 in The Journal of Neuroscience, the team ran experiments with two male carrion crows ( Corvus corone), during which the birds sat on a wooden perch and interacted with a computer monitor in front of them. The birds' brain activity patterns also support the idea that zero falls before "1" on crows' mental number line, so to speak. And now, by peering into the brains of crows, Nieder and his colleagues have discovered that the birds' nerve cells, or neurons, encode "zero" as they do other numbers. thinking that is detached from empirical reality," Nieder said. Zero represents that emptiness, the absence of apples, and "that obviously requires very abstract thinking.
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