Crows and ravens are famously brainy birds, but a new study suggests they possess a kind of consciousness, something once thought to be the exclusive domain of humans and some primates, reports Sharon Begley for Stat.
By measuring brain activity in crows performing a visual task, the researchers found that on top of the crows’ basic sensory experience, the birds have another layer of awareness. In the journal Science, the authors argue that these two layers of perception constitute a form of what humans call subjective experience.
Until now, this type of consciousness has only been witnessed in humans and other primates, which have completely different brain structures to birds.
"The results of our study opens up a new way of looking at the evolution of awareness and its neurobiological constraints," says Andreas Nieder, an animal physiologist at the University of Tübingen and the study’s lead author, in a statement.
The experiments involved monitoring the brain activity of two crows trained to peck at a colored light if they saw a figure appear on a screen. The majority of these visual stimuli were bright and unambiguous, but some were so faint the crows couldn’t always make them out. The crows were trained to report whether they had seen anything using red and blue lights. In some trials, a red light meant the crows should peck the screen if they saw something and a blue light meant no response was required. In other trials, the blue light was used to tell the bird to peck the screen if they hadn’t seen anything and the red light meant they could just sit there.
Electrodes hooked up to the crows’ brains showed that if the crow’s answer was “yes,” there was elevated brain activity in the time between when the stimulus appeared and when the crow pecked the screen, reports Michelle Starr for Science Alert. If the answer was “no,” nerve cell activity was flat. The correlation between elevated brain activity in this time interval between the stimulus and the bird’s answer was so reliable that the researchers could use the crows’ brain activity to predict their responses.
What’s more, the crows’ responses didn’t simply correspond with the brightness and clarity of the figure on the screen. Faint figures of equal intensity still managed to elicit varying responses from the pair of crows. This observation suggests the presence of some secondary mental process that occurred when the crows noticed the figures.
"Nerve cells that represent visual input without subjective components are expected to respond in the same way to a visual stimulus of constant intensity," says Nieder in the statement. "Our results, however, conclusively show that nerve cells at higher processing levels of the crow's brain are influenced by subjective experience, or more precisely produce subjective experiences."
The crows’ neurons “have activity that represents not what was shown to them, but what they later report...to have seen—whether or not that is what they were shown,” Suzana Herculano-Houzel, a neurobiologist at Vanderbilt University who published an analysis of the study in Science, tells Stat. This secondary layer of processing of the visual stimulus occurs in the time between when the stimulus appears on the screen and when the crow pecks its answer.
“That’s exactly what one would expect from neurons that participated in building the thoughts that we later report,” Herculano-Houzel tells Stat, adding that it suggests these birds “are as cognitively capable as monkeys and even great apes.”