Bird Brains And Sensory Consciousness
The English language has many synonyms for a lack of intelligence, and one of those is “birdbrain.” Needless to say, our society doesn’t think much of birds’ intelligence, and in particular doesn’t think much of their brains. But a recent study shows that we may have under-estimated them. The study finds evidence of sensory consciousness–a trait we like to think of as unique to certain mammals–in the brains of birds.
Sensory consciousness is the ability to have subjective experiences (experiences that are influenced by our own perceptions) that can be clearly assessed. Primates and humans have a cerebral cortex, which is where scientists have found and studied sensory consciousness.
But birds dont have a layered cerebral cortex like we do. They diverged from mammals 320 million years ago, so their brains look very different from the brains of primates. Instead, birds have the nidopallium caudolaterale (NCL), which is considered to be the bird equivalent.
To study sensory consciousness in primates, scientists look at how brain activity changes depending on whether or not the subject recognizes a stimulus. In primates, sensory consciousness is a two-stage process: first, the neurons mostly fire in response to the visual stimulus itself (unconscious vision), and then later, the neurons primarily correspond to the primate’s awareness of the stimulus.
In this study, the researchers looked at sensory consciousness in the NCL of birds (specifically corvids, a family that includes crows and ravens). In order to determine whether birds have sensory consciousness, they replicated what we know about primates. In the experiment, they showed a visual stimulus to the crows, and then after a pause, the birds were given a cue that told them how to report whether or not they saw the stimulus. The pause is important because it meant that the birds couldn’t prepare their motor responses prior to being given the cue. The scientists grouped the stimuli into high-intensity, threshold intensity (meaning, the level of intensity needed for the crow to be aware of the stimulus), and no-stimulus.
The scientists identified neurons that responded to the stimuli, and these were the neurons that they watched. They wanted to answer these questions: Were the neurons responding to the visual stimulus, or to the bird’s perception of the stimulus? And did this change over time—from when the stimulus was presented (“stimulus presentation”) to the pause before the crow’s report (“delay period”)?
They found that during stimulus presentation, the neurons were mostly responding to the intensity of the stimulus. They determined this because the neuron firing patterns were similar for all trials of the same stimulus intensity, regardless of the crow’s final report. In other words, in a no-stimulus trial, the neuron responses were similar regardless of whether the crow correctly reported that there was no stimulus or if the crow reported a false positive.
But this changed during the delay period. Instead, the neurons mostly responded to the crow’s impending report, and not to the stimulus intensity. They determined this by finding that all “yes” responses (regardless of whether it was a high-intensity stimulus, threshold stimulus, or no stimulus) showed higher neuron firing rates than all “no” responses (again, regardless of what the stimulus actually looked like).
The scientists found that the information about the physical stimulus and the later report were carried out by the same NCL neurons; these NCL neurons switched from primarily encoding the stimulus intensity at the beginning, to then mostly encoding the crows’ subjective experience. But while the neurons switched their main focus, the scientists found that there was a population of neurons that had information about the crows’ subjective experience throughout the trial.
This shift in how the neurons respond to a visual stimulus corresponds to our understanding of how neurons perform visual consciousness, so this finding is evidence for sensory consciousness in birds. The two-stage process in awareness is very similar to what has been found in primates, and could be how vertebrates in general achieve sensory consciousness.
The finding also provides evidence for the evolution of consciousness, because we know now that sensory consciousness extends past primates and other mammals. Perhaps birds and mammals both inherited sensory consciousness from their last common ancestor 320 million years ago, or maybe consciousness emerged independently in the two species.
The evidence of sensory consciousness in birds is just one of many recent findings that demonstrate how similar we are to other animals — and adds to the evidence suggesting that we should not be keeping them captive and using them in experiments. Unfortunately, humans are bound up in anthropocentrism, and we wield it as a justification for treating animals as beneath us. As knowledge of our similarities strengthens, our rationale to elevate ourselves and belittle animals weakens.