This picture above (I am unable to find the original source) has turned out to be my most viewed tweet. I believe it to be a kookaburra bird, but I am not an expert ornithologist. The striking feature of this photo is that, with a small effort, you can also see a rabbit (if you choose to perceive the beak of the bird as the ears of a rabbit!). Obviously one percept is in rivalry with the other, so you will either see a bird or a rabbit but never both of them.
This reminds me of the 100-year-old optical illusion used by Joseph Jastrow to demonstrate that perception is a product of our mental activity. What happens in the brain when we perceive this kind of bistable stimuli?
Three years ago, Tomohiro Ishizu and Semir Zeki published an interesting article in Neuroimage in which they presented the results of an fMRI investigation on bistable images such us the bird-rabbit. What did they found? Interestingly, the brain seems to adopt similar strategies when faced with unstable stimuli that offer a transition between objects belonging to the same category (e.g. face–face) or different categories (e.g. face–body). Therefore, the authors suggested the existence of a “common reversal-related neural circuitry, which includes fronto-parietal cortex and primary visual cortex”.
Additional brain areas (ACC and the STG) were also recruited in cross-categorical reversals alone that required more radical action selection processes. These results cannot give a definitive answer about the many questions that unstable or bistable images trigger. Moreover, this kind of endeavour is not trivial as it may shed light on the how the brain resolves sensory conflicts. For now, we know that primary perceptual areas provide signals to the brain suggesting that the conflict between the two interpretation is perceptually driven. Higher areas, instead, seem to be suppressed indicating the conflict is strictly perceptual. Further work, the authors suggest, should explore the complexity of cognitively unstable stimuli. Let’s wait for further results and meanwhile enjoy these beautiful and intriguing images.