The rats were conditioned to push the button. Whether the rats were conditioned to take a selfie is unclear. The sugar is an unconditioned stimulus, the rat's pleasure of receiving sugar is an unconditioned response, and pushing the button is a conditioned response. The button is a conditioned stimulus. Is the selfie a conditioned stimulus as well? Melodramatically put, is the selfie more than a pearl cast in front of a swine?
My killjoy answer is: give the rat two tunnels. The rat can see partway into the tunnel from the entrance. One tunnel has a selfie inside. The other tunnel has a picture of the wall in the background of the selfie. Add more tunnels and check which tunnels the rat prefers.
My artistic answer: give yourself the pleasure of assuming that the rats like the selfies.
My killjoy answer is: give the rat two tunnels. The rat can see partway into the tunnel from the entrance. One tunnel has a selfie inside. The other tunnel has a picture of the wall in the background of the selfie. Add more tunnels and check which tunnels the rat prefers.
My artistic answer: give yourself the pleasure of assuming that the rats like the selfies.