Sure, same experiment as this one, but instead of introducing another rectangle for the third object, introduce a circle. If they favor that over the rectangle they're clearly gamblers.
Actually, I have a snake that eats one mouse per week. Maybe I'll slowly try this over the course of a year or so.
The dopamine is triggered by the cheese it gets right? At the time when the mouse is making the decision, it knows it'll get the cheese and dopamine from the old rectangle, yet it doesn't go there.
So what does this have to do with anything? Why is the mouse preferring something new and unknown over a known cheese/dopamine event?
My statement is that the mouse may be gambling, not that it's afraid. It knows the square is no cheese, the rectangle is one piece, and thinks maybe an unknown object of any shape is 10.
You asked why the learning is not taken as an aversion response to lack of reward. My interpretation is because your baseline (square) is zero: that in itself is not aversive; it's just nothing.
I think the new rectangle is evidence of hyperstimulation, where a stronger response is elicited when features are exaggerated. To establish this case, we'd have to test how strongly the rat perceives prototypes of squares vs. rectangles; my guess is square-ness vs. rectangle-ness is easily discernable. That is to say, I am pretty convinced this is just hyperstimulation, in which case a random shape would not do. It would have to resemble a former stimulus X but exhibit features that are even more X-like than the previous Xs. If perception of a rectangle is encoded as "right-angled shapes where height != width" (a plausible encoding, because our shape detectors operate on angles and lines, while curves utilize different detectors), then when height !!!=== width, then you get BAM!