On and Off Again: A look into how we perceive “backwards” motion

Every time you watch a movie, you might think you’re capturing every mo­ment unfold. In reality, however, the brain is fooled into perceiving this rapid sequence of still images as continuous motion. This is “phi motion” in action. Due to this effect, a bright point seems to move right­ward when it disappears and reappears to the right of its former position. Surprisingly, when a point moves rightward while alter­nating between dark and bright states, it will appear to “move” leftward via the “reverse-phi” effect.

Previously, researchers had known that the motion of bright­ening and darkening light is processed separately in “ON” and “OFF” channels in the eyes and brain. Since “reverse-phi” motion involves both brightening and darkening, it was unclear where it’s processed. A recent study led by Yale professor Damon Clark has explained how the visual system processes this illusion by studying Drosophilia, which perceives many of the same motion illusions as humans. Interestingly, they discovered that T4 and T5 neurons can, in fact, process light-dark mixing, an essential feature for visual processing.

Additionally, although several neurons collectively encode visu­al motion, only one neuron is active at a time. “This organization can be a good way to do it: it may conserve energy and make it easier for downstream neurons to figure out what’s going on,” Clark explained. While the direction reversal of the “reverse-phi” illusion seems counterintuitive, it suggests the specialization of motion detection channels. Thus, studying this illusion has led to a better understanding of how the brain works, especially how crucial light-dark mixing is for extracting visual motion informa­tion from the world.