The optical illusion you're referring to is known as the "phantom nose illusion" or the "thumb trick." It creates the perception that an extra nose appears in the person's field of view when they focus on their own nose while someone moves their finger back and forth in front of it. The scientific explanation for this illusion involves several factors related to our visual perception and the way our brain processes sensory information.
The illusion primarily relies on the phenomenon called "motion-induced blindness" or "perceptual fading." When a moving stimulus repeatedly passes over a particular region of our visual field, our brain tends to "fill in" the missing information or ignore it altogether. In the case of the phantom nose illusion, the moving finger acts as the stimulus, repeatedly passing over the same area of the person's field of view where their actual nose is located.
As the finger moves back and forth, the brain selectively attends to the finger's motion, causing a temporary suppression of the person's awareness of their own nose. Consequently, the brain attempts to fill in the missing visual information by creating the perception of an extra nose, as if it were a continuation of the finger's motion. This phenomenon occurs due to the brain's tendency to integrate and interpret visual information in a way that creates a coherent and meaningful representation of the world.
Another contributing factor to this illusion is the "phi phenomenon," which relates to our perception of motion when presented with a series of still images in rapid succession. The brain perceives this sequence of static images as continuous motion. In the case of the phantom nose illusion, the movement of the finger combined with the suppression of the actual nose's perception may lead the brain to interpret the rapid succession of finger positions as a continuous motion, further enhancing the illusion of an extra nose.
In summary, the phantom nose illusion is a result of the brain's interpretation and filling-in of missing visual information caused by motion-induced blindness. By focusing on the moving finger and momentarily suppressing the perception of the actual nose, the brain creates the illusion of an additional nose. It's a fascinating example of how our brain processes visual stimuli and constructs our perception of reality.