Final answer:
The shift in perspective when alternately closing your eyes, with your finger held out in front of your face, is attributed to retinal disparity, a binocular depth cue that helps the brain perceive depth by combining two slightly different images from each retina.
Step-by-step explanation:
When you hold your finger out in front of your face approximately 12 inches and alternately shut your right eye, then your left, the shift in what you see is due to retinal disparity. This is a binocular depth cue resulting from the slight difference between the two images on each of our retinas because our eyes are spaced apart. This retinal disparity allows our brain to extract depth information from the scenery we observe, helping us perceive the world in three dimensions. Retinal disparity occurs because objects at different distances will fall on varying spots on our two retinas. When you focus on your finger in front of your face, with a backdrop at a different distance, each eye sees a slightly different image. Your brain combines these images to give you a sense of depth, which is an effect known as stereopsis.