Final answer:
The concept being referred to is depth perception, more specifically the binocular depth cue called binocular disparity. As an object moves closer, the angles from which each eye views the object differ more significantly, and the brain uses this information to perceive depth.
Step-by-step explanation:
The cue your instructor is referring to involves the concept of depth perception, which is related to the field of Biology. When you bring your writing utensil closer to your nose and notice your eyes crossing more, you are experiencing a binocular depth cue known as binocular disparity. This occurs because each of your eyes sees the world from a slightly different angle. As an object moves closer to you, the difference in the angle from which each eye views the object increases. Your brain uses this information to calculate how far away the object is, allowing you to perceive depth and the three-dimensional aspect of the world.
To illustrate this, by extending an arm and alternating the closure of each eye, you can observe that the position of an object like a pen appears to shift. This shift, known as parallax, is more noticeable when the object is closer. As the object is moved further away, the shift becomes less apparent, and at a certain distance, it becomes imperceptible. This is a demonstration of how our eyes and brain work together to provide us with a sense of how far away things are in our environment.