Summary
Highlights
Vision is the most understood human sense, largely due to animal research. The physical energy for vision is light. Without light reflecting off objects, we cannot see them. Humans can only detect a small part of the electromagnetic spectrum, from gamma rays to AC electricity, enabling us to perceive color. Other animals, like snakes and bees, can detect different parts of the spectrum, such as infrared and ultraviolet rays.
The human eye shares similarities with a camera, particularly having a lens. When light enters the eye, it projects an inverted image onto the retina. Before reaching the retina, light passes through the cornea (a protective coating), the iris (the colored part of the eye), and the pupil (which adjusts based on light and arousal). The brain then re-orients the image so we don't see things upside down.
The retina is a richly condensed area filled with specialized photoreceptor cells called rods and cones. Cones are responsible for color vision and detailed visual information, while rods are for peripheral vision and vision in dim light conditions. Light stimulates these cells, which then activate bipolar cells and ganglion cells, sending information to the optic nerve.
The human eye has a blind spot where the optic nerve leaves the eye, meaning there are no rods or cones in that area. Despite this, we don't perceive a gap in our vision due to a process called sensory completion, where our brain fills in the missing information based on expectations and surrounding visual data. A test for the blind spot is demonstrated.
Information from the eye travels through the optic nerve to the thalamus, which acts as a relay station, and then to the occipital lobe at the back of the brain. The occipital lobe is crucial for feature detection, processing shapes, patterns, colors, and motion. This is where the brain perceives and makes sense of what was sensed by the eyes.