Human vision is a sensory process that allows individuals to interpret their surroundings. It involves a continuous collaboration between the eyes and the brain to construct a coherent understanding of the world. The eyes detect light, which is then transformed into nerve signals that the brain decodes, ultimately creating the images we perceive.
How the Eye Gathers Light
Vision begins when light rays reflect off objects and enter the eye through the cornea, the clear, dome-shaped outer layer. The cornea bends, or refracts, these light rays to help focus them. Following the cornea, light passes through a watery substance called the aqueous humor before reaching the pupil, which is the adjustable opening in the center of the iris. The iris, the colored part of the eye, controls the amount of light entering the eye by adjusting the pupil’s size; it constricts in bright conditions and dilates in dim light.
Behind the pupil, light encounters the lens, a clear and flexible structure. The lens works with the cornea to further focus light rays onto the retina, located at the back of the eye. The lens changes its shape to adjust the focal distance, allowing the eye to focus on objects at various distances. At this point, the image projected onto the retina is inverted.
The retina, a light-sensitive tissue, contains millions of specialized cells called photoreceptors. These photoreceptors come in two types: rods and cones. Rods detect light intensity and enable vision in dim conditions, primarily processing black, white, and shades of gray. Cones, concentrated in the central part of the retina known as the macula, detect color and provide detailed vision in bright light. Both rods and cones capture light signals and convert them into electrochemical impulses.
The Brain’s Role in Seeing
Once photoreceptors in the retina convert light into electrical signals, these signals are transmitted to the optic nerve. The optic nerve then carries this visual information from the retina to various parts of the brain for processing. A relay station for this information is the lateral geniculate nucleus (LGN) in the thalamus. The LGN further relays these signals to the primary visual cortex (V1), located in the occipital lobe at the rear of the brain.
The primary visual cortex is the initial area where the brain begins to interpret raw visual input. This region contains specialized cells that respond to basic features such as lines, edges, and their orientations. It also possesses a retinotopic map, which spatially organizes information from the visual field, effectively projecting the image from the retina onto V1. This early processing builds the foundational elements of our visual experience.
From the primary visual cortex, visual information is processed in a hierarchical manner, with increasingly complex features extracted at subsequent stages. Two main processing streams emerge: the ventral pathway, often referred to as the “What Pathway,” and the dorsal pathway, or the “Where Pathway.” The ventral pathway is involved in object recognition. The dorsal pathway processes information related to motion, location, and spatial organization.
How We Perceive the World
The brain integrates processed visual information to construct a coherent perception of the world. This involves combining basic features detected in the visual cortex with higher-level cognitive processes. For instance, the perception of color is linked to specific brain regions, like area V4 within the fusiform gyrus. Our perception of color can also be subjective, influenced by factors like lighting and the surrounding colors.
Depth perception relies on both binocular and monocular cues. Binocular vision, using input from both eyes, allows the brain to merge slightly different images to create a three-dimensional view and accurately judge distance. Monocular cues, which can be perceived with a single eye, include aspects like relative size, interposition, and linear perspective, helping to infer depth. The brain also processes motion, which is partly handled by the dorsal pathway.
Recognizing objects and faces involves higher-level visual areas, such as the inferior temporal (IT) cortex, which can represent complete objects. A specific part of the IT cortex, the fusiform face area, is specialized for face recognition. Beyond raw sensory input, our past experiences and stored knowledge significantly influence our perception. The brain integrates current sensory information with past images to form our perceived reality. This means what we “see” is not just a direct reflection of reality, but a construction shaped by our prior learning and expectations.