Human vision is a complex process that extends beyond the eyes. While the eyes capture light and convert it into signals, the brain interprets these signals, transforming raw data into the rich, detailed images we perceive. This process involves the brain receiving, processing, and making sense of visual information.
The Visual Pathway
The journey of visual information begins in the retina at the back of the eye, where specialized cells called photoreceptors convert light into electrical signals. These signals are then processed by other retinal cells before being sent to ganglion cells. The axons of these ganglion cells converge to form the optic nerve.
Each optic nerve carries signals from one eye towards the brain. At a structure called the optic chiasm, located at the base of the brain, fibers from the inner (nasal) halves of each retina cross over to the opposite side of the brain. This crossing ensures that the right side of the brain receives visual information from the left visual field, and the left side of the brain receives information from the right visual field.
After the optic chiasm, visual information travels to the thalamus, a relay station for sensory inputs. The lateral geniculate nucleus (LGN) within the thalamus receives these visual signals. The LGN organizes and filters the information before transmitting it to the brain’s primary visual processing centers.
The Primary Visual Hub
From the lateral geniculate nucleus, visual signals are relayed to the primary visual cortex, often referred to as V1. This area is also known as the striate cortex. V1 is located at the very back of the brain, within the occipital lobe.
The primary visual cortex is the first cortical area where visual information arrives for conscious processing. It contains a detailed map of the visual field, with specific parts of V1 corresponding to specific locations in our field of view. Neurons in V1 are highly specialized for detecting basic visual features.
This initial processing involves identifying simple elements such as edges, lines, and their orientations, as well as detecting motion and spatial location. V1 breaks down complex visual scenes into these fundamental components, shaping our perception.
Specialized Visual Processing Areas
Beyond the primary visual cortex, visual processing continues in a network of specialized regions known as extrastriate visual areas. These areas, including V2, V3, V4, and V5 (also called MT), receive input from V1 and are responsible for more complex aspects of vision. The extrastriate cortex is involved in analyzing intricate visual features like color, form, and motion.
Visual information is processed through distinct pathways, often described as two main streams: the dorsal and ventral streams. The dorsal stream, sometimes called the “where” or “how” pathway, extends from V1 towards the parietal lobe. It processes spatial relationships, motion, and guides visually-based actions.
The ventral stream, known as the “what” pathway, projects from V1 towards the temporal lobe and is specialized in object recognition. This stream enables us to identify objects, faces, and understand their properties, such as shape and color. Both streams operate in parallel, allowing the brain to simultaneously process different attributes of a visual scene, contributing to our perception.