The brain constructs a coherent image from light signals hitting the eye through a complex process. The visual cortex converts these raw signals into the perception of shapes, movement, and the surrounding world. This process is often studied in the mouse, which serves as an effective model organism for understanding mammalian brain function due to its well-mapped neural circuitry and genetic tractability. By tracing the visual signal, researchers can explain the specific steps the mouse brain takes to process and interpret its environment, involving initial filtration, hierarchical analysis, and continuous adaptation.
The Journey of Light: From Eye to Primary Cortex (V1)
The initial phase of visual processing begins in the retina, where light is captured by photoreceptors and then relayed by retinal ganglion cells. These cells already initiate a form of signal separation by encoding visual information into distinct pathways, primarily separating signals related to light onset (ON) and light offset (OFF). This divided signal then travels along the optic nerve to the thalamus, the brain’s central relay station.
The primary destination for visual information within the thalamus is the dorsal Lateral Geniculate Nucleus (dLGN). The dLGN acts as a filter and modulator of the input before it reaches the cortex. Research in mice has shown that the dLGN encodes a diverse array of features, including early signs of orientation and direction selectivity, rather than just simple center-surround receptive fields.
After the dLGN refines and organizes the signal, it sends projections directly to the Primary Visual Cortex (V1), the first cortical area to receive visual input. V1 neurons transform the circular center-surround input from the dLGN by combining it into elongated receptive fields. This enables the initial cortical extraction of basic features in the visual world, such as the orientation of edges and the detection of contrast changes.
Hierarchical Processing and Feature Extraction
The V1 region serves as the starting point for cortical computation, where simple features are first identified. The resulting information is then distributed for further, more complex analysis. V1 broadcasts its output to a network projecting to more than 18 distinct brain areas, forming the basis of the visual processing hierarchy and ensuring that different aspects of the visual scene are channeled to specialized regions for parallel processing.
As information progresses from V1 into secondary visual areas (V2, V3, and others), the complexity of the features being analyzed increases. Unlike the highly segregated visual streams found in primates, the mouse visual cortex exhibits a less strictly defined but still hierarchical arrangement. This organization includes pathways loosely analogous to the primate ventral stream, concerned with object features, and the dorsal stream, which specializes in spatial location and motion.
The lateral higher visual areas are generally associated with object recognition, while the anterior and medial areas process movement information. This specialization is evident even in the V1 output. For example, V1 neurons that project to the posteromedial area (PM) prefer slower moving stimuli, whereas those projecting to the anterolateral area (AL) prefer faster motion.
Neurons in these higher areas exhibit an expansion of their receptive fields, responding to a larger portion of the visual space than V1 neurons do. Furthermore, the temporal integration of signals increases along this hierarchy, meaning correlation timescales become longer in higher cortical areas. This lengthening of the timescale suggests that these regions integrate sensory input over a more extended period, which is necessary for recognizing complex, temporally evolving features like object motion or entire shapes.
The Dynamic Brain: Experience-Dependent Plasticity
Visual processing is not a fixed, hardwired process but a dynamic one, constantly shaped by the environment through neural plasticity. The mechanism underlying this adaptability is synaptic plasticity, which refers to the strengthening or weakening of connections between neurons based on their activity. This change in connectivity allows the visual cortex to fine-tune its circuits to match the specific visual world the animal experiences.
A prominent example of this is the “critical period,” a time early in life when the visual cortex is maximally flexible and sensitive to visual input. In the mouse, this period for ocular dominance plasticity—the relative responsiveness of cortical neurons to input from each eye—peaks around Postnatal Day 28 (P28). During this window, briefly manipulating the visual input, such as by temporarily depriving one eye, can cause a rapid shift in the cortex, where neurons become significantly more responsive to the non-deprived eye.
This experience-dependent modification is essential for the correct development and alignment of the visual system. While the critical period eventually closes due to the accumulation of certain molecular “brakes,” the ability to adapt does not entirely vanish. Adult mice retain a form of plasticity, enabling ongoing learning and adaptation to visual changes, though the mechanisms differ from those in the young brain.