The concept of the analyzer and the working principles of sensory systems
According to the classical definition by I. P. Pavlov, an analyzer is a part of the nervous system consisting of three sections: the peripheral part (receptors), pathways, and the cortical part, which performs higher analysis and synthesis of information to form conscious perception.
Modern physiology views the analyzer as a complex self-regulating system with a multilevel hierarchy. Visual information is processed not only by the cortex but also by subcortical structures, primarily the thalamic nuclei, which act as a relay station and filter. A key feature of such systems is the presence of feedback loops: signals travel not only from receptors to the cortex but also in the opposite direction. Due to this, the brain actively "queries" the external world, selecting biologically significant stimuli and blocking insignificant ones.
Coding of visual information
The transformation of information about the external world into nerve impulses is based on coding. Since the action potential obeys the all-or-none law, information is transmitted not by the amplitude of the impulse, but by frequency changes, the number of active neurons, and temporal activity patterns.
- Modality (light) is coded by the receptor type — the labeled line principle.
- Intensity is coded by the firing frequency in the optic nerve.
- Spatial parameters (shape, size, localization) are processed via retinotopic organization: retinal points are projected onto specific areas of the cortex.
- Temporal parameters (onset, offset, stimulus duration) are processed via specific neuronal response patterns.
The visual sensory system provides over 90% of information about the external environment by converting visible electromagnetic radiation into nerve impulses and subsequently into an integrated subjective image.
Peripheral part: the optical apparatus of the eye
The eye is an optical device that focuses light rays and forms a reduced, inverted image on the retina. The dioptric apparatus includes the cornea, aqueous humor, crystalline lens, and vitreous body. The greatest refractive power is in the cornea, while fine-tuning is provided by lens accommodation.
According to Helmholtz's theory, for distance vision, the ciliary muscle is relaxed, the suspensory ligaments are taut, and the lens is flattened. For near vision, the ciliary muscle contracts, the tension of the ligaments decreases, and the lens becomes more convex due to its own elasticity, increasing refractive power.
The pupil acts as a diaphragm. Pupillary reflexes include:
- direct light reflex — constriction of the pupil in the illuminated eye;
- consensual light reflex — constriction of the pupil in the opposite eye;
- convergence and accommodation reflex when looking at a near object.
Retina and phototransduction
The retina is a part of the brain extended to the periphery. It has an inverted structure: light passes through all layers of neurons before reaching the photoreceptors in the layer furthest from the light. Phototransduction — the conversion of light into a neural signal — takes place in the photoreceptors.
- Rods (~120 million) contain rhodopsin, provide scotopic black-and-white vision, have high light sensitivity but low acuity due to the convergence of many rods onto a single bipolar cell.
- Cones (~6–7 million) contain iodopsins, are responsible for photopic and color vision, and have high acuity. In the fovea centralis, each cone is connected via a midget bipolar cell to practically a single ganglion cell.
According to the Lomonosov–Young–Helmholtz theory, color vision is trichromatic: there are three types of cones — S (blue-violet), M (green), and L (red-yellow). Information from photoreceptors is transmitted to bipolar and then ganglion cells, whose axons form the optic nerve. The flow is modulated by horizontal cells (lateral inhibition, border contrast enhancement) and amacrine cells (motion detection).
Pathways and cortical processing
Ganglion cell axons form the optic nerve. At the chiasm, fibers from the nasal halves of the retinas decussate, while those from the temporal halves remain uncrossed. As a result, information from the left half of the visual field enters the right hemisphere and vice versa — the basis of binocular and stereoscopic vision. At the optic disc, the blind spot is formed, which the brain fills in via interpolation (Mariotte's experiment).
Next, the optic tract leads to several structures:
- The lateral geniculate nucleus of the thalamus is the main relay station that filters and directs the flow of information.
- The superior colliculi of the midbrain serve as an ancient center for involuntary reflex reactions to visual stimuli.
- The visual cortex of the occipital lobes is the final destination for conscious processing.
Cortical processing is hierarchical. The primary visual cortex (V1, Brodmann area 17) detects elementary features — lines, borders, orientations, directions of movement, preserving retinotopic projection. Secondary areas (V2, V3, V4) analyze complex shapes, surfaces, and color (V4 is especially important for color perception). Association areas (V5, etc.) in the parietal and temporal cortex are responsible for recognizing faces, objects, and perceiving space and motion.
Main visual functions
- Visual acuity is the ability to distinguish two points separately. It depends on cone density in the fovea centralis and the size of receptive fields. An angle of 1 minute is considered normal (Sivtsev and Golovin charts).
- The visual field is the space perceived by an immobile eye. It is examined by perimetry; narrowing or scotomas are diagnostic signs of glaucoma, retinal lesions, and optic nerve damage.
- Color vision is based on three types of cones. Impairments (color blindness) are detected using Rabkin's tables: a distinction is made between anomalous trichromacy and dichromacy.
- Adaptation: dark adaptation (up to 50 minutes, switch to rod vision, regeneration of rhodopsin, increase in summation) and light adaptation (5–10 minutes, decrease in sensitivity). It is measured using an adaptometer.
- Contrast sensitivity is provided by lateral inhibition in the retina. Simultaneous and successive contrast are distinguished.
Key points
- The visual analyzer includes the peripheral part, pathways, and cortical part with a multilevel hierarchy and feedback loops.
- Information is coded by impulse frequency, the number of active neurons, and temporal activity patterns.
- Helmholtz accommodation is provided by changing the curvature of the lens through the action of the ciliary muscle.
- Rods are responsible for scotopic vision (rhodopsin), while cones are responsible for photopic and color vision (iodopsins, three types: S, M, L).
- At the chiasm, fibers from the nasal halves of the retinas decussate, which provides binocular vision.
- The lateral geniculate nucleus of the thalamus is the main relay and filtering station of the visual system.
- The visual cortex operates hierarchically: from line detection in V1 to the recognition of complex patterns in association areas.
- The main visual functions — acuity, visual field, color perception, adaptation, and contrast sensitivity — have a clear physiological basis and diagnostic significance.