Visual system
The visual system is the part of the nervous system that is involved in the processing of visual information. The visual system comprises the eye with the retina ( retina), the optic nerve, parts of the thalamus and the brain stem as well as the visual cortex.
Optical apparatus
The optical apparatus of the eye ( transparent cornea, eye chambers, iris, lens, vitreous ) generated on the retina upside-down and reversed image of the area in the visual field.
The fovea is the site with the highest possible image sharpness. However, the eye can only during the dwell time of about 0.2 to 0.3 seconds, such a fixation point remain ( Optokinetic nystagmus).
From the fovea, about half of the optic nerve goes (below in yellow) to the center of vision. The remaining half of the optic nerve are reserved for the peripheral system, which captures up to 90 compressed images of the visual field per second.
Visual pathway
The light stimuli are registered by the sensory cells of the retina, the rods and cones and the changes in their membrane potential shown ( receptor potential). About the bipolar cells forwarded to the ganglion cells, action potentials can be triggered here. The projections of the retinal ganglion cells form the cranial nerve II ( optic nerve ), which forwards the action potentials.
The visual pathway directs the action potentials to the visual cortex: After entering the cranial cavity cross the optic nerves of both eyes in the optic chiasm ( optic chiasm ). Uncrossed extend the outer ( temporal ) fibers further, while the inner ( nasal ) cross to the opposite side. In the way the fibers run the left half of the retina of both eyes in the left brain and the retina of the right halves in the right. In the two optic tracts these nerve fibers extend to the Lateral geniculate lateral geniculate body of the thalamus, where they have the wide range of optic radiation ( optic radiation ) to the visual cortex ( visual cortex) are forwarded.
Visual perception
Visual perception means - beyond just "seeing" beyond going - a " explicit symbolic description of the observed scene " given the image projected on the retina by the optical apparatus image of the scene is already in the retina analyzed (brightness, color, contrast, movement) and edited ( brightness balance, contrast enhancement). When forwarding via the optic nerve and geniculate the spatial location relationships of the receptors in the positional relationships of nerves and synapses are retained ( so-called retinotopy ). This positional relationship is detectable in the visual cortex as a neural map. The activity of the nerve cells of this card represents the perception of the scene, but in a distorted form: left and right halves are separated from each other in the right and left brain, the center of the scene ( the fixation point ) as the main part of the scene is represented by a larger region than the edge.
The identification of individual elements and their importance probably occurs by comparison with previously stored experiences (scenes associated with body sensation, emotion, smell, noise, and much more ).
Eye reflexes
Associated with the visual pathway are the nerves of the eye reflexes. Reflexes that control the automatic fixing of static or moving objects, use the already processed spatial information from the visual cortex.
Protective reflexes
- Pupillary reflex - Compensation of abrupt light -dark cycle by changing the aperture size of the pupil.
- Blink reflex to protect from bright light and desiccating wind and approaching foreign bodies.
Fixation reflexes
- Accommodation: Adaptation of the optical properties of the lenses at the distance of the object being focused by changing the lens curvature
- Convergence: alignment of the visual axes of the two eyes at the distance of the focused object. At distant objects, the axes are aligned in parallel. The closer the object, the more need to be rotated inside the eye. This is done simultaneously by the respective nasal underlying eye muscle ( medial rectus ).
- Eye rotation movement: alignment of the visual axes to compensate for head movements or in the pursuit of moving objects. This is done by a coordinated action of the eye muscles, made possible by connections between the nuclei of the eye muscles in the " center view " of the top two hills of the tectum (see also optokinetic nystagmus).
- Saccadic eye movements ( saccades ) are rapid, jerky eye movements. You used to supplement the peripheral perception and the existing ideas. They also occur in the dreams and the imagining of visual ideas.