The pupil appears black because of the layer of black pigmented cells that line the back of the eye and absorb the light. The diameter of the pupil is controlled by the iris , a circular muscle whose pigmentation gives the eye its colour and whose contraction lets the eye adapt continuously to changing light conditions.
On a dark night, your pupils are big and black, because your irises open wide to let in as much as possible of the little light available. This reaction is called the pupillary reflex. You can observe it easily yourself, by watching your eyes in a mirror while you turn a nearby light on and off.
After passing through the pupil, the light goes on through the lens , which is suspended between the aqueous humour and the vitreous humour , the fluid that fills the inside of the eye.
The lens in turn focuses the light rays onto the retina, lining the back of the eye. The retina converts the image formed by the light rays into nerve impulses. The optic nerve, composed of the axons of the retina's ganglion cells , then transmits these impulses from the eye to the first visual relay in the brain. The axons of the retina's ganglion cells collect in a bundle at the optic disc and emerge from the back of of the eye to form the optic nerve.
The optic nerve is the pathway that carries the nerve impulses from each eye to the various structures in the brain that analyze these visual signals. The vast majority of the nerve fibres in the optic tract project to the lateral geniculate nucleus LGN in the dorsal part of the thalamus. The LGN is the main relay in the pathway to the primary visual cortex.
The projection from the LGN to the visual cortex is called the optic radiation. Because damage at any point along the pathway from the retina to the cortex results in some degree of blindness, this is clearly the pathway through which conscious visual perception takes place in human beings.
Brodmann Areas. Seeing without knowing it : the strange phenomenon of blindsight. People whose primary visual cortexes have been damaged consider themselves to be blind and unable to discern anything in their visual environment. But if you ask these people to "take a chance" and point their finger at a dot of light in space, they will point straight at this target.
And the data show that this result is not random. V3 communicated directly with the respective dorsal and ventral subsystems of V2.
Dorsal V3 seems to play a role in processing motion, while ventral V3 may play a role in color sensitivity. V3 as a whole is less well-defined compared to other areas of the visual cortex.
V4 receives information from V2 and is part of the ventral processing stream. Cells in V4 are very responsive to color. The inferotemporal cortex is located along the lower inferior portion of the temporal lobe.
This area of the brain is part of the ventral processing stream and seems to respond best so simple shapes circle, square, etc. Over Vivid Vision Providers prescribe virtual reality alongside patching and vision therapy to treat your lazy eye. Sign up through our doctor locator to see if Vivid Vision is right for you.
Visual Cortex The visual cortex is located in the occipital lobe of the brain and is primarily responsible for interpreting and processing visual information received from the eyes. Primary Visual Cortex V1, striate cortex, Brodmann area 17 The brain is filled with depressions or grooves sulci and elevations gyri.
Visual Area Three V3 V3 communicated directly with the respective dorsal and ventral subsystems of V2. Visual Area Four V4, extrastriate cortex V4 receives information from V2 and is part of the ventral processing stream. Visual Area Five V5, middle temporal cortex V5 is part of the dorsal processing pathway and contains cells highly sensitive to motion. Inferotemporal Cortex The inferotemporal cortex is located along the lower inferior portion of the temporal lobe.
Information concerning moving targets and information governing scanning of the eyes travels to a second site in the brainstem, a nucleus called the superior colliculus. The superior colliculus is responsible for moving the eyes in short jumps, called saccades.
Saccades allow the brain to perceive a smooth scan by stitching together a series of relatively still images. Most projections from the retina travel via the optic nerve to a part of the thalamus called the lateral geniculate nucleus LGN , deep in the center of the brain. The LGN separates retinal inputs into parallel streams, one containing color and fine structure, and the other containing contrast and motion. Cells that process color and fine structure make up the top four of the six layers of the LGN; those four are called the parvocellular layers, because the cells are small.
Cells processing contrast and motion make up the bottom two layers of the LGN, called the magnocellular layers because the cells are large.
The cells of the magnocellular and parvocellular layers project all the way to the back of the brain to primary visual cortex V1. Cells in V1 are arranged in several ways that allow the visual system to calculate where objects are in space.
First, V1 cells are organized retinotopically, which means that a point-to-point map exists between the retina and primary visual cortex, and neighboring areas in the retina correspond to neighboring areas in V1. This allows V1 to position objects in two dimensions of the visual world, horizontal and vertical. The third dimension, depth, is mapped in V1 by comparing the signals from the two eyes.
Those signals are processed in stacks of cells called ocular dominance columns, a checkerboard pattern of connections alternating between the left and right eye.
A slight discrepancy in the position of an object relative to each eye allows depth to be calculated by triangulation. Finally, V1 is organized into orientation columns, stacks of cells that are strongly activated by lines of a given orientation.
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