Visual Pathway & Visual Processing

Visual field

What is seen with one eye alone; with both eyes open, the visual field can be divided into right and left visual fields around a central focal point.

Binocular vision

Center of the visual field that is viewed by both eyes.

Monocular vision

Edges of the visual field seen only by the eye on the same side.

Nasal retina

Portion of the retina nearest the nose.

Temporal retina

Portion of the retina nearest the temple.

Left visual field retinal detection

Detected by the nasal retina of the left eye and temporal retina of the right eye.

Right visual field retinal detection

Detected by the temporal retina of the left eye and nasal retina of the right eye.

Retinal ganglion cell axons

Axons that leave the retina and form the optic nerve.

Optic nerve

Cranial nerve formed by axons of retinal ganglion cells carrying visual information from an eye toward the brain.

Optic chiasma

X-shaped structure where the two optic nerves meet and some retinal axons cross to the opposite side.

Fibers that cross at the optic chiasma

Axons originating from the nasal retina of each eye.

Fibers that do not cross at the optic chiasma

Axons originating from the temporal retina of each eye remain on the same side.

Purpose of crossing at the optic chiasma

Allows all information from one visual field to be processed by the opposite cerebral hemisphere.

Left visual field processing

All stimuli from the left visual field are processed by the right cerebral hemisphere.

Right visual field processing

All stimuli from the right visual field are processed by the left cerebral hemisphere.

Optic tract

Bundle of visual axons leaving the optic chiasma and traveling toward brain visual-processing centers.

Main destination of optic tract fibers

Most terminate in the lateral geniculate nucleus of the thalamus.

Lateral geniculate nucleus

Thalamic nucleus that receives most visual information from the optic tracts and relays it toward the primary visual cortex.

Other destinations of optic tract fibers

About 10% travel to the superior colliculus or pretectal area of the midbrain.

Optic radiations

Axons of lateral geniculate neurons that fan through the temporal and parietal lobes toward the primary visual cortex.

Primary visual cortex

Visual cortex in the medial portion of the occipital lobe where conscious awareness of visual stimuli occurs.

Main conscious visual pathway

Retina → optic nerve → optic chiasma → optic tract → lateral geniculate nucleus of thalamus → optic radiations → primary visual cortex.

Superior colliculus

Midbrain structure that coordinates eye movement to focus on new objects and communicates with neurons controlling head and neck movements.

Pretectal area

Midbrain area involved in the consensual pupillary response.

Consensual pupillary response

Reflex in which shining light into one eye causes both pupils to constrict.

Optic tract fibers in the pupillary response

Some axons from each optic tract terminate on both sides of the midbrain in the pretectal area.

Pretectal pathway to the pupils

Pretectal neurons contact neurons whose axons travel through the oculomotor nerves to the pupillary muscles of both eyes.

Result of the consensual pupillary response

Both pupils constrict even when light is directed into only one eye.

Clinical importance of the consensual pupillary response

Failure of the reflex may indicate damage to the retina, optic nerve, or brainstem.

Stereoscopic vision

Depth perception produced when the brain compares the slightly different images received by the two eyes.

Depth perception

Ability to perceive the distance of an object from the eye.

Why binocular vision allows depth perception

Overlapping visual fields allow the brain to compare the images from the two eyes and use their differences to estimate distance.

Effect of losing vision in one eye on stereoscopic vision

The brain can no longer compare images from the two eyes, so stereoscopic vision is lost.

Visual mapping in the brain

Retinal axons project so that visual images are mapped onto the lateral geniculate nucleus and primary visual cortex.

Representation of photoreceptor-rich retinal areas in the brain

Areas of the retina containing more photoreceptors occupy more processing area in the brain.

Primary visual cortex processing

Processes location, movement, shape, and color and combines images for binocular vision.

Two general visual-processing pathways

Posterior (dorsal) pathway and anterior (ventral) pathway.

Posterior (dorsal) visual pathway

Involves the parietal lobe and is involved in interpreting motion.

Anterior (ventral) visual pathway

Ends in the inferior portion of the temporal lobe and processes color and objects such as people's faces.

Dorsal pathway key function

Interpreting motion.

Ventral pathway key functions

Processing color and recognizing objects such as faces.

Prosopagnosia

Inability to recognize people by their faces caused by damage involving the anterior/ventral visual-processing pathway.

Achromatopsia

Complete loss of color perception resulting from damage involving the anterior/ventral visual-processing pathway.

Additional uses of processed visual information

Visual pathways connect with other brain regions to form visual memories, read aloud, direct extrinsic eye muscles, and make behavioral decisions based on vision.

Visual pathway hemisphere rule

Left visual field → right hemisphere; right visual field → left hemisphere.

Optic chiasma crossing rule

Nasal retinal fibers cross; temporal retinal fibers stay on the same side.