visual pathway Flashcards
What are the two kinds of blindness?
Normal blindness (eye dysfunction) and cortical blindness (brain dysfunction).
What are the main pathways from the primary visual cortex (V1)
The “What” pathway to the temporal cortex (identifying objects) and the “Where” pathway to the parietal cortex (locating objects)
What is visual agnosia?
Visual agnosia is the inability to identify objects and/or people, caused by damage to the inferior temporal lobe, disrupting the “what” pathway
What is visual neglect?
isual neglect is the inability to see objects in the left visual field, caused by damage to the right parietal lobe, disrupting the “where” pathway.
What are the main components of the visual system?
The eye (optics of image formation), retina (light transduction), lateral geniculate nucleus (waystation), area V1 (primary visual cortex), higher cortical areas (features), and cortical pathways (what/where).
Describe the optic chiasm.
The optic chiasm is where ganglion cell axons from the nasal parts of the retina cross, transmitting information from contralateral visual fields beyond the chiasm as part of the optic tract.
What happens to optic tract fibers?
Optic tract fibers terminate in the lateral geniculate nucleus (LGN) of the thalamus, each tract carrying input from the contralateral visual field.
What are optic radiations?
Axons of LGN neurons travel to the primary visual cortex (Area 17) via optic radiations, with upper visual fields taking a looping course into the temporal lobe and lower visual fields taking a more direct route.
What is the function of Area V1 (Primary Visual Cortex)?
Area V1, located on either side of and within the calcarine fissure, processes visual information, with lesions in this area resulting in blindness in the contralateral visual field.
What is the pupillary light reflex?
The pupillary light reflex is the reflex constriction of the pupil in response to light exposure, involving both the afferent (sensory) and efferent (motor) pathways.
What is the afferent limb of the pupillary light reflex?
The afferent limb is the optic nerve (cranial nerve II), which detects light and transmits the information to the brain.
What is the efferent limb of the pupillary light reflex?
The efferent limb is the oculomotor nerve (cranial nerve III), which controls the constriction of the pupil via the parasympathetic fibers.
What is a consensual pupillary light reflex?
The consensual pupillary light reflex is the constriction of the pupil in the eye opposite to the one exposed to light.
What happens in the case of an afferent pupillary defect?
In an afferent pupillary defect (e.g., Marcus Gunn pupil), the affected eye shows reduced or no constriction in response to light, while the consensual response in the unaffected eye remains intact.
How can the pupillary light reflex be tested clinically?
The pupillary light reflex can be tested using a flashlight to observe direct and consensual responses in both eyes, and the swinging flashlight test to detect relative afferent pupillary defects.
What is the role of the pretectal area in the pupillary light reflex?
The pretectal area in the midbrain receives input from the optic nerve and projects to the Edinger-Westphal nucleus, which initiates the parasympathetic response for pupil constriction.
What is the Edinger-Westphal nucleus?
The Edinger-Westphal nucleus is part of the oculomotor nucleus complex in the midbrain, responsible for sending parasympathetic fibers to the eye for pupil constriction.
What is the significance of the pupillary light reflex?
The pupillary light reflex is important for protecting the retina from excessive light exposure and is used clinically to assess the integrity of the optic and oculomotor nerves, as well as brainstem function.
What results from a lesion in the right optic nerve?
Blindness in the right eye.
What is the visual effect of a lesion in the optic chiasm?
Bitemporal heteronymous hemianopsia (loss of vision in the outer half of both visual fields
What occurs with a lesion in the right optic tract?
Left homonymous hemianopsia (loss of the left visual field in both eyes)
What is the consequence of a lesion in the left temporal lobe affecting Meyer’s loop?
Right upper homonymous quadrantanopia (loss of vision in the upper right quadrant of both visual fields).
What visual defect is associated with a lesion in the left parietal lobe?
Right lower homonymous quadrantanopia (loss of vision in the lower right quadrant of both visual fields).
What results from a lesion in the primary visual cortex (Area V1)
Contralateral homonymous hemianopsia (loss of vision in the opposite visual field of both eyes).
What is macular sparing and when does it occur?
Macular sparing is the preservation of central vision despite a lesion in the visual cortex, often due to dual blood supply
What is the visual outcome of a lesion in the lateral geniculate nucleus (LGN)?
Contralateral homonymous hemianopsia (loss of vision in the opposite visual field of both eyes).
What is a scotoma?
A scotoma is an area or “island” of visual loss within the visual field.
What is the difference between homonymous and heteronymous defects?
Homonymous defects affect the same side of the visual field in both eyes, while heteronymous defects affect opposite sides of the visual fields in both eyes.
