Visual Pathways Flashcards
Define the term ‘visual field’ and explain why it consists of a binocular zone and two monocular crescents. Describe how the visual field is projected onto each retina and the relationship of each retinal image to the visual field.
a visual field is the view percieved by the eyes
the binocular zone is central and large because both eyes can easily see straight ahead (it’s the overlap of the two eye’s visions)
monocular zones are really crescents surrounding the binocular zone
the left visual field is projected onto the right side of the retina and vice versa
top and bottom are also flipped onto the retina
List the input source, sensitivities and conductance properties that distinguish M and P type retinal ganglion cells.
M ganglion cells get input from the peripheral retina, get rod input, are large and fast-conducting, respond to movement and high contrast, and conduct to layers 1 & 2 of the Lateral Geniculate Body
P ganglion cells are in central retina, get input from cones, respond to fine detail and color, are slow conducting, and conduct to layers 3 4 5 and 6 of LGB
m is magnocellular, p is parvocellular
Sketch the pathway by which visual information in the left visual hemifield reaches the primary visual cortex. Start with the projection of the light onto each retina, then, trace the pathway of the neural signal from each retina to the primary visual cortex. Indicate which neurons in the pathway receive monocular and which receive binocular input.
left visual field hits right side of both retinas, projects through optic nerves to right LGB and then to right primary visual cortex
neurons of the right LGB recieve monocular input b/c each layer recieves only one eye, sends axons through optic radiations to primary visual cortex (superior bundle carries lower visual quadrant while lower bundle loops around inferior horn of 4th ventricle and carries upper visual quadrant)
Identify the site(s) of lesions that would result in: (1) bitemporal heteronymous hemianopsia; (2) right homonymous hemianopsia; (3) left superior quadrantanopsia; and (4) right inferior quadrantanopsia. For each deficit, sketch how each eye’s visual field would be perceived by the patient.
1) midline of optic chiasm
2) left optic tract
3) the right side lower bundle of optic radiation
4) the left side upper bundle of optic radiations
Describe the two pathways from the Primary Visual Cortex (V1) to higher visual association areas and their roles in visual processing.
the M layer of the layer IV of V1 projects through V5 up to the superior parietal cortex, this does “where is it” vision: motion, spatial analysis, visual attention
the P layer of layer IV of V1 projects through V4 down to the inferior temporal lobe, does “what is it” vision: recognizing colors, objects, visual memory
Describe the following disorders of central visual processing (visual agnosias): simultanagnosia; prosopagnosia; akinetopsia; cortical color blindness.
simultagnosia: inability to recognize multiple objects at once or only able to recognize single objects within a whole, often bilateral lesion of superior parietal cortex
prosopagnosia: can’t recognize faces, a P pathway lesion
akinetopsia: loss of motion perception, like viewing world through a strobe light, M pathway lesion
achromatopsia: cortical color blindness, P pathway
where do the axons of the optic tract terminate?
the lateral Geniculate body, the superior colliculus via the brachium of the superior colliculus, and the pretectal area of the midbrain, and the hyposthalamic suprachiasmatic nucleus (which regulates circadian rhythm)
what does the superior colliculus do?
it mediates eye reflexes to visual stimuli
what does the pretectal area do?
mediates the pupillary light reflex
what is the input of neurons to each layer of the LGB?
it has 6 layers
1 and 2 are Magnocellular (rod input, motion and contrast) while layers 3 4 5 6 are Parvocellular (cone input, color and acuity)
1, 4 and 6 are crossed while 2 3 5 are ipsilateral eye
this is monocular input
what layer do the axons of LGB neurons terminate in at V1?
layer IV
