Neuroscience of Vision

what condition is blindness a major complication in?

diabetes mellitus

why is vision important in the context of this unit?

major contributor to motor behavior, can be used as a tool to treat some movement disorders

how many people worldwide have a visual impairment or blindness?

2.2 billion

Prevalence of diabetic retinopathy

80% in type 1 and 30% in type 2

what factors are associated w/ the development of diabetic retinopathy?

length of disease, mean A1C

where/how does visual processing begin?

at the retina w/ activation of photoreceptors and transduction along specific pathways

light passes through the _____ and forms an image on the ______

lens, retina

on the retina, how does the image appear?

inverted and reversed

central retinal region w/ highest visual acuity

fovea

the eye is focused so that central fixation point falls on _______

fovea of the retina

retinal region that surrounds the fovea, also has high visual acuity

macula

axons leaving the retina in form of the optic nerve, “blind spot” w/ no photoreceptors

optic disc

sensory receptors that allow transduction of visual wavelengths into electrical potentials

retinal photoreceptors

2 classes of retinal hotoreceptors

rods and cones

photoreceptors for vision in low light- more numerous, poor spacial/temporal resolution, do not detect color, concentrated in periphery

rods

photoreceptors for vision in bright light- high spatial/temporal resolution, detect color, concentrated in fovea

cones

which photoreceptors are used for more detailed vision?

cones

what happens to the retina during diabetic retinopathy

bvs damage that leads to micro-hemorrhages and retinal cell death

what happens once photoreceptors in the retina transduce light photons into electrical signals?

series of synaptic connections relay the signal throughout the retina, then the axons of the retinal ganglion cells form the optic and pass info on to central visual pathways

what does the visual field refer to?

the regions of the visual image detected by each eye (divided into quadrants

how are visual field deficits named?

according to visual field loss (not area of the retina)

deficit in half of the VF

hemianopia

defect in quarter of VF

quadrantanopia

VF deficits for both eyes are similar

homonymous

eyes have non-overlapping VF losses

Heteronymous

specific/discrete region of visual loss

scotoma

what damage could cause a monocular scotoma?

damage to specific area of the retina

what damage could cause monocular visual loss?

damage/trauma to optic nerve

what damage could cause bitemporal hemianopia?

damage at optic chiasm

what damage could cause a left contralateral homonymous hemianopia?

right optic tract, right LGN, regions of optic radiation or primary visual cortex

minority of fibers in the optic tract that bypass LGN and enter superior colliculus and pretectal area, associated w/ attention and orientation

extrageniculate pathways

what are the extrageniculate pathways important for?

pupillary light reflex and gaze control

where does the superior colliculus receive info from?

extrageniculate pathways, auditory systems, somatosensory systems

determines amount of light reaching retina, under autonomic control

pupillary reflex

Describe steps of parasympathetic control on the pupillary reflex

light optic tract synapse in sup colliculus nad pretectal areas→ projects to edinger-westpal nucleus→ preganglionic axons travel via CN III to ciliary ganglion and synapse on postganglionic neurons→ control pupillary constrictor muscles

basic path of the central visual pathways

optic nerve→ optic chiasm→ optic tract→ thalamus→ primary visual cortex

axons of ganglion cells enter the CNS w/ partial crossing at chiasm

optic nerve

Partial crossing of info detected by nasal portions of each retina

optic chiasm

Continuation of axons to lateral geniculate nucleas of thalamus after optic chiasm

optic tract

the left visual field is represented in the _______ optic tract

right

areas of the visual field detected by retinal regions of both eyes

binocular zones

areas of the visual field detected by retinal regions of only one eye

monocular zones

layers 1 and 2 of the LGN, relay info from specific retinal ganglion cells that detect motion and spatial analysis

magnocellular layers

layers 3-6 of the LGN, relay info from retinal ganglion cells that detect fine detain and color of visual image

parvocellular layers

axons leaving the LGN that terminate in the cortex adjacent to calcarine sulcus

optic radiations

where is the primary visual cortex located?

posterior region of the occipital lobe

cortical area above the calcarine sulcus where inferior VF are represented

cuneus

cortical area below the calcarine sulcus where superior VF are represented

lingula

receive primary projections from primary visual cortex to temporal and parietal areas

visual association areas

axons that project to the parietooccipital cortex and represent the “where” of visual info (motion, spatial relationships)

dorsal stream

originates in primary visual cortex and axons project to occipitotemporal association cortex, represent the “what” of visual image (form and detail of objects)

ventral stream

upper limb movement to reach for object in visual field, integrated through activation of primary and association cortices (visual, sensory, motor)

skilled reaching

why is visual input needed for grasping/skilled reaching?

object location, grasp aperature, reaching accuracy

at what points of grasping should you examine visual system?

start, visual attention, grasp, disengagement

how does vision contribute to postural control and gait?

avoid obstacles, proper foot placement, irregualr surfaces

motion of the visual environment projected onto the retina during movement in environment

optic flow

vision is integrated with ______ to control posture and gait

vestibular and proprioception