Sensory Physiology
Vision, Hearing, and Orientation
Light Refraction
• Light is refracted whenever it
passes between material of
different densities
• Light passing through the eye
is refracted by…
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cornea
aqueous humor
lens
vitreous humor
• Focus light on fovea centralis
Ciliary Muscles and Lens
• Lens
– solid but pliable transparent body
– used to focus light on the retina
• Ciliary Muscle
– ring-shaped smooth muscle
– linked to lens by suspensory
ligaments
– adjusts shape of lens to focus light
Accommodation
• Changing lens shape to focus light from
objects at different distances
• Far objects
– light from narrow range of angles
– ciliary muscles relax, lens stretched
– less convex, less bending of light
• Near objects
– light from wide range of angles
– ciliary muscles contract, lens recoils
– more convex, more bending of light
Refractive Power
• Strength by which a lens bends light
• In eye, only lens has variable refractive power
–  lens convexedness,  refractive power
– Focus light from objects different distances on the
fovea
• Refractive Power (diopters) = 1 / focal length (m)
– focal length = 0.25 m
• RP = 4 diopters
– focal length = 0.50 m
• RP = 2 diopters
Refractive Power of Eye
• Distance from lens to fovea ~1.5 cm
– RP = 67 diopters for light from distant
objects
– RP can be increased to 79 diopters by
thickening lens to observe close objects
• Focus light on retina
• Enhance visual acuity for objects at
different distances
– Ability to discriminate between points in
the visual field
Refractive Power and Visual
Disorders
• myopia (nearsightedness)
• distant object brought into focus in front of the retina
– Elongated eyeball
– Abnormally high convexedness to cornea or lens
• Too much refractive power
• corrected w/ concave lenses
Refractive Power and Visual
Disorders
• hyperopia (farsightedness)
• close object brought into focus behind of the retina
– Shortened eyeball
– Abnormally low convexedness to cornea or lens
• too little refractive power
• corrected w/ convex lenses
Refractive Power and Visual
Disorders
• Astigmatism
– Oblong shape to cornea or lens (not perfect hemisphere)
– refraction of light in horizontal plane ≠ that in the vertical plane
• Corrective lens prescriptions
– +3 (diopters) = convex lens for hyperopia
– -2 (diopters) = concave lens for myopia
– astigmatisms include strength of lens and axis of defect
• e.g. +2 axis 90 = horizontal plane
Age-related Changes in
Accommodation
• Throughout, continuous stretching of lens
• Lens loses elasticity with age
– Remains in “stretched” state
– Loses ability to increase refractive power
• Presbyopia (aka presbyopta)
– Far-sightedness associated with age
– Analyzed with near point of vision test
• 8 cm at age 10, 100 cm at age 70
Experiments:
Visual Accommodation
• Snellen Eye Chart (myopia)
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20’ from chart
Test one eye at a time
Read smallest font possible
Determine visual acuity based on distance associated
with each font size
• Astigmatism Chart (astigmatism)
– Test one eye at a time
– If astigmatism present, one set of lines (axis of
astigmatism) will be sharper and darker than the others
Experiments:
Visual Accomodation
• Near Point of Vision (Presbyopia)
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Test one eye at a time
Place meter stick on bridge of nose
Focus on pencil tip
Draw tip along meter stick towards eye
Point at which tip just begins to become fuzzy
= near point of vision.
Retina
• Inner layer of the eye
• Contains photoreceptors
– Rods – light intensity (scotopic)
– Cones – color, high acuity (photopic)
• Fovea centralis
– point where light is focused
– high density of cones
– High acuity
• Optic disk
– where optic nerve joins the eye
– no photoreceptors - “blind spot”
Blind Spot Experiment
• Cover right eye
• Hold paper in right hand at arm length, with + sign sticking
out to the right
• Looking directly at black spot, move paper toward eye
• Note that at one point the + sign disappears from
peripheral vision
Stereoptic Vision and Depth
Perception
• Visual fields of eyes
overlap
• Viewing of object in both
visual fields allows depth
perception
– Near objects – lateral
projection on retinas
– Far objects – projection at
center of retinas
3-D Vision
• One person holds test
tube at arms length
• Other holds pencil in
arm upright
• Try to swing down
lower arm to place
pencil directly in test
tube
• Repeat, with one eye
closed
Sound Conduction and Deafness
• Sound can be perceived from vibrations of the
skull as well as conducted through the ear
• Vibrations to skull can be used to diagnose basic
type of deafness
– Conductive deafness
• damage to conduction system (tympanic membrane, ear bones,
etc.)
• Can hear skull vibrations
– Sensorineural deafness
• damage to sensors or nerves (cochlea, auditory nerve, etc.)
• Cannot hear skull vibrations
Tests
• Rhinne Test
– Place tuning fork on mastoid process
• Webers test
– Place tuning fork on midsagittal line
• Binaural sound
– Follow direction of sound with eyes closed
Orientation, Balance and
Coordination
• Orientation and balance rely on numerous inputs
– Vestibular apparatus – detects movement and
orientation of head
– Touch, pressure and proprioception
• Indicate mechanical forces acting on rest of body
– Vision
Orientation, Balance and
Coordination
• Experiment – time how long you can stand on one
foot without losing your balance while…
– Keeping your eyes open
– Keeping your eyes closed without touching furniture,
counters, etc.
– Keeping your eyes closed and touching one finger to a
countertop