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…– 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)– 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)– 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