2. Sensory Physiology Vision, Hearing, and Orientation

3. 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

4. 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

5. 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

6. 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

7. 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

8. 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

9. 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

10. 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

11. 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

12. 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

13. 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.

14. 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”

15. 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

16. 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

17. 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

18. 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

19. 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

20. 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

21. 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