1. Sensation and Perception - duplicity.ppt © 2001 Dr. Laura Snodgrass, Ph.D.1 Duplicity Theory Two independent visual systems Photopic system – day vision (high light) composed primarily of cones Scotopic system –a night vision (low light) composed primarily of rods

2. Sensation and Perception - duplicity.ppt © 2001 Dr. Laura Snodgrass, Ph.D.2 8 Differences Light sensitivity Spectral sensitivity Distribution on retina Interconnections Acuity Response to movement CFF - critical flicker frequency Signaling properties

3. Sensation and Perception - duplicity.ppt © 2001 Dr. Laura Snodgrass, Ph.D.3 Light Sensitivity Total range of sensitivity is 100,000 Rods have an absolute threshold 1-10K lower than cones The range of sensitivity is accomplished by dark and light adaptation –photopigment “bleaches” (breaks down) in the light –photopigment regenerates in the dark –so in the dark, more pigment is available and the eyes are more sensitive

4. Sensation and Perception - duplicity.ppt © 2001 Dr. Laura Snodgrass, Ph.D.4 Dark Adaptation Curve

5. Sensation and Perception - duplicity.ppt © 2001 Dr. Laura Snodgrass, Ph.D.5 Dark And Light Adaptation Dark adaptation –as more photopigment regenerates, switch from the less sensitive cones to the more sensitive rods. End up using only rods because not enough light intensity to trigger cone photopigment Light adaptation –use cones because all rod photopigment is broken down

6. Sensation and Perception - duplicity.ppt © 2001 Dr. Laura Snodgrass, Ph.D.6 Spectral Sensitivity

7. Sensation and Perception - duplicity.ppt © 2001 Dr. Laura Snodgrass, Ph.D.7 Purkinje Shift

8. Sensation and Perception - duplicity.ppt © 2001 Dr. Laura Snodgrass, Ph.D.8 Differences In Spectral Sensitivity Purkinje shift –different colors appear brighter because of diff in sensitivity, which is brighter depends on overall illumination Scotopic NOT sensitive to deep red light –use deep red for dark adaptation Scotopic NOT process color –photopic has three cone types needed for color processing

9. Sensation and Perception - duplicity.ppt © 2001 Dr. Laura Snodgrass, Ph.D.9 Three Cone Types The interaction of the three cone types is what signals color differences

10. Sensation and Perception - duplicity.ppt © 2001 Dr. Laura Snodgrass, Ph.D.10 Distribution On The Retina Cones in fovea No rods in fovea No receptors in blind spot What does this mean for color perception, acuity, and motion detection?

11. Sensation and Perception - duplicity.ppt © 2001 Dr. Laura Snodgrass, Ph.D.11 Interconnections Photopic system –one-to-one wiring, direct line to brain –produces spatial discrimination Scotopic system –one-to-many wiring, convergence –produces spatial summation

12. Sensation and Perception - duplicity.ppt © 2001 Dr. Laura Snodgrass, Ph.D.12 Spatial Discrimination

13. Sensation and Perception - duplicity.ppt © 2001 Dr. Laura Snodgrass, Ph.D.13 Spatial Summation

14. Sensation and Perception - duplicity.ppt © 2001 Dr. Laura Snodgrass, Ph.D.14 Acuity Photopic acuity –measured by Snellen charts –degree of spatial discrimination Problems: –Presbyopia - loss of lens elasticity –astigmatism - corneal imperfections –cataracts - cloudy lens

15. Sensation and Perception - duplicity.ppt © 2001 Dr. Laura Snodgrass, Ph.D.15 Opias Acuity errors caused by length of the eyeball or curvature of the lens. Myopia - Hyperopia -

16. Sensation and Perception - duplicity.ppt © 2001 Dr. Laura Snodgrass, Ph.D.16 Response To Movement Scotopic system –wired to detect motion, very sensitive –very sensitive to motion in the periphery Photopic system –less sensitive to motion Night time driving

17. Sensation and Perception - duplicity.ppt © 2001 Dr. Laura Snodgrass, Ph.D.17 CFF (critical flicker frequency) Frequency at which a flickering light appears fused –index of temporal discrimination Photopic –light fuses at lower frequency –temporal summation - add up light over time Scotopic –higher frequency –temporal discrimination

18. Sensation and Perception - duplicity.ppt © 2001 Dr. Laura Snodgrass, Ph.D.18 Signaling Properties Photopic –sustained signal –continues to fire as long as stimulus is present Scotopic –transient signal –fires at onset and offset, brief signal Contributes to difference in CFF