Psychophysics of measurements, weber’s law, visual threshold & sensitivity

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1. Psychophysics of measurements, Introduction to Weber’s law, visual threshold & sensitivity Mohammad Arman Bin Aziz Optometry Faculty ICO

2. What is Vision Science? The study of mechanism of transformation of light entering the eye into the percept of vision and appreciation of the world around us. Based on many different scientific disciplines

3. Why Study Vision Science? Vision Science Helps us understand Many Clinical Aspects. Vision Tests. Patients Diseases

4. Vision Science and Visual Tests Why we use particular tests? How to choose appropriate tests? What these tests reveal about the visual system? Why they are done in the specific way they are?

5. Vision Science and Patients Why patients respond they way they do on vision tests and what their responses really mean What you, the clinician, can do to treat the vision loss or help the patients

6. Vision Science and Patients Why and how certain diseases affect vision. We can predict where a physical problem or lesion might be located from the visual signs and symptoms that the patient presents .

7. Vision Science Studies the Brain The visual system makes up about 40% of the brain!!! The sensory parts of the eyes develop as an outgrowth of the brain and are composed of neural tissue.

8. Our Model of Vision Four main sources contribute to our understanding of vision. Neuro anatomy Neurophysiology Psychophysics Clinical Data

9. Studying Visual Science Four main sources contribute to our understanding of vision. Neuro anatomy Neurophysiology Psychophysics methods Computational approach Clinical approach

10. Anatomical Approach Study of structure of the visual system Morphology Connections How does each structure contribute to the function of the visual system? Structure of function when different parts are damaged.

11. Neuro-physiological approach Study of the flow of information through individual neurons or brain areas in the visual system via. Receptive field recordings Single unit recording Neuro imaging MRI

12. Psychophysical Methods Study of psycho-physical sensation evoked by a particular physical stimulus What we ‘see’ Visual system is often studied as a ‘black box’.

13. Computational approach Use of mathematics or computer programs to study the calculations the visual system uses to process visual information Development of computer- based visual systems with robotics, artificial intelligence and image processing algorithms.

14. Clinical approach Everything we learn scientifically must be modified by what real patients do in the clinic. Some of our knowledge of the visual system comes from patients with disorders in which the visual system malfunctions.

15. Which Approach Do We Use? The most important approach Most Optometric tests are really psycho-physical tests of visual function. We measure thresholds (the limits of what we can see) to determine the sensitivity of our visual systems. Example visual acuity.

16. Stages of Visual Processing Encoding the retinal image Conversion of light energy into neural signals Imperfections with this process limit what we see Representation within the visual system How the neural signals change as they travel through the vision system. Adaptation helps correct some imperfections Parallel processing steams. Interpretation of the visual How we put together all of this information into a single view of the world around us.

17. Visual Information Our visual system modifies and rearranges information as it is processed. Integration of information Combining information together to simplify it Segregation of information Sending information to different neurons to process different aspects of it Allows parallel processing (faster than serial processing) THIS TRANSFORMKS INTO WHAT WE SEE.

18. Encoding the Intensity of Light Absolute threshold for Light The dimmest light that can be detected How many quanta of light are needed to change the membrane potential? How many quanta of light do we need to ‘see’ the light?

19. Key Points Only one quanta of light must be absorbed by a photoreceptor to yield an electrical response A total of 7 photoreceptors must be excited to produce a visual sensation of light .

20. Threshold What is Threshold? The minimum amount of energy required for detection of a stimulus Clinical examples Visual Acuity Testing Visual field testing

21. Visual Thresholds The minimum amount of energy required for a patient to detect a stimulus Low threshold = high sensitivity. Threshold = 1/Sensitivity Scotopic Threshold: threshold of a patient measured in dim light conditions (night) Photopic threshold: threshold of a patient measured in bright light conditions (sunny)

22. Sensitivity What is Sensitivity? How well the subject can detect a stimulus Threshold = 1/(Sensitivity) A low threshold indicates a high sensitivity.

23. Absolute Sensitivities Highest Possible Sensitivity of a system. How do we differentiate the system? Individual is placed in a dark room for 45 min. All photo pigments fully regenerate

24. Photochromatic Interval When the stimulus is detected by the cones (Photopic system), color will be perceived The photochromic interval is the difference in sensitivity between the Photopic and Scotopic systems. Scotopic system is more sensitive to all wavelengths except the long wavelength region (red color) Photopic system is more sensitive to wavelengths > 650 nm

25. Purkinje Shift Scotopic system : stimuli of 507 nm are perceived brighter than other stimuli. Photopic system : stimuli of 555 nm are perceived brighter than other stimuli. The difference in the peak sensitivity of the 2 systems is the ‘ purkinje shift’. The relative brightness of different wavelength as from cone to rod vision

26. Weber’s Law Describes threshold while considering the background illumination. Many of psychophysical tests require the patients to distinguish the background and stimulus from the background alone visual field testing.

27. Weber’s Law “equal relative increments of stimuli are proportional to equal increments of sensation“ Weber's Law states that the ratio of the increment threshold to the background intensity is a constant Weber's Law is not always true, but it is good as a baseline to compare performance and as a rule-of-thumb.

28. Weber's Law Weber's Law is related to the Just Noticeable Difference (also known as the difference threshold), which is the minimum difference in stimulation that a person can detect 50 percent of the time. the stimuli must differ by a constant "proportion" not a constant "amount".

29. What does This mean Clinically? Visual Acuity Testing Stimulus background lighting . Dark optotype on a light background vs. light optotype on a dark background. Simultaneous Contrast Phenomenon that demonstrates that the brightness of a stimulus depends on the background.

30. References Visual Perception by Steven H. Schwartz Sensation and Perception by E. Bruce Goldstein Visual perception: An introduction Nicholas J.Wade and Michael T.Swanston