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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