Vision science studies the mechanisms by which light is transformed into visual perception and appreciation of the world. It draws on many disciplines like neuroanatomy, psychophysics, and clinical data. Psychophysics methods measure visual thresholds and sensitivity to understand the limits of human vision. Weber's law states that the minimum detectable difference between a stimulus and background is proportional to the background intensity. This relates to clinical tests like visual acuity which use different background light levels and stimulus contrasts.

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