Psychophysics of measurements, Introduction to Weber’s law, visual threshold & sensitivity
Mohammad Arman Bin AzizOptometry Faculty
ICO
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
Why Study Vision Science?
• Vision Science Helps us understand Many Clinical Aspects.
–Vision Tests.–Patients–Diseases
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?
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
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.
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.
Our Model of Vision
• Four main sources contribute to our understanding of vision.–Neuro anatomy –Neurophysiology–Psychophysics–Clinical Data
Studying Visual Science
• Four main sources contribute to our understanding of vision. –Neuro anatomy–Neurophysiology– Psychophysics methods– Computational approach– Clinical approach
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.
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
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’.
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.
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.
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.
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.
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.
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?
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.
Threshold
• What is Threshold?– The minimum amount of energy required for
detection of a stimulus• Clinical examples– Visual Acuity Testing– Visual field testing
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)
Sensitivity
• What is Sensitivity? – How well the subject can detect a stimulus
• Threshold = 1/(Sensitivity) – A low threshold indicates a high sensitivity.
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
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
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
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.
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.
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".
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.
References
Visual Perception by Steven H. SchwartzSensation and Perception by E. Bruce
GoldsteinVisual perception: An introduction Nicholas
J.Wade and Michael T.Swanston