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