3. The Light Sense II 2324.pptx

The Light Sense II Introduction. Objectives and Readings. Light adaptation. Increment thresholds – threshold vs intensity (tvi) curves. Weber’s Law Ricco’s Law. Application. SFO1004 Dr Sarah J Waugh© Objectives and readings The student should be able to: • Understand the term increment thresholds...

The Light Sense II Introduction. Objectives and Readings. Light adaptation. Increment thresholds – threshold vs intensity (tvi) curves. Weber’s Law Ricco’s Law. Application. SFO1004 Dr Sarah J Waugh© Objectives and readings The student should be able to: • Understand the term increment thresholds and know how they are measured. • Describe the essential parts of the tvi curve in photopic vision. • Define Weber’s law of visual sensitivity. • Define Ricco’s area and Ricco’s law. SFO1004 Optometry: Science, Techniques and clinical management. Rosenfield and Logan. • Chapter 5 (pages 71-73) https://webvision.med.uta h.edu/ • Part VII: Light and dark adaptation • Part VIII: Visual acuity Dr Sarah J Waugh© Light adaptation and increment thresholds • We know about how the eye can adapt to the dark. • What about the ability to detect small light changes under different conditions? • The eye has to manage significant changes in luminance in daylight in order to operate effectively. • This can be studied SFO1004 through measurement of Lb ΔL Dr Sarah J Waugh© Light adaptation and increment thresholds Increment Lb ΔL ΔL Intensity • The increment threshold is the smallest amount of energy which must added to a stimulus in order to detect a change in the stimulus. • Consider a small spot of light, ΔL, that is flashed on a background of luminance Lb. • The observer has to just discriminate the spot (increment) on the background (this is called the ‘just noticeable difference’ or jnd). • SFO1004 This is procedure is reminiscent Front-on view of stimulus Backgrou nd Distance Cross-sectional view of stimulus Dr Sarah J Waugh© Lb Light adaptation and increment thresholds • As the background intensity increases the intensity of the increment (or the jnd) has to increase to be detected. • However the ratio of the jnd (ΔL) to the background (Lb) is constant. • This relationship is called Weber’s law and can be expressed by the equation seen. • ΔL = increment threshold (JND) • SFO1004 Lb = background luminance Lb ΔL Δ𝐿 =𝑘 𝐿𝑏 Dr Sarah J Waugh© Weber’s law • The ratio of the jnd (ΔL) to the background (Lb) is constant. • ΔL = increment threshold (JND). • Lb = background luminance. • K = Weber’s constant (or fraction). • Weber’s law applies to a number of senses in addition to vision. SFO1004 ΔL Lb Δ𝐿 =𝑘 𝐿𝑏 Dr Sarah J Waugh© Increment thresholds and tvi curves • Present a background luminance, Lb to adapt the eye (which may be coloured). • Small test field ΔL, added to the background to measure the smallest perceptible light increment (jnd). • Repeat with different backgrounds. • Derive a threshold vs intensity curve (‘tvi’ curve). SFO1004 Lb ΔL Dr Sarah J Waugh© • Log threshold intensity is plotted against background luminance. • At high background luminances, cones determine the threshold • At low luminances rods are the determining factor. • Weber’s law: – ΔL/L = constant. SFO1004 Log threshold intensity (DL) Threshold vs Intensity (tvi) curve L Δb L L Δb L Straight lines = Weber’s Law (Lb) https://webvision.med.utah.edu/ book/part-viii-psychophysics-of-v ision/light-and-dark-adaptation/ Dr Sarah J Waugh© Effect of stimulus size on increment threshold Ricco’s Law • For small stimuli sizes, the jnd threshold, ΔL, is inversely proportional to the area of the stimulus. • This means that as the stimulus size increases, the ability to detect the stimulus improves (threshold gets smaller). • Varies depending on retinal location. SFO1004 Lb ΔL Lb Δ L Dr Sarah J Waugh© Effect of stimulus size on increment threshold Ricco’s Law • For small stimuli sizes, the jnd threshold, ΔL, is inversely proportional to the area of the stimulus. • This means that as the stimulus size increases, the ability to detect the stimulus improves (threshold gets smaller). • This applies up to different sizes of stimuli and retinal locations and we call this SFO1004 Ricco’s area. Lb ΔL Lb Δ L Dr Sarah J Waugh© Ricco’s area and spatial summation • Ricco’s area can be represented graphically. • The log (logarithm) of the spot luminance is plotted against the log of the spot area. • Ricco’s law says that the luminance of the increment required for detection decreases as the size of the spot increases – up to a certain critical point called Ricco’s area. SFO1004 • Knowledge of Ricco’s area is https://webvision.med.utah.ed u/book/part-viii-psychophysics -of-vision/visual-acuity/ Dr Sarah J Waugh© Ricco’s area and pooling • Ricco’s area represents the underlying pooling or summation occurring in the neural representation. • In the fovea Ricco’s area is about 5 min arc. • For more peripheral retinal locations, Ricco’s area becomes larger, indicating SFO1004 greater pooling of Increment DL Background (Lb) G G G Dr Sarah J Waugh© Ricco’s area and pooling More central retinal G SFO1004 G More peripheral retinal G not anatomically correct G G G Dr Sarah J Waugh© Ricco’s area and pooling More peripheral retinal More central retinal Size will change threshold as pooling for G is changing. No change to threshold as pooling onto same G does not change. G SFO1004 G G not anatomically correct G G G Dr Sarah J Waugh© Clinical use of increment thresholds: visual field measurements • Static perimetry is used to measure sensitivity of the eye to small light increments across the visual field. • Background luminance fixed at predetermined level. • Patient fixates on a small (red) target light with chin on rest. • Responds to small lights that are flashed across the visual field. The intensity of the lights is varied to measure sensitivity. • Detects ocular abnormality based on analysis of responses. SFO1004 Dr Sarah J Waugh© Questions? SFO1004 Dr Sarah J Waugh©