2. The Light Sense I 2324.pptx

The Light Sense I Introduction Objectives and Readings Spectral sensitivity of the eye Dark adaptation The absolute threshold of vision SFO1004 Dr Sarah J Waugh © Objectives and readings The student should be able to: • Define the term spectral sensitivity and Vλ curves. • Understand and appreci...

The Light Sense I Introduction Objectives and Readings Spectral sensitivity of the eye Dark adaptation The absolute threshold of vision SFO1004 Dr Sarah J Waugh © Objectives and readings The student should be able to: • Define the term spectral sensitivity and Vλ curves. • Understand and appreciate the differences in spectral sensitivity curves for both cone and rod vision. • Understand the process of dark adaptation. • Define the absolute threshold of scotopic vision. SFO1004 Optometry: Science, Techniques and clinical management. Rosenfield and Logan. • Chapter 5 (pages 71-73). Visual Perception: a clinical orientation. Schwartz. • Chapter 3 (The Duplex Retina) https://webvision.med.utah.edu/ • Part VII: Light and dark adaptation. Dr Sarah J Waugh© The visible spectrum of light • The sensitivity to the eye varies according to the wavelength of light and overall luminance because of differences in rod and cone sensitivity. This change in sensitivity is described by the Vλ The electromagnetic spectrum and visible light. Nanometre (10-9m) is the most common unit for wavelength SFO1004 (‘vee lambda’) curve (for cones) and V’λ (‘vee prime lambda’) for rods. Dr Sarah J Waugh© Spectral sensitivity of the eye - Vλ • Cones operate under photopic light levels – a luminance level of more that about 10 cd/m2. • Rods operate under dark adapted conditions known as scotopic (less than about 10-3 cd/m2). • Under mesopic lighting conditions both rods and cones operate (between about 10-3 and 10 cd/ m2). • See page 71 of reference (Optometry) for examples of different lighting values. SFO1004 Dr Sarah J Waugh© Spectral sensitivity of the eye - Vλ • Different responses of the eye depending on the wavelength of light. • Sensitivity refers to the ability of the photoreceptor to detect light. • Rods and cones have different spectral sensitivity functions - Vλ curves. In other words they respond differently to different wavelengths of light. SFO1004 Dr Sarah J Waugh© Spectral sensitivity – Vλ measurements • The Vλ curve can be derived by comparing the relative brightness of 2 halves of a circular visual field. • The eye (as the measurements are monocular) is adapted (by a period of sitting in the dark) and a bipartite (or split) field is presented to the observer. The field has a white reference and a comparison containing the wavelength (λ) to be measured. • Observer adjusts brightness of the λ field until the two fields appear equally bright. • The energy of the 2 fields is then measured. • Measurement of Vλ for cones (photopic) and V’λ for SFO1004 Dr Sarah J Waugh© rods (scotopic). Spectral sensitivity – experimental setup • The observer makes adjustments to the brightness of the test field to determine a ‘threshold’. Here we mean the minimum adjustment needed to obtain equality. • Once the 2 halves are matched (equal in brightness), the radiant energy of each half is measured. • The procedure is then repeated for different SFO1004 test field – vary energy to appear equally bright λ Standard white light (reference) Dr Sarah J Waugh© Here sensitivity is plotted on a linear scale Scaling in measurements (linear) formation change difficult to see on linear scale SFO1004 Dr Sarah J Waugh© 100 0 10 0 1 0 1 SFO1004 Here sensitivity is plotted on a log scale Scaling in measurements (logarithmic) Dr Sarah J Waugh© Log and linear scales Here sensitivity is a linear scale Here sensitivity is a log scale A lot of measurements in vision science uses logarithmic scaling in part because of the range of sensitivity of the eye. This includes measurements of visual acuity (e.g. logMAR scale). SFO1004 Dr Sarah J Waugh© Photopic and scotopic spectral sensitivity • Vλ (for cones) and V’λ (for rods). • Rods are 100 times more sensitive than cones in the middle of the spectrum. • Cones are a little more sensitive for very long wavelengths. • Uses a ‘log’ scale for sensitivity. is is Wavelength in nm or 10-9m (millimicrons m =10-3x10-6=10-9) SFO1004 Dr Sarah J Waugh© is is Sensitivity. Vλ (for cones) and V’λ (for rods). Purkinje Shift Can compare the 2 curves by ‘normalising’ so that the peak for each equals 1. Peak sensitivity is about 510 nm for rods and 555 nm for cones. 510nm Photopic Mesopic Scotopic SFO1004 555n m > 10cd/m2 0.001 – 10 cd/m2 < 0.001 cd/m2 As overall luminance decreases from photopic to mesopic levels, there is an enhancement to shorter wavelengths, as the peak of the Vλ curve shifts from cone to rod function. This is known as the Purkinje shift. Dr Sarah J Waugh© Purkinje shift From photopic to mesopic/scotopic light levels SFO1004 Dr Sarah J Waugh© Spectral sensitivity of the eye - Vλ • Practical applications: – e.g. traffic lights. – For the same radiant energy lights will look different to observers – yellow will look brighter than the red or (maybe) the green, so relative energy will need to be adjusted to ensure the colours have the same brightness. – e.g. important in visual field measurements where different SFO1004 Dr Sarah J Waugh© Threshold and sensitivity in vision • A threshold in vision science can be defined as the minimum amount of a stimulus that produces a sensation. • The more sensitive you are the better you will be at detecting small changes, so your threshold is lower. SFO1004 • Sensitivity is defined as: 1/threshold – In the spectral sensitivity example, the threshold radiance or energy to give equal brightness of the two fields. • Can you think of another threshold that is commonlyDrused in Sarah J Waugh© optometry? Interval SFO1004 Dr Sarah J Waugh© Dark adaptation and night vision • When we move from a well lit area (e.g. outside sunlight) into a dark area, our eyes take some time to ‘adapt’. • The more time spent in the dark, the more the sensitivity of the eye increases. • Within the retina, the photosensitive cells, rods and cones contain specialised pigments. SFO1004 These pigments are Dr Sarah J Waugh© Dark adaptation and night vision • Rods – contain a single photopigment, called rhodopsin. • Cones – have one of 3 different photopigments, either a red (long wavelength sensitive), green (medium wavelength sensitive) or blue (short wavelength sensitive) type. • Photopigments undergo a chemical reaction upon absorption of light i.e. SFO1004 called bleaching. Dr Sarah J Waugh© Measuring dark adaptation • This method measures the ‘recovery’ of the sensitivity of cones and rods. • Threshold for detection was measured, i.e. the physical light intensity just sufficient for the subject to see the test field. • First the observer eye is adapted to a bright white light (~5000cd/m2) for 3 minutes. The pigments in the photoreceptors absorb SFO1004 light (quanta) and become Dr Sarah J Waugh© Measuring dark adaptation • The light is turned off and using one eye, the observer fixates onto a red fixation light directly ahead. • A small test light (3° in size and short wavelength – violet) is briefly flashed (for 0.2sec) in the periphery. • The observer’s task is to detect the test light by slowly adjusting its apparent brightness until it is just visible. • SFO1004 This value is recorded and the procedure repeated over a set 7° Dr Sarah J Waugh© First measure after bleachin g The dark adaptation curve • The (log) threshold (light energy) is then plotted against the time in the dark. • This produces a characteristic dark adaptation function. • The function comprises two (2) distinct parts representing cone then rod function. • The more sensitive cones recover first, then after SFO1004 about 8-10 mins in the Cones operate From Hecht and Shlaer, 1938 Rod-cone break Rods operate Dr Sarah J Waugh© The dark adaptation curve • As the cones recover their sensitivity first, the target stimulus appears violet. • Then, as rod sensitivity improves, the stimulus becomes colourless – can you explain why? • Peak sensitivity of the rods occurs after about 30-35 mins in the dark. SFO1004 Appears violet Rod/cone break Colourles s Peak at 30-35 mins From Hecht and Shlaer, 1938 Dr Sarah J Waugh© Dark adaptation curve • Although there is a variation in individual thresholds, the main features of the dark adaptation curve are similar across individuals with normal vision. • The curve will vary depending on the eccentricity in the visual field. • What would the curve look like if the test SFO1004 stimulus was also set at Cone function Rod function From Hecht and Mandelbaum, 1939 (110 normal subjects) Dr Sarah J Waugh© Normal Control Dark adaptation curve – clinical applications • How could knowledge of dark adaptation function be useful in a clinical application? • Patients that suffer from symptoms of night blindness. • Often hereditary disorders of vision (e.g. retinitis pigmentosa, CSNB or congenital stationary night blindness). SFO1004 • No rod/cone break CSNB2: Rod function impaired Cone function only CSNB1: Rod function absent https://journals.plos.org/plosone/articl e?id=10.1371/journal.pone.0062927 Dr Sarah J Waugh© Interval SFO1004 Dr Sarah J Waugh© How sensitive is the eye? • We have seen that time in the dark increases the eye’s sensitivity to detecting light. • But what is the least amount of light energy that the eye can detect? • Hecht, Shlaer and Pirenne (1942) wanted to know the minimum number of neural SFO1004 impulse required for • Their aim was to measure the absolute threshold. • The absolute threshold is the lowest light energy level of a stimulus that can be perceived. • The number of quanta at the threshold for seeing = number of rhodopsin molecules required = number of neural impulses. Dr Sarah J Waugh© The absolute threshold of About 20 deg scotopic vision • The number of rods and cones varies across the retina. • Cone density (i.e. the number of cones per unit of retinal area) is highest more centrally and reduces into the periphery. • Rod density peaks at about 20o in the periphery. •SFO1004 Note there are no rods at the fovea. About 7 deg http:// webvision.med.utah.edu/ Dr Sarah J Waugh© The absolute threshold of scotopic vision • Observer was dark adapted for 30-40 mins. • Recall that rod sensitivity reaches a peak after about 30-35 mins in the dark. • The observer fixated on a small red spot while a stimulus was presented at 20° in the temporal retinal periphery. SFO1004 • At this point on the Dr Sarah J Waugh© The absolute threshold of scotopic vision • Used a test λ = 510 nm – peak of rod V’λ sensitivity and stimulus size of 10’. • Flashed the stimulus 1msec. • Threshold for detection was measured, i.e. the physical light intensity just sufficient for the subject to see the test field. • Threshold intensity was defined as the energy when the stimulus was seen 6 out 10 times correctly i.e. SFO1004 20° Dr Sarah J Waugh© The absolute threshold of scotopic vision • At the plane of the eye the energy from the light source was 2.1 x1017 to 5.7 x10-17 joules. • This represents 54-143 quanta (recall previous equation) at the plane of the eye. • Cornea reflects 4% of light. • Crystalline lens absorbs 50% of light at 510nm. • 80% of light getting to the retina passes between the rod photoreceptors. • SFO1004 Resulting in 5-14 quanta absorbed Dr Sarah J Waugh© Absolute threshold conclusions • At absolute threshold, 5-14 quanta absorbed by the rod photopigments. • A stimulus 10’ in size falls on about 300 rods at 20° in the periphery; hence, the threshold for seeing is determined by a small number of rods each absorbing a single quantum (bearing in mind the stimulus timing and size and probability of seeing correctly). • A single quantum is sufficient to activate a rod. • However, a single activated rod is not sufficient for seeing. SFO1004 Dr Sarah J Waugh© Absolute threshold conclusions • However, a single activated rod is not sufficient for seeing. – The signal generated in the rod cell passes through different neural elements in the retina to the retinal ganglion cells. – The ganglion cell combines the input from a number of rod cells over a relatively large area. – This combination is called ‘pooling’. – It allows the scotopic rod system to have very good sensitivity to detect light but poorer or lesser ability to know how many individual lights are visible (i.e. poor spatial resolution). SFO1004 Dr Sarah J Waugh© Pooling of rod signals onto Ganglion Cell (blue lines) Back of eye Rod cells Other cells Ganglion cell not anatomically correct G Single quanta Light entering eye towards periphery SFO1004 Front of eye Dr Sarah J Waugh© Questions? • Remember you can ask questions via chat function on Teams. SFO1004