Vision Practicals.pdf

BSMS Module 202 (2021) Human Vision Lab – Before Friday 2nd December Location: Online on My Studies Recommended books: •Human Physiology - The Basis of Medicine. G Pocock and CD Richards. Oxford University Press. 4th edition, 2013. Chapter 12. •Neurophysiology. RHS Carpenter & B Reddi. Hodder Arnol...

BSMS Module 202 (2021) Human Vision Lab – Before Friday 2nd December Location: Online on My Studies Recommended books: •Human Physiology - The Basis of Medicine. G Pocock and CD Richards. Oxford University Press. 4th edition, 2013. Chapter 12. •Neurophysiology. RHS Carpenter & B Reddi. Hodder Arnold. 5th edition, 2012. Chapter 7. This practical class explores the performance of the human (i.e. your own!) visual system. Work together in pairs. Please make notes and graphs of the observations and measurements that you make and answer the questions in the handout. *Use a calculator or calculator app* 2 Vision Experiment 1 – Spatial resolution of the human retina (adapted from a practical by Professor Mike Land) The ability of the eye to resolve fine detail depends on two things: the angular spacing of the receptors and the quality of the image. In this experiment you will investigate how the cone spacing in the retina is related to your ability to resolve a gap in a circle. To determine the angle between two cones in the fovea of the eye we need to know their actual separation (s) and the focal length of the eye (f) (Figure 1). The angle between them is then given by s/f radians, or 57.3 x s/f degrees, or 60 x 57.3 x s/f minutes of arc (a radian is the angle made by an arc of a circle whose length is equal to the circle’s radius, so there are 2π of them in 360º, hence the value of 57.3º, which is 360/2π). Figure 1. Angular separation of the cone photoreceptors From the image below (Figure 2) of the photoreceptor distribution in the human fovea, we can estimate the separation (s) between neighbouring cones in the fovea. Figure 2. Cones in the human fovea, from Ahnelt et al. (1987) Q1. Seeing that there are six cones in 0.01 mm, and assuming this is measured from centre to centre of the cones at each end, calculate the separation between adjacent cones in units of μm (10-6 m). A: ● Take the focal length of the eye as 17 mm Q2. What is the angle (Δφ) between two cones in the fovea (in minutes of arc)? A: Visual acuity is tested in the clinic by your ability to resolve the gaps in the circles or letters of a test chart, at a fixed distance of typically 6 m from the chart. The chart with circles is known as the Landolt C test chart, and normally these would be available in the lab. Instead, you can test your visual acuity with the pdf file provided 3 separately (Landolt C screen.pdf). View the file on your computer screen (the best option is to click View on the ribbon, then select Full Screen Mode). You need to self-evaluate about whether you can accurately see the gap positions in the C- like characters. To measure your threshold, try and position your screen with a decent space behind you. To measure your threshold separately for each eye, close or cover one eye, then repeat for the other eye. Do wear your glasses or contact lenses if applicable. Start quite far away (where you can’t perceive the gap positions in the C-like characters) and move slowly inwards. The point at which you can clearly see the gaps in a particular row of the Cs is your threshold. You then need to measure the distance from the screen to your eyes (estimates are fine) and measure the size of the gap in the Cs for your target row. These need to be in the same unit of measurement. You can then use the above calculation to estimate your visual acuity. You have essentially measured the smallest object that you can accurately perceive. Q3. What is your visual acuity expressed as an angle (in minutes of arc)? Measure each eye separately. A: Q4. Note the similarity (or otherwise) of this angle with the angular spacing of the cones in the retina. How many cones do you think are needed to resolve the gap in the smallest C-like characters that you can see? A: Vision Experiment 2 – Colour adaptation and colour opponency If you look for a long time at a bright, saturated colour this will bleach some of the cone photopigment and/or fatigue a subsequent neural mechanism. In either case, the colour ‘channel’ in question works less well afterwards. View the supplied pdf file (colour adaptation.pdf) on your computer screen. The best option is to click View on the ribbon, then select Full Screen Mode. Look at the cross in the centre of the yellow and green stripe pattern for 30 seconds. Q5. What do you see when you switch your gaze below, to the cross in the centre of the white background? Account for the colours you see in terms of colour opponency. A: Vision Experiment 3 – Colour vision and colour deficiency Congenital disorders of colour vision are due to the loss of either the red, green, or blue cone pigment. These conditions are known respectively as protanopia, deuteranopia and tritanopia. The first two of these X-linked recessive disorders are 4 much more common than the third. The Ishihara test cards are designed to identify people with protanopia or deuteranopia. Look at the six test cards shown below (Figure 3) and identify the numbers on each card. Figure 3. Photographs of images from Ishihara’s Tests for Colour Deficiency (Dr Shinobu Ishihara) Q6. What are the numbers you see on each card? A: Q7. How do you think this test works? A: Q8. People with a colour deficiency see a number in one of the cards where people with normal colour vision do not. Why do you think this is? A: 5 Vision Experiment 4 – Retinal blood vessels and blind spot One of the peculiarities of vision is that we see the world through a maze of blood vessels which overlie the retina, yet we don’t see these vessels! It is possible to see your own blood vessels by the trick of shifting them slightly to a position where you do not normally see them. Close your eyes and turn them hard to the left or right and with a penlight or the light on your mobile phone illuminate a portion of the white sclera where it is exposed. This spot illuminates the retina from the side and the blood vessels cast an easily visible series of shadows that look like lightning, or tree roots (this must be done in dim light). Notice that your fixation point is almost devoid of vessels, which flow away from this region. Also notice that the blood vessels thicken over to one side, begin to converge and then disappear from view. This invisible point is the blind spot, towards which many of the blood vessels appear to converge. Q9. Explain with a diagram how this experiment works and sketch the appearance of your blood vessel pattern. Note: if you don’t have a penlight or phone light, feel free to skip this question. A: The blind spot can be found by inspection of Figure 4 below with the right eye only. Look at the cross from about 40 cm away (this will vary with the size of your computer screen), move your head towards and away from the page and notice the smiley disappear and reappear. Measure the actual distance that your eye is away from the screen with a tape measure or ruler – an approximate measurement is fine. + ☻ 6 Figure 4. Measuring the position of the blind spot on the retina Q10. Measure the distance between the cross and the smiley face. How many degrees is the blind spot from the fovea (see Experiment 1 for the method)? From this experiment, what can you conclude about the position of the optic disc on the retina, relative to the fovea? A: Vision Experiment 5 – Retinal ganglion cells and lateral inhibition in the retina (adapted from a practical by Professor Paul Graham) The third cell layer in the visual system consists of retinal ganglion cells. One type of ganglion cell is the on-centre off-surround. With this type of cell, a stimulus in the central region increases the activity of the neurone (on-centre: excitation). The central region is surrounded by a circular region in which a stimulus would decrease the activity (off-surround: inhibition). The process carried out by this receptive field structure is often also referred to as 'lateral inhibition' (Figure 5). Figure 5. Action potential responses of an on-centre off-surround retinal ganglion cell. The mosaic (top right) represents photoreceptors contributing to excitation (light shading) and inhibition (dark shading) of retinal ganglion cells. Example of on-centre off-surround and off-centre on-surround arrangements are shown. Q11. What would be the change in cell activity for these patterns of light falling onto an on-centre, off-surround retinal ganglion cell? 7 A: A: This non-linear receptive field means that retinal ganglion cells enhance information at visual boundaries. This can be seen in the famous Mach band illusion (Figure 6) where we can see that within each equiluminant stripe the left edge appears lighter and the right edge darker. Figure 6. The Mach band illusion We can create a simple model of an on-centre off-surround ganglion cell by assigning numbers (weights) to the different regions of its receptive field. We can also represent each of the Mach bands as a different light intensity. The light intensity increases as the stripes get lighter. To predict the response level of a retinal ganglion cell to the Mach bands, we can move our model receptive field across the stimulus and investigate how the output changes. The response level of our model cell is the sum (∑) of the weights multiplied by light intensity. Output = ∑(Intensity x weight) So at location a in the Figure 7: output = (-1x1)+(4x1)+(-1x1) = 2. At location h: output = (-1x3)+(4x3)+(-1x4) = 5. 8 Figure 7. A model of computation of light intensity by retinal ganglion cells Q12. Complete the calculations for each location. How does the Perceived Brightness curve differ from the Light Intensity curve? A: