Introduction
The purpose of this lab report is to reflect on an experiment carried out on 30/01/2020. The experiment's objective “was to measure the minimum amount of luminance of a test spot required to produce a visual sensation which is the absolute intensity threshold (1/sensitivity) of the visual system to light in dark conditions as a function of time using the standard instrument for this purpose. By viewing the target extra-foveally, using part of the retina containing both cones and rods, both phases of dark adaptation will be measured.” 1
To establish the dark adaptation (DA) mechanism of the eye and to measure an individual dark adaptation capacity, Goldmann-Weekers Adaptometer was used, with the psychophysical method of limits adopted. This instrument shows that rods and cones recover at different rates as shown in the graphs below. The use of this experiment has been proven useful in the clinical diagnosis of various retinal disorders. 2
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The ability to adapt to light and dark conditions is a crucial ability of the eye. Dark adaptation is a phenomenon that the eyes adjust to scotopic conditions, for example walking into a cinema and the eyes slowly adjusting to see in the dark. Dark adaptation is not a new concept, it has been the subject of many research studies and towards the end of the 19th century, it was considered a mechanism of the eyes and not the brain. Evidence of DA is that when one eye of a dark-adapted subject to bright light, there is a decrease in sensitivity of the exposed eye but not that of the shielded eye. Suggesting that a dual method of light and dark adaptation is present in the eye. 3
The duplex nature of our visual system is shown by the dark adaptation curve below (image 1). This theory indicates that the retina utilizes two photoreceptors types, rods, and cones. The first part of the curve shows a rapid reduction until 5-10 mins when the curve plateaus (cones), this represents the photopic thresholds. The rod-cone break is where there is an abrupt change in the slope, this is when the rods have become more sensitive than the cones. This is followed by the slow reduction in the second part of the curve which represents the skeptics' threshold to around 30 mins where the curve again plateaus (rod).2
The spectrum of light adsorbed in rods, peaks about 500nm when light excites the rhodopsin molecule, it’s isomerized and the transduction process occurs. This then splits from rhodopsin to retinal and opsin. The physical effect is that it becomes lighter in color, known as visual pigment bleaching. In a process called visual pigment regeneration, retinal and opsin are converted back to rhodopin. For this process to happen so efficiently there is always a normal level of bleached molecules in the retina. To measure the visual pigment regeneration one must look to William Rushton's research, who devised a procedure that concluded cones pigment can take 6 minutes to regenerate and rods up to thirty minutes. Rushton's results also demonstrated that there is an important link between perception and physiology.
Sensitivity to light depends on the concentration of visual pigment
The speed at which sensitivity is adjusted to the dark depends on the regeneration of the visual pigment.4
Results
To create the dark adaptation curve, three individuals are exposed to a pre-adapting light in one eye for different durations, and the intensity of the light was kept constant. Then the light is turned off. The subject, now sitting in the dark, is exposed to a succession of dim light targets approximately 11o below fixation. The intensity of the test light is controlled by a neutral density filter. Pre-adaptation plays an important role in the regulation of the experiment and to verify that a bi-phasic curve is achieved and indicates the Duplex retina theory. (Kolb et al., 2011)6
- Pre- Bleaching times
- Shóna light adapted: 7 mins
- Saroj light adapted: 6mins
- Rita light adapted: 5mins
- Shóna Hendrick
- Blue=cones
- Red= rods
Shóna was bleached for 7 minutes and a rod-cone break is seen at ~7 ½ minutes. This graph shows the rod threshold extending to 17mins. The graph does not start at 0 minutes as an error occurred in recording initially. Due to this, the results are not completely accurate for this trial.
- Saroj Khatri
- Blue=cones
- Red= rods
As seen in the graph for Saroj, the first section(cones) shows a rapid threshold reduction followed by a plateau. Then the Cone-Rod break is noticed at approx 5 minutes and moves on to the second part of the graph(rod) where the reduction in rod thresholds extends to 26mins.
- Rita Jennings
- Blue= Rod
- Red= cones
Rita was bleached for 5 minutes and a rod-cone break is seen at approx. 5 ½-6 minutes, this is the first cure and phase one of the dark adaptation curve. The rod sensitivity increases and can be seen in the second part of the curve, phase 2 of the dark adaptation curve. This graph shows the rod threshold extending to 25mins which means the minimum ( absolute threshold) is at 10−5 cd/m2 after 25 minutes in the dark. (Kolb et al., 2011)
Discussion
The dark adaptation experiment gave insight into the importance of the photoreceptor cells and their regeneration. During this experiment, we learned how to measure a person’s ability to dark adapt. The resulting graphs from our experiment showed a short cone phase, confirming that the cones were not active in scotopic conditions. The rod-cone break is seen at roughly 5,7 ½, and 5 ½ minutes respectively on our graphs, as the rods become more sensitive than the cones.
Factors that affect dark adaptation include:
1. Intensity and duration of the pre-adapting light.
Shóna was bleached for the longest and Saroj and Rita had shorter bleaching times. The graphs prove the duration of the pre-adapting light affects the curve. The weaker the pre-adaptation light, the faster the recovery as seen through Saroj and Rita's graphs. Weak or short pre-adaptation bleaching, can cause negligible photopigment bleaching and can raise the threshold, therefore decreasing the sensitivity, as per who the law is this?
2. Size and position of the retinal are used in measuring dark adaptation.
A small test spot with an eccentricity of 0o would reveal only a rod curve is obtained versus that of 2o which shows a normal DA curve.
The opposite is true with the size of the spot, as only a single cone break is found because these are the only photoreceptors found at the fovea. Increasing size incorporates the peripheral retina containing rods.
3. Wavelength distribution of the light used. For example, the shorter the wavelength the more obvious the rod-cone break is as rods are much more sensitive to short wavelengths once dark adapted.
In this experiment, only monochromatic light was used. To further investigate the hypothesis that the wavelength of light affects dark adaptation, other wavelengths need to be introduced into this experiment. 5
Both strengths and limitations were noted during this experiment:
- Using and understanding the equipment posed a problem initially and therefore resulted in Shóna’s results being slightly inaccurate. The graph was restarted once we familiarised ourselves with the method. For this reason, we recommend a trial before commencing the real experiment.
- Age must be taken into consideration as rod degeneration occurs in older patients before degeneration cones. This will inevitably affect the result of the curve and make the length of the test longer
- Another restriction to this method of measuring DA is the time it takes. Each patient takes ~30mins and therefore it would not be realistic for this test to be carried out in daily optometric practice.
- One strength is the graphing of the curve by the adaptometer, as immediately after ceasing the experiment it is possible to see results and estimate the rod-cone break.
- Another advantage is that any abnormalities on the graph can be seen promptly and assist in diagnosing/ more accurate referral letters.
Recent studies have shown that the AdaptDx dark adaptometer is a more accurate and efficient way of measuring DA. AdaptDx is used more widely now as test time is < 6.5mins rather than >30mins when using Goldmann-Weekers. 7
Measuring and recording the DA curve has proven to be a diagnosing instrument for AMD, slow adaptation can indicate subclinical AMD at least 3 years before drusen are visible on the retina. 8
In conclusion, the DA curve recorded by Goldmann-Weekers Dark Adaptometer can be used to evaluate how a patient’s photoreceptors are functioning. It assesses night blindness, degenerative retinal diseases (eg: retinitis pigmentosa), hereditary cone degeneration, and rod monochromats. It is also frequently used to test for and diagnose AMD. 7
- Grainne Scanlon, Physiology of Vision 1 Lab Manual, 2019/20; Pg3-6.
- Chapter 3, Dark Adaptation(pg 37-43),Visual Perception, A Clinical Orientation, 4th edition,Steven H. Schwartz, ISBN: 978-0-07-160462-8
- Reuter, T. (2011). Fifty years of dark adaptation 1961-2011. Vision Research. https://doi.org/10.1016/j.visres.2011.08.021
- Goldstein, E. Bruce. The Blackwell Handbook Of Sensation And Perception. 1st ed. John Wiley & Sons, 2008.
- M. Kalloniatis and C. Luu, Light and Dark Adaptation; 2007
- Light and Dark Adaptation by Michael Kalloniatis and Charles Luu Kolb, et al. 2011
- Jackson GR, Scott IU, Kim IK, Quillen DA, Iannaccone A, Edwards JG. Diagnostic sensitivity and specificity of dark adaptometry for detection of age-related macular degeneration. Invest Ophthalmol Vis Sci. 2014;55(3):1427–1431. Published 2014 Mar 10. doi:10.1167/iovs.13-13745
- AdaptDx Dark Adaptometer, 2019: AdaptDx® Dark Adaptometer
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