23 Mar 2015
This section describes the aim of this practical, what is Dark Adaptometry and its history. The purpose of this practical is to measure dark adaptation on a normal observer. It is to determine the amount of time that have past, before the eye regains its maximum sensitivity to low intensity of luminance, when going from conditions of bright light to total darkness.
1.2 What is Dark Adaptation?
Â Dark Adaptation is the ability of the eye to recover its sensitivity in the dark after being exposed to bright light, making vision possible in relative darkness.
Historically, Hermann Rudolph AubertÂ was the one who introduce a phenomenon known as 'Dark Adaptation'. He was the first person to systematically investigate the sensitivity of eye in the dark following exposure to bright lights.Â
In our eyes, there are photoreceptors, a specialized neuron that is able to detect and react to light. They can be in the form of rod cells or cone cells. Within these rod and cone cells, there are photopigments. Thus, when they absorb light these photopigments will undergo a chemical change.
Thus when the eye is exposed to bright light, bleaching of the photopigments will occur. These photopigments need to be regenerated before photoreceptors can regain its function.
Rod cells is responsible for night (scotopic) vision, it has slower recovery time but has higher sensitivity. As opposed to rod cells, cones cell is responsible for day vision and has a faster recovery time but lower sensitivity.
There are two types of experiment carried out; dark adaptation in using a "white" stimulus and dark adaption in using a long wavelength stimulus. A machine, Goldman-Weekers Dark Adaptometer (Figure 1), was used in both experiments to measure the threshold.
Figure 1 - Goldmann-weekers Dark Adaptometer
This procedure must take place in a light proof room. Furthermore, ensure that all cracks under the doors are blocked and windows are covered.
Firstly, the subject will be asked to occlude one of his/her eyes and his/her chin positioned on the chin rest. A chin rest is being used so that the subject's head does not shift in position during the test.
Next, a recording sheet is inserted firmly in the drum of the Goldman-Weekers Dark Adaptometer, with the spike positioning at approximately 4 minutes behind the zero point. All light sources are then switched off.
Following that, the pre-adapting light/field is then switched on; this is to let the subject being exposed to about 4 minutes of pre-adaptive light as the drum rotates towards the zero point. When the spike reaches the zero point of the recording sheet, after about 4 minutes, the pre-adaptive light was switched off.
The tester will then turn the dark adaptometer's knob anti-clockwise at a moderate consistent speed to increase the light intensity. Once the subject sees a light source, he/she will signal to the tester by knocking on the table twice. Hence, allowing the tester to immediately pull the knob to the right, which then allows the spike to puncture on the recording sheet.
Finally, the tester then spins the knob clockwise to return the light to zero intensity. After that, he/she will turn the knob anti-clockwise again to repeat the experiment. It is repeated for 25 minutes with an interval of approximately 15 seconds between every recording.
This experiment will measure the dark adaptometry curve for the same observer/subject. However, a red stimulus will be used for this procedure. Similar to the first experiment, this procedure must also take place in a light proof room and the procedures are repeated.
Firstly, a red filter is placed in front of the stimulus in the Goldman-Weekers Dark Adaptometer.
Next, a recording sheet is inserted firmly in the drum of the machine, as usual, with the spike positioning at approximately 4 minutes behind the zero point.
Once the subject had occluded one of his/her eyes and his/her chin positioned on the chin rest, all light sources will be switched off.
Following that, the subject will then be exposed to about 4 minutes of pre-adaptive light as the drum rotates towards the zero point. When the spike reaches the zero point of the recording sheet, the pre-adaptive light will be switched off.
Once the subject sees a light source, he/she will signal to the tester by knocking on the table twice. However, before that, the tester has to increase the light intensity by turning the dark adaptometer's knob anti-clockwise at a moderate consistent speed.
Likewise, the tester will immediately pull the knob to the right, allowing the spike to puncture on the recording sheet. Finally, the light will be adjusted back to zero intensity. After that, the tester will turn the knob anti-clockwise again to repeat the experiment.
This experiment is also repeated for 25 minutes with an interval of approximately 15 seconds between every recording.
The results are recorded in terms of threshold, absolute threshold which is the log intensity plotted over time in minutes. The moment when a stimulus is of sufficient intensity that can produce an effect is known as the threshold.
Therefore, the thresholds for both of the experiment are the moment where the subject is able to see the light source from the machine, which are the pinholes previously punctured by the machine. The pinholes on the recording sheet are marked with a dark pen, and a best fitted curve is plotted. This dark adaptation curve demonstrates the recovery of sensitivity following the bleaching of photoreceptors in our eye.
Figure 2 - A threshold measurement plotted in a best-fitted curve on a recording sheet for the "white" stimulus experiment
The shape of the dark adaptation curve obtained shows that the curve is in decreasing trend. From the above Figure 2, three features are seen in the dark adaptation curve, namely the cone branch, rod-cone break and the rod branch.
After turning off the pre-adapting light, cone photoreceptors will have a high threshold as they have just been bleached. When the threshold is high, sensitivity will be low. Thus, a high intensity of light stimulus will be needed in order for the eye to detect light. As discussed previously, the cones photoreceptors have a faster recovery time than the rods photoreceptors. For that reason, cones will recover first before the rods. In Figure 2, a rapid reduction in threshold was seen in the cone branch, which was denoted by the steep descending curve. This shows that the cones are quickly regenerating, as the sensitivity is increased. At approximately 2.5 minutes, the curve then gradually becomes gentler as it reaches its cone plateau, this represent the threshold for cone system. The result obtained for the cone threshold is log 103.2.
At approximately 2.5 minutes, there is prominent dip in the slope of the curve, causing a distinct break between the two curves and it is known as the rod-cone break. Prior to this point, the cones detect the stimulus. After this point, the rods detect it. This is due to the rods that are becoming more sensitive than the cones. Therefore, another curve which is less steep than the cone branch was formed after 2.5 minutes, this curve is known as the rod branch.
As compared to cones, rods need to take a much longer time to regenerate fully. This is why the rod branch is longer than the cone branch. The rod branch then gradually forms a straight line at approximately 14 minutes to the end of the experiment (25 minutes). This straight line is known as the rod plateau, it represent the threshold for the rod system. The result obtained for the rod threshold is log 101.3.
There is a recovery of sensitivity, and this is partly due to the regeneration of photoreceptor and photopigment, which was bleached by the pre-adapting light.
Figure 3 - A threshold measurement plotted in a best-fitted curve on a recording sheet for the long wavelength stimulus experiment
The shape of the dark adaptation curve obtained shows that the curve is in decreasing trend too. From the Figure 3 above, we can see that there is no significant dip (rod-cone break) as compared to Figure 2. It is because for this long wavelength stimulus experiment, we had used a red filter paper. Hence, 'red' stimulus is used, instead of 'white' stimulus. This leads to the threshold reached quickly, as the 'red' stimulus is of smaller diameter. The light will therefore, falls mainly on the central fovea, where there are mainly cones photoreceptors.
As only cones were used throughout this experiment, there was no rod-cone break seen in the graph. Thus, the threshold is quickly obtained- denoted by a straight line with no gradient
The sensitivity was quickly regained from the start of the experiment until the third minute, as seen in the steep negative gradient which is the decrease of threshold from log 104.4 to log 104. For another 22 minutes until the end of the experiment, threshold decreased slowly to log 103.2, hence a more relaxed gradient.
In this section, the factors that will affect dark adaptometry will be discussed, as well as the advantages and disadvantages of this experiment.
Adapting to different ambient levels of luminance dependant in a number of factors: intensity of pre-adapting light, duration of pre-adapting light, size of retina used, location of retina used and wavelength of stimulus light used
Figure 4 - The different intensities of pre-adaptive luminance for dark adaptation curve Adapted from < http://webvision.med.utah.edu/light_dark.html#intensity>
If the intensity of the pre-adapting luminance increases; the cone branch will be much longer, whereas the rod branch will be prolonged. Therefore, the absolute threshold takes a longer time to reach.
When the intensity of the pre-adapting light decrease or at low levels; the rod thresholds will reach absolute threshold rapidly.
The decrease duration of pre-adapting light will cause an immediate decrease in dark adaptation. If the duration of pre-adaptation is very short, only a rod curve will be seen. A cone and rod branched are obtained, only when there is a long duration of pre-adaptation.
Figure 6 - The distribution of rod and cones in the retina Adapted from < http://webvision.med.utah.edu/light_dark.html#intensity>
The dark adaptation curve will be affected, due to the allocation of the photoreceptors in the retinal. Cones are found closely packed together in the center of fovea. On the other hand, rods are dominated heavily in the periphery region.
If the fovea (eccentricity of 0o) detected a small test spot, mostly cone sensitivity is recorded which means that only one branch is seen. The cone branch will also reach threshold very quickly, as it has poor sensitivity in the dark.
On the other hand, if the peripheral retina detects the same size test spot, the rod-cone break will be seen in the curve denoting the cone and the rod branch. The rod branch will reach maximum sensitivity slowly for about 35 minutes, as it has higher sensitivity in the dark.
To elucidate, it is due to cones being allocated at the center of fovea, and with the rods being heavily dominated in the periphery region.
Figure 8 - The measurement of dark adaptometry at different angular distance. Adapted from < http://webvision.med.utah.edu/light_dark.html#intensity>
During dark adaptation if a small test spot is used, the test spot will be found at the fovea. Hence, a single cone branch is seen.
However when a bigger test spot is used, it will lead to the activation of both cones and rods. So, rod-cone break will be present.
As the test spot used increase in size, more rods will be incorporated; the sensitivity of the eye in the dark will increase.
Figure 9 - The different test stimuli of different wavelengths in measuring dark adaptometry Adapted from < http://webvision.med.utah.edu/light_dark.html#intensity>
From Figure 9 above, when using a stimulus of long wavelengths such as red, no rod-cone break is seen. This is largely due to the photoreceptors (rods and cones), having similar sensitivities to light of long wavelengths.
On the contrary, when using a stimulus of short wavelength, there will be a distinct rod-cone break seen, as to short wavelengths the rods are more sensitive than the cones.
During the course of dark adaptation, it makes it possible to carry out the examination of visual acuity, such as finding out the visual acuity in reduced illumination and examinations for sensitivity to dazzle.
As dark adaptation is aÂ test that we can determine the adjustment of the eye that occurs under low intensity of illumination, we will be able to know how well can the rods handle vision in low light conditions and how well can the cones handle color vision and detail. Both the photoreceptors react differently during the experiment and are measured on a graph. Thus, the tests allow us to determine the threshold and the minimum light intensity required to produce an effect in the eye.
The main disadvantage of this experiment is that we may need to perform the experiments a few times with the same or different subject, in order to get the average, for the best result. This is due to the results obtained differing from theoretically results. As from our results, the irregularities in points plotted show that our subject and tester are not a perfect subject/tester. Hence, it makes it impossible to achieve a smooth graph without drawing a best fit curve.
Theoretically, the difference between the cone and rod plateau should be 3 log units. However our subject's difference in log 103.2 and log 101.3 only gave us 1.9 log units. This tells us that compared to average; our subject's rod photoreceptors took a longer time to regenerate so as to become more sensitive than the cone photoreceptors.
The results we obtained differ from theoretical results, because humans are a complex system and neural noise can easily obstruct us from attaining 'perfect results'.
There are other factors that may have lead to these irregularities are the subject/tester's lack of motivation and concentration; the subject may be fatigue as the subject had to the experiment without resting and especially when he/she could not move his/her head at all. As well as the speed at which the Dark Adaptometer's knob for operating the revolving diaphragm was turned inconsistently.
To conclude, dark adaptometry demonstrates the ability of the eye to the recovery of sensitivity following bleaching of cones and rods by a high intensity of luminance, with cones recovering faster than rods. In addition, the rods taking a much longer time to recover, as compared to the cones. This is why it will take a while for our eye to adapt to darkness after being in a bright environment, for example, finding our seats in the movie theater.
Although, the results were different from the theoretical results, it is inevitable. As we, human, are not prefect observers, there would surely be differing in the results in all kinds of experiments.
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