Visual Thresholds and Adaptation

Psychophysical isolation of different classes of cone

Project Report 2013 7 Pages

Psychology - Methods



1. Introduction

2. Methods
2.1. Apparatus
2.2. Procedure

3. Results

4. Discussion

5. Conclusion References

1. Introduction

Neural coding is based on three principles, which enable us to identify how visual mechanisms work. First of all, neurons are preferentially activated by particular features, e.g. photoreceptors are activated by different wavelengths of light. The peak sensitivities for the three differ- ent cones are 420 nm, 530 nm and 560 nm and for the rods 496 nm.1 Secondly, the principle of univariance states that, although individual neurons are more responsive to some properties than others, their re- sponses are ambiguous regarding the presence or absence of a particular feature with their response only varying along one dimension. Hence, the response of multiple cells needs to be considered in order to disam- biguate the signal (Pattern Coding). Thirdly, the principle of adaptive independence accounts for our ability to adapt to different features independently of each other, yielding perceptual distortions such as Date: October 29, 2013.


afterimages and aftereffects. Here we explore this third principle in more detail by isolating particular neural channels (π mechanisms1 ) within the visual system: We adapt middle- and long-wave cones independently of short-wave cones and compare their t.v.i. (threshold vs intensity) curves.1 1

2. Methods

Similarly to Stiles two-colour increment-threshold procedure2, a monochromatic-test flash is presented in the centre of a larger, con- centric, adapting field and the threshold for detecting the test flash is measured. The background field in this experimental setup is yellow adapting middle- and long-wave cones independently of short-wave cones and the test flashes were foveated (suppressing interference with rods) and either blue or green.

2.1. Apparatus. The apparatus consists of two half-silvered and two normal mirrors that enable light to be passed through two different kinds

of filter: one background filter and and one variable circular light filter that allowed the subject to adjust the intensity of the test patch until it reached the threshold of visibility. The value of the corresponding optical density and thus the proportion of transmitted light (given that Optical density = log10[ proportionof transmittedlight ] could be recorded by means of the pointer and scale on the optical wedge.

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Figure 1. The observer looks through the artificial pupil and detects the combined image of two beams of light (one bachground beam and one test-flash beam).


2.2. Procedure.

As dark adaptation lasts significantly longer than light adaptation (20-30min vs 5min), the thresholds were first measured on a dim background and then on increasingly brighter ones. At each intensity, two measurements were made with a green filter followed by a blue filter. Their respective average was used for the data evaluation. The following steps were taken to avoid systematic errors: a) The subject had to look straight into the test flash when recording the thresholds for different background intensities in order to avoid rod intrusion. b) To focus the subject’s attention entirely on the investigated eye, a patch had to be worn over the unused one throughout the whole experiment. c) While filters were being changed, she had to close her eyes and then adapt to each new background intensity for at least one minute.

3. Results

Overall, we observe that at low yellow background light intensity or “radiances” enhancing the sensitivity of middle- and long-wave cones only, the middle-wave cones also dominate the threshold of detecting blue test flashes in contrary to normal conditions. At a background optical density between 1.5 and 1.7 (corresponding to a background radiance of Rbg ≈ 0 . 03 W m 2 sr) the sensitivity of middle-wave cones as well as long-wave cones (irrelevant in this experiment) is significantly reduced due to the continuous light adaptation. Hence, we observe a second branch in the data that indicates the shift to measuring the threshold of the short-wave cones (see Figure 3). For the first part of the experiment, when middle-wave cones dominated the detection of the test flashes, our results obey Weber’s law3 2 (see Figure 2). The results are listed in the table of Figure 4.


Figure 2. Weber’s Law:Δ L k, where Δ L and Lb are the lumi-

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nances of test patch and background and k is a constant. L 0 represents the retinal noise and determines the absolute threshold of detection for the individual subject.

4. Discussion

Compared to the predicted shift from a higher sensitivity of middle- wave cones to short-wave cones to detecting the blue test flashes at a value of optical density of around 4 (corresponding to a background W radiance of Rbg = 10 4 m 2 sr) our subject seemed to have more sensitive middle-wave cones than an average person, as it occurred between 1 . 5 and 1 . 7 (corresponding to a background radiance of Rbg ≈ 0 . 03 W

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Another reason for this disagreement might be the subject’s inability to find the thresholds adequately as the threshold of visibility varied very gradually over large differences in optical density in the subject’s perception. Hence, the subject’s criterion of classifying thresholds might have changed throughout the experiment.

5. Conclusion

Overall, our results show that Weber’s law and especially the principle of adaptive independence are indeed valid.


1. Horace Basil Barlow and John D Mollon, The senses, vol. 3, CUP Archive, 1982.

2. WS Stiles, Investigations of the scotopic and trichromatic mechanisms of vision by the two-colour threshold technique., Archives d’ophtalmologie et revue générale d’ophtalmologie 28 (1949), no. 4, 215.

3. Ernst Heinrich Weber, Eh weber: The sense of touch, Academic Press for Exper- imental Psychology Society, 1978.


1 Regarding visual mechanisms the principle of adaptive independence states that the sensitivity of each class of photoreceptor depends only on the number of photons per unit time they absorb from the background and not on their retinal environment.

2 The just-noticeable difference between two stimuli is proportional to the magnitude of the stimuli.


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University of Cambridge – Department of Experimental Psychology
visual thresholds adaptation psychophysical



Title: Visual Thresholds and Adaptation