RESUMEN RECUENTO LEUCOCITARIO
VI. CONCLUSIONES Y RECOMENDACIONES
The deuteranope shows similar pupil responses to the bluish stimulus, but very different responses to the greenish stimulus when compared to the normal subject. This suggests that the deuteranope might use the same chromatic mechanism for
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the bluish stimulus, but very different mechanism for the greenish stimulus. Figure 4-7(A) shows that the measured afterimage thresholds for the deuteranopes are different when compared to results for normal subjects in two ways – a very large initial afterimage threshold when t = 0 and abnormal high thresholds following an exponential trend for the rest (t = 1, 2 … 10).
Possible mechanism for the large initial afterimage threshold
Due to the lack of M cones, the stimulus is no longer d-isoluminant to a deuteranope. In fact, to generate the isoluminant greenish stimulus in a normal trichromat, the M cone signal is increased whereas the L cone signal is decreased to balance the overall luminance change. Because the deuteranope subject only has L- and S-cones, he only responds to L- and S- cone signals. When the stimulus is presented, the deuteranope detects L-and S- cone signal decrements and the offset of the stimulus causes L- and S- cone increments. The L cone increment may well result in an overall luminance increment at stimulus offset.
Figure 4-15. A schematic diagram shows perceived reference stimulus from the deuteranopes. The gray rectangle shows the L cone signal decrement and the red arrow indicates the sudden increment at stimulus offset.
For the standard observer, the luminance signal is defined as the sum of the L and M cone signals (L+M) and the yellow-blue colour signal is defined as the difference between the S cone signal and the sum of L and M cone signals (S - (L+M)). Due to the absence of M cones in deuteranopes, both the (L+M) and S - (L+M) signals yield different results. Therefore, the gray rectangle shown in Figure 4-15 can
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indicate either a luminance decrement, a yellow blue colour signal or the combination of the two.
Nevertheless, in all these cases, the reference stimulus can be viewed as two components (the gray region and the red arrow in Figure 4-15). So the generated afterimage can also be modelled as two stages.
Figure 4-16. A deuteranope may well match the perceived ‘afterimage’ in two stages – (A) the curve shows the match of the afterimage created in the gray rectangle and (B) the red rectangle illustrates the matching signal of the impulse signal. (C) The final result is the sum of the signals produced by the two mechanisms. Notice that, the test stimulus is the exact opponent colour of the reference stimulus, i.e., the chromatic angle is reversed by 180o. Therefore, a deuteranope subject only responds to the L-cone increment in the test stimulus. Both colours are seen the same by the deuteranopes. The colour used in the figure is intended only for illustration purposes.
Figure 4-16 (A & B) shows how the afterimage perceived by a deuteranope can be modelled and separated in two parts – one part to match the afterimage caused by a stimulus (either a luminance decrement, a blue colour signal or the combination or the two) and one part to match the offset increment signal. The final outcome is the sum of the two matches shown in Figure 4-16(C). The model therefore explains the large initial afterimage threshold shown in Figure 4-7(A) and Figure 4-9(A) for the deuteranope. When the bluish stimulus (249o) is employed (Figure 4-9(B)), the deuteranope subject yields just a little higher initial threshold than the normal
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trichromat. This is because for this particular angle, the L and M cone contrasts are approximately equal to 0%. Therefore, it is much more isoluminant to the deuteranope. Also, the yellow-blue chromatic sensitivity is very similar between a normal subject and a deuteranope. Hence, the measured thresholds have similar values and shape.
Mechanism for the abnormally high final thresholds
Figure 4-9 (B) shows that along the chromatic displacement of 249o, the results for the normal subject are similar to the deuteranope. The deuteranope’s final threshold is, however, higher than the corresponding threshold in a normal trichromat. The deuteranope’s colour thresholds are shown in Figure 4-14 (B & C) and his yellow- blue threshold is approximately 0.015 which is only slightly more than his smallest afterimage thresholds (around 0.011). This suggests that the deuteranope may also use some luminance signal to match the test stimulus with the background.
A similar pattern is observed for the greenish stimulus (125o) in Figure 4-9(A), where the normal’s thresholds are much lower than that of the deuteranope’s. However, the deuteranope shows very similar thresholds (within 1 standard deviation) in both figures in the tail part of the curves (x > 6s). This suggests that the deuteranope uses the same mechanism for both stimuli. Because they only have two types of cones, the signal generated provide the inputs to all three channels – red-green, yellow-blue and luminance channel.
As demonstrated in Figure 1-22, Brown reported that the luminosity functions from deuteranopes are very similar to those of normal trichromats in the long wavelength
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region (>590nm) whereas in the short wavelength region (<590nm), the deuteranopes are less sensitive to luminance (Brown and Wald, 1964). This is not surprising since the luminance channel depends only on L cone signals in deuteranope. These findings confirm these expectations and also show that the afterimage results for the deuteranope subjects can be accounted for using a single system colour discrimination, that is S – L.