A Paradox of Cerebral Achromatopsia

Transcription

1 A Paradox of Cerebral Achromatopsia

2 A Case of Cerebral Achromatopsia From Haywood, Cowey & Newcombe, The patient, M.S., had bi-lateral damage to the temporal-occipital regions of cortex. He was classified as densely achromatopsic which means that he fails the standard tests given for colour blindness. He could not name the colours of objects or sort colours by category. He scored at chance on the Farnsworth-Munsell 100 hue discrimination test. He could not discriminate between two colour samples matched for brightness. He could not identify the numbers in pseudo-isochromatic Ishihara plates

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4 Farnsworth Munsell 100 Hue Test: MACB020 How it works The test consists of four trays containing a total of 85 removable colour reference caps (incremental hue variation) spanning the visible spectrum. Colour vision abnormalities and aptitude are detected by the ability of the test subject to place the colour caps in order of hue. Under simulated daylight conditions, each tray is opened separately, and the discs are spread out on a table randomly. The subject is then asked, using the stable pilot discs for reference, to align the 85 colour disks in order of a colour scale (hue) order. This test provides a good indication of an individual s accuracy in judging small differences in colour. Hue discrimination ability is estimated from the total error score and the type of colour deficiency is determined from a graphical representation of the results. Colour vision anomalies and colour aptitude are detected by the ability of tester to place colour caps in hue order. This test consists of four 20" x 1.75" x 1.25" trays, housed in a 21.5" x 6.12" x 2.5" carrying case. Colour reference size is 7/16" diameter. Each chip is numbered in order to facilitate scoring. Please note the test must be administered under daylight conditions as provided by a daylight viewing booth (see related products).

5 Different brightness, different hue Same hue & different brightness Same brightness, different hue

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7 What M.S. could do: Could sort a series of grays, from light to dark. Normal VEP s to an isoluminant sine-wave of red and green gratings. Has normal chromatic contrast sensitivity He can detect motion of an isoluminant chromatic grating phase shifted 90 o either left or right. He can detect coloured shapes embedded in either static or dynamic grey backgrounds.

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12 Stimulus A. Low luminance contrast background + desaturated (pink) square.

13 Stimulus B. Low luminance background + saturated (red) square.

14 Stimulus C. High contrast background + saturated (red) square.

15 Stimulus D. High contrast background + desaturated cross and square.

16 A Paradox of Phenomenology 1. Either M.S. sees in colour or in black and white. 2. M.S. does not see in colour. 3. Therefore, M.S. sees in black and white. 4. The cross differs from its background only in colour. 5. If one cannot see the cross as differing in colour from its background, one cannot see the cross. 6. If one sees in black and white, then one cannot see the cross as differing in colour from its ground. 7. Therefore, M.S. cannot see the cross.

17 A Paradox of Phenomenology 1. Either M.S. sees in colour or in black and white. 2. M.S. does not see in colour. 3. Therefore, M.S. sees in black and white. 4. The cross differs from its background only in colour. 5. If one cannot see the cross as differing in colour from its background, one cannot see the cross. 6. If one sees in black and white, then one cannot see the cross as differing in colour from its ground. 7. Therefore, M.S. cannot see the cross.

18 One way out... Contrary to initial appearances, it is true that M.S. does not see the cross. Hence there is no paradox to be dissolved. Either M.S. does not have a conscious visual perception of the cross or M.S. s s experience of cross, while conscious, is not visual per se.

19 Neither of these options is tenable... What M.S. has is not a form of colour blindsight,, the prototypical case of perceptual knowledge of colour without visual consciousness. M.S. is clearly conscious of the presence of the cross he says he can see it, he does not have to guess whether he sees it, he does not have to be prompted to guess M.S. s s experience seems to be clearly visual given that: His experience clearly directs his motor behaviour he exhibits visually directed motor behaviour. He himself is in no doubt as to the modality of his experience.