A Paradox of Cerebral Achromatopsia

A Case of Cerebral AchromatopsiaA Case of Cerebral Achromatopsia

From Haywood, Cowey & Newcombe, 1994.From Haywood, Cowey & Newcombe, 1994.

The patient, M.S., had bi-lateral damage to the temporal-occipital regions of cortex.The patient, M.S., had bi-lateral damage to the temporal-occipital regions of cortex.He was classified as He was classified as ““densely achromatopsicdensely achromatopsic”” which means that he fails the which means that he fails thestandardstandard teststests given for colour blindness.given for colour blindness.

❍❍ He could not name the colours of objects or sort colours by category.He could not name the colours of objects or sort colours by category.❍❍ He scored at chance on the He scored at chance on the Farnsworth-Munsell Farnsworth-Munsell 100 hue discrimination100 hue discrimination

test.test.❍❍ He could not discriminate between two colour samples matched forHe could not discriminate between two colour samples matched for

brightness.brightness.❍❍ He could not identify the numbers in pseudo-isochromatic Ishihara platesHe could not identify the numbers in pseudo-isochromatic Ishihara plates

Farnsworth Munsell 100 Hue Test: MACB020

How it worksThe test consists of four trays containing a total of 85 removable colourreference caps (incremental hue variation) spanning the visible spectrum.Colour vision abnormalities and aptitude are detected by the ability of thetest subject to place the colour caps in order of hue.

Under simulated daylight conditions, each tray is opened separately, andthe 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 inorder of a colour scale (hue) order. This test provides a good indication ofan individual’s accuracy in judging small differences in colour.

Hue discrimination ability is estimated from the total error score and thetype of colour deficiency is determined from a graphical representation ofthe results. Colour vision anomalies and colour aptitude are detected bythe 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" x6.12" x 2.5" carrying case. Colour reference size is 7/16" diameter. Eachchip is numbered in order to facilitate scoring. Please note the test must beadministered under daylight conditions as provided by a daylight viewingbooth (see related products).

Same hue & different brightness Same brightness, different hue

Different brightness, different hue

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 greengratings.

•Has normal chromatic contrast sensitivity

•He can detect motion of an isoluminant chromatic grating phaseshifted 90o either left or right.

•He can detect coloured shapes embedded in either static or dynamicgrey backgrounds.

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

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

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

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

A Paradox of PhenomenologyA Paradox of Phenomenology

1. Either M.S. sees in colour or in black and white.1. Either M.S. sees in colour or in black and white.2. M.S. does not see in colour.2. M.S. does not see in colour.3. Therefore, M.S. sees in black and white.3. Therefore, M.S. sees in black and white.

4. The cross differs from its background only in colour.4. The cross differs from its background only in colour.5. 5. If one cannot see the cross as differing in colour from its background,If one cannot see the cross as differing in colour from its background,

one cannot see the cross.one cannot see the cross.6. If one sees in black and white, then one cannot see the cross as6. If one sees in black and white, then one cannot see the cross as

differing in colour from its ground.differing in colour from its ground.7. Therefore, M.S. cannot see the cross.7. Therefore, M.S. cannot see the cross.

A Paradox of PhenomenologyA Paradox of Phenomenology

1. Either M.S. sees in colour or in black and white.1. Either M.S. sees in colour or in black and white.2. M.S. does not see in colour.2. M.S. does not see in colour.3. Therefore, M.S. sees in black and white.3. Therefore, M.S. sees in black and white.

4. The cross differs from its background only in colour.4. The cross differs from its background only in colour.5. 5. If one cannot see the cross as differing in colour from its background,If one cannot see the cross as differing in colour from its background,

one cannot see the cross.one cannot see the cross.6. If one sees in black and white, then one cannot see the cross as6. If one sees in black and white, then one cannot see the cross as

differing in colour from its ground.differing in colour from its ground.7. Therefore, M.S. cannot see the cross.7. Therefore, M.S. cannot see the cross.

One way out...One way out...

•• Contrary to initial appearances, it is true that M.S. does Contrary to initial appearances, it is true that M.S. does notnot see the see thecross. Hence there is no paradox to be dissolved.cross. Hence there is no paradox to be dissolved.

•• Either M.S. does not have a Either M.S. does not have a conscious conscious visual perception of the crossvisual perception of the cross or or M.S. M.S.’’s experience of cross, while conscious, is not s experience of cross, while conscious, is not visualvisual per se. per se.

Neither of these options is tenable...Neither of these options is tenable...

•• What M.S. has is not a form of What M.S. has is not a form of ““colour blindsightcolour blindsight””, the prototypical case of, the prototypical case ofperceptual knowledge of colour without visual consciousness. M.S. is clearlyperceptual knowledge of colour without visual consciousness. M.S. is clearlyconscious of the presence of the crossconscious of the presence of the cross——he says he can see it, he does nothe says he can see it, he does nothave to guess whether he sees it, he does not have tohave to guess whether he sees it, he does not have to be prompted to guessbe prompted to guess……

•• M.S.M.S.’’s experience seems to be clearly visual given that:s experience seems to be clearly visual given that:

–– His experience clearly directs his motor behaviour His experience clearly directs his motor behaviour —— he exhibits visually he exhibits visuallydirected motor behaviour.directed motor behaviour.

–– He himself is in no doubt as to the modality of his experience.He himself is in no doubt as to the modality of his experience.