The Colorful Case of the Philosophical Zombie?

By Neuroskeptic | October 20, 2013 5:18 am

The philosophical zombie, or p-zombie, is a hypothetical creature which is indistinguishable from a normal human, except that it has no conscious experience.

Whether a p-zombie could exist, and whether it even makes sense to ask that question, are popular dinner-table topics of conversation amongst philosophers of mind.

A new case report from Swiss neurologists Antonio Carota and Pasquale Calabrese will give them something to chew over: The Achromatic ‘Philosophical Zombie’

The ‘zombie’ is a 48 year old man who suffered a brain haemorrhage leading to damage to the occipitotemporal cortex. He survived, but reported that he could no longer see colors: his vision was entirely colourless, though otherwise normal. This is known as cerebral achromatopsia.

Remarkably, however, he was able to name colours. Shown hundreds of circles in 12 basic colors, the patient correctly named most of them (NB – the graph and colors are mine):


Given that random picks would get you just 9% correct, this is most impressive performance for a man who ‘can’t see colors.’

So is this patient really a chromozombie, behaving as if he can see colours but having no conscious experience of them?

My first reaction is: he doesn’t always behave as though he can see colours – his verbal behaviour (self-report) is that he doesn’t. Whether that makes him less of a zombie is one for philosophers to ponder. But he still makes for an interesting case, although as the authors point out, it is not entirely new : the principle of perception-without-reported-awareness is called blindsight and is quite well studied.

This seems to be one of the most dramatic cases of pure color blindsight on record, though.

ResearchBlogging.orgCarota A, & Calabrese P (2013). The achromatic ‘philosophical zombie’, a syndrome of cerebral achromatopsia with color anopsognosia. Case Reports in Neurology, 5 (1), 98-103 PMID: 23687498

  • Robert Kybird

    Interesting that higher accuracy for blue-yellow opponent channel processed more slowly by koniocellular pathway

  • Robert Kybird

    Fast visual processing appears to consist mainly of input from Red-Green photoreceptors and includes input to fusiform face area. Evolutionary driver for extracting potential threat from local scene, and has higher processing density in lower visual hemifield. Conversely Blue-yellow processed slower by a different subset of photoreceptors with higher processing in upper visual hemifield, corresponding to above the horizon and almost always more distant as regards threat, e.g sun, blue sky etc. Could be considered the root of blue skies thinking i.e. slower, more considered and serially processed.

    • Jyothish George

      Red light has highest wavelength in visible spectrum.should it come from faraway places than local area?

    • Rob Neff

      Interesting theory Robert, is this based on peer-reviewed data? Humans have the best ability to differentiate color in the blue-green part of the spectrum, but the difference between blue skies and white or grey clouds is pretty obvious. So blue being tied to studying the sky seems to be a bit of overkill. More likely it was needed to determine if fruit and maybe grain was ripe, which is not necessarily in the upper part of your vision.

  • Nick

    Great share! Thanks!

  • Bill Skaggs

    Very interesting. A couple of minor points: (1) Although he is clearly above chance, a normal person would be at essentially 100% on this task. (2) The colors he is best at are ones that stand out in terms of luminance.

    It’s still a very interesting dissociation though.

    • John McIntire

      I instantly thought the same thing about luminance, which the paper says quite honestly it was not controlled for. So it’s not a safe inference that they are using color as a cue, when it could be luminance (same reason color blind people often can name colors above chance, and may not even know they are color blind).

      • Neuroskeptic

        True. Perhaps it’s interesting that performance on blue, green, red was close to 33%, or in other words, what you’d get if you could tell, from luminance, that it was one of those three, and then picked one randomly?

        But this is speculation…

        • John McIntire

          No you are on to something with that…notice he is best with white, yellow, and black, which have either relatively high or low luminances. All the other colors are easily confusible in terms of luminance and have similar accuracy (~25-50%). In the paper, it says he failed the Farnsworth Munsell color test, which is the most comprehensive. So he is color blind.
          It also says he was able to pass the Ishihara plates, which can be passable by color blind individuals, especially if light is not very strictly controlled (because luminance differences can be spotted).

        • Rob Neff

          I’m not so sure. I just converted the graph in the article (although I can’t be sure it’s the exact colors used in the test, but probably close) to grey-scale. Looking at the grey-scale versions of those colors, black and white are obvious, but yellow is not much different than grey or pink (grey and pink he had a hard time identifying, but he did well with yellow). And red is noticeably different than blue and green, so it’s not a 1/3 chance amongst those 3 colors. Especially as orange is very similar to blue and green, but red is more similar to brown and violet.

          So it appears to me that something more is going on than just grouping colors based on luminance and then randomly guessing.

          • Neuroskeptic

            Actually the colours are mine. I made a graph based on the accuracy data in the paper (which are just in a table) and picked colours in MS Paint. Sorry. I should have made that clear!

  • Tanaka Masashi

    It would be interesting if some pathways in the occipitotemporal robe are required for color perception and other pathways may be also engaged in processing color information to which our consciousness cannot access. But, I wonder if the patient “could no longer see colors” because he is more than 50% confident that the tested color is black/white or yellow/blue. I deem that partial damage in general color processing pathways can account for the deficits in the patient. I don’t see any mysterious (I mean philosophical) issues in this report anyway.

  • Zachary Stansfield

    He cannot “see” colour, but he can perform colour discrimination–likely, as Bill suggests by noting achromatic differences between each of these. Clearly these are two different statements, which makes me wonder exactly what creative rhetorical flourish made the authors think this has anything to do with a “p-zombie”.

    Notably, the p-zombie is a great example of how easily the philosophy of mind lends itself to ridicule. How exactly could there be a non-conscious being who cannot be distinguished from a conscious one? This paradox is easily resolved: such an individual could never exist. These two ideas are mutually exclusive.

  • disqus_RR3RwdvVj4

    This study is absurd and tells us nothing. Luminance and chromatic information are mediated by separate pathways in the brain. The colors were not equated for luminance, as stated in the methods. You have no way to tell whether the subject was using chromatic or luminance information to do this task. If you put a conventionally completely colorblind person (lacking all cones and having only rods) to the task they would do above chance too (if not similarly to this particular patient) using just luminance information.

    • Rob Neff

      I just converted the colors in the graph above to grey scale. Assuming they’re similar to what the patient saw and comprehended, I would not expect those results. Black and white are obvious, yellow looks like pink, red is like violet but not orange. So if his results were based only on luminance and then random guessing, it would not be ordered anything like that.
      It is more likely the the chromatic information is getting to a part of his brain, but not to the conscious recognition section.

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  • hypnosifl

    I wonder how he would do on those tests for colorblindness with the numbers made of different-colored circles from the circles in the background–would the shape made by the different-colored circles “pop out” for him, or can he only decide on the color of a single object when he focuses on it? Likewise, I wonder how he would do on the “stroop test”, where you have to read a series of color names, but the colors they’re printed in may not match the color name (and if they don’t, people with normal color vision make more errors or have to read more slowly).

  • Elitist

    You guys are so smart. Lol just kidding.

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  • Bob Randolph

    I’m what is called color blind- I can evaluate graduations of shade of red but can’t see the difference in the hues. There may be slight miscalculations of color mixtures w/ red but it is not very evident.

  • Naomi Morris

    As we cannot experience it ourselves, how do we know that the patient is experiencing a lack of colour vision and not an altered perception in the way colour is viewed? I agree that these findings are complicated by the self-reported nature of the condition – it may be difficult to discover a genuine case of blindsight, as a result.

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  • Rodrigo

    Considering the fact that brain achromatopsia occur in the same part where the brain process color, this is 100% proof that color is OBJECTIVE. this is amazing.



No brain. No gain.

About Neuroskeptic

Neuroskeptic is a British neuroscientist who takes a skeptical look at his own field, and beyond. His blog offers a look at the latest developments in neuroscience, psychiatry and psychology through a critical lens.


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