Neuropsychologia 44 (2006) 1437–1443 Covert colour processing in colour agnosia Tanja C.W. Nijboer ∗ , Martine J.E. van Zandvoort, Edward H.F. de Haan Psychological Laboratory, Helmholtz Instituut, Universiteit Utrecht Heidelberglaan 2, NL-3584 CS Utrecht, The Netherlands Received 9 September 2005; received in revised form 4 December 2005; accepted 8 December 2005 Available online 18 January 2006 Abstract Patients with colour agnosia can perceive colours and are able to match coloured patches on hue, but are unable to identify or categorise colours. It is a rare condition and there is as yet no agreement on the clinical definition or a generally accepted explanation. In line with observations from object agnosia and prosopagnosia, we hypothesised that (some of) these patients might still be able to process colour information at an implicit level. In this study, we investigated this possibility of implicit access to colour semantics and colour names in a man (MAH) who suffers from developmental colour agnosia. We designed two experimental computer tasks: an associative colour priming task with a lexical decision response and a reversed Stroop task. The results of these experiments suggest that there is indeed automatic processing of colour, although MAH was unable to explicitly use colour information. © 2005 Elsevier Ltd. All rights reserved. Keywords: Implicit colour knowledge; Colour priming; Reversed Stroop interference 1. Introduction Colour vision is a powerful aide for distinguishing and iden- tifying objects. It enriches subjective visual experience and enables object- and pattern recognition, especially when the object possesses characteristic or ‘diagnostic’ colours. The cortical processing of colour fractionates into a number of specific sub-processes, each with their own neuro-anatomical substrates. Humans have the ability to match, classify, name, memorise and imagine colour. Each of these abilities can be lost as a result of brain damage and the occurrence and the co-occurrence of deficits suggest that each may be func- tionally independent. Neuropsychological studies have demon- strated selective colour impairments, e.g. cerebral achromatop- sia (selective impairment in colour perception; e.g. Heywood & Cowey, 1999; Meadows, 1974; for review, see Zeki, 1990), colour agnosia (selective impairment in colour recognition; e.g. Beauvois & Saillant, 1985; Davidoff, 1996), and colour anomia (selective impairment in colour naming; e.g. Davidoff & Ostergaard, 1984; Geschwind & Fusillo, 1966). In this paper, we will focus on colour agnosia. ∗ Corresponding author. E-mail address: t.c.w.nijboer@fss.uu.nl (T.C.W. Nijboer). One of the first clinical descriptions of patients with a selec- tive impairment in retrieving the colour of familiar objects was reported by Lewandowsky (1908); (for translation, see Davidoff & Fodor, 1989). This patient could not name or indicate the colour of objects, even when presented with the object name or an uncoloured drawing. He also experienced problems in sorting colours, and naming or pointing to colours that were named by the examiner. Lewandowsky (1908) attributed the dissociation of colour and form to colour anomia, a view shared by Damasio, McKee, and Damasio (1979). Colour agnosia is often accom- panied by alexia and homonymous hemianopia (Beauvois & Saillant, 1985; Woodward, Dixon, Mullen, Christensen, & Bub, 1999). However, others have suggested that colour agnosia can result from a disturbed access to the colour lexicon or to imagery disturbances (Beauvois & Saillant, 1985; de Vreese, 1991). Poor object-colour retrieval with good colour naming is very rare. Beauvois and Saillant (1985) studied two patients with visual, verbal and visuo-verbal tests and differentiated two syn- dromes: colour agnosia (specific impairments on the visual tests) and optic aphasia for colours (specific impairments on the visuo- verbal tests). Further dissociations have been found between poor object-colour retrieval with preserved ability to categorise colours and to name colours (e.g. Beauvois & Saillant, 1985) and without the ability to name colours (Levine, 1978). This shows that knowledge about colour is neither strictly verbal nor visual. The suggestion that colour agnosia may not be a unitary con- 0028-3932/$ – see front matter © 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.neuropsychologia.2005.12.004