Language and perceptual categorisation.

In a pioneering set of experiments, Rosch investigated the colour processing of a remote traditional culture. It was concluded that colours form universally natural and salient categories. However, our own cross-cultural research, backed up by neuropsychological data and interference studies, indicates that perceptual categories are derived from the words in the speaker's language. The new data support a rather strong version of the Whorfian view that perceptual categories are organized by the linguistic systems of our mind.

A particular difficulty in discriminating between mirror images.

We investigated the selective impairment of mirror image discrimination in a patient with bilateral parieto-occipital lesions (FIM). We report a difficulty with the discrimination between mirror images more selective than has been previously reported (Turnbull OH, McCarthy RA. Failure to discriminate between mirror-image objects: a case of viewpoint-independent object recognition? Neurocase 1996;2:63). FIM was asked to judge, in five same/different experiments, whether pairs of simultaneously presented line drawings of objects were identical. FIM demonstrated only a minor impairment in discriminating between orientations in the picture plane but was at chance in making discrimination between mirror images. An experiment with normal observers established that our results were not due to differences in task difficulty. Two further experiments investigated the effects of rotation on the discrimination of letters and geometric shapes. FIM's impairment extended to geometric shapes but not to letters. These results would be consistent with the preservation of an abstract representation for object recognition that did not code the difference between mirror image views.

The categorical perception of colors and facial expressions: the effect of verbal interference.

A series of five experiments examined the categorical perception previously found for color and facial expressions. Using a two-alternative forced-choice recognition memory paradigm, it was found that verbal interference selectively removed the defining feature of categorical perception. Under verbal interference, there was no longer the greater accuracy normally observed for cross-category judgments relative to within-category judgments. The advantage for cross-category comparisons in memory appeared to derive from verbal coding both at encoding and at storage. It thus appears that while both visual and verbal codes may be employed in the recognition memory for colors and facial expressions, subjects only made use of verbal coding when demonstrating categorical perception.

Color categories are not universal: replications and new evidence from a stone-age culture.

The authors sought to replicate and extend the work of E. Rosch Heider (1972) on the Dani with a comparable group from Papua, New Guinea, who speak Berinmo, which has 5 basic color terms. Naming and memory for highly saturated focal, non-focal, and low-saturation stimuli from around the color space were investigated. Recognition of desaturated colors was affected by color vocabulary. When response bias was controlled, there was no recognition advantage for focal stimuli. Paired-associate learning also failed to show an advantage for focal stimuli. Categorical Perception effects for both English and Berinmo were found, but only at the boundaries of existing linguistic categories. It is concluded that possession of linguistic categories facilitates recognition and influences perceptual judgments.

Failure at object identification improves mirror image matching.

In a previous report ([5]: Davidoff J & Warrington EK. The bare bones of object recognition: implications from a case of object recognition impairment. Neuropsychologia 1999;37:279-92) the inability to differentiate between mirror images was recorded in a patient with excellent canonical view recognition. We now extend our investigation to a patient (JBA) with probable Alzheimer's disease in whom canonical view recognition was compromised. The reciprocal inhibition of two aspects of object processing are demonstrated in JBA. The patient's ability to detect mirror image rotations was dependent on her inability to identify the object. Paradoxically, her performance was more impaired for those stimuli that she was able to identify than those she was not.

Similarity and categorisation: neuropsychological evidence for a dissociation in explicit categorisation tasks.

A series of experiments are reported on a patient (LEW) with difficulties in naming. Initial findings indicated severe impairments in his ability to freesort colours and facial expressions. However, LEW's performance on other tasks revealed that he was able to show implicit understanding of some of the classic hallmarks of categorical perception; for example, in experiments requiring the choice of an odd-one-out, the patient chose alternatives dictated by category rather than by perceptual distance. Thus, underlying categories appeared normal and boundaries appeared intact. Furthermore, in a two-alternative forced-choice recognition memory task, performance was worse for within-category decisions than for cross-category decisions. In a replication of the study of Kay and Kempton [Kay, P., Kempton, W., 1984. What is the Sapir-Whorf hypothesis? American Anthropologist 86, 65-78], LEW showed that his similarity judgements for colours could be based on perceptual or categorical similarity according to task demands. The consequences for issues concerned with perceptual categories and the relationship between perceptual similarity and explicit categorisation are considered; we argue for a dissociation between these kinds of judgements in the freesort tasks. LEW's inability to make explicit use of his intact (implicit) knowledge is seen as related to his language impairment.

The bare bones of object recognition: implications from a case of object recognition impairment.

Three experiments were designed to investigate the performance of a patient (RK) who could name objects when presented in conventional views but showed catastrophic failures in identification from unconventional views. The aim of all three experiments was to assess the properties of the central representations that allow recognition of objects presented in conventional but not unconventional views. All three experiments showed that RK had problems in object identification not apparent from his naming performance. In the first experiment, RK was found to be extremely impaired at recognising the parts of objects even though he could name the whole object. In the second experiment, alterations in colour, shape and parts of objects were undetected in stimuli that he could name. In the third experiment, RK showed considerable difficulty with mirror-images and inversion tasks. The explanation for RK's impaired object recognition could not be attributed to defects to his early visual processing. We argue that RK's recognition is achieved through abstract (object-centred) representations that are global rather than local, and quite independent of their spatial framework. These abstract representations we take to be the essential bare bones for object recognition.

Higher-level cortical processing of colour.

Wavelength information serves both to organise shapes and to provide the appearance of surfaces. These two roles are considered within a model that outlines the role of colour in picture naming (Davidoff and de Bleser, 1993). An introduction to the papers in this issue is given within the framework of the model. The topics in this issue cover the role of colour in scene segmentation, colour constancy and the memory for the colours of objects.

Brain events related to normal and moderately scrambled faces.

The neural basis of normal and scrambled face processing was investigated by recording evoked potentials from 21 electrodes at standard EEG sites, with respect to a nose reference. Temporal negativities were found that result from two overlapping phenomena: they arise from the polarity reversal on temporal electrodes of the vertex P2, a positive wave peaking about 170-200 ms after the onset of a face stimulus, and also from an overlapping 'processing negativity' of long duration associated with the processing difficulty of the scrambled face stimulus. The comparisons of scalp potential and current density mappings support the proposal that some neuronal networks are active both for faces and scrambled faces and are compatible with the involvement of the superior temporal sulcus, the inferotemporal cortex and the parahippocampal and fusiform gyri, whereas the processing negativity would only involve the deepest generators of this network. Furthermore, the encoding of both faces and scrambled faces seems to take place predominantly in the right hemisphere.

Human colour discrimination based on a non-parvocellular pathway.

BACKGROUND: Traditionally, colour information is assumed to be carried by neural channels in the parvocellular pathway and to be encoded in an opponent manner, while other, non-parvocellular, spectrally non-opponent channels are thought to play no part in colour vision. But is the parvocellular pathway the only way that colours can be discriminated in human vision? We studied two patients with cerebral achromatopsia, who lack conscious colour perception but are nevertheless able to make use of colour information. In particular, we investigated whether, in these patients, colour discrimination is mediated by the parvocellular pathway. RESULTS: The achromatopsic patients carried out a forced-choice colour- and luminance-discrimination task, and showed clear evidence of unconscious colour processing, consistent with previous studies. We added different types of luminance noise to see when this unconscious colour information could be masked. The results of the colour-discrimination-with-noise and the brightness-non-additivity experiments showed a double-dissociation between patients. This indicates that, in one patient, unconscious colour discrimination may be subserved by a spectrally non-opponent mechanism, which does not have the characteristics of the parvocellular pathway and which is responsive to fast flicker. Spectral sensitivity, contrast sensitivity and motion perception experiments confirmed that this patient lacks a working opponent parvocellular system. The second achromatopsic patient showed evidence of a residual parvocellular system. CONCLUSIONS: Our results show that chromatic discrimination need not be mediated by neural mechanisms, the parvocellular system in particular, normally assumed to subserve conscious colour perception. Such discrimination may be mediated by a neural subsystem which responds to fast flicker, is spectrally non-opponent, and supports normal motion perception.

Selective impairments of object knowledge in a case of acquired cortical blindness.

A patient (N.B.) is described, who displays distinct deficits of object knowledge related to knowledge type (Functional/Associative vs Visual) and also to knowledge category (Animate vs Inanimate). The patient was first given an orally presented forced-choice test devised to assess orthogonal combinations of knowledge type and knowledge category. In the production of this test it was found that normals took longer to respond to Visual questions than to Functional/Associative questions; therefore, sets of questions were compiled that were matched for both accuracy and latency. There were two main findings concerning N.B.'s semantic memory. First, with careful matching of difficulty level, the patient showed selective preservation of Functional/Associative knowledge of Animate objects compared with Visual knowledge of Animate objects and also compared with Functional/Associative knowledge of Inanimate objects. Second, there was a qualitative difference in patterns of knowledge retrieval for Visual compared with Functional/Associative knowledge. Retrieval of Visual knowledge, both Animate and Inanimate, was inconsistent and, in a word-pair recall test, a high degree of connection of a Visual property to an object did not promote paired-associate learning. In contrast, retrieval of Functional/Associative knowledge (both Animate and Inanimate) was consistent and paired-associate learning was influenced by connection strength. This study provides strong evidence to support the validity of both "knowledge type" and "category" based accounts of the organisation of semantic memory.

Impaired retrieval of object-colour knowledge with preserved colour naming.

Two cases (G.G. and A.V.) are described of cognitive impairment resulting from herpes simplex infection. Both cases demonstrated anomic disorders and impairments in drawing but only in G.G.'s drawings was there a reliable selective impairment for items from natural categories. Both cases, however, showed an impairment for the retrieval of knowledge concerning the colours of objects. The impairment has, in the past, been ascribed to interference from colour anomia; this was not so for the present cases. For G.G. and A.V., impairments in object-colour retrieval were related to errors in picture naming. More errors were associated with items that induced circumlocutions than to those that were correctly named. The impairment was also present for some items that were named correctly. The patients' impairments are discussed within a model in which object-colour knowledge is functionally situated between an object's shape description and its output phonology but on a separate route from other associated object knowledge.

Impaired picture recognition with preserved object naming and reading.

A case study is reported on a left posterior cerebrovascular accident involving the infracalcarine cortex. The patient (HG) exhibited a marked color anomia and picture naming deficit (photograph anomia) without alexia. However, objects were named successfully from either visual or tactile inspection. Despite successful object categorization in several tasks, it is argued that HG's picture naming deficit is dependent on a disorder of recognition (access to the stored structural descriptions for objects). A similar functional impairment can account for HG's impaired color naming. The site of HG's cortical damage implies that recognition disorders can result from a unilateral left-sided lesion.

Partial recovery from visual object agnosia: a 10 year follow-up study.

We describe a young woman, J.R., who sustained a very severe head injury in 1981 at the age of 17 years. She was assessed in 1982 and found to have visual agnosia. Since then J.R. has been assessed on several occasions over a period of ten years. Her agnosia for real objects has resolved and she has improved on the identification of other classes of stimuli. However, she still has some problems with the identification of line drawings, photographs and model animals. Her drawing from memory remains particularly poor and she has difficulty with visual imagery. We consider her residual deficits in the light of Farah's (1990) theoretical framework; this proposes that associative agnosia could be due to a disconnection syndrome, a loss of stored visual representations or to the loss of knowledge of how to perceive objects. J.R.'s residual impairments appear to be mainly due to a loss of access to visual representations in the absence of visual input.

The colour cognition of children.

It is well known that young children have great difficulty in stating the appropriate colour name when presented with a colour stimulus, even though they may be effective at matching one colour stimulus with another and possess a good vocabulary of colour terms. The present studies demonstrate a further, but distinct, impairment in dealing with coloured stimuli. Three- to four-year-old children were given the task of distinguishing an appropriately coloured option from one with an inappropriate colour. Despite displaying a preference for the appropriate colour over the inappropriate one, children found the task very difficult. In contrast, they succeeded on the task more frequently when the alternatives were presented verbally, rather than in terms of two pictures. We discuss the results within a theoretical framework which distinguishes the access to object-colour knowledge from colour naming. The model makes divisions within the mental templates required for object matching. In particular, colour detail is functionally separated from shape detail and appears to be represented in more than one code.

A dissociation of shape discrimination and figure-ground perception in a patient with normal acuity.

We report the case of a 21-year-old man who sustained brain damage due to anoxia. His visual acuity was normal and his colour perception was adequate if not completely normal. However, his shape discrimination was so impaired that he had great difficulty distinguishing a square from an oblong or a large square from a smaller square. At the same time, other aspects of his shape perception were remarkably intact. His performance on a shape detection task was good. He had no difficulty on tests of figure-ground (boundary) segmentation and his perception of subjective and illusory contours appeared to be normal. S.M.K.'s performance on a number of tasks is compared with that of a patient (F.G.P.) previously reported who showed the opposite pattern of deficit (Kartsounis and Warrington, Neuropsychologia 29, 969-980, 1991). This evidence of a double dissociation between two components of shape perception is discussed in the context of a neurophysiological model of the organization of the visual system.

A dyscalculic patient with selectively impaired processing of the numbers 7, 9, and 0.

A patient (J.C) had particular difficulties with the numbers 7, 9, and 0 in aurally and visually presented calculations. These problems were explained in terms of a modified version of the McCloskey, Caramazza, & Basili (1985) model of calculation processes. His calculation difficulties were attributed to degraded semantic representations of the numbers 7 and 9 and to lost knowledge of rules governing calculations involving the number 0. Use of "noncalculation" number processing tasks confirmed that his 7/9 problem was not at the level of acoustic analysis, the auditory input lexicon, the phonological output lexicon, visual analysis, or the orthographic output lexicon.