Loss of visual imagery and loss of visual knowledge—A case study

As a sequel of a left posterior cerebral artery infarction a patient had severely defective mental imagery of shapes and colours of objects. Imagery of faces, letters, and topological relationships was preserved. The impairment of imagery of object colours was associated with colour agnosia and colour anomia. For colours, there was no difference between performance on tasks calling for imagery of object colours and tasks affording a distinction between correctly and incorrectly coloured objects. For shapes of objects, imagery appeared to be below the level of the patient's knowledge about the visual appearance of objects as manifested by the ability to identity objects and to distinguish correctly from incorrectly drawn pictures. The apparently selective image generation deficit for shapes of objects could be a sequel of loss of knowledge about visual attributes of objects, if superior performance on shape recognition tasks was afforded by perceptual entry level representation which enable a rapid identification of objects but are inaccessible to introspective consciousness.     

Neural correlates of imagined and synaesthetic colours

The experience of colour is a core element of human vision. Colours provide important symbolic and contextual information not conveyed by form alone. Moreover, the experience of colour can arise without external stimulation. For many people, visual memories are rich with colour imagery. In the unusual phenomenon of grapheme-colour synaesthesia, achromatic forms such as letters, words and numbers elicit vivid experiences of colour. Few studies, however, have examined the neural correlates of such internally generated colour experiences. We used functional magnetic resonance imaging (fMRI) to compare patterns of cortical activity for the perception of external coloured stimuli and internally generated colours in a group of grapheme-colour synaesthetes and matched non-synaesthetic controls. In a voluntary colour imagery task, both synaesthetes and non-synaesthetes made colour judgements on objects presented as grey scale photographs. In a synaesthetic colour task, we presented letters that elicited synaesthetic colours, and asked participants to perform a localisation task. We assessed the neural activity underpinning these two different forms of colour experience that occur in the absence of chromatic sensory input. In both synaesthetes and non-synaesthetes, voluntary colour imagery activated the colour-selective area, V4, in the right hemisphere. In contrast, the synaesthetic colour task resulted in unique activity for synaesthetes in the left medial lingual gyrus, an area previously implicated in tasks involving colour knowledge. Our data suggest that internally generated colour experiences recruit brain regions specialised for colour perception, with striking differences between voluntary colour imagery and synaesthetically induced colours.     

Colour-naming defects in dysphasic patients. A qualitative analysis

An analysis of the performance of 20 dysphasic subjects and 14 patients with cerebral lesions but with no dysphasia in a variety of colour-naming tasks was undertaken. In addition the performance of a group of blind subjects in tests requiring the verbalization of colour names in response to verbal commands is presented, in order to evaluate the significance of the use of colour names derived exclusively from verbal associations. The results show that the dysphasic subjects differ from non-dysphasic subjects in three aspects of colour naming behaviour, namely in their ability to name unstructured colours, coloured drawings of objects and in the number of colour names that can be cited spontaneously in a limited time. The findings also show that the dysphasic subjects have greater difficulty in naming coloured objects than they have in naming unstructured colours objects alone. Some theoretical interpretations of the findings are discussed.     

Cerebral correlates of imagining colours, faces and a map--II. Negative cortical DC potentials

Cortical activation patterns as measured by negative shifts of the scalp-recorded cortical steady potential ("DC shifts") were assessed in 28 normal subjects during imagining colours, faces, and a spatial map. Imaging resulted in sustained negative DC shifts at temporal, parietal and particularly at occipital sites. The topographic distributions of such DC shifts was modulated as a function of whether spatial or visual imagery was performed. During imaging the spatial map, a parietal maximum was observed, as opposed to a distribution in favour of temporal and occipital sites during imagining faces and colours. Results suggest a neuroanatomical dissociation between visual and spatial imagery. Since a similar visual-spatial dichotomy exists in perception, the finding is interpreted as further evidence of a shared cerebral substrate for images and percepts. The results are discussed in conjunction with the joint blood flow study.     

Contributions of occipital and temporal brain regions to visual and acoustic imagery--a spect study

Regional cerebral blood flow (rCBF) was assessed by means of HMPAO-SPECT in two experimental groups. In a control condition both groups listened to abstract words, in the experimental condition they heard five names of objects. One group was advised to form visual images of the objects, the other group was advised to form acoustic images of the sounds made by these objects. Post-experimental questionnaires revealed that most of the subjects in the acoustic imagery condition had had visual images in addition to the acoustic ones. Both imagery conditions lead to approximately equal increases of rCBF in the left inferior occipital region and in the left thalamus. Flow increases in both hippocampal regions and the right inferior and superior temporal regions were larger in the acoustic than in the visual imagery condition. It is concluded that only the activation of left inferior occipital and left thalamic regions can be interpreted as being related to modality-specific visual aspects of imagery.     

A pet study of visual and semantic knowledge about objects

The distinctiveness of temporal lobe regions activated during the retrieval of knowledge regarding structural, colour and associative (encyclopaedic) aspects of familiar objects was investigated using PET. These three types of knowledge were contrasted using well matched tasks requiring the detection, in a series of coloured-in line drawings, of occasional anomalous objects (in the three conditions: structurally incorrect chimeras composed of parts of real objects; inappropriately coloured objects; familiar objects that do not exist in the modern world). Relative to a resting baseline condition, all semantic retrieval tasks yielded extensive bilateral activations in occipital and temporal areas, extending anteriorly on the ventral surface of the brain, plus an area in the right superior parietal lobe. In direct semantic-task comparisons focussing on the temporal lobe: (i) structural relative to associative decisions activated the right posterior middle/inferior temporal gyrus; (ii) colour decisions relative to structural judgements were associated with a region in the right superior temporal gyrus; (iii) the associative decision task selectively activated the left anterior middle/superior temporal gyrus and temporal pole relative to both object structure and colour, and also the homologous right temporal pole relative to colour only. These results indicate that each type of stored knowledge involves at least partially distinct cortical areas, and suggest that both anterior/posterior and left/right temporal regions have specialised roles.     

Neural correlates of implicit object identification

The present study sought to assess neural correlates of implicit identification of objects by means of fMRI, using tasks that require matching of the physical properties of objects. Behavioural data suggests that there is automatic access to object identity when observers attend to a physical property of the form of an object (e.g. the object's orientation) and no evidence for semantic processing when subjects attend to colour. We evaluated whether, in addition to neural areas associated with decisions to specific perceptual properties, areas associated with access to semantic information were activated when tasks demanded processing of the global configuration of pictures. We used two perceptual matching tasks based on the global orientation or on the colour of line drawings. Our results confirmed behavioural data. Activations in the inferior occipital cortex, fusiform and inferior temporal gyri in both tasks (orientation and colour) account for perceptual and structural processing involved in each task. In contrast, activations in the posterior and medial parts of the fusiform gyrus, shown to be involved in explicit semantic judgements, were more pronounced in the orientation-matching task, suggesting that semantic information from the pictures is processed in an implicit way even when not required by the task. Thus, this study suggests that cortical regions usually involved in explicit semantic processing are also activated when implicit processing of objects occurs.     

Covert colour processing in colour agnosia

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.     

Tactile agnosia and tactile aphasia: symptomatological and anatomical differences.

Two patients with tactile naming disorders are reported. Case 1 (right hand tactile agnosia due to bilateral cerebral infarction) differentiated tactile qualities of objects normally, but could neither name nor categorize the objects. Case 2 (bilateral tactile aphasia after operation of an epidural left parietal haematoma) had as severe a tactile naming disturbance as Case 1, but could categorize objects normally, demonstrating that tactile recognition was preserved. Case 1 may be the first case of tactile agnosia clearly differentiated from tactile aphasia. CT scans of Case 1 revealed lesions in the left angular gyrus, and in the right parietal, temporal, and occipital lobes. Case 2 had lesions in the left angular gyrus and of posterior callosal radiations. Our findings suggest that tactile agnosia appears when the somatosensory association cortex is disconnected by a subcortical lesion of the angular gyrus from the semantic memory store located in the inferior temporal lobe, while tactile aphasia represents a tactual-verbal disconnection.     

Cerebral correlates of imagining colours, faces and a map--I. SPECT of regional cerebral blood flow

The distribution of regional cerebral blood flow (rCBF) was assessed by single photon emission computerized tomography (SPECT) in subjects during a resting state and during imagining either colours or faces or a route on a map. Twelve out of 30 subjects reported the spontaneous occurrence of mental visual images during the resting state. In these subjects flow in both orbitofrontal regions was higher than in those subjects who had not experienced spontaneous imagery. Voluntary imagery led to an increase of regional flow indices in basal temporal regions of both hemispheres and to a rightwards shift of global hemispheric asymmetry. The local changes were distinctly more marked with faces than with any of the other two stimuli. Imagining faces was also the only condition that led to an increase of activity in the left inferior occipital region which has been suggested by previous studies as being a crucial area for visual imagery. It is concluded that the observed differences of rCBF patterns between imagery conditions are related to the amount of information conveyed by the mental image. In contrast to the results of a companion study on DC-shifts accompanying imagery there was no effect of the visual versus spatial character of the images.     

Physiologic mechanisms of the constancy of color perception]

The color constancy is the ability to recognize correctly the colours of the objects under different illuminations. For this purpose the visual system must identify the character of illumination and estimate the colour of the object, using the light reflected from this object according to the spectrum of illumination. The behavioral experiments with fishes showed that they possess the colour constancy. The electrophysiological studies on colour-coding ganglion cells showed that the simplest mechanisms of estimating the colours of objects according to illumination exist at the retinal level. The presence of colour vision facilitates the recognition of the volume form of the objects. The generally accepted view on the place and role of colour vision in the general function of the visual system of animals has to be changed on the basis of these facts.     

Does colour say something about emotions?: Laypersons’ assessments of colour drawings

This study examines the use of colours in estimating psychological states. Sixteen artworks were produced by two different populations: clients with a psychological condition and psychologically healthy people. Their drawings were presented in pairs on screen either chromatically or achromatically. Two hundred and twenty-four subjects were asked to choose a picture which they thought had been produced by a psychologically unhealthy client. The rate of correct answers was higher for the chromatic condition. When artworks were presented achromatically, errors increased. Analysis of colour factors and post-inquiry indicated that the number of colours used, list of colours, and colour connotation might have affected choices. This study confirms that colour can be effective in estimating psychological states. Its implications and limitations are discussed and suggestions made for future study.     

Chromatic Discrimination in a Cortically Colour Blind Observer

We tested the ability of a subject with cerebral achromatopsia to discriminate between colours and to detect chromatic borders. He was unable to identify colours or to arrange them in an orderly series or choose the odd colour out of an array or even to pick out a colour embedded in an array of greys. Nevertheless, he could select the odd colour when the colours were contiguous, even when they were isoluminant, and could discriminate an ordered from a disordered chromatic series as long as the colours in each row abutted one other. His verbal replies showed that he did so by detecting an edge between two stimuli that were, to him, perceptually identical. Introducing a narrow isoluminant grey stripe between adjacent colours abolished or greatly impaired this ability. As long as isoluminant colours were contiguous the patient could identify the orientation of the chromatic borders. Photopic spectral sensitivity showed evidence both for activity of three cone channels and for chromatic opponent processing, indicating that postreceptoral chromatic processing is occurring despite the absence of any conscious awareness of colour. The results indicate that both parvocellular colour opponent and magnocellular broad-band channels are active and that the cortical brain damage has selectively disrupted the appreciation of colour but not the ability to detect even isoluminant chromatic borders, which would be invisible to a retinal achromat. The subject's performance on non-colour tasks involving the discrimination of shape, texture, greyness and position was excellent. His disorder is therefore not like that of macaque monkeys in which cortical area V4 has been removed, and which are much more severely impaired at discriminating shape than colour.     

The neural correlate of colour distances revealed with competing synaesthetic and real colours

Synaesthetes claim to perceive illusory colours when reading alphanumeric symbols so that two colours are said to be bound to the same letter or digit (i.e., the colour of the ink, e.g., black, and an additional, synaesthetic, colour). To explore the neural correlates of this phenomenon, we used a Stroop single-letter colour-naming task and found that distances in colour space between the illusory and real colours of a letter target (as computed from either the RGB or CIExyY coordinates of colours) systematically influenced the degree of neuronal activation in colour-processing brain regions. The synaesthetes also activated the same fronto-parietal network during the classic colour-word Stroop task and single-letter tasks. We conclude that the same neural substrate that supports the conscious experience of colour, as triggered by physical wavelength, supports the experience of synaesthetic colours. Thus, two colour attributes (one that is wavelength-dependent and one that is illusory) can be bound to the same stimulus position and simultaneously engage the colour areas in proportion to their similarity in colour space.     

Detection and discrimination of chromatic targets in hemianopic macaque monkeys and humans

We measured the ability of three macaque monkeys with unilateral removal of primary visual cortex to detect 9 degrees, 200-ms targets presented at random in the upper or lower quadrants of the normal and hemianopic visual fields. The white or variously coloured target could differ from the background in both colour and luminance, or in either of them. Blue and red targets were detectable at any luminance contrast, but green and white targets were barely or not at all detectable at and near isoluminance in the hemianopic field. Blue and red targets were better detected than white targets at the same luminance difference. However, when both the target and the background were dynamically luminance-masked, detection in the hemianopic field failed at isoluminance whatever the colour. In addition, the monkeys were unable to discriminate between simultaneously presented red and green or blue and green targets in the hemianopic field when both targets had similar luminance contrast with the background. Two hemianopic patients tested on a subset of the tasks performed similarly to the monkeys. Together, the results indicate that the residual colour-opponent system that survives damage to V1 is involved in the detection of chromatic changes but cannot sustain simultaneous discrimination between pairs of colours.     

Colour knowledge in semantic dementia: it is not all black and white

In three experiments we assessed the colour knowledge of patients with semantic dementia, a neuro-degenerative condition that gradually erodes conceptual knowledge. In Experiment 1, the patients’ colour naming performance correlated strongly with their object naming for frequency-matched items, with no patient showing better-than-expected naming of colours relative to objects. In Experiment 2, where patients were asked to colour black-and-white line drawings of common objects, all patients were impaired relative to controls, and performance correlated strongly with degree of semantic deficit. The fact that patients often erroneously selected green for fruits or vegetables, and brown for animals, suggests some preservation of general knowledge about the colours that typify a given domain. In Experiment 3, patients were given pairs of identical line drawings of familiar animals, fruits and vegetables—one of each pair coloured correctly, and one incorrectly—and were asked to choose the correct one. When the target's colour was characteristic of the domain, patients scored well; but when the distractor had a typical hue and the target's colour was unusual (e.g. a green versus an orange carrot), performance was far poorer. The results are discussed with reference to alternative theories about the neural basis of conceptual knowledge.     

Temporal segregation deficit in visual perception: A single case study

Abstract

The patient (LD) presents a visual deficit In temporal segregation. PET scan showed a bilateral hypometabolism in the superior parietal, temporal and occipital cortex. Neuropsychological data revealed that memory, attention and language functions were almost completely preserved. Visual sensory tests showed that visual acuity, depth and colour perception were normal, whereas detection of apparent motion direction, flicker fusion and saccadic eye movements were abnormal. Shape drawing was impaired whereas shape recognition (and naming), with either static or dynamic stimuli, was good. LD failed in tasks requiring Integration of elements to produce a whole image (line drawings, degraded and textured shapes). On the other hand, LD's performance was good in tasks not involving spatial integration, such as visual search and letter-by-letter reading. The main finding is that LD failed in tasks requiring temporal segregation. These tasks involved shape recognition when different shapes (Experiment la) or different parts of either a word or a shape presented successively on the same retinal locus with (Experiments 2b, 3a, 3b) and without (Experiments 4a, 4b) eye movements. Finally, LD has no problems in tasks (4c) involving temporal integration alone. Taken together, these results suggest damage to a general mechanism involved in temporal segregation.     

Spatial dissociation of early and late colour evoked components

Evoked potentials to the primary colours red, green, yellow and blue were recorded and compared with those evoked by white. The unpatterned 10 degrees X13 degrees stimuli were generated with the aid of a colour monitor. Activity was depicted with 5 electrodes, the central electrode 5 cm above the inion and two on each side 5 and 10 cm apart from the central electrode. With equally bright colour stimuli a previously described early negative colour-dominated component N87 was localized in all subjects at the central occipital electrode while the following positivity P100 was clearly more lateralized to the peripheral electrodes. With half-field stimulation N87 showed a similar--paradoxical--lateralization to the ipsilateral electrodes as has been demonstrated for pattern reversal. The existence and localization of N87 could be confirmed also for blue colours, its amplitude independent of the blue luminance, its latency decreasing for definite additional brightness increments and decrements. The N87 to blue was of the same latency as the N87 components to other colours. N87 is mainly generated foveally and parafoveally, its amplitude saturates with stimuli larger than 6-8 degrees in diameter.     

Visual and spatial working memory: from boxes to networks

It is shown that visuo-spatial working memory is better characterized as processes operating on sensory information (visual appearance) and on spatial location (environmental coordinates) in a distributed network than as unitary slave system. Results from passive (short-term) and active memory tasks (imagery) disclose the properties (capacity, content) and the components of this network. The prefrontal cortex is a control structure (dorsal prefers active, ventral passive tasks) and it contributes to spatial memory by a prospective spatial code (eye movements). Visual appearance (including dynamic aspects) is represented as features and object files (bound features) within content-specific areas in the ventral occipital cortex. Spatial coordinates are represented in the parietal cortex (modality-unspecific), when used in spatio-temporal tasks (Corsi) they are closely related to attention. Imagery of objects activates occipito-temporal structures, spatial transformations and mental rotation the parietal cortex (specifically the intraparietal sulcus). Perception, working memory, and imagery use the same neural network. Differences between the tasks are explained by different demands and states of the neural network, and differences in the configuration of the anterior–posterior neural circuits.