Preserved imagery for colours in a patient with cerebral achromatopsia

Visual mental imagery: Evidence for a heterarchical neural architecture

Theories of Visual Mental Imagery (VMI) emphasize the processes of retrieval, modification, and recombination of sensory information from long-term memory. Yet, only few studies have focused on the behavioral mechanisms and neural correlates supporting VMI of stimuli from different semantic domains. Therefore, we currently have a limited understanding of how the brain generates and maintains mental representations of colors, faces, shapes - to name a few. Such an undetermined scenario renders unclear the organizational structure of neural circuits supporting VMI, including the role of the early visual cortex. We aimed to fill this gap by reviewing the scientific literature of five semantic domains: visuospatial, face, colors, shapes, and letters imagery. Linking theory to evidence from over 60 different experimental designs, this review highlights three main points. First, there is no consistent activity in the early visual cortex across all VMI domains, contrary to the prediction of the dominant model. Second, there is consistent activity of the frontoparietal networks and the left hemisphere's fusiform gyrus during voluntary VMI irrespective of the semantic domain investigated. We propose that these structures are part of a domain-general VMI sub-network. Third, domain-specific information engages specific regions of the ventral and dorsal cortical visual pathways. These regions partly overlap with those found in visual perception studies (e.g., fusiform face area for faces imagery; lingual gyrus for color imagery). Altogether, the reviewed evidence suggests the existence of domain-general and domain-specific mechanisms of VMI selectively engaged by stimulus-specific properties (e.g., colors or faces). These mechanisms would be supported by an organizational structure mixing vertical and horizontal connections (heterarchy) between sub-networks for specific stimulus domains. Such a heterarchical organization of VMI makes different predictions from current models of VMI as reversed perception. Our conclusions set the stage for future research, which should aim to characterize the spatiotemporal dynamics and interactions among key regions of this architecture giving rise to visual mental images.

Colors in the mind's eye

The famous "Piazza del Duomo" paper, published in Cortex in 1978, inspired a considerable amount of research on visual mental imagery in brain-damaged patients. As a consequence, single-case reports featuring dissociations between perceptual and imagery abilities challenged the prevailing model of visual mental imagery. Here we focus on mental imagery for colors. A case study published in Cortex showed perfectly preserved color imagery in a patient with acquired achromatopsia after bilateral lesions at the borders between the occipital and temporal cortex. Subsequent neuroimaging findings in healthy participants extended and specified this result; color imagery elicited activation in both a domain-general region located in the left fusiform gyrus and the anterior color-biased patch within the ventral temporal cortex, but not in more posterior color-biased patches. Detailed studies of individual neurological patients, as those often published in Cortex, are still critical to inspire and constrain neurocognitive research and its theoretical models.

Probing the unimaginable: The impact of aphantasia on distinct domains of visual mental imagery and visual perception

Different individuals experience varying degrees of vividness in their visual mental images. The distribution of these variations across different imagery domains, such as object shape, color, written words, faces, and spatial relationships, remains unknown. To address this issue, we conducted a study with 117 healthy participants who reported different levels of imagery vividness. Of these participants, 44 reported experiencing absent or nearly absent visual imagery, a condition known as "aphantasia". These individuals were compared to those with typical (N = 42) or unusually vivid (N = 31) imagery ability. We used an online version of the French-language Battérie Imagination-Perception (eBIP), which consists of tasks tapping each of the above-mentioned domains, both in visual imagery and in visual perception. We recorded the accuracy and response times (RTs) of participants' responses. Aphantasic participants reached similar levels of accuracy on all tasks compared to the other groups (Bayesian repeated measures ANOVA, BF = .02). However, their RTs were slower in both imagery and perceptual tasks (BF = 266), and they had lower confidence in their responses on perceptual tasks (BF = 7.78e5). A Bayesian regression analysis revealed that there was an inverse correlation between subjective vividness and RTs for the entire participant group: higher levels of vividness were associated with faster RTs. The pattern was similar in all the explored domains. The findings suggest that individuals with congenital aphantasia experience a slowing in processing visual information in both imagery and perception, but the precision of their processing remains unaffected. The observed performance pattern lends support to the hypotheses that congenital aphantasia is primarily a deficit of phenomenal consciousness, or that it employs alternative strategies other than visualization to access preserved visual information.

Colour perception deficits after posterior stroke: Not so rare after all?

Cerebral achromatopsia is an acquired colour perception impairment caused by brain injury, and is generally considered to be rare. Both hemispheres are thought to contribute to colour perception, but most published cases have had bilateral or right hemisphere lesions. In contrast to congenital colour blindness that affects the discrimination between specific hues, cerebral achromatopsia is often described as affecting perception across all colours. Most studies of cerebral achromatopsia have been single cases or case series of patients with colour perception deficits. Here, we explore colour perception deficits in an unbiased sample of patients with stroke affecting the posterior cerebral artery (N = 63) from the Back of the Brain project. Patients were selected based on lesion location only, and not on the presence of a given symptom. All patients were tested with the Farnsworth D-15 Dichotomous Colour Blindness Test and performance compared to matched controls (N = 45) using single case statistics. In patients with abnormal performance, the patterns of colour difficulties were qualitatively analysed. 22% of the patients showed significant problems with colour discrimination (44% of patients with bilateral lesions, 28% with left hemisphere lesions and 5% with right hemisphere lesions). Lesion analyses identified two regions in ventral occipital temporal areas in the left hemisphere as particularly strongly related to impaired performance in colour perception, but also indicated that bilateral lesions are more strongly associated with impaired performance that unilateral lesions. While some patients only had mild deficits, colour perception impairments were in many cases severe. Many patients had selective deficits only affecting the perception of some hues. The results suggest that colour perception difficulties following PCA stroke are common, and that they vary in severity and expression. In addition, the results point towards bilateral processing of colour perception with a left hemispheric domination, contradicting previous reports.

Predictive processing and perception: What does imagining have to do with it?

Predictive processing (PP) accounts of perception are unique not merely in that they postulate a unity between perception and imagination, but in claiming that (i) perception should be conceptualised in terms of imagination and (ii) the two involve an identity of neural implementation. This paper argues against these claims, on both conceptual and empirical grounds. Conceptually, the manner in which PP theorists link perception and imagination belies an impoverished account of imagery as cloistered from the external world in its intentionality, akin to a virtual reality, as well as endogenously generated. Yet this ignores a whole class of imagery whose intentionality is directed on the actual environment-projected mental imagery-and also ignores the fact that imagery may be triggered crossmodally in a bottom-up, stimulus-driven way. Empirically, claiming that imagery and perception share neural circuitry ignores relevant clinical results in this area. These evidence substantial perception/imagery neural dissociations, most notably in the case of aphantasia. Taken together, the arguments here suggest that PP theorists should substantially temper, if not outright abandon, their claim to a perception/imagination unity.

Visual objects and their colors

The ventral temporal cortex hosts key regions for the high-level visual processing of object shape and color. These areas represent nodes of large-scale neural circuits dedicated to object recognition. In the language-dominant hemisphere, some of these regions communicate with the language systems; by assigning verbal labels to percepts, these circuits speedup stimulus categorization, and permit fast and accurate interindividual communication. By impairing the functioning of these circuits, neurological damage may provoke disabling disorders of the processing of visual objects and of their colors. Brain damage of vascular, degenerative, toxic, or traumatic origin can induce deficits at different levels of visual processing, from the building of shape- or wavelength-invariant percepts, to their connections with semantic knowledge and with the appropriate lexical entry. After an overview of the neuroimaging of domain-preferring regions for object shape and color in the ventral temporal cortex, this chapter reviews evidence from historical and recent cases of acquired visual agnosia and color processing deficits. A recurrent motif emerging from patients' patterns of performance and lesion locations is the existence of caudo-rostral gradients in the ventral occipito-temporal cortex, spanning from more perceptual to more cognitive stages of processing.

Color Vision Deficits

PURPOSE OF REVIEW

Color provides important information about the identity of the objects we encounter. After early processing stages in the retinal cones, thalamus, and occipital cortex, retinal signals reach the ventral temporal cortex for high-level color and object processing, which links color perception with top-down expectations and knowledge. In the language-dominant hemisphere, some of these regions communicate with the language systems; by assigning verbal labels to percepts, these circuits speed up stimulus categorization, and permit fast and accurate inter-individual communication. This paper provides a review of color processing deficits, from dysfunction of wavelength discrimination in the retinal photoreceptors to deficits of high-level processing in the ventral temporal cortex.

RECENT FINDINGS

Neuroimaging evidence defined the existence and localization of color-preferring domains in the ventral occipito-temporal cortex. Evidence from the performance of a brain-damaged patient with color anomia but preserved color categorization demonstrated the independence of color categorization from color naming in the adult brain. Evidence from patients with brain damage suggests that high-level color processing may be divided into at least three functional domains: perceptual color experience, color naming, and color knowledge.

Hemifield-specific color perception deficits after unilateral V4α lesions

Functional neuroimaging and patient studies demonstrated significant involvement of ventral area V4α, located in the anterior ventral pathway, in color vision. A low number of case studies reported lesions in close vicinity to this region leading to symptoms of hemiachromatopsia indicating hemifield-specific processing of color information. With the present study, we present the first group study investigating hemiachromatopsia after injury to anterior ventral brain areas. In lateral stimulus presentations with several color perception tasks, we observed symptoms of hemiachromatopsia, which were specific to patients with unilateral lesions to the ventral pathway. Particularly, we identified unilateral lesions to area V4α as an important contribution to color perception deficits under demanding viewing conditions. Our results suggest that color information processed along the anterior ventral path is hemifield-specific and that selective deficits in color perception cannot be fully compensated by the intact contralesional visual stream.

Color vision

Color is a fundamental aspect of normal visual experience. This chapter provides an overview of the role of color in human behavior, a survey of current knowledge regarding the genetic, retinal, and neural mechanisms that enable color vision, and a review of inherited and acquired defects of color vision including a discussion of diagnostic tests.

What Cognitive Neurology Teaches Us about Our Experience of Color

Color provides valuable information about the environment, yet the exact mechanisms explaining how colors appear to us remain poorly understood. Retinal signals are processed in the visual cortex through high-level mechanisms that link color perception with top-down expectations and knowledge. Here, we review the neuroimaging evidence about color processing in the brain, and how it is affected by acquired brain lesions in humans. Evidence from patients with brain-damage suggests that high-level color processing may be divided into at least three modules: perceptual color experience, color naming, and color knowledge. These modules appear to be functionally independent but richly interconnected, and serve as cortical relays linking sensory and semantic information, with the final goal of directing object-related behavior. We argue that the relations between colors and their objects are key mechanisms to understand high-level color processing.

The Organization and Operation of Inferior Temporal Cortex

Inferior temporal cortex (IT) is a key part of the ventral visual pathway implicated in object, face, and scene perception. But how does IT work? Here, I describe an organizational scheme that marries form and function and provides a framework for future research. The scheme consists of a series of stages arranged along the posterior-anterior axis of IT, defined by anatomical connections and functional responses. Each stage comprises a complement of subregions that have a systematic spatial relationship. The organization of each stage is governed by an eccentricity template, and corresponding eccentricity representations across stages are interconnected. Foveal representations take on a role in high-acuity object vision (including face recognition); intermediate representations compute other aspects of object vision such as behavioral valence (using color and surface cues); and peripheral representations encode information about scenes. This multistage, parallel-processing model invokes an innately determined organization refined by visual experience that is consistent with principles of cortical development. The model is also consistent with principles of evolution, which suggest that visual cortex expanded through replication of retinotopic areas. Finally, the model predicts that the most extensively studied network within IT-the face patches-is not unique but rather one manifestation of a canonical set of operations that reveal general principles of how IT works.

Localization and patterns of Cerebral dyschromatopsia: A study of subjects with prospagnosia

OBJECTIVE

Cerebral dyschromatopsia is sometimes associated with acquired prosopagnosia. Given the variability in structural lesions that cause acquired prosopagnosia, this study aimed to investigate the structural correlates of prosopagnosia-associated dyschromatopsia, and to determine if such colour processing deficits could also accompany developmental prosopagnosia. In addition, we studied whether cerebral dyschromatopsia is typified by a consistent pattern of hue impairments.

METHODS

We investigated hue discrimination in a cohort of 12 subjects with acquired prosopagnosia and 9 with developmental prosopagnosia, along with 42 matched controls, using the Farnsworth-Munsell 100-hue test.

RESULTS

We found impaired hue discrimination in six subjects with acquired prosopagnosia, five with bilateral and one with a unilateral occipitotemporal lesion. Structural MRI analysis showed maximum overlap of lesions in the right and left lingual and fusiform gyri. Fourier analysis of their error scores showed tritanopic-like deficits and blue-green impairments, similar to tendencies displayed by the healthy controls. Three subjects also showed a novel fourth Fourier component, indicating additional peak deficits in purple and green-yellow regions. No subject with developmental prosopagnosia had impaired hue discrimination.

CONCLUSIONS

In our subjects with prosopagnosia, dyschromatopsia occurred in those with acquired lesions of the fusiform gyri, usually bilateral but sometimes unilateral. The dyschromatopsic deficit shows mainly an accentuation of normal tritatanopic-like tendencies. These are sometimes accompanied by additional deficits, although these could represent artifacts of the testing procedure.

Color perception involves color representations firstly at a semantic level and then at a lexical level

Studies and models have suggested that color perception first involves access to semantic representations of color. This result leads to two questions: (1) is knowledge able to influence the perception of color when associated with a color? and (2) can the perception of color really involve only semantic representations? We developed an experiment where participants have to discriminate the color of a patch (yellow vs. green). The target patch is preceded either by a black-and-white line drawing or by a word representing a natural object associated with the same or a different color (banana vs. frog). We expected a priming effect for pictures because, with a 350-ms SOA, they only involve access to semantic representations of color, whereas words seem only elicit an access to lexical representations. As expected, we found a priming effect for pictures, but also for words. Moreover, we found a general slowdown of response times in the word-prime-condition suggesting the need of an additional processing step to produce priming. In a second experiment, we manipulated the SOA in order to preclude a semantic access in the word-prime-condition that could explain the additional step of processing. We also found a priming effect, suggesting that interaction with perception occurs at a lexical level and the additional step occurs at a color perception level. In the discussion, we develop a new model of color perception assuming that color perception involves access to semantic representations and then access to lexical representations.

Colour, face, and visuospatial imagery abilities in low-vision individuals with visual field deficits

This study investigates to what extent visual perception integrity is necessary for visual mental imagery. Sixteen low-vision participants with severe peripheral visual field loss, 16 with severe central field loss, 6 left brain-damaged patients with right homonymous hemianopia, 6 right brain-damaged patients with left homonymous hemianopia, and 16 normally sighted controls performed perceptual and imagery tasks using colours, faces, and spatial relationships. Results showed that (a) the perceptual and mental image>ry disorders vary according to the type of visual field loss, (b) hemianopics had no more difficulties imagining spatial stimuli in their contralesional hemispace than in their ipsilesional one, and (c) the only hemianopic participant to have perceptual and mental imagery impairments suffered from attentional deficits. Results suggest that (a) visual memory is not definitively established, but rather needs perceptual practice to be maintained, and (b) that visual mental imagery may involve some of the attentional-exploratory mechanisms that are employed in visual behaviour.

[Presentation of an assessment battery for visual mental imagery and visual perception]

INTRODUCTION

The relationship between visual perception and visual mental imagery are at the center of a lively theoretical debate between those postulating common neurocognitive processes between perception and imagery and those who emphasize the differences between these two entities. Neuropsychology can make an important contribution to this debate, by assessing associations and dissociations between perceptual and imaginal deficits in patients with brain damage. However, currently there is no standardized test battery available for such assessments.

MATERIAL AND METHODS

Here we present a battery of paper-and-pencil tests assessing different domains of visual mental imagery and visual perception abilities: object form and color, animals, orthographic material, numbers, faces, and space. We also explored the effects of age, educational level and gender on performance on a group of 103 participants free of neurological damage.

RESULTS

The battery includes two parts: one composed of 14 tests assessing mental imagery and the second part composed of eight tests assessing the abilities of visual perception. We calculated the correlations between the tests, and found that, with the exception of orthographic material, there were generally poor correlations between imagery and perceptual tests.

CONCLUSION

This result seems inconsistent with hypotheses postulating a strict correspondence between perceptual and imagery abilities.

Contribution of right hemisphere to visual imagery: a visual working memory impairment?

Visual Imagery is the ability to generate mental images in the absence of perception, that is, We describe a patient, IM, who suffered from an acute ischemic stroke in the right anterior choroidal artery who appeared to demonstrate relatively isolated impairment in visual imagery. Her cognitive function, including her performance on tests of semantic function, was at ceiling, apart from a deficit in visual memory. IM failed in tasks involving degraded stimuli, object decision involving reality judgments on normal animals, and drawings from memory. By contrast, she was able to match objects seen from an unfamiliar viewpoint and to perform tasks of semantic and visual association. We hypothesize that IM has a visual working memory deficit that impairs her ability to generate full visual representations of objects given their names, individual feature, or partial representations. The deficit appears to be the result of damage to connections between the right thalamus and the right temporal lobe. Our findings may help to clarify the role of the thalamus in the cortical selective engagement processes that underlie working memory.

Can 'football-team color-code' compensate for anomia? The case study of FN, a patient with color anomia

A case study is reported on large ischemic infracts involving cortical and subcortical areas of the parietal lobes bilaterally, especially left temporo-parietal and right parietal. On examination, the diagnosis of vascular dementia with color anomia, optic aphasia for colors, was established. The patient (FN) showed great difficulty in understanding a scene as a whole and in describing complex scenes. FN's oral comprehension skills at word and sentence level were satisfactory and he exhibited communicative effectiveness during conversation. He could read letter by letter, but could not make simple judgments of shapes. FN exhibited a marked inability to name colors presented to him visually and to indicate or point to the color requested from the examiner. The most interesting of all the patient's characteristics was the strategy--a football-team color-code--he had developed for compensating for his inability to name colors.

Behavioral Deficits and Cortical Damage Loci in Cerebral Achromatopsia

Lesions to ventral occipital cortex can produce severe deficits in color vision, a syndrome known as cerebral achromatopsia. Because most studies examine relatively few cases, however, uncertainty remains about precisely which cortical loci, when damaged, produce the syndrome. In addition, the extents of the associated perceptual deficits remain unclear. To address these issues, we performed a meta-analysis of 92 case reports from the literature. The severity of color vision deficits of the cases varied greatly, although nearly all showed some deficit in color discrimination. Almost all cases tested also showed some loss of spatial vision. Lesion overlap analyses revealed a relatively small region of high overlap in ventral occipital cortex. The region of high overlap was located near areas identified by neuroimaging studies as important for color perception. For comparison, we performed a similar analysis of prosopagnosia, a disorder of face perception, and found several regions of high lesion overlap adjacent to the region associated with achromatopsia. Because the behavioral deficits in achromatopsia are often incomplete and never restricted to color vision, the region of high lesion overlap may be one critical stage within a stream of many visual areas that participate nonexclusively in color perception.

Visual imagery in cerebral visual dysfunction

Many sorts of deficits in imagery follow brain damage, but the relation between the site of damage and the type of deficit is not simple or straightforward. The dissociations in performance after brain damage provide hints regarding the processing system underlying imagery, but difficulties in interpretation urge caution in mapping these findings to theoretic models. Neuroimaging techniques, such as PET and fMRI, open a window into the working brain and offer valuable information not easily accessible through the study of patients, who, as noted, may have deficits beyond those observable and may rely on compensation and neural reorganization. As we come to understand the mental imagery system more fully, such issues as the laterality of image generation are likely to prove too coarse and vague. The brain is an enormously intricate organ, and even within a circumscribed domain such as imagery it seems to process information in complex and subtle ways.

Visually- and motor-based knowledge of letters: evidence from a pure alexic patient

We describe a patient, VSB, whose reading was impaired as a consequence of a left temporal-parietal lesion, whereas writing was relatively preserved. At variance with other pure alexic patients described in the literature, VSB claimed to have become unable to mentally visualise letters and words. Indeed, his performance on a series of tests tapping visual mental imagery for orthographic material was severely impaired. However, performance on the same tests was dramatically ameliorated by allowing VSB to trace each item with his finger. Visual mental imagery for non-orthographic items was comparatively spared. The pattern of dissociation shown by VSB between impaired visual mental imagery and relatively preserved motor-based knowledge for orthographic material lends support to the view that separate codes, respectively based on visual appearance and on motor engrams, may be used to access knowledge of the visual form of letters and words.

The relationship between visual perception and visual mental imagery: a reappraisal of the neuropsychological evidence

Visual perception and visual mental imagery, the faculty whereby we can revisualise a visual item from memory, have often been regarded as cognitive functions subserved by common mechanisms. Thus, the leading cognitive model of visual mental imagery holds that visual perception and visual imagery share a number of mental operations, and rely upon common neural structures, including early visual cortices. In particular, a single visual buffer would be used "bottom-up" to display visual percepts and "top-down" to display internally generated images. The proposed neural substrate for this buffer consists of some cortical visual areas organised retinotopically, that is, the striate and extrastriate occipital areas. Empirical support for this model came from the report of brain-damaged patients showing an imagery deficit which parallels a perceptual impairment in the same cognitive domain. However, recent reports of patients showing double dissociations between perception and imagery abilities challenged the perception-imagery equivalence hypothesis from the functional point of view. From the anatomical point of view, the available evidence suggests that occipital damage is neither necessary nor sufficient to produce imagery deficits. On the other hand, extensive left temporal damage often accompanies imagery deficits for object form or colour. Thus, visual mental imagery abilities might require the integrity of brain areas related to vision, but at an higher level of integration than previously proposed.

Anomalous perception in synaesthesia: a cognitive neuroscience perspective

An enduring question in cognitive neuroscience is how the physical properties of the world are represented in the brain to yield conscious perception. In most people, a particular physical stimulus gives rise to a unitary, unimodal perceptual experience. So, light energy leads to the sensation of seeing, whereas sound waves produce the experience of hearing. However, for individuals with the rare phenomenon of synaesthesia, specific physical stimuli consistently induce more than one perceptual experience. For example, hearing particular sounds might induce vivid experiences of colour, taste or odour, as might the sight of visual symbols, such as letters or digits. Here we review the latest findings on synaesthesia, and consider its possible genetic, neural and cognitive bases. We also propose a neurocognitive framework for understanding such anomalous perceptual experiences.

The dissociation of color from form and function knowledge

We report on two brain-damaged subjects who exhibit the uncommon pattern of loss of object color knowledge, but spared color perception and naming. The subject P.C.O., as in previously reported patients, is also impaired in processing other perceptual and functional properties of objects. I.O.C., in contrast, is the first subject on record to have impaired object color knowledge, but spared knowledge of object form, size and function. This pattern of performance is consistent with the view that semantic information about color and other perceptual properties of objects is grounded in modality-specific systems. Lesion analysis suggests that such grounding requires the integrity of the mesial temporal regions of the left hemisphere.

Covert processing of faces in prosopagnosia is restricted to facial expressions: evidence from cross-modal bias

We present a single case study of a brain-damaged patient, AD, suffering from visual face and object agnosia, with impaired visual perception and preserved mental imagery. She is severely impaired in all aspects of overt recognition of faces as well as in covert recognition of familiar faces. She shows a complete loss of processing facial expressions in recognition as well as in matching tasks. Nevertheless, when presented with a task where face and voice expressions were presented concurrently, there was a clear impact of face expressions on her ratings of the voice. The cross-modal paradigm used here and validated previously with normal subjects (de Gelder & Vroomen, 1995, 2000), appears as a useful tool in investigating spared covert face processing in a neuropsychological perspective, especially with prosopagnosic patients. These findings are discussed against the background of different models of the covert recognition of face expressions.

Computer-based training of stimulus detection improves color and simple pattern recognition in the defective field of hemianopic subjects

In a previously conducted randomized placebo-controlled trial, we were able to demonstrate significant visual field enlargement induced by restitution therapy in patients with cerebral lesions [Kasten, E., Wuest, S., Behrens-Bamann, W., & Sabel, B. A. (1998c). Computer-based training for the treatment of partial blindness. Nature Medicine, 4, 1083-1087.]. Visual field training was performed on a computer monitor for 1 hr per day over a period of 6 months. Since the procedure included only stimulation with white light, in the present study we investigated if this simple detection training had a transfer effect on color or form recognition in the trained area (i.e., in the absence of modality specific training). Answering this question would be crucial for planning optimal restitution therapy: In case there is no transfer of training effects to other visual modalities, a specific treatment of each visual function must be performed in order to achieve maximum benefit. Therefore, we analyzed the data from 32 patients with visual field defects who had participated in the original trial and whose form and color recognition had been investigated. The experimental group (n = 19, restitution training) experienced not only an increase of 12.8% correctly detected stimuli (PeriMa program, p <.05), but also an improvement of 5.6% in pattern recognition (PeriForm) and of 6.1% in color perception (PeriColor), respectively. In contrast, the placebo group (n = 13, fixation training) showed no significant changes from baseline to final outcome in any of the visual modalities (PeriMa: 0.3%; PeriForm: -0.3%; PeriColor: 0.4%). Conventional perimetry yielded an increase of 7.8% detected stimuli in the experimental group, but only of 1.2% in the placebo group (p <.05). For form recognition and color perception, the differences between the results of the experimental and the placebo groups narrowly missed significance. However, correlations of diagnostic results showed that mainly those patients who had achieved visual field enlargement also improved in color and form perception: r =.67 (p <.05) between PeriMa and PeriForm and r =.32 between PeriMa and PeriColor. We conclude that visual restitution training using a simple white light stimulus has at least some influence on improving other visual functions such as color and pattern recognition. This result supports the "bottleneck theory" of visual restitution, i.e., training effects can be explained as a process of perceptual learning and increased processing of information by residual structures surviving lesions of the primary visual pathways.

The architecture of the colour centre in the human visual brain: new results and a review

We have used the technique of functional magnetic resonance imaging (fMRI) and a variety of colour paradigms to activate the human brain regions selective for colour. We show here that the region defined previously [Lueck et al. (1989) Nature, 340, 386-389; Zeki et al. (1991) J. Neurosci., 11, 641-649; McKeefry & Zeki (1997) Brain, 120, 2229-2242] as the human colour centre consists of two subdivisions, a posterior one, which we call V4 and an anterior one, which we refer to as V4alpha, the two together being part of the V4-complex. The posterior area is retinotopically organized while the anterior is not. We discuss our new findings in the context of previous studies of the cortical colour processing system in humans and monkeys. Our new insight into the organization of the colour centre in the human brain may also account for the variability in both severity and degree of recovery from lesions producing cerebral colour blindness (achromatopsia).

Odor memory induces brain activation as measured by functional MRI

PURPOSE

Our goal was to use functional MRI (fMRI) to measure brain activation in response to imagination of odors in humans.

METHOD

fMR brain scans were obtained in 21 normal subjects (9 men, 12 women) using multislice FLASH MRI in response to imagination of odors of banana and peppermint and to the actual smells of the corresponding odors of amyl acetate and menthone, respectively, in three coronal sections selected from anterior to posterior temporal brain regions. Similar studies were obtained in two patients with hyposmia using FLASH MRI and in one patient with hyposmia using echo planar imaging, both before and after theophylline treatment, which returned smell function to or toward normal in each patient. Activation images were derived using correlation analysis, and ratios of areas of brain activated to total brain areas were calculated.

RESULTS

Activation was present in each section in all normal subjects and in each patient after imagination of each vapor. In normal subjects, brain activation in response to imagination of odors was significantly less than that in response to the actual smell of these odors, and activation following imagination of banana odor was significantly greater in men than in women, as was previously reported for the actual smell of the odor of amyl acetate. However, in relative terms, albeit at an absolute lower brain activation level, the ratio of brain activation by imagination of banana to activation by actual amyl acetate odor was about twice as high in women as in men. Before treatment, in patients with hyposmia, brain activation in response to odor imagination was greater than after presentation of the actual odor itself. After treatment, in patients with hyposmia in whom smell acuity returned to or toward normal, brain activation in response to odor imagination was not significantly different quantitatively from that before treatment; however, brain activation in response to the actual odor was significantly greater than that in response to imagination of the corresponding odor. Brain regions activated by both odor imagination and actual corresponding odor were similar and consistent with regions previously described as responding to odors.

CONCLUSION

These studies indicate that (a) odors can be imagined and similar brain regions are activated by both imagined and corresponding actual odors; (b) imagination of odors elicits quantitatively less brain activation than do actual smells of corresponding odors in normal subjects; (c) absolute brain activation in men by odor imagination is greater than in women for some odors, but on a relative basis, the ratio for odor imagination to actual smell in women is twice that in men; (d) odor imagination, once the odor has been experienced, is present, recallable, and capable of inducing a relatively constant degree of brain activation even in the absence of the ability to recognize an actual corresponding odor.

Inversion superiority in visual agnosia may be common to a variety of orientation polarised objects besides faces

Selective impairment in recognition of faces (prosopagnosia) resulting from certain localized cortical lesions has been advanced as an argument for a face specific brain module. The argument is claimed to be strengthened by the discovery of an inversion superiority effect in the recognition of faces by a prosopagnosic patient (Farah et al., Vis Res 1995b;35:2089-2093). The present paper reports an inversion superiority effect in the recognition of faces and shoes in a visual agnosic patient. The finding raises the possibility that several classes of orientationally polarized objects, of which shoes and faces are examples, will exhibit inversion superiority.

Multiple-domain dissociation between impaired visual perception and preserved mental imagery in a patient with bilateral extrastriate lesions

A brain-damaged patient is described whose pattern of performance provides insight into both the functional mechanisms and the neural structures involved in visual mental imagery. The patient became severely agnosic, alexic, achromatopsic and prosopagnosic following bilateral brain lesions in the temporo-occipital cortex. However, her mental imagery for the same visual entities that she could not perceive was perfectly preserved. This clear-cut dissociation held across all the major domains of high-level vision: object recognition, reading, colour and face processing. Our findings, together with other reports on domain-specific dissociations and functional brain imaging studies, provide evidence to support the view that visual perception and visual mental imagery are subserved by independent functional mechanisms, which do not share the same cortical implementation. In particular, our results suggest that mental imagery abilities need not be mediated by early visual cortices.