Associative agnosia and optic aphasia: qualitative or quantitative difference?

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.

Language and visual perception associations: meta-analytic connectivity modeling of Brodmann area 37

BACKGROUND

Understanding the functions of different brain areas has represented a major endeavor of neurosciences. Historically, brain functions have been associated with specific cortical brain areas; however, modern neuroimaging developments suggest cognitive functions are associated to networks rather than to areas.

OBJECTIVE

The purpose of this paper was to analyze the connectivity of Brodmann area (BA) 37 (posterior, inferior, and temporal/fusiform gyrus) in relation to (1) language and (2) visual processing.

METHODS

Two meta-analyses were initially conducted (first level analysis). The first one was intended to assess the language network in which BA37 is involved. The second one was intended to assess the visual perception network. A third meta-analysis (second level analysis) was then performed to assess contrasts and convergence between the two cognitive domains (language and visual perception). The DataBase of Brainmap was used.

RESULTS

Our results support the role of BA37 in language but by means of a distinct network from the network that supports its second most important function: visual perception.

CONCLUSION

It was concluded that left BA37 is a common node of two distinct networks-visual recognition (perception) and semantic language functions.

Humans and macaques employ similar face-processing strategies

Primates developed the ability to recognize and individuate their conspecifics by the face. Despite numerous electrophysiological studies in monkeys, little is known about the face-processing strategies that monkeys employ. In contrast, face perception in humans has been the subject of many studies providing evidence for specific face processing that evolves with perceptual expertise. Importantly, humans process faces holistically, here defined as the processing of faces as wholes, rather than as collections of independent features (part-based processing). The question remains to what extent humans and monkeys share these face-processing mechanisms. By using the same experimental design and stimuli for both monkey and human behavioral experiments, we show that face processing is influenced by the species affiliation of the observed face stimulus (human versus macaque face). Furthermore, stimulus manipulations that selectively reduced holistic and part-based information systematically altered eye-scanning patterns for human and macaque observers similarly. These results demonstrate the similar nature of face perception in humans and monkeys and pin down effects of expert face-processing versus novice face-processing strategies. These findings therefore directly contribute to one of the central discussions in the behavioral and neurosciences about how faces are perceived in primates.

Neural regions essential for distinct cognitive processes underlying picture naming

We hypothesized that distinct cognitive processes underlying oral and written picture naming depend on intact function of different, but overlapping, regions of the left hemisphere cortex, such that the distribution of tissue dysfunction in various areas can predict the component of the naming process that is disrupted. To test this hypothesis, we evaluated 116 individuals within 24 h of acute ischaemic stroke using a battery of oral and written naming and other lexical tests, and with magnetic resonance diffusion and perfusion imaging to identify the areas of tissue dysfunction. Discriminant function analysis, using the degree of hypoperfusion in various Brodmann's areas--BA 22 (including Wernicke's area), BA 44 (part of Broca's area), BA 45 (part of Broca's area), BA 21 (inferior temporal cortex), BA 37 (posterior, inferior temporal/fusiform gyrus), BA 38 (anterior temporal cortex) and BA 39 (angular gyrus)--as discriminant variables, classified patients on the basis of the primary component of the naming process that was impaired (defined as visual, semantics, modality-independent lexical access, phonological word form, orthographic word form and motor speech by the pattern of performance and types of errors across lexical tasks). Additionally, linear regression analysis demonstrated that the areas contributing the most information to the identification of patients with particular levels of impairment in the naming process were largely consistent with evidence for the roles of these regions from functional imaging. This study provides evidence that the level of impairment in the naming process reflects the distribution of tissue dysfunction in particular regions of the left anterior, inferior and posterior middle/superior temporal cortex, posterior inferior frontal and inferior parietal cortex. While occipital cortex is also critical for picture naming, it is likely that bilateral occipital damage is necessary to disrupt visual recognition. These findings provide new evidence that a network of brain regions supports naming, but separate components of this network are differentially required for distinct cognitive processes or representations underlying the complex task of naming pictures.

Optic aphasia: a case study

Optic aphasia is a rare syndrome in which patients are unable to name visually presented objects but have no difficulty in naming those objects on tactile or verbal presentation. We report a 79-year-old man who exhibited anomic aphasia after a left posterior cerebral artery territory infarction. His naming ability was intact on tactile and verbal semantic presentation. The results of the systematic assessment of visual processing of objects and letters indicated that he had optic aphasia with mixed features of visual associative agnosia. Interestingly, although he had difficulty reading Hanja (an ideogram), he could point to Hanja letters on verbal description of their meaning, suggesting that the processes of recognizing objects and Hanja share a common mechanism.

A metanalysis of impaired and spared naming for different categories of knowledge in patients with a visuo-verbal disconnection

The study of the neuroanatomical correlates of category-specific semantic disorders has strongly supported the 'sensory/motor model of semantic knowledge,' which assumes that the cortical areas that have critically contributed to the development of various categories are also implicated in their semantic representation. However, if the anatomo-clinical correlates are consistent with the model, less clearcut results have been obtained by functional neuroimaging experiments. In the present metanalysis, I addressed the question from a different viewpoint, shifting attention from the anatomical lesion in patients with category-specific semantic disorders to the pattern of naming impairment shown by patients suffering from a disconnection between visual areas and lexical output mechanisms. According to the model, living entities should be particularly impaired, since their semantic representations are mainly based upon visual perceptual attributes. On the contrary, actions and body parts (and to a lesser extent artefacts) should be relatively spared, as their semantic representations are mainly based upon motor, somato-sensory or functional attributes. These predictions were checked by reviewing the categorical pattern of naming impairment shown by patients with a visuo-verbal disconnection and a category-specific naming impairment published in the last 20 years. The pattern of impaired and spared categories observed in these patients was consistent with the hypothesis, since: (1) 'actions' and 'body parts' were systematically spared in comparison to all the other categories; (2) 'artefacts' were relatively spared with respect to the 'living categories'; and (3) within the biological categories, 'plants' were usually more impaired than animals.

The influence of misnaming on object recognition: a case of multimodal agnosia

We present a case of multimodal agnosia in the visual and tactile modality due to an infarction in the territory of the left posterior cerebral artery. The patient's ability to recognize objects fluctuated depending on his verbal activity. When he misnamed presented objects, he tended to use them and to draw them in keeping with the wrong name. We submit that the mechanism causing associative agnosia is more dynamic than it was hitherto considered. It originates from the rivalry between top-down central regulation and bottom-up peripheral flow.

The more you know the less you can tell: inhibitory effects of visuo-semantic activation on modality specific visual misnaming

WH, a 77-years old right-handed psychoanalyst, displayed modality specific visual misnaming as a sequel of an embolic stroke in the left posterior cerebral artery. WH's errors in visual object naming consisted mainly of semantic paraphasias and perseverations. His verbalizations during testing sometimes manifested a conflict between correct responses and perseverations. Analysis of the stream of information from visual perception via semantics to phonology suggested incomplete access from vision to semantics as the source of errors. The disconnection did not affect verbo-visual matching, though he was unable to reject names that did not correspond to visual stimuli. Action naming was relatively spared, but naming of pictures of actions with objects was worse than naming of pictures of intransitive actions. Tactile naming worsened with simultaneous vision of objects. In visual object naming the error rate increased with increasing familiarity of objects. We propose that an interaction of excitation and inhibition within a single semantic system can explain the clinical phenomena of modality specific visual misnaming.

Associative visual agnosia resulting from a disconnection between intact visual memory and semantic systems

We report the case of a patient (RC) who developed a severe visual agnosia, associated to alexia without agraphia, color anomia and amnesia, following an ischemic stroke in the territory supplied by the left posterior cerebral artery. Based on his proficient performance on tests evaluating analysis of elementary visual features, formation of viewer-centered and object-centered representations of visual stimuli and discrimination between drawings representing real and unreal objects, we concluded that the critical locus of deficit was a disconnection between the normally functioning visual memory store and the semantic system. RC's disturbance in visual processing of human faces paralleled his recognition disorder of other classes of objects. The possible contribution of neurobiological factors in determining RC's agnosic deficit is discussed.

Optic aphasia with pure alexia: a mild form of visual associative agnosia? A case study

A single-case study is reported of a naming disorder selective to the visual modality. The patient showed intact access to structural knowledge of objects and letters, but impaired access to complete semantic knowledge of objects and alphabetical knowledge of letters from visual input. The impairment was most striking when the patient had to discriminate between semantically similar objects or within a given symbolic repertoire, i.e. letters. The co-occurrence of a partial deficit of visual recognition for objects and for letters indicated features of optic aphasia and pure alexia. This symmetric performance between object and letter processing may also constitute a mild form of visual associative agnosia.