Bradford Z. Mahon

WHAT ARE THE FACTS OF SEMANTIC CATEGORY-SPECIFIC DEFICITS? A CRITICAL REVIEW OF THE CLINICAL EVIDENCE

In this study we provide a critical review of the clinical evidence available to date in the field of semantic category-specific deficits. The motivation for undertaking this review is that not all the data reported in the literature are useful for adjudicating among extant theories. This project is an attempt to answer two basic questions: (1) what are the categories of category-specific deficits, and (2) is there an interaction between impairment for a type of knowledge (e.g., visual, functional, etc.) and impairment for a given category of objects (e.g., biological, artefacts, etc.). Of the 79 case studies in which the reported data are sufficiently informative with respect to the aims of our study, 61 presented a disproportionate impairment for biological categories and 18 presented a disproportionate impairment for artefacts. Less than half of the reported cases provide statistically and theoretically interpretable data. Each case is commented upon individually. The facts that emerge from our critical review are that (1) the categories of category-specific semantic deficits are animate objects, inanimate biological objects, and artefacts (the domain of biological objects fractionates into two independent semantic categories: animals, and fruit/vegetables); (2) the types of category-specific deficits are not associated with specific types of conceptual knowledge deficits. Other conclusions that emerge from our review are that the evidence in favour of the existence of cases of reliable category-specific agnosia or anomia is not very strong, and that the visual structural description system functions relatively autonomously from conceptual knowledge about object form.

Concepts and Categories: A Cognitive Neuropsychological Perspective

One of the most provocative and exciting issues in cognitive science is how neural specificity for semantic categories of common objects arises in the functional architecture of the brain. More than two decades of research on the neuropsychological phenomenon of category-specific semantic deficits has generated detailed claims about the organization and representation of conceptual knowledge. More recently, researchers have sought to test hypotheses developed on the basis of neuropsychological evidence with functional imaging. From those two fields, the empirical generalization emerges that object domain and sensory modality jointly constrain the organization of knowledge in the brain. At the same time, research within the embodied cognition framework has highlighted the need to articulate how information is communicated between the sensory and motor systems, and processes that represent and generalize abstract information. Those developments point toward a new approach for understanding category specificity in terms of the coordinated influences of diverse regions and cognitive systems.

Category-Specific Organization in the Human Brain Does Not Require Visual Experience

Distinct regions within the ventral visual pathway show neural specialization for nonliving and living stimuli (e.g., tools, houses versus animals, faces). The causes of these category preferences are widely debated. Using functional magnetic resonance imaging, we find that the same regions of the ventral stream that show category preferences for nonliving stimuli and animals in sighted adults show the same category preferences in adults who are blind since birth. Both blind and sighted participants had larger blood oxygen-level dependent (BOLD) responses in the medial fusiform gyrus for nonliving stimuli compared to animal stimuli and differential BOLD responses in lateral occipital cortex for animal stimuli compared to nonliving stimuli. These findings demonstrate that the medial-to-lateral bias by conceptual domain in the ventral visual pathway does not require visual experience in order to develop and suggest the operation of innately determined domain-specific constraints on the organization of object knowledge.

What drives the organization of object knowledge in the brain

Various forms of category-specificity have been described at both the cognitive and neural levels, inviting the inference that different semantic domains are processed by distinct, dedicated mechanisms. In this paper, we argue for an extension of a domain-specific interpretation to these phenomena that is based on network-level analyses of functional coupling among brain regions. On this view, domain-specificity in one region of the brain emerges because of innate connectivity with a network of regions that also process information about that domain. Recent findings are reviewed that converge with this framework, and a new direction is outlined for understanding the neural principles that shape the organization of conceptual knowledge.

What is the role of motor simulation in action and object recognition? Evidence from apraxia

An important issue in contemporary cognitive neuroscience concerns the role of motor production processes in perceptual and conceptual analysis. To address this issue, we studied the performance of a large group of unilateral stroke patients across a range of tasks using the same set of common manipulable objects. All patients (n = 37) were tested for their ability to demonstrate the use of the objects, recognize the objects, recognize the corresponding object-associated pantomimes, and imitate those same pantomimes. At the group level we observed reliable correlations between object use and pantomime recognition, object use and object recognition, and pantomime imitation and pantomime recognition. At the single-case level, we document that the ability to recognize actions and objects dissociates from the ability to use those same objects. These data are problematic for the hypothesis that motor processes are constitutively involved in the recognition of actions and objects and frame new questions about the inferences that are merited by recent findings in cognitive neuroscience.

The orchestration of the sensory-motor systems: Clues from Neuropsychology.

Research over the last several decades has led to clear and empirically tractable proposals about the representation of conceptual knowledge in the brain. Here we argue that there are already sufficient data from neuropsychology to strongly constrain extant hypotheses about the representation of conceptual knowledge. One constraint imposed by these neuropsychological data is that recognition of actions and understanding of objects do not necessarily depend on the ability to produce object-associated actions. This conclusion compels a reconsideration of the role played by motor planning and/or execution processes in action and object recognition and understanding.

Unconscious processing dissociates along categorical lines

Visual object recognition is subserved by ventral temporal and occipital regions of the brain. Regions comprising the dorsal visual pathway have not been considered relevant for object recognition, despite strong categorical biases for tool-related information in those regions. Here, we show that dorsal stream processes influence object categorization. We used two techniques to render prime pictures invisible: continuous flash suppression (CFS), which obliterates input into ventral temporal regions, but leaves dorsal stream processes largely unaffected, and backward masking (BM), which allows suppressed information to reach both ventral and dorsal stream structures. Categorically congruent primes suppressed under CFS facilitate categorization of tools but have no effect on nonmanipulable objects; in contrast, primes rendered invisible through BM facilitate target categorization for both tools and nonmanipulable things. Our findings demonstrate that information computed by the dorsal stream is used in object categorization, but only for a category of manipulable objects.

The Role of the Dorsal Visual Processing Stream in Tool Identification

The dorsal visual processing stream subserves object-directed action, whereas the ventral visual processing stream subserves visual object recognition. Little is known about how information computed by dorsal-stream structures influences object recognition. We used continuous flash suppression to functionally separate information computed by the dorsal stream from that computed by the ventral stream. We show that information originating from the dorsal stream influences not only decisions requiring the selection of superordinate category labels, but also decisions that entail the selection of a basic-level object. We further show that information computed by the dorsal stream does not carry specific functional information about objects. Our results indicate that the dorsal stream, in isolation from the ventral stream, is agnostic as to the identity of the objects that it processes. We suggest that structures within the dorsal visual processing stream compute motor-relevant information (e.g., graspability), which influences the identification of manipulable objects, and is not either about the function of the object or function-specific.

The organisation of conceptual knowledge in the brain: The future's past and some future directions

We review the development and current status of theories of the organisation and representation of conceptual knowledge in the human brain. The currently known facts from optic aphasia, category-specific semantic deficits, and functional neuroimaging are consistent with a framework in which the first-order constraint on the organisation of conceptual knowledge is domain. Data from functional neuroimaging suggests additionally a framework characterised by both domain- and modality-specific constraints. Work in congenital disorders and in apraxia indicate that the content of conceptual knowledge is not exhausted by modality-specific input/output processes. It is concluded that future empirical and theoretical work on the organisation and representation of conceptual knowledge will profit from a reorientation of the problem from the organisation of distinct processing systems to the content of information represented internal to such systems.

The cumulative semantic cost does not reflect lexical selection by competition

The cumulative semantic cost describes a phenomenon in which picture naming latencies increase monotonically with each additional within-category item that is named in a sequence of pictures. Here we test whether the cumulative semantic cost requires the assumption of lexical selection by competition. In Experiment 1 participants named a sequence of pictures, while in Experiment 2 participants named words instead of pictures, preceded by a gender marked determiner. We replicate the basic cumulative semantic cost with pictures (Exp. 1) and show that there is no cumulative semantic cost for word targets (Exp. 2). This pattern was replicated in Experiment 3 in which pictures and words were named along with their gender marked definite determiner, and were intermingled within the same experimental design. In addition, Experiment 3 showed that while picture naming induces a cumulative semantic cost for subsequently named words, word naming does not induce a cumulative semantic cost for subsequently named pictures. These findings suggest that the cumulative semantic cost arises prior to lexical selection and that the effect arises due to incremental changes to the connection weights between semantic and lexical representations.