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Category-Specific Semantic Deficit
Category-specific semantic deficits are impairments that unequally affect some aspects of conceptual knowledge over others. The most commonly reported deficit in brain damaged individuals who have difficulty distinguishing between living and non-living things was the source of interest into category specific knowledge deficits. Animals and plants all relate to the category of living things whereas things like tools and furniture are non-living objects. The living and non-living distinction impairment has thus been widely studied as a result of the numerous reports of this impairment. The most common dysfunction found in those with category-specific deficits is a larger impairment in classifying living things rather than non-living things. However, a proportion (though smaller) does report the opposite with a greater impairment with non-living things instead of living things. Reports dating from 100 years ago of difficulty naming objects demonstrate that this category-specific deficit is not a new phenomenon. Further, deficits relating to the living and non-living distinction is not the only type of deficit that may exist. Documentation of deficits show that impairments occur regarding abstract and concrete words, body parts and the naming of colours. This variation in cases with an individual's impairment makes generating a complete definition complex for neuroscientists. Advances in technologies have allowed cognitive scientists to observe the places of impairment within the brain and to generate greater understanding of how the brain is organized.

Specific Semantic Impairment Disorders
Specific-semantic deficits occur in many patients and differ on all kinds of variations which makes it difficult to generate a complete understanding of its effect on those individuals with the deficit. The existence of semantic dementia helps to highlight aspects of category-specific semantic deficits. Individuals with semantic dementia show a decline in semantic memory that gets worse over time. Further, this decline is selective. Individuals with Alzheimer's dementia show a greater impairment with living things versus non-living things as the severity of the disorder worsens. However, the majority of those with semantic dementia do not show a difference between living and non-living things. This semantic disorder also demonstrates the existence of an on-going relationship between episodic memory and semantic memory. Understanding semantic memory allows further insight to the complexities of category-specific deficits.

Organization of Semantic Knowledge
There are numerous accounts of how the brain is organized. In the past, phrenology was used in order to obtain information about the human brain. Advances in technologies allowed neuroscientists to better understand the complexities of the human brain and the locations of various functioning processes. Specifically, semantic knowledge organization has received considerable attention as a highly organized and complex system within the brain. Researchers disagree as to how semantic information is represented in the brain. However, the majority of researchers are in agreement with regards to the fact that all concepts arise from features. It should be noted that currently there is no theory recognized that completely accounts for all category-specific-semantic deficits. This further demonstrates the complexity of category-specific semantic deficits.

Modality-Specific Hypothesis
The assumption made by the modality-specific hypothesis is that semantic processing is organized into modality-specific semantic subsystems. Thus, each aspect of semantic information has a specific area that controls that type of functioning. For example, an area of the brain is specifically associated with each of visual, motor, and auditory processes only.

Sensory/Functional theory
The theory of sensory/functional thinking has been greatly studied as a form of the modality-specific hypothesis. This theory was originally created by Warrington and colleagues. As mentioned previously, semantic information is organized in specific modules that only process that type of semantic information. In this particular theory, two systems are involved with processing semantic information. The first is visual and the second is functional. The visual system is activated when processing living things because in order to recognize a living thing it is most important to be able to access visual information about the object. Visual information includes information relating to the size, colour and shape of an object. The functional system involves the use of the objects. This system is activated more for non-living things because humans need to be able to know how to use objects in their environment. For example, functional information includes the interaction of sitting on a chair or lifting a chair. In this theory, it is therefore possible to have damage to visual information and have functional information intact. If a living thing impairment exists, it should still be possible to maintain visual information of a non-living thing and thus have it accessible.

The main problem associated with this theory is that it fails to account for the phenomenon of greater deficits occurring for living things than non-living things. As well, in reported case studies, the theory is unable to provide a complete explanation for visual impairment relating to non-living things. Findings are inconsistent as to what relationship exists as some findings show no impairment in visual knowledge and some show moderate impairment.

Domain-Specific Hypothesis
The importance of evolution is outlined within the domain-specific hypothesis. Semantic categories exist because of their importance for evolutionary survival of human beings. Domains such as 'animals', 'fruit/vegetables', 'conspecifics', and possbily 'tools' are the main neural domains. It should be noted that the original proposal did not include the category of 'conspecifics' but this was later added. Knowledge of animals and fruits/vegetables are important because of their survival value for the species. This survival value is reflected in the fact that human fitness depended on being able to quickly and efficiently spot predators (animals) and sources of food(fruits/vegetables). Thus, neural connections within the brain are stronger for those categories that aided in human survival. This hypothesis also suggests that there is no specific module that corresponds to a conceptual category. Therefore, unlike the sensory functional hypothesis where there was a corresponding system for specific knowledge, the domain-specific hypothesis does not have such a system.

Damage to one of these domains results in complete loss of that knowledge. However, a complication with this hypothesis is that this has not been observed in all patients with category-specific semantic deficit. For example, it has been reported that individuals may have a deficit that is not related to being unable to recognize all living things as a whole. This theory does account for the fact that the ability to process visual information may be unable to recognize a visual object (visual agnosia). Further, processes relating to object recognition have domain specific constraints. Further support for this is documented in those who have impairments in both visual and functional information but are unable to recognize objects as living things versus non-living.

The Organized-Unitary-Content Hypothesis (OUCH)
OUCH theory suggests that conceptual features that are related to one another for recognizing objects will be stored beside each other in semantic memory. When damage occurs in a semantic area the result will be a category-specific semantic deficit. One possible reason for this deficit to occur is that because related semantic information is stored in close proximity, this results in damage to those areas as well. Also, because some areas are highly correlated to one another, these areas will experience greater damage.

The main problem with this account of category-specific-semantic deficit is that it does not provide specific explanations for the deficit. For this reason, an extension of the OUCH was developed to provide a better explanation of category specific semantic deficits.

Conceptual-Structure Approach
This conceptual-structure approach (developed by Tyler, Moss and collaborators) explains the cause of how this deficit occurs without using a modality-specific explanation. Under this approach, living-things are thought to have more features in common than features relating to non-living things. More specifically, there are more features that set non-living things apart than living things. An increased advance in the development of various tools has led to finer variations in types of functions that each object does. For example, distinguishing functionality between a marker, a pen and a crayon needs to be very specific. Therefore, living things share information that is highly correlated with shared perceptual properties. Perceptual properties consist of things related to the senses (sight, taste, sound, touch and smell). Non-living things, however, are highly correlated with functional information that have distinctive perceptual properties. The more correlated the features are, the more resistant they are to damage since there are other routes to access information necessary for object recognition. These other routes make recognition more likely to occur due to the fact that pieces of features can be combined. When damage increases in severity, larger proportions of intercorrelated features are lost. Therefore, a larger impairment occurs for living-things because of the greater correlations that exist between features. Dementia of Alzheimer's type (DAT) represents support for this theory as individuals show a larger impairment with living instead of non-living things.

A shortcoming of this theory of a conceptual-structure is the finding of a patient that has a larger impairment for non-living things versus living things. As noted previously, this is a more rare case, but its existence creates a problem for using the conceptual structured approach theory. Living-things should have more highly correlated features and therefore be more affected by damage.

Functional Imaging Studies
Neural connections as depicted with fMRI allow insight into what part of the brain is activated during different processes. More specifically, the strength of the neural connections informs researchers about the degree of activation. A common finding is activation in the brain occurs in the same region but occurs within different knowledge categories. .Further, these imaging studies allow researchers to be more certain of the location, as well as the degree of these impairments in the brain. This is important for gaining information about perceptual abilities and role played in object recognition.

An experiment using fMRI individuals with a category-specific semantic deficit was used where the individuals were asked simple yes or no questions regarding the nature of various objects (living and non living things). The researchers were concerned with whether there would be activation in the left ventral occipital-temporal lobe because of its association for the presentation of visual objects. Not only was this area activated for the visual questions of non-living things but it was activated as well for questions involving visual and functional questions for living things. This demonstrates that it is possible for areas of activation commonality to occur for specific perceptual modules (in this case, visual).

Current/Future Research
Individuals with category-specific semantic deficits are tested with various measures. They may be assessed by naming things in coloured pictures or line drawings. A picture naming task may include pointing to the image that represents a certain object or giving a definition of the object shown.

Research has focused on the living and non-living distinction in order to understand what areas of impairments are occurring in category-specific semantic deficit. However, the current trend in neuroscience research in this area is to narrow these two broad distinctions with more specific distinctions to classify objects. For example, future research questions recognizing the need for more specific classifications look towards the possibility of non-living things being broken down into smaller types of specific deficits. What further makes research in the area of semantic deficit complicated is the issue of finding well-controlled sets of stimuli for use in research experiments. In order to avoid confounding variables, a larger set of images of familiarity matching living and non-living things has been created.