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The ability to recognize an object facilitates an individual’s capacity to describe its appearance, uses, and functions, allowing effective interaction with the environment (Eysenck & Keane, 2015). In this paper, object recognition refers to the ability to determine whether a line drawing represents something that is real. Recognizing a familiar object requires visual features to be processed and match knowledge stored in long-term memory (Tree & Playfoot, 2015). However, the knowledge required for object recognition has been debated in the literature with the ‘Localist’ and ‘Connectionist’ perspectives emerging as two major theories. Traditional models of object recognition proposed structural descriptions of an object’s subcomponents and semantic descriptions, that incorporate meaning, and are stored in long-term memory (Hillis & Caramazza, 1995). The main distinction between the competing theories is that ‘Localists’ state object recognition requires a structural description to match a visual featural representation existing in memory (Coltheart, 2004), whilst ‘Connectionists’ argue retrieval of semantic resources is essential (Rogers, Lambon Ralph, Hodges & Patterson, 2003).
The proposition of separate structural and semantic systems was derived from the performance of agnosia patients (Lissauer, 1890) in the Object Decision task (Riddoch & Humphreys, 1993), which involves deciding whether a picture represents a real object or chimera – recombination of parts from real objects. Successful performance requires matching visual features to structural descriptions, without retrieving semantic knowledge. Apperceptive agnosia is the dysfunction of perceptual processing which produces structural descriptions, making it difficult to discriminate between shapes and processes for meaning. Contrastingly, associative agnosia consists of impaired associative processing and the formation of semantic descriptions, making it difficult to relate objects to meaning. Several studies observed associative agnosia patients could perform the Object Decision task accurately (Carlesiomo, Casadio, Sabbadini & Caltagrirone, 1998; Hillis & Caramazza, 1995). This challenges the ‘Connectionist’ perspective, implying that only structural descriptions are required. However, Rogers et al. (2003) argued object recognition was not preserved, but the result of poorly constructed tasks in which chimeras were obviously impossible. Therefore, patients completed the task using general knowledge, without retrieving semantic resources.
Neuropsychological research has questioned the independence of the structural and semantic systems, involving Semantic Dementia (SD) causes – progressive neurodegeneration of the semantic system (Tree & Playfoot, 2015). Several studies observed a correlation between SD severity and object decision performance (Patterson, Lambon Ralph, Jefferies & Woollams, 2006; Rogers et al. 2003), inferring semantic deficits are responsible for poor object decision performance. However, ‘Connectionists’ advocate damage to the semantic system will disrupt the processing of information from any sensory modality and developed the ‘Hub and Spoke’ model, using a computerized simulation, to demonstrate how semantic representations are the result of mapping between visual representations and verbal descriptions (Rogers, Lambon Ralph, Garrad, Bozeat, McClelland, Hodges & Patterson, 2004a). This model assumes damage to the semantic system will reduce semantic task performance and atypical objects are more dependent on the semantic system. In contrast, some SD cases performed the Object Decision task accurately (Hovius, Kellenbach, Graham, Hodges & Patterson, 2003), implying semantic information is not essential for object recognition and challenging the ‘Connectionist’ approach.
Rogers, Lambon Ralph, Hodges, and Patterson (2004b) explained this discrepancy by the strength of connections relating to item familiarity and typicality. To certify this, Rogers et al. (2003;2004b) developed a two-alternative forced-choice decision task – real object and chimera presented simultaneously. Supporting the connectionist approach, SD patients’ performance was significantly impaired in low familiarity atypical trials which required the greatest semantic input. Conversely, Tree and Playfoot (2015) observed no predictable relationship between semantics and object recognition accuracy, supporting the ‘Localist’ perspective that only structural descriptions are necessary. Therefore, there is a vast range of contrasting evidence for both theories.
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