Humphreys et al 1995 model

Humphreys, Lamote, and Lloyd-Jones (1995) produced an interactive activation and competition model of object recognition and naming, which has also been applied to visual agnosia. The model contains pools of units of four kinds (see Figure 4.10):

1. Stored structural descriptions of objects.

2. Semantic representations.

3. Name representations.

4. Superordinate units or category labels.

Activation from the structural units proceeds initially to semantic units before proceeding to name representations. There are bi-directional excitatory connections between related units at adjacent levels of the model. In addition, there are mutually inhibitory connections between units within each level. According to the model, the structural descriptions of objects visually similar to the object actually presented are activated to some extent. Of particular importance, it is assumed that living things are typically more visually similar to other members of the same category than is the case with non-living things.

Evidence

According to the model, living things should generally be named more slowly than non-living things, but should be categorised more rapidly. Why is this so? Living things are more visually similar to each other than are non-living things. This causes more activation of irrelevant structural representations and name representations, which inhibits naming living things and slows performance. In contrast, the additional activation of irrelevant representations from the same category as the presented object for living objects increases activation of the appropriate category label and so speeds up categorisation. Both predictions were confirmed in simulations of the model, and correspond to findings on people (Humphreys et al., 1995).

Humphreys, Riddoch, and Quinlan (1988) found that objects with common names were named faster than objects with less common names, and that this frequency effect was greater for non-living things than for living things. Humphreys et al. (1995) found that their model produced the same pattern of findings. According to the model, the activation from semantic representations to name representations is greater for objects with more common names, and this produces the overall frequency effect. The greater activation of irrelevant structural and name representations when living objects are presented reduces this advantage.

Associative agnosics typically show worse identification of living things than of non-living things, but are reasonably good at categorising objects (e.g., Sheridan & Humphreys, 1993). When the model was "lesioned" in various places, this reduced its ability to name objects and especially living objects. The greater effect on living objects occurred because the presentation of a living object tends to activate the structural representations of various visually similar objects, and this makes naming more difficult.

Patients with category-specific anomia have selective impairment in the ability to name certain categories of objects (typically living objects), in spite of being able to access much semantic information about objects (e.g., Farah & Wallace, 1992). Humphreys et al. (1995) tried to mimic the effects of category-specific anomia by "lesioning" connections between the semantic and name representations in their model. They found that the model showed worse naming performance for living things than for non-living things, in line with the evidence from patients. The model is an interactive one, with the consequence that the greater activation of irrelevant structural representations when living objects are seen has knock-on effects that influence naming.

Evaluation

The interactive activation and competition model of Humphreys et al. (1995) provides accounts of object recognition in both normal individuals and in those with various visual disorders. This is an advance on the model of Farah and McClelland (1991), which was designed only to simulate performance in patients suffering

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