No abstract

The dominant view in the field of lexical access in speech production maintains that selection of a word becomes more difficult as the levels of activation of nontarget words increase--selection by competition. The authors tested this prediction in two sets of experiments. First, the authors show that participants are faster to name pictures of objects (e.g., "bed") in the context of semantically related verb distractors (e.g., sleep) compared with unrelated verb distractors (e.g., shoot). In the second set of experiments, the authors show that target naming latencies (e.g., "horse") are, if anything, faster for within--category semantically close distractor words (e.g., zebra) than for within--category semantically far distractor words (e.g., whale). In the context of previous research, these data ground a new empirical generalization: As distractor words become semantically closer to the target concepts--all else being equal--target naming is facilitated. This fact means that lexical selection does not involve competition, and consequently, that the semantic interference effect does not reflect a lexical level process. This conclusion has important implications for models of lexical access and interpretations of Stroop-like interference effects.

The principles driving the organization of the ventral object-processing stream remain unknown. Here, we show that stimulus-specific repetition suppression (RS) in one region of the ventral stream is biased according to motor-relevant properties of objects. Quantitative analysis confirmed that this result was not confounded with similarity in visual shape. A similar pattern of biases in RS according to motor-relevant properties of objects was observed in dorsal stream regions in the left hemisphere. These findings suggest that neural specificity for "tools" in the ventral stream is driven by similarity metrics computed over motor-relevant information represented in dorsal structures. Support for this view is provided by converging results from functional connectivity analyses of the fMRI data and a separate neuropsychological study. More generally, these data suggest that a basic organizing principle giving rise to "category specificity" in the ventral stream may involve similarity metrics computed over information represented elsewhere in the brain.

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.