Neural Correlates of Semantic Memory: from Neuropsychology to Neuroimaging

This thesis will describe two functional Magnetic Resonance Imaging (fMRI) experiments and one Voxel-Based Morphometry (VBM) study, each investigating how the human brain identifies objects and their associated properties. In particular, we used three different categories of objects – living (animals), nonliving (tools and nontools) and faces (famous and non-famous) – to examine the type of knowledge attribute in question: one perceptual (movement) and two semantic attributes (typical object location and biographic knowledge). We know from neuropsychological literature that the most anterior portions of the temporal cortices critically support human conceptual knowledge. Unfortunately, the Anterior Temporal Lobe (ATL) is a challenging region for fMRI due to susceptibility artifacts, especially at high fields. For these reasons we established an optimized fMRI protocol (described in the second Chapter) by adjusting key acquisition parameters like phase-encoding gradient polarity, slice thickness, echo time, and slice angle. The protocol gave reliable Blood-Oxygen-Level Dependence (BOLD) signal sensitivity in the ATL. Clinical data describe patients with specific semantic impairments at the level of category (living, nonliving) as well as disproportionate deficits for a modality or type of knowledge (e.g., visual/perceptual knowledge or manipulation knowledge). Functional neuroimaging studies on semantic organization with normal subjects found an “action network†specific for tools rather than living items. In the first experiment (Chapter 3) we devised an fMRI paradigm to investigate the processing of movement (action) and place (encyclopedic) features, and their influence on category-specific activations. Within the “movement network†statistical analyses did not show any significant interaction between categories. These findings suggest that the visuomotor “action network†is not specific for tools because it is also activated when the action related knowledge is elicited for other categories, such as animals. The second and the third experiment (Chapter 4) focus on the processing of faces. Neuropsychological literature attributes semantic and lexical retrieval deficits in patients to ATL lesions. In Part I of Chapter 4, we report data from a VBM study on patients with known lesions in the temporal lobe. Unfortunately, as far as we know, data on patients and functional neuroimaging in healthy individuals has not clarified the differential role of this area in the two mental operations because semantic and lexical processes usually occur simultaneously and automatically. In Part II, we devised an event-related fMRI activation paradigm that allowed us to study the identification (i.e., association of semantic biographical information) of celebrities, with and without the ability to retrieve the proper name. While semantic retrieval reliably activated the ATL, only more posterior areas in the left temporal and temporal-parietal junction were significantly modulated by covert lexical retrieval. These results support findings from patients with ATL lesions and suggest that their anomia is due to semantic rather than lexical retrieval impairment.