Abstract
There is considerable debate about the neuroanatomic localization of semantic memory, the knowledge of culturally shared elements such as objects, concepts, and people. Two recent meta-analyses of functional imaging studies (fMRI and PET) sought to identify cortical regions involved in semantic processing. Binder and colleagues (Binder et al., 2009) identified several regions of interest, widely distributed throughout the frontal, parietal, and temporal cortices. In contrast, Lambon Ralph and colleagues (2010) focused on the anterior temporal lobe, and found that when the potential for signal loss is accounted for (due, for example, to distortion artifact or field of view restriction), significant regional activation is detected. We set out to determine whether the anterior temporal lobe plays a significant role in picture naming, a task which relies on semantic memory. We examined a relatively large sample of patients with early Alzheimer's disease (N=145), a multifocal disease process typically characterized in the early stages by problems with episodic memory and executive function. Hypothesis-driven analyses based on regions of interest derived from the meta-analyses as well as exploratory analyses across the entire cerebral cortex demonstrated a highly specific correlation between cortical thinning of the left anterior temporal lobe and impaired naming performance. These findings lend further support to theories that include a prominent role for the anterior temporal lobe in tasks that rely on semantic memory.
Highlights
Semantic memory is a form of declarative memory, distinct from working and episodic memory, that supports factual knowledge about elements of the world
The primary goal of Alzheimer’s Disease Neuroimaging Initiative (ADNI) has been to test whether serial magnetic resonance imaging (MRI), positron emission tomography (PET), other biological markers, and clinical and neuropsychological assessment can be combined to measure the progression of mild cognitive impairment (MCI) and early Alzheimer’s disease (AD)
AD is a widely distributed multifocal neurodegenerative disease process, and is not typically characterized by prominent early impairment of semantic memory, we found here that cued naming impairment in mild AD is correlated with cortical thinning of the left anterior temporal lobes (ATL)
Summary
Semantic memory is a form of declarative memory, distinct from working and episodic memory, that supports factual knowledge about elements of the world. An alternative view postulates that there is a hub localized in the anterior temporal lobes (ATL) that serves a central integrating function for information processed in the distributed set of sensory, motor, and linguistic zones (Patterson et al, 2007) Each of these views is supported by evidence from both functional neuroimaging studies of healthy subjects and investigations of patients with brain lesions (Chertkow et al, 2008; Lambon Ralph et al, 2010a; Tranel, 2009). Binder and colleagues analyzed 120 functional magnetic resonance imaging (fMRI) and positron emission tomography (PET) studies carefully chosen to include only those with tasks and contrasts that differentiate semantic processing from other linguistic or cognitive processes (Binder et al, 2009) These authors identified seven regions that were consistently and engaged during semantic processing: posterior inferior parietal lobule (angular gyrus), middle temporal gyrus, fusiform and parahippocampal gyri, dorsomedial prefrontal cortex, inferior frontal gyrus, ventromedial prefrontal cortex, and posterior cingulate gyrus. Their study highlighted the imperative for a careful analysis of ATL activation in functional neuroimaging experiments: if this region is not included in the field of view, or distortion artifact is not accounted for, the activation is missed
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