Frontiers in Psychology (Apr 2014)
The neural bases of orthographic working memory
Abstract
Introduction: There is a fundamental distinction in memory systems between long-term memory and working memory. This distinction is prominent in virtually every cognitive domain including written (and spoken) word production, where the long-term storage and retrieval of lexical information is distinguished from working memory processes deployed during production. These latter processes are responsible for maintaining the activation level of the representations retrieved from long-term memory, serial selection of segments, inhibition of produced segments, etc. In the orthographic domain, evidence regarding the processes and representations involved in orthographic working memory (OWM) has come primarily from investigations of acquired dysgraphia subsequent to brain injury (Buchwald & Rapp, 2009; Caramazza & Miceli, 1990; Costa, et al, 2011). However, the neural bases of OWM (also referred to as the graphemic buffer) has not been extensively examined. In this research we present findings from the first evaluation of brain lesion overlap in a set of individuals with well-documented OWM impairments. Methods: The lesions of 9 right-handed individuals with left-hemisphere strokes and well-documented OWM deficits were examined. All individuals had deficits affecting OWM and not orthographic lexical or sublexical processes. There were 6 males, 6 native English speakers, and 3 native Italian speakers. Neuroimaging modalities included 3 CT scans and 6 T1-weighted MRI scans. MRIcron was used to map each lesion. Normalization was carried out in SPM8 with non-linear normalization parameters derived from the intact tissue, not the lesion volume (Brett, Leff, Rorden, & Ashburner, 2001). Normalized lesion maps were overlaid on a template brain, and regions of high density lesion overlap were defined as regions with 4 or more overlapping lesions. Results: The analysis (see Figure 1) revealed three left hemisphere regions of high density lesion overlap (geometric centers in MNI coordinates): (1) along the intraparietal sulcus (IPS), with primary concentration in the anterior region (-36, -49, 39); (2) the inferior frontal junction (IFJ;-44, -11, 38) and (3) the insula (-43, 5, 0). Eight of the nine individuals had lesions affecting the IPS region, while both the IFJ and the insula regions were affected by 4 of the lesions; there was one individual whose lesion only impacted the insula region. General Discussion: First, these results reveal a neurotopography of OWM lesion sites that is well-aligned with results from neuroimaging of orthographic working memory in neurally intact participants (Rapp & Dufor, 2011). Second, the dorsal neurotopography of the OWM lesion overlap is clearly distinct from what has been reported for lesions associated with either lexical or sublexical deficits (e.g., Henry, Beeson, Stark, & Rapcsak, 2007; Rapcsak & Beeson, 2004); these have, respectively, been identified with the inferior occipital/temporal and superior temporal/inferior parietal regions. These neurotopographic distinctions support the claims of the computational distinctiveness of long-term vs. working memory operations. The specific lesion loci raise a number of questions to be discussed regarding: (a) the selectivity of these regions and associated deficits to orthographic working memory vs. working memory more generally (b) the possibility that different lesion sub-regions may correspond to different components of the OWM system.
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