Current voxel-based lesion-symptom mapping techniques cannot answer the above questions because they: (i) search for the most significant effects across all lesions rather than identifying a set of regions that are each sufficient to cause a deficit, (ii) may miss lesion sites where the effect of damage is inconsistent across patients 22 and (iii) do not identify lesion sites that are not associated with persistent VSTM impairments. Moreover, we need new lesion identification methods. Answering these questions requires large samples of patients who do, and do not, have VSTM impairments. Taken together, it is clear that VSTM impairments can be caused by several different lesion sites but it is unclear: (i) how consistently damage to each of the identified regions impairs VSTM (ii) whether there are other, as yet unidentified sites, where damage can also impair VSTM and (iii) which lesion sites are seldom associated with persistent VSTM or cause only transient VSTM impairments. 16 Other patient studies have associated VSTM with the left posterior superior temporal gyrus, 8, 13, 17–20 the left inferior frontal gyrus 14, 16, 19 and the bilateral fronto-polar cortex. 3–5, 8–11 This conclusion was further endorsed by group studies of patients, 12–15 and a study of direct cortical stimulation. Early case studies of patients with post-stroke aphasia highlighted the importance of the left inferior parietal lobe (BA40). Previous lesion studies have associated VSTM impairments with damage to several different brain regions. Such information is essential for predicting which patients will recover from early VSTM impairments and which patients may need sustained therapeutic intervention. The goal of our study was to investigate which lesion sites do, and do not, cause persistent VSTM impairments. 3–6 Importantly, preliminary work suggests that VSTM impairments are amenable to treatment 7 which may also facilitate language comprehension. Many studies suggest that a deficient VSTM can lead to difficulties with language comprehension. It affects the ability to immediately recall long sequences of auditory verbal material such as lists of words or digit strings. 1, 2 Impaired VSTM is common in patients with aphasia. It requires minimal attentional and other executive resources. It is limited in its capacity and in the time it can hold information. Verbal short-term memory (VSTM) is the passive storage of verbal information. These findings have clinical implications for predicting recovery of verbal short-term memory after stroke. Sparing of these regions very rarely results in persistently poor verbal short-term memory. These data suggest that verbal short-term memory impairments are most consistently associated with damage to left temporo-parietal and basal ganglia structures. However, when the same regions were spared, only 5% (23/455) presented with digit span impairments. Even complete damage to one or more of these five regions was not consistently associated with persistent digit span impairment. Almost all (199/222 = 90%) had left hemisphere damage to five small regions in basal ganglia and/or temporo-parietal areas. Digit span impairments were observed in 222/816 patients. Third, we examined how often these lesion sites were observed in patients who either had no digit span impairments or digit span impairments that co-occurred with difficulties in speech perception and/or production tasks. Second, we studied the lesion sites in a subgroup of these patients ( n = 39) with left hemisphere damage and selective digit span impairment-defined as impaired digit span with unimpaired spoken picture naming and spoken word comprehension (tests of speech production and speech perception, respectively). First, we identified the incidence of digit span impairments in a sample of 816 stroke survivors (541 males/275 females age at stroke onset 56 ± 13 years time post-stroke 4.4 ± 5.2 years). Here we asked: How many different lesion sites can account for selective impairments in verbal short-term memory that persist over time, and how consistently do these lesion sites impair verbal short-term memory? We assessed verbal short-term memory impairments using a forward digit span task from the Comprehensive Aphasia Test. Prior studies have reported inconsistency in the lesion sites associated with verbal short-term memory impairments.
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