Components of Short-Term Memory and Their Relation to Language Processing Evidence From Neuropsychology and Neuroimaging

Verbal working memory consists of separable capacities for the retention of phonological and semantic information. Within the phonological domain, there are independent capacities for retaining input-phonological codes and output-phonological codes. The input-phonological capacity does not appear to be critical for language comprehension but is involved in verbatim repetition and long-term learning of new words. The semantic capacity is critical for both comprehension and production and for the learning of new semantic information. Different neural structures appear to underlie these capacities, with a leftparietal region involved in input-phonological retention and a left-frontal region involved in semantic retention. KEYWORDS—short-term memory; sentence comprehension; language production; verbal learning; neuropsychology Most cognitive psychologists take for granted the notion that language comprehension draws on working memory (the system that stores information temporarily and allocates attention to the processing of this information). Traditionally, memory span (or the number of words that can be recalled in order) has been used as a measure of the storage-capacity component of working memory, often referred to as short-term memory (STM) capacity. Neurally intact individuals have a span of about five words. Thus, one might expect that a brain-damaged patient or a child with a developmental disorder who had a memory span of only one to two words would, as a result, have problems with understanding language. Many studies from our lab have shown, however, that whether or not there is a connection between reduced span and difficulties with language processing depends on the specific source of the STM deficit and the specific type of language task (Freedman & Martin, 2001; Hanten & Martin, 2000; Martin & He, 2004). We have developed a multiplecomponent model of verbal STM to accommodate these findings (Martin, in press). This multiple-component approach was first applied to understanding the relation between STM and sentence comprehension and repetition, but has been expanded more recently to encompass the relation between STM and language production and verbal learning. We have also begun to investigate more closely the neural architecture that underlies these different capacities through lesion studies and functional neuroimaging. PHONOLOGICALVERSUS SEMANTIC STORAGE COMPONENTS A large body of findings indicates that individuals use a phonological (i.e., speech-sound) code to maintain information in short-term memory. For example, memory span is smaller for lists of phonologically similar words (e.g., rhyming words) than for phonologically dissimilar words and smaller for lists of multisyllable words than for single-syllable words. These effects hold irrespective of whether the lists are presented auditorily or visually. These findings have led to models of verbal STM in which words are stored in a phonological form and rehearsed subvocally. A number of individuals with brain damage or developmental disorders whose memory-span performance implicates a disruption of phonological storage have been reported (Hanten & Martin, 2000; Martin, in press; Vallar & Papagno, 1995). That is, despite showing accurate perception and production of speech sounds, these patients fail to show the standard phonological effects on span, and have particular difficulty repeating lists of nonwords (e.g., ‘‘dember,’’ ‘‘plim,’’ etc.)—an Address correspondence to: Randi C. Martin, Psychology, MS-25, Rice University, P.O. Box 1892, Houston, TX 77251; e-mail: [email protected]. CURRENT DIRECTIONS IN PSYCHOLOGICAL SCIENCE 204 Volume 14—Number 4 Copyright r 2005 American Psychological Society ability that depends mainly on retaining phonological information. For example, patient EA had a word span of one to two items, but could only repeat a nonword if it had three phonemes or fewer. However, not all patients with reduced span show this pattern. Some show standard phonological effects but fail to show normal retention of semantic information, despite having excellent knowledge of word meanings. For example, these patients fail to show the normal advantage for words over nonwords on span tasks—suggesting that they cannot make use of the lexical and semantic information in words to aid their recall (Martin & He, 2004). These differing STM patterns indicate that the retention of phonological and semantic information depends on separate capacities, which may be differentially affected by brain damage. A further differentiation of STM capacities appears to be required within the phonological domain. We and other researchers have argued that there is one capacity for the retention of ‘‘input’’ phonological codes—those derived from speech perception—and another for the retention of ‘‘output’’ phonological codes—those generated internally that serve as the basis for speech production (Martin, Lesch, & Bartha, 1999). Supporting this contention are patients who perform well on STM tasks when they do not have to reproduce the list (as on probe tasks), but perform poorly on all list-repetition tasks. A case study that we reported (Martin et al., 1999) showed this pattern. Patient MS is a classic anomic patient in that he is a fluent speaker but has difficulty producing names for objects and producing nouns in spontaneous speech, though he shows good comprehension of the words he cannot produce. In picture naming, he often produces circumlocutions—that is, he accurately describes the object to be named, but cannot produce its name. On list-memory tasks that did not involve reproducing the list (e.g., deciding whether two lists were the same or different), he performed at a normal level. However, when asked to reproduce lists, he performed poorly and often produced circumlocutions like those he produced in naming. Thus, MS demonstrated preserved input-phonological capacity (supporting his excellent performance on probe and matching tasks), but poor access to output phonology, which impaired his performance in overt