The Search for the Phonological Store: From Loop to Convolution

The phonological loop system of Baddeley and colleagues' Working Memory model is a major accomplishment of the modern era of cognitive psychology. It was one of the first information processing models to make an explicit attempt to accommodate both traditional behavioral data and the results of neuropsychological case studies in an integrated theoretical framework. In the early and middle 1990s, the purview of the phonological loop was expanded to include the emerging field of functional brain imaging. The modular and componential structure of the phonological loop seemed to disclose a structure that might well be transcribed, intact, onto the convolutions of the brain. It was the phonological store component, however, with its simple and modular quality, that most appealed to the neuroimaging field as the psychological “box” that might most plausibly be located in the brain. Functional neuroimaging studies initially designated regions in the parietal cortex as constituting the “neural correlate” of the phonological store, whereas later studies pointed to regions in the posterior temporal cortex. In this review, however, we argue the phonological store as a theoretical construct does not precisely correspond to a single, functionally discrete, brain region. Rather, converging evidence from neurology, cognitive psychology, and functional neuroimaging argue for a reconceptualization of phonological short-term memory as emerging from the integrated action of the neural processes that underlie the perception and production of speech.

Verbal Memory Impairments in Children after Cerebellar Tumor Resection

This study was designed to investigate cerebellar lobular contributions to specific cognitive deficits observed after cerebellar tumor resection. Verbal working memory (VWM) tasks were administered to children following surgical resection of cerebellar pilocytic astrocytomas and age-matched controls. Anatomical MRI scans were used to quantify the extent of cerebellar lobular damage from each patient's resection. Patients exhibited significantly reduced digit span for auditory but not visual stimuli, relative to controls, and damage to left hemispheral lobule VIII was significantly correlated with this deficit. Patients also showed reduced effects of articulatory suppression and this was correlated with damage to the vermis and hemispheral lobule IV/V bilaterally. Phonological similarity and recency effects did not differ overall between patients and controls, but outlier patients with abnormal phonological similarity effects to either auditory or visual stimuli were found to have damage to hemispheral lobule VIII/VIIB on the left and right, respectively. We postulate that damage to left hemispheral lobule VIII may interfere with encoding of auditory stimuli into the phonological store. These data corroborate neuroimaging studies showing focal cerebellar activation during VWM paradigms, and thereby allow us to predict with greater accuracy which specific neurocognitive processes will be affected by a cerebellar tumor resection.

Anatomy of the Episodic Buffer: A Voxel-Based Morphometry Study in Patients with Dementia

In 2000 Baddeley proposed the existence of a new component of working memory, the episodic buffer, which should contribute to the on-line maintenance of integrated memory traces. The author assumed that this component should be critical for immediate recall of a short story that exceeds the capacity of the phonological store. Accordingly, patients with Alzheimer’s dementia (AD) should suffer of a deficit of the episodic buffer when immediate recall of a short story is impossible. On the other hand, the episodic buffer should be somewhat preserved in such patients when some IR can occur (Baddeley and Wilson, 2002). We adopted this logic for a voxel-based morphometry study. We compared the distribution of grey-matter density of two such groups of AD patients with a group of age-matched controls. We found that both AD groups had a significant atrophy of the left mid-hippocampus; on the other hand, the anterior part of the hippocampus was significantly more atrophic in patients who were also impaired on the immediate prose recall task. Six out of ten patients with no immediate recall were spared at “central executive” tasks. Taken together our findings suggest that the left anterior hippocampus contributes to the episodic buffer of the revised working memory model. We also suggest that the episodic buffer is somewhat independent from the central executive component of working memory.

Semantic encoding of spoken sentences: Adult aging and the preservation of conceptual short-term memory

Traditional models of human memory have postulated the need for a brief phonological or verbatim representation of verbal input as a necessary gateway to a higher level conceptual representation of the input. Potter has argued that meaningful sentences may be encoded directly in a conceptual short-term memory (CSTM) running parallel in time to such a phonological store. The primary aim of the current study was to evaluate two main tenets of the CSTM model: that linguistic context biases selection of information entering the conceptual store, and that information not integrated into a coherent structure is rapidly forgotten. Results confirmed these predictions for spoken sentences heard by both young and older adults, supporting the generality of the model and suggesting that CSTM remains stable in normal aging.

Enhancement of Phonological Memory Following Transcranial Magnetic Stimulation (TMS)

Phonologically similar items (mell, rell, gell) are more difficult to remember than dissimilar items (shen, floy, stap), likely because of mutual interference of the items in the phonological store. Low-frequency transcranial magnetic stimulation (TMS), guided by functional magnetic resonance imaging (fMRI) was used to disrupt this phonological confusion by stimulation of the left inferior parietal (LIP) lobule. Subjects received TMS or placebo stimulation while remembering sets of phonologically similar or dissimilar pseudo-words. Consistent with behavioral performance of patients with neurological damage, memory for phonologically similar, but not dissimilar, items was enhanced following TMS relative to placebo stimulation. Stimulation of a control region of the brain did not produce any changes in memory performance. These results provide new insights into how the brain processes verbal information by establishing the necessity of the inferior parietal region for optimal phonological storage. A mechanism is proposed for how TMS reduces phonological confusion and leads to facilitation of phonological memory.

Phonological sequence learning and short-term store capacity determine second language vocabulary acquisition

Two studies examined individual cognitive differences affecting the acquisition of second language word forms. Experiment 1 measured 40 undergraduates' ability to learn phonological sequences, their phonological short-term store capacity as indexed by ability to repeat nonwordlike nonwords, and their learning of novel foreign language vocabulary (German) in an experimental task. Phonological sequence learning predicted receptive vocabulary learning. Phonological sequence learning and phonological store capacity made independent additive contributions to productive vocabulary learning. Experiment 2 determined the interactions of phonological sequence learning ability, phonological store capacity, and second language acquisition during a longitudinal field study of 44 novice undergraduate learners of Spanish during a 10-week course. Students' initial skill in phonological sequence learning predicted their final levels of Spanish receptive language and their eventual ability to repeat Spanish-wordlike nonwords. The results suggest that phonological store capacity and phonological sequence learning ability are initially separable constraints on second language vocabulary acquisition and that sequence learning ability underpins the acquisition of long-term phonological knowledge. Subsequent apprehension and consolidation of a novel word form is a product not only of phonological short-term store capacity but also of this long-term knowledge of the phonological regularities of language.