Experiment 1 presents data from a patient with a semantic short-term memory deficit, patient ML, that indicate profound susceptibility to interference. For example, although this patient cannot reliably recall three items in a serial recall task, he paradoxically shows exaggerated effects of proactive interference in short-term memory. However, this patient's difficulty with interference appears to be limited to tasks involving verbal stimuli---other data show that patient ML performs normally on two nonverbal tasks that require resolution of interference.
Experiment 2 attempted to identify ERP components related to proactive interference in one of the tasks administered in Experiment 1. This task, the recent negatives task, is a convenient measure of susceptibility to proactive interference. Moreover, Experiment 2 added an additional manipulation motivated by a unique effect discovered during the testing of patient ML (Hamilton, 2004), whereby the patient performed much better on the recent negatives task when repetition was minimized and the number of stimuli presented within the task were expanded. Two ERP components, a frontally distributed N400 effect and parietally distributed late positive component (LPC), were found to respond to the manipulation of recency and repetition.
Experiment 3 and Experiment 4 examined a language comprehension paradigm known to differentiate between patients with semantic and phonological short-term memory deficits. This task requires detection of semantic anomalies in phrases in which multiple adjectives appear before or after a noun---multiple adjectives appearing before a noun are believed to place greater demands on semantic short-term memory relative to when adjectives appear after a noun. Thus, patients with semantic short-term memory deficits are especially poor at detecting anomalies in the before condition. Experiment 3 uses the parietally distributed N400 as an indirect measure of short-term memory demands to corroborate behavioral and patient data. Relative to the "after" condition, the "before" condition did elicit smaller N400s, consistent with the idea that integration of adjectives in the before condition differs from integration in the after condition.
Experiment 4 uses a modification of the sentence anomaly task employed in Experiment 3 to identify brain areas engaged in short-term maintenance of semantic representations. It was hypothesized that the before condition would produce greater activation in the left inferior frontal gyrus, a region that has been related to short-term maintenance of semantic representations. Results from Experiment 4 are discussed in terms of the organization of maintenance and control processes important in semantic short-term memory.
Finally, data from Experiments 1--4 are discussed in terms of their implications for theories of semantic short-term memory deficits and the associations of semantic short-term memory deficits with particular deficits of language comprehension and production.