The magical number 4 in short-term memory: a reconsideration of mental storage capacity

Miller (1956) summarized evidence that people can remember about seven chunks in short-term memory (STM) tasks. However, that number was meant more as a rough estimate and a rhetorical device than as a real capacity limit. Others have since suggested that there is a more precise capacity limit, but that it is only three to five chunks. The present target article brings together a wide variety of data on capacity limits suggesting that the smaller capacity limit is real. Capacity limits will be useful in analyses of information processing only if the boundary conditions for observing them can be carefully described. Four basic conditions in which chunks can be identified and capacity limits can accordingly be observed are: (1) when information overload limits chunks to individual stimulus items, (2) when other steps are taken specifically to block the recording of stimulus items into larger chunks, (3) in performance discontinuities caused by the capacity limit, and (4) in various indirect effects of the capacity limit. Under these conditions, rehearsal and long-term memory cannot be used to combine stimulus items into chunks of an unknown size; nor can storage mechanisms that are not capacity-limited, such as sensory memory, allow the capacity-limited storage mechanism to be refilled during recall. A single, central capacity limit averaging about four chunks is implicated along with other, noncapacity-limited sources. The pure STM capacity limit expressed in chunks is distinguished from compound STM limits obtained when the number of separately held chunks is unclear. Reasons why pure capacity estimates fall within a narrow range are discussed and a capacity limit for the focus of attention is proposed.     

Phonological short-term memory contributions to sentence processing in young children

Two experiments investigated the contribution of phonological short-term memory to the processing of spoken sentences by 4- and 5-year-old children. In Experiment 1, sentences contained either short or longer words, and varied in syntactic structure. Overall, repetition but not comprehension of the sentences was significantly influenced by word length. In Experiment 2. children selected on the basis of their high phonological short-term memory ability were founded to be superior at repeating sentences to children of lower phonological short-term memory ability, although the two groups did not differ in their comprehension accuracy for the same sentences. In both experiments, comprehension and repetition performance were differently influenced by particular sentence structures. It is proposed that sentence repetition in children is constrained by phonological memory capacity, and is therefore directly influenced by memory-related factors that include the length and number of words in sentences, and individual differences in memory skills.     

Semantic short-term memory and its role in sentence processing: a replication

Previous studies have shown that an aphasic patient (AB) with a semantic short-term memory deficit (STM) had difficulties comprehending and producing sentences with structures that demanded the simultaneous retention of several individual word meanings (Martin & Freedman, 2001a, 2001b; Martin & Romani, 1994; Martin, Shelton, & Yaffee, 1994). The present study provides a replication of these findings with an additional case (ML) who shows a striking dissociation between preserved semantic knowledge and disrupted semantic STM. ML performed poorly on comprehension tasks for sentences that required the retention of single word meanings across several intervening words. In contrast, he did not show an effect of intervening words on processing grammatical relations. ML had difficulty producing adjective-noun phrases, though able to produce the individual nouns and adjectives. These findings support the contention that there is a semantic retention capacity, involved in both comprehension and production, that is separate from semantic knowledge representations.     

Interference with short-term visual memory produced by concurrent central processing

Double-letter memory and test stimuli were used in two experiments on a speek comparison task. Faster decision times were found when memory and test stimuli were physically identical than when they were the same in name only. This finding was true even with retention intervals as long as 12 sec and even when difficult tasks filled the retention intervals. However, the decision-time advantage of physically identical comparisons was greatest when the interval was not filled with a task likely to interfere with rehearsals. High verbal subjects had a smaller advantage for the physically identical comparisons than did low verbal subjects but were affected in the same way as low verbal subjects in terms of which conditions raised the overall correct comparison times, raised the error rates, and reduced the advantage of physically identical comparisons.     

Interference in short-term auditory memory

Although interference is a well-established forgetting function in short-term auditory memory, an adequate understanding of its underlying mechanisms and time course has yet to be attained. The present study therefore aimed to explore these issues in memory for timbre. Listeners compared standard and comparison complex tones, having distinct timbres (four components varying in frequency), over a 4.7-s retention interval and made a same–different response. This interval either was silent or included one of 15 distractor tones occurring 0 ms, 100 ms, or 1,200 ms after the standard. These distractors varied in the extent to which the frequencies of their component tones were shared with the standard. Performance in comparing the two tones was significantly impaired by distractors composed of novel frequencies, regardless of the temporal position at which the distractor occurred. These results were fully compatible with the recent timbre memory model (McKeown & Wellsted, 2009 McKeown, D. and Wellsted, D. 2009. Auditory memory for timbre. Journal of Experimental Psychology: Human Perception and Performance, 35: 855–875. [Crossref], [PubMed], [Web of Science ®] , [Google Scholar]) and suggested that interference in auditory memory operates via a feature-overwriting mechanism.     

Interference effects in short-term memory

Abstract.— Two experiments exploring the modality relation between memory task and interference task are reported. With visual and verbal tasks, variations in modality relation did not influence the total amount recalled. The usual serial position effect was observed when memory and interference task were of different modalities. The amount of recall was found to be a function of the memory strategies used by the subjects, 'active' strategies producing highest recall.     

Interference from short-term memory processing on encoding and reproducing brief durations

A temporal reproduction task is composed of two temporal estimation phases: encoding of the interval to be reproduced, followed by its reproduction. The effect of short-term memory processing on each of these phases was tested in two experiments. In Exp. 1, a memory set was presented, followed by two successive tones bounding the target interval to be reproduced. During the reproduction of the target interval, a probe was presented, and the subject ended the reproduction by pressing one of two keys, depending on the presence or absence of the probe in the memory set. In Exp. 2, probe recognition was required during the encoding of the interval to be reproduced. Whereas in Exp. 1 reproductions lengthened as a function of memory-set size, in Exp. 2 temporal reproductions decreased with set size. These results support attentional models of time estimation and suggest that short-term memory processing interrupts concurrent accumulation of temporal information. Received: 11 September 1997 / Accepted: 2 March 1998     

Short-term memory and sentence processing: Evidence from neuropsychology

Traditional models of memory assume that short-term memory, as measured by memory span, plays an important role in linguistic processing and the learning ofverbal information. Contradicting this view are findings from a brain-damaged patient, E.A., who, despite a verbal memory span of about two items, demonstrated normal sentence comprehension in a variety oftasks. She was, however, impaired whenever verbatim phonological information had to be maintained or learned. These results and those from other patients with reduced span suggest that the phonological storage capacity that is critical to memory span plays only a limited role in language processing, specifically in the maintenance and learning of phonological forms. Implications for models of short-term memory are discussed. It is argued that short-termmemory should be seen as deriving from the processing and retentive capacities of language processing modules, with span tasks drawing on only a subset of these modules.     

Two modality effects in verbal short-term memory: Evidence from sentence recall

This paper investigates the mechanisms underlying the standard modality effect (i.e., better recall performance for auditorily presented than for visually presented materials), and the modality congruency effect (i.e., better memory performance if the mode of recall and presentation are congruent rather than incongruent). We tested the assumption that the standard modality effect is restricted to the most recent word(s) of the sentences but occurs in both verbatim and gist recall (Experiments 1 and 2), whereas the modality congruency effect should be evident for the rest of the sentence when using verbatim recall (Experiment 3) but not when using gist recall (Experiment 4). All experiments used the Potter-Lombardi intrusion paradigm. When the target word was the most recent word of the sentence, a standard modality effect was found with both verbatim recall and gist recall. When the target word was included in the middle of the sentences, a modality congruency effect was found with verbatim recall but not with gist recall.     

The spatial or temporal organization of short-term memory

Children were shown three visually displayed digits. The digits were exposed successively in three windows in such a way, that the left-to-right order never corresponded with the temporal-sequential order. When asked to recall or recognize the digits, normal children responded in terms of temporal order. Deaf, autistic and some subnormal children, recalled or recognized the spatial, i.e. the left-to-right order. The relationship between hearing and/or speech and the temporal ordering of visual displays is discussed.     

Long-term semantic memory versus contextual memory in unconscious number processing

Subjects classified visible 2-digit numbers as larger or smaller than 55. Target numbers were preceded by masked 2-digit primes that were either congruent (same relation to 55) or incongruent. Experiments 1 and 2 showed prime congruency effects for stimuli never included in the set of classified visible targets, indicating subliminal priming based on long-term semantic memory. Experiments 2 and 3 went further to demonstrate paradoxical unconscious priming effects resulting from task context. For example, after repeated practice classifying 73 as larger than 55, the novel masked prime 37 paradoxically facilitated the "larger" response. In these experiments task context could induce subjects to unconsciously process only the leftmost masked prime digit, only the rightmost digit, or both independently. Across 3 experiments, subliminal priming was governed by both task context and long-term semantic memory.     

Binding or prioritization: The role of selective attention in visual short-term memory

Whether selective attention binds features in visual short-term memory or prioritizes selection for memory consolidation and decision was investigated with a change detection paradigm. Two types of change were manipulated: Feature or conjunctions of features. Previous work suggests that the allocation of attentional resources affects binding; hence attentional shifts during retention should affect the detection of conjunction changes more than feature changes. The results of Experiments 1 and 2 showed that attention shifts had a similar impact on detecting feature and conjunction changes. Experiment 3 showed a performance benefit with a post-cue occurring 200 or 550 ms after stimulus offset, but no improvement was found when prioritization occurred with a delay of 800 ms. The results of Experiment 4 suggested that signals from both feature changes and conjunction changes contribute to detection. The theoretical implications are discussed.     

Stimulus control of information processing in pigeon short-term memory

Six pigeons were trained initially on a delayed successive matching-to-sample task using red and green fields as sample and test stimuli. Following acquisition, each sample was followed either by a vertical line (“remember” cue), which indicated that sample memory would be tested, or by a horizontal line (“forget” cue), which indicated that sample memory would not be tested. During the experiments, sample memory on forget trials was tested occasionally. A series of five experiments revealed: (a) better retention on remember trials than on forget trials, (b) increased effectiveness of a forget cue when it followed closely sample offset, (c) more rapid forgetting over a retention interval ranging from 3 to 6 sec on forget trials than on remember trials, (d) a “cancellation” effect in which a remember cue which followed immediately the offset of a forget cue attenuated markedly the effectiveness of the forget cue, and (e) an “insulation” effect in which the effectiveness of a forget cue was reduced considerably when presented after a remember cue. It was concluded that pigeons actively process or rehearse the sample memory during the retention interval.     

Age, working memory, and on-line syntactic processing in sentence comprehension

One hundred twenty-seven individuals who ranged in age from 18 to 90 years were tested on a reading span test and on measures of on-line and off-line sentence processing efficiency. Older participants had reduced working-memory spans compared with younger participants. The on-line measures were sensitive to local increases in processing load, and the off-line measures were sensitive to the syntactic complexity of the sentences. Older and younger participants showed similar effects of syntactic complexity on the on-line measures. There was some evidence that older participants were more affected than younger participants by syntactic complexity on the off-line measures. The results support the hypothesis that on-line processes involved in recognizing linguistic forms and determining the literal, preferred, discourse-coherent meaning of sentences constitute a domain of language processing that relies on its own processing resource or working-memory system.     

The effects of word co-occurrence on short-term memory: associative links in long-term memory affect short-term memory performance

In immediate serial recall tasks, high-frequency words are recalled better than low-frequency words. This has been attributed to high-frequency words' being better represented and providing more effective support to a redintegration process at retrieval (C. Hulme et al., 1997). In studies of free recall, there is evidence that frequency of word co-occurrence, rather than word frequency per se, may explain the recall advantage enjoyed by high-frequency words (J. Deese, 1960). The authors present evidence that preexposing pairs of low-frequency words, so as to create associative links between them, has substantial beneficial effects on immediate serial recall performance. These benefits, which are not attributable to simple familiarization with the words per se, do not occur for high-frequency words. These findings indicate that associative links between items in long-term memory have important effects on short-term memory performance and suggest that the effects of word frequency in short-term memory tasks are related to differences in interitem associations in long-term memory.     

Interference in implicit memory caused by processing of interpolated material

This study addresses the susceptibility of implicit memory to interference. Interference is manipulated by presenting interpolated lists of words that do or do not have word stems in common with previously studied target words (e.g., target word paragraph followed by interpolated words such as paradise or vicinity). Interference in a word stem completion task occurred only when words had similar word stems (Experiment 1). Increasing the number of interpolated words with corresponding word stems (e.g., not only paradise but also parking, pardon, and parliament) produced increasing amounts of interference (Experiment 2). Interference in implicit memory appears to be a simple response competition phenomenon that occurs when cues simultaneously activate primed targets and primed competing responses. The amount of interference can be explained by a quantitative model of the relative strengths of target and competing responses.