List-strength Effect Research Articles

Retrieving effectively from source memory: Evidence for differentiation and local matching processes

The ability to distinguish between different explanations of human memory abilities continues to be the subject of many ongoing theoretical debates. These debates attempt to account for a growing corpus of empirical phenomena in item-memory judgments, which include the list strength effect, the strength-based mirror effect, and output interference. One of the main theoretical contenders is the Retrieving Effectively from Memory (REM) model. We show that REM, in its current form, has difficulties in accounting for source-memory judgments – a situation that calls for its revision. We propose an extended REM model that assumes a local-matching process for source judgments alongside source differentiation. We report a first evaluation of this model’s predictions using three experiments in which we manipulated the relative source-memory strength of different lists of items. Analogous to item-memory judgments, we observed a null list strength effect and a strength-based mirror effect in the case of source memory. In a second evaluation, which relied on a novel experiment alongside two previously published datasets, we evaluated the model’s predictions regarding the manifestation of output interference in item and lack of it in source memory judgments. Our results showed output interference severely affecting the accuracy of item-memory judgments but having a null or negligible impact when it comes to source-memory judgments. Altogether, these results support REM’s core notion of differentiation (for both item and source information) as well as the concept of local matching proposed by the present extension.

Sparse attentional subsetting of item features and list-composition effects on recognition memory

Although knowledge is extremely high-dimensional, human episodic memory performance appears extremely low-dimensional, focused largely on stimulus-features that distinguish list items from one another. A cognitively plausible way this tension could be addressed is if selective attention selects a small number of features from each item. I consider an ongoing debate about whether stronger items (better encoded) interfere more than weaker items (less well encoded) with probe items during old/new episodic recognition judgements. This is called the list-strength effect, concerning whether or not effects of encoding strength are larger in lists of mixed strengths than in pure lists of a single strength. Analytic derivations with Anderson’s (1970) matched filter model show how storing only a small subset of features within high-dimensional representations, and assuming those same subsets tend to reiterate themselves item-wise at test, can support high recognition performance. In the sparse regime, the model produces a list-strength effect that is small in magnitude, resembling previous findings of so-called null list-strength effects. When the attended feature space is compact, such as for phonological features, attentional subsetting cannot be sparse. This introduces non-negligible cross-talk from other list items, producing a large-magnitude list-strength effect, similar to what is observed for the production effect (better recognition when reading aloud). This continuum-based account implies the existence of a continuous range of magnitudes of list-composition effects, including occasional inverted list-strength effects. This lays the foundation for propagating effects of task-relevant attention to sparse subsets of features through a broad range of models of memory behaviour.

Testing a Strategy-Disruption Account of the List-Strength Effect.

Presenting items multiple times on a study list increases their memorability, a process known as item strengthening. The list-strength effect (LSE) refers to the finding that, compared to unstrengthened (pure) lists, lists for which a subset of the items have been strengthened produce enhanced memory for the strengthened items and depressed memory for the unstrengthened items. Although the LSE is found in free recall (Tulving & Hastie, 1972), it does not occur in recognition (Ratcliff et al., 1990). In free recall, the LSE in mixed lists is attributed to a sampling bias promoting priority recall of strong items and consequent output interference affecting weak items. We suggest that, in recognition, the disruption of this pattern through the randomization of test probes is responsible for the null LSE. We present several pilot experiments consistent with this account; however, the registered experiment, which had more statistical power, did not support this account.

Modeling list-strength and spacing effects using version 3 of the retrieving effectively from memory (REM.3) model and its superimposition-of-similar-images assumption.

Shiffrin and Steyvers (1997) introduced a model of recognition memory called retrieving effectively from memory (REM) and successfully applied it to a number of basic memory phenomena. REM incorporates differentiation, wherein item repetitions are accumulated in a single mnemonic trace rather than separate traces. This allows REM to account for several benchmark findings, including the null list-strength effect in recognition (Ratcliff, Clark, & Shiffrin, 1990). The original REM treated massed and spaced repetitions identically, which prevents it from predicting a mnemonic advantage for spaced over massed repetitions (i.e., the spacing effect). However, Shiffrin and Steyvers discussed the possibility that repetitions might be represented in a single trace only if the subject identifies that the repeated item was previously studied. It is quite plausible that subjects would notice repetitions more for massed than for spaced items. Here we show that incorporating this idea allows REM to predict three important findings in the recognition memory literature: (1) the spacing effect, (2) the finding of slightly positive list-strength effects with spaced repetitions, as opposed to massed repetitions or increased study time, and (3) list-strength effects that have been observed using very large strong-to-weak ratios (see Norman, 2002).

List-strength effects in older adults in recognition and free recall.

The present investigation provides a novel extension of the retrieving effectively from memory (REM) model to examine free-recall and recognition memory in older adults to inform our understanding of age-related cognitive decline. When some items on a list are strengthened through distributed repetitions, memory for the nonstrengthened items on that list may become impaired depending on how memory is tested-a phenomenon known as the list-strength effect (LSE; e.g., Tulving & Hastie, Journal of Experimental Psychology, 92, 297-304, 1972). When the strengthening operation involves distributed repetitions, LSE is robust in free recall, but it is reliably absent in recognition (Malmberg & Shiffrin, Journal of Experimental Psychology, 31, 322-336, 2005; Ratcliff, Clark, & Shiffrin, Journal of Experimental Psychology, 16, 163-178, 1990). Two experiments investigated LSE in recall and recognition in older adults and young adults who encoded items with full or divided attention. Despite showing impaired recall and recognition, older adults showed patterns of LSEs across both experiments that were similar to young adults with full attention rather than young adults with divided attention. In recognition, there was a null LSE in older and young adults, but a positive LSE was observed under divided attention. In contrast, in free recall, there was a positive LSE in older and young adults, but a null LSE under divided attention. Collectively, the results suggest that older adults do not have impaired encoding of context information (evidenced by intact LSE in recall), and they do not have impaired differentiation of item representations (evidenced by the null LSE in recognition). Age-related impairment in both memory tasks can be accounted for by sparse encoding of item-based information.

The list strength effect in source memory: Data and a global matching model

A critical constraint on models of item recognition comes from the list strength paradigm, in which a proportion of items are strengthened to observe the effect on the non-strengthened items. In item recognition, it has been widely established that increasing list strength does not impair performance, in that performance of a set of items is unaffected by the strength of the other items on the list. However, to date the effects of list strength manipulations have not been measured in the source memory task. We conducted three source memory experiments where items studied in two sources were presented in a pure weak list, where all items were presented once, and a mixed list, where half of the items in both sources were presented four times. Each experiment varied the nature of the testing format. In Experiment 1, in which each study list was only tested on one task (item recognition or source memory), a list strength effect was found in source memory while a null effect was found for item recognition. Experiments 2 and 3 showed robust null list strength effects when either the test phase (Experiment 2) or the analysis (Experiment 3) was restricted to recognized items. An extension of the Osth and Dennis (2015) model was able to account for the results in both tasks in all experiments by assuming that unrecognized items elicit guess responses in the source memory task and that there was low interference among the studied items. The results were also found to be consistent with a variant of the retrieving effectively from memory model (REM; Shiffrin & Steyvers, 1997) that uses ensemble representations.

Category-length and category-strength effects using images of scenes.

Global matching models have provided an important theoretical framework for recognition memory. Key predictions of this class of models are that (1) increasing the number of occurrences in a study list of some items affects the performance on other items (list-strength effect) and that (2) adding new items results in a deterioration of performance on the other items (list-length effect). Experimental confirmation of these predictions has been difficult, and the results have been inconsistent. A review of the existing literature, however, suggests that robust length and strength effects do occur when sufficiently similar hard-to-label items are used. In an effort to investigate this further, we had participants study lists containing one or more members of visual scene categories (bathrooms, beaches, etc.). Experiments 1 and 2 replicated and extended previous findings showing that the study of additional category members decreased accuracy, providing confirmation of the category-length effect. Experiment 3 showed that repeating some category members decreased the accuracy of nonrepeated members, providing evidence for a category-strength effect. Experiment 4 eliminated a potential challenge to these results. Taken together, these findings provide robust support for global matching models of recognition memory. The overall list lengths, the category sizes, and the number of repetitions used demonstrated that scene categories are well-suited to testing the fundamental assumptions of global matching models. These include (A) interference from memories for similar items and contexts, (B) nondestructive interference, and (C) that conjunctive information is made available through a matching operation.

Divided attention during encoding causes separate memory traces to be encoded for repeated events

Abstract Strengthening some items on a list adversely affects memory for the remaining items on the list – a phenomenon known as the list-strength effect (LSE; e.g., Tulving & Hastie, 1972). Whether the LSE is observed depends on how memory is tested and how items are strengthened (Malmberg & Shiffrin, 2005; Ratcliff, Clark, & Shiffrin, 1990), with free recall producing robust LSE, whereas recognition test typically producing null LSE. In this report, we examined the LSE in free recall and recognition when items were learned with full attention or under divided attention at encoding. In free recall, the results showed a robust LSE under full attention, but a null LSE in divided attention. In contrast, in recognition a null LSE was observed under full attention, but a positive LSE emerged under divided attention. Within REM theoretical framework, the combination of these findings suggests that DA reduces the tendency to accumulate information across repetitions in a single trace, thereby reducing the influence of differentiation.

The list strength effect in cued recall

Abstract Episodic memory is the process by which information about experienced events is encoded and retrieved. Successful retrieval of episodic memories is dependent on the way in which memory is tested and as a result many effects and theories of episodic memory are task dependent. One such finding is the list strength effect. In free recall, a positive list strength effect is observed; memory for a given item is harmed by the presence of other strongly encoded items and helped by the presence of other weakly encoded items. In recognition, a null list strength is observed; memory for a given item is unaffected by the strength of other items. Such differential empirical findings are crucial to understanding memory, but it is undesirable to have multiple task-specific theories rather than a unified theory of memory. Here we use cued recall, a task that shares properties of both free recall and recognition, to move toward that goal. In a series of 5 experiments, we observed a null list strength effect in cued recall. We suggest that a successful theory would entail the use of both item and context information during retrieval, consistent with the approach of the Search of Associative Memory model.

Stimulus Type and the List Strength Paradigm

In recognition memory, increasing the strength of studied items does not reduce performance on other items, an effect dubbed the null list strength effect (LSE). While this finding has been replicated many times, it has rarely been tested using stimuli other than single words. Kinnell and Dennis ( 2012 ) recently tested for the presence of list length effects using non-word stimulus classes while controlling for the confounds that are present in list length designs. Small list length effects were found for fractal and face images. We adopted the same paradigm and stimuli used by Kinnell and Dennis to test whether these stimuli would be susceptible to list strength effects as well. We found significant LSEs for fractal images, but null LSEs for face images and natural scene photographs. Stimuli other than words do appear to be susceptible to list strength effects, but these effects are small and restricted to particular stimulus classes, as is the case in list length designs. Models of memory may be able to address differences between these stimulus classes by attributing differences in representational overlap between the stimulus classes.

Individual differences in contextual storage: Evidence from the list-strength effect.

Strengthening some items in a list of words impairs free recall of the remaining items in the list-a phenomenon known as the list-strength effect (LSE; e.g., Tulving & Hastie, 1972). Research indicates that whether the LSE is observed depends on the nature of the strengthening manipulation, and the effect is attributed to the enhancement of the contextual information in the memory trace of the items (e.g., Malmberg & Shiffrin, 2005). We investigated the magnitude of the LSE as a function of individual differences in working memory capacity (WMC). The findings indicate that low-WMC participants do not show the LSE, suggesting that they do not accumulate as much contextual information in the memory trace as the remaining participants do. These results suggest that the low-spans' deficits in utilizing contextual cues during retrieval (e.g., Spillers & Unsworth, 2011) could be partly linked to their deficits in encoding and storing contextual information. Implications for global theories of memory are discussed.

Associative recognition and the list strength paradigm

When a subset of list items is strengthened, the discriminability of the nonstrengthened items is unaffected. This regularity has been dubbed the null list strength effect (LSE), and despite its many replications in item recognition, little research has investigated whether an LSE occurs in associative recognition. We conducted two experiments in which a set of pairs were studied once and a set of interference pairs were studied either once (pure-weak-list condition) or four times (mixed-list condition). Equivalent levels of performance for the nonstrengthened pairs were observed in both the pure-weak and mixed conditions using both yes-no and two-alternative forced choice testing. Additionally, equivalent false alarm rates were observed between rearranged pairs composed of weak and strong items. Both sets of results were found to be consistent with a matrix model that has no overlap among its item representations.

Rethinking inhibition theory: On the problematic status of the inhibition theory for forgetting

The standard textbook account of interference and forgetting is based on the assumption that retrieval of a memory trace is affected by competition by other memory traces. In recent years, a number of researchers have questioned this view and have proposed an alternative account of forgetting based on a mechanism of suppression. In this inhibition account, such forgetting is due to an inhibitory control process that operates whenever non-target information hinders the retrieval of a specific target item. It is assumed that the memory traces of these non-target items are suppressed or inhibited in order to overcome their interfering effects and it is claimed that this inhibition has a longer-lasting effect on the strength of the suppressed memory traces. In this paper we critically review the claim that the inhibition theory provides a better account of forgetting than more traditional competition-based theories. We discuss the explanations that have been proposed to account for retrieval induced forgetting, the think/no-think paradigm, directed forgetting, the part-list cuing effect, output interference and list-strength effects. We conclude that the theoretical status of inhibition as an explanation for interference and forgetting is problematic. We show that the claim that these findings cannot be explained by standard competition-based accounts is incorrect.

The list-strength effect in recall: Relative-strength competition and retrieval inhibition may both contribute to forgetting.

According to the principle of relative-strength competition, stronger items in memory block the retrieval of weaker items. This principle, integral to many theories of forgetting over the years, derives much of its support from the list-strength effect (LSE), in which strengthening some items in a study list makes it more difficult to recall other items. Work in the retrieval-induced forgetting literature has challenged the existence of relative-strength competition, 1st by offering many examples of a null LSE and 2nd by proposing that extant observations of the LSE can be explained by retrieval inhibition. In the present study, a series of experiments produced a robust LSE in cued recall under conditions meant to control the contribution of retrieval inhibition. Simulations of the SAM-REM model of recall (K. J. Malmberg & R. M. Shiffrin, 2005) showed that a model based on relative-strength competition can accommodate both the presence and absence of an LSE. The empirical results and model simulations together make a case for the role of strength-based competition in forgetting.

List Strength Effect without List Length Effect in Recognition Memory

The study of list length effects (adding items to a list affects memory for the other items) and list strength effects (strengthening some items in a list affects memory for the nonstrengthened items) is important to constrain models of memory. In recognition memory, a list length effect is generally found, whereas a list strength effect is not. Using the switched-plurality procedure in an old-new recognition task (e.g., study banana; test bananas), we found the opposite pattern. Length manipulations caused no change in memory performance, whereas strength manipulations did. The list strength effect was found when recollection was likely to operate at test (with switched-plurality lures). When recollection was unlikely to operate (with unrelated lures), the strength effect disappeared. The result was observed using both a size judgement task (which has previously produced positive list strength effects) and a pleasantness judgement task (which has not yielded list strength effects before).

Exploring a recognition-induced recognition decrement.

Four experiments were performed to explore a recognition decrement that is associated with the recognition of a word from a short list. The stimulus material for demonstrating the phenomenon was a list of words of different syntactic types. A word from the list was recognized less well following a decision that a word of the same type had occurred in the list than following a decision that such a word had not occurred in the list. A recognition decrement did not occur for a word of a given type following a positive recognition decision to a word of a different type. A recognition decrement did not occur when the list consisted exclusively of nouns. It was concluded that the phenomenon may reflect a criterion shift but, probably, does not reflect a list strength effect, suppression, or familiarity attribution consequent to a perceived discrepancy between actual and expected fluency.

The list strength effect: a contextual competition account.

Research on the list strength effect (LSE) has shown that learning some words on a list more strongly than others impairs memory for the weakly learned words when tested with a recall task. Norman (2002) demonstrated that the LSE also occurs within the recollection process of a recognition test. In this study, a mechanistic dual-process account of the LSE was tested that simultaneously makes predictions concerning additional sources of context in interference effects. In two experiments, we attempted to replicate Norman's (2002) findings and provide the basis for our modeling efforts. We found evidence for a recollection LSE in raw measures of responding, with memory performance also benefiting from reinstatement of perceptual characteristics at test. However, large differences in the hits between the lists were accompanied by small differences in false alarms, such that when d' is calculated, differences between the lists are not significant. We propose an account of the LSE whereby the actual effect of competition between items on the list is small, although present, and difficult to distinguish from large effects of bias due to the strength manipulations. We argue that our findings provide support for a mechanistic explanation of LSE that is based on competition of source activation and changes in the thresholds for responses.

The "One-Shot" Hypothesis for Context Storage.

In 3 experiments motivated by the implicit memory literature, the authors investigated the effects of different strengthening operations on the list strength effect (LSE) for explicit free recall, an effect posited by R. M. Shiffrin, R. Ratcliff, and S. E. Clark (1990) to be due to context cuing. According to the one-shot hypothesis, a fixed amount of context is stored when an item is studied for at least 1 or 2 s. Beyond the initial context storage, increases in study time or different orienting tasks do not influence the amount of context that is stored, and thus only spaced repetitions should produce a positive LSE. Consistent with prior findings, spaced repetitions always produced a positive LSE, but increases in depth of processing, study time, and massed repetitions did not. A model implements the one-shot hypothesis, and a role for context storage as a link between episodic and semantic memory is discussed.

Strong memories obscure weak memories in associative recognition

The list strength effect, in which strengthening some memories has a detrimental effect on the retrieval of other memories, has generally not been found in item recognition. The present study shows that the list strength effect does occur in associative recognition. Study materials were sets of overlapping word pairs (A-B, A-C, D-B, etc.). Within critical sets of words, strong pairs were presented three times at study, as compared with one presentation for weak pairs. In Experiment 1, associative recognition for weak pairs was less accurate than that for baseline pairs, and response times for hits were slower. In Experiment 2, receiver-operating characteristic curve data provided further evidence of poor accuracy for weak pairs. These findings support a qualitative distinction between item and associative recognition.

Differential effects of list strength on recollection and familiarity.

Numerous studies have found a null list strength effect (LSE) for recognition sensitivity: Strengthening memory traces associated with some studied items does not impair recognition of nonstrengthened studied items. In Experiment 1, the author found a LSE using receiver operating characteristic-based measures of recognition sensitivity. To account for the discrepancy between this and prior research, the author (a) argues that a LSE occurs for recollection but not for discrimination based on familiarity, and (b) presents self-report data consistent with this hypothesis. Experiment 2 tested the dual-process hypothesis more directly, using switched-plurality (SP) lures to isolate the contribution of recollection. There was a significant LSE for comparisons involving SP lures; the LSE for discrimination of studied items and nominally unrelated lures (which can be supported by familiarity) was not significant.