Visual working memory (VWM) has long been considered to be limited in capacity, but the way in which it is limited remains unclear. One of the theoretical debates in visual working memory concerns whether the number of objects that can be stored is fixed (discrete slot models) or variable (flexible resource models). Recent research on the resolution of VWM has helped elucidate this debate by acknowledging an important trade-off between number and resolution: as the number of items stored increases, the resolution of representation declines. Yet, a different conception suggests that the number and resolution may represent distinct aspects of visual working memory, evidenced by both behavioral and neuroimaging data. In this thesis, I examined three theoretical questions regarding the relationship between the number and the resolution of items in VWM. First, how does set size affect high- & low-resolution representations (differentially)? If an item limit can be evidenced in the high-resolution measure, but not in the low-resolution measure, my second research question emerges. That is, how much resolution do we have for the remaining objects when the item limit is exceeded? Third, if both high- & low-resolution representations of an item exist in VWM, are they stored together or independently? In a series of five experiments, I addressed these questions using an adapted continuous report paradigm, in which participants were asked to remember a mixture of objects from two categories and respond firstly to the category of the item-to-report (low-resolution measure), followed by a second within-category response (high-resolution measure) which was contingent on the first. In Experiments 1-2, only performance in the low-resolution, but not in the high-resolution, measure was largely indifferent to set size, which was not compatible with either discrete slot or flexible resource models, but was largely consistent with predictions from the two-factor model and the neural object-file theory. In Experiments 3-4, precision of high-resolution representations declined monotonically until the set size reached around four items, fitting to the predictions from discrete slot models. The overall accuracy in low-resolution measure, however, remained relatively high, suggesting differential set size influence on high- and low-resolution representations. In Experiment 5, capacity comparison revealed no significant difference when the low-resolution task was absent. Taken together, I demonstrate that 1) both low-resolution ensemble representations and high-resolution individual item representations exist in VWM, and 2) high-resolution representations (i.e. object identity) and low-resolution representations (i.e. objects’ categories, configural information and perhaps some coarse feature information) of an object might be stored independently. / published_or_final_version / Psychology / Master / Master of Philosophy

published_or_final_version / abstract / Linguistics / Master / Master of Arts

The activity of neurons in the prefrontal cortex (PFC) may underlie working memory processes in the brain. Both the performance of working memory tasks and the activity of PFC neurons are modulated by dopamine. The goal of the present thesis was to gain insight into the neural basis of working memory by studying the PFC, and the DA system in the PFC, from both a behavioral and cellular perspective. The functional contribution of the PFC to working memory processes in the rat was assessed in Chapter 2 of the present thesis using memory-based foraging tasks on an 8-arm radial maze. The results of these studies indicated that lidocaine-induced inactivations of the PFC selectively disrupted the ability to use mnemonic information to guide foraging, but not the ability to acquire or retain such information. The ability to use mnemonic information to guide foraging was also disrupted by microinjection of a D1 but not D2 receptor antagonist into the PFC. Chapters 3-5 investigated how PFC neurons process synaptic inputs to their dendrites to produce spike output. The intrinsic membrane properties and synaptic responses at the soma and dendrites of deep layer PFC pyramidal neurons were recorded using sharp intracellular or whole-cell patch-clamp techniques in a brain-slice preparation. Different passive and active membrane properties of the soma and dendrites of PFC neurons were observed. The distal dendrites of PFC neurons responded most effectively to strong, highly coincident synaptic inputs. Ca²⁺currents near the soma both amplified the effects of these inputs and modulated the spike output pattern. Spike output at the soma was also controlled by the interplay of slowly-inactivating Na⁺ and K⁺ currents. Chapter 6 investigated the modulation of PFC neurons by DA. DA or a D1 but not D2 receptor agonist increased the evoked firing of PFC neurons via a D1- mediated modulation of slowly-inactivating Na⁺ and K⁺ currents. Concurrently, D1 receptor activation reduced burst firing in PFC neurons, due to a attenuation of Ca²⁺ currents. D1 receptor activation also increased both GABA[sub A] IPSPs and NMDA EPSPs. The final chapter of this thesis integrated these data into a cellular model of PFC function and its modulation by DA. It is proposed that DA may tune PFC neurons such that they respond selectively to strong synchronized inputs from other cortical areas. In the presence of DA, working memory processes mediated by the PFC may be influenced selectively by stimuli of behavioral significance.

No description available.

In an immediate serial recall task, participants are asked to recall lists of items in order. In the Hebb repeating-list variant of the task, subjects are read a series of lists, and every third list is repeated. Performance improves across repetitions but is stable for the non-repeated trials. The repetition advantage—the increased accuracy for the repeated list—is known as the Hebb effect. Several models have been advanced to explain how participants order successive items, but how participants take advantage of the repetition has largely been ignored. Although the task is usually discussed in terms of recall of verbal items, the Hebb effect has been observed with sequences of visuo-spatial positions. The present work assesses whether immediate serial recall of verbal material benefits from visuo-spatial context. Sequences of letters were presented in different spatial positions in a visual version of the Hebb task. Presenting lists in random spatial positions on the periphery of an imaginary circle did not boost performance, but if the sequence was predictable, overall accuracy increased. The spatial path of successive items influenced the Hebb effect. When the distance between successive positions was minimized, participants were able to exploit the repetition early in practice. The results deny an account based on item distinctiveness. I discuss the results in terms of contemporary models of ISR. / Thesis (Master, Psychology) -- Queen's University, 2007-10-03 13:01:00.716

No description available.

The major purpose of this study was to determine the effect of two mediational strategies, labeling and rehearsal, on the short-term memory of prelingual deaf children. The research question answered by this investigation examined whether inducing the use of a mediator was affected by age and the serial position of stimulus items presented.The population of prelingual, severely and profoundly deaf children of normal intelligence were screened for overt production of existing mediational techniques. Thirty-three nonproducing subjects were randomly assigned to three treatment groups at four age levels. Two groups were taught memory strategies, and the third group acted as a control. A single null hypothesis was tested using a 3x4x4 analysis of variance with repeated measures on the last factor. The .05 level of significance was predetermined as the critical probability level for rejecting the hypothesis.FINDINGS1. Differences in recall performance of prelingual deaf children who (a) were induced to label, (b) were induced to cumulatively rehearse, and (c) had no induced strategy did not vary as a function of age and serial position.2. Deaf children's performance on short-term, visual sequential memory tasks is a function of the type of preferred memory strategy and age.3. No differences in performance at the serial positions occurred as a function of age.4. The type of memory strategy used by prelingual deaf children did not result in differences in performance as a function of serial position.5. The youngest deaf children who rehearsed and labeled enhanced recall significantly better than those children who were taught no strategy.6. Most older subjects taught to rehearse recalled significantly better than the children taught to label and those who were not taught a memory technique.7. Late intermediate deaf children who rehearsed recalled better than those children taught to label, but not significantly different from those taught no strategy.8. The relationship among serial position levels showed primacy and recency effects on the memory curve.CONCLUSIONS1. The cognitive processes used by deaf children are similar to those used by hearing children.2. Cognitive processes used by deaf children are utilized for similar purposes as those used by hearing children.3. Deaf children appear to display a production deficiency with regard to the use of mediational strategies. / Department of Special Education

This thesis is about the nature of a man's immediate memory for a short sequence of verbal items. Two components of immediate memory may be distinguished: an 'active' memory component, which is progressively destroyed within a few seconds if, subsequent to presentation of the sequence, the man is required to perform a distracting task which prevents rehearsal of the items, and a residual 'inactive' component which is more permanent. My concern is with behavioural evidence relating to the question of how the attributes of a verbal sequence are represented in active memory subsequent to identification of the constituent items. It is possible selectively to probe the retrieval of different attributes of the sequence by the use of appropriate tasks. Evidence on the performance of normal adult subjects in various short-term memory tasks, from the literature and from my own experiments, is extensively reviewed. There emerges a fundamental contrast between two groups of tasks: on the one hand, tasks which require the subject to indicate whether a test item was present in the memorised sequence (item recognition) or to judge the relative recency with which items have occurred; on the other,tasks which require the subject to base his response on the order or precise position of items in the sequence. To account for the nature of the contrast I put forward a general hypothesis which is an amended and elaborated version of parts of a general model of memory described by Morton (1970). It is argued that subsequent to identification of an item, representations of it may temporarily be held in active memory by two distinct storage mechanisms. Thefirst is as a decaying trace at the neural unit, called (following Morton) a 'logogen', responsible for the identification of that item. This trace may be treated as possessing a unidimensional 'strength' whose magnitude is dependent on how recently the item has occurred. Item recognition and judgements of recency are held to be mediated by assessment of the test item's trace strength with respect to a decision criterion located on the strength continuum. Secondly, the item may also be represented as one of several maintained in a serially- organised limited-capacity storage mechanism called (following Morton) the 'response buffer' . This holds a small number of the verbal responses recently made available by the logogens, coded as a string of articulatory descriptions or commands. It has two crucial properties. Firstly, items represented in it as potential responses may be fed back in sequence to the logogens for re-identification (the process of sub-vocal rehearsal). Secondly, since the serial order in which items enter the response buffer is retained as an intrinsic property of its structure, retrieval from it provides a ready mode of access to the order or position of items in the memorised sequence, which item-traces at logogens do not. The thesis falls into two parts. The first three chapters contain a theoretical review of the literature. In Ch.1, the active/ inactive distinction is introduced with some reference to the historical background (1.1). Some points of technique are raised (1.2), the aims, of the present work are outlined (1.3) and the twostore hypothesis is described (1.4) In Ch.2, evidence is reviewed from short-term memory experiments in which accuracy is the main dependent variable, beginning with experiments in which either serial recall of the whole sequence or probed recall of a single item is required. The hypothesis that the order of items is retained in active memory as an intrinsic property of the memory structure is contrasted with theories (e.g. Wickelgren, 1972) emphasising the formation of temporary associations between the representations of items adjacent in the list and/or between item and position representations. It is concluded that inter-item associations play no major role in active memory, and that item-position associations cannot account both for the partial independence of order and item errors and for the relationship between order errors and phonemic similarity. Conrad's (1965) model is introduced as a precursor of my own. It is concluded that items are recalled primarily from the (ordered) response buffer, but that traces at logogens influence the availability of responses as guesses when items are wholly or partly lost from the response buffer (2.11). Some evidence is then described which suggests that variables may be identified which differentially influence retention of information about the order of items and about their occurrence (2.12). Experiments comparing the effectiveness of position, context or both,as cues for recall are argued to imply that order and position are coded by the same mechanism, but that access to items retained by it may be more direct given a position rather than a contextual cue (2.13). We then turn to experiments on short-term recognition and judgements of recency, which are discussed in relation to the trace strength theory of Wickelgren and Norman (1966) (2.21). It is concluded that an item's recent identification is represented as an exponentially-decaying trace located at a unique permanent representation of that item, to which access is direct (2.22). But order-recognition experiments provide little evidence that associations between items are represented in active memory in the same way (2.23). The two-store hypothesis is then applied to some general problems of the functions of active memory: in speech comprehension (2.31) as an 'address register' for inactive memory (cf. Broadbent, 1971) (2.32) and as a working memory. Finally, the logogen system is discussed in relation to Sperling's (1967) 'recognition buffer' (2.34) and evidence for articulatory as opposed to acoustic coding in immediate memory is reviewed (2.35). In Ch.3, I introduce the experimental paradigms pioneered by Sternberg (1966), which involve presentation on each trial of a different sub-span list for memorisation, followed by a probe to which reaction-time (RT) is measured. Several models are outlined of the nature, dynamics and format of the representations mediating performance in Sternberg's item-recognition paradigm (IRn), in which the subject must indicate whether the probe was or was not a member of the list. Predictions are derived from Sternberg's two scanning models and two versions of a trace strength hypothesis and compared to data available in the literature. The evidence favours the hypothesis that subjects perform the IRn task by judging the strength of the trace at the amodal representation (logogen) to which the probe provides direct access (3.1). Evidence from a version of the IRn paradigm in which the memory set remains constant from trial to trial is then reviewed; it is argued that the results obtained are necessarily equivocal with respect to active memory; nevertheless, the concept of trace strength may be of use in this context also (3.2). Next, we see that Sternberg's own experiments on contextual recall (CR), in which the subject must respond with the item following the probed item in the list, yield a pattern of results very different from that obtained in IRn. His own explanation is in terms of different strategies of search within a single store. In view of the failure of the exhaustive scanning model of IRn, the differences may be better accounted for by the two-store model. Since these RT paradigms provide a relatively pure and sensitive way of probing the retrieval of different attributes from active memory, a strategy is suggested of looking for factors which differentially influence performance in IRn, CR and related tasks. Evidence already available suggests that these factors may include serial position, phonemic similarity and the availability of a position cue. The second part of the thesis describes my own experiments.

No description available.

Working memory, or the ability to maintain and manipulate information in mind when it is no longer physically present, is a pervasive yet severely capacity-limited component of cognition. Visual working memory, also known as visual short-term memory (VSTM), is limited to three or four items on average, with individual differences that range from roughly two to up to six items. Despite agreement that capacity is functionally limited, the current literature is split with respect to the nature of VSTM representations on two key questions: (1) What information is maintained in VSTM? (2) How is information stored in VSTM? The studies presented here address these questions using an event-related potential (ERP) task and a series of behavioural experiments that incorporate attentional selection via retrospective cueing (retro-cues). Experiment 1 manipulated both the number of features and the number of locations to be remembered in a lateralized change-detection task, with differences in the amplitude and topography of the resulting contralateral delay activity (CDA) indicating separate stores for features and locations. Experiments 2a-c established the basic effects of retro-cues on change-detection tasks, showing that attentional selection operated on one system at a time, with overall shorter response times and increased capacity estimates once baseline capacity was exceeded. Experiments 3a-b demonstrated that retro-cues biased VSTM resources to the cued item at the expense of representational strength of the other, non-cued items, showing flexible reallocation of resources. Experiments 4a-b presented multiple retro-cues to further examine the flexible reallocation of resources in VSTM, showing that capacity benefits depended on spatial specificity of retro-cues and that VSTM weights could be reallocated multiple times before probe comparison. Experiment 5 discounted the potential role of a general alerting mechanism in boosting capacity estimates, showing again that the retro-cue benefit required specificity of the cue. Experiment 6 showed that flexible reallocation of resources within one system did not change the online maintenance of representations within the other system. Thus, the studies collectively address the questions of (1) what and (2) how information is stored by supporting a two-system model of VSTM in which (1) locations and features are stored (2) independently via flexibly allocated resources.