Serial Position Functions Research Articles

Rapid processing of letters, digits and symbols: what purely visual‐attentional deficit in developmental dyslexia?

Visual-attentional theories of dyslexia predict deficits for dyslexic children not only for the perception of letter strings but also for non-alphanumeric symbol strings. This prediction was tested in a two-alternative forced-choice paradigm with letters, digits, and symbols. Children with dyslexia showed significant deficits for letter and digit strings but not for symbol strings. This finding is difficult to explain for visual-attentional theories of dyslexia which postulate identical deficits for letters, digits and symbols. Moreover, dyslexics showed normal W-shaped serial position functions for letter and digit strings, which suggests that their deficit is not due to an abnormally small attentional window. Finally, the size of the deficit was identical for letters and digits, which suggests that poor letter perception is not just a consequence of the lack of reading. Together then, our results show that symbols that map onto phonological codes are impaired (i.e. letters and digits), whereas symbols that do not map onto phonological codes are not impaired. This dissociation suggests that impaired symbol-sound mapping rather than impaired visual-attentional processing is the key to understanding dyslexia.

Neural Correlates of Sequence Encoding in Visuomotor Learning

To examine the neural basis of sequence learning, a fundamental but poorly understood human ability, we recorded event-related potentials (ERPs) while subjects viewed and memorized randomly directed sequences of motions for later imitation. Previously, we found that the amplitude of ERPs elicited by successive motion segments decreased as a function of each segment's serial position. This happened when subjects were required to remember the sequence, but not when they were performing a perceptual task. Here, to study the functional significance of this amplitude gradient in sequence learning, we presented each sequence several times in succession and examined changes in ERP amplitude as subjects learned the sequence through repeated observation and imitation. Behaviorally, with each repetition subjects grew more accurate in reproducing what they had seen. At the same time, ERPs grew smaller with each successive presentation, replicating and extending previous demonstrations of repetition suppression. Importantly, a comparison of ERPs to segments occupying different serial positions within a sequence revealed a decreasing amplitude gradient that grew steeper with sequence repetition. This sharpening of the amplitude gradient may reflect an explicit encoding process that relies on a magnitude code for serial order.

Short Article: Single-probe serial position recall: Evidence of modularity for olfactory, visual, and auditory short-term memory

The present study examined and compared order memory for a list of sequentially presented odours, unfamiliar faces, and pure tones. Employing single-probe serial position recall and following a correction for a response bias, qualitatively different serial position functions were observed across stimuli. Participants demonstrated an ability to perform absolute order memory judgments for odours. Furthermore, odours produced an absence of serial position effects, unfamiliar faces produced both primacy and recency, and pure tones produced recency but not primacy. Such a finding is contrary to the proposal by Ward, Avons, and Melling (2005) that the serial position function is task, rather than modality, dependent. In contrast, the observed functions support a modular conceptualization of short-term memory (e.g., Andrade & Donaldson, 2007; Baddeley & Hitch, 1974), whereby separate modality-specific memorial systems operate. An alternative amodal interpretation is also discussed wherein serial position function disparities are accommodated via differences in the psychological distinctiveness of stimuli (Hay, Smyth, Hitch, & Horton, 2007).

Serial position effects in the identification of letters, digits, and symbols.

In 6 experiments, the authors investigated the form of serial position functions for identification of letters, digits, and symbols presented in strings. The results replicated findings obtained with the target search paradigm, showing an interaction between the effects of serial position and type of stimulus, with symbols generating a distinct serial position function compared with letters and digits. When the task was 2-alternative forced choice, this interaction was driven almost exclusively by performance at the first position in the string, with letters and digits showing much higher levels of accuracy than symbols at this position. A final-position advantage was reinstated in Experiment 6 by placing the two alternative responses below the target string. The end-position (first and last positions) advantage for letters and digits compared with symbol stimuli was further confirmed with the bar-probe technique (postcued partial report) in Experiments 5 and 6. Overall, the results further support the existence of a specialized mechanism designed to optimize processing of strings of letters and digits by modifying the size and shape of retinotopic character detectors' receptive fields.

Serial position effects in 2-alternative forced choice recognition: Functional equivalence across visual and auditory modalities

Two experiments examined Ward, Avons, and Melling's (2005) proposition that the serial position function is task, rather than modality, dependent. Specifically, they proposed that for backward testing the 2-alternative forced-choice (2AFC) recognition paradigm is characterised by single-item recency irrespective of the modality of the stimulus presentation. In Experiment 1 the same nonword sequences, presented both visually or auditorily, produced qualitatively equivalent serial position functions with 2AFC testing. Forward testing produced a flat serial position function, while backward testing produced two-item recency in the absence of primacy. In order to rule out the possibility that the serial position functions for visual stimuli were the product of sub-vocal rehearsal, Experiment 2 employed articulatory suppression during the presentation phase. Serial position function equivalence was again observed together with a modest impairment in overall recognition rates. Taken together, these data are consistent with the Ward et al. proposition and further support the existence of a visual memory that can facilitate storage of visual-verbal material (e.g. Logie, Della Sella, Wynn, & Baddeley, 2000). However, the observation of two-item recency contradicts the original duplex account of single-item recency traditionally observed for backwards recognition testing of visual stimuli (Phillips & Christie, 1977).

Recall latencies, confidence, and output positions of true and false memories: Implications for recall and metamemory theories

Recall latency, recall accuracy rate, and recall confidence were examined in free recall as a function of recall output serial position using a modified Deese–Roediger–McDermott paradigm to test a strength-based theory against the dual-retrieval process theory of recall output sequence. The strength theory predicts the item output sequence to be in the descending order of memory strength. The dual-retrieval process theory postulates two phases in a free recall, a first direct access phase in which items are output verbatim in the weakest-to-strongest order (cognitive triage) and a second reconstructive phase in which reconstructed items are output in the strongest-to-weakest order. In four experiments, all three indicators of memory strength (latency, accuracy, and confidence) consistently showed a descending-strength order of recall both for true and false memories. Additionally, false memory was found to be output in two phases and subjects’ confidence judgment of their own memory to be unaccountable by retrieval fluency (recall latency).

Position distinctiveness, item familiarity, and presentation frequency affect reconstruction of order in immediate episodic memory

Three experiments examined the effects of position distinctiveness, item familiarity, and frequency of presentation on serial position functions in a task involving reconstructing the order of a subset of 12 names in a list of 20 names. Three different serial position conditions were compared in which the subset of names occurred in Positions 1–12, 5–16, or 9–20, with all subsets including Positions 9–12. The serial positions were defined temporally in Experiments 1 and 2 and spatially in Experiment 3. The serial position functions in all three experiments were well predicted by Murdock’s [Murdock, B. B., Jr. (1960). The distinctiveness of stimuli. Psychological Review, 67, 16–31] account in terms of the distinctiveness of the absolute positions. Experiment 3 also revealed significant effects of item familiarity and frequency of presentation on order reconstruction.

Eye Movements and Serial Memory for Visual-Spatial Information

This research investigated the nature of encoding and its contribution to serial recall for visual-spatial information. In order to do so, we examined the relationship between fixation duration and recall performance. Using the dot task--a series of seven dots spatially distributed on a monitor screen is presented sequentially for immediate recall--performance and eye-tracking data were recorded during the presentation of the to-be-remembered items. When participants were free to move their eyes at their will, both fixation durations and probability of correct recall decreased as a function of serial position. Furthermore, imposing constant durations of fixation across all serial positions had a beneficial impact (though relatively small) on item but not order recall. Great care was taken to isolate the effect of fixation duration from that of presentation duration. Although eye movement at encoding contributes to immediate memory, it is not decisive in shaping serial recall performance. Our results also provide further evidence that the distinction between item and order information, well-established in the verbal domain, extends to visual-spatial information.

Serial-probe recognition in rhesus macaques: Effects of midazolam

A serial-probe recognition task was used to assess the effects of midazolam on visual attention and short-term memory in three rhesus monkeys. On each trial, six unique alphanumeric sample stimuli (list items) were presented sequentially followed by a choice period. Choosing the ‘probe’ stimulus was correct if the probe matched one of the list items; otherwise, choosing the ‘default’ stimulus (a white square) was correct. Behavior was examined under a range of doses of midazolam (0.065, 0.13, 0.26, and 0.52 mg/kg IM). Midazolam did not significantly reduce choice accuracy or change the shape of the serial position function and did not significantly reduce choice responding. However, choice reaction time was significantly increased by the two highest doses of midazolam. Responding directed at the sample stimuli was reduced at the two highest doses of midazolam. Furthermore, 0.52 mg/kg midazolam significantly increased sample-stimulus reaction time at all six serial positions. Overall, these data suggest that the two highest doses of midazolam tested increase reaction time, but do not directly impair short-term visual recognition memory. This is noteworthy because such doses appear capable of protecting against nerve agent-induced seizures.

Modeling age-related differences in immediate memory using SIMPLE

In the SIMPLE model (Scale Invariant Memory and Perceptual Learning), performance on memory tasks is determined by the locations of items in multidimensional space, and better performance is associated with having fewer close neighbors. Unlike most previous simulations with SIMPLE, the ones reported here used measured, rather than assumed, dimensional values. The data to be modeled come from an experiment in which younger and older adults recalled lists of acoustically confusable and nonconfusable items. A multidimensional scaling solution based on the memory confusions was obtained. SIMPLE accounted for the overall difference in performance both between the two age groups and, within each age group, the overall difference between acoustically confusable and nonconfusable items in terms of the MDS coordinates. Moreover, the model accounted for the serial position functions and error gradients. Finally, the generality of the model’s account was examined by fitting data from an already published study. The data and the modeling support the hypothesis that older adults’ memory may be worse, in part, because of altered representations due to age-related auditory perceptual deficits.

Oscillatory correlates of the primacy effect in episodic memory

Both intracranial and scalp EEG studies have demonstrated that oscillatory activity, especially in the gamma band (28 to 100 Hz), can differentiate successful and unsuccessful episodic encoding [Sederberg, P.B., Kahana, M.J., Howard, M.W., Donner, E.J., Madsen, J.R., 2003. Theta and gamma oscillations during encoding predict subsequent recall. Journal of Neuroscience, 23(34), 10809–10814; Fell, J., Klaver, P., Lehnertz, K., Grunwald, T., Schaller, C., Elger, C.E., Fernandez, G., 2001. Human memory formation is accompanied by rhinal–hippocampal coupling and decoupling. Nature Neuroscience, 4 (12), 1259–1264; Gruber, T., Tsivilis, D., Montaldi, D., and Müller, M. (2004). Induced gamma band responses: An early marker of memory encoding and retrieval. Neuroreport, 15, 1837–1841; Summerfield, C., Mangels, J.A., in press. Dissociable neural mechanisms for encoding predictable and unpredictable events. Journal of Cognitive Neuroscience]. Although the probability of recalling an item varies as a function of where it appeared in the list, the relation between the oscillatory dynamics of successful encoding and serial position remains unexplored. We recorded scalp EEG as participants studied lists of common nouns in a delayed free-recall task. Because early list items were recalled better than items from later serial positions (the primacy effect), we analyzed encoding-related changes in 2 to 100 Hz oscillatory power as a function of serial position. Increases in gamma power in posterior regions predicted successful encoding at early serial positions; widespread low-frequency (4–14 Hz) power decreases predicted successful memory formation for later serial positions. These results suggest that items in early serial positions receive an encoding boost due to focused encoding without having to divide resources among numerous list items. Later in the list, as memory load increases, encoding is divided between multiple items.

Serial Position Memory in the Visual—Spatial Domain: Reconstructing Sequences of Unfamiliar Faces

In two studies we presented pictures of unfamiliar faces one at a time, then presented the complete set at test and asked for serial reconstruction of the order of presentation. Serial position functions were similar to those found with verbal materials, with considerable primacy and one item recency, position errors that were mainly to the adjacent serial position, a visual similarity effect, and effects of articulatory suppression that did not interact with the serial position effect or with the similarity effect. Serial position effects were found when faces had been seen for as little as 300 ms and after a 6-s retention interval filled with articulatory suppression. Serial position effects found with unfamiliar faces are not based on verbal encoding strategies, and important elements of serial memory may be general across modalities.

The recall of missing items

In two experiments, we studied the recall of missing items. Short lists of common words were presented once and were followed immediately by a random permutation of all but one of the presented items. The task of the subject was to recall the missing item--that is, the item present in the study set but missing from the probe set. Experiment 1 replicated the high accuracy with five-item lists originally reported by Yntema and Trask (1963) and showed that the latencies were quite short (about 750 msec). Experiment 2 varied list length unpredictably and showed that accuracy was a function of both list length (four, five, or six items) and serial position. Latency was again quite short but was essentially independent of list length and serial position. It was possible to simulate most of the effects with the power set model with no free parameters (i.e., parameters that varied with the experimental manipulations). The results seemed to be more consistent with a direct access model (the power set model of TODAM; Murdock, 1995) than with a simple search or serial-scanning model.

Executive dysfunction can explain word-list learning disability in very mild Alzheimer's disease: the Tajiri project.

Elderly people with questionable dementia (i.e. a Clinical Dementia Rating (CDR) of 0.5) have been focused on as representing the borderline zone condition between healthy people and dementia patients. Many of them are known to have pathologic traits of very mild Alzheimer's disease (AD). Although they present mild memory disorder, the underlying mechanism has not been fully investigated. Herein is reported the mechanism of learning disability in very mild AD. Eighty-six CDR 0.5 participants and 101 age- and education-matched healthy controls (CDR 0) were randomly selected from a community in the town of Tajiri, Miyagi Prefecture. The word-recall task of the Alzheimer Disease Assessment Scale-Japanese (i.e. learning and recall of 10 words) was administered. The numbers of words recalled in each trial and those never recalled throughout the trials were compared for the two CDR groups. The serial-position function was depicted for three parts (i.e. primary, middle, and recency). The CDR 0.5 group recalled significantly fewer words than the CDR 0 group. The number of never-recalled words was greater in the CDR 0.5 group. A remarkable difference was found in the middle part of the word list. The number of never-recalled words of the CDR 0.5 group was greater in the middle part. The large number of never-recalled words accounted for the poor learning performance of very mild AD participants. The results suggested that very mild AD participants have difficulty in learning and retaining words in the middle part of the word-list because of a functional decline of the central executive system.

Interference processes in monkey auditory list memory.

A rhesus monkey's memory was tested for single items and four-item lists of natural and environmental sounds. Memory items were presented from a center speaker, followed by a retention delay and then a choice response to a test sound presented simultaneously from two side speakers. Recognition of the last item of four-item lists was much poorer than that of single items at 0-, 1-, and 2-sec delays, despite there being the same temporal relations between study and test. This result showed that the first three items proactively interfered with memory of the last list item. Proactive interference dissipated after 2 sec, revealing a recency effect that eventually equaled single-item performance. Recognition of the first item of four-item lists was much poorer than single items at 20- and 30-sec delays, showing that the last three items retroactively interfered with memory of the first list item. The results point to the critical nature of interference processes in the understanding of serial position functions.

Forward and backward recall: different response time patterns, same retrieval order.

How do people retrieve information in forward and backward recall? To address this issue, we examined response times in directional recall as a function of serial position and list length. Participants memorized lists of four to six words and entered responses at the keyboard. Recall direction was postcued. Response times exhibited asymmetry in terms of direction. In forward recall, response times peaked at the first position, leveling off for subsequent positions. Response times were slower in backward recall than in forward recall and exhibited an inverse U-shaped function with an initial slowdown followed by a continuous speedup. These asymmetries have implications for theoretical models of retrieval in serial recall, including temporal-code, rule-based, and network models. The response time pattern suggests that forward recall proceeds in equal steps across positions, whereas backward recall involves repeated covert cycles of forward recall. Thus, retrieval in both directions involves a forward search.

Monkey auditory list memory: tests with mixed and blocked retention delays.

A rhesus monkey was tested in an auditory list memory task with blocked and mixed retention delays. Each list of four natural or environmental sounds (from a center speaker) was followed by a retention delay (0, 1, 2, 10, 20, or 30 sec) and then by a recognition test (from two side speakers). The monkey had been tested for 12 years in tasks with blocked delays. An earlier (4 years prior) blocked-delay test was repeated, with virtually identical results. The results from the mixed-delay test were likewise similar. Thus, the peculiarities of blocked-delay testing, such as delay predictability or differences in list spacing, apparently do not alter this monkey's memory for auditory lists. It is concluded from this and other evidence that the monkey's serial position functions reflect mnemonic processes that change with changes in retention delay and are not artifacts of the blocked-delay procedure. The nature of the monkey's auditory memory is discussed.

Dynamics of serial position change in probe-recognition task

Relationship between practice and serial position effects was investigated, in order to obtain more evidence for underlying short-term memory processes. The investigated relationship is termed the dynamics of serial position change. To address this issue, the present study investigated mean latency, errors, and performed Ex-Gaussian convolution analysis. In six-block trials the probe-recognition task was used in the so-called fast experimental procedure. The serial position effect was significant in all six blocks. Both primacy and recency effects were detected, with primacy located in the first two blocks, producing a non-linear serial position effect. Although the serial position function became linear from the third block on, the convolution analysis revealed a non-linear change of the normal distribution parameter, suggesting special status of the last two serial positions. Further, separation of convolution parameters for serial position and practice was observed, suggesting different underlying mechanisms. In order to account for these findings, a strategy shift mechanism is suggested, rather then a mechanism based on changing the manner of memory scanning. Its influence is primarily located at the very beginning of the experimental session. The pattern of results of errors regarding the dynamics of serial position change closely paralleled those on reaction times. Several models of short-term memory were evaluated in order to account for these findings.

Parkinsonian Patients Reduce Their Stroke Size with Increased Processing Demands

Parkinson's disease (PD) patients often show reductions in writing size (micrographia) as the length of the text they produce increases. The cause for these reductions in stroke size are not well understood. Reductions in stroke size could be associated with either concurrent processing demands that result from the coordination and control of fingers, wrist, and arm during writing and the processing of future words or increased extension of the wrist joint as the execution of the writing progresses to the right across the page, resulting in increased stiffness in the pen-limb system. Parkinson's patients and elderly controls wrote phrases of different lengths with target patterns in various serial positions. When the number of words to be written increased, PD patients reduced their stroke size of the initial target pattern, while the elderly controls did not reduce their stroke size. There was no systematic change in stroke size of the second pattern as function of serial position. This result suggests that PD patients reduce the size of their handwriting strokes when concurrent processing load increases.

Short-term memory for serial order in schizophrenia: A detailed examination of error types.

Cognitive deficits in schizophrenia have been associated with working memory problems. Schizophrenic patients (n = 24) and controls (n = 29) participated in simple short-term memory tasks, recalling a list of letters from the first to last item in the order of presentation. The authors hypothesized that deficient sequential representations would increase movement errors (e.g., ABCD being recalled as ABDC) or intrusion errors (e.g., ABCD being recalled as ABCX), whereas simple trace decay would lead to omission errors (e.g., ABCD being recalled as ABC_). Patients made disproportionately more omissions toward the end of 6-item lists. There were no group differences in movements or intrusions as a function of serial position. Schizophrenic patients' limited short-term memory span may be due to greater forgetting during recall and not to a selective deficit in the mechanisms responsible for maintaining serial order information.