Probe Letter Research Articles

Age-Related Declines in Context Maintenance and Semantic Short-Term Memory

This study reports age-related declines in context maintenance (Braver et al., 2001) and semantic short-term memory (STM) and evidencc for a relation between the two. A group of younger and older adults completed a context maintenance task (AX-CPT), a semantically oriented STM task (conceptual span), a phonologically oriented STM task (digit span), and a meaning integration task (semantic anomaly judgement). In the AX-CPT task, a target response is required to the probe letter "X" but only when it is preceded by the letter "A" (the context). Either three (short interference) or six distractor letters (long interference) were presented between the cue and the probe. Results indicated an age-related deficit in context maintenance. Age-related declines were also observed for conceptual span and semantic anomaly judgement but not for digit span. Context maintenance was correlated with conceptual span and semantic anomaly judgement but not with digit span. These correlations were largely mediated by age differences, which also explained variance that was unique to (and not shared among) context maintenance, conceptual span, and semantic anomaly judgement.

An event-related fMRI study of the neurobehavioral impact of sleep deprivation on performance of a delayed-match-to-sample task

Eighteen subjects (ages 18–35) underwent event-related functional magnetic resonance imaging (efMRI) while performing a delayed-match-to-sample (DMS) task before and immediately after 48 h of sustained wakefulness. The DMS trial events were: a 3-s study period of either a one-, three-, or six-letter visual array; a 7-s retention interval; and a 3-s probe period, where a button press indicated whether the probe letter was in the study array. Ordinal Trend Canonical Variates Analysis (OrT CVA) was applied to the data from the probe period for trials with six-letter study lists prior to and immediately following sleep deprivation to find an activation pattern whose expression decreased with sleep deprivation in as many subjects as possible, while being present in both conditions. The first principal component of the OrT analysis identified a covariance pattern whose expression decreased as a function of sleep deprivation in 17 of 18 subjects ( p<0.001). While overall expression of the pattern showed a systematic decrease with sleep deprivation, the brain regions that make up the pattern show covarying increases and decreases in activation. Regions that decreased their activation were noted in the parietal (BA 7 and 40), temporal (BA 37, 38 and 39) and occipital (BA 18 and 19) lobes; regions that increased their activation were noted in the cerebellum, basal ganglia, thalamus and the anterior cingulate gyrus (BA 32). The reduction in pattern expression with sleep deprivation for each subject was related to the change in performance on the DMS task. Subject decreases in pattern expression were correlated with reductions in recognition accuracy ( p<0.05), increased intra-individual variability in reaction time ( p<0.005) and increased lapsing ( p<0.005).

Visual memory for shape-colour conjunctions utilizes structural descriptions of letter shape

Visual recognition memory for shape-colour associations is superior when the colours were originally perceived to belong to the shapes, rather than to the backgrounds against which the shapes appeared. Recognition is at chance in the latter case. In two experiments, the nature of the shape representations supporting visual memory for shape-colour conjunctions is examined. Letters are used as to-be-remembered shapes and the impact of two types of change in their appearance as recognition probes is assessed. When letters reappear in the same case, recognition performance is unaffected by a change in font (Experiment 1). However, when letters reappear in the same font, recognition performance drops to chance if letter case is changed (Experiment 2). The contrast between these two types of change was not confounded with differences in the visual similarity of the memory and probe letters. It seems that colour is linked to structural descriptions of letter shape in visual memory. The relationship between these descriptions and other forms of representation, including abstract representations of letter identity, is discussed.

Frontal activations associated with accessing and evaluating information in working memory: an fMRI study

To investigate the involvement of frontal cortex in accessing and evaluating information in working memory, we used a variant of a Sternberg paradigm and compared brain activations between positive and negative responses (known to differentially tax access/evaluation processes). Participants remembered two trigrams in each trial and were then cued to discard one of them and maintain the other one as the target set. After a delay, a probe letter was presented and participants made decisions about whether or not it was in the target set. Several frontal areas—anterior cingulate (BA32), middle frontal gyrus (bilateral BA9, right BA10, and right BA46), and left inferior frontal gyrus (BA44/45)—showed increased activity when participants made correct negative responses relative to when they made correct positive responses. No areas activated significantly more for the positive responses than for the negative responses. It is suggested that the multiple frontal areas involved in the test phase of this task may reflect several component processes that underlie more general frontal functions.

Frontal activations associated with accessing and evaluating information in working memory: an fMRI study

To investigate the involvement of frontal cortex in accessing and evaluating information in working memory, we used a variant of a Sternberg paradigm and compared brain activations between positive and negative responses (known to differentially tax access/evaluation processes). Participants remembered two trigrams in each trial and were then cued to discard one of them and maintain the other one as the target set. After a delay, a probe letter was presented and participants made decisions about whether or not it was in the target set. Several frontal areas—anterior cingulate (BA32), middle frontal gyrus (bilateral BA9, right BA10, and right BA46), and left inferior frontal gyrus (BA44/45)—showed increased activity when participants made correct negative responses relative to when they made correct positive responses. No areas activated significantly more for the positive responses than for the negative responses. It is suggested that the multiple frontal areas involved in the test phase of this task may reflect several component processes that underlie more general frontal functions.

Dissociable neural mechanisms underlying response-based and familiarity-based conflict in working memory.

Cognitive control requires the resolution of interference among competing and potentially conflicting representations. Such conflict can emerge at different points between stimulus input and response generation, with the net effect being that of compromising performance. The goal of this article was to dissociate the neural mechanisms underlying different sources of conflict to elucidate the architecture of the neural systems that implement cognitive control. By using functional magnetic resonance imaging and a verbal working memory task (item recognition), we examined brain activity related to two kinds of conflict with comparable behavioral consequences. In a trial of our item-recognition task, participants saw four letters, followed by a retention interval, and a probe letter that did or did not match one of the letters held in working memory (positive probe and negative probe, respectively). On some trials, conflict arose solely because of the current negative probe having a high familiarity, due to its membership in the immediately preceding trial's target set. On other trials, additional conflict arose because of the current negative probe having also been a positive probe on the immediately preceding trial, producing response-level conflict. Consistent with previous work, conflict due to high familiarity was associated with left prefrontal activation, but not with anterior cingulate activation. The response-conflict condition, when compared with high-familiarity conflict trials, was associated with anterior cingulate cortex activation, but with no additional left prefrontal activation. This double dissociation points to differing contributions of specific cortical areas to cognitive control, which are based on the source of conflict.

Interhemispheric Integration of Letter Stimuli Presented Foveally or Extra-Foveally

Two experiments are reported in which participants decided whether a single target letter presented below and to the left or right of fixation matched either of two probe letters presented above the fixation cross to the left and right. The level of matching required was either physical (A-A) or abstract (A-a). All three letters were presented either extra-foveally (Experiment 1) or within the fovea (Experiment 2). In both experiments, physical matching was faster than abstract matching. Physical matching was faster within than across visual fields while abstract matching showed the opposite pattern. Matching was faster when the matching probe letter was in the LVF than when it was in the RVF. Importantly, the pattern of results was the same for extra-foveal and foveal presentations, supporting the theory that the representation of the fovea is split down the middle into two visual fields rather then being bilateral. The practical implication following this study is that lateralization studies can be performed with closer to fixation stimuli presentation.

Temporal processing of global and local information varies with global precedence

The concept of global precedence, which suggests that the global aspect of a scene is processed more rapidly than local details, was examined using the attentional blink paradigm. Eighteen adult subjects observed multiple sequences of complex global-local letter figures to see whether the attentional blink duration would be affected by the visual angle size of the stimulus. Within each sequence, the subject was directed to identify either a global or local red target letter and to detect whether a global or local probe letter (X) was presented in the sequence following the target letter. Stimuli were presented at three different sizes. Results showed significantly higher probe detection rates for global probes than for local at small stimulus sizes. However, using large stimulus sizes, mean correct probe detection was significantly higher in conditions requiring local attention compared to global. No significant difference in probe detection performance was observed between global and local conditions at medium stimulus sizes. The results suggest that the rate of visual information processing varies according to the visual angle of the particular information. The results support the suggestion that the precedence of information is an important factor in the temporal processing of global-local information.

No negative semantic priming from unconscious flanker words in sight.

In replicating 1 of the within-language conditions of E. Fox's (1996) Experiment 1, the authors confirmed that unattended words presented 2.4 degrees above and below fixation are mostly unavailable to awareness. However, no negative semantic priming was observed in a lexical decision on a probe letter string appearing about 1 s later, which does not replicate Fox's finding. These results are compatible with the hypothesis underlying the present study, according to which positive semantic priming, if any, rather than negative semantic priming is expected in Fox's situation. The reason is that unavailability to awareness of the parafoveal words is not achieved by means of an act of selective inhibition combined with attentional diversion through masking but is achieved simply by means of perceptual degradation.

Age differences in prefrontal cortical activity in working memory.

Working memory (WM) declines with advancing age. Brain imaging studies indicate that ventral prefrontal cortex (PFC) is active when information is retained in WM and that dorsal PFC is further activated for retention of large amounts of information. The authors examined the effect of aging on activation in specific PFC regions during WM performance. Six younger and 6 older adults performed a task in which, on each trial, they (a) encoded a 1- or 6-letter memory set, (b) maintained these letters over 5-s. and (c) determined whether or not a probe letter was part of the memory set. Comparisons of activation between the 1- and 6-letter conditions indicated age-equivalent ventral PFC activation. Younger adults showed greater dorsal PFC activation than older adults. Older adults showed greater rostral PFC activation than younger adults. Aging may affect dorsal PFC brain regions that are important for WM executive components.

Order information in working memory: fMRI evidence for parietal and prefrontal mechanisms.

Working memory is thought to include a mechanism that allows for the coding of order information. One question of interest is how order information is coded, and how that code is neurally implemented. Here we report both behavioral and fMRI findings from an experiment involved comparing two tasks, an item-memory task and an order-memory task. In each case, five letters were presented for storage, followed after a brief interval by a set of probe letters. In the case of the item-memory task, the two letters were identical, and the subject responded to the question, "Was this letter one of the items you saw?". In the case of the order-memory task, the letters were different, and subjects responded to the question, "Are these two letters in the order in which you saw them?". Behaviorally, items that were further apart in the sequence that elicited faster reaction times and higher accuracy in the Order task. Areas that were significantly more activated in the Order condition included the parietal and prefrontal cortex. Parietal activations overlapped those involved in number processing, leading to the suggestion that the underlying representation of order and numbers may share a common process, coding for magnitude.

A distinction between the initiation and the continuation of response preparation

Previous findings suggest that motoric response preparation cannot be initiated in parallel with memory scanning. In the present study, response preparation was initiated with the aid of a precue to examine whether such preparation can be maintained or continued while memory scanning is active. In Experiment 1, each trial began with a colored square indicating which hand might be needed to respond. A probe letter's memory set membership determined whether the primed response should be made or withheld. Lateralized readiness potentials were initiated by the square precue and continued to increase after letter presentation, suggesting that once response preparation had been initiated it was continued in parallel with memory scanning. Experiment 2 suggested that the difficulty of the concurrent memory search had little effect on the continuation of response preparation. The results support the view that motoric response preparation consists of at least two qualitatively distinct phases--initiation and continuation.

Load-Dependent Roles of Frontal Brain Regions in the Maintenance of Working Memory

Brain imaging studies have suggested a critical role for prefrontal cortex in working memory (WM) tasks that require both maintainenance and manipulation of information over time in delayed-response WM tasks. In the present study, functional magnetic resonance imaging (fMRI) was used to examine whether prefrontal areas are activated when only maintenance is required in a delayed-response WM task, without the overt requirement to manipulate the stored information. In two scans, six subjects performed WM tasks in which, on each trial, they (1) encoded 1, 3, or 6 to-be-remembered letters, (2) maintained these letters across a 5-second unfilled delay, and (3) determined whether a single probe letter was or was not part of the memory set. Activation of left caudal inferior frontal gyrus was observed, relative to the 1-letter task, when subjects were required to maintain 3 letters in WM. When subjects were required to maintain 6 letters in WM, additional prefrontal areas, most notably middle and superior frontal gyri, were activated bilaterally. Thus, increasing the amount of to-be-maintained information, without any overt manipulation requirement, resulted in the recruitment of wide-spread frontal-lobe regions. Inferior frontal gyrus activation was left-hemisphere dominant in both the 3- and 6-letter conditions, suggesting that such activation reflected material-specific verbal processes. Activation in middle and superior frontal gyri appeared only in the 6-letter condition and was right-hemisphere dominant, suggesting that such activation reflected material-independent executive processes.

Age Differences in Entropy: Primary Versus Secondary Memory

We report a spatial - memory scanning experiment that was used to measure age differences in entropy . A target grid consisting of four adjacent letters followed by the presentation of a single probe letter was presented on each trial. Half of the trials presented the probe stimulus in the same spatial position was the target letter (i.e., the probe letter was always a member ofthe positive set), and half of the trails transposed the target letter one, two , or three spaces of the right or left of the original target display position (i.e., different trials). The experiment involved blocks of primary - memory and secondary - memory tasks . Reaction - time and error - rate data , as well as entropy analyses and the fitting of an entropy model ( based on Allen , Kaufman , Smith , and Propper , in press ) to the empirical data indicated that older adults showed higher entropy levels than young adults. These results are interpreted in a "computa tional temperature" framework in which older adults' higher computa tional temperatures result in less efficient spatial , episodic memory functioning .

Inhibition in verbal working memory revealed by brain activation.

There are many occasions in which humans and other animals must inhibit the production of some behavior or inhibit the processing of some internal representation. Success in inhibitory processing under normal circumstances can be revealed by the fact that certain brain pathologies render inhibitory processing ineffective. These pathologies often have been associated with damage to frontal cortex, including lateral and inferior aspects. We provide behavioral evidence of a verbal working memory task that, by hypothesis, engaged inhibitory processing, and we show (by using positron emission tomograpny) that the inhibitory processing is associated with a lateral portion of the left prefrontal cortex. The task in which subjects engaged was item-recognition: Four target letters were presented for storage followed, after a brief interval, by a probe letter that could match a target letter or not. On some trials, when the probe did not match a target letter and therefore required a "no" response, the probe had matched a target letter of the previous trial, so on these trials a "yes" response was prepotent and had to be inhibited, by hypothesis. Compared with a condition in which no prepotent response was created, this condition yielded brain activation in left inferior frontal gyrus, in the region of Brodmann's area 45.

Bilateral stimulus presentation in memory scanning.

Abstract Participants performed a memory-scanning task (Steinberg, 1966, 1969a) in which probe letters were displayed unilaterally or bilaterally after sets of two, four, or six letters were memorized. The mean response time (RT) to bilateral presentations was significantly longer than the mean RT to unilateral presentations, but the slope of the set-size function was not affected, suggesting that presenting stimuli bilaterally affected stages other than memory scanning. There were no significant visual-field effects in either the bilateral or unilateral conditions, suggesting that memory scanning is not a lateralized process. There was no evidence that bilateral presentation increased visual-field differences. This is not consistent with Boles' (1983, 1990) hypothesis that visual-field asymmetry effects are more pronounced with bilateral than with unilateral presentation of stimuli. Two methods are often used to assess visual-field differences in perception. One is the unilateral method, which involves presenting a single stimulus either in the left or the right visual field. A less frequently used method is bilateral presentation of stimuli, which involves presenting two different stimuli, one in each visual field, with an indication to the subject as to which stimulus to respond to. A response time (RT) or accuracy advantage for a visual field is then taken to indicate contralateral hemispheric specialization for the task. Boles (1983, 1990) has claimed that bilateral presentations of stimuli produce visual-field asymmetries larger than those of unilateral presentations. His explanation for this effect is that bilateral stimulation effectively disrupts communication between the two hemispheres, preventing information transfer. If information cannot be transferred between the hemispheres, each hemisphere's capability can be assessed more accurately, and any hemispheric differences would be accentuated. In the experiment reported here, subjects performed a memory-scanning task (Sternberg, 1966, 1969a) in which probe stimuli were presented both unilaterally and bilaterally. In a typical memory-scanning task, subjects memorize a set of between one and seven items and then decide whether a probe item is in the memorized list. Sternberg found that RT increased linearly with the number of items in the set and that the slope of this function was the same for Yes and No responses, which was taken to suggest that subjects perform an exhaustive, serial memory scan. Sternberg (1966) elaborated his memory-scanning theory by differentiating among four stages of processing: stimulus encoding, memory scanning, response selection, and response execution. The zero-intercept of the linear set size function is a measure of the total duration of all processes that occur just once, regardless of the size of the memory set -- that is, stimulus encoding, response selection, and execution. The slope, on the other hand, measures the rate at which members of the memory set are scanned. Introducing a new factor in a memory-scanning experiment could influence the RT function in two ways. It could influence the memory-scanning stage only, reflected in a change in slope and not in the zero-intercept. Alternatively, it might influence the stimulus encoding, response selection or execution stages, altering the zero-intercept and not the slope. The aim of the present experiment was to investigate the effect of bilateral stimulus presentation on memory scanning. Two different aspects were of interest. The first had to do with the effects that bilateral as opposed to unilateral presentation has on the different stages of a memory-scanning task. If bilateral and unilateral presentation have different effects on the memory-scanning stage, it would be reflected in a difference in slopes for unilateral and bilateral presentations. Alternatively, if presenting stimuli bilaterally only had an effect on stages other than memory scanning, there would be only an intercept difference between unilateral and bilateral presentations. …

Semantic priming in visual word recognition: Activation blocking and domains of processing

The fact that letter search on a prime eliminates the typically robust semantic priming effect in lexical decision is often attributed to the "shallowness" of the prime-processing task. In three experiments we investigated this claim by using two different "shallow" prime-processing tasks: letter search and color identification. Consistent with previous reports, lexical decisions to semantically related targets were not facilitated when subjects searched the prime for a probe letter. In contrast, semantic priming was observed following a color discrimination task on the prime. We suggest that a levels-of-processing interpretation is not an adequate framework for understanding these data. Instead, a domain-specific processing account is offered in which explicit processing at the letter level (as in letter search) makes demands on resources (e.g., activation) that drives processing at the semantic level. This competition is resolved by establishing a temporary activation block at the lexical-semantic interface, which results in the elimination or attenuation of semantic priming. In contrast, global judgment of color is viewed as a domain that does not make demands on the resources that drive the visual word recognition machinery. There is therefore no need for an activation block, and semantic priming is not prevented.

Search for letter identity and location by disabled readers.

Reading-disabled boys, reading- and age-matched controls, and adults searched letter arrays for the identity or location of a probe letter. Response time (RT) and accuracy were examined as a function of the temporal relation between probe and array letters (probe first, simultaneous, array first), and array size (1-5 letters). Although disabled readers closely resembled age controls in RT, their accuracy differed significantly when large letter arrays were tested. In the letter identification task, this was only evident when the array letters preceded the probe; in the letter location task, it occurred in all three probe conditions. Correlational analyses showed that all subjects were influenced by the visual, but not the phonological, similarity between letters. Thus, a reading-related impairment is evident in both letter identification and letter location processes, even when the phonological coding of letters has been minimized.

On Age Differences in Processing Variability and Scanning Speed

This investigation examined adult age differences on a recognition memory taks for order information. Experiment 1 consisted of a modified memory-scanning task in which a memory set of four letters was presented in boxes. Subjects judged whether a subsequent probe letter was or was not transposed from its original presentation position. One block of primary memory trials (i.e., no distractor task) and one block of secondary memory trials (i.e., a 10-second distractor task) were used. Experiment 2 used basically the same task, except both blocks of trials were primary memory tasks, and acoustic similarity was manipulated. A processing variability model predicted that both experiments should show distance effects (i.e., transposing letters a distance of one box should result in more errors and/or longer reaction times than a transposition of two, or three boxes) for both right and left transposition directions. However, a generalized slowing model predicted that distance effects should only occur for left transpositions (because of left-to-right scanning order). The present data supported the processing variability model but not the generalized slowing model.

Effect of imagery ability on letter-level and word-level processing

In a letter-identification task, subjects matched a probe letter to the initial letter of a subsequently presented probe word. We varied word frequency and predicted that the performance of vivid imagers would resemble that of typical young adults, whereas the performance of poor imagers would fall in between that of typical young adults and of older adults, or would simply resemble the performance of typical older adults (a linear decrease in reaction time [RT] with word frequency; see Allen & Madden, 1989). The in-between function for young and older adults combined predicts a dip in RT for very-high-frequency words compared to medium-high-, low-, and very-low-frequency words. As predicted, vivid imagers exhibited increased latencies for medium-high-frequency words relative to the other three word-frequency categories, whereas poor imagers exhibited a dip.