Functional organization of spatial and nonspatial working memory processing within the human lateral frontal cortex

The present study used functional magnetic resonance imaging to demonstrate that performance of visual spatial and visual nonspatial working memory tasks involve the same regions of the lateral prefrontal cortex when all factors unrelated to the type of stimulus material are appropriately controlled. These results provide evidence that spatial and nonspatial working memory may not be mediated, respectively, by mid-dorsolateral and mid-ventrolateral regions of the frontal lobe, as widely assumed, and support the alternative notion that specific regions of the lateral prefrontal cortex make identical executive functional contributions to both spatial and nonspatial working memory.     

Medial septal lesions disrupt spatial, but not nonspatial, working memory in rats.

In Exp 1, rats with small medial septal lesions were less able than were control rats to remember the location of the arm of a Y maze they had been forced to enter on the preceding sample run. Moreover, as the retention interval between the sample and choice runs on this spatial delayed nonmatching-to-sample (DNMTS) task was increased to 1 and 2 min, the magnitude of the deficit increased. In contrast, these same lesioned rats were not deficient in Exp 2 in their ability to remember the object they had encountered in the straight alley on the sample run. In fact, when the retention interval was increased to 1 min on this nonspatial DNMTS task, the rats with medial septal lesions were more accurate than were the controls. This pattern of results did not appear to be due to task difficulty, recovery of function, or sequence of training. Rather, these results indicate that damage to the septohippocampal system disrupts spatial working memory more than it disrupts nonspatial working memory. (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Hippocampal and amygdaloid involvement in nonspatial and spatial working memory in rats: Effects of delay and interference.

Parametric manipulations of the task demand were used to examine the role of the hippocampus and amygdala in nonspatial and spatial working memory in male rats. Hippocampal lesions produced an immediate and long-lasting impairment of nonspatial working memory in an operant task. The memory deficits increased as the delay interval and the amount of proactive interference increased. Hippocampal lesions severely impaired spatial working memory in spatial alternation. Extensive postoperative testing reduced the magnitude of impairment of nonspatial but not spatial working memory. Amygdaloid lesions did not impair any aspect of performance in 2 tasks. The results suggest that the hippocampus, but not the amygdala, is involved in working memory and the task demand is a critical determinant for observing impairments of nonspatial working memory following hippocampal lesions. (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Hippocampal and amygdaloid involvement in working memory for nonspatial stimuli.

Hippocampal lesions in rats lead to an impairment of performance in spatial delayed conditional discriminations. The effect of such lesions on nonspatial tasks is controversial. In monkeys, both the hippocampus and the amygdala are involved in nonspatial delayed conditional discriminations. The effect of amygdaloid lesions in rats on this type of task has not been studied. To clarify the role of hippocampus and amygdala in a cue-relevant/space-irrelevant delayed conditional discrimination, rats were trained on a delayed match-to-sample task with visual and tactile cues as discriminative stimuli. Rats were then given one of five lesions: control, complete fimbria-fornix, partial fimbria-fornix, complete amygdala, or partial amygdala. Amygdaloid lesions, partial or complete, did not impair choice accuracy. Fimbria-fornix lesions did impair choice accuracy, and the magnitude and duration of the impairment was a function of the size of the lesion. Partial fimbria-fornix lesions produced a slight impairment that disappeared with continued testing. Complete fimbria-fornix lesions produced chance performance throughout postoperative testing. These results indicate that the fimbria-fornix, but not the amygdala, is involved in nonspatial delayed match-to-sample. (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Temporal isolation of the neural correlates of spatial mnemonic processing with fMRI

The use of cognitive subtraction to study the neural substrates of the maintenance component of spatial working memory in humans relies upon the assumptions of the pure insertion of cognitive processes and a linear transform of neural activity to neuroimaging signal. Here, functional changes attributable to the memory requiring phase (referred to as the retention delay) of a spatial working memory task were temporally discriminated from those attributable to other behavioral subcomponents within trials using an experimental design that is argued to obviate these assumptions, as well as permit a joint test of their validity. The hypothesis that the assumptions of cognitive subtraction (as applied to neuroimaging) hold in general was not supported. Functional changes attributable to the retention delay were detected in the dorsolateral prefrontal cortex as well as in other cortical regions in a subset of the subjects, and in the right frontal eye field and right superior parietal lobule of all subjects (n=5). These results support models in which these regions are involved in maintaining spatial representations in humans. In addition, nearly all regions that evidenced such functional changes during the retention delay also evidenced functional changes during behaviors that did not require spatial working memory. This result tends to dispute models which posit the existence of gross neuroanatomical regions involved in solely mnemonic function.     

Effects of ethanol on nonspatial working memory and attention in rats.

Investigation into the neural basis for ethanol-induced cognitive dysfunction requires the use of valid animal models. An operant signal detection procedure was developed to assess simultaneously the processes of sustained attention and working memory in rats, and to determine the effects of ethanol on these cognitive functions. Ethanol, at 0.75 g/kg intraperitoneal/ly (ip), produced delay- and stimulus length-dependent decreases in choice accuracy, effects that are consistent with deficits in both working memory and sustained attention. Local infusion of ethanol directly into the medial septal area resulted in a selective loss of choice accuracy at the long delay. The impairment by intraseptal ethanol did not interact with stimulus length. Thus, the working memory impairment, but not the decrement in sustained attention, was mimicked by intraseptal ethanol. The current model provides a foundation for studying the neural basis of ethanol's cognitive effects. (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Rehearsal in spatial working memory

This article reports 3 experiments that tested a hypothesis regarding the nature of rehearsal in spatial working memory, one in which discrete shifts of spatial selective attention mediate the maintenance of location-specific representations. Experiment 1 demonstrated increases in visual processing efficiency for locations held in working memory, which suggested that attention was oriented toward these locations. Experiment 2 eliminated key alternative explanations for Experiment 1 by using an identical stimulus display with a nonspatial memory task, and little or no facilitation of processing at memorized locations was found under these conditions. Finally, Experiment 3 showed that spatial working memory was impaired when participants were hindered in their ability to attend to memorized locations. It is argued that these results implicate selective spatial attention as a rehearsal mechanism for spatial working memory.     

A dissociation between perceptual explicit and implicit memory processes

Patient M.S., who underwent right-occipital lobe resection to treat intractable epilepsy, has intact recall and recognition memory for words, but impaired repetition priming in word identification and visual stem-completion tasks. This mirror dissociation to amnesia suggests that explicit recognition and visuoperceptual repetition priming are mediated by distinct neural systems. In prior studies, however, M.S.' recognition memory was tested only with tasks that drew upon his intact verbal knowledge. The present study examined M.S.' recognition memory for nonverbal perceptual information, namely, the modality and font of word presentation and line patterns. M.S.' recognition memory was intact, providing further evidence that perceptual explicit and implicit memory processes are subserved by functionally and neurally independent memory systems.     

Coordination processes in visual working memory

Starting with a discussion on the coordination problem from different points of view within cognition the question is raised whether there is a dissociation of coordination demand of a task and the mental load for executing mental procedures. Three investigations are carried out with the paradigm of sequential mental transformations of patterns. Results of the first experiment show that coordination really requires additional mental effort beside the time necessary for mental transformation. In a further study the transfer of a coordination mechanism is proved. The influence of segmented information presentation demonstrates that amount of coordination is related to the stability and clarity of mental representations. Plans for further experiments are presented. The results are related to the current literature on working memory research.     

The mnemonic value of perceptual identification.

In four experiments, subjects were required to name words presented on a CRT screen. On generate trials, the words were presented quickly, at a point where roughly half could be identified correctly; on read trials, the items were presented for a full second, allowing for rapid and easy naming. A surprise recognition test for the presented items then revealed a substantial retention advantage for the briefly presented items, but no similar advantage was produced in recall. It is argued that under rapid viewing conditions subjects may fail to extract enough visual features to allow for immediate resolution, requiring the initiation of a kind of data-driven generation process. This latter process then produces a generation effect for the briefly presented items compared with the read items, but only on a retention test that shows sensitivity to data-driven processing. These results are discussed from the standpoint of current theoretical views on the generation effect. (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

On the relationship between recognition familiarity and perceptual fluency: evidence for distinct mnemonic processes

Fluent reprocessing of perceptual aspects of recently experienced stimuli is thought to support repetition priming effects on implicit perceptual memory tests. Although behavioral and neuropsychological dissociations demonstrate that separable mnemonic processes and neural substrates mediate implicit and explicit test performance, dual-process theories of memory posit that explicit recognition memory judgments may be based on familiarity derived from the same perceptual fluency that yields perceptual priming. Here we consider the relationship between familiarity-based recognition memory and implicit perceptual memory. A select review of the literature demonstrates that the fluency supporting implicit perceptual memory is functionally and anatomically distinct from that supporting recognition memory. In contrast to perceptual fluency, recognition familiarity is more sensitive to conceptual than to perceptual processing, and does not depend on modality-specific sensory cortices. Alternative possible relationships between familiarity in explicit memory and fluency in implicit memory are discussed.     

Performance of four-seed caching corvid species in operant tests of nonspatial and spatial memory.

The performance of 4 seed-caching corvid species was tested using 2 different operant nonmatching tasks. These species differ in their dependence on stored food, and differences in spatial memory tests have been correlated with better performance by the more cache-dependent species. Acquisition and retention of a color non-matching-to-sample task was tested in Experiment 1. Acquisition of the color task was not correlated with cache dependence, and no differences between species in performance during memory testing were found. Acquisition and retention of an operant spatial non-matching-to-sample task was tested in Experiment 2. Species differences in the spatial task were found for acquisition and during retention testing. The influence of natural history on the evolution of memory is discussed. (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Sex differences in prefrontal cortical brain activity during fMRI of auditory verbal working memory.

Functional imaging studies of sex effects in working memory (WMEM) are few, despite significant normal sex differences in brain regions implicated in WMEM. This functional MRI (fMRI) study tested for sex effects in an auditory verbal WMEM task in prefrontal, parietal, cingulate, and insula regions. Fourteen healthy, right-handed community subjects were comparable between the sexes, including on WMEM performance. Per statistical parametric mapping, women exhibited greater signal intensity changes in middle, inferior, and orbital prefrontal cortices than men (corrected for multiple comparisons). A test of mixed-sex groups, comparable on performance, showed no significant differences in the hypothesized regions, providing evidence for discriminant validity for significant sex differences. The findings suggest that combining men and women in fMRI studies of cognition may obscure or bias results. (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Temporal limits of spatial working memory in humans

An essential feature attributed to working memory is the labile and transient nature of its representations. Using an oculomotor task, we examined the stability of spatial working memory in 16 normal human subjects. Eye movements towards remembered spatial cues (memory-guided saccades) were electro-oculographically recorded after memorization delays that varied unpredictably between 0.5 and 30s. A peaked time-course of saccadic targeting errors, with maximal errors around 20s delay, was found, showing that delay-dependent decay of spatial information in working memory occurs, but is time-limited and reverts significantly beyond delays of about 20s. These data (i) indicate temporal limits of spatial working memory and (ii) provide the first behavioural evidence for the existence of two parallely generated mental representations of space that successively control memory-guided behaviour in humans.     

The comparison of visual working memory representations with perceptual inputs.

The human visual system can notice differences between memories of previous visual inputs and perceptions of new visual inputs, but the comparison process that detects these differences has not been well characterized. In this study, the authors tested the hypothesis that differences between the memory of a stimulus array and the perception of a new array are detected in a manner that is analogous to the detection of simple features in visual search tasks. That is, just as the presence of a task-relevant feature in visual search can be detected in parallel, triggering a rapid shift of attention to the object containing the feature, the presence of a memory–percept difference along a task-relevant dimension can be detected in parallel, triggering a rapid shift of attention to the changed object. Supporting evidence was obtained in a series of experiments in which manual reaction times, saccadic reaction times, and event-related potential latencies were examined. However, these experiments also showed that a slow, limited-capacity process must occur before the observer can make a manual change detection response. (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Two dissociable updating processes in working memory.

The authors show that the updating of working memory (WM) representations is carried out by the cooperative act of 2 dissociable reaction time (RT) components: a global updating process that provides stability by shielding WM contents against interference and a local process that provides flexibility. Participants kept track of 1?3 items (digits or Gibson figures). In each trial, the items either were similar to those in the previous trial or were different in any or all of the items. Experiments 1 and 2 established the existence of 2 independent RT components representing the 2 updating processes. Global updating cost was sensitive to total number of items in WM (set size), regardless of the number of items that actually were modified. Local updating cost was sensitive to the number of modified items, regardless of the set size. Experiment 3 showed that participants had to dismantle the representation formed by previous global updating in order to carry out new updating. (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Functional neuroanatomy of spatial working memory in children.

Functional magnetic resonance imaging (fMRI) was used to examine spatial working memory in 8- to 11-year-old children tested under three conditions. In the visual condition, children were asked to examine the location of a dot on a screen. In the motor condition, children were instructed to push a button that corresponded to the location of a dot presented on a screen. In the memory condition, children were asked to remember the location of a dot presented 1 or 2 trials previously. Subtracting the activation of the motor condition from the memory condition revealed activity in the dorsal aspects of the prefrontal cortex and in the posterior parietal and anterior cingulate cortex. These findings were also obtained in the analysis of the memory minus visual conditions except that motor cortex activation was also observed. These findings parallel those reported in comparable studies of adults and suggest that fMRI may be a useful means of examining function–structure relations in developmental populations. (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Age-related deficits in component processes of working memory.

Working memory deficits in normal aging have been well documented, and studies suggest that high memory load plus the presence of distraction negatively impacts successful memory performance to a greater degree in older individuals. However, characterization of the component processes that are impaired by these task manipulations is not clear. In this behavioral study, younger and older subjects were tested with a delayed-recognition and recall task in which the encoding and delay period were both manipulated. During the encoding period, the subjects were presented with either a single letter or multiple letters at their predetermined forward letter span, and the delay period was either uninterrupted or interrupted with a visual distraction. There was an age-related impairment of working memory recognition accuracy only in the combination of high memory load and distraction. These results suggest that when working memory maintenance systems are taxed, faulty recognition processes may underlie cognitive aging deficits in healthy older individuals. (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Preexposure effects in the spatial domain: Dissociation between latent inhibition and perceptual learning.

In 4 experiments, rats were given intermixed or blocked preexposure to an array of landmarks that subsequently defined the location of a hidden goal in a Morris pool task. Previous research has shown that intermixed preexposure to pairs of adjacent landmarks retards learning whereas preexposure to individual landmarks facilitates subsequent learning (J. Prados, V. D. Chamizo, & N. J. Mackintosh, 1999). Accordingly, in Experiment 1, intermixed and blocked preexposure to pairs of adjacent landmarks was found to retard learning. In Experiment 2, however, a scheduling effect was found: Rats given intermixed preexposure to the individual landmarks learned faster than rats given blocked or no preexposure. Experiment 3 showed that intermixed (but not blocked) preexposure to pairs of landmarks resulted in a facilitatory effect when preexposure and test were carried out in different contexts. Experiment 4 replicated within a single experiment the main results observed in Experiments 1 and 3. This pattern of results suggests that intermixed preexposure engages learning processes other than latent inhibition that facilitate subsequent learning of the navigation task. (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Dissociations between familiarity processes in explicit recognition and implicit perceptual memory.

Dual-process theories of recognition posit that a perceptual familiarity process contributes to both explicit recognition and implicit perceptual memory. This putative single familiarity process has been indexed by inclusion–exclusion, remember–know, and repetition priming measures. The present studies examined whether these measures identify a common familiarity process. Familiarity-based explicit recognition (as indexed by the inclusion–exclusion and the independence remember–know procedures) increased with conceptual processing. In contrast, implicit word-identification priming and familiarity-based word-stem completion (as indexed by inclusion–exclusion) increased with study–test perceptual similarity. These dissociations indicate that familiarity-based explicit recognition may be more sensitive to conceptual than to perceptual processing and is functionally distinct from the perceptual familiarity process mediating implicit perceptual memory. (PsycINFO Database Record (c) 2010 APA, all rights reserved)