Christy Marshuetz

The Role of Parietal Cortex in Verbal Working Memory

Neuroimaging studies of normal subjects and studies of patients with focal lesions implicate regions of parietal cortex in verbal working memory (VWM), yet the precise role of parietal cortex in VWM remains unclear. Some evidence (Paulesu et al., 1993; Awh et al., 1996) suggests that the parietal cortex mediates the storage of verbal information, but these studies and most previous ones included encoding and retrieval processes as well as storage and rehearsal of verbal information. A recent positron emission tomography (PET) study by Fiez et al. (1996) isolated storage and rehearsal from other VWM processes and did not find reliable activation in parietal cortex. This result suggests that parietal cortex may not be involved in VWM storage, contrary to previous proposals. However, we report two behavioral studies indicating that some of the verbal material used by Fiez et al. (1996) may not have required phonological representations in VWM. In addition, we report a PET study that isolated VWM encoding, retrieval, and storage and rehearsal processes in different PET scans and used material likely to require phonological codes in VWM. After subtraction of appropriate controls, the encoding condition revealed no reliable activations; the retrieval condition revealed reliable activations in dorsolateral prefrontal, anterior cingulate, posterior parietal, and extrastriate cortices, and the storage condition revealed reliable activations in dorsolateral prefrontal, inferior frontal, premotor, and posterior parietal cortices, as well as cerebellum. These results suggest that parietal regions are part of a network of brain areas that mediate the short-term storage and retrieval of phonologically coded verbal material.

Effects of frontal lobe damage on interference effects in working memory

Working memory is hypothesized to comprise a collection of distinct components or processes, each of which may have a unique neural substrate. Recent neuroimaging studies have isolated a region of the left inferior frontal gyrus that appears to be related specifically to one such component: resolving interference from previous items in working memory. In the present study, we examined working memory in patients with unilateral frontal lobe lesions by using a modified version of an item recognition task in which interference from previous trials was manipulated. In particular, we focused on patient R.C., whose lesion uniquely impinged on the region identified in the neuroimaging studies of interference effects. We measured baseline working memory performance and interference effects in R.C. and other frontal patients and in age-matched control subjects and young control subjects. Comparisons of each of these groups supported the following conclusions. Normal aging is associated with changes to both working memory and interference effects. Patients with frontal damage exhibited further declines in working memory but normal interference effects, with the exception of R.C., who exhibited a pronounced interference effect on both response time and accuracy. We propose that the left inferior frontal gyrus subserves a general, nonmnemonic function of selecting relevant information in the face of competing alternatives and that this function may be required by some working memory tasks.

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 that 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 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.

Facial attractiveness is appraised in a glance.

Those who are physically attractive reap many benefits--from higher average wages to a wider variety of mate choices. Recent studies have investigated what constitutes beauty and how beauty affects explicit social judgments, but little is known about the perceptual or cognitive processing that is affected by aesthetic judgments of faces and why beauty affects our behavior. In this study, the authors show that beauty is perceived when information is minimized by masking or rapid presentation. Perceiving and processing beauty appear to require little attention and to bias subsequent cognitive processes. These facts may make beauty difficult to ignore, possibly leading to its importance in social evaluations.

Working Memory for Complex Scenes: Age Differences in Frontal and Hippocampal Activations

Age differences in frontal and hippocampal activations in working memory were investigated during a maintenance and subsequent probe interval in an event-related fMRI design. Younger and older adults either viewed or maintained photographs of real-world scenes (extended visual or maintenance conditions) over a 4-sec interval before responding to a probe fragment from the studied picture. Behavioral accuracy was largely equivalent across age and conditions on the probe task, but underlying neural activations differed. Younger but not older adults showed increased left anterior hippocampal activations in the extended visual compared with the maintenance condition. Onthesubsequent probeinterval, however, older adultsshowed more left and right inferior frontal activations than younger adults. The increased frontal activations at probe in older adults may have been compensatory for the decreased hippocampal activations during maintenance, but alternatively could have reflected the increased difficulty of the probe task for the older subjects. Thus, we demonstrate qualitatively different engagement of both frontal and hippocampal structures in older adults in a working memory task, despite behavioral equivalence.

Order information in working memory : An integrative review of evidence from brain and behavior

Evidence about memory for order information comes from a number of different methodologies: human cognition, patient studies, neuroimaging studies, and animal lesion and behavioral studies. The present article discusses (a) evidence that order and item memory are separable; (b) proposed mechanisms for order memory (interitem associations, direct codes, hierarchical codes, feature codes, and magnitude codes) and evidence for each; (c) evidence for the neural substrates of order memory from patient, neuroimaging, and animal lesion and single-cell recording studies; (d) barriers to integration between the disciplines; and (e) suggestions for better coordination of efforts between the different disciplines.

Remembering “what” brings along “where” in visual working memory

Does a behavioral and anatomical division exist between spatial and object working memory? In this article, we explore this question by testing human participants in simple visual working memory tasks. We compared a condition in which there was no location change with conditions in which absolute location change and absolute plus relative location change were manipulated. The results showed that object memory was influenced by memory for relative but not for absolute location information. Furthermore, we demonstrated that relative space can be specified by a salient surrounding box or by distractor objects with no touching surfaces. Verbal memory was not influenced by any type of spatial information. Taken together, these results indicate that memory for “where” influences memory for “what.” We propose that there is an asymmetry in memory according to which object memory always contains location information.

Working memory for order and the parietal cortex: An event-related functional magnetic resonance imaging study

Memory for order information has been tied to the frontal lobes, however, parietal activation is observed in many functional neuroimaging studies. Here we report functional magnetic resonance findings from an event-related experiment involving working memory for order. Five letters were presented for storage, followed after a delay by two probe items. Probe items could be separated by zero to three positions in the memory set and subjects had to indicate whether the items were in the correct order. Analyses indicate that activation in left parietal cortex shows a systematic decrease in activation with increasing probe distance. This finding is consistent with an earlier study in which we suggested that parietal cortical regions mediate the representation of order information via magnitude codes.

Working memory for order information: Multiple cognitive and neural mechanisms

Working memory for order information is mediated by different cognitive mechanisms that rely on different neural circuits. Here we discuss evidence that order memory involves mechanisms that range from general supervisory processes to process that maintenance fine-grained temporal position information. We suggest that neural regions-including the prefrontal cortex, motor cortex, parietal cortex and medial temporal structures-operate at different levels and processing stages to give rise to working memory for order information.

Functional Neuroimaging and the Prefrontal Cortex: Organization by Stimulus Domain?

Working memory is the set of cognitive operations that maintains and processes information “on-line”. It has been characterized both as a mental workspace (Baddeley, 1986) and as a set of operations that allow the efficient allocation of cognitive resources (Carpenter et al., 1990, 1999). Working memory typically is thought to be of limited capacity, between 4–7 items (Miller, 1956; Cowan, 2000), of limited duration, on the order of seconds (Peterson and Peterson, 1959) and as involving a number of separable sub-mechanisms, among these, rehearsal processes, domain-specific storage buffers, and a set of executive processes that are thought to operate on currently active information (Baddeley, 1986; Smith et al., 1996). This chapter focuses on the cognitive operations mediated by the frontal lobes in the service of working memory tasks.