Carly J. Leonard

The Relationship Between Working Memory Capacity and Broad Measures of Cognitive Ability in Healthy Adults and People With Schizophrenia

OBJECTIVE Working memory (WM) capacity, typically measured with cognitively complex span tasks, is correlated with higher order cognitive abilities in healthy adults. The goals of this study were to determine whether a more focused measure of visual WM storage capacity would show similar higher order ability correlations in healthy adults and in people with schizophrenia (PSZ), thereby demonstrating the importance of simple storage capacity; determine whether the illness alters the pattern of correlations across cognitive domains; and evaluate whether between-groups differences in WM capacity could account for the generalized cognitive impairment in PSZ. METHOD Ninety-nine PSZ and 77 healthy controls (HCs) completed a visual WM change-localization task, the Wechsler Abbreviated Scale of Intelligence (WASI), and the MATRICS Consensus Cognitive Battery (MCCB). RESULTS PSZ performed more poorly than HCs on all cognitive measures. The between-groups effect size for WM capacity was large (d = 1.11). WM robustly correlated with WASI and MCCB performance, with no significant differences in the magnitude or pattern of correlations across groups. When the groups were pooled, WM capacity correlated at r = .68 with MCCB composite score and at r = .56 with WASI estimated Full Scale IQ. WM capacity accounted for approximately 40% of the between-groups variance across the WASI and MCCB. CONCLUSIONS A simple measure of WM storage capacity is robustly associated with the higher order cognitive abilities assessed by the WASI and MCCB in HCs and PSZ. WM capacity reduction may be a critical determinant of the general cognitive impairment in PSZ.

Attentional guidance in singleton search: An examination of top-down, bottom-up, and intertrial factors

While it is clear that the goals of an observer change behaviour, their role in the guidance of visual attention has been much debated. In particular, there has been controversy over whether top-down knowledge can influence attentional guidance in search for a singleton item that is already salient by a bottom-up account (Theeuwes, Reimann, & Mortier, 2006). One suggestion is that passive intertrial priming accounts for what has been called top-down guidance (e.g., Maljkovic & Nakayama, 1994). In the present study, participants responded to the shape of a singleton target among homogenous distractors in a trial-by-trial cueing design. We examined the influence of target expectancy, trial history, and target salience (which was manipulated by changing the number of distractors). Top-down influence resulted in fast RTs that were independent of display size, even on trials that received no priming. Our findings show there is a role for top-down guidance, even in singleton search. The designation of intertrial priming as a bottom-up factor, rather than an implicit top-down factor (Wolfe, Butcher, Lee, & Hyle, 2003), is also discussed.

Failure of schizophrenia patients to overcome salient distractors during working memory encoding.

BACKGROUND Prior demonstrations of impaired attentional control in schizophrenia focused on conditions in which top-down control is needed to overcome prepotent response tendencies. Attentional control over stimulus processing has received little investigation. Here, we test whether attentional control is impaired during working memory encoding when salient distractors compete with less salient task-relevant stimuli. METHODS Patients with schizophrenia (n = 28) and healthy control subjects (n = 25) performed a visuospatial working memory paradigm in which half of the to-be-encoded stimuli flickered to increase their salience. After a 2-second delay, stimuli reappeared and participants had to decide whether or not a probed item had shifted location. RESULTS In the unbiased condition where flickering and nonflickering stimuli were equally likely to be probed, both groups displayed a trend toward better memory for the flickering items. In the flicker-bias condition in which the flickering stimuli were likely to be probed, both groups displayed a robust selection advantage for the flickering items. However, in the nonflicker-bias condition in which the nonflickering stimuli were likely to be probed, only healthy control subjects showed selection of the nonflickering items. Patients displayed a trend toward preferential memory for the flickering items, as in the unbiased condition. CONCLUSIONS Both groups were able to select salient over nonsalient stimuli, but patients with schizophrenia were unable to select nonsalient over salient stimuli, consistent with impairment in the effortful control of attention. These findings demonstrate the generality of top-down control failure in schizophrenia in the face of bottom-up competition from salient stimuli as with prepotent response tendencies.

The role of attention in subitizing: Is the magical number 1?

Subitizing, the fast and accurate enumeration of up to about 3 or 4 objects, has often been thought to be dependent on limited-capacity preattentive mechanisms. We used an attentional blink paradigm to investigate the extent to which subitizing requires attentional resources. On each trial, subjects identified a target letter in an RSVP stream and then enumerated dots presented in the stream that were either simultaneous with the target letter or followed it by up to 400 ms. For numerosities from 2 to 9, evidence of an attentional blink was observed; only enumeration of 0 or 1 elements was independent of lag. Thus, even enumeration of 2–3 objects, which is within the traditional subitizing range, appears to require attentional resources. The relation of this work to studies on the attentional requirements of detecting a unique item among distractors, a supposedly preattentive discrimination, is briefly discussed.

Direct Evidence for Active Suppression of Salient-but-Irrelevant Sensory Inputs

Researchers have long debated whether attentional capture is purely stimulus driven or purely goal driven. In the current study, we tested a hybrid account, called the signal-suppression hypothesis, which posits that stimuli automatically produce a bottom-up salience signal, but that this signal can be suppressed via top-down control processes. To test this account, we used a new capture-probe paradigm in which participants searched for a target shape while ignoring an irrelevant color singleton. On occasional probe trials, letters were briefly presented inside the search shapes, and participants attempted to report these letters. Under conditions that promoted capture by the irrelevant singleton, accuracy was greater for the letter inside the singleton distractor than for letters inside nonsingleton distractors. However, when the conditions were changed to avoid capture by the singleton, accuracy for the letter inside the irrelevant singleton was reduced below the level observed for letters inside nonsingleton distractors, an indication of active suppression of processing at the singleton location.

Toward the Neural Mechanisms of Reduced Working Memory Capacity in Schizophrenia

People with schizophrenia (PSZ) demonstrate reliable reductions in working memory (WM) capacity (i.e., the number of objects that can be held in memory). The present study asked whether WM impairments in PSZ can be explained by the same neural mechanisms that underlie individual differences in WM capacity among healthy individuals. Specifically, we examined event-related potentials in PSZ and healthy matched controls during a change detection task that required the storage of multiple objects in WM. The amplitude of contralateral delay activity (CDA), which correlates with WM capacity in healthy individuals, was larger in controls than in PSZ for memory loads of 3 and 5 objects, but larger in PSZ than in controls for a memory load of 1. This same pattern was found in the subgroups of PSZ and controls with an equivalent WM capacity. Moreover, the increase in CDA amplitude was correlated with individual differences in capacity in controls, but not in PSZ. These results demonstrate that WM impairment in PSZ is not associated with the same patterns of neural activity that characterize low WM capacity in healthy individuals. We propose that WM impairment in PSZ instead reflects a specific impairment in the ability to distribute attention broadly.

Visuospatial attention in schizophrenia: deficits in broad monitoring.

Although selective attention is thought to be impaired in people with schizophrenia (PSZ), prior research has found no deficit in the ability to select one location and withdraw attention from another. PSZ and healthy control subjects (HCS) performed a stimulus detection task in which one, two, or all four peripheral target locations were cued. When one or two locations were cued, both PSZ and HCS responded faster when the target appeared at a cued than uncued location. However, increases in the number of validly cued locations had much more deleterious effects on performance for PSZ than HCS, especially for targets of low contrast whose detection was more dependent on attention. PSZ also responded more slowly in trials with four cued locations relative to trials with one or two invalidly cued locations. Thus, visuospatial attention deficits in schizophrenia arise when broad monitoring is required rather than when attention must be focused narrowly.

Why salience is not enough: Reflections on top-down selection in vision

The target article represents a distillation of nearly 20 years of work dedicated to the analysis of visual selection. Throughout these years, Jan Theeuwes and his colleagues have been enormously productive in their development of a particular view of visual selection, one that emphasizes the role of bottom-up processes. This work has been very influential, as there is substantial merit to many aspects of this research. However, this endeavor has also been provocative—the reaction to this work has resulted in a large body of research that emphasizes the role of top-down processes. Here we highlight recent work not covered in Theeuwes’s review and discuss how this literature may not be compatible with Theeuwes’s theoretical perspective. In our view this ongoing debate has been one of the most interesting and productive in the field. One can only hope that in time the ultimate result will be a complete understanding of how visual selection actually works.

Impaired Working Memory Capacity Is Not Caused by Failures of Selective Attention in Schizophrenia

The cognitive impairments associated with schizophrenia have long been known to involve deficits in working memory (WM) capacity. To date, however, the causes of WM capacity deficits remain unknown. The present study examined selective attention impairments as a putative contributor to observed capacity deficits in this population. To test this hypothesis, we used an experimental paradigm that assesses the role of selective attention in WM encoding and has been shown to involve the prefrontal cortex and the basal ganglia. In experiment 1, participants were required to remember the locations of 3 or 5 target items (red circles). In another condition, 3-target items were accompanied by 2 distractor items (yellow circles), which participants were instructed to ignore. People with schizophrenia (PSZ) exhibited significant impairment in memory for the locations of target items, consistent with reduced WM capacity, but PSZ and healthy control subjects did not differ in their ability to filter the distractors. This pattern was replicated in experiment 2 for distractors that were more salient. Taken together, these results demonstrate that reduced WM capacity in PSZ is not attributable to a failure of filtering irrelevant distractors.

Control of working memory content in schizophrenia

People with schizophrenia (PSZ) exhibit signs of reduced working memory (WM) capacity. However, this may reflect an impairment in managing its content, e.g. preventing irrelevant information from taking up available storage space, rather than a true capacity reduction. We tested the ability to eliminate and update WM content in 38 PSZ and 30 healthy control subjects (HCS). Images of real-world objects were presented consecutively, and a tone cued the item most likely to be tested for memory. On half the trials, randomly intermixed, a second tone occurred. Participants were informed that the item cued by the second tone was now the most likely to be tested, and the item cued by the first tone now the least likely, providing incentive to eliminate the first cued item from WM. Both HCS and PSZ displayed a robust performance advantage for cued items. Unexpectedly, PSZ more efficiently removed the no-longer-essential item from WM than HCS. The magnitude of the WM clearance of this first cued item correlated with memory performance for the newly prioritized second cued item in PSZ, indicating that it was adaptive. However, WM clearance was not associated with WM capacity, ruling out the need to budget limited resources as an explanation for greater clearance in PSZ. A robust correlation between WM clearance and poverty of speech in PSZ instead suggests that the propensity to rapidly clear non-essential information and minimize the number of items in WM may be the reflection of a negative symptom trait. This finding may reflect a more general tendency of PSZ to focus processing more narrowly than HCS.