The construct of attention in schizophrenia

Abstract

Schizophrenia is widely thought to involve deficits of attention. However, the term "attention" can be defined so broadly that impaired performance on virtually any task could be construed as evidence for a deficit in attention, and this has slowed cumulative progress in understanding attention deficits in schizophrenia. To address this problem, we divide the general concept of attention into two distinct constructs: input selection, the selection of task-relevant inputs for further processing; and rule selection, the selective activation of task-appropriate rules. These constructs are closely tied to working memory, because input selection mechanisms are used to control the transfer of information into working memory and because working memory stores the rules used by rule selection mechanisms. These constructs are also closely tied to executive function, because executive systems are used to guide input selection and because rule selection is itself a key aspect of executive function. Within the domain of input selection, it is important to distinguish between the control of selection--the processes that guide attention to task-relevant inputs--and the implementation of selection--the processes that enhance the processing of the relevant inputs and suppress the irrelevant inputs. Current evidence suggests that schizophrenia involves a significant impairment in the control of selection but little or no impairment in the implementation of selection. Consequently, the CNTRICS (Cognitive Neuroscience Treatment Research to Improve Cognition in Schizophrenia) participants agreed by consensus that attentional control should be a priority target for measurement and treatment research in schizophrenia.

Figures

Figure 1

(A) Example of the spatial cuing task. In this version, a cue (a thickening of one of four location marker boxes) is used to direct attention to one location. After a brief delay, a target array appears, and the subject indicates whether it contains the letter T. In most experiments, the target is more likely to appear at the cued location (valid trials) than in an uncued location (invalid trials). This is largely an input selection task because the cue indicates which input source is relevant. (B) Example of the visual search task. In this version, the target is a green vertical bar, and the subjects makes a presence/absence response. This task can be performed by focusing attention to one item at a time, in random order, until the target is found. It can also be performed by limiting search just to the green items, which is much more efficient. In either case, this is largely an input selection task because—at any given moment—attention is used to select a subset of the objects for further processing. (C) Distinction between the control of selection (i.e., limiting attention to the circled items) and the implementation of selection (i.e., creating an annulus of suppression around the currently selected item).

Figure 2

Example of the relationship between competition and attention. When a target object (the letter T) has no competition (A), it will be processed without any intervention by attention mechanisms even if it is weakly represented. When multiple objects are present but the target object is more salient than the nontarget objects (B), the target will automatically inhibit the other objects and can win the competition even without any help from attention. If, however, the target is weaker than the nontargets (C), but does not receive any help from attention, it will be inhibited by the stronger nontargets and lose the competition for representation. Attention can overcome the low salience of the target object by boosting its signal, allowing it to inhibit the stronger objects, and thus win the competition (D). According to this view, attention is necessary only when the relevant object is weaker than, or equal in strength to, the irrelevant objects.

Figure 3

(A) Example of the Stroop task. Subjects are asked to name the ink color in one condition and to read the word in another condition. This is primarily a rule selection task, because the rules for the two tasks interfere with each other rather than the inputs. (B) Example of the flankers task. In this version, subjects press a left-hand button if the central letter is an A, and they press a right-hand button if the center letter is a B. On the trial shown here, the task-irrelevant flanking letters are incompatible with the central letter, which leads to slowed responses if the flanking letters are not adequately filtered. Although the flankers task is quite similar to the Stroop task, the flankers task is primarily an input selection because it stresses the need to apply a single rule to one input and preventing other inputs from being given access to this rule.

Figure 4

Visual search tasks used by Fuller et al. (55). The number of items in each stimulus array varied across trials, and performance was quantified as the slope of the function relating reaction time to the number of items in the stimulus array. The tasks are shown in order of ascending difficulty. In tasks A, B, and C, the target was a square with a gap on the left or right side, and subjects were required to press one of two buttons to indicate the side of this gap. In task A, the target was the only item with a gap, and performance could be optimized by controlling the search so that attention was directed immediately to this item rather than shifting randomly from item to item. In Task B, each item had a gap, making it impossible to use feature information to guide attention directly to the target. Thus, attention shifts randomly in this condition even in healthy individuals, and the need for precise attentional control is minimized. Task C was identical to Task B except that the gaps were smaller, making the task more difficulty without increasing the need for precise attentional control. In task C, the stimulus arrays consisted of pairs of items in the left and right visual fields, and subjects were required to find the one pair of identical items and report the direction of the gap in this pair. This task requires precise attentional control because an item in one visual field must be compared with a specific item in the opposite field, require a precise shift of attention to this item. Schizophrenia patients were most impaired in tasks A and C, the two tasks that stressed the control of selection.