Impaired working memory capacity is not caused by failures of selective attention in schizophrenia

Abstract

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.

Keywords: schizophrenia; selective attention; working memory.

Fig. 1.

Task sequence depicting a 3:2 trial from experiment 1. Following a fixation period with 12 empty placeholders, a memory array appears for 300ms. After a 2000-ms delay period, one of the placeholders is probed and participants indicate whether or not the space was occupied by a target.

Fig. 2.

Number of items stored in WM (K ± standard error of the mean) from experiment 1. (A) The mean number of items stored for each condition, (B) the filtering cost. 3:0 = 3 targets, 0 distractors; 3:2 = 3 targets, 2 distractors; 5:0 = 5 targets, 0 distractors. K was lower in PSZ compared with HCS for all 3 trial types (P < .001) and did not decline significantly between the 3:0 and the 3:2 conditions for either group (P’s > .13). PSZ and HCS did not differ in K _DIFF (P = .12) or K _RATIO (P = .21). HCS = healthy control subject; PSZ = people with schizophrenia; WM = working memory.

Fig. 3.

Number of items stored in WM (K ± standard error of the mean) from experiment 2. (A) The mean number of items stored for each condition, (B) the filtering cost for each of the 3 distractor conditions. 3:0 = 3 Targets, 0 Distractors; RR = 3 Targets, 2 Rotating-Red Distractors; RY = 3 Targets, 2 Rotating-Yellow Distractors; SY = 3 Targets, 2 Stationary-Yellow Distractors. K was lower in PSZ compared with HCS for all 4 conditions (P < .001), yet PSZ and HCS did not differ in K _DIFF for any of the 3 distractor types (P’s > .23). PSZ exhibited larger K _RATIO in the RY condition only (P = .03). HCS = healthy control subject; PSZ = people with schizophrenia; WM = working memory.