Episodic memory impairment is a robust correlate of familial risk for schizophrenia (SZ) and bipolar disorder (BD); still much is unknown about the processes that underlie this deficit and how they may be implicated in BD and SZ. We examined the possibility that (a) episodic memory impairment may arise from abnormalities in the cognitive control of interference between task-relevant and task-irrelevant memories during retrieval; inability to suppress task-irrelevant representations could give rise to intrusions of inappropriate memories and increased rate of forgetting, (b) cognitive control deficits during retrieval may be differentially affected by familial predisposition to SZ or BD. We examined episodic memory in relatives of patients with SZ (SZ-R) (n = 15) or BD (BD-R) (n = 17) compared to healthy controls (n = 23) using the California Verbal Learning Test (CVLT) and the Doors and People Test (DPT). All relatives were free of any psychiatric morbidity and were matched to controls on age, sex, educational achievement and general intellectual ability. During the CVLT, both relatives' groups made significantly more perseverative recall errors than controls. However, intrusion errors were significantly increased in SZ-R only. SZ-R also showed increased rate of forgetting in the DPT while BD-R were comparable to controls. Familial predisposition to SZ, compared to that of BD, was associated with significantly greater impairment in cognitive control processes during episodic memory retrieval with some evidence of specificity for SZ in connection with mechanisms relating to increased forgetting.
Disruption in memory systems is a replicable finding in patients with schizophrenia (SZ) and bipolar disorder (BD) particularly with respect to episodic memory (Aleman et al., 1999 and Torres et al., 2007). Episodic memory refers to the ability to recall information through personal experience (Tulving, 1983). Effect sizes for episodic memory deficits have been found to be large for patients with SZ (range 1–1.27) and moderate to large for patients with BD (range 0.59–0.85) (Skelley et al., 2008, Bora et al., 2009 and Lefebvre et al., 2010). Episodic memory impairment is also associated with familial predisposition as unaffected relatives of patients with SZ or BD also show deficits of moderate effect size (range 0.44–0.65 for SZ and 0.33–0.53 for BD) (Snitz et al., 2003, Whyte et al., 2005, Arts et al., 2008, Skelley et al., 2008, Bora et al., 2009, Glahn et al., 2010 and Lefebvre et al., 2010).
Episodic memory tasks typically involve a learning condition, where participants are given information (usually pictures or words) to ‘encode’ into memory, and a retrieval condition, where participants are requested to recall this information. Successful retrieval is thought to rely on processes of cognitive control (Anderson and Neely, 1996) implemented through the coordinated engagement of the hippocampus (and adjacent cortical regions), the anterior cingulate and the prefrontal cortex (Nyberg et al., 1996 and Fletcher et al., 1997; Tulving, 2002, Kuhl et al., 2008 and Schott et al., 2011). Within this network the ability to recall information correctly relies on cognitive control processes in order to overcome competition (or interference) between task-relevant and task-irrelevant memories (Monsell, 1978 and Jonides et al., 1998). Inability to suppress task-irrelevant representations could give rise to intrusions of inappropriate material during recall as well as increased rate of forgetting (Kuhl et al., 2007). It has been proposed that the mechanisms underpinning interference control during episodic memory retrieval are not specific to this domain but reflect shared inhibitory processes that manage interference across tasks (Hamilton and Martin, 2005).
We (Frangou et al., 2006) and others (Krabbendam et al., 2005 and Bora et al., 2010) have previously provided evidence for dysfunction across a number of cognitive control tests being significantly greater in patients with SZ than in patients with BD. More recently, we have reported that unaffected relatives of SZ patients demonstrate significantly greater impairment in cognitive control than relatives of BD patients as implied by increased intrusion errors during cued word production and reduced inhibition of task-irrelevant responses (Christodoulou et al., 2011). Here we extend the comparative examination of the effect of familial predisposition to SZ or BD in the same sample by focusing on aspects of cognitive control during episodic memory using the California Verbal Learning Test (CVLT; Delis et al., 1987) and the Doors and People Test) (DPT; Baddeley et al., 1994) as probes. The CVLT is a popular test of episodic memory which allows the examination of susceptibility to interference based on measures of recall errors while the DPT provides estimates of the rate of forgetting. We hypothesised that unaffected relatives of patients with SZ will show greater susceptibility to interference as indexed by increased recall errors and increased rate of forgetting compared to unaffected relatives of BD patients.
