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Although false memories and confabulation have been linked to both executive dysfunction and greater suggestibility, similar associations with the emergence of delusional thinking remain unexamined. We therefore compared healthy individuals who scored high and low on the Peters Delusional Inventory (PDI: Peters et al., 1999) on measures of set-shifting (the intra–extradimensional set shift task: IED) planning (the Stockings of Cambridge Task: SOC). Additionally, we examined whether high delusion-prone individuals show greater suggestibility on the Gudjonsson Suggestibility Scale (GSS 2: Gudjonsson, 1987). On the IED task, the high group made more pre-extradimensional shift errors than the low PDI group, and this was especially notable for reversal learning. By contrast, no differences emerged on any aspect of the SOC. Finally, and intriguingly, the high PDI group was less likely than the low PDI group to change their responses after receiving suggestive negative feedback. We propose that delusional-style thinking may be underpinned by an orbitofrontal-based reversal learning difficulty affecting the flexibility to adapt responses to changing contingencies and external pressure.

Delusions are present in virtually all persons with schizophrenia at some time and delusions are surprisingly common in the general nonclinical population. Indeed, estimates suggest that 1%–3% of the nonclinical population have delusions comparable in severity to clinical cases, with a further 5%–6% having a delusion of lesser severity, and a further 10%–15% have fairly regular delusional ideation (Freeman, 2006). Little consensus exists about the psychological or neuropsychological basis of delusions in clinical cases (Gilleen and David, 2005). Nonetheless, the study of delusion-prone healthy individuals provides an approach to the study of delusional thinking that is uncontaminated by other symptoms, medication, and the cognitive deficits typically linked to full-blown psychosis; and potentially a means to examine how certain cognitive problems may predate and lead to actual symptom formation (Laws et al., 2008). Functional imaging studies reporting hypofrontality (for a review, see Hill et al., 2004) and poor performance on executive tests such as the Wisconsin Card Sort Test (WCST), word fluency, Stroop and Trail Making (Heinrichs and Zakzanis, 1998 and Laws, 1999) point to the frontal lobes as a key area of brain dysfunction in schizophrenia. Almost as widespread is the belief that dysfunction of the frontal lobes underpins some of the main symptoms of the disorder (Liddle, 1987; for a review, see Dibben et al., 2009). Indeed, neuroimaging studies report reduced prefrontal and anterior cingulated cortex activity in individuals who experience delusions (Schröder et al., 1996, Erkwoh et al., 1997, Sabri et al., 1997, Blackwood et al., 2004 and Lahti et al., 2006). Poorer frontal/executive test performance has also been documented in people who score highly on psychometric measures of schizotypy, with evidence of increased WCST perseverative errors (Spaulding et al., 1989, Raine et al., 1992, Poreh et al., 1995, Suhr, 1997, Daneluzzo et al., 1998, Gooding et al., 1999 and Tallent and Gooding, 1999), fewer completed categories and more failures in maintaining set (Lyons et al., 1991 and Gooding et al., 1999), as well as deficits on Trail making (Poreh et al., 1995) and the Stroop (Suhr, 1997). Despite these findings, no previous study has examined whether executive/frontal dysfunction is associated with the reporting of specific schizophrenia-like symptoms in healthy individuals. Recent studies have, however, reported findings that would appear to be consistent with an executive/frontal dysfunction in delusion-prone individuals. These include reports of significantly greater false recall and false recognition in healthy individuals who score highly on the Peters Delusional Inventory (PDI: Peters et al., 1999) measure of delusion proneness (Laws and Bhatt, 2005, Dehon et al., 2008 and Bhatt et al., 2010) as well as a greater confidence in such false memories (Laws and Bhatt, 2005 and Bhatt et al., 2010). Parallels have been drawn between delusions and the confabulatory memories that sometimes emerge in neurological patients especially following frontal lobe injury. For example, Gilboa and Moscovitch (2002) reviewed studies of confabulators and reported that 81% had damage to the prefrontal cortex with orbitofrontal and ventromedial damage being the most common (see also Schnider, 2003). In the context of false memories, it is also worth considering the role of suggestibility (Gudjonsson, 2003) which has been linked to false memory, but remains unexamined as a factor in delusional thinking. In this context, a relevant recent study by Gudjonsson and Young (2010) did find a positive relationship between suggestibility and confabulation, while confabulation and IQ were inversely related. Although the clinical presentations of delusions and confabulations can be quite similar (Turner and Coltheart, 2010) differences do exist with some arguing that confabulation typically involves the executive system, while delusions are more thematic and unrelated to the executive system (Kopelman, 2010).

Table 2 shows that the high PDI group made significantly more pre-ED errors. The total number of reversal errors from the first three reversal stages (sr, cdr, idr)1 was also significantly greater in the high PDI group. All of the participants passed all of the pre-ED stages and it was only at the ED stage that failures occurred (see Fig. 3). At the extradimensional shift and extradimensional shift reversal stages, no significant differences emerged between the high and low PDI groups in terms of errors although a greater number of high PDI subjects failed these stages (see Fig. 3). The high PDI group also made more errors than the low PDI group at these stages (see Fig. 4), although again these differences did not reach significance. Table 2. Group mean (S.D.) comparisons between the high and low PDI groups on the IED task. Low PDI High PDI Mean (S.D.) Mean (S.D.) F-value ηp2 Stages completed (n = 9) 8.7 (0.7) 8.60 (0.8) F1, 98 < 1.00, P = 0.46 0.005 Pre-ED errors 7.7 (5.3) 10.6 (7.7) F1, 98 = 4.67, P = 0.03 0.046 Reversal errors (sr, cdr and idr) 4.3 (1.9) 5.7 (4.1) F1, 98 = 5.48, P = 0.02 0.053 EDS errors 8.5 (9.1) 10.4 (9.5) F1, 98 = 1.02, P = 0.31 0.010 Completed stage trials 76.8 (20.9) 84.3 (24.0) F1, 98 = 2.81, P = 0.10 0.028 Total trials 89.3 (33.3) 98.9 (34.1) F1, 98 = 2.04, P = 0.16 0.020 Table options Full-size image (20 K) Fig. 3. Cumulative percentage of high and low PDI subjects achieving each IED stage. Figure options Full-size image (22 K) Fig. 4. Mean number of IED errors at each stage for low and high PDI groups. Note: sd = simple discrimination; sr = simple reversal; cda = compound discrimination adjacent; cds = compound discrimination superimposed; cr = compound reversal; ids = intradimensional shift; idsr = intradimensional shift reversal; eds = extradimensional shift; edsr = extradimensional shift reversal. Figure options 3.2. Stockings of Cambridge (SOC) The high and low PDI groups did not differ significantly on any of the outcome measures of this test i.e. initial thinking time, subsequent thinking time, the number of correct solutions and the number of problems solved in the minimum number of moves (each for 2, 3, 4 and 5 moves). For minimum move solutions (2, 3, 4 and 5), all F < 1 except 3 moves F = 1.34, P = 0.25). For initial thinking time, all F values were nonsignificant (2 and 3 moves F < 1; 4 moves F = 1.88, P = 0.17; 5 moves F = 3.16, P = 0.08). For subsequent thinking times, all were nonsignificant and all F < 1 (see Fig. 5). All latency measures were also analysed following using the square root transform of the reaction time; however this made no difference to the main findings. Full-size image (30 K) Fig. 5. Initial thinking times (top) and subsequent thinking times (bottom) to solve 2, 3, 4 and 5 move problems from the Stockings of Cambridge. Error bars = SE. Figure options 3.3. Gudjonsson Suggestibility Scale (GSS2) The high PDI group showed better immediate recall and were slightly less suggestible than the low PDI group though both only approached significance. Yet the low PDI group displayed a higher propensity than the high PDI group to give in to leading questions both prior to and post receiving the negative comment although this difference was not significant (see Table 3). However, the low PDI group had a significantly higher amount of shift in their responses highlighting that individuals with a low measure of delusional ideation have a greater tendency to change their responses after receiving negative feedback (P = 0.02).