Similarities and differences between parietal and frontal patients in autobiographical and constructed experience tasks
Section snippets
Neuropsychological participants
All patients were obtained through the Center for Cognitive Neuroscience Patient Database at the University of Pennsylvania (Fellows, Stark, Berg, & Chatterjee, 2008). All patients and controls signed informed consent documents for each experimental protocol. The Institutional Review Board of the University of Pennsylvania approved all experimental protocols. The bilateral PPC patients EE555 and TQ591 have been described in detail previously (Berryhill and Olson, 2008, Berryhill et al., 2007).
Experiment 1: autobiographical memory following PFC damage
We previously reported that bilateral PPC lesions created a specific deficit in the ability to freely recall autobiographical memories (Berryhill et al., 2007). A limitation of that study was that it did not include a brain damaged control group to account for the possibility that deficits were side effects of brain damage. Here, we tested a diverse group of PFC patients in the autobiographical memory task described previously (Levine et al., 2002).
There is considerable reason to suspect that
Participants
The data from the 10 control participants (age: mean 46.7 years, range 37–57, 6 males; education: mean 15 years, range 10–19) and the PPC patients (M age = 44.0, range 39–49, 0 males; M education = 15.5, range 15–16) reported in Berryhill et al. (2007) were compared with four PFC patients (M age = 57.3, range 35–81, 1 male; M education = 17.5, range 14–20). Data from PFC patient BX384 was unavailable. There were no significant differences in age or education between patient and control groups as
Quantitative analysis: PFC versus controls
The free recall data were subjected to a permutation analysis with the factors of group (PFC, control) and detail type (internal, external; see Fig. 2 and Table 1). The PFC patients performed normally (F1, 13 < 1, p = n.s.) with one exception. The interaction of group and detail type was significant (F1, 13 = 5.15, p = .04) due to the fact that the PFC patients reported relatively more internal details (M control = 86.35, M PFC = 104.38), but relatively fewer external details (M control = 105.80, M PFC =
Discussion
Experiment 1 showed that across multiple measures, a group of PFC patients did not exhibit impaired autobiographical memory retrieval. Indeed, the PFC patients performed similarly to the control population in both the free recall and specific probe phases. The interaction between control subjects and PFC patients who freely recalled a greater number of internal details and a smaller number of external details also counters age concerns. Previous work has found that older adults tend to exhibit
Experiment 2: constructed experience
The goal of Experiment 2 was to examine whether PPC damage leads to impaired performance on constructed experience tasks. Since the same cortical networks are observed during episodic memory and constructed experience tasks, we predicted that the PPC patients would perform poorly. Because the PPC group described past events in impoverished detail, we predicted they would also describe imaginary events in impoverished detail. In contrast, since the PFC group performed normally in the
Participants
A second group of ten normal, healthy controls (M age = 47.8, M years of education = 15.4, 3 males) with normal neurological histories were tested. Both PPC patients (M age = 44.0, M education = 15.5) and five PFC patients (M age = 59.6, M education = 16.4) participated. There were no significant differences in age or education between patient and control groups as determined by non-parametric Kruskal–Wallis tests (both , both p's > .41).
Constructed experience procedure
Our protocol followed Hassabis et al., 2007a, Hassabis et
Experiential index (EI)
The overall ability to imagine a scenario was comprehensively measured by the EI; see Fig. 3. A non-parametric Kruskal–Wallis's one-way ANOVA was performed using the data from the three groups. The analysis indicated a significant effect of group (, p = .002). In order to determine whether each patient group was significantly impaired with regard to the control group, we conducted Mann–Whitney's tests. These comparisons indicate that both patient groups had significantly lower EI
Discussion
Experiment 2 confirmed that the PPC patients were impaired across most measures. Unexpectedly, the PFC group was also impaired across most measures. Both the PPC and PFC patients mentioned fewer items in the environment, and described fewer sensory and spatial details, suggesting that their constructed experiences were impoverished. The PPC and PFC patients made relatively fewer comments about their own thoughts, emotions, and actions, suggesting that their representation of themselves in a
General discussion
The question posed by this study was whether portions of the PPC play a functional role in constructed experience. To assess this possibility, we compared patients with bilateral PPC damage to control participants and to a lesion control group, patients with unilateral lesions in ventrolateral or dorsal PFC.
We first tested whether the PFC patient group exhibited the same pattern of episodic memory retrieval deficits that we had previously observed in the PPC patients (Berryhill et al., 2007).
Acknowledgments
We would like to thank Eleanor Maguire for providing testing materials. We would also like to thank Marianna Stark and Elizabeth Roy for help with scheduling test participants. Last, we would like to thank Anjan Chatterjee for kindly allowing us access to the University of Pennsylvania Patient Database. This research was supported by NIH RO1 MH071615-01 to I.O. and NRSA NS059093 to M.B.
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