True Memory, False Memory, and Subjective Recollection Deficits after Focal Parietal Lobe Lesions

There is growing evidence that the posterior parietal cortex (PPC) plays an important role in episodic memory retrieval. Functional MRI studies have reported lateral and medial PPC activations across a wide range of episodic memory retrieval tasks (reviewed in Cabeza, 2008; Cabeza, Ciaramelli, Olson, & Moscovitch, 2008; Ciaramelli, Grady, & Moscovitch, 2008; Wagner, Shannon, Kahn, & Buckner, 2005; (Vilberg & Rugg, 2008). The functional involvement of the PPC in episodic memory has been verified by a small number of focal lesion studies (Berryhill, Phuong, Picasso, Cabeza, & Olson, 2007; Davidson, et al., 2008; Simons, Peers, Mazuz, Berryhill, & Olson, 2009; reviewed in Olson & Berryhill, 2009). For instance, we previously tested two patients with bilateral parietal lobe damage, one patient with damage that was mostly in the inferior parietal lobe (IPL), the other had damage extending into the superior parietal lobe, on a study examining autobiographical memory (Berryhill, Phuong, Picasso, Cabeza, & Olson, 2007). Patients and matched controls were asked to select a memory from five designated epochs in their lifetimes and then describe the memory in as much detail as possible. Following this free recall stage, a series of specific probe questions were asked that were aimed at elaborating on the number of details mentioned. The free recall and specific probe data were subjected to a detailed text analysis to categorize, tally, and rate the details mentioned by study participants (Levine, Svoboda, Hay, Winocur, & Moscovitch, 2002). The results demonstrated a surprising dissociation in the patients' performance between the free recall and the specific probe stages. Their freely recalled memories were significantly impoverished in terms of the number of details they referenced. In contrast, during the specific probe stage, patients responded with as much detail as the control subjects. Because the task did not involve encoding, the patients' abnormally low performance is best characterized as a memory retrieval deficit. In another study (Davidson, et al., 2008), a patient with unilateral left PPC damage was tested in a series of tasks including the classic Deese-Roediger-McDermott (DRM) false-memory paradigm (Deese, 1959; Roediger & McDermott, 1995). In the standard DRM, participants are presented with lists of conceptually related words, and later, they are asked to perform an old/new recognition memory task. At retrieval, most participants claim to have seen or heard words that in fact, had never appeared on the list (“false memories”). This occurs when the words are closely related to the core theme of the words on the learned list. It is thought that such false memories arise from automatic activation of conceptually related words, or “gist” information (Schacter & Slotnick, 2004). Such false memories are typically accompanied by a strong subjective sense of recollection. Meta-memory processes operating at retrieval can also shape expectations about what true memories feel like, further modulating false memory rates. The Davidson, et al., (2008) report described a unique pattern of deficits in their unilateral PPC patient: she remembered relatively fewer items and made relatively fewer false alarms. The authors interpret this as evidence that the patient was not activating the ‘gist’ information associated with each semantically related list. Davidson's patient also reported a decreased sense of subjective recollection, a finding that has been reported by two other groups using different tasks and stimuli (Ally, Simons, McKeever, Peers, & Budson, 2008; Simons, Peers, Mazuz, Berryhill, & Olson, 2009). It is important to point out that there are several negative findings in this small literature. For instance, three studies have reported that unilateral and bilateral parietal lobe damage has no deleterious effect on source memory accuracy (Davidson et al., 2008; Simons et al., 2009) even when the lesions overlap with regions activated by a source-memory task in an fMRI study (Simons et al., 2008). There is also evidence that unilateral parietal lobe damage or has no measureable effect on item-recognition memory, such as recognition of a long series of words, pictures, or sounds (Haramati, Soroker, Dudai, & Levy, 2007). In summary, the existing data indicates that parietal lobe damage affects episodic memory in some cases but not others. Because there have only been a small number of studies on this topic, and those that exist offer uneven evidence for parietal involvement in episodic memory, it is difficult to make informed hypotheses about the parietal lobes' functional role in episodic memory. Also, with a few exceptions, most reported findings have not been replicated. The goal of this study is to bolster the literature by replicating and extending the DRM findings reported by Davidson and colleagues. Davidson's finding is valuable because the DRM task has been used on several different patient populations, thus allowing one to compare the effects of different focal lesions on task performance. In the present study, two patients with bilateral parietal lobe damage are tested on two different DRM tasks. After conducting our primary analyses, we conduct a brief qualitative meta-analysis in which the performance of our patients is compared to the performance of patients with medial temporal lobe (MTL) damage.