The current results are consistent with previous group analyses. Deciding whether a trait adjective applies to oneself recruit the medial regions associated with theory of mind—the medial precuneus and MPFC—but not the TPJ.
Caution in interpreting the observed overlap is appropriate. These data do not completely overcome methodological limitations. First, a single voxel in the current study reflects the average response over 3 × 3 × 4
mm of tissue, much lower spatial resolution than the functional organization of cortex. Higher resolution functional imaging is currently becoming available, and may yet reveal that activations associated with the ‘self’ and ‘false belief’ tasks are neighboring but distinct (e.g. Schwarzlose et al
). Second, even within single voxels, neurons subserving distinct functions may be interspersed. One approach to disentangling such overlap is functional adaptation, which relies on the reduction of activity observed when two successive stimuli are processed by the same sub-population of neurons within a voxel, but not when the stimuli recruit different sub-populations (Kourtzi and Kanwisher, 2001
; Krekelberg et al
). Third, although overlapping voxels were observed in each individual, in most individuals we also observed non-overlapping voxels in medial regions, in which the t-value of one task was higher than 3.6, and the second task did not reach t
> 0.5 (corresponding to a P
-value of 0.3). These non-overlapping voxels provide evidence that at least some aspects of the medial regions’ contribution to theory of mind are not shared by the self-attribution task and vice versa.
Still, the current data provide the strongest evidence to date that sub-regions of medial precuneus and MPFC are recruited both when subjects reason about a character's thoughts, and when they attribute a personality trait to themselves. Recently, research has revealed a third cognitive function associated with very similar regions of medial cortex: autobiographical episodic memory (e.g. Shannon and Buckner, 2004
; Wheeler and Buckner, 2004
; Wagner et al
; Ries et al
; see also Fossati et al
; Lou et al
Interestingly, these same three tasks—theory of mind, self reflection and autobiographical episodic memory—are correlated in child development (Moore and Lemmon, 2001
). One measure of self-reflection in childhood is Povinelli and colleagues’ (1996
) delayed self-recognition task. In this task, an experimenter is videotaped covertly placing a large sticker on the child's head. Three minutes later, the child is shown the video tape. Although all children between 2 and 4 years correctly identify themselves in the video, only children over 3.5 years reach up to retrieve the sticker. Performance on this task specifically reflects children's developing conception of the connection between their past and present selves; given a mirror, children at all these ages successfully retrieve the sticker. Children's performance on the delayed-self recognition task is correlated with scores on episodic memory and false belief tasks (Moore and Lemmon, 2001
Developmental and neural data thus converge on a triad of interrelated tasks. The next challenge is to establish the causal and dependence relations between these tasks, and/or the common cognitive function(s) underlying them. Both careful studies of individual subjects’ functional data, and careful task analyses, will be necessary. Many different models are consistent with the current evidence ():
- Autobiographical memory may depend on theory of mind. Perner (2001) has proposed that autobiographical/episodic memory inherently depends on theory of mind: that is, on the ability to identify the source of a current experience (a recollection) in the previously experienced event. Developmental evidence is consistent with such a dependence of autobiographical memory on theory of mind. Children's theory of the origin of epistemic states—that is, of how beliefs and knowledge are acquired or caused—develop along with performance on false belief tasks (Wimmer et al., 1988; O'Neill et al., 1992). Three year olds, for example, but not four year olds, expect that people (including themselves) can distinguish between a heavy ball and a light ball just as well by looking at the balls as by lifting them (Burr and Hofer, 2002). Success on tests of theory of mind predicts success on episodic memory tasks (Perner, 2001), and helps children become resistant to contamination of their autobiographical memories through suggestion (Welch-Ross, 1999).
- Theory of mind may depend on autobiographical memory. Some theorists have suggested that in order to understand the causal relations between another person's experiences, thoughts, and behaviors, observers bring to mind specific, and relevantly similar, past experiences of their own (Adams, 2001). There is at least some evidence that empathy is affected by the observer's prior experiences (Batson et al., 1996).
- Self-reflection may depend on autobiographical memory. During the self-attribution task, subjects are asked to judge whether trait words (‘logical’, ‘reckless’, ‘rebellious’) apply to themselves. To answer this question, subjects may retrieve autobiographical memories of specific incidents in which their actions merited (or did not merit) the target description.
- Autobiographical memory and self-reflection may both depend on recognizing the self as an enduring entity, with persisting causal and social properties (Povinelli and Simon, 1998). Povinelli (2001) therefore proposed that delayed self-recognition is a necessary precursor to autobiographical memory.
- Theory of mind may depend on self-reflection. ‘Simulation Theory’ proposes that an observer attributes mental states to another person by using her own mind as a model of the other mind (e.g. Gallese and Goldman, 2004; but see Saxe, 2006). The observer would adjust (i) the input, using the other person's (hypothesized) perceptual environment, rather than her own and (ii) the output, generating a prediction rather than an action (Nichols and Stich, 2003). Identifying the output as a prediction for someone else's action might involve a kind of self-attribution, and be a necessary component of theory of mind.
- Theory of mind and self-trait attribution may share a common conception of human agents as enduring entities, with persisting causal and social properties. This distinction lies at the core of recent proposals that there is a general domain of ‘social cognition’, distinct from all forms of ‘non-social cognition’ (Jenkins and Mitchell, in press; Mitchell et al., 2005b).
Fig. 5 Six models of the relationships between theory of mind, autobiographical memory and self-reflection that are consistent with current data. Arrows depict causal or developmental dependence of the top box on the bottom box. The models are described further (more ...)
One further observation may illuminate (or complicate) this picture: patients with Alzheimer's disease show amyloid deposition, hypo-metabolism, hypo-activation and tissue atrophy in these midline regions (Greicius et al
; Shannon and Buckner, 2004
; Buckner et al
; Rombouts et al
; Wang et al
). This pattern converges with the evidence that medial precuneus and MPFC are involved in autobiographical memory, which is impaired in Alzheimer's disease. However, performance on false belief tasks is preserved in patients with Alzheimer's disease (Gregory et al
; Zaitchik et al
; Zaitchik et al
), and there is also evidence for preserved self-attribution of traits (Klein et al
; Cotrell and Hooker, 2005
; Rankin et al
). In contrast, both theory of mind and self-attribution task performance is impaired in a different degenerative disorder, fronto-temporal dementia (Gregory et al
; Rankin et al
). One study recently directly compared recruitment of the medial precuneus regions for autobiographical memory and for self-trait attribution in healthy subjects and in patients with mild cognitive impairment (MCI), a risk factor for Alzheimer's disease. MCI patient showed hypo-activation of the medial precuneus and MPFC for the memory task but normal activation for the self-trait attribution (Ries et al
). Thus while theory of mind, self-attribution and episodic memory are correlated in development, and recruit common brain regions in healthy adults, the three tasks appear to be dissociable in degenerative disease. Any full account of the role of the medial precuneus and MPFC should aim to explain all of these results.