Self-specific processing in the default network: a single-pulse TMS study

Abstract

In examining neural processing specific to the self, primarily by contrasting self-related stimuli with non-self-related stimuli (i.e., self vs. other), neuroimaging studies have activated a consistent set of regions, including medial prefrontal cortex (MPFC), precuneus, and right and left inferior parietal cortex. However, criticism has arisen that this network may not be specific to self-related processing, but instead reflects a more general aspect of cortical processing. For example, it is almost identical to the active network of the resting state, the "default" mode, when the subject is free to think about anything at all. We tested the self-specificity of this network by using transcranial magnetic stimulation (TMS) to briefly disrupt local cortical processing while subjects rated adjectives as like or unlike themselves or their best friend. Healthy volunteers show a self-reference effect (SRE) in this task, in which performance with self-related items is superior to that with other-related items. As individual adjectives appeared on a monitor, single-pulse TMS was applied at five different times relative to stimulus onset (SOA: stimulus onset asynchrony) ranging from 0 to 480 ms. In 18 subjects, TMS to left parietal cortex suppressed the SRE from 160 to 480 ms. SRE suppression occurred at later SOA with TMS to the right parietal cortex. In contrast, no effects were seen with TMS to MPFC. Together with our previous work, these results provide evidence for a self-specific processing system in which midline and lateral inferior parietal cortices, as elements of the default network, play a role in ongoing self-awareness.

Fig. 1

Schematic depiction of two trials of the TMS portion of the adjective task. Word stimuli are presented until the subject responds with a button push, or until 4 s have elapsed. A blank screen follows for an intertrial interval (ITI) of 5 s. In each trial, a TMS pulse occurs at one of five times relative to stimulus onset—in the figure, at 80 and 480 ms

Fig. 2

Mean group efficiency scores (and standard error bars) for Self and Best Friend conditions for a separate sample of 23 subjects performing the adjective task without TMS

Fig. 3

Mean group efficiency scores (with SE error bars) between Self and Best Friend conditions. Scores are shown for each SOA, for TMS to a midline prefrontal cortex, b left lateral parietal cortex, and c right lateral parietal cortex. The plot in d shows the change in efficiency with TMS to precuneus from data collected in a different group (Lou et al. 2004)

Fig. 4

Change in efficiency scores between Self and Best Friend conditions. A positive value indicates an SRE. The error bars for each SOA represent the standard error of the difference between Self and Best Friend. Scores are shown for each SOA, for TMS to a midline prefrontal cortex, b left lateral parietal cortex, and c right lateral parietal cortex. The plot in d shows the change in efficiency with TMS to precuneus from data collected in a different group (Lou et al. 2004) and included for purposes of comparison. The change score was zero at 240 ms with TMS to left lateral parietal cortex, at 480 ms for right lateral parietal cortex, and at 160 ms for precuneus