Neuropsychological evidence has associated self/other processing with the right parietal lobe. Symptoms following damage to this area frequently involve disorders of self-awareness, examples of which include: hemi-spatial neglect, a disorder in which a patient is unaware of the side of space contralateral to the lesion, often including their own body parts (Vallar and Perani, 1986; Driver and Mattingley, 1998); anosognosia, in which the patient is unaware of their contralesional disabilities (for example, their hemiplegia) (Paysant et al., 2004; Jehkonen et al., 2006) asomatognosia, in which patients can deny ownership of their own limb (Pia et al., 2004) and some cases of alien limb syndrome (Groom et al., 1999) in which patients report the absence of volitional control over the affected limb—often referring to it in the third person. Another patient group with a disrupted sense of agency are those presenting with passivity (delusions of control). This is one of the first-order positive symptoms in schizophrenia and involves the patient believing that his or her actions are being controlled or influenced by an external agent. This specific symptom has been associated with abnormal activity in the right inferior parietal lobe (rIPL). For example Spence et al. (1997) observed hyperactivity in the rIPL using positron emission tomography (PET) in schizophrenic patients experiencing passivity compared with patients without this symptom and a non-schizophrenic control group. This specific patient group has also been found to have reduced cortical volume in the rIPL (Maruff et al., 2005).

While attribution errors in brain-damaged and schizophrenic individuals have been studied extensively, reports of misattribution in healthy people are rare. However, evidence associating the rIPL with successful self/other action discrimination has been demonstrated in neurologically intact individuals using functional imaging. Farrer et al. (2003a) tested participants making joystick movements in a self/other judgment task while measuring brain activity using PET. Participants were given visual feedback of their movements by the presentation of a virtual hand and joystick on a mirror positioned in front of their moving hand. In some trials, the visual feedback deviated from their actual movements by 25° or 50° (defined as other) while the remaining trials were left unperturbed (defined as self). The results of this experiment revealed an increase of blood flow to the rILP when participants made other judgments and an increase in blood flow to the insula (primarily right hemisphere) when participants made self judgments. The PET 2003 study was a follow-up to a previous functional magnetic resonance imaging (fMRI) study that also highlighted the involvement of the anterior insula and rIPL for self- vs other-generated movements, respectively (Farrer and Frith, 2002). However, it is not clear from the study of Farrer et al. (2003a), whether the activation detected in rIPL reflected the locus of primary mechanisms involved in the sense of agency or simply the detection of spatial discordance between the seen and felt positions of the joystick which might then be used to inform self/other judgments.

Schizophrenic patients experiencing passivity can accurately carry out movements as intended, but feel as if their movements are under the control of another (external) agent. The comparator model proposed by Frith et al. (2000), supported by the current results, helps to create an understanding of the processes that might underlie this experience (Figure 3). Whenever the central nervous system (CNS) plans a movement, a copy of the motor command is generated (efference copy) and this can be used by the CNS to predict the consequences of that movement. Such a prediction mechanism can be used in many ways, but importantly it allows the CNS to anticipate and correct for movement errors, filter expected sensory input and help maintain the estimate of the current state of the motor system. An accurate representation of one's own current limb position depends on accurate sensory feedback as well as accurate current state predictions.

Frith and colleagues suggest that schizophrenic patients with passivity symptoms have impaired predicted state representations and as a consequence they perceive a false discordance between the predicted and actual states of their movement. As a result they feel as though an external agent is controlling their actions even though the intended goal is still achieved (the rest of the system remains intact so the patient can still successfully construct and execute the desired movement and their intended goals match their perceived outcome). The misattribution of agency observed in the current experiment, mirrors that seen in schizophrenic delusions of control and is also explained by a disruption of predicted state mechanisms. As outlined above, the comparator model predicts that disrupting predicted state representations would lead to an increase in other judgments that is precisely what happens in the current experiment when the rIPL is disrupted by TMS. A unique and important factor in the current study relates to the timing of the TMS pulse: crucially it occurred when vision of the virtual hand was not available to the participant. Correct self/other judgments in a task such as this, requires that the participant accurately predict where their hand will re-emerge from behind the occluding bar. Occluding the hand for a portion of the reach places an extra burden on predictive mechanisms at precisely the time at which the TMS pulse is delivered. In relation to previous studies, it has been argued that the rIPL activation observed in the Farrer et al. (2003a) study might simply reflect the detection of spatial discordance rather than the sense of agency itself. In the current experiment however, TMS is delivered at a time at which there is no sensory discordance. The felt position of the limb remains unperturbed and the seen position of the limb is occluded. While the rIPL may indeed be heavily implicated in the detection of sensory discordance, that is not the process that is being disrupted by TMS in the current experiment.

It is interesting to note that the planned comparison between TMS and NO-TMS at the vertex stimulation site also approached significance (P = 0.07) with perturbations of four degrees. The direction of the difference here was in the same direction as the equivalent TMS vs NO-TMS comparison at the rIPL stimulation site. This probably reflects a general effect of TMS on the frequency of other responses (that is, participants might generally report ‘other’ more frequently when receiving TMS regardless of stimulation location). However, unlike rIPL TMS, the difference for zero-degree perturbations at the vertex did not approach significance. Thus, the effect of TMS over the rIPL (significantly affecting responses at both the zero- and four-degree perturbations) suggests that parietal TMS has an effect over and above any general effects of TMS. In addition, there appears to be a substantial self response bias which is most likely a consequence of the inherent difficulty of the task: previous research (Farrer et al., 2003b) has demonstrated that whereas participants can easily detect perturbations of around 10–15 degrees, they find perturbations as small as 5 degrees particularly difficult and tend to give many more self than other responses. If the default response, when uncertain, is self, then more self responses would be expected if TMS increased uncertainty. This, however, was not the outcome of the current experiment: more other, rather than self, responses were observed. In an attempt to disentangle the source of this change in bias, data were subsequently re-analyzed in the following manner: The original results were converted into percent self responses and re-entered in a 2 × 2 × 2 ANOVA as before. Planned comparisons between TMS and No-TMS for each condition revealed a significant reduction in self responses at the rIPL site for both zero- [F(1, 9) = 12.7, P < 0.01] and four- [F(1, 9) = 6.8, P < 0.05] degree perturbations, a marginal effect at the vertex site for the four-degree perturbation [F(1, 9) = 4.5, P < 0.06] and no effect at the vertex site for the zero-degree perturbation [F(1, 9) = 0.4, P = 0.57]. Taken together with the initial results, it seems as though TMS over rIPL might preferentially affect the ability to detect self on self trials (i.e. when there is no perturbation) If, as argued here, the comparator's representation of the predicted state of the motor system is disrupted by rIPL TMS, then the CNS would no longer have access to what self is, which would, indeed, lead to an increase in the number of inaccurate other responses on self trials.

Interestingly, other non-action-related forms of self/other discrimination have been associated with this same region. In an fMRI study, Uddin et al. (2005) found an increase in activity in the rIPL during a self-face recognition task in which participants had to make self/other judgments of faces morphed to different degrees between the participants own face and that of a gender-matched familiar other. The activity observed in this instance, however, was an increase when making self judgments (opposite to that observed with self/other action discrimination) and a follow-up TMS study (Uddin et al., 2006) demonstrated that disruption to the rIPL lead to a reduction in other judgments. These findings highlight one of the inherent problems with self-other judgment tasks, as they involve two different processes: a lower-level feeling of agency and a higher-level judgment of agency. This is a key criticism of both self/other judgment tasks and comparator model explanations in which there is no distinction between the lower-order sensations of otherness from higher-order overt categorical judgments (Gallagher, 2007; Synofzik et al., 2007).

There are, however, two crucial differences between the Uddin et al. (2006) study and the current experiment: first, in Uddin et al.' s work TMS was applied prior to the experimental procedure for 20 min at 1 Hz which has the effect of depressing the stimulated area for a prolonged period rather than at a specific time period during an individual process; second, Uddin et al.' s task would not have engaged the neural motor comparator mechanism as there was no motor component as in the current task. It is difficult to determine whether depressing rIPL in Uddin et al.' s experiment depressed the activity of lower-order face processing mechanisms or depressed part of the higher-order network involved in judgments of agency. Due to the transitory nature of the stimulation in the current experiment, however, it was more likely to have influenced lower-order mechanisms than higher-order networks.