Discussion of Experiment 1

To shed light on the laterality debate of motor imagery, the effect of tDCS on mental rotation of hands and objects was explored. Across three sessions, electrodes were placed bilaterally over the parietal cortices, with either cathodal-inhibitory stimulation over the left parietal lobe while anodal-excitatory stimulation was applied to the right parietal lobe (LPc/RPa), anodal-excitatory stimulation over the left parietal lobe while cathodal-inhibitory stimulation was applied over the right parietal lobe (LPa/RPc), or sham stimulation. It was anticipated that if motor imagery was left hemisphere dominant, cathodal-inhibitory stimulation of the left parietal cortex would reduce reaction time and response accuracy when mentally rotating hands. As visual imagery is heavily right lateralised, it was also predicted that cathodal-inhibitory stimulation of the right parietal cortex would reduce reaction time and response accuracy during object mental rotation.

70 75 80 85 90 95 100 1 2 3 4 A cc ur acy ( %) Difficulty Rank 70 75 80 85 90 95 100 1 2 3 4 A cc u ra cy (% ) Difficulty Rank

Hands Sham Hands LPc/RPa Hands LPa/RPc

Objects Sham Objects LPc/RPa Objects LPa/RPc *

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Exploring the effect of direct-current stimulation on task performance confirmed that cathodal- inhibitory stimulation over the left parietal lobe with anodal-excitatory stimulation over the right parietal lobe (LPc/RPa) enhanced response accuracy. Likewise, anodal-excitatory stimulation over the left parietal lobe with cathodal-inhibitory stimulation over the right parietal lobe (LPa/RPc) reduced response accuracy. Reaction times were not affected by stimulation.

Examining performance during hand mental rotation, both tDCS protocol affected response accuracy, but neither protocol affected reaction times. The effect of tDCS on response accuracy manifested when mentally rotating the most difficult hand orientations (Rank 4); response accuracy was enhanced during cathodal-inhibitory stimulation over the left parietal lobe with anodal-excitatory stimulation over the right parietal lobe (LPc/RPa) compared to reduced accuracy during anodal-excitatory stimulation over the left parietal lobe with cathodal-inhibitory stimulation over the right parietal lobe (LPa/RPc). Based on the implication of the left hemisphere during motor imagery (Haaland et al., 2004; Johnson-Frey et al., 2005; Muhlau et al., 2005), it is possible that the stimulation effect on performance accuracy was caused by modulation of the left parietal cortex. However, these modulatory effects were unexpected, as they did not adhere to the anticipated polarity effects of anodal and cathodal stimulation (i.e. excitatory and inhibitory respectively). As described by Jacobson and colleagues (2012) the inhibitory effects of left parietal anodal stimulation and excitatory effects of left parietal cathodal stimulation on task performance can be explained by the highly variable nature of tDCS during cognitive tasks. Depending on the duration and amplitude of stimulation, the anode and cathode have been shown to have the opposite polarity effects. Consequently, left parietal anodal stimulation may reduce accuracy during motor mental rotation and left parietal cathodal stimulation may improve accuracy.

Instead, it is also possible that the stimulation applied to the right hemisphere is driving the effect. As the current electrode montage does not allow the source of the stimulation effects to be teased apart (i.e. whether performance changes are driven by inhibition of the left hemisphere or excitation of the right hemisphere), a unilateral stimulation protocol was explored in Experiment 2; the target electrode was applied to the left or right parietal cortex while the reference electrode was placed over a neutral frontal reference site. That said it is important to note that task accuracy during both stimulation protocols were comparable to sham. Therefore it is likely that both protocols were having mild effects on task performance, which were only markedly different when compared to each other as opposed to compared to baseline performance. This suggests that motor mental rotation may rely on both motor and spatial processes from the left and right parietal cortices (Vingerhoets, de Lange, Vandemaele, Deblaere, & Achten, 2002).

The marginal and somewhat unpredicted stimulation effects on behaviour may be due to the stimulation sites being too close together. As reported in Wagner and colleagues (2007), there is a greater risk of “shunting” the electrical current over the scalp with increased electrode proximity, resulting in minimal stimulation penetrating cortical tissue. If this is the case, the current may be running over the surface scalp area instead of through the cortical regions of interest. The effect of shunting is of particular relevance to this task due to the bilateral parietal placement of electrodes. Specifically, there was a distance of approximately two to three centimetres between the electrodes, whereas to minimise the risk of shunting it may be more appropriate to separate electrodes by approximately eight centimetres (Wagner et al., 2007). As the current study was exploring the role of parietal regions in motor and visual imagery with bilateral stimulation, it was not possible to extend the distance between electrodes by much to reach the desirable separation between the electrodes.

Taking this into account, it cannot be determined whether one stimulation protocol was more effective than the other, given that performance during both protocols were comparable to sham. Likewise, due to bilateral tDCS electrode placement, it is uncertain whether accuracy was affected by modulation of the left or right parietal cortex. In other words, accuracy may have improved during left parietal cathodal-inhibitory and right parietal anodal-excitatory stimulation either due to the effects of the cathode on the left hemisphere, the effects of the anode on the right hemisphere, or a relationship between both left and right parietal stimulation (i.e. modulating the balance between parietal cortices). This also applies to the reduced performance found during left parietal anodal-excitatory and right parietal cathodal-inhibitory stimulation. Nevertheless, it can be concluded that placing the electrodes bilaterally over both parietal cortices may modulate performance accuracy during motor mental rotation tasks.

The lack of stimulation effects on reaction time may be due to task difficulty masking the effects of stimulation. If participants were responding slowly overall, it would be difficult to detect subtle changes in reaction time due to stimulation. Further, if participants try to maintain their response speed in more difficult trials, it might result in speed-accuracy trade-off compromising performance accuracy as opposed to speed. However this is speculative. Task difficulty may also explain why neither stimulation protocol (right parietal cathodal-inhibitory with left parietal anodal-excitatory or right parietal anodal-excitatory with left parietal cathodal-inhibitory) affected performance during the visual imagery control task, object mental rotation. Based on results indicating that object mental rotation is right lateralised (Corballis, 1997; Bricolo et al., 2000; Dong et al., 2000; Rumiati et al., 2001; Tomasino et al., 2003a; Tomasino et al., 2003b; Zacks et al., 2003a; Zacks et al., 2003b), it was anticipated that modulation of the right parietal cortex using tDCS would affect reaction time or accuracy performance during this task. Although

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observable stimulation effects are expected when stimuli are presented in their most difficult orientations, reaction times during object mental rotation were considerably longer than hand mental rotation; on average participants took approximately three to five seconds to respond during object mental rotation compared to one and a half seconds when rotating hands. It is possible that participants are taking too long for the subtle stimulation effects to be observed. Likewise, the average accuracy during object mental rotation was approximately eight percent less than hand mental rotation. It may therefore be necessary to reduce task difficulty in order to confirm whether stimulation is affecting object mental rotation performance.

Based on the points listed above, it is important to explore the efficacy of obtaining a robust effect of direct-current stimulation during cognitive tasks exploring motor and visual imagery. Experiment 2 used different electrode montages to establish the optimum tDCS application to produce modulatory effects and to shed light on uncertainties highlighted in Experiment 1. In particular, given the unexpected effects of cathodal-inhibitory and anodal-excitatory stimulation in Experiment 1, the use of different electrode montages would indicate whether performance differences found here are driven by facilitation of the left hemisphere or inhibition of the right hemisphere (and vice versa). The stimuli used in the object mental rotation task were also changed.