Importantly, we found that the TMS over the right FPA modulated the pursuit gain of 200 to 320 ms after the stimuli movement onset in the high load condition. The FPA is directly involved in the control of the pursuit in human (e.g.
Petit & Haxby, 1999;
Rosano et al., 2002). Our finding of reduced pursuit gain by the FPA TMS supports that the FPA contributes to the gain control of the pursuit. First, it is important to check the reliability of the FPA TMS effect found in the present study because of the small magnitude of the FPA TMS effect. We found about 3.5% of decrease in the 200 to 320 ms time window when the TMS was applied to the rFPA in the high-demanding condition. In spite of the small magnitude, the FPA TMS effect is comparable to other studies in magnitude.
Nuding et al. (2009) applied 10 Hz TMS for 500 ms over the left and right FPA and they found that the TMS over the FPA induced 4.3% decrease in steady-state gain (mean eye velocity reduced from 16.1 to 15.4 deg/s) for visually guided pursuit, which was small in magnitude but highly significant. In addition,
Gagnon et al. (2006) found about 4.7% of gain decrease when the TMS was delivered during mid-cycle movement, although they found 43.7% increase in the new pursuit direction when the TMS applied at the target turnaround. In the study of
Gagnon et al. (2006), the FPA TMS site was localized according to individual's fMRI signal while the participants performed smooth pursuit and fixation task. Besides,
Sack et al. (2009) compared four different ways of determining the parietal TMS site, guided by individual fMRI, MRI, Talairach coordinates from a previous literature, and the 10–20 EEG System (P4), and investigated optimal sample size for these different ways. They found that 5 participants were sufficient for the fMRI-guided approach, 9 participants for the MRI-guided approach,13 participants for the Talairach coordinates, and 47 participants were necessary for the P4 TMS stimulation. According to this study, 16 participants used in the current study are enough for investigating the role of the rFPA. Taken together, the small magnitude of the FPA TMS effect in the present study is reliable. Finding of reduced eye velocity gain is consistent with previous TMS studies that demonstrated the role of the FPA in the pursuit gain control (
Gagnon et al., 2006;
Nuding et al., 2009). In these previous studies, observers made smooth pursuit eye movements only without any secondary attentional task. In contrast, our observers conducted a letter detection task in addition to the pursuit task. Hence, the TMS over the FPA enabled us to study the role of the FPA in the pursuit and attention concurrently. Behavioral studies have established close relationship between the pursuit and attention (
Acker & Toone, 1978;
Clementz et al., 1990;
Hutton & Tegally, 2005;
Kerzel et al., 2009;
Stubbs et al., 2018). Here, attention on the pursuit target was successfully manipulated by the letter detection task, as shown by the lower accuracy of letter detection in the high load condition. Importantly, we found the effect of the rFPA TMS on the pursuit gain in the high load condition, where the letter detection task required high level of attentional resources. In contrast, no effect of the rFPA TMS on neither the letter detection nor the pursuit was observed in the low load condition. Considering the role of the FPA in the pursuit gain control (
Gagnon et al., 2006;
Nuding et al., 2009), the pursuit gain control by the FPA may be compensated by attention in the low load condition. However, in the high load condition, there may be no sufficient attentional resources that could compensate for the FPA TMS effect on the pursuit due to high level of attentional consumption by the letter detection task, resulting in evident pursuit gain reduction by the FPA TMS. This argument is acceptable because endowing attention on the pursuit target is not necessary to mean that attention is allocated to the pursuit task. Average data over the participants showed the FPA stimulation tended to benefit the letter detection only if the letter detection task had high level of attentional load, establishing a beneficial role of the FPA in the letter detection. It seems that the suppression of the FPA, induced by the cTBS method (
Huang et al., 2005;
Chung et al., 2016), biased attentional resources to the detection task, consistent with attention resource sharing mechanism (
Navon & Gopher, 1979). Interestingly, the FPA TMS effects correlation analysis showed that individuals who showed less impairment on the pursuit gain induced by the rFPA TMS demonstrated bigger benefit on the detection performance induced by the rFPA TMS. It seems to contradict the finding from the mean data. However, these individual different results also demonstrated the rFPA TMS effect with high attentional demanding task, because no correlations were found for low demanding tasks. Moreover, the TMS over the rFPA may have modulated either the pursuit gain or the letter detection performance. In other words, the rFPA is involved in both the pursuit and attention demanding task and can flexibly contribute to the pursuit or attention demanding task. In all, our results demonstrate that the gain control by the FPA may be mediated by attention.