On average, the ratings of subjective awareness (scale 0–3) were below 1, indicating that the participants were unconscious of the orientation of the prime (1 = ‘I saw something but I did not have any trace of the orientation of the prime) (Fig. 3). The Stimulation Site (2) × VF (2) × TMS (7) anova on the ratings of subjective awareness in the masked prime visibility task revealed a main effect for TMS (F6,78 = 9.89, P < 0.001) which was modified by TMS × Site (F6,78 = 5.02, P = 0.001) and TMS × Site × VF (F6,78 = 3.20, P = 0.016) interactions.
Figure 3. Masked prime visibility task. (Top) The subjective rating of awareness (scale 0–3) and (bottom) recognition accuracy in the contra- and ipsilateral visual field (relative to the position of the TMS coil) when no TMS was applied or when TMS was applied 30, 60, 90, 120, 150 or 180 ms after the onset of the prime.
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In the V1/V2 condition, the effect of TMS (F6,42 = 10.26, P < 0.001) was retinotopic, as indicated by the VF × TMS interaction, F6,42 = 3.73, P = 0.013. TMS influenced subjective awareness of masked primes in the contralateral VF (F6,42 = 8.06, P < 0.001) by reducing it at the SOAs of 60 ms (P = 0.022) and 90 ms (P = 0.019) relative to no TMS and by increasing it at the SOA of 180 ms above the level observed in no TMS baseline (P = 0.043). TMS over V1/V2 influenced subjective awareness also in response to masked primes in the ipsilateral VF (F6,42 = 2.54, P = 0.043). This effect is due to the decrease of awareness at the shortest (30 ms) SOA (P = 0.009) relative to no TMS. In the ipsilateral VF, TMS did not improve awareness above the no TMS baseline level at any of the SOAs. Stimulation of LO did not have any statistically significant effects on subjective visibility of masked primes.
Thus, as can be expected on the basis of the findings in the conscious recognition task, TMS on V1/V2 impaired visibility of masked primes presented to the contralateral VF in the time windows (60 and 90 ms SOAs) where it also had the strongest effects on conscious perception of the unmasked prime. In later time windows, TMS increased the visibility of masked primes. This effect must be an indirect one in which TMS influenced processing of the target (i.e. mask) by reducing its masking effectiveness and hence indirectly enhanced the visibility of the prime (see also Ro et al., 2003). Figure 3 reveals that this effect seems to begin even at an SOA of 120 ms where the visibility of masked primes in the contralateral VF increases nonsignificantly above that of primes in the ipsilateral VF. It is important to keep in mind that the SOAs here refer to the interval between the onsets of prime and TMS. The asynchrony between the onsets of the prime and target was 67 ms. Thus, at the 120-ms prime–TMS SOA, the TMS pulse is delivered 53 ms after the onset of the target; the results of the conscious recognition task indicated that at this latency range TMS over V1/V2 impaired conscious recognition of the prime.
Stimulation Site (2) × VF (2) × TMS (7) anova on accuracy (Fig. 3) in the masked prime visibility task showed only a Site × VF interaction (F1,13 = 4.81, P = 0.047). This result was due to the fact that in the V1/V2 condition performance was better in the ipsilateral than in the contralateral VF (F1,7 = 13.90, P = 0.007), while there was no differences between VFs in the LO condition. In the contralateral field of V1/V2 condition, forced-choice performance did not exceed the 50% chance level in any TMS conditions (one-sample t-tests, P > 0.11); in the ipsilateral field, forced-choice accuracy was higher than expected by chance in the no TMS baseline (P = 0.014) and at the SOAs of 30 ms (P = 0.005) and 180 ms (P = 0.03). In the LO condition, accuracy never exceeded the chance level (P > 0.05). Note that the subjective visibility rating must be considered as the primary measure of awareness because it more directly assesses the conscious experience of observers and because forced-choice accuracy in the absence of reported awareness may exceed the chance level under some circumstances (Boyer et al., 2005; Koivisto et al., 2010, 2011b).
Analyses of the conscious recognition of unmasked primes and masked prime visibility task suggest that TMS over V1/V2 at the SOAs of 30, 60 and 90 ms did not indirectly influence the processing of primes via affecting the processing of the target (i.e. mask). Therefore, the primarily statistical analyses of the V1/V2 condition focused on these SOAs. In the LO condition, all six SOAs were included as there was no evidence that TMS over LO would influence processing of primes indirectly via affecting the processing of the targets. Priming in RT measures and accuracy was defined as the difference between responses to incongruent and congruent trials. Accuracy data replicated the RT results and are available online as Supporting Information (Fig. S1).
The 2 × 2 × 4 anova on RTs in the V1/V2 condition (Fig. 4) shows a main effect of congruency (F1,7 = 21.45, P = 0.002), indicating that priming occurred. RTs were longer in response to targets in the contralateral VF as compared with those in the ipsilateral VF (F1,7 = 9.63, P = 0.017). VF × Congruency interaction shows (F1,7 = 27.44, P = 0.001) that there was larger priming in the ipsi- than contralateral VF. Most importantly, TMS influenced priming effects differently in the VFs (VF × Congruency × TMS: F3,21 = 3.64, P = 0.035). Separate analysis of the data from the contralateral VFs showed a Congruency × TMS interaction (F3,21 = 5.79, P = 0.005), indicating that TMS to V1/V2 reduced priming. Post hoc comparisons indicated that priming effects were reduced by TMS relative to no TMS condition at SOAs of 30 ms (P = 0.006), 60 ms (P = 0.024) and 90 ms (P = 0.026). Further analyses comparing RTs to congruent and incongruent targets showed that the priming effect in the contralateral VF was significant in the no TMS condition (t7 = 4.28, P = 0.004), but not at SOAs of 30 ms (t7 = 1.07) and 60 ms (t7 = 1.46). At an SOA of 90 ms, the priming effect was significant (t7 = 3.15, P = 0.016), although it was smaller than that in the no TMS condition. In the ipsilateral VF, congruency did not interact with TMS (F3,21 = 2.27, P = 0.133), suggesting that TMS to V1/V2 did interfere with priming only in the contralateral VF relative to the position of the TMS coil.
Figure 4. Response times in the priming task to targets presented in the contralateral and ipsilateral visual field (relative to the position of the TMS coil) when no TMS was applied or when TMS was applied 30, 60, 90, 120, 150 or 180 ms after the onset of the prime. In congruent trials the prime and target had the same left–right orientation, whereas in incongruent trials it was different. The priming effect is the difference in response times between congruent and incongruent trials. Asterisks indicate the latencies at which TMS impaired priming relative to no TMS baseline. The data points on the left side of the dotted vertical line (upper-left panel) represent data points in which TMS to early visual cortex did not impair processing of targets which served also as masks.
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Analysis of the longer SOAs of 120, 150 and 180 ms (at which TMS over V1/V2 directly affected the processing of the target) revealed a reliable effect for congruency (i.e. priming) (F1,7 = 2.27, P = 0.009), which did not interact with TMS or VF. It is important to note that there is no indication (Fig. 4, upper-left panel) that TMS would interfere with priming at these long SOAs. It seems to slow down specifically the RTs to incongruent targets in the contralateral field while the responses to congruent targets are less affected. This pattern is expected because the incongruent condition is more demanding and hence more vulnerable to interference.
The 2 × 2 × 7 anova on the LO condition revealed a highly significant main effect of congruency (F1,6 = 125.11, P < 0.001), but no interactions. Thus, it appeared that TMS applied to LO did not affect priming. However, as each participant was tested in two separate sessions, we observed after the first sessions that all the participants in the LO condition showed TMS-induced suppression of priming in the contralateral field at an SOA of 120 ms (62 ms priming effect in the no TMS condition, 30 ms at an SOA of 120 ms; TMS × Congruency: F1,6 = 16.42, P = 0.007), while there was no corresponding suppression in the ipsilateral VF. This unexpected pattern suggests that the role of LO in priming may change as a function of training. This hypothesis deserves further testing.
To perform a stronger and independent test of whether the contribution of LO was critical for priming only in the beginning of the priming task, we analysed separately the RTs in the beginning of the task (the first two TMS and no TMS blocks of the first session) and in the end of the task (the last two TMS and no TMS blocks of the second session) (Fig. 5). Thus, the trials in the middle of the task were excluded from the analyses. The Period (2: beginning vs. end) × VF (2) × Congruency (2) × TMS (7) anova on RTs in the LO stimulation condition showed a significant four-way interaction (F6,36 = 3.39, P = 0.015), confirming that TMS had a retinotopic effect on priming only at the beginning of the task. At the beginning of the priming task, TMS suppressed priming in relation to no TMS baseline in the contralateral VF at SOAs of 90 ms (F1,6 = 10.50, P = 0.018) and 120 ms (F1,6 = 12.50, P = 0.012), and marginally significantly at an SOA of 150 ms (F1,6 = 5.87, P = 0.053). In the ipsilateral VF, the priming effects did not differ significantly from those in the no TMS condition at any of the SOAs. At the end of the task, TMS did not significantly suppress priming either in the contra- or the ipsilateral field at any of the SOAs. Thus, TMS to LO interfered with priming only at the beginning of the task. Analysis of V1/V2 stimulation condition did not reveal this kind of phenomenon.
Figure 5. Response times to targets in the contralateral and ipsilateral visual field (relative to the position of the TMS coil) at the beginning and at the end of the priming task in the lateral occipital cortex stimulation condition. The priming effect is the difference in response times between congruent and incongruent trials. Asterisks indicate the latencies at which TMS impaired priming relative to no TMS baseline.
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