We assessed the effects of rTMS delivered at the onset of a spatial cue directing attention to a peripheral location on the accuracy and reaction time (RTs) of target identification ~1.5 seconds later (). The effects of rTMS were different at different cortical sites both in terms of RTs (F (3,45)=11.19; p<0.0001) and accuracy of target identification (F (3,45)=10.88; p<0.0001). Magnetic stimulation on right FEF (544 ms ± 31) and right IPS (560 ms ± 32) significantly slowed down target responses as compared to Sham (524 ms ± 29; p<0.001 vs. FEF; p<0.0001 vs. IPS) or right PrCe stimulation (502 ms ± 31; p<0.001 vs. FEF; p<0.0001 vs. IPS). There was no significant difference between right PrCe and Sham stimulation (). Correct responses also occurred less frequently after rTMS on right FEF (86.3 % ±1.7) and right IPS (85.9 % ± 1.9) than after Sham (91.1 % ± 1.1; p<0.0001 vs. FEF; p<0.0001 vs. IPS) or right PrCe stimulation (88.7 % ± 1.9; p<0.01 vs. FEF; p<0.01 vs. IPS)().
Behavioral effects of rTMS at different cortical sites
rTMS did not disrupt the observers’ ability to direct spatial attention to the target location. In fact there was an overall significant main effect of target validity (RTs: valid, 503 ms ± 30; invalid, 552 ms ± 32; F (1,15)=14.84 p<0.002; accuracy: valid, 90.4% correct ± 1.5; invalid, 85.6% correct ± 2 F (1,15)=16.74 p<0.001). However, rTMS of right FEF and right IPS more strongly impaired target detection at unattended (invalidly cued) locations (Accuracy: Validity × rTMS Condition, F(3,45)=2.74 p=0.054) (Supplementary Figure 1
The effect of rTMS at different cortical sites was not differential for left (contralateral) or right (ipsilateral) visual field targets. However, targets presented in the right visual field were identified overall more accurately and more rapidly than targets presented in the left visual field (left VF: 507 msec ± 30.5; right VF: 499 msec ± 30 F (1,15)=18.64 p<0.0005; accuracy: left VF: 90% correct ± 1.6; right VF: 91% correct ± 2 F (1,15)=4.97 p<0.05)(Supplementary Figure 2
; see Supplementary Figure 3
for a complete picture of behavioral results divided by site of stimulation, target visual field, and cue validity).
The visual field lateralization may relate to the well-known superiority of the right visual field (left hemisphere) for alphabetical material (Rizzolatti et al., 1971
). To insure that this effect did not depend on magnetic stimulation, we ran a novel group of healthy volunteers (N=9) without rTMS. Once again right visual field targets were detected faster and more accurately than left visual field targets (RTs: left VF: 663 msec ± 37.4; right VF: 612 msec ± 40 F (1,8)=30.52 p<0.001; accuracy: left VF: 86% correct ± 2.75; right VF: 93% correct ± 1.8 F (1,8)=5.49 p<0.05).
Finally, to verify that the behavioral deficits induced by rTMS in prefrontal and posterior parietal cortex did not reflect a cumulative effect building up over many trials, but actually reflected interference with preparatory processes on a trial-by-trial basis, we checked whether the size of the deficit differed in the first, second, third, and fourth quartile of each block of trials, and found no difference. Although this null result does not rule out that a cumulative effect occurred, it is more consistent with the notion that rTMS mainly interfered on the trial in which it was applied.
Overall these findings indicate that interference with preparatory processes in FEF and IPS during anticipatory visuo-spatial attention significantly altered visual perception of targets presented two seconds later in both visual fields.
The EEG signals chosen for the analysis of alpha rhythms (+0.5 s to +1.5 s after cue onset) were free of rTMS artifacts. Supplementary figure 4a
shows EEG data at parietal and occipital electrodes of interest (P3, P4, O1, O2) from a single subject in the four conditions (Sham, right FEF, right IPS, right PrCe). The rTMS artifact practically lasted the stimulation period (150 msec) plus about 10 msec. Supplementary figure 4b
shows EEG power spectra (3–40 Hz, 1 Hz resolution) for the ‘baseline’ (−1.5 s to −0.5 s before the cue stimulus onset) and the “cue event” period (+0.5 s to 1.5 s). The alpha frequency peak is clearly recognizable at all electrodes of interest, and the profile of the EEG spectra looks regular.
The main question of the study was whether anticipatory alpha rhythms in occipital visual cortex were affected by interference with neural activity in control regions, IPS and FEF, during a delay in which subjects covertly attended to a target location. illustrates the topography of parieto-occipital alpha ERD/ERS in the four conditions (Sham, right FEF, right IPS, right PrCe), during the anticipation of the target. During Sham, we observed a robust bilateral ERD (desynchronization) at both low- and high-frequency alpha sub-bands in parietal-occipital cortex. A weak anticipatory alpha ERD was also observed after rTMS on right FEF and right PrCe. In contrast, right IPS-rTMS abolished the normal desynchronization which was substituted by a paradoxical synchronization, or bilateral increase of alpha power (ERS). This qualitative impression was confirmed by statistical analysis. For the low-frequency alpha ERD/ERS (), an ANOVA showed a significant main effect of site of stimulation (F (3,45)=5.96; p<0.002). This was accounted for by a greater anticipatory alpha power (ERS) for right IPS than Sham (p<0.001), right PrCe (p<0.02), or right FEF stimulation (p<0.02) regardless of electrodes of interest (occipital, parietal) or hemisphere (left, right). The same effect was observed for the high-frequency alpha ERD/ERS (F (3,45)=6.10; p<0.002) with greater anticipatory alpha ERS for right IPS than Sham (p<0.001), right PrCe (p<0.03), or right FEF (p<0.03) stimulation (). In summary, interference with right IPS preparatory activity during spatial attention abolished the normal anticipatory desynchronization of alpha rhythms in parieto-occipital cortex.
Topography of alpha power as function of rTMS conditions
Next, we examined whether IPS- or FEF-rTMS changed the spatially selective topography of alpha desynchronization in parieto-occipital cortex (Worden et al., 2000
; Yamagishi et al., 2005
; Sauseng et al., 2005
; Thut et al., 2006
). As predicted by previous studies, anticipatory alpha ERD in the high-frequency sub-band was stronger over the hemisphere contralateral to the side of attention during Sham (F (1,15)=13.60; p<0.003). Interestingly, a significant contralateral topography was still present after right PrCe stimulation (F (1,15)=6.37; p<0.03), but was completely disrupted by stimulation of both right FEF and right IPS ().
Contralateral spatial selectivity of alpha power by rTMS condition
If the physiological disruption of anticipatory alpha rhythms in occipital cortex after disruption of attention-delay activity in right FEF and right IPS is functionally significant, then we would expect a positive relationship between changes in alpha ERD/ERS and visual performance. A Pearson correlation analysis was run between parieto-occipital alpha ERD/ERS (at electrodes P3, P4, O1, O2) during the cue period, and RTs to target stimuli separately for each stimulation site. Only in the case of right IPS stimulation we found a positive correlation between low-frequency alpha ERD/ERS at the P3 electrode (left parietal region contralateral to the rTMS stimulation) and RTs (r = 0.61 p< 0.01) (). A second positive correlation was found between high-frequency alpha ERD/ERS at the O1 electrode (left occipital region contralateral to the rTMS stimulation) and RTs (r=0.58 p< 0.02) (). These findings suggest that subjects with higher paradoxical synchronization of occipito-parietal cortex after right IPS stimulation tend to identify the target letters more slowly. This relationship is predictive in the sense that the ERD/ERS changes produced by rTMS precede in time the identification of the target.
Across-subject correlation between alpha ERD/ERS and RTs
To more stringently correlate disruption of anticipatory alpha rhythms with target discrimination, we carried out, separately for each stimulation site (Sham, IPS, FEF, PrCe), a within-subject ANOVA with Stimulus (canonical, rotated), RT (fast, slow), and Electrode (P3, O1, P4, O2) as factors (see methods). Only in right FEF we found a significant interaction of Stimulus × RT × Electrode (F(3,45)=2.99; p<0.04) in the high-frequency alpha ERD/ERS (). For targets that were more difficult to discriminate (rotated L or T), there was a stronger paradoxical synchronization after right FEF-rTMS on right parietal (P4) and right occipital (O2) electrodes when subjects were slower to respond; in contrast, a normal desynchronization occurred when subjects responded more quickly to the same stimuli (). In contrast, for canonical letters associated with faster responses and likely requiring less attentional scrutiny, a normal desynchronization was observed in parieto-occipital cortex (). A predictive relationship between disruption of anticipatory alpha rhythms and behavioral performance after right FEF-rTMS was also confirmed across subjects, with a positive correlation between parieto-occipital high-frequency alpha ERD/ERS (at electrodes P4, O2) and slow RTs for rotated targets. As shown in , across-subjects higher alpha power at the P4 electrode (right parietal) after right FEF stimulation was positively correlated with slower (r = 0.49, p= 0.05; red dots, ), but not faster RTs (black dots, ).
Within-subject relationship between alpha ERD/ERS and RTs
Overall these results suggest a strong link between disruption of right FEF preparatory activity, interference with parieto-occipital anticipatory alpha rhythms, and target discrimination. While disruption of IPS preparatory activity may have a more general effect on alpha desynchronization and target detection, preparatory activity in FEF may play an especially important role when visual selection/discrimination is more demanding.
A potential alternative interpretation of our results is that rTMS affects visual perception not by disrupting preparatory processes during the delay, but target evoked activity. We performed a control analysis on the earlier components of parieto-occipital potentials evoked by the target stimulus (P1, N1). The amplitude and latency of the P1-N1 complex were not affected by rTMS during the cue period (p>0.6, Sup. ) at any of the sites. Hence visual discrimination impairment was not related to impaired target processing, but disrupted selection.
A second control analysis ruled out that changes in alpha power during the attention delay were not due to changes of the baseline before cue onset (see methods). This result was confirmed for both low and high-frequency alpha sub-bands (p>0.5 in both cases).
Given the rTMS train was presented coincident with the onset of the cue, an alternative explanation of the behavioral deficits induced by right IPS (and FEF) stimulation was that rTMS disrupted the sensory processing of the cue, the interpretation of its directional information, or the initial shift of attention. Although in the main experiment all subjects reported seeing clearly the cue stimulus, and all modulations on alpha rhythms (and related behavioral correlation) were measured from 0.5 to 1.5 seconds after the onset of the cue, these alternative interpretations, proposed by one of the reviewers, could not be ruled out with the present data set.
In a control experiment on 8 new right-handed healthy volunteers rTMS was delivered as either Sham or on right IPS at two different times: simultaneously to the onset of the cue stimulus (R-IPS t(0)), or 350 msec after the cue (R-IPS t(350)). If the behavioral deficits reported in the main experiment reflect ongoing preparatory processes during the delay, then similar effects should be measured at both intervals. Alternatively, if the behavioral deficits underlie transient processes occurring at the onset of the cue (e.g. cue encoding or shift of attention) then weaker effects should be obtained when the rTMS train is delivered later in the delay.
Irrespective of timing, we replicated the deficits produced on response speed (RTs: F(2,14)=4.92; p<0.025)) and identification accuracy (%correct: F(2,14)=7.03; p<0.01) by right IPS-rTMS (Sup. ). When the stimulation was delivered simultaneously to the onset of the cue (RIPS t(0): 596 ms ± 44) or later in the delay (R-IPS t(350): 603 ms ± 40), target RTs were significantly slower as compared to Sham (560 ms ± 38; p<0.03 vs. R-IPS t(0); p<0.03 vs. R-IPS t(350)). There was no significant difference between R-IPS t(0) and R-IPS t(350) (). Correct responses were also less frequent after R-IPS t(0) (79.4 % ±7.7) and R-IPS t(350) (81.0.x % ± 6.1) stimulation than after Sham (86.8% ± 4.7; p<0.01 vs. R-IPS t(0); p<0.02 vs. R-IPS t(350)) (). Similarly to the main experiment, there was an overall significant effect of target validity (RTs: valid, 554 ms ± 42; invalid, 619 ms ± 40; F (1,7)=6.91 p<0.04; accuracy: valid, 88.2% correct ± 5.3; invalid, 76.6% correct ± 7.0 F (1,7)=7.43 p<0.03). The results of this control experiment closely replicate the main experiments and demonstrate that the behavioral deficits are not due to impaired sensory encoding of the cue, abnormal directional encoding, or early shifts of attention. Rather they support our interpretation of a deficit of spatial maintenance and selection.
Behavioral effects of rTMS as function of time of stimulation during delay