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Ten right-handed participants (4 male; mean age: 31.5 years; standard deviation: 8.1; Edinburgh Handedness inventory score (mean ± SD) of 78±16.8) with no history of neurological illness, normal or corrected-to-normal vision and reported normal hearing took part in the study after giving written informed consent. Because of technical failure two participants did not participate in the experiments with occipital TMS stimulation. The current comparison between TMS to occipital and parietal cortices therefore focuses on 8 participants that took part in both parts. The study was approved by the Human Research Ethics Committee of the Medical Faculty at the University of Tübingen. The data from IPS stimulation have been included in a previous report [15].
In a sustained spatial attention paradigm, participants detected visual targets that were presented in a placeholder in the left lower quadrant on 50% of the trials. Across sessions we manipulated whether parietal, occipital or Sham-TMS was applied. Moreover, we also manipulated the presence vs. absence of concurrent task-irrelevant auditory inputs across runs. In short, the paradigm conformed to a 2x2x3 factorial design with factors: (i) task-relevant visual input (V present, V absent) (ii) auditory context (A present, A absent) and (iii) TMS condition (right occipital cortex (Occ), right anterior IPS, Sham) (Fig 1A). Hence, our design included the following 4 trial types: (i) visual target present, without sound (V), (ii) visual target absent, without sound (¬V)), (iii) visual target present with sound (AV) and (iv) visual target absent with sound (A). Each trial type was presented with Occ-, IPS- and Sham-TMS resulting in 12 conditions in total. We limited the presentation of the visual target to the left hemifield because right parietal and occipital TMS has previously been shown to elicit different effects for contra- and ipsi-lateral visual stimuli (e.g. [16, 17, 18]). In all trial types, participants reported whether they had detected the visual target (see Visual and auditory stimuli) via a two choice key press. They were instructed to use a strict decision criterion and report that they had detected a target only when being confident and to report ‘unseen’ otherwise. Participants fixated a cross presented in the centre of the screen throughout the entire scanning session.
Figure data removed from full text. Figure identifier and caption: 10.1371/journal.pone.0181438.g001 Experimental design. (A) 2x2x3 factorial design manipulating (i) task-relevant visual input (V present, V absent), (ii) auditory context (A present, A absent) and (iii) TMS condition (Occ, IPS, Sham). (B) Timeline example of stimuli presentation. Blocks of 12 trials started and ended with a grey fixation cross and were interleaved with baseline periods, during which the fixation cross turned red. A trial began when the fixation cross turned blue. In target present trials, the visual stimulus was presented 100 ms after trial begin. After a total period of 600 ms the fixation cross turned back to grey and remained like this until the next trial. (C) Illustration of the concurrent TMS-fMRI protocol and stimuli presentation timing during auditory present runs. Within a block the fixation cross was grey during volume acquisition and blue during the acquisition gaps. At 100 ms after trial begin (i.e. 2790 ms after begin of volume acquisition), the task-relevant visual stimulus was either present (first depicted trial) or absent (second depicted trial). Bursts of 4 TMS pulses were applied during acquisiton gaps at 10 Hz and started 100 ms after the target onset time (i.e. 2890 ms after begin of volume acquisition). (D) Illustration of approximate coil positions during i) occipital and ii) parietal stimulation.
At the beginning of each trial, the fixation cross changed its colour from grey to blue (Fig 1B). After 100 ms the visual target was presented for a duration of 16 ms with 50% probability. On each trial, we applied bursts of 4 TMS pulses (or Sham-TMS) 200 ms after trial onset (i.e. 100 ms after target onset in V and AV trials) (see Data acquisition and TMS procedures; Fig 1C). At 600 ms of trial onset, the fixation cross turned back to grey for a duration of 2690 ms until a change in the colour of the fixation cross indicated the onset of the next trial. The interstimulus interval amounted thus to 3290 ms, equalling one TR of the EPI acquisition (see Data acquisition and TMS procedures). In half of the runs, a sound (see Visual and auditory stimuli) was presented synchronously with target onset, regardless of the presence/absence of the task-relevant visual input. Hence, the sound did not predict the presence of the visual stimulus. Nevertheless, the presence of the sounds may have reduced participants’ uncertainty about the temporal onset of the visual target. Auditory sounds were included because it has been shown that the presence of a synchronously presented auditory sound can facilitate the detection of contra-laterally presented visual signals in hemianopia and neglect patients [19]. Yet, the effect of auditory stimuli will not be examined further in this report, which focuses on the methodological aspects of stimulating lower sensory and higher-order association cortices. Blocks of twelve trials were interleaved with fixation baseline periods of 13 seconds. Each block contained an initial period of 2790 ms to signal the upcoming appearance of the first trial of the block (see Fig 1C). Additionally, to avoid carry-over effects of the response to the last trial into the baseline periods, an extra 3290 ms were introduced at the end of each block. These time lengths were chosen based on the EPI acquisition (see Data acquisition and TMS procedures; Fig 1C). Therefore, each block effectively consisted of 13 TRs of the EPI acquisition, resulting in a block length of about 43 seconds. The fixation periods were indicated by a red fixation cross. The beginning and end of the activation blocks were indicated by a grey fixation cross and the detection trials by a blue fixation cross. Hence, the colour of the fixation cross indicated changes in the attentional settings: while blue and grey were associated with a high attentional load, red indicated little attentional demands. Each run encompassed seven activation blocks with 42 target present trials and 42 target absent trials (i.e. 84 trials per run). The data of the main experiment were acquired in three sessions on different days with each session including eight runs. Across days/sessions, we manipulated whether Occ-, IPS- or Sham-TMS was applied. On each day, we manipulated the auditory context across runs within an ABBAABBA design counterbalanced across participants (i.e. 4 ‘auditory context present (A)’ and 4 ‘auditory context absent (B)’ runs per day). The visual target presence was randomized as trials within and across runs. Hence, we obtained a total of 168 trials for each condition (e.g. number of trials for condition ‘visual target present, auditory context present, IPS-TMS: 42 target-present trials per run x 4 runs with auditory context present x 1 TMS session for IPS). Each participant was trained in a minimum of six runs prior to the actual fMRI experiment.
The task-relevant visual stimulus consisted of a small (9x9 pixels, visual angle: 0.52°) square presented for one frame (i.e. 16 ms) on a grey background. The visual stimulus was presented in the centre of a blue placeholder (40x40 pixels, visual angle: 2.3°) that was positioned 12° left and 5° down relative to the fixation cross. The placeholder was displayed throughout the entire run (i.e. including fixation periods). The ‘overall grey level of the target square’ was adjusted with the help of dithering for each participant in a Quest Procedure [20] inside the scanner aiming at a detection threshold of 70% and using the same parameters as in the main experiment. In other words, the ‘overall grey level’ was adjusted by manipulating the density of white pixels within the square. This detection threshold was selected to place increased demands on cognitive resources such as spatial attention. Importantly, identical grey levels were used across Occ, IPS and Sham stimulation and across auditory contexts.
To ensure that auditory stimuli were easily segregated from the scanner noise and TMS clicks we generated an auditory stimulus by adding sinusoidal tones with base frequencies of 130.81 Hz, 164.81 Hz and 196 Hz and the following six terms of their respective geometric progressions (i.e. adding the terms 2n*f, where f represents each of the three base frequencies and 1 ≤ n ≤ 6). Hence, the auditory sound spanned a total of seven octaves and ranged from 130.81 Hz to 12543.58 Hz. The duration of each auditory stimulus was 40 ms. Next, we convolved this auditory signal with spatially specific head-related transfer functions (HRTFs) to create a left localized stimulus. This will provide participants with audiovisual spatial localization cues and thereby enhance audiovisual integration. The HRTFs were pseudo-individualized by matching participants’ head width, height, depth and circumference to the anthropometry of participants in the CIPIC database [21].
Visual and auditory stimuli were presented using Psychophysics Toolbox version 3.0.10 [22, 23] running on MATLAB 7.9 (MathWorks Inc, MA, USA) and a Macintosh laptop running OS-X 10.6.8 (Apple Inc, CA, USA). The visual stimulus was back-projected onto a frosted Plexiglas screen using a LCD projector (JVC Ltd., Yokohama, Japan; resolution: 800x600 pixels, refresh rate: 60 Hz, viewing distance: 48 cm) visible to the participant through a mirror mounted on the MR head coil. Auditory stimuli were presented via MR-compatible electrodynamic headphones at a sampling frequency of 44100 Hz (MR Confon GmbH). Furthermore, earplugs were used to attenuate both scanner and TMS noise. Participants indicated their response (i.e. visual target seen or unseen) with their right hand using an MR-compatible custom-built button device connected to the stimulus computer.
TMS was applied over the right anterior IPS and the right occipital cortex (Occ) as experimental sites and Sham TMS was included as a control condition. For the parietal stimulation site, we selected the MNI coordinates (x = 42.3, y = -50.3, z = 64.4) that had previously been associated with impairment of visuospatial processing by Oliver et al [24]. For the occipital stimulation site, we determined the MNI coordinates in an initial pilot hunting procedure outside the MRI scanner in seven additional participants. The hunting procedure used the same visual display and task as in the main experiment, established protocols for visual suppression (i.e. single TMS pulse applied 100 ms after target onset; [25]) and a 3x3 grid of coil positions with an equidistant spacing of 1 cm, placed over the right hemisphere guided by Thielscher et al. [26]. The positions showing larger decrements in detection performance across participants were located in the middle row of the grid (hit rates (± SD) from left to right were 41 ± 26%, 43 ± 26% and 42 ±16%, respectively). As the final location for occipital TMS stimulation we selected the MNI coordinates that were associated with the maximal decrement in detection performance across participants (MNI coordinates: x = 19.42, y = -102.35, z = 13.4). Based on cytoarchitectonic probabilistic mapping [27] this position is located in BA17/18. With the bursts of 4 pulses and the stimulation intensity used in the TMS-fMRI experiment (see Data acquisition and TMS procedures), this position did not induce peripheral nerve stimulation that could cause discomfort to the participants. However, occipital stimulation inside the scanner inevitably involves lying on the TMS coil, which is associated with additional discomfort. Individual stimulation coordinates for both experimental TMS sites were determined by inverse transforming the MNI coordinates for parietal and occipital targets into native space using the parameters obtained from spatial normalization. These coordinates were entered in the neuronavigation system, which was used to mark the desired position on the participants’ skull. For occipital stimulation, the coil was oriented so that the electric field in the stimulation hotspot was oriented from lateral to medial for the second half of the biphasic stimuli. For IPS stimulation the coil was oriented so that the current flow was from anteromedial to posterolateral in the second half of the stimulus (Fig 1D). A posteriori coil reconstruction of the coil position was based on custom-written MATLAB (MathWorks Inc, MA, USA) scripts. The centre of the TMS coil and its circumference were marked with Vitamin E capsules and a water tube, respectively, to enable the automatic co-registration of the coil representation in the FLASH images with a pre-acquired reference image of the coil. The coil representation included a line passing perpendicularly (to coil surface) through the centre of the coil, which allowed determining the coordinates where it first touched the cortical surface. In addition, the subject’s head in the FLASH images was co-registered to the high-resolution structural scan. Thereby, we were able to determine the coil position inside the scanner with respect to an individual’s structural MRI. Across participants, the target IPS coordinates were obtained with a mean deviance of 9 mm ± 2.5 (mean, SD). The across-participants mean coordinate in MNI space was (x = 34.7, y = -52.8, z = 63.5). The across-participants mean of the target Occ coordinates in MNI space was (x = 18.8, y = -100.9, z = 11.4) with a mean deviance of 5.1 mm ± 1.7 (mean, SD). Due to the quadratic decay of the TMS-induced magnetic field and the use of a fixed stimulation intensity across all participants and stimulation sites (see Data acquisition and TMS procedures), it is important to make sure that coil-cortex distances do not significantly differ between the two TMS conditions, as these could confound the comparisons of the TMS-induced cortical effects between the two sites. For each participant and each stimulation location, we calculated the Euclidean distance between the centre of the coil and the reached cortical coordinates (i.e. the location where the normal to the coil intersects with the brain surface). A paired t-test showed that the distances between coil and this location defined on the brain surface did not differ significantly for the two stimulation sites (t(8) = -1.764, p = 0.12). These results suggest that differences in TMS effects for the different TMS sites cannot be explained by differences in the coil-brain surface distances across participants. In the sham condition, 2 cm thick plastic plates were fixed between the TMS coil and the skull. Given the quadratic decay of the TMS-induced magnetic field, this Sham condition precluded the effects of direct brain stimulation. Indeed, when tested over the finger region of the motor cortex, this Sham condition did not induce muscular twitches on pre-activated finger muscles even at 100% of total output intensity. During the Sham condition the coil was placed over the right hemisphere in a middle position between experimental locations, given the space constraints inside the MR coil. Critically, the Sham-TMS condition tightly controlled for the TMS side effects such as the TMS-noise and feelings of vibrations. Furthermore, comparing IPS-TMS or Occ-TMS with Sham-TMS did not elicit significant activations in the auditory cortex. Hence, we concluded that this particular application of Sham-TMS inside the scanner is a better control than low intensity TMS that does not control effectively for auditory confounds (see Data acquisition and TMS procedures and [14]).
Data acquisition and TMS procedures: A 3T TIM Trio System (Siemens, Erlangen, Germany) was used to acquire both a T1-weighted three-dimensional high-resolution structural image (MPRAGE, 176 sagittal slices, TR = 2300 ms, TE = 2.98 ms, TI = 1100 ms, flip angle = 9°, FOV = 240 mm x 256 mm, image matrix = 240 x 256, voxel size = 1 mm x 1 mm x 1 mm, using a 12-channel head coil) and T2*-weighted axial echoplanar images (EPI) with blood oxygenation level dependent (BOLD) contrast (GE-EPI, TR = 3290 ms, TE = 35 ms, flip angle = 90°, FOV = 192 mm x 192 mm, image matrix 64 x 64, 40 axial slices acquired sequentially in ascending direction, slice thickness = 3 mm, interslice gap = 0.3 mm, voxel size = 3 mm x 3 mm x 3.3 mm, using a 1-channel Tx/Rx head coil). Each participant took part in a total of eight experimental runs per TMS condition. A total of 124 volume images were acquired for each run. After each EPI run, a fast structural image (fast low-angle shot [FLASH], 100 axial slices, TR = 564 ms, TE = 2.46 ms, FOV = 256mm x 256 mm, image matrix = 256x256, voxel size = 1x1x3 mm) was acquired to enable a posteriori reconstruction of the TMS coil position inside the scanner, as described elsewhere [14]. The EPI sequence was adapted for concurrent TMS-fMRI experiments by introducing gaps of 600 ms after every volume acquisition. Each gap was introduced to allow the delivery of four TMS pulses without interference with image quality [28, 29]. While this fixed relationship between TMS and MR acquisition does not enable continuous sampling of the hemodynamic response function, it is a common practice in concurrent TMS-fMRI study to minimize image artefacts caused by the application of a TMS pulse. Bursts of four pulses at 10 Hz were applied every trial, with the first pulse applied 2890 ms after begin of volume acquisition, i.e., 100 ms after stimulus onset (Fig 1C). TMS pulses were applied after stimulus onset in order to minimize crossmodal interaction effects between our stimuli and the TMS induced auditory and somatosensory side effects [14, 30]. Similar TMS protocols have been used over the parietal cortex in TMS studies outside the scanner [24, 31] and in concurrent TMS-fMRI studies [10–12, 32–34] investigating the effects of parietal TMS on visuospatial processing. However, occipital TMS studies have mainly been performed using a single or double pulse TMS, in which an appropriate timing relative to stimulus onset (effective time window: 80–110 ms) is crucial in the generation of suppression effects [35, 36]. To allow for comparison between parietal and occipital TMS conditions, we also applied bursts of 4 TMS pulses in the occipital stimulation with the first pulse being applied within the classical effective suppression time window and the remaining three pulses in a wider window that may affect higher order visual processing (see [25]). Likewise, the TMS intensities of each pulse were the same (see below) across parietal and occipital stimulation conditions. Consequently, the intensity of the occipital stimulation was most likely in a range where it perturbs activity locally under the coil and in remote brain areas as well as effective connectivity between brain areas rather than actively impairing visual detection performance. Indeed, previous studies have shown that visual evoked potentials can be influenced at TMS intensities that do not yet affect participants’ visual discrimination performance (e.g. [37]). Biphasic stimuli were delivered using a MagPro X100 stimulator (MagVenture, Denmark) and a MR-compatible figure of eight TMS coil (MRi-B88), using the same coil-holding device as described in Moisa et al [29]. To prevent the propagation of RF noise into the MR room, the TMS stimulator was placed outside the MR room and was connect to the TMS coil via a high-current filter (E-LMF-4071; ETS- Lindgren, St. Louis, MO, USA) attached to the copper shielding of the MR room [29]. During occipital stimulation the coil was directly placed on a cushion in the RF coil with the major axis oriented parallel to the scanner bore axis and the cable pointing to the right relative to participants’ heads ([38] and Fig 1D). During IPS and Occ stimulation, a fixed TMS intensity of 69% of total stimulator output was used for all participants. This corresponded to 125% of the mean resting motor threshold, as determined across twenty-four participants of prior studies using the same coil. To ensure similar somatosensory side effects between experimental conditions and Sham-TMS the TMS intensity was increased to 75% of total stimulator output during the Sham condition based on the subjective report of two naïve participants that participated in a pilot test. Extensive image quality tests of our setup are reported elsewhere ([29], [39]: Supplementary Material). For completeness, we acquired EPI data with a phantom using the same experimental design. After realignment, data were entered in a first level analysis using the same model as for the real participants. Computing all the relevant contrasts (height threshold: p < 0.01 uncorrected) yielded only a spurious and randomly distributed pattern.
The fMRI data were analysed using SPM8 (Wellcome Department of Imaging Neuroscience, London; www.fil.ion.ucl.ac.uk/spm) [40]. Scans from each participant were realigned using the first as a reference, unwarped, spatially normalized into MNI space, resampled to a spatial resolution of 2 x 2 x 2 mm3, and spatially smoothed with a Gaussian kernel of 8 mm full-width at half-maximum. The time series of all voxels were high-pass filtered to 1/128 Hz. The first 3 volumes were discarded to allow for T1-equilibration effects. The fMRI experiment was modeled as a mixed block-event-related design. Individual trials were modeled as events and entered into a design matrix after convolution with a canonical hemodynamic function and its first temporal derivative. Each run included separate regressors for visual present vs. visual absent trials as two types of events. In addition to modeling these two trial types, our statistical model modeled block begin and end (i.e. the periods during which the fixation cross was grey at the beginning and end of a block; see Experimental design and task) as mini blocks of 2.69 s and 3.29 s duration, respectively. These additional regressors were included to explicitly account for the increased attentional demands at the beginning and end of the task blocks (note that models that did not include these additional regressors provided basically equivalent results for contrasts combining the remaining regressors). To allow for a more efficient estimation we concatenated the four runs for each of the 3 (Occ, IPS, Sham-TMS) x 2 (auditory present vs. absent) combinations and modeled the run-specific means as separate regressors. Hence, the factors of auditory context and TMS were manipulated across runs and sessions, respectively. Nuisance covariates included the realignment parameters to account for residual motion artifacts. For each participant, condition specific effects were estimated according to the general linear model by creating contrast images of each condition (i.e. limited to the canonical hemodynamic response) relative to the arbitrary baseline. Hence, the baseline controls for non-specific effects caused by the positioning of the coil (e.g. discomfort during the session with occipital TMS). The statistical comparisons (see details listed below) were entered into independent second-level one-sample t-tests to allow for random effects analyses and inferences at the population level [41]. For full characterization of the data, we report activations at the cluster level at p < 0.1 corrected for multiple comparisons (family-wise error rate) within the entire brain based on non-parametric permutation testing [42, 43] using an auxiliary uncorrected voxel threshold of p = 0.01. However, we only discuss activations that are significant at p<0.05 corrected for multiple comparisons in the entire brain. Moreover, based on our apriori hypothesis and questions we report activations at the voxel level corrected for multiple comparisons within right IPS and right lower level visual areas (i.e. V1 + V2) as our regions of interest (ROI) where Occ and IPS-TMS were applied. The regions of interest were defined based on cytoarchitectonic probabilistic mapping [27]. Right IPS combined right hIP1, hIP2 and hIP3, while low level right visual regions included right hOC1 and hOC2. Main effects of task-relevant visual input: Main effects of task-relevant visual input were tested for by comparing visual target present (= AV + V) and visual target absent trials (= A + ¬V) pooled (i.e. summed) over TMS conditions (i.e. we performed the comparisons “target present > target absent” and “target absent > target present” pooled over the remaining factors). Since TMS effects in the absence of a neutral control condition may result in interpretational ambiguities, comparisons between TMS conditions were implemented in a paired-wise fashion by comparing each experimental condition (IPS or Occ) with the control condition (Sham). While not the main focus of this study, comparisons between each experimental condition with each other were also computed for completeness. Hence, effects of IPS-TMS were identified by comparing IPS > Sham and Sham > IPS pooled (i.e. summed) over conditions. Equivalent contrasts were calculated for Occ-TMS and for direct comparisons between the two experimental conditions. Please note that formally these ‘main effects of TMS’ test for an interaction (e.g. main effect of Occ-TMS > Sham can be more precisely written as: (TMS stimulation—Baseline) under Occ-TMS > (TMS stimulation—Baseline) under Sham-TMS). Hence, this interaction contrast controls for non-specific TMS effects such as discomfort that may have been increased for Occ-TMS.
State-dependent TMS effects: Interaction effects between visual input and TMS: Likewise, interaction effects between task-relevant visual input and TMS conditions were evaluated through direct comparisons between TMS conditions. Consequently, interaction contrasts for (target present > target absent)IPS (or Occ) > (target present > target absent)Sham and (target absent > target present)IPS (or Occ) > (target absent > target present)Sham were estimated. Interaction effects between Occ- and IPS-TMS were directly evaluated in the same manner.
It is well established that in addition to being able to disrupt visual perception, TMS over the occipital cortex can elicit transient perceptions of light known as phosphenes. The ability to perceive phosphenes is quite variable across participants [44–46] and strongly depends on the amount of attention given to them [26, 36]. On the other hand, the intensity needed to elicit phosphenes is generally lower than the one necessary to induce effective suppression effects [26, 47, 48]. As phosphenes are elicited contra-laterally to the stimulated hemisphere, they could have potentially interfered with task performance [47–49]. To evaluate the effect of phosphenes in our study, at the end of the Occ-TMS session we asked participants whether they had perceived anything else apart from the standard visual display and requested them to draw what they saw. Only three participants reported having seen something, of which only two described image distortions near the placeholder. However, the reported distortions by one of these two participants were not specific to the vicinity of the placeholder but extended throughout the entire visual display comprising also the right visual field (i.e. ipsi-laterally to TMS stimulation), which suggests that the reported distortions were not ‘authentic’ phosphenes. The remaining participant that reported visual distortions adjacent to the placeholder also reported that these distortions were distinct from the task-relevant visual stimulus. Hence, in total it is unlikely that phosphene perception influenced our results on the group level both behaviourally and for the BOLD measurements.
Eye monitoring (outside the scanner): To ensure that the observed activation pattern did not result from eye movements, twitches, or startle effects, 8 additional participants (one left-handed; 2 male; mean age: 26.9 years; standard deviation: 5.3) took part in a supplementary TMS-psychophysics experiment outside the scanner performed with equivalent parameters and comparable durations. One experimental run was acquired per participant for each TMS condition. To account for the absence of the high-current filter used in the concurrent TMS-MRI setup [29], the TMS intensity was reduced to 63% of total output. Horizontal and vertical eye movements were recorded using an iView XTM RED-III remote eyetracker system (SensoMotoric Instruments Inc., Needham/Boston, MA, USA) (50 Hz sampling rate). The eyetracking system was calibrated using a 13-point calibration. Eye position data were automatically corrected for blinks and converted to radial velocity. For each trial condition the mean distance (degrees) from the fixation cross, the number of saccades (defined by a radial eye velocity threshold > 30°/s for a minimum of 60 ms duration and radial amplitude larger than 5°), and the proportion of blinks were computed for the entire trial duration separately for each individual condition and for baseline periods. Across all participants, saccades were almost completely absent, precluding further statistical analyses for this index. For the two remaining indices we first evaluated whether they differed for activation trials and baseline periods. We therefore compared each index pooled (i.e. averaged) over all activation conditions with baseline periods in a paired t-test. The mean distance from the fixation cross was significantly greater during baseline periods (1.38° ± 0.44) than task trials (0.92° ± 0.25; paired t-test: t(7) = -3.038, p = 0.019). We also observed a non-significant (t(7) = -1.349, p = 0.22) increase in eye blinks for fixation baseline periods (1.79 ± 0.61) relative to activation trials (1.39 ± 0.54). Second, to test for differences across individual task conditions, the two indices were independently entered into 2 (task-relevant visual input: V present, V absent) x 2 (auditory context: A present, A absent) x 3 (TMS: Occ, IPS, Sham) RM-ANOVAs. Importantly, neither of the two RM-ANOVAs revealed any significant main effects or interactions, thereby confirming that there was no significant difference in eye movements amongst our experimental conditions. Collectively, these results demonstrate that differences in eye movements across task conditions are unlikely to account for activation differences across the eight task conditions. However, activation differences between task conditions and fixation baseline may result from eye movements that participants made during the fixation conditions when they recovered from the very demanding sustained attention task. Activations and deactivations relative to fixation baseline may in part be accounted for by eye movement confounds. Hence, the parameter estimate plots for the fMRI data should be interpreted only by comparing the eight task conditions, while the activation or deactivation relative to fixation should not be further interpreted.
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