Modulation of the spontaneous brain activity and functional connectivity in the triple resting-state networks following the visual oddball paradigm

The default mode network (DMN), the salience network (SN), and the central executive network (CEN) could be considered as the core resting-state brain networks (RSN) due to their involvement in a wide range of cognitive tasks. Despite the large body of knowledge relating to their regional spontaneous activity (RSA) and functional connectivity (FC) of these networks, less is known about the influence of task-associated activity on these parameters and on the interaction between these three networks. We have investigated the effects of the visual-oddball paradigm on three fMRI measures (amplitude of low-frequency fluctuations for RSA, regional homogeneity for local FC, and degree centrality for global FC) in these three core RSN networks. A rest-task-rest paradigm was used and the RSNs were identified using independent component analysis (ICA) on the resting-state data. We found that the task-related brain activity induced different patterns of significant changes within the three RS networks. Most changes were strongly associated with the task performance. Furthermore, the task-activity significantly increased the inter-network correlations between the SN and CEN as well as between the DMN and CEN, but not between the DMN and SN. A significant dynamical change in RSA, alongside local and global FC within the three core resting-state networks following a simple cognitive activity may be an expression of the distinct involvement of these networks in the performance of the task and their various outcomes.


146
resting-state conditions (pre-and post-task resting-state, R1 and R2) and the task 147 condition composed of three subtasks of the visual oddball paradigm (VOP).

149
During RS conditions, the subjects were instructed to close their eyes and not focus on 150 any specific thoughts. All the fMRI data were acquired in a single scanning session and 151 instructions were given to the subjects in-between each condition via a microphone.

153
The visual oddball paradigm comprises of three subtasks: passive (T1), count (T2), and 154 respond (T3). Two different colored circles were established as frequent (yellow circles) and target 155 (blue circles) stimuli. During the passive condition, the subjects were asked to simply keep the 156 stimuli under observation. During the count condition, the subjects were asked to count the target 157 stimuli (blue circles), and during the respond condition, the subjects were instructed to press a 158 button with their right index finger as soon as they recognised the target stimuli.

177
Structural images were acquired using a magnetization prepared rapid gradient echo (MP-

R2
Voxel-level fMRI measures during the second (post-task) RS.

T1, T2, T3
Voxel-level fMRI measures during the three subtasks of the visual oddball paradigm.

RS difference (RSD)
Difference between post-and pre-task RS (R2 -R1) in the voxel-level fMRI measures for each subject.

RS similarity (RSS)
Correlation coefficient between R1 and R2 for each subject.

Task effect at the group level
Correlation coefficients between the differences (Task (whole) -R1) and (R2 -R1).
All correlation coefficients were computed using Pearson's correlation coefficients at a significance level of p < 0.05.

253
The inter-network FC of the three networks were calculated by first extracting the mean of 254 the BOLD signal time series from the binarized mask of each network, followed by the computation of the Pearson's correlation coefficient between each pair of networks. Fisher r to z 256 transformation was performed to improve the normal distribution. A paired t-test was used to 257 examine the difference of FC between the pre-and the post-task RS.

259
To investigate the relationship between the behavioral data (e.g. reaction time) and the 260 fMRI measures, the correlation coefficients between the RSD and subject's reaction time in the 261 response condition was performed. Having checked the normality of the data using the 262 Kolmogorov-Smirnov test, a paired-sample t-test was used in order to find the differences 263 between the pre-and post-task RS in each fMRI measure.

265
Behavioural data 266 267 The mean reaction time of the respond condition was 477ms (SD = 13).

268
Imaging data -task data 269 270 The task data were initially analysed and reported following the examination of the first 16  The triple network was identified using group independent component analysis (Fig 2). The fMRI measures showed different values in the RSA and the local and global FC during 297 the different brain-states (rest-task-rest) (Fig 3). These values differed significantly at the group 298 level. A pairwise comparison between the pre-and post-task RS (R1 and R2) revealed significant 299 differences in each network and for each fMRI measure at a significance level of p < 0.05, with 300 exception of the ReHo measure in the SN (Table 2).  The correlation between the differences between post-task and pre-task RS parameters 336 (RSD = R2 -R1) and the fMRI measures resulting from the pure task effects (task (Whole) -R1) are 337 depicted in (Fig 4).

383
Our findings indicate dynamic, disparate alterations in the post-task resting-state brain 384 networks as a function of immediately preceding cognitive experiences. Thereby, the extent of 385 the changes in the RS networks can be said to be closely associated with the magnitude of the 386 direct task-effects measurable during the task performance.   and during creative idea production [85]. Furthermore, a significant interaction between the DMN 445 and the CEN has also been shown during the RS condition [86]. Thereby, this interaction seems 446 to fluctuate dynamically across short time scales [87], indicating that the temporal relationships