Cortical excitability controls the strength of mental imagery
- School of Psychology, University of New South Wales, Australia
- Department of Neurophysiology, Max Planck Institute for Brain Research, Germany
- Brain Imaging Center Frankfurt, Goethe-University Frankfurt, Germany
Figures
Figure 1 with 2 supplements
Timeline of the basic imagery experiment.
Participants were cued to imagine a red-horizontal or a green-vertical Gabor patch for 6–7 s by the letter R or G (respectively). Following this, they were presented with a brief binocular rivalry …
Figure 1—figure supplement 1
Imagery vividness results.
(A) Data shows the correlation between mean vividness ratings (x-axis) and visual imagery priming (y-axis) for participants from both the MRI and tDCS experiments (tDCS experiments 2, 3 and 4). All …
Figure 1—figure supplement 2
Re-test reliability for imagery strength (A) and Phosphene Thresholds (B).
(A) Scatterplot shows participants’ imagery strength measured by percent of binocular rivalry displays primed before tDCS stimulation across 2 days of testing (pre-anodal and pre-cathodal …
Figure 2 with 3 supplements
Surface-based whole brain analysis of data from two different fMRI resting-state measurements: negative associations with imagery strength in the occipital cortex.
Two columns on the left: results of the main resting-state fMRI data set with a TR of 2 s (TR2). Two columns on the right: results of an additional resting-state fMRI data set with a TR of 1 s (TR1);…
Figure 2—figure supplement 1
Retinotopic ROI anslysis of resting-state fMRI data and it's realtionship with imagery strength.
(A and B) Lateral view of the pial surface (A) and posterior view of the inflated surface (B) of the visual areas that showed a significant negative relationship with imagery. Red = V1, Orange = V2, …
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Figure 2—figure supplement 1—source data 1
fMRI resting state correlation data.
- https://cdn.elifesciences.org/articles/50232/elife-50232-fig2-figsupp1-data1-v1.csv
Figure 2—figure supplement 2
Surface-based brain analysis of data from two different fMRI resting-state measurements and imagery: positive associations with imagery strength in the frontal cortex.
Results of the main resting-state fMRI data set with a TR = 2 s are shown on the left (TR2); results of an additional resting-state fMRI data set with a TR = 1 s are shown on the right (TR1); in …
Figure 2—figure supplement 3
Visual cortex relationships with imagery strength and the number of EPI volumes discarded at the beginning of the run.
To allow for longitudinal magnetization equilibration to stabilize, it is standard to remove at least the first two (or more) volumes from the measurement. With a TR of 2 s, our analyses were …
Figure 3
Scatterplots for TMS phosphene thresholds and mock rivalry data.
(A) Correlation between the inverse phosphene threshold and imagery strength. Individuals with lower cortical excitability in visual cortex tended to have stronger imagery. (B) Correlation between …
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Figure 3—source data 1
TMS inverse phosphene correlation data.
- https://cdn.elifesciences.org/articles/50232/elife-50232-fig3-data1-v1.csv
Figure 4 with 3 supplements
Visual cortex stimulation data.
(A) Effect of visual cortex stimulation on imagery strength at 1mA. The top image shows the tDCS montage, with the active electrode over Oz and the reference electrode on the supraorbital area. The …
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Figure 4—source data 1
1mA occipital tDCS data.
- https://cdn.elifesciences.org/articles/50232/elife-50232-fig4-data1-v1.csv
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Figure 4—source data 2
1.5mA occipital tDCS data.
- https://cdn.elifesciences.org/articles/50232/elife-50232-fig4-data2-v1.csv
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Figure 4—source data 3
1.5mA occipital and sham tDCS data.
- https://cdn.elifesciences.org/articles/50232/elife-50232-fig4-data3-v1.csv
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Figure 4—source data 4
1.5mA occipital TMS + tDCS data.
- https://cdn.elifesciences.org/articles/50232/elife-50232-fig4-data4-v1.csv
Figure 4—figure supplement 1
Raw tDCS imagery strength and difference scores as a function of block for experiments 1, 2 and 4.
(A) Experimental timeline for all tDCS experiments. Spread of individual data points for raw data and difference scores for experiment 1 (1mA Occipital: B and C), experiment 2 (1.5mA Occipital: D …
Figure 4—figure supplement 2
Raw tDCS imagery strength and difference scores as a function of block for experiments 3 and 5.
(A) Experimental timeline for all tDCS experiments. Spread of individual data points for raw data and difference scores for experiment 3 (Occipital stimulation: B and D), experiment 5 (Occipital + …
Figure 4—figure supplement 3
tDCS modulation of phosphene thresholds.
(A) Data shows phosphene thresholds (PT) before cathodal (left-hand side, blue data points) and before anodal (left-hand side, red data points) and after cathodal (right-hand side, blue data points) …
Figure 5
Data for prefrontal cortex stimulation.
(A) Effect of left prefrontal (pFC) cortex stimulation on imagery strength at 1.5mA. The left image shows the tDCS montage, with the active electrode between Fz and F3 and the reference electrode on …
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Figure 5—source data 1
1.5mA Prefrontal tDCS data.
- https://cdn.elifesciences.org/articles/50232/elife-50232-fig5-data1-v1.csv
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Figure 5—source data 2
1.5mA combined tDCS data.
- https://cdn.elifesciences.org/articles/50232/elife-50232-fig5-data2-v1.csv
Tables
Table 1
Summary of montage, intensity, duration, and significance of each tDCS experiment.
| Experiment # | Montage (EEG Coordinates) | Intensity + duration | Notes | Significant |
|---|---|---|---|---|
| 1 Occipital | Active: Inion (Oz) Reference: Supraorbital (Fpz) | 1 mA 15 min | Tested effect on imagery strength | ✗ |
| 2 Occipital | Active: Inion (Oz) Reference: Right Cheek | 1.5 mA 15 min | Tested effect on imagery strength | ✓ |
| 3 Occipital | Active: Inion (Oz) Reference: Right Cheek | 1.5 mA 15 min | Tested effect on imagery strength (additional sham condition) | ✓ |
| 4 Prefrontal | Active: Between F3-Fz Reference: Right Cheek | 1.5 mA 15 min | Tested effect on imagery strength | ✓ |
| 5 Occipital + Prefrontal | Active: Inion (Oz) Active: Between F3-Fz | 1.5 mA 15 min | Tested effect on imagery strength | ✗ |
| Additional control Occipital | Active: Inion (Oz) Reference: Right Cheek | 1.5 mA 15 min | Tested effect on phosphene threshold | ✓ |
Table 2
Exclusion criteria for tDCS experiments.
| Exclusion | Explanation |
|---|---|
| Mock Priming (Higher than 66%) | Mock displays are fake binocular rivalry displays – priming on these trials indicates that participants are showing a response/demand characteristic and as such we cannot trust their priming scores, as they may either be responding in a way that they think we want them too, or they are not attending to the task correctly. A score of more than 66% indicates that the participant has primed on these mock trials more than once. |
| Low Priming (lower than 40%) | Participants whose imagery scores were lower than 40% were excluded, as the score becomes difficult to interpret: The measure of imagery strength is predicated on how the energy of a stimulus impacts on binocular rivalry. Very weak perceptual stimuli prime binocular rivalry up unto a certain point. At this tipping point, as the image becomes stronger, it begins to suppress binocular rivalry (Brascamp et al., 2007). For this reason, very low priming can either mean that the participant’s visual imagery is so strong that it leads to suppression, or that the opposite is the case, and imagery is in fact very weak. Alternatively, it may also be that a participant is not completing the task correctly. 40% was chosen as the cut off as this is 10% lower than chance values (50%). |
| Mixed Percepts (higher than 1/3 or 33%) | We analyse our priming data as percent primed,that is the percentage of trials a participant imagined an image, then saw that image in the following display. Mixed trials cannot be labelled as either ‘primed’ or ‘not primed’, and as such these trials are not included in the analysis. A high number of mixed trials reduces the number of usable trials for analysis. This can lead to large changes that may be spurious (much larger percentage changes due to a single trial) and not due to the stimulation parameters. We excluded an individual’s data set if more than a third of the trials were mixed percept’s. |
| Attrition | Attrition occurred when a participant did not turn up to or cancelled the second and/or third day of testing. |
| Impedance (Exceeding voltage: impedance greater than 55 kΩ) | For safety reasons, our tDCS machine was programmed to shut off once the impedance exceeded 55 kΩ (this is pre-programmed into the tDCS machine). As the participants completed the experiment in a blackened room by themselves we could not monitor the impedance of the machine in real-time and as such the machine could switch off during a block of the experiment. As we cannot know how much stimulation this participant received we are unable to use them reliably in our analysis – as different stimulation durations will lead to different excitability changes. The second (1.5mA Occip + cheek) and fourth (1.5mA pFC + cheek) tDCS experiments were run at the same time. There were a large number of cases of impedance being exceeded in these studies – it was discovered that this was due to a faulty wire, which was replaced halfway through the experimental data collection. |
| Phosphene Determination | If participants reported phosphenes in the wrong visual hemifield (e.g. left visual hemisphere was stimulated and participants reported phosphenes in the left visual hemifield) their data was excluded. Participants who had very large eye-blinks in the REPT procedure were excluded from the experiment, due to this potentially resulting in a missed phosphene leading to incorrect phosphene threshold estimation. |
Table 3
Number of participants excluded per exclusion criteria for tDCS experiments.
| Exclusion criteria | Exp 1 | Exp 2 | Exp 3 | Exp 4 | Exp 5 | Exp 6 | Total |
|---|---|---|---|---|---|---|---|
| Mock priming | 1 | 2 | 2 | 5 | |||
| Low priming | 2 | 2 | 1 | 3 | 8 | ||
| Mixed percept’s | 3 | 2 | 1 | 6 | |||
| Attrition | 1 | 1 | |||||
| Impedance | 9 | 4 | 1 | 2 | 16 | ||
| Incorrect buttons | 1 | 3 | 4 | ||||
| Technical issues | 3 | 2 | 5 | ||||
| Phosphenes | 6 | 6 | |||||
| Total | 5 | 13 | 6 | 7 | 9 | 11 | 51 |
Additional files
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Source code 1
LME code for r analysis.
- https://cdn.elifesciences.org/articles/50232/elife-50232-code1-v1.rtf
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Supplementary file 1
fMRI tables.
- https://cdn.elifesciences.org/articles/50232/elife-50232-supp1-v1.docx
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Transparent reporting form
- https://cdn.elifesciences.org/articles/50232/elife-50232-transrepform-v1.docx
