3.1 Results of fMRI Imaging Study
Before training, increased BOLD responses to the perception of background scenes without concealed objects vs. inter-stimulus baseline (fixation cross) were highly significant across a wide range of brain areas (), including much of bilateral occipital, temporal and parietal cortex, cerebellum, thalamus and lenticular nuclei ().
Figure 2 BOLD fMRI responses to stimuli. The left hand column shows responses to scenes that do not contain concealed objects significantly greater than the inter-stimulus fixation crosshair for subjects at the novice learning stage, before training. The right (more ...)
BOLD responses to scenes containing concealed objects were compared to those without, which revealed significant differences across a range of brain areas, including bilateral anterior caudate and putamen, occipital cortex, anterior and posterior insulae, parahippocampal gyri (BA 34), cingulate gyri (BA 31 and 32), superior temporal gyri, inferior and superior parietal lobules.
After training to the intermediate learning stage, a more restricted set of brain regions responded with significantly more positive BOLD response to scenes containing concealed objects vs. scenes without, located primarily in anterior brain areas. In addition, a set of posterior brain regions showed significantly more positive BOLD responses to scenes without concealed objects vs. those with, ranging in location from x= +55 to -33, y=-100 to -67 and z=-13 to +36.
For the 7 subjects who were imaged after training to the expert learning stage, a more spatially restricted pattern of activity was found overall, with more positive BOLD response to scenes with concealed objects vs. scenes without extending from the left middle frontal gyrus to the superior temporal gyrus, and medially to the insula, along with other regions.
No significant BOLD response differences were found in the amygdala for scenes containing concealed objects that indicated possible threats vs. those without. At the expert learning stage, the difference between BOLD responses to stimuli with and without concealed objects in the right and left amygdala were T=1.421 and T=1.122, respectively. To examine the possibility that the amygdalae responded both to scenes with and without concealed objects, differences between scenes and inter-stimulus crosshair were examined. However, no significant differences were observed. Taken together, this suggests that the amygdalae are not sensitive to concealed objects that represent potential threats in this laboratory based study.
3.1.4 Effects of Training
ANOVA was used to examine responses to concealed stimuli (concealed object present vs. absent) and training level (novice vs. intermediate). The effects of training, in which responses to stimuli changed significantly with improved performance after training, were found primarily in the right hemisphere (), with the most significant focus located in right middle frontal gyrus.
Figure 3 Results of 2X2 ANOVA of training level (novice vs intermediate) and stimulus type (camouflaged cue present vs absent). Significant F values for training level effect are shown. Arrows indicate significant foci in parahippocampal gyri (PHG), right middle (more ...) 3.1.5 Dynamic Bayesian Network Analysis
The DBN analysis was performed using fMRI data obtained at the novice and intermediate learning stages. DBN revealed some brain networks that were present both before and after training, and others that were only present after training. One network present before training included left supramarginal cortex and right caudate (p=0.0014) and another included right superior frontal and right ventral cortex (p=0.027). Networks found after training but not before included a variety of brain regions. Three of these included right hippocampus, one combined with left middle frontal cortex (p=0.027), the second that included a combination of left fusiform and frontal middle orbital cortex (p=0.036), and the third with left middle temporal cortex (p=0.041). Another network included right fusiform and right inferior parietal cortex (p=0.029).
This imaging study was followed by a series of behavioral learning studies using tDCS. Based on the published literature of object perception and selective attention, and the results of our fMRI studies of concealed object detection described above, we chose the right inferior frontal (found to be most significant using GLM-based methods) and right parietal brain areas (found using DBN) as the most accessible targets for brain stimulation with the highest likelihood of accelerating the learning process.
3.2 Results of tDCS Learning Studies
A total of 83 healthy participants (mean age 24.1 years, range 18 to 38 years, 53 male, 30 female) performed the same learning task with different groups of participants receiving varying levels of tDCS current over different scalp locations. For all tDCS learning studies, a series of four, 12-minute long training blocks were presented during the one-hour learning phase (see ), with tDCS applied during the first two training blocks. For tDCS Learning Experiments 1 and 2, anodal current was applied over the right inferior frontal scalp, located nearby electrode site F10 over the right sphenoid bone, or right temple, with the cathode on the contralateral arm. For 26 subjects in the “full-current” tDCS group, 2.0 mA of current was used. In 37 subjects in the “low-current” tDCS group, 0.1 mA current was used.
shows the change in accuracy obtained across the four, 15-minute training blocks in Experiment 1 and replication of this in a separate group of subjects in Experiment 2. For both Experiments 1 and 2, subjects receiving full-current reached 77% mean accuracy by the 4th training block. The 13 subjects receiving low-current in Experiment 1 reached 69.4% accuracy (SE 3.3%), and the 23 subjects receiving low-current in Experiment 2 reached 67.5% accuracy (SE 3.4%), which was not significantly different between experiments (F(1,35)=1.01, N.S.). Subsequently, data was pooled across both experiments for analysis. During training, accuracy changed significantly across the four training blocks in both the full- and low-current tDCS groups of Experiments 1 and 2 (F(3,183)=89.86, p=8.78×10-36). The 26 subjects receiving full-current tDCS learned significantly more over the one hour of training, compared with the 36 subjects receiving low-current tDCS, as shown by the difference in accuracy between groups during the final training block (F(1,61)=13.16, p=5.86×10-4).
Figure 4 Shows change in accuracy with training across the four tDCS training blocks for tDCS Learning Experiments 1 and 2. Mean accuracy within each training block is indicated for subjects receiving full-current (2.0 mA) tDCS over right inferior frontal cortex (more ...)
For the 36 subjects in Experiment 2 (23 of these subjects receiving low-current tDCS and 13 receiving full-current tDCS), detection accuracy was tested immediately before training, immediately after training, and then again after a one-hour rest period. Pre-training test data were obtained in order to ascertain baseline levels of accuracy resulting from prior experience before training, and to verify that these stimuli were impossible to discriminate without training. Before training, accuracy was close to chance (50.75% mean accuracy, SE 0.64%), confirming that scenes containing concealed objects were initially indistinguishable from scenes without concealed objects, and that training was required to perform this task.
Subjects receiving low-current tDCS improved their accuracy by 14.2% (SE 2.84%) between the pre-training and immediate post-training test blocks, as shown in . Accuracy decreased to 10.5% (SE 2.66%) after a one-hour rest period. When full-current tDCS was applied during training, this resulted in a 26.6% (SE 2.51%) increase in accuracy immediately after training relative to before training. This was an 87% greater increase in performance accuracy with training relative to that found for the low-current tDCS group (F(1,35)=12.23, p=0.001). After the one-hour delay, mean accuracy for the full-current tDCS group was 21.3% (SE 1.87%). There were no significant differences between the full-current and low-current groups in the amount of forgetting over the one hour delay, as determined by the reduction in accuracy over the 1-hour interval between the immediate and 1 hour delay post-tests (F(1,35)=0.199, N.S.). This suggests that the greater increase in performance obtained with training in the full-current tDCS group did not degrade more quickly than the low-current tDCS group after training was ended. The greater amount of learning during training for the full-current group, combined with a similar amount of forgetting between groups after this, resulted in an overall increase in between-group learning differences by the end of the 1 hour rest period, or 104% greater change in performance accuracy (F(1,35)=11.09, p=0.002).
Figure 5 Shows difference in accuracy obtained with training across the four tDCS conditions for tDCS Learning Experiments 2-4. Columns shown on the left side of the figure show changes in accuracy immediately after training, columns on the right side of the figure (more ...)
TDCS Learning Experiment 3 tested the hypothesis that the amount of learning would scale with tDCS current strength. All subjects from Experiments 2 and 3 were entered into a regression analysis using current as an independent variable. Performance accuracy of the 8 subjects that received intermediate-current (0.6 mA) over right inferior frontal cortex (16.8%, SEM 3.0%) fell between the low- and full-current groups. This relationship between current strength and learning as measured by change in performance with training was well predicted by a linear model of current strength (r=0.437, p=0.0015). This suggests that there was a strong relationship between the amount of current administered during training and the amount of learning, within the range of current strengths tested here.
In tDCS Learning Experiment 4, we examined the effect of full-current anodal tDCS over right parietal cortex (over electrode site P4) in 12 additional subjects, and compared this to full-current and low-current anodal tDCS over right inferior frontal cortex from Experiment 2. There was a significant effect of tDCS over electrode site P4 on learning, improving accuracy by 22.5% (SEM 2.6%; F(1,34)=4.40, p=0.035) relative to low-current stimulation over F10. This magnitude of learning acceleration was somewhat smaller for P4 relative to full-current F10 stimulation immediately after training (F(1,25)=3.46, p=0.075). No significant difference between P4 and F10 full-current tDCS was present by the 1-hour delayed posttest.
3.3 Relationship Between Skin Sensation and Learning
Although subjects were blind to the presence of a manipulation of current intensity, and were naïve to tDCS upon entering the study, differences in skin sensation between groups may have influenced results, through differences in level of distraction or arousal, or some other indirect mechanism. To examine the relationship between self-reported skin sensation and learning, we collected skin sensation data from 73 subjects. While there were significant differences in self-reported skin sensation between the full-current and low-current groups (F(4,68)=6.30, p=0.003), learning rate was not associated with sensation (F(3,195)=0.541, N.S.). Taken together, this suggests that while skin sensation did vary with current amplitude, individual differences in reported skin sensation had no relationship with learning rate. This suggests that tDCS may have an effect on behavior through a direct effect on neural activity, rather than indirectly through stimulation of the skin.