Time-frequency dynamics of cortico-striatal interactions between nucleus accumbens (NAc) and frontal cortex (FC).

<p><b>A</b> Time-frequency coherence spectrum. In the prestimulus period (-1000 ms to 0 ms), both alpha and theta band coherence were strong. After stimulus onset (0 ms), alpha band coherence decreased while theta band coherence increased. <b>B</b> Granger causality from frontal electrodes to nucleus accumbens was highest in the alpha range (9–14 Hz) and diminished upon stimulus processing (after 0 ms). <b>C</b> Granger causality from nucleus accumbens to frontal electrodes was strongest in the theta range and increased upon stimulus processing (after 0 ms).</p

Significant interaction effects of the factorial ANOVA on the Granger estimates.

<p><b>A</b> The <i>time</i> by <i>frequency</i> interaction (F(1, 5) = 20.92, p<0.01, Cohen’sf = 0.17) indicates that there was a significant theta band increase poststimulus compared to prestimulus Granger causality (t(5) = 3.52, p<0.05). No such change was present in the alpha-band. In the poststimulus window, Granger causality in the theta band was significantly stronger than in the alpha band (t(5) = 4.27, p<0.01). <b>B</b> The ANOVA showed a significant frequency by directionality interaction (F(1, 5) = 9.68, p<0.05, Cohen’s f = 0.17). In post-hoc tests, we found that the granger-causal influence from nucleus accumbens on cortex was stronger in the theta-band than in the alpha-band (t(5) = 4.84, p<0.01). In contrast, there was no significant difference in Granger causality in the alpha-band between cortex-to-nucleus accumbens and nucleus accumbens-to-cortex direction (t(5) = 1.75, p = 0.14). However, for five out of six subjects Granger estimates in the alpha-band were stronger for the cortex-to-nucleus accumbens direction than the other way around (see <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0138685#pone.0138685.g005" target="_blank">Fig 5</a>). Data are represented as mean ± SEM. * = p<0.05, ** = p<0.01. = p<0.01.</p

Granger causality values for the theta- and the alpha-band in the two directions calculated for the full time interval (-1 to 0.5 s).

<p>The solid line indicates the main diagonal to help illustrating in how many participants the Granger results were stronger in the one compared to the other direction A In half of the subjects, theta-band oscillations were stronger from cortex to nucleus accumbens (NAc) than the other way around. B Alpha-band oscillations were stronger from the cortex to the nucleus accumbens than from nucleus accumbens to the cortex for five out of six subjects (permutation test across subjects, p<0.05). This indicates that cortical alpha-band activity granger-cause activity in the nucleus accumbens.</p

The paradigm.

<p>The attended side was initially indicated by a cue. Subjects had to fixate at the central cross and by button press indicate the color of the attended squares (left button for red and right button for blue). The 1200 ms prestimulus period was followed by the colored stimuli flashed for 33 ms. Subjects had to respond within 2500 ms. If there was a color change in the square of the unattended hemifield, attention had to switch to that direction (‘switch-trial). After the response there was a 1000 ms window for eye blinking. A Example of an explicit cue followed by a repeat trial. The subject had to covertly attend to the left and subsequently report the color of the stimuli by pressing the corresponding button (here: blue, right button). B Example of a switch trial. In the previous repeat trials, the subject had to attend to the left, because of the initially shown spatial cue. Upon stimulus presentation, the subject correctly switched attention and indicated so by reporting the color of the stimulus at the formerly unattended side (here: right, red color). If the subject responded according to the formerly attended side (here: left, blue), the switch trial would repeat up to four times. Repetitions of switch trials were removed from the analysis. If the subject did not switch after the fourth consecutive switch trial, another explicit spatial cue pointing to the formerly unattended side was presented (here: a rightward pointing arrow).At the beginning of each block, subjects were explicitly cued to which side to attend. From then on, the attended side was determined by stimuli properties alone. A central fixation point was presented during the entire experiment. Colored squares were flashed 1200ms after the beginning of each trial for about 33 ms (two frames = 2/60Hz). These stimuli were presented with six degrees eccentricity and two degrees lower than the fixation cross (measured from the fixation cross to the center of the stimuli). The squares were two degrees wide.</p

Partial coherence results.

<p>Ordinary coherence between the frontal and parietal cortex showed clear peaks in the theta and alpha frequency band. When partializing out the coherence from the nucleus accumbens, the frontoposterior coherence in the 9–14 Hz alpha-band (t(5) = 3.00, p = 0.03) decreased significantly.</p

Coherence between the nucleus accumbens (NAc) electrodes and the scalp EEG.

<p><b>A</b> Z-transformed coherence spectrum between nucleus accumbens and frontal scalp electrode Fz. The horizontal dotted line (Z = 2.8) indicates the threshold of the coherence calculated from data shuffled over trials (see <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0138685#sec003" target="_blank">Methods</a>). Coherence values between 4 Hz and 14 Hz are above this threshold. Electrodes Fz and Oz are circled. <b>B</b> Coherence in the theta-band showed a strong frontal topography and a weaker parieto-occipital topography. <b>C</b> Coherence in the alpha-band was strongest around frontal electrodes. The *’s indicate scalp electrodes that were significantly coherent between 4–14 Hz with nucleus accumbens when controlling for multiple comparisons over electrodes and frequency bins using a cluster randomization approach. A strong frontal and a weaker parietal cluster emerged. <b>D</b> Example of coherence in the parietal cluster. The Z-transformed coherence spectrum between nucleus accumbens and occipitoparietal scalp electrode Oz is depicted.</p

Granger causality ANOVA results.

<p>Granger causality ANOVA results.</p