Finding thalamic BOLD correlates to posterior alpha EEG

Abstract

Oscillatory electrical brain activity in the alpha (8-13 Hz) band is a prominent feature of human electroencephalography (EEG) during alert wakefulness, and is commonly thought to arise primarily from the occipital and parietal parts of the cortex. While the thalamus is considered to play a supportive role in the generation and modulation of cortical alpha rhythms, its precise function remains controversial and incompletely understood. To address this, we evaluated the correlation between the blood oxygenation level dependent (BOLD) functional magnetic resonance imaging (fMRI) signals in the thalamus and the spontaneous modulation of posterior alpha rhythms based on EEG-fMRI data acquired concurrently during an eyes-closed task-free condition. We observed both negative and positive correlations in the thalamus. The negative correlations were mostly seen within the visual thalamus, with a preference for the pulvinar over lateral geniculate nuclei. The positive correlations were found at the anterior and medial dorsal nuclei. Through functional connectivity analysis of the fMRI data, the pulvinar was found to be functionally associated with the same widespread cortical visual areas where the fMRI signals were negatively correlated with the posterior alpha modulation. In contrast, the dorsal nuclei were part of a distinct functional network that included brain stem, cingulate cortex and cerebellum. These observations are consistent with previous animal electrophysiology studies and the notion that the visual thalamus, and the pulvinar in particular, is intimately involved in the generation and spontaneous modulation of posterior alpha rhythms, facilitated by its reciprocal and widespread interaction with the cortical visual areas. We further postulate that the anterior and medial dorsal nuclei, being part of the ascending neuromodulatory system, may indirectly modulate cortical alpha rhythms by affecting vigilance and arousal levels.

Figure 1

Single-subject spectral contrast between eyes-closed and eyes-open periods. a) Spectrogram for the Oz channel during the voluntary eyes-close-eyes-open task; b) Spectral difference between the eyes-closed and eyes-open periods for all EEG channels; c) Spectral difference for the Oz channel with the IAF determined from the peak; d) The spatial distribution of the spectral difference specifically at the IAF.

Figure 2

Group-level map of BOLD correlates to posterior alpha power. The map shows the distribution of the mean voxel-wise correlation (represented in z score) averaged across 15 subjects, thresholded by a t-value that yields p<0.02 (corrected for FDR). The MNI z coordinate is shown in the left-bottom corner of each slice.

Figure 3

Comparison between BOLD activations with visual stimulation (a) and BOLD correlates to posterior alpha power (b). Both maps are not thresholded. Two peak activations in the thalamus are indicated by the red arrows as LGN in a), or by the green arrows in b). The gray bar and MNI slice positions indicated in b) also applies to a).

Figure 4

Comparison between BOLD activations with visual stimulation (a) and BOLD correlates to posterior alpha power (b) with the underlay of high-resolution GRE phase images. The gray and color bars apply to the underlay and overlay, respectively for both a) and b). The three slices (from left to right) shown in both a) and b) are at z=−2, 2 and 13mm in MNI space.

Figure 5

a) BOLD activations with visual stimulation (red) and negative BOLD correlates to posterior alpha power (blue) overlaid on top of the histologically defined atlas of the thalamus, for a horizontal slice parallel to and 2.7mm below the intercommissural plane. The activation and correlation are thresholded with p<0.02 (corrected for FDR). b) ROC curves with respect to Pul, c) ROC curves with respect to LGN.

Figure 6

Functional connectivity with the seed region at Pul (a) or AN/MDN (b), as defined from the thalamic regions where BOLD signals are correlated either negatively or positively with posterior alpha power, respectively.