Comparison of simultaneously recorded [H2(15)O]-PET and LORETA during cognitive and pharmacological activation

Abstract

The complementary strengths and weaknesses of established functional brain imaging methods (high spatial, low temporal resolution) and EEG-based techniques (low spatial, high temporal resolution) make their combined use a promising avenue for studying brain processes at a more fine-grained level. However, this strategy requires a better understanding of the relationship between hemodynamic/metabolic and neuroelectric measures of brain activity. We investigated possible correspondences between cerebral blood flow (CBF) as measured by [H2O]-PET and intracerebral electric activity computed by Low Resolution Brain Electromagnetic Tomography (LORETA) from scalp-recorded multichannel EEG in healthy human subjects during cognitive and pharmacological stimulation. The two imaging modalities were compared by descriptive, correlational, and variance analyses, the latter carried out using statistical parametric mapping (SPM99). Descriptive visual comparison showed a partial overlap between the sets of active brain regions detected by the two modalities. A number of exclusively positive correlations of neuroelectric activity with regional CBF were found across the whole EEG frequency range, including slow wave activity, the latter finding being in contrast to most previous studies conducted in patients. Analysis of variance revealed an extensive lack of statistically significant correspondences between brain activity changes as measured by PET vs. EEG-LORETA. In general, correspondences, to the extent they were found, were dependent on experimental condition, brain region, and EEG frequency.

Figure 1

Top: Comparisons between conditions performed within each imaging modality separately (PET and LORETA). Bottom: The different levels of analysis to compare results across imaging modalities (PET vs. LORETA, shown as superposed layers). Levels 1 and 2: comparison of raw images; levels 3–5: comparison of differences images. See text for further explanation.

Figure 2

Brain activity during the control task after administration of a placebo, shown in transverse, coronal, and sagittal view. Top row: CBF as measured by [H₂O]‐PET. Following rows: Neuroelectric activity as computed by LORETA for seven frequency bands. Brighter hues indicate stronger activity. Images are sliced at z = 22 mm (transverse), y = −26 mm (coronal), and x = 0 mm (sagittal), with the coordinate origin located at the anterior commissure.

Figure 3

Correlations between CBF and neuroelectric activity in the different frequency bands during the control task under placebo (left) and MDMA (right). Positive correlations above the significance threshold of r = 0.6 are shown in red. No negative correlations emerge. Except for the delta band, images are sliced at z = 22 mm (transverse), y = −26 mm (coronal), and x = 0 mm (sagittal), with the coordinate origin located at the anterior commissure. The delta band transverse views are sliced at z = −14 mm in order to better show the regions of correlations.

Figure 4

Color‐coded images showing the differences between placebo and MDMA during the control task (left) and the difference between the control task and the CPT during placebo (right), expressed as t‐value distribution. Orange of increasing intensity indicates increasing positive t‐values, blue of increasing intensity indicates increasing negative t‐values. Top rows: CBF as measured by PET. Following rows: Neuroelectric activity as computed by LORETA. The LORETA t‐images of different frequency bands are scaled to a common maximum to make them directly comparable. PET images are sliced at z = 22 mm (transverse), y = −26 mm (coronal), and x = 0 mm (sagittal), with the coordinate origin located at the anterior commissure. LORETA images are sliced at the anterior commissure.

Figure 5

PET‐LORETA correlations between images of difference between the control task and the CPT during placebo (left), between placebo and MDMA during the control‐task (middle), and between control task and the CPT during MDMA (right). Red colors indicate positive, blue colors indicate negative supra‐threshold correlations (r ≥ |0.6|). Images are sliced at z = 22 mm (transverse), y = −26mm (coronal), and x = 0 mm (sagittal), with the coordinate origin located at the anterior commissure. The transvers delta and theta band images in the middle display are sliced at z = − 8 mm to better show regions of correlations.