Abstract
The relationship between blood oxygenation level dependent-functional magnetic resonance imaging (BOLD-fMRI) and magnetoencephalography (MEG) metrics were explored using low-level visual stimuli known to elicit a rich variety of neural responses. Stimuli were either perceptually isoluminant red/green or luminance-modulated black/yellow square-wave gratings with spatial frequencies of 0.5, 3, and 6 cycles per degree. Neural responses were measured with BOLD-fMRI (3-tesla) and whole head MEG. For all stimuli, the BOLD response showed bilateral activation of early visual cortex that was greater in the contralateral hemisphere. There was variation between individuals but weak, or no evidence, of amplitude dependence on either spatial frequency or the presence of luminance contrast. In contrast, beamformer analysis of MEG data showed activation in contralateral early visual cortex and revealed: (i) evoked responses with stimulus-dependent amplitude and latency; (ii) gamma and high-beta oscillations, with spatial frequency dependent peaks at approximately 30 and 50 Hz, but only for luminance-modulated gratings; (iii) The gamma and beta oscillations appeared to show different spatial frequency tuning profiles; (iv) much weaker gamma and beta responses, and at higher oscillation frequencies, for isoluminant compared to luminance-modulated gratings. The results provide further evidence that the relationship between the fMRI-BOLD response and cortical neural activity is complex, with BOLD-fMRI being insensitive to substantial changes in neural activity. All stimuli were clearly visible to participants and so the paucity of gamma oscillations to isoluminant stimuli is inconsistent with theories of their role in conscious visual perception.
Highlights
Neuroimaging techniques such as magnetoencephalography (MEG) and functional magnetic resonance imaging provide enormous potential for the non-invasive study of human brain function
This study demonstrates that the two neuroimaging techniques, MEG and blood oxygenation level dependent (BOLD)-functional magnetic resonance imaging (fMRI), provide complementary but not interchangeable data on brain activity
MEG revealed substantial differences in oscillatory activity in response to different stimuli types (Figures 3 and 6), whereas BOLD responses were similar for all stimuli (Figures 2 and 3)
Summary
Neuroimaging techniques such as magnetoencephalography (MEG) and functional magnetic resonance imaging (fMRI) provide enormous potential for the non-invasive study of human brain function. Interpretation of neuroimaging data is limited by a lack of knowledge of how neural (and non-neural, e.g., glial) activity contributes to the signals being measured. To explore these relationships, results obtained with one neuroimaging technique can be compared with results from another neuroimaging technique, or to invasive recordings from animals. One such study in anesthetized macaque demonstrated that stimuli contrast positively correlated with the fMRI blood oxygenation level dependent (BOLD) response and with local field potentials (LFPs) measured simultaneously in primary visual cortex (Logothetis et al, 2001). A study in cat showed fluctuations of the hemodynamic response, as measured with optical imaging, correspond with fluctuations in the gamma response (Niessing et al, 2005)
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