Neurophysiological investigation of the basis of the fMRI signal


Functional magnetic resonance imaging (fMRI) is widely used to study the operational organization of the human brain, but the exact relationship between the measured fMRI signal and the underlying neural activity is unclear. Here we present simultaneous intracortical recordings of neural signals and fMRI responses. We compared local field potentials (LFPs), single- and multi-unit spiking activity with highly spatio-temporally resolved blood-oxygen-level-dependent (BOLD) fMRI responses from the visual cortex of monkeys. The largest magnitude changes were observed in LFPs, which at recording sites characterized by transient responses were the only signal that significantly correlated with the haemodynamic response. Linear systems analysis on a trial-by-trial basis showed that the impulse response of the neurovascular system is both animal- and site-specific, and that LFPs yield a better estimate of BOLD responses than the multi-unit responses. These findings suggest that the BOLD contrast mechanism reflects the input and intracortical processing of a given area rather than its spiking output.

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Figure 1: Neural and BOLD responses to pulse stimuli.
Figure 2: Time-dependent frequency analysis for population data.
Figure 3: Simultaneous neural and haemodynamic recordings from a cortical site showing transient neural response.
Figure 4: Correlation analysis for the estimation of the impulse response of the neurovascular system and validation of data collected with a pulse or a variable-contrast stimulus.
Figure 5: MRI responses to pulse stimuli at four different contrasts (12.5, 25, 50 and 100%).
Figure 6: Recording hardware.
Figure 7: Elimination of residual interference by applying PCA (see Methods).


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We thank D. Leopold, G. Rainer and N. Sigala for reading the manuscript and for many useful suggestions. We also thank H. Mandelkow for writing some of the Matlab code; K. Lamberty for the drawings; D. Blaurock for English corrections and editing; and S. Weber for fine-mechanic work. This research was supported by the Max Planck Society.

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Correspondence to Nikos K. Logothetis.

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Logothetis, N., Pauls, J., Augath, M. et al. Neurophysiological investigation of the basis of the fMRI signal. Nature 412, 150–157 (2001).

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