The neural basis of the blood-oxygen-level-dependent (BOLD) functional magnetic resonance imaging (fMRI) signal is discussed in this review. The BOLD signal is derived from changes in blood oxygenation levels, which are linked to neural activity. While fMRI is widely used to study brain function, its physiological basis remains poorly understood. This review outlines the principles of MRI and fMRI, and examines the relationship between the BOLD signal and neural activity. The authors conducted simultaneous intracortical recordings of neural signals and BOLD responses, finding a moderate to strong correlation between neural activity and the BOLD signal. However, the BOLD signal showed higher variability than neural activity, suggesting that fMRI may underestimate the reliability of neuronal activity. The study also compared local field potentials (LFPs), single- and multi-unit activity (MUA) with high spatio-temporal fMRI responses in monkey visual cortex. LFPs were found to be the most correlated with the haemodynamic response, and were better at predicting fMRI responses than MUAs. These findings suggest that the BOLD signal primarily measures the input and processing of neuronal information within a region, rather than the output signal transmitted to other brain regions. The review also discusses the principles of MRI, including nuclear magnetism, relaxation processes, and imaging techniques. It highlights the importance of MRI in studying the monkey brain and its application in understanding the neural basis of the BOLD signal. The review concludes that MRI is a valuable tool for studying brain function and that further research is needed to fully understand the relationship between the BOLD signal and neural activity.The neural basis of the blood-oxygen-level-dependent (BOLD) functional magnetic resonance imaging (fMRI) signal is discussed in this review. The BOLD signal is derived from changes in blood oxygenation levels, which are linked to neural activity. While fMRI is widely used to study brain function, its physiological basis remains poorly understood. This review outlines the principles of MRI and fMRI, and examines the relationship between the BOLD signal and neural activity. The authors conducted simultaneous intracortical recordings of neural signals and BOLD responses, finding a moderate to strong correlation between neural activity and the BOLD signal. However, the BOLD signal showed higher variability than neural activity, suggesting that fMRI may underestimate the reliability of neuronal activity. The study also compared local field potentials (LFPs), single- and multi-unit activity (MUA) with high spatio-temporal fMRI responses in monkey visual cortex. LFPs were found to be the most correlated with the haemodynamic response, and were better at predicting fMRI responses than MUAs. These findings suggest that the BOLD signal primarily measures the input and processing of neuronal information within a region, rather than the output signal transmitted to other brain regions. The review also discusses the principles of MRI, including nuclear magnetism, relaxation processes, and imaging techniques. It highlights the importance of MRI in studying the monkey brain and its application in understanding the neural basis of the BOLD signal. The review concludes that MRI is a valuable tool for studying brain function and that further research is needed to fully understand the relationship between the BOLD signal and neural activity.