BOLD and spiking activity

Abstract

Viswanathan and Freeman 1 claim that oxygen concentration and, by inference, blood oxygen level-dependent (BOLD) functional magnetic resonance imaging (fMRI) reflect synaptic activity more than spiking activity. As this is a fundamental and controversial issue in fMRI research, this claim, if incorrect, may erroneously bias the interpretation of a large body of data.The authors simultaneously recorded multi-unit activity (MUA), local field potentials (LFP) and tissue oxygen concentration in primary visual cortex of anesthetized cats stimulated with moving gratings. During high temporal-frequency stimulation, when thalamic inputs were active, but few cortical neurons responded, oxygen signals were observed without MUA. Therefore, the authors concluded that oxygen responses reflect synaptic inputs more than spiking. However, careful inspection of their results leads to the opposite conclusion and supports a tight coupling between oxygen signals and local cortical spiking.Tissue oxygen responses show an initial decrease that is attributed to local oxygen consumption (negative peak) and a delayed increase that is attributed to more global changes in blood flow (positive peak). The authors showed that the initial negative peak was greater than zero during high-frequency stimulation (when spiking activity was absent), but it was in fact 80-90% smaller to high-frequency stimulation than to low-frequency stimulation (calculated from ref. 1, see red arrow in Fig. 1). Given that roughly the same thalamic input is expected on stimulation of either temporal frequency 2 , we conclude that the initial negative oxygen response depended only slightly (10-20%) on thalamocortical synaptic activity and mostly (80-90%) on cortical spiking.What about the more widespread delayed positive oxygen response? This component was evident during stimulation at high frequencies, but only in one of their two experiments that used large stimuli. A previous study demonstrated that delayed positive oxygen responses were associated with spiking outside the field of view of the electrode when using such large stimuli 3 . There is, therefore, a mismatch between the spatial extents of MUA (which

The Insular Cortex and the Regulation of Cardiac Function

“…There is evidence for good intrasubject as well as intersubject reliability (329) and good test/retest reliability (289). fcMRI responses (particularly those acquired in the resting state), are probably not the product of random unpropagated synaptic activity but reflect neuronal activity which has informational content (132,227).…”

Decoupling of the brain's default mode network during deep sleep

“…This spontaneous activity has spatial and temporal characteristics that resemble those of intracortical electrical and optical recordings (9)(10)(11). In particular, a network of brain regions has been identified in which activity is actually increased in absence of cognitively demanding tasks (12,13).…”

A BOLD window into brain waves

“…In this view, the cortex would be more like a sea pierced by sharp islands. On the other hand, the slow hemodynamic response function underlying the BOLD signal may make fMRI partly blind to the distinction between slow, low-amplitude fluctuations in firing and fast, high-amplitude bursts of activity (4). If there are two modes of neural activity, it bears keeping in mind that neurons in the READY mode would be as necessary as neurons in the GO mode in specifying different cognitive states, just as the background is as necessary as the foreground.…”

“…though the precise mechanism underlying the BOLD signal is still being investigated (3)(4)(5), it is becoming clear that spontaneous BOLD fluctuations are not just noise, but are tied to fluctuations in neural activity. In this issue of PNAS, He et al (6) have been able to directly investigate the relationship between BOLD fluctuations and fluctuations in the brain's electrical activity in human subjects.…”