Abstract
Time-domain analysis of blood-oxygenation level-dependent (BOLD) signals allows the identification of clusters of voxels responding to photic stimulation in primary visual cortex (V1). However, the characterization of information encoding into temporal properties of the BOLD signals of an activated cluster is poorly investigated. Here, we used Shannon entropy to determine spatial and temporal information encoding in the BOLD signal within the most strongly activated area of the human visual cortex during a hemifield photic stimulation. We determined the distribution profile of BOLD signals during epochs at rest and under stimulation within small (19–121 voxels) clusters designed to include only voxels driven by the stimulus as highly and uniformly as possible. We found consistent and significant increases (2–4% on average) in temporal information entropy during activation in contralateral but not ipsilateral V1, which was mirrored by an expected loss of spatial information entropy. These opposite changes coexisted with increases in both spatial and temporal mutual information (i.e., dependence) in contralateral V1. Thus, we showed that the first cortical stage of visual processing is characterized by a specific spatiotemporal rearrangement of intracluster BOLD responses. Our results indicate that while in the space domain BOLD maps may be incapable of capturing the functional specialization of small neuronal populations due to relatively low spatial resolution, some information encoding may still be revealed in the temporal domain by an increase of temporal information entropy.
Highlights
The understanding of information encoding in the brain, i.e., how the brain represents and processes information, is an important goal for neuroscience [1]
Note that quantitatively only mean and variance rely on the actual values of the blood-oxygenation level-dependent (BOLD) signals, whereas the other quantities only depend on the profile of the underlying distribution
Except for the clear-cut relation between rest and stimulated values of spatial or temporal information entropy and the corresponding functional Magnetic Resonance Imaging (fMRI) signals (Figure 4A), we found no correlation between the stimulation-induced changes of information and BOLD response (Figure 4B, p > 0.46)
Summary
The understanding of information encoding in the brain, i.e., how the brain represents and processes information, is an important goal for neuroscience [1]. Measurement of brain activity is paramount for attempting a quantitative analysis of information encoding, a goal that can be approached with different experimental techniques that operate at different spatiotemporal scales. Functional Magnetic Resonance Imaging (fMRI) measures macroscopic changes in blood-oxygenation level-dependent (BOLD) signals. FMRI studies have commonly been used to extract information (e.g., activations and/or deactivations) about relatively slow processes (e.g., task-related vascular responses) taking place in tissue volumes relatively larger than the underlying functional units (e.g., cortical areas vs columns). The spectral properties of temporal autocorrelation of BOLD oscillations have been convincingly proposed as a metric for its temporal organization, these concepts were developed for the analysis of fluctuations of the resting brain, and do not directly apply to the study of localized, evoked responses [6]
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