Inter-Trial Variability in Sensory-Evoked Cortical Hemodynamic Responses: The Role of the Magnitude of Pre-Stimulus Fluctuations

Mohamad Saka,Jason Berwick,Myles Jones

doi:10.3389/fnene.2012.00010

Mohamad Saka, Jason Berwick + Show 1 more

Open Access

https://doi.org/10.3389/fnene.2012.00010

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Abstract

Brain imaging techniques utilize hemodynamic changes that accompany brain activation. However, stimulus-evoked hemodynamic responses display considerable inter-trial variability and the sources of this variability are poorly understood. One of the sources of this response variation could be ongoing spontaneous hemodynamic fluctuations. We recently investigated this issue by measuring cortical hemodynamics in response to sensory stimuli in anesthetized rodents using 2-dimensional optical imaging spectroscopy. We suggested that sensory-evoked cortical hemodynamics displayed distinctive response characteristics and magnitudes depending on the phase of ongoing fluctuations at stimulus onset due to a linear superposition of evoked and ongoing hemodynamics (Saka et al., 2010). However, the previous analysis neglected to examine the possible influence of variability of the size of ongoing fluctuations. Consequently, data were further analyzed to examine whether the size of pre-stimulus hemodynamic fluctuations also influenced the magnitude of subsequent stimulus-evoked responses. Indeed, in the case of all individual trials, a moderate correlation between the size of the pre-stimulus fluctuations and the magnitudes of the subsequent sensory-evoked responses were observed. However, different correlations between the size of the pre-stimulus fluctuations and magnitudes of the subsequent sensory-evoked cortical hemodynamic responses could be observed depending on their phase at stimulus onset. These analyses suggest that both the size and phase of pre-stimulus fluctuations in cortical hemodynamics contribute to inter-trial variability in sensory-evoked responses.

Highlights

The changes in blood flow, volume, and oxygenation that accompany brain activation are collectively referred to as the hemodynamic response
METRICS OF THE SIZE OF PRE-STIMULUS FLUCTUATIONS AND RESPONSE MAGNITUDE Our previous investigation demonstrated that the phase of the pre-stimulus fluctuations in cerebral hemodynamics at stimulus onset influenced the magnitude of subsequent stimulus-evoked hemodynamics (Saka et al, 2010)
If the size of pre-stimulus cortical hemodynamics is examined alone, a moderate positive correlation can be observed between the size of the pre-stimulus hemodynamic fluctuation for each trial and the magnitude of the evoked hemodynamic response

Summary

Introduction

The changes in blood flow, volume, and oxygenation that accompany brain activation are collectively referred to as the hemodynamic response. Subsequent subtraction of coherent spontaneous BOLD fluctuations from the cortex contralateral to that activated by the task, reduced inter-trial variability and suggested a superposition of evoked and ongoing hemodynamics. We previously investigated this phenomenon in a well defined animal model, the whisker barrel somatosensory cortex of the rodent (Saka et al, 2010). As in the somatosensory system of the anesthetized rodent sensory stimuli have been shown to elicit changes in metabolic and hemodynamic parameters in cortex both ipsilateral and contralateral to the sensory stimulus (Devor et al, 2008; Boorman et al, 2010) adopting a similar approach to Fox and colleagues in this

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