Temporal spatial differences observed by functional MRI and human intraoperative optical imaging

Pre-operative functional magnetic resonance imaging (fMRI). cortical evoked potentials (EPs) and intraoperative optical imaging of intrinsic signals ( iOIS) were employed to relate the temporal-spatial characteristics of senso rimotor responses in human brain. Peripheral somasthetic stimulation (2 s) was provided either by a 110 Hz finger vibrator or transcutaneous median ne rve stimulation in eight patients undergoing neurosurgical procedures. Each technique provided unique spatial patterns and temporal response profiles. EPs and iOIS activities were observed over the surface of pre- and post-ce ntral gyri (at the level of the superior genu) with very similar spatial di stributions. In contrast, fMRI spatial distributions depended upon the mode l used for statistical correlation analysis. Using a monophasic response mo del, fMRI primarily localized within the central sulcus and did not demonst rate large signal changes over the pre- and post-central gyri (areas with i OIS/EP activity). However, as initial negative responses were incorporated into the response model, fMRI progressively localized closer to the iOIS an d somatosensory EP maps. Temporally, responses to single stimuli differed b etween the fMRI and iOIS techniques. Using a monophasic model for fMRI anal ysis, the total fMRI response was delayed by 2-3 s relative to iOIS. As ini tial negative responses were incorporated in the analysis, the fMRI time co urse developed temporal characteristics similar to iOIS. Ultimately, when f MRI time courses were examined over pixels co-localizing with iOIS activati on (without using statistical correlation analysis), the fMRI temporal prof ile included an initial decrease in signal (an initial dip) that closely re sembled the time course of iOIS response. This is the first study to experi mentally cc-localize (temporally and spatially) iOIS and fMRI signals in hu man subjects. The spatial/temporal differences in this study likely reflect the capillary versus venous contributions of iOIS and fMRI, respectively. The temporal/spatial cc-localization of the iOIS signal and the fMRI initia l dip suggests the initial fMRI dip and the iOIS signal may result from sim ilar physiologic events.