Visualizing and quantifying evoked cortical activity assessed with intrinsic signal imaging

Intrinsic signal imaging (ISI) measures changes in light reflectance from t he illuminated cortex (intrinsic signals or IS) attributed to various vascu lar and metabolic sources that, when using illumination in the 600 nm range , appear to co-localize with neuronal activity. Given the multiple sources contributing to the collected IS, the common practice of averaging across a n extended post-stimulus time epoch before dividing by baseline data typica lly visualizes evoked IS overlying both the cortical tissue and the large s urface blood vessels. In rat PMBSF, the contribution from these vessels are problematic as they do not co-localize with known PMBSF function. Determin ing a means for quantifying the evoked IS area poses an additional challeng e. Here, we describe how exploiting IS collected shortly after stimulus ons et (within 1.5 s), which coincides with fast oxygen consumption of active n eurons, visualizes evoked IS overlying the cortical tissue without the larg e surface vessels. We also describe how the use of absolute thresholds comb ined with a baseline determined from data collected immediately prior to st imulus onset (within 1 s) targets most precisely a specific evoked IS ampli tude, a method that should be especially useful when evoked areas are expec ted to occupy a substantial portion of the total imaged area and/or when pe ak activity is expected to differ between subjects. (C) 2000 Elsevier Scien ce B.V. All rights reserved.