Abstracts

Optical recording of intrinsic signals (ORIS) has been used to map the onset and spread of epileptiform events in animal and human cortex. However it is unclear how well these perfusion based signals correlate with neuronal activity. It has been proposed that the optical signals correlate with the extent of subthreshold activity (EPSPs). We use voltage sensitive dye (VSD) RH-795 to measure excitatory and inhibitory activity associated with interictal spikes induced with iontophoresis of bicuculline into rat somatosensory cortex in vivo (n=4). ORIS at 570nm and 610nm was then used to measure changes of cerebral blood volume (CBV) and deoxygenated hemoglobin (Hbr) respectively. VSD recordings revealed that neuronal excitation peaked [sim]50ms after the onset of the interictal spikes and spread to a local region of cortex with a mean area of [sim]3mm². The initial dip in hemoglobin oxygenation peaked at [sim]1.3s and spread to an area [sim]7mm². CBV peaked at [sim]1.6s and spread to [sim]6.9mm². Although the epicenter of each signal was overlapping, the perfusion based signals were twice as large. An inhibitory surround was imaged with the VSD that peaked at 140-200ms. The onset of the inhibitory surround partially overlapped with the decay in the excitation peak and extended to an area larger than the initial dip and CBV. We conclude that the [ldquo]point spread[rdquo] of perfusion-based optical signals overestimates the extent of neuronal excitation (including subthreshold activity) by [sim]200%, but does not reach the extent of the more delayed inhibitory surround. (Supported by NIH NS043799-04 and NS049482-01.)