Schedule Aug 20, 2003
Intrinsic Optical Signal Imaging of Focal Seizures in the Rat Neocortex
Theodore Schwartz (New York Presbyterian Hospital)

The intrinsic optical signal (IOS), a change in light reflectance from neural tissue that correlates with the underlying electrophysiology, holds great promise as a high-resolution tool for seizure imaging. It has been shown that different information predominates in the IOS at different wavelengths, but the optimal wavelength for imaging epileptiform events has not yet been determined. We investigate the IOS response to 4-aminopyridine (4AP)-induced focal neocortical seizures in the rat at various wavelengths of illuminating light. The skulls of adult urethane-anesthetized rats were thinned; a small area of dura was opened and two electrodes, one for field potential recording and one through which 50 mM 4AP was injected, were placed ~500 μm deep in the somatosensory cortex with tips <1 mm apart. A CCD camera was focused 500 μm below the cortical surface, which was illuminated with light of various wavelengths. Images, taken every 600 msec, were divided by a baseline image prior to seizure onset. Percent change in light reflectance (ΔR/R) was calculated over the time-course of each seizure. In a 1mm2 region surrounding the injection site, the largest ΔR/R was obtained at 546 nm (15.5 + 9.6%, N=17 seizures). At higher wavelengths, maximal ΔR/R was 7.1 + 2.6% (605 nm, N=7), 6.8 + 0.9% (630 nm, N=6) and 2.4 + 1.7% (700 nm, N=10). The spatial extent of the seizures was 21.7 + 7.2 mm2 (546 nm), 17.9 + 6.4 mm2 (605 nm), 19.4 + 2.6 mm2 (630 nm) and 11.2 + 7.0 mm2 (700 nm). For all seizures, an increase in light reflectance was observed in the region surrounding or adjacent to the seizure focus. Here, reflectance change was 5.0 + 4.2% (546 nm), -6.4 + 4.5% (605 nm), -3.7 + 3.0% (630 nm) and 2.8 + 1.8% (700 nm). This inverted optical signal may correspond to an inhibitory surround.  Hence, although the signal amplitude is greater at lower wavelengths, the larger area of the signal change may be an overestimation of the size of the epileptic focus.

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