Abstracts

Optical imaging of intrinsic signal is a powerful technique for mapping epileptiform events with high spatial and temporal resolution. Depending on the wavelength of light measured, the intrinsic optical signal (IOS) carries information about blood flow (546 nm), hemoglobin oxygenation (605 and 630 nm), and cell volume (700 nm). These various components have different time courses and amplitudes. The optimal wavelength for imaging epileptiform events has yet to be determined.
Adult urethane-anesthetized rats were placed in a stereotactic frame and two glass electrodes were placed through a thinned skull into layer 2-3 of the somatosensory cortex. A 10-bit CCD camera was focused 500 [mu]m below the surface of the brain and illuminated with 546 nm, 605 nm, 630 nm and 700 nm wavelengths. 5 mM bicuculline methiodide (BMI) was iontophoresed through one of the two glass electrodes to induce interictal spikes (IIS), which were recorded with a local field potential using the second electrode. Images were obtained at frame rates of up to 100 msec/frame. Images obtained after individual IIS were divided by images obtained immediately prior to the IIS.
Maximal percent change in reflectance following the IIS at 546 nm was 1.83 [plusmn] 0.77% (N= 5 rats), at 605 nm 0.59 [plusmn] 0.25 % (N = 5 rats), at 630 mm 0.54 [plusmn] 0.48 % (N= 11 rats), and at 700 nm 0.22 [plusmn] 0.13 % (N= 8 rats). The maximal change in reflectance occurred typically 1-2 seconds following the IIS. However, a statistically significant change in reflectance with respect to baseline was observed much earlier. A statistically significant change was observed 533 [plusmn] 219 msec following the IIS at 546 nm (N= 5 rats). At 605 nm (N= 5 rats), a significant change was observed at 433 [plusmn] 55 msec, at 630 nm (N= 11 rats) the latency was 447 [plusmn] 236 msec and at 700 nm (N= 8 rats) the latency was 465 [plusmn] 191 msec. The spatial extent of reflectance change was greatest at 546 nm (24.25 [plusmn] 24.58 mm², N= 4 rats) and reduced at longer wavelengths: at 605 nm (16.68 [plusmn] 23.62 mm², N= 5 rats), at 630 nm (15.82 [plusmn] 22.71 mm², N= 5 rats) and at 700 nm (9.90 [plusmn] 16.04 mm², N= 5 rats).
The amplitude of the change in reflectance of light associated with an IIS is greatest at 546 nm, likely caused by changes in blood flow. The earliest signal, however, is recorded at longer wavelengths and has a smaller area. This early response, representing increases in oxygen metabolism and cell swelling, likely correlates more closely with the population of epileptic neurons.
[Supported by: The NINDS.]