Frequency and luminance-dependent blood flow and K+ ion changes during flicker stimuli in cat optic nerve head

The purpose of this study was to investigate whether blood flow in the cat optic nerve head (ONH) is related to increased neuronal activity elicited by diffuse luminance flickering light stimulation.

ONH blood flow was measured by laser Doppler flowmetry in anesthetized cats during 1 to 3 minutes of flickering light stimulation at controlled luminance and frequency (n = 227 measurements in 18 cats) using either a conventional visual stimulator (repetitive short flashes) or a sinusoidally varying light stimulator. Potassium ion concentration ([K+]) changes in the vitreous humor immediately in front of the optic disk were measured with neutral carrier K+ ionophore liquid membrane microelectrodes. Effects of varying flicker frequency (2 to 80 Hz) at constant luminance were quantified. Effects of luminance were quantified by varying the modulation depth of the stimulus at constant frequency.

Both ONH blood flow and [K+] increased during flicker stimulus with an average slope of 0.305% +/- 0.064% (SE)/microM [K+] (257 measurements in 18 cats). The peak ONH blood flow increase was 59% +/- 11% above baseline at 33.3 +/- 3.1 Hz. The peak [K+] increase was 188 +/- 42 microM above baseline at 38.3 +/- 3.3 Hz. Both ONH blood flow and [K+] changes had similar bandpass characteristics with frequency, first increasing, then dropping off at higher frequencies (122 measurements in 10 cats). Both frequency responses were described by power law functions (y = af"). Luminance responses for both ONH blood flow and [K+] changes could be fit by a modified Hill model and were 50% of maximum at light modulation depths of 21.2% +/- 4.6% and 22.5% +/- 3.7%, respectively (53 measurements in 5 cats).

Increases in ONH blood flow were correlated with changes in [K+]. Both responses were remarkably similar, with no significant differences in the frequency for peak responses in ONH blood flow or [K+], in low- and high-frequency power law exponents of the two responses, or in the 50% response to light modulation. The results are consistent with close coupling of neuronal activity and ONH blood flow.