FREQUENCY AND LUMINANCE-DEPENDENT BLOOD-FLOW AND K-NERVE HEAD( ION CHANGES DURING FLICKER STIMULI IN CAT OPTIC)

Purpose. The purpose of this study was to investigate whether blood fl ow in the cat optic nerve head (ONH) is related to increased neuronal activity elicited by diffuse luminance flickering light stimulation. M ethods. ONH blood flow was measured by laser Doppler flowmetry in anes thetized 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 flashe s) or a sinusoidally varying light stimulator. Potassium ion concentra tion ([K+]) changes in the vitreous humor immediately in front of the optic disk were measured with neutral carrier K+ ionophore Liquid memb rane microelectrodes. Effects of varying flicker frequency (2 to 80 Hz ) at constant luminance were quantified. Effects of luminance were qua ntified by varying the modulation depth of the stimulus at constant fr equency. Results. Both ONH blood flow and [K+] increased during flicke r stimulus with an average slope of 0.305% +/- 0.064% (SE)/mu M [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 mu M above baseline at 38.3 +/- 3.3 Hz. Both ONH blood flo w and [K+] changes had similar bandpass characteristics with frequency , first increasing, then dropping off at higher frequencies (122 measu rements in 10 cats). Both frequency responses were described by power law functions (y = af(n)). Luminance responses for both ONH blood flow and [K+] changes could be fit by a modified Hill model and were 50% o f maximum at light modulation depths of 21.2% +/- 4.6% and 22.5% +/- 3 .7%, respectively (53 measurements in 5 cats). Conclusions. Increases in ONH blood flow were correlated with changes in [K+]. Both responses were remarkably similar, with no significant differences in the frequ ency for peak responses in ONH blood flow or [K+], in low- and high-fr equency power law exponents of the two responses, or in the 50% respon se to light modulation. The results are consistent with close coupling of neuronal activity and ONH blood flow.