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1 From the Institut de Recherche en Ophtalmologie, Sion, Switzerland; and 2 Istituto di Oftalmologia, Università Cattolica S. Cuore, Rome, Italy.
PURPOSE. To determine the response of human optic nerve head blood flow (RFonh) to heterochromatic equiluminant flicker modulation and compare it to the response induced by pure luminance flicker.
METHODS. In five normal volunteers, Fonh measured at the neuroretinal rim was monitored continuously by laser Doppler flowmetry. Stimuli were generated by green and red light emitting diodes and delivered to the fundus in Maxwellian view (field of 25o). Both green (G) and red (R) illuminances were square-wave modulated, 180o out of phase, with a maximum value of 10.4 for G and 2.64 lux for R. Flicker frequency was varied from 2 Hz to 40 Hz. RFonh was defined as the change in Fonh during stimulation relative to the prestimulus Fonh.
RESULTS. Defining the color ratio r as R/(R + G), the RFonh, measured for a 15-Hz flicker, was largest at pure luminance (r = 0 and 1), declined at mixed luminance and chromatic modulations, and reached a secondary maximum at r = 0.45, the value of psychophysical equiluminance. RFonh versus flicker frequency displayed the characteristics of a low-pass function for the equiluminance flicker stimulus and of a band-pass function, with a maximum at intermediate frequencies, for the luminance flicker stimulus.
CONCLUSIONS. RFonh in humans can be evoked by heterochromatic flicker, modulated either in luminance or chromatic equiluminant conditions. RFonh may be specific for luminance and chromatic modulations, similar to neural responses dominated by the magno- and parvocellular activity, respectively. These findings offer a new approach to study the neurovascular coupling at the optic nerve head in both physiological and diseased conditions involving predominantly or selectively the magno- and parvocellular pathways.
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