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This relationship between RA and RFonh, the so-called neurovascular coupling as originally postulated for the brain (Roy and Sherrington, 1890), has led to a large number of studies in the brain aimed at exploring the physiological parameters, as well as the clinical conditions which, in humans, might influence this coupling. Similar investigations in the optic nerve, a tissue consisting of axons and glia (white matter) but deprived of neuron somata (Ransom and Orkand, 1996), are more recent, although this tissue, which can be accessed noninvasively, is involved in the pathogenesis of a number of severe ocular diseases.

The aim of the present review is to report the current status of knowledge on the RA-induced RFonh, the latter measured by LDF. This functional LDF (FLDF) has been employed to investigate the dependence of RFonh upon the characteristics of the visual stimuli, to quantitatively measure the coupling between RFonh and RA, and to determine whether, and to what extent, pathologic conditions such as glaucoma may affect this coupling. The findings obtained so far indicate that (i) visual stimulation is a powerful modulator

of Fonh; (ii) a coupling can be quantified in the optic nerve by simultaneously recording RFonh and

electrical activity changes in the neural tissue of the retina; (iii) RFonh may be altered in certain stress conditions and in an ocular pathological condition involving the optic nerve head microcirculation, i.e. ocular hypertension and chronic glaucoma.

Technology

Laser Doppler flowmetry of the optic nerve

This technique is based on the Doppler effect, which describes the change in frequency (Df ) of an electromagnetic wave of frequency f emitted from a source moving toward or away from an observer. When a laser beam impinges on red blood cells (RBCs) moving in a small region of the tissue of the optic nerve, the multiplicity of directions and magnitudes of the velocity of the RBCs, as well as the scattering of the laser light in many directions by the nonmoving tissue structures give rise to a distribution of Doppler shift frequencies, the

so-called Doppler shifts power spectrum (DSPS). As discussed elsewhere (Riva et al., 2000), from which the following text has been borrowed with permission from the publisher, it can be said that

(i) since blood occupies only 1–5% of the sampled tissue volume, only a small fraction of the scattered light is Doppler frequency-shifted through interaction with the RBCs. The rest of the scattered light has been predominantly scattered by nonmoving structural components of the tissue; (ii) the DSPS is generated by the heterodyne mixing of the nonshifted light with the shifted scattered light at the surface of the photodetector. In accordance with Bonner and Nossal’s (1990) theory of LDF, the following parameters are extracted from the DSPS:

Velonh is the mean Doppler shift frequency (unit Hz), which is proportional to the mean velocity of the RBCs moving in the sampled volume. Volonh is proportional to the number of moving RBCs.

Fonh ¼ Velonh Volonh. Both Fonh and Volonh are expressed in relative units and Df in Hz. P(Df ) is

the power of the DSPS at each Df and Adc is the amplitude of the direct current produced by the light incident on the detector. Doppler shifts below 30 Hz are filtered out to avoid slow tissue motion artifacts. In the optic nerve, the Doppler shifts are below 3000 Hz.

Fonh varies linearly with the actual flux of the RBCs, which is identical to blood flow if the hematocrit does not vary during the experiment (Riva and Petrig, 1997). This condition has been assumed to be fulfilled in all experiments to be discussed in this review.

The LDF data presented in this review has been obtained with a near-infrared LDF system mounted on a fundus camera (Fig. 1) (Riva et al., 2000;

Logean et al., 2005). Briefly, with this system, the laser beam (wavelength between 750 and 810 nm) followed the optical path of the fundus illumination system and was aimed at a site on the rim of the optic disc, avoiding the main retinal vessels. The light scattered by the RBCs and tissue structures was picked up by an optical fiber whose aperture at the disc is E160 mm and guided to an avalanche photodetector. The fundus was illuminated in nearinfrared light (826 nm) and viewed on the screen of a CCD camera, together with the aperture of the fiber collecting the scattered light. The aperture of an optical fiber (50 mm diameter) mounted on an x-y-z micromanipulator placed in one of the retinal planes of the fundus camera was illuminated by a red light to provided a fixation target. By moving

this target, the operator, guided by photographs of the disc and the pitch of the Doppler sound, aimed the laser at the desired site of measurement.

The output signal of the detector was analyzed using software implemented on a NeXT computer and the LDF parameters were displayed on a monitor at a rate of 21 values per second (Petrig and Riva, 1988). Figure 2 shows recordings of

Velonh, Volonh, and Fonh obtained from the optic disc of a human volunteer.

Visual stimulation

Two types of stimulation were used in humans: (A) constant luminance (red/black or green/black flicker) and chromatic red/green flicker. These