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Fig. 3. Group average time course of RVelonh, RVolonh, and RFonh to photopic diffuse luminance flicker from the temporal rim of the optic disk in a group of 15 normal volunteers. The time constants of the increases at onset and decreases at cessation of flicker are given in the text. Adapted with permission from Riva et al. (2004a).

Fig. 4. RFonh plotted as a function of stimulus color ratio (see text) for a 15 Hz flicker. Adapted with permission from Riva et al. (2001).

suggest a mechanism involving predominantly capillary dilatation (Riva et al., 2005).

Varying the parameters of the stimulus on RFonh

Luminance versus chromatic modulation

RFonh as a function of the color ratio r [ ¼ R/(R+G)] for a 15 Hz flicker was largest at pure luminance (r ¼ 0 and 1), declined at mixed luminance and chromatic modulations, and reached a secondary maximum at r ¼ 0.45, the value of psychophysical equiluminance (Fig. 4) (Riva et al., 2001). Thus, in humans, an RFonh can be evoked by heterochromatic flicker, modulated either in luminance or in chromatic equiluminance. Presumably, RFonh is specific for luminance and chromatic modulation, similar to neural responses dominated by the M- and P-cellular activities, respectively.

Frequency

RFonh versus flicker frequency (2–40 Hz) displayed the characteristics of a band-pass function, with a maximum at intermediate frequencies, for the luminance flicker stimulus and of a low-pass function for the equiluminant flicker stimulus (Fig. 5) (Riva et al., 2001).

Effect of stimulus area and site of measurement of

RFonh

RFonh increases linearly with the area of an 8 Hz diffuse luminance field centered at the disk (Fig. 6) (Riva et al., 2005). The stimulation consisted of a circular field centered at the optic disk. A small response was detectable when the stimulation just covered the optic disk. The data suggest that in humans, RFonh is generated mainly from the retinal area directly stimulated and therefore shows spatial summation properties, similarly to that observed for the pattern ERG (Hess and Baker, 1984).

Effect of pattern stimulation

Recordings of Fonh in response to checkerboard pattern reversed in counterphase at 8 Hz (check

Fig. 5. RFonh versus flicker frequency for luminance red–black flicker (top) and chromatic red–green flicker (bottom). Adapted with permission from Riva et al. (2001).

Fig. 6. Effect of increasing the area of the flicker stimulation on RFonh. ( ) The flicker field was centered at the ONH. (r) The flicker field of the size of the optic disk was moved from the disk to the fovea. Adapted with permission from Riva et al. (2005).

size 32 min arc, 151 151 field, approximately centered on the disk) were performed using the setup shown in Fig. 1. After a baseline measurement of Fonh at constant, uniform luminance, the

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black–white checkerboard pattern was presented for 1 min and then switched off to the constant uniform luminance (same mean luminance as the pattern). The experiment was repeated four times (Fig. 7). ERG was measured simultaneously with

Fonh and the 2P component was determined. Average RFonh (7SD) (%) was 3876%. After the stimulus was turned off, Fonh returned back to

the baseline in about 10 s (Riva et al., 2005). The P-ERG data recorded simultaneously with Fonh, show a clear and reliable second harmonic component, with a temporal period of 60 ms (corresponding to the contrast reversal period) and a peak-to-peak amplitude of 1.9 mV (SD error 0.49 mV).

Neurovascular coupling in humans

Measurements in five normal observers (age

between 28 and 61 years), in which Fonh was recorded simultaneously with the ERG in response to counterphased red (R) and green (G) flickering lights (301 field at the retina), revealed a significant correlation between RFonh and the changes of the ERG components, particularly with the 2F harmonic, further supporting the presence of a coupling between RFonh and inner retinal activity

(Fig. 8) (Falsini et al., 2002). RFonh and the ERG changes were documented as a function of (a) the frequency (2.3–64 Hz) of luminance and chromatic modulations, (b) the mean illuminance of 10 Hz luminance modulation (1–13 lux), and (c) the color ratio r of a 15 Hz chromatic modulation. The first harmonic 1Fand 2F-ERG amplitudes were determined. In both observers, RFonh and 2F amplitude displayed similar frequency-dependent changes, for either luminance or chromatic modulations. Both RFonh and 2F amplitude increased and then saturated with increasing mean illuminance of 10 Hz luminance modulation, and decreased with a similar functional shape as the 15 Hz chromatic modulation approached the R-G equiluminance value. In each observer, under specific experimental conditions (frequency response for the equiluminant red–green flicker, effect of mean illuminance for the luminance green flicker), RFonh was positively correlated with corresponding 1F or 2F amplitudes.