checkerboards. Experimental evidence in cats and monkeys (Maffei and Fiorentini, 1986) indicates that the P-ERG is associated with the activity of inner retinal neurons (perhaps the ganglion cells themselves). In some clinical situations, the F-ERG and the P-ERG second harmonic dissociate and there are differences between generators for the second harmonic of the ERG evoked by modulation of luminance versus spatial contrast (Porciatti et al., 1989; Falsini et al., 1991).

P-ERG can be recorded in both primates and humans by red–green equiluminant patterns, alternated in counterphase (Porciatti et al., 1989; Falsini et al., 1991; Morrone et al., 1994a). The response displays characteristics comparable to those of retinal P-ganglion cells (see below), such as low-pass response function as the spatial and temporal frequency of the stimulus are varied, and a significant latency increase compared to responses evoked by luminance contrast. There is also evidence that the P-ERG to chromatic contrast is altered in glaucoma (Korth et al., 1994; Porciatti et al., 1997). When compared to the P-ERG elicited by luminance contrast, the chromatic P-ERG in glaucoma can be altered even to a greater extent, provided that the optimal spatiotemporal parameters are used. P-ERG abnormalities involve both amplitude and latency of the response (Porciatti et al., 1997).

The Parvo (P)- and Magno (M)-cellular pathways

Psychophysical studies have shown that luminance and chromatic modulations may allow differential assessment of the activity of the M- and P-cellular pathways, the two main cellular systems of the primate visual pathway. The P-system consists of tonic, wavelength-opponent retinal ganglion cells projecting to the P-cellular layers of the lateral geniculate nucleus (Lee, 1990). Most neurons belonging to this system receive antagonistic input from medium-wavelength-sensitive and long- wavelength-sensitive cones. The M-system is made up of phasic, nonopponent ganglion cells, which project to the M-cellular layers of the lateral geniculate nucleus. M-cellular ganglion cells receive combined input from mediumand

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long-wavelength-sensitive cones at both center and surround. Uniform field stimuli, whose luminance is varied periodically over time (i.e. the luminance flicker modulation) at relatively high temporal frequencies, are known to be optimal, though not specific, for evoking the response of the M-neurons. On the other hand, low temporal frequency, counterphase modulation of red and green fields, whose luminance has been matched by heterochromatic flicker photometry (i.e. the equiluminant chromatic flicker modulation), represents a strong stimulus for the P-cellular neurons, eliciting predominantly their activity. At both retinal and postretinal levels, sensitivity of the human visual system to luminance modulation is maximal at intermediate temporal frequencies (10–20 Hz) and falls-off at both low and high frequencies, whereas sensitivity to equiluminant chromatic modulation is maximal at low temporal frequencies (o5 Hz) and falls-off at frequencies greater than 5–10 Hz ( Lee, 1990; Korth et al., 1994; Morrone et al., 1994a, b; Fiorentini et al., 1996; Porciatti et al., 1997).

Physiology

Magnitude and time course of RFonh in humans

Figure 3 shows the group average time course of

RVelonh, RVolonh, and RFonh to photopic diffuse luminance flicker obtained from the temporal rim

of the optic disk in a group of normal subjects (Riva et al., 2004a). Time constants of the increases (ti) and decreases (td) of each LDF parameter after onset and offset of the stimulation were obtained using a two-parameter exponential

function. For the RVelonh, RVolonh, and RFonh, ti were 3.4, 12.7, and 9.1 s, respectively, and td were

3.8, 9.8, and 7.5 s, respectively (Riva et al., 2005). RFonh at onset of stimulation and after stimulation stop closely resembles that of cerebral blood flow in response to motor and visual stimulation (Kru¨ger et al., 1999; Miller et al., 2001). Although the local vasodilatation reflected in RVolonh could be due to capillary recruitment, capillary dilatation, or venous dilatation, various considerations