SPATIAL RELATIONS OF FLICKER SIGNALS IN THE 2 ROD PATHWAYS IN MAN

Flicker signals originating from the human rod photoreceptors seem to have access to two retinal pathways: one slow and sensitive, the other fast and insensitive. The phase lag between signals in the two pathwa ys grows monotonically with frequency, reaching 180 deg near 15 Hz. 2. At 15 Hz, destructive interference between the dow and the fast signa ls can cause two related phenomena: (i) a suprathreshold intensity reg ion - the perceptual null - within which the perception of flicker van ishes, and (ii) a double branching of the 15 Hz rod-detected flicker t hreshold versus intensity (TVI) curve. 3. Here we investigate the effe ct of changing target size on these phenomena in normal human observer s. We find that the double-branched flicker TVI curve and the perceptu al null are found for all targets larger than 2 deg in diameter. For s maller diameter targets, however, neither the lower branch of the doub le-branched flicker TVI curve nor the null are found. 4. While this mi ght suggest that the slow rod signals are selectively disadvantaged by the use of small targets, phase measurements relative to a cone stand ard reveal that the slow signals are always present. For targets less than or equal to 2 deg in diameter, however, they remain below detecti on threshold because of destructive interference with the fast rod sig nals. Thus, for small targets, the perceptual null is not absent, but has merged with (and therefore obliterated) the lower branch of the do uble-branched flicker TVI function. 5. This situation could arise if d ecreasing the target size causes a parallel reduction in the sensitivi ties of both pathways, rather than a selective reduction in the sensit ivity of either one. Our findings are therefore consistent with a mode l in which the large-scale spatial organization of the two rod pathway s is roughly similar.