Light responses of primate and other mammalian cones

Retinal cones are photoreceptors for daylight vision. For lower vertebrates, cones are known to give monophasic, hyperpolarizing responses to light flashes. For primate cones, however, they have been reported to give strongly biphasic flash responses, with an initial hyperpolarization followed by a depolarization beyond the dark level, now a textbook dogma. We have reexamined this primate-cone observation and, surprisingly, found predominantly monophasic cone responses. Correspondingly, we found that primate cones began to adapt to steady light at much lower intensities than previously reported, explainable by a larger steady response to background light for a monophasic than for a biphasic response. Similarly, we have found a monophasic cone response for several other mammalian species. Thus, a monophasic flash response may in fact be the norm for primate and other mammalian cones as for lower-vertebrate cones. This revised information is important for ultimately understanding human retinal signal processing and correlating with psychophysical data.Previous suction-pipette recordings have demonstrated that, unlike the typically monophasic flash responses of lower-vertebrate cones (1–8), those of monkey and human cones are distinctly biphasic (9–13). This surprising finding has raised an unanswered question of how retinal circuitry would process such biphasic responses (14). In contrast, rods of mammals and nonmammals alike show monophasic flash responses. More recently, human data extracted from paired-flash electroretinographic (ERG) recordings in conjunction with modeling have suggested, albeit indirectly, that in situ primate-cone responses may actually be monophasic (15, 16). Accordingly, we have reexamined this important question directly with single-cell, suction-pipette recordings, which is the same experimental method as used in previous work (9–13).