SUMMARY1. Visual transduction in rods of the cynomolgus monkey, Macacafa8cicularis, was studied by recording membrane current from single outer segments projecting from small pieces of retina.2. Light flashes evoked transient outward-going photocurrents with saturating amplitudes of up to 34 pA. A flash causing twenty to fifty photoisomerizations gave a response of half the saturating amplitude. The response-stimulus relation was of the form 1 -e-x where x is flash strength.3. The response to a dim flash usually had a time to peak of 150-250 ms and resembled the impulse response of a series of six low-pass filters.4. From the average spectral sensitivity of ten rods the rhodopsin was estimated to have a peak absorption near 491 nm.5. The spectral sensitivity of the rods was in good agreement with the average human scotopic visibility curve determined by Crawford (1949), when the human curve was corrected for lens absorption and self-screening of rhodopsin.6. Fluctuations in the photocurrent evoked by dim lights were consistent with a quantal event about 0-7 pA in peak amplitude. 7. A steady light causing about 100 photoisomerizations s-1 reduced the flash sensitivity to half the dark-adapted value. At higher background levels the rod rapidly saturated. These results support the idea that dim background light desensitizes human scotopic vision by a mechanism central to the rod outer segments while scotopic saturation may occur within the outer segments.8. Recovery of the photocurrent after bright flashes was marked by quantized step-like events. The events had the properties expected if bleached rhodopsin in the disks occasionally caused an abrupt blockage of the dark current over about one-twentieth of the length of the outer segment. It is suggested that superposition of these events after bleaching may contribute to the threshold elevation measured psychophysically.9. The current in darkness showed random fluctuations which disappeared in bright light. The continuous component of the noise had a variance of about 0-03 pA2 and a power spectrum that fell to half near 3 Hz. A second component, consisting * Present address:

SUMMARY1. Visual transduction in macaque cones was studied by measuring the membrane current of single outer segments projecting from small pieces of retina.2. The response to a brief flash of light was diphasic and resembled the output of a bandpass filter with a peak frequency near 5 Hz. After the initial reduction in dark current there was a rebound increase which resulted from an increase in the number of open light-sensitive channels. The response to a step of light consisted of a prominent initial peak followed by a steady phase of smaller amplitude.3. Responses to dim light were linear and time-invariant, suggesting that responses to single photons were linearly additive. From the flash sensitivity and the effective collecting area the peak amplitude of the single photon response was estimated as about 30 fA.4. With flashes of increasing strength the photocurrent amplitude usually saturated along a curve that was gentler than an exponential but steeper than a Michaelis relation. The response reached the half-saturating amplitude at roughly 650 photoisomerizations.5. The response-intensity relation was flatter in the steady state than shortly after a light step was turned on, indicating that bright light desensitized the transduction with a delay. This desensitization was not due to a reduction in pigment content. In the steady state, a background of intensity I lowered the sensitivity to a weak incremental test flash by a factor 1/(1 +I/IO), where Jo was about 2-6 x 104 photoisomerizations so1, or about 3.3 log trolands for the red-and green-sensitive cones.6. Bleaching exposures produced permanent reductions in flash sensitivity but had little effect on the kinetics or saturating amplitude of subsequent flash responses. The sensitivity reductions were consistent with the expected reductions in visual pigment content and gave photosensitivities of about 8 x 10' /tm2 (free solution value) for the red-and green-sensitive pigments. During a steady bleaching exposure the final exponential decline of the photocurrent had a rate constant given by the product of the light intensity and the photosensitivity.7. In some cells it was possible to measure a light-induced increase in current noise. The power spectrum of the noise resembled the spectrum of the dim flash NIS 8019 J. L. SCHNAPF AND OTHERS response and the magnitude of the noise was consistent with a single photon response roughly 20 fA in size.8. The membrane current recorded in darkness was noisy, with a variance near 0 12 pA2 in the band 0-20 Hz. The power spectrum of the dark noise resembled the spectrum of the dim flash response. Noise with the observed magnitude and spectral composition would be generated by photoisomerizations occurring at a rate of about 2400s-l.

1. Spectral sensitivities of cones in the retina of cynomolgus monkeys were determined by recording photocurrents from single outer segments with a suction electrode. 2. The amplitude and shape of the response to a flash depended upon the number of photons absorbed but not the wave‐length, so that the 'Principle of Univariance' was obeyed. 3. Spectra were obtained from five 'blue', twenty 'green', and sixteen 'red' cones. The wave‐lengths of maximum sensitivity were approximately 430, 531 and 561 nm, respectively. 4. The spectra of the three types of cones had similar shapes when plotted on a log wave number scale, and were fitted by an empirical expression. 5. There was no evidence for the existence of subclasses of cones with different spectral sensitivities. Within a class, the positions of the individual spectra on the wave‐length axis showed a standard deviation of less than 1.5 nm. 6. Psychophysical results on human colour matching (Stiles & Burch, 1955; Stiles & Burch, 1959) were well predicted from the spectral sensitivities of the monkey cones. After correction for pre‐retinal absorption and pigment self‐screening, the spectra of the red and green cones matched the respective pi 5 and pi 4 mechanisms of Stiles (1953, 1959).

SUMMARY1. A new method is described for determining the effects of rapid changes in ionic concentration on the light-sensitive currents of rod outer segments.2. Replacing Na with another monovalent cation caused a rapid change in current followed by an exponential decline of time constant 05-2 s.3. From the magnitude of the initial rapid change in current we conclude that Li, Na, and K and Rb ions pass readily through the light-sensitive channel in the presence of 1 mM-Ca, whereas Cs crosses with difficulty and choline, tetramethylammonium and tetraethylammonium not at all.4. The effect of reducing Ca in the external medium indicates that the residual inward current recorded for a few seconds when Na is replaced by an impermeant ion is carried largely by Ca ions.5. With 1 /SM-Ca in the external medium the relative ability of monovalent cations to carry light-sensitive current is Li:Na:K:Rb:Cs = 1P4:1:0'8:0-6:0-15. The same order applied in the physiological region but the values are less certain.6. Large transient inward currents are seen if external Ca is raised from 1 /.M to 5 mm or more; these currents which are maximal in an isotonic Ca solution are presumably carried by Ca. 7. The effect of monovalent cations on the number of open light-sensitive channels was tested by adding the cation to a solution containing 55 mm-Na. Na ions open light-sensitive channels with a delay, probably by promoting Na-Ca exchange; K and Rb close channels by inhibiting exchange; Li and Cs seem inert in the exchange mechanism.8. The rate at which inward current declines in low [Na]o or high [Ca]o is accelerated by weak background lights and slowed by 3-isobutyl-1-methylxanthine (IBMX), which inhibits the hydrolysis of cGMP.9. On returning to Ringer solution after a period in low [Na]0 the current recovers with a delay of about 1 s which decreases as the Ca concentration of the low [Na]0 medium is reduced.10. We conclude that intracellular Ca has a strong effect on the number of open light-sensitive channels. None the less, several observations are inconsistent with channel closure being dependent simply on combination with internal Ca.

SUMMARY1. The exponential decline of light-sensitive current seen after a switch from Na+ to Li+ in the presence of Ca2+ probably depends on the activity of the phosphodiesterase (PDE) which hydrolyses cyclic GMP.2. This probability is supported by experiments with suction electrodes which show that in toad and salamander rods the rate constant, b, of the exponential decline of current was increased at least 10-fold by moderate light intensities and decreased about 10-fold by 3-isobutyl-1-methylxanthine (IBMX), an inhibitor of PDE.3. The rate constant b is about 3 times more sensitive to weak lights or to IBMX than the membrane current. This may be explained by a feed-back involving calcium ions which tends to hold current constant, perhaps by calcium inhibition of guanylate cyclase.4. The time course of b, which probably represents the changes in PDE activity, was measured by switching from Na+ to Li+ at various times after a flash. The results suggest that a moderate flash (140 Rh*) increased b about 7 times in 0-5 s and that b then declined with a time constant of 1-5-2 s.5. Extrapolated values of the parameter b suggest that strong flashes (5000-10 000 Rh*) increased b from 1 s-1 in the dark to perhaps 60 s-and that b continued to increase with flash strength for several log units after the current had reached saturation.6. The observations in 4 and 5 fit well with the idea that b is related to PDE activity and that changes in the latter are sufficient to account for the rising phase of the flash response.7. After a flash the light-sensitive current recovers much more rapidly than the time constant b-1, a discrepancy which is explained if a light flash causes a delayed increase in guanylate cyclase activity.