Dark adaptation of separate cone systems studied with psychophysics and electroretinography

An adaptive ERG technique to measure normal and altered dark adaptation in the mouse

The time-course of dark adaptation provides valuable insights into the function and interactions between the rod and cone pathways in the retina. Here we describe a technique that uses the flash electroretinogram (ERG) response to probe the functional integrity of the cone and rod pathways during the dynamic process of dark adaptation in the mouse. Retinal sensitivity was estimated from the stimulus intensity required to maintain a 30 microV criterion b-wave response during a 40 min period of dark adaptation. When tracked in this manner, dark adaptation functions in WT mice depended upon the bleaching effects of initial background adaptation conditions. Altered dark adaptation functions, commensurate with the functional deficit were recorded in pigmented mice that lacked cone function (Gnat2 ( cplf3 )) and in WT mice injected with a toxin, sodium iodate (NaIO(3)), which targets the retinal pigment epithelium and also has downstream effects on photoreceptors. These data demonstrate that this adaptive tracking procedure measures retinal sensitivity and the contributions of the rod and/or cone pathways during dark adaptation in both WT control and mutant mice.

Analysis of retinal light adaptation with the flicker electroretinogram

To study the retinal light adaptation we measured and analyzed the flicker electroretinogram response to stimuli that varied in temporal frequency, retinal illuminance, and modulation depth. The responses measured at 100% modulation showed the classic adaptation pattern, being independent of mean retinal illuminance at low temporal frequencies, consistent with Weber adaptation, and increasing in proportion to mean retinal illuminance at high temporal frequencies, consistent with linearity. At 25% modulation, however, high-frequency linearity was not found. The response amplitude consistently showed a minimum at 40-48 Hz. When modulation was systematically varied, response amplitudes measured at 16 and 22 Hz showed Weber adaptation at all modulations and response phase was relatively constant with modulation, whereas response amplitudes at 40 and 48 Hz showed adaptation at low modulations but linearity at high modulations and response phase varied with modulation. We conclude that retinal gain controls also operate at high temporal frequencies.

Surface color naming in dichromats

In previous experiments, Montag and Boynton [(1987) Vision Research, 27, 2153-2162] found that many dichromats can categorize colors using color naming in fair agreement with color-normal subjects. The contribution of rods to color vision was suspected as underlying this ability. Here we follow up on these experiments by having dichromats name colors under various conditions. When the stimuli are limited to a brief presentation time (60 msec) the dichromats' categorization in the three dimensions of the OSA color space is impaired. Using high light levels so that the rods are saturated does not impair performance. The dichromats named colors during the period of the cone plateau following a rod bleach. Contrary to Montag and Boynton (1987) there was no deficit. These results suggest that an anomalous third cone pigment is responsible for the categorization in three dimensions. It is concluded that the receptors containing the anomalous pigment require greater temporal and spatial summation in order to contribute to the dichromats' color categorization.

Red-green flicker photometry and nonlinearities in the flicker electroretinogram

We investigated whether responses from different cone classes are combined before or after the nonlinearity that is responsible for generating nonlinear response components of the flicker electroretinogram (ERG). We measured the nonlinear response of the retina while systematically varying the modulation in the long-wavelength-sensitive and middle-wavelength-sensitive cones by changing the proportions of flickering 633- and 543-nm lights that compose a sum-of-sinusoids temporal waveform. We found that at high temporal frequencies the ERG responses are best accounted for by a model in which the principal retinal nonlinearity is located before the convergence of signals from the two cone classes. At low temporal frequencies the ERG signal is dominated by cone-antagonistic responses. At frequencies of 30 Hz and higher, the flicker ERG and psychophysical flicker photometry give similar estimates of the relative proportions of long- and middle-wavelength-sensitive cones. The ERG photometric null is frequency dependent, whereas the psychophysically determined ratio is much less sensitive to changes in frequency.

Spatial and chromatic interactions in the human pattern electroretinogram

The spectral sensitivity and the spatial selectivity was studied both psychophysically and electroretinographically using the pattern onset-offset paradigm. All measurements were made under intensive yellow adaptation. The spectral sensitivity functions of both measures were in close agreement. They showed a peak at 460 nm (blue-sensitive mechanism) and a shoulder around 550 nm (red-green-sensitive mechanism). The luminance curves of the pattern onset ERG obtained with long wavelengths had a steeper slope and reached larger amplitudes than those obtained with short wavelengths. In addition the response-peak times were longer with short wavelengths. When the spatial frequency of the pattern was varied the 460 nm-onset responses showed very little or no spatial tuning and long peak times (around 60 msec). This was ascribed to the contribution from only one type of ganglion cell, namely the blue-yellow opponent receptive fields lacking a center-surround organization. The 550 nm-onset responses showed a clear spatial tuning (4 c/deg) and an increase in peak time (40-50 msec) with increasing spatial frequency (0.26-9.2 c/deg). This was ascribed to different types of receptive fields having a center-surround structure.

Dark adaptation and increment threshold in rhesus monkey and man

Dark adaptation and increment thresholds were determined for a trained rhesus monkey and a human observer under identical conditions. The measurements were made with a 1.5' diameter, 100 ms target at 10 degrees and 40 degrees eccentricity along the temporal horizontal meridian. Qualitatively, the results are similar for the two species: Both rhesus monkey and man have photopic and scotopic branches, which cross at approximately the same time in the dark and at approximately the same background luminance.

Electroretinograms evoked by sinusoidal excitation of human cones

The amplitude and phase of the fundamental Fourier component of the human electroretinogram (e.r.g.) were recorded with a synchronous detection method under conditions in which each of the three species of cones can be assumed most sensitive in turn. Weber-Fechner behaviour is well established at, or more distal in the retina than, the source of these voltages. Results over the frequency range 7-50 Hz exclude a diffusion model of human flicker perception. The e.r.g. phase vs. frequency plot found with a 'red' test differs from that obtained with a 'green'. The shapes of the e.r.g. field sensitivity action spectra agree with those of the subject's IIj(mu) (j = 3, 4 and 5) mechanisms of Stiles and with in situ measurements of the absorbance spectra of human cone pigments. Threshold phase with each test was independent of background wave-length but, consistent with the results in 3, the phase of the response to the 'red' test (25 Hz) differed significantly from that to the 'green'. If these differences resulted from the absorption of test photons of different colours at different points along the outer segment (independent of cone spectral sensitivity), they would be as clear on dichromats as on trichromats. Results on a protanope are inconsistent with this prediction. We infer that differences in phase are due to the different kinetics of different cone mechanisms and that the e.r.g.s studied here are evoked by exciting only the most sensitive cone mechanism, even though dark-adaptation studies prove that at e.r.g. threshold the test is well above psychophysical threshold for all three cone species. If the inference in 5 is correct, studies of sensitivity across the retina suggest that the spatial distribution of long-, middle- and short-wave-sensitive cones in the human retina differ remarkably.

Factors influencing threshold of the fundamental electrical response to sinusoidal excitation of human photoreceptors

The amplitude and phase of the fundamental Fourier component of the electroretinogram (e.r.g.) in response to sinusoidally modulated light were studied in the range 7-50 Hz. Sensitivity was best at the lowest frequency. The threshold-frequency relationship divided into two parts. A weak steady background depressed sensitivity of the low, but increased sensitivity of the high, frequency component. At 8 Hz a small test spot was 0.7 log10 units more effective on the most sensitive part of the retina than on the optic disk. On the fovea, it was 0.1-0.2 log10 units less effective than on the disk. The fovea was 0.7 log10 units more sensitive to 25 Hz than the blind spot. Psychophysical and e.r.g. dark-adaptation curves were similar, but the former was 10(4) times more sensitive than the latter. Four sets of experiments examined the possibility that the Fourier component of the e.r.g. response at the modulation frequency of 8 Hz during the 'rod' phase of the e.r.g. dark-adaptation curve arose from excitation of rods alone. The only hint of a possible cone contribution was a very small but systematic increase in phase delay with increase in background wave number found while measuring the field sensitivity action spectrum. No suggestion was found that the fundamental Fourier component of threshold e.r.g. responses at the modulation frequency of 25 Hz was influenced by photons absorbed in rods.

Sinusoidal flicker characteristcis of primate cones in response to heterochromatic stimuli

Electrophysiological recordings of primate photorecptors have been obtained and frequency response characteristics of the red, green and blue cones have been determined and compared to previous psychophysical findings. Using cynomolgus monkeys, we recorded the foveal local electroretinogram, which is dominated by the late receptor potential, and obtained criterion-response threshold data for sinusoidally flickered test stimuli complementary chromatic adapting backgrounds. Our results support the hypotheses that (a) the shapes of the MTFs of the red and green cone systems are identical and are determined solely by the photoreceptors at high frequencies, and (b) the blue cones have an MTF with a lower corner frequency than the red- and green-cone systems.

White noise analysis of Phycomyces light growth response system. II. Extended intensity ranges

By means of white gaussian noise stimulation, the Wiener kernels are derived for the Phycomyces light growth response for a variety of intensity conditions. In one experiment the intensity I, rather than log I, is used as the input variable. Under the very limited dynamic range of that experiment, the response is fairly linear. To examine the dependence of the kernels on dynamic range, a series of experiments were performed in which the range of log I was halved and doubled relative to normal. The amplitude of the kernels, but not the time course, is affected strongly by the choice of dynamic range, and the dependence reveals large-scale nonlinearities not evident in the kernels themselves. In addition kernels are evaluated for experiments at a number of absolute intensity levels ranging from 10(-12) to 10(-3) W/cm2. The kernel amplitudes are maximal at about 10(-6) W/cm2. At 10(-12) W/cm2, just above the absolute threshold, the respond is very small. The falloff at high intensity, attributable to inactivation of the photoreceptor, is analyzed in the framework of a first-order pigment kinetics model, yielding estimates for the partial extinction coefficient for inactivation epsilonI455 = (1.5 +/- 0.2) X 10(4) liter/mol-cm and a regeneration time constant of tau = (2.7 +/- 0.6) min. A model is introduced which associates the processes of adaptation and photoreceptor inactivation. The model predicts that the time constants for adaptation and pigment should be identical. This prediction is consistent with values in this and the preceding paper. The effects of pigment inactivation are simulated by a linear electronic analog circuit element, which may be cascaded with the linear simulator circuit in the preceding paper.