Attenuation of the frog's cone system during rapid dark-adaptation

Suppression of cone-driven responses by rods in the isolated frog retina

The sensitivity of rod- and cone-driven responses was studied in the isolated frog retina during the period of rapid dark adaptation following a conditioning flash which bleached a negligible amount of visual pigment. Following a conditioning flash, cone-driven b-wave responses were first enhanced and then depressed. The time courses of the enhancement and subsequent depression of cone-drive responses varies greatly with the intensity and wavelength of the conditioning flash, but were identical when the conditioning flashes were matched for equal excitation of 502 nm rods. These changes in cone-driven response sensitivity were correlated with the desensitization and recovery of rods following the conditioning flash. When signal transmission from rods to second-order cells was interrupted by the addition of L-glutamate, the conditioning flash did not produce the above-described enhancement and subsequent depression of long-wavelength receptor potential responses. The suppression of cone-driven response therefore appears to be due to a synaptically mediated influence from 502 nm rods which is maximal when the rods are in the dark-adapted state, with little or no contribution from 433 nm rods, and no involvement of the pigment epithelium.

Stimulation of rods can increase cone flicker ERGs in man

Recent psychophysical studies in man and electrophysiological studies in lower vertebrates show that dark adapted, unstimulated rods inhibit cone mediated flicker. This investigation uses comparable psychophysical and ERG procedures in man to demonstrate rod-cone interaction of this type. With either procedure the rod cone interaction cannot be demonstrated with Ganzfeld stimulation. A single small, red, flickering test field, which is a common psychophysical stimulus for testing rod-cone interaction, elicits an immeasurably small cone ERG. But an array of many such targets, flickering synchronously, is an effective psychophysical stimulus and produces an ERG with larger cone than rod components. With such an array, it can be shown that a steady, rod-stimulating background selectively enhances cone ERG components.

Rod-cone interaction in flicker perimetry: evidence for a distal retinal locus

The luminance threshold for the detection of 25 Hz flicker was measured in nine patients with retinal disorders under stimulus conditions that have been shown previously to involve an interaction between rod and cone systems. The disorders studied included congenital stationary nightblindness, X-linked juvenile retinoschisis, hereditary dominant optic atrophy, optic atrophy found in association with neurofibromatosis, retinitis pigmentosa, choroideremia, and an acquired diffuse photoreceptor disorder, all of which involve pathologic changes that are presumed to occur primarily at specific levels of the retina. The results are consistent with a distal (outer) retinal locus for the rod-cone interaction.

Rod-cone interaction in flicker detection

There is considerable evidence in the literature that rod-cone interaction occurs when both rods and cones simultaneously detect a test target. More recent evidence, however, has shown a parafoveal rod-cone interaction during dark adaptation for a purely cone-detected flickering test stimulus; this influence on cone threshold appears to be mediated by surrounding rods. In this study, we demonstrate a similar rod-mediated influence on parafoveal cone-detected flicker threshold. More surprisingly, foveal cone-detected thresholds are also influenced by rods. This effect occurs over at least a 2 log unit intensity range of mesopic background level; cone-detected 25 Hz flicker sensitivity is enhanced by increasing the radiance of the background. The action spectrum of this effect fits the scotopic spectral sensitivity curve. At higher background levels, this rod-cone interaction disappears and surrounding cone activity then influences the cone flicker threshold. The results suggest that, as rods recover sensitivity, they reduce cone-detected flicker sensitivity, even at the fovea. The rod influence on cone flicker is most apparent for long wavelength test stimuli. Our results, in agreement with recent reports, suggest that the rod-cone interaction is laterally-mediated and may be specific for the long wavelength-sensitive cone type.

Inhibitory influence of unstimulated rods in the human retina: evidence provided by examining cone flicker

In the parafoveal retina of human observers, cone-mediated sensitivity to flicker decreases as rods become progressively more dark-adapted. This effect is greatest when a rod response to flicker is precluded. These results indicate that rods tonically inhibit cone pathways in the dark.