Franklin Naarendorp

Summation of Rod and S Cone Signals at Threshold in Human Observers

We examined whether signals from rods and S cones can combine to produce a threshold response. Test flashes of specific wavelengths superposed on a long wavelength adapting field were used to isolate threshold responses from the two receptor systems, simultaneously and at the same retinal location. Dark adaptation experiments and spectral sensitivity determinations indicated that, in the adaptational range from about 1.6 to 2.8 log scot td, 530 nm and 440 nm flashes were detected by rod and S cone photoreceptors, respectively. The intensities of the 530 nm and 440 nm flashes were mixed in various ratios and the increment threshold was then measured with these mixture flashes using the method of constant stimuli. The effects of rod and S cone excitation were found to summate linearly at threshold, under these experimental conditions. Summation occurred presumably at an early stage of the visual process.

Bright flash response recovery of mammalian rods in vivo is rate limited by RGS9

The temporal resolution of scotopic vision is thought to be constrained by the signaling kinetics of retinal rods, which use a highly amplified G-protein cascade to transduce absorbed photons into changes in membrane potential. Much is known about the biochemical mechanisms that determine the kinetics of rod responses ex vivo, but the rate-limiting mechanisms in vivo are unknown. Using paired flash electroretinograms with improved signal-to-noise, we have recorded the amplitude and kinetics of rod responses to a wide range of flash strengths from living mice. Bright rod responses in vivo recovered nearly twice as fast as all previous recordings, although the kinetic consequences of genetic perturbations previously studied ex vivo were qualitatively similar. In vivo, the dominant time constant of recovery from bright flashes was dramatically reduced by overexpression of the RGS9 complex, revealing G-protein deactivation to be rate limiting for recovery. However, unlike previous ex vivo recordings, dim flash responses in vivo were relatively unaffected by RGS9 overexpression, suggesting that other mechanisms, such as calcium feedback dynamics that are strongly regulated by the restricted subretinal microenvironment, act to determine rod dim flash kinetics. To assess the consequences for scotopic vision, we used a nocturnal wheel-running assay to measure the ability of wild-type and RGS9-overexpressing mice to detect dim flickering stimuli and found no improvement when rod recovery was speeded by RGS9 overexpression. These results are important for understanding retinal circuitry, in particular as modeled in the large literature that addresses the relationship between the kinetics and sensitivity of retinal responses and visual perception.

The scotopic electroretinogram of the sugar glider related to histological features of its retina

The flash electroretinogram (ERG) was used to characterize the scotopic retinal function in a marsupial. Key parameter values of the a- and b-waves of adult male sugar gliders, Petaurus breviceps breviceps, elicited with ganzfeld flashes were determined under dark- and light-adapted conditions. Using standard histological methods, the thicknesses of the major layers of the retina were assessed to provide insight into the nature of the ERG responses. The ERG and histological results were compared to corresponding data for placental C57Bl/6 mice to establish whether the functional retinal specialization that underlies scotopic visual function in a marsupial parallels that of a placental mouse. The sensitivity of the a-wave assessed with the Lamb and Pugh (Invest Ophthalmol Vis Sci 47:5138–5152, 2006) “model” and that of the b-wave assessed with standard methods were lower in the sugar glider compared to the mouse. The thickness of the sugar glider retina was two-third of that of the mouse. The high-intensity flash ERG of the sugar glider substantially differed in shape from that of the mouse reflecting perhaps structural and functional differences between the two species at the level of the inner retina.

S-cone light adaptation: Effects of moderately intense adapting flashes

The course of light adaptation of parafoveal S-cones to a moderately intense steady red field to which a pedestal had been added was studied by psychophysical means. Upon exposure to the red field, the observer adjusted the intensity of a 430 nm probe (flashed with the pedestal) to threshold visibility continuously until equilibrium was reached. With a short wavelength pedestal, S-cone sensitivity decreased initially. About 10 seconds after onset of the adapting stimuli, sensitivity began to increase and reached a maximum about 40-50 seconds later. Then, in the second minute of adaptation, S-cone sensitivity again decreased and stablilized eventually (below maximum sensitivity) in the fourth minute of adaptation. The transient increase in sensitivity during the early stage of adaptation became more pronounced with increased intensity of the short wavelength pedestal. With a long wavelength pedestal present, S-cone sensitivity decreased considerably at onset of the steady field and equilibrium was reached gradually without any sign of a transient increase in sensitivity. The results are discussed in terms of a shift in the balance of short and long wavelength signals reaching a chromatically opponent site.