An eletrical potential recorded from the cornea, the a-wave of the ERG, is evaluated as a measure of human photoreceptor activity by comparing its behavior to a model derived from in vitro recordings from rod photoreceptors. The leading edge of the ERG exhibits both the linear and nonlinear behavior perdicted by this model. The capability for recording the electrical activity of humans photoreceptors in in vivo opens new avenues for assessing normal and abnormal receptor activity in humans. Furthermore, the quantitative model of the receptor response can be used to isolate the inner retinal contribution, Granit's PII, to the gross ERG. Based on this analysis, the practice of using the trough-to-peak amplitude of the b-wave as a proxy for the amplitude of the inner nuclear layer activity is evaluated.

Kang Derwent and Linsenmeier (2000) reported that the a- and b-waves of the cat electroretinogram (ERG) are resistant to hypoxemia, at least for PaO2 above 40 mm Hg. The a-wave may be resistant because the photoreceptor can increase glycolysis during hypoxemia. This hypothesis was tested by making the animal hypoglycemic, which should minimize the ability of the photoreceptor to switch to glycolysis. The ERG of dark-adapted anesthetized cats was recorded between an Ag/AgCl electrode in the vitreous humor and a reference electrode near the eye. Responses to bright flashes of diffuse white light were recorded at 3-min intervals during hypoxic episodes at three levels of blood glucose. The moderate hypoglycemia (50 to 70 mg/dl) had a relatively small effect on the a-wave amplitude. Furthermore, the a-wave amplitude increased transiently by 16 ± 7% within 30–40 min of the start of severe hypoglycemia (20 to 40 mg/dl), before recovering to near normal. Severe hypoglycemia alone decreased the b-wave amplitude by 15 ± 13%. Combined hypoxemia and hypoglycemia decreased the b-wave amplitude more than the a-wave amplitude. At all levels of blood glucose, b-wave decreases began at a higher PaO2 than the a-wave changes. For both the a- and b-waves, hypoxemic effects began at higher PaO2 when the animal was hypoglycemic. The increased sensitivity to hypoxemia during severe hypoglycemia suggests that a switch from oxidative to glycolytic metabolism ordinarily protects the retina from moderate hypoxemia. Because the a- and b-waves were affected to different degrees and at different PaO2s, it is unlikely that inner retinal changes are caused completely by changes in the photoreceptor.

It has often been assumed that the recovery of the a-wave from its trough is caused by the intrusion of the b-wave. This study examined the recovery following the a-wave trough using intraretinal recordings in dark-adapted intact cat retina. Adult cats were anesthetized and paralyzed. The vitreal ERG was recorded between the vitreous humor and a reference electrode near the eye. Intraretinal recordings were made by referencing a microelectrode to the vitreal electrode. Bright flashes of diffuse white light were used to elicit a- and b-waves. Intravitreal injections of 2-amino-4-phosphonobutyrate (APB), cis 2,3-piperidine dicarboxylic acid (PDA), and kynurenic acid (KYN) were used to block the responses of bipolar and horizontal cells. Intravitreal injections of UL-FS 49 or DK-AH 269 were used to block Ih, a hyperpolarization-activated potassium current. Since the microelectrode was referenced to the vitreal electrode, recordings from the inner retina showed only the oscillatory potentials and b-waves. In the inner retina, the potential was flat until the b-wave became measurable, ∼17 ms from the onset of the flash. The a-wave started to appear as the microelectrode reached the photoreceptors and its amplitude increased with depth until the microelectrode reached the choroid. The a-wave peaked at ∼8 ms in response to flashes that saturated its amplitude and then began to recover well before any inner retinal responses were apparent. After injections of APB, PDA, and KYN, vitreal and intraretinal recordings showed only the a-wave, which consisted of an increase to peak at ∼10 ms followed by a recovery to a plateau which was reached at ∼25 ms. Blockers of Ih reduced the recovery, but did not eliminate it. The a-wave peaks and partially recovers before the b-wave intrudes. Both phases survive blockers of second-order neurons which implies that the photoreceptors generate both the rising and recovery phase of the a-wave. The recovery phase may be due to a current generated by the inner segment of photoreceptors.

The electroretinogram (ERG) of the rhodopsin knockout (rho−/−) mouse of Humphries et al. (1997) (Humphries et al., 1997) was studied for evidence of light-evoked rod activity and to describe the cone function. The rho−/− retina develops normal numbers of rod and cone nuclei, but the rods have no outer segments, and no rhodopsin is found by immunohistochemistry. The dark-adapted ERG threshold was elevated 4.7 log units above wild-type (WT) control mice, indicating that any residual rod responses were reduced >50,000-fold, consistent with a complete functional knockout. The dark-adapted rho−/− ERG had a cone waveform, and the spectral sensitivity peaked near 510 nm for both dark-adapted and light-adapted conditions, without evidence of a Purkinje shift. The light-adapted ERG b-wave amplitude of young rho−/− mice was the same as WT. The amplitude remained steady up to postnatal day P47, but thereafter it declined to only 1–2% by P80 when no cone outer segments remained. Cone b-wave threshold of dark-adapted rho−/− mice was −1.07 ± 0.39 log cd-s/m2 (n = 17), which is 1.27 log units more sensitive than light-adapted thresholds against a rod-suppressing Ganzfeld background of 1.61 log scotopic cd/m2. This indicates that dark-adapted WT responses to still dimmer stimuli are exclusively rod driven with minimal cone intrusion. Above this cone threshold intensity, the dark-adapted b-wave of WT will be a summation of rod and cone responses. Threshold versus intensity (TVI) studies gave no evidence of a rod influence on the mouse cone b-wave.

The paper deals with the one-dimensional modified random walk in the presence of partially reflecting barriers at a and –b (a, b > 0). The simple one-dimensional random walk on a line is the motion-record of a particle which may extend over (–∞, + ∞) or be restricted to a portion of it by absorbing and/or reflecting barriers. Here we introduce the possibility of a particle staying put along with its moving a unit step to the right or to the left and find the bivariate generating functions of the probabilities of a particle reaching m (0 <m <a) under different conditions.