Luminance-duration relationship in the electric response of the human retina

Temporal Summation and Phenomenal Simultaneity: Experiments with the Radius Display

This study explores the phenomenon of subjective simultaneity between multiple successive positions of a target, which consisted here of an illuminated radial line in both stepwise and continous rotation around a fixation point. In four experiments the span of phenomenal simultaneity ( M) was measured under varying intensities of target and surround, and with two different estimation procedures. M was found to change as a power function of target intensity, with an exponent of −0.13 or −0.16, for both stroboscopic and continuous illumination of the target. Variation of surround intensity affected M only for 9 out of 15 subjects (Experiment III.) For these subjects M varied inversely with log I of the surround; for the remaining 6 subjects M remained approximately constant over a range of 5 log units of I. Auditory noise tended slightly to decrease M in the former group and to increase it in the latter. Under simultaneous continuous and stroboscopic illumination of the target a paradoxical, single-valued M was obtained (Experiment IV). Higher values of M were found when subjects attempted to estimate the number of apparently simultaneous radii (Experiment I). The function relating M to target intensity closely parallels that for the critical duration of temporal brightness summation. The possibility that both measures reflect the same underlying process is discussed.

The neural retina in retinopathy of prematurity

Retinopathy of prematurity (ROP) is a neurovascular disease that affects prematurely born infants and is known to have significant long term effects on vision. We conducted the studies described herein not only to learn more about vision but also about the pathogenesis of ROP. The coincidence of ROP onset and rapid developmental elongation of the rod photoreceptor outer segments motivated us to consider the role of the rods in this disease. We used noninvasive electroretinographic (ERG), psychophysical, and retinal imaging procedures to study the function and structure of the neurosensory retina. Rod photoreceptor and post-receptor responses are significantly altered years after the preterm days during which ROP is an active disease. The alterations include persistent rod dysfunction, and evidence of compensatory remodeling of the post-receptor retina is found in ERG responses to full-field stimuli and in psychophysical thresholds that probe small retinal regions. In the central retina, both Mild and Severe ROP delay maturation of parafoveal scotopic thresholds and are associated with attenuation of cone mediated multifocal ERG responses, significant thickening of post-receptor retinal laminae, and dysmorphic cone photoreceptors. These results have implications for vision and control of eye growth and refractive development and suggest future research directions. These results also lead to a proposal for noninvasive management using light that may add to the currently invasive therapeutic armamentarium against ROP.

Temporal summation in children with a history of retinopathy of prematurity

PURPOSE

To assess temporal summation in children with a history of retinopathy of prematurity (ROP) by determining the critical duration (tCRIT) for complete temporal summation under rod-mediated conditions. From prior ERG studies, it is known that the kinetics of activation of phototransduction are prolonged in the ROP rod photoreceptor.

METHODS

Dark-adapted thresholds for detecting 10° diameter stimuli with durations from 10 to 640 ms were measured. A two-alternative, spatial, forced-choice psychophysical procedure was used. The tCRIT for complete summation was estimated in former preterm subjects with a history of severe ROP (n = 7), mild ROP (n = 23), and no ROP (n = 15). The subjects ranged in age from 10.4 to 17.6 (median 15.6) years. Age-similar term-born control subjects (n = 5) were also tested.

RESULTS

Critical duration was significantly longer in subjects with a history of ROP than in subjects who never had ROP or who were born at term. Mean tCRIT in the mild ROP group [127.5 (SD = 19.9) ms] and severe group [147.6 (SD = 18.9) ms] did not differ significantly, but both were significantly longer than in former preterms who never had ROP [101.1 (SD = 16.5) ms] and in term-born controls [101.0 (SD = 19.5) ms].

CONCLUSIONS

In ROP subjects, tCRIT is significantly prolonged. This is likely due to abnormal kinetics in the rod outer segment.

Violation of Bloch's Law that specifies reciprocity of intensity and duration with brief light flashes

For more than a century researchers have been reporting that the visual impact of a very brief flash is determined by the quantity of photons that the flash delivers. This has been variously described as the Bunsen-Roscoe Law or Bloch's Law, often specified as reciprocity of intensity × duration. Prior research found no evidence for such reciprocity when microsecond-duration flashes from a light-emitting diode array were used to display the major contours of nameable shapes. The present work tested with flash durations ranging up to 100 ms and also found no reciprocity. This departure from classic principles might be due to the specific range of wavelengths of the light-emitting diodes and to a mesopic level of ambient light, which together would preclude activation of rods. The reciprocity of intensity and duration may only be valid with full dark adaptation and very dim flashes that activate rods.

The electroretinogram and visual evoked potential of freely moving rats

The vascularised rat retina could be one of the most useful experimental objects in visual neuroscience to understand human visual physiological and pathological processes. We report here on a new method of implantation for studying the visual system of freely moving rats that provides a rat model for simultaneous recording at corneal and cortical level and is stable enough to record for months. We implanted light emitting diodes onto the skull behind the eyeball to stimulate the eye with flashes and to light adapt the retina with constant light levels. A multistrand, stainless steel, flexible fine wire electrode placed on the eyeball was used for electroretinogram recording and screw electrodes (left/right visual and parietal cortical) were used to record the visual evoked potential and the electroencephalogram. In the present report we focus on the new method of implantation for recording the corneal flash electroretinogram of normal, freely moving rats simultaneously with the visual evoked cortical potential showing examples in various visual experiments. We also introduce a program for retinogram and visual evoked potential analysis, which defines various measures (latencies, areas, amplitudes, and durations) and draw attention to the benefits of this method for those involved in visual, functional genomic, pharmacological, and human ophthalmologic research.

THE RAT ELECTRORETINOGRAM. II. BLOCH'S LAW AND THE LATENCY MECHANISM OF THE B-WAVE

Electroretinograms were obtained from the all-rod eye of the rat with uniform illumination of the entire retina and stimulus flashes of less than 3 msec. duration. Bloch's law of temporal summation was verified for the b-wave latency by varying the time between two equal intensity flashes and observing that no change occurred in the latency when measured from the midpoint of the two flashes. The results of this and other experiments are described in terms of a simple but general model of the latency-determining mechanism. It is shown that this latency mechanism acts as if it depends on a linear additive process; and also that a hypothetical excitatory substance which triggers activity in the sources of the b-wave must accumulate rapidly in time after the flash, approximately as t⁸. The rate at which this substance accumulates is accurately represented by the diffusion equation for more than 4 to 6 log units in the flash intensity. This suggests that the rate-determining step in the latency mechanism may be diffusion-limited.

The 'OFF' response of the human electroretinogram does not contribute to the brief flash 'b-wave'

It has been assumed that the 'ON' and 'OFF' responses of the human electroretinogram (ERG) interact to produce a single waveform when brief flashes are used. To test this assumption, we examined the separate effects of stimulus intensity, the level of retinal illuminance and stimulus duration on the ERG. Both 'ON' and 'OFF' response amplitude decreased as stimulus intensity was decreased and as the level of retinal illumination was reduced. When stimulus duration was reduced, the amplitude of the 'OFF' response decreased; however, the amplitude of the 'ON' response increased. Summing of the 'ON' and 'OFF' response waveforms could not account for the increased amplitude in response to brief stimuli or for the changes in ERG wave shape. These results indicate that there is not a significant corneally recordable 'OFF' response elicited by brief stimuli, such as those commonly used to record the flash ERG.

Temporal summation in dark-adapted 10-week old infants

The effect of stimulus duration on the b-wave and psychophysical responses of dark-adapted 10-week-old infants and adult control subjects is reported. Both infant and adult b-wave sensitivities vary with stimulus duration, show summation for brief duration stimuli, critical durations estimated at 88-155 msec, and little variation in sensitivity for longer durations. There are however, substantial differences between the infant and adult psychophysical temporal summation functions. The infant function is described by a straight line, slope about -0.5, across all flash durations while adults show summation at durations less than 100 msec and critical durations of 136 to 151 msec. Adult, but not infant, b-wave integration times and b-wave rise and fall times show duration-dependent changes. Thus, both ERG and psychophysical measures demonstrate immaturities in the rod mediated function of the infant retina.

Comparative study of temporal summation and response form in hymenopteran photoreceptors

1. Temporal summation was measured in green-sensitive photoreceptors of seven hymenopteran species with various life styles: three bees, Melipona quadrifasciata quadrifasciata, Trigona spinnipes and Bombus morio; one wasp, Polistes canadensis; and three ants, Pseudomyrmex phyllophilus, Camponotus rufipes, and Atta sexdens rubropilosa. In all species approximate agreement with Bloch's law was confirmed. 2. Critical durations (tc), which varied from 10 ms (Pseudomyrmex) to 46 ms (Atta), are discussed in relation to the life styles of the species and to the mechanisms causing the differences. 3. The direct measures of critical duration obtained are compared to estimates made by convolution or integration of impulse responses measured here in one species and from published data. Linear convolution of typical impulse responses is shown to result in significant departures from Bloch's law, a fact that seems to have been overlooked in the literature. 4. The method used to measure temporal summation involved recording responses to 300-ms stimuli at various intensities; the form of these responses varied greatly from species to species. Possible causes of these variations are discussed.

Temporal information processing in cones: effects of light adaptation on temporal summation and modulation

We have studied the temporal information processing of turtle cones in steady states of light adaptation using intracellular recording techniques. We measured the linear range incremental sensitivity of cones as a function of the stimulus duration. Linear range incremental sensitivity is a function of the background intensity. It is also proportional to the duration of short duration stimuli but is independent of duration for long duration stimuli. The plot of log sensitivity versus log stimulus duration displays two straight line asymptotes; a slope of one for short durations and a slope of zero for long durations. These asymptotes intersect at a time, the critical duration, which decreases with increasing background intensity. Linear systems theory was used to predict these results in addition to the interdependence of critical duration, response kinetics, and sensitivity for any state of adaptation. We have also calculated cone sensitivity as a function of sinusoidal frequency for a variety of background intensities. Correlations between these results and psychophysical studies suggest that the limits on temporal summation established by the cones appear not to be substantially altered by the rest of the retina.