SLO radiant power and brightness.

Available output in the Scanning Laser Ophthalmoscope (SLO) may be expressed as radiant power at the beam pivot (SLO exit pupil), in units of microwatts (microW). This power corresponds to dimensions of brightness (like luminance and retinal illuminance) and to a range of related measures (like cd/m2, lm/m2, and the troland value) in both free and Maxwellian views. We demonstrate that the conversion factor power/troland=1.26*10(-3) microW and 3.15*10(-4) microW for SLO nominal visual angles 40 degrees and 20 degrees, respectively. The factor permits measured SLO power to be expressed in units of brightness and (inversely) brightnesses of everyday objects to be expressed in units of SLO power. Examples of both conversions are given. Reference to the literature demonstrates the importance of expressing SLO power in brightness terms common to everyday activities and to visual function-testing instruments besides the SLO.

SLO power calibration.

We present a method for calibrating the Scanning Laser Ophthalmoscope (SLO) that predicts radiant power at any of 256 grayscale values (gsv) and 12 polarized filter (polarizer) levels. Predicted power values, p(gsv), were determined by substitution into polynomials linearly transformed to old or new power at p(0) and p(255). This was compared with observed power values at 125 levels of attenuation/session. Prediction accuracy was the proportion of nonsignificant pairwise comparisons (t-test, p=0.0001). We found that power transformation between polarizers and within sessions has both linear and nonlinear characteristics. Within polarizer and between sessions, however, power transformation has linear characteristics. A 5th-degree polynomial was individually fit, at each polarizer, to session 1 power distributions of 9 gsv steps (0, 31, 63, 95, 127, 159, 191, 223, 255). When adjusted to p(255) and p(0) in new sessions, we obtained p(gsv) that predicted power at 25 gsv * 5 polarizers for 18 days with an accuracy of about 0.84. When only adjusted to p(255), predictive accuracy was 0.81.

Refractive changes induced by intraocular lens tilt and longitudinal displacement.

There is a tendency for the posterior chamber intraocular lens to tilt about the points of insertion. This is found clinically and in postmortem studies of eyes that have had intraocular lenses surgically implanted. One end is often located in the lens capsule and the other end is located in the ciliary sulcus. The possibility of anteroposterior displacement also exists. Using a modified Gullstrand schematic model eye, we have computed the amount of spherical and cylindrical errors that are induced due to the tilt and/or displacement of the intraocular lens. If a cylindrical error of approximately 90 degrees from the axis of insertion (lens plus haptic loops) and/or axis of tilt is detected, further careful investigation is warranted.

Frequency dependence in scotopic flicker sensitivity.

Sensitivity to rod-mediated (scotopic) flicker was parametrically studied in the parafoveal retina of human observers. Confirming prior studies, the present results show that sensitivity to scotopic flicker has many similarities to that at photopic levels. Specifically, our results show that the frequency response function for scotopic flicker is characterized by both low- and high-frequency cutoffs and that sensitivity to low frequencies is described by Weber's law. Overall, however, scotopic flicker sensitivity is characterized by higher increment thresholds and lower frequency tuning than photopic flicker. The influences of spatial factors and the prevailing level of illuminance on sensitivity is sufficiently different for relatively low (less than 3 Hz) and relatively high (greater than 5 Hz) temporal frequencies to suggest that they may be mediated by different channels. This possibility is also suggested by selective adaptation experiments. These show that adaptation to flicker frequencies of 3, 5, and 7 Hz have a similar influence on sensitivity to subsequent flicker which is different from the influence on 1 Hz flicker adaptation. Results are compared with prior evidence for channeling within both the scotopic and photopic visual systems.

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.

The signal-to-noise characteristics of rod-cone interaction.

1. The influence of rods on cone-mediated vision was assessed in eight human observers. To this end, increment threshold functions were obtained by determining thresholds of a cone-detected test flash (25 ms duration, 655 nm wave-length, 13' diameter) as a function of the illuminance of larger, 500 ms duration, rod-detected masking flashes. The type of photoreceptor influenced by each stimulus was carefully checked by means of a series of control procedures involving action spectra and selective rod adaptation.2. When the rod mask was 512 nm in wave-length, 40' in diameter, and less than one scotopic td in illuminance, increment threshold functions show that [Formula: see text], where I(Cth) is cone test threshold, I(R) is rod mask illuminance, and D is a dark noise term similar to that used by Barlow (1956). Further increases in I(R) have no additional influences on cone test threshold until threshold is influenced by the combined action of the mask on both rods and cones. If I(R) is expressed in terms of scotopic flux rather than illuminance, the functional relationship obtained with all rod masks 2 degrees have negligible influence on cone test threshold. We propose that I (inhibitory spatial summator), a neural locus which responds to scotopic flux provided over a very large area, attenuates the activity of E. The combined action of E and I is designated a rod channel. The response of cones and the rod channel summate at a detector. Within the detector, cone signals are distinguished from rod-related activity and intrinsic dark noise on the basis of signal-to-noise discriminations.5. The neural substrate for this rod channel most probably involves the combined action of several neurones which synapse within the inner plexiform layer of the retina. The relationship of this rod channel to other perceptual phenomena is discussed.

LEDs: convenient, inexpensive sources for visual experimentation.

This article presents the electrical and optical properties of LEDs for use as visual stimulators in neurophysiological and psychophysical research. one particular circuit is considered in detail which should enable an investigator with minimal technical expertise to build a high quality and versatile stimulator within a few hours and at a minimal cost. Improved circuits and anticipated developments in LED technology are discussed.