The effects of image degradation on retinal illuminance and pattern responses to checkerboard stimuli

Abstract

The contrast of a retinal image is less than that of the external stimulus owing to a process of optical degradation. Theoretical studies have shows that this affects the pattern and illuminance detectors of the retina differently and provides a new insight into the nature of contrast stimulation and the mechanisms responsible for the pattern electroretinogram. Consensus data on the optical transfer function of the eye are applied to the Fourier transform of the pattern stimulus and the retinal illuminance distribution is determined. The checkerboard image is shown to undergo substantial degradation for those check sizes used in experimental and clinical observations. Current concepts of contrast and modes of stimulation are examined and methods are described for quantifying the effects on stimulation of illuminance and pattern detectors. The findings are applied to experimental data on spatial tuning functions of electroretinograms elicited by checkerboard pattern stimulation. It is concluded that the signal predominantly originates from local illuminance, but that this can account for only part of the response with small check sizes. The remainder must be a highly selective response to spatial frequency. If the predicted degradation is accepted, a similar conclusion must be reached for many previously reported tuning functions of the pattern electroretinogram as well as to the present experiment on eight normal eyes.

Focal macular photopic negative response in patients with optic neuritis

“…40,41 Some researchers recorded pattern ERG for patients with optic neuritis. [42][43][44][45] The positive peak of the pattern ERG is produced by retinal structures that are not myelinized, 46 whereas the negative component of the pattern ERG is mainly affected in optic nerve diseases if the pattern ERG is abnormal. 47 Berninger and Heider 43 showed that the positive component of the pattern ERG (P50) was reduced in all acutely affected eyes in comparison with the fellow eye in acute retrobulbar optic neuritis.…”

Distortion of spatial selectivity by pattern onset stimulation

“…We have previously reported methods of analysis of the PERG [9,16], in which the retinal illuminance response is obtained by multiplying the lowest spatial frequency PERG by the space-averaged temporal contrast of the pattern at each spatial frequency. The pattern-specific response is then obtained by subtracting the illuminance response from the PERG.…”

“…Since the pattern response is proportional in amplitude to the retinal contrast [15] it is possible to calculate an iso-contrast pattern-specific response by dividing the signal by the modulation transfer value. The effect of this correction applied to our original data [16] is to increase the amplitude of the pattern-specific response to higher spatial frequencies and thus also to displace the peak of its contrast function toward the higher spatial frequency end of the scale. When this correction is applied the spatial tuning curves derived from different areas of retina are more clearly defined than in our previous reports and seem to represent more precisely the expected characteristics of retinal neurones (Fig.…”

Differences between pattern onset and pattern reversal retinal responses

“…The different effects of luminance-and color-contrast stimuli on the amplitude and peak latencies of the responses to onset and reversal stimuli could result from the combination of three factors: 1) As pointed out by Drasdo and associates [23], pattern-reversal stimuli of the luminancecontrast type are associated with a higher local luminance modulation depth than onset stimuli. This is probably true also for color-contrast patterns.…”