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Department of Clinical Ophthalmology, Moorfields Eye Hospital, City Road, London EC1V 2PD
Institute of Ophthalmology, Judd Street, London WC1H 9QS
1. We have recorded electroretinograms (e.r.g.s) in normal subjects. Television monitors were used as stimulators. The screens were surrounded by brightly lit white reflecting surfaces to ensure that the responses were developed by defined retinal areas.
2. Various types of stimuli were employed. Either (i) a pattern of dark and bright squares was reversed, to evoke a pattern e.r.g. (p.e.r.g.), (ii) the luminance of the uniform screen was abruptly increased and decreased to evoke a focal on—off e.r.g. or (iii) a pattern was made to appear and disappear from a uniform background. In each of these cases, the sequence of changes of luminance at any one point could be made identical. The aim of the experiments was to determine whether the e.r.g. was modified by the spatial organization of the stimulus.
3. In other experiments a colour monitor was used so that (i) a red—green flicker, (ii) red—green pattern reversal or (iii) the appearance of a red—green pattern from a yellow background could be used as a stimulus. The responses were caused by the changes in hue, since all the colours were equiluminant.
4. With black and white patterns the p.e.r.g. peaks 5 msec later than the focal on—off e.r.g. The largest response is produced by squares of 0.5-1° subtense.
5. The ratio of the amplitudes of the p.e.r.g. to the focal on—off response is largest for stimuli confined to the macula and smallest for those projected onto peripheral retina.
6. The amplitude of responses to chequerboard reversing patterns increases nearly linearly with contrast up to the maximum contrast available.
7. When patterns appear or disappear from a uniform screen, and there is an associated change in the quantity of light entering the eye, recognizable b-waves occur when the average screen luminance increases, independently of whether pattern contrast increases (appearance) or decreases (disappearance).
8. When a pattern appears or disappears with no change in luminance, e.r.g.s are evoked at both `on' and `off'. The disappearance of the dark parts of the pattern causes the largest logarithmic increase in local retinal illumination. For patterns of square size > 4° the pattern disappearance response is larger than for pattern appearance. As the square size is reduced, the appearance response grows and the disappearance response decreases. The e.r.g.s evoked by the appropriate changes in luminance of a uniform screen are no longer the same as those caused by the appearance and disappearance of the pattern.
9. The responses to change of hue are 70% as large as those produced by black and white patterns. The same ratio occurs for pattern and focal on—off e.r.g.s.
10. When coloured patterns appear from and disappear to a uniform field, the e.r.g.s. evoked are very similar to those recorded when the appropriate changes of hue occur in a uniform field. This result is quite different to the findings for black and white patterns (see 8 above).
11. The results suggest that it is the change in local adaptation caused by the black and white patterns which modifies the e.r.g. and not the presence of contrasting borders.
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