Influence of the adapting background on the Broca-Sulzer phenomenon

The influence of the state of adaptation of the eye on the Broca-Sulzer phenomenon has been investigated by measuring equal-brightness curves as a function of flash duration for circular 1°-fields, projected against a photopic and a dark background. The results show that the Broca-Sulzer effect is present in both conditions. In the dark-field condition the Broca-Sulzer effect was found to shift to shorter durations with increasing flash luminance . For flashes presented against the photopic background, however, this shift is absent. Furthermore, the equalbrightness curves were found to be isomorphous. They coincide after translation along the luminance axis in the photopic background condition and after translation along the duration and luminance axes in the dark-field condition. It is argued that both the isomorphism of the equal-brightness curves and the absence of the shift in the photopic background condition confirm our earlier conclusion (De Ridder & Theelen, 1983, 1984) that the exponent of the power function, relating brightness to luminance, is independent of flash duration. The shift of the Broca-Sulzer effect in the dark-field condition is attributed to disturbances of the adaptation level

Brightness of time-dependent stimuli

An earlier study (De Ridder & Theelen , 1983) showed that the brightness of point sources, flashed on an extended photopic background, can be predicted from the impulse response, measured at threshold level. Here we have duplicated the brightness matching experiments, using large (1° ) fields instead of point sources. The results show once again that equal-brightness curves, determined by matching the brightness peaks of a constant reference flash and a test flash of variable luminance and duration, can be predicted from the impulse response. The results imply that the exponent of the power function, which is assumed to describe the relationship between brightness and luminance, is independent. of the flash duration for both point sources and large fields. The fact that the shape of the equal-brightness curve did not depend on flash intensity supports our earlier suggestion that the initial part of the visual system is linear even for large amplitude changes (more than 3 log units above threshold). The brightness matching of a test flash of constant luminance but variable duration with a reference flash of constant duration can also be predicted from the impulse response. It is argued that the deviations, obtained near detection threshold, are due to the fact that the brightness impression is determined by the effective luminance, i.e. luminance above threshold