Psychophysical and electrophysiological experiments suggest that visual image processing is effectuated successively as regard as its spatial frequency (SF) content, low SF-information being processed faster than high SF-information. In an investigation of the effect of exposure time on the detection threshold of a test grating superimposed on a high-contrast masking grating Mezrich (1978) has obtained data which he interprets as suggesting the reverse spatio-temporal relationship in processing of suprathreshold masking stimuli, namely, that processing latency decreases with increasing SF. The experiments described in the present paper confirmed his data. It was, however, shown that his results are not related to the processing rate of the masking stimulus and can be explained by an impaired temporal summation due to reduced signal-to-noise ratio or to interchannel interaction.