Studies on human visible persistence

The visual system has limited temporal resolution, with the intensity of brief stimulus presentations being summed, or integrated over time. This temporal integration may manifest itself in various ways, including timeintensity reciprocity at threshold (Bloch's Law), flicker fusion, some masking phenomena, and visible persistence. The degree of visual temporal resolution varies with a number of conditions including luminance, field size, and the spatial frequency of gratings. Many of these temporal properties may possibly be explained in terms of interactions between two parallel visual mechanisms - \sustained\" and \"transient\" channels. These appear to analyse pattern and movement respectively. This thesis reports investigations into properties of one of these temporal characteristics visible persistence. It attempts to relate persistence to threshold measures of temporal summation especially time-intensity reciprocity. It is argued that persistence may occur at more than one level of the visual system and that the more peripheral component may result from processes similar to those responsible for the \"critical duration\" of time-intensity reciprocity. Using a separation threshold method persistence duration was found to increase with both increasing spatial frequency and decreasing contrast. By measuring persistence over several stimulus durations it was possible to show that it consisted of two components. One occurred only at brief stimulus durations and declined linearly with increasing stimulus duration. The other appeared to be present and approximately constant at every stimulus duration. Varying stimulus conditions such as orientation contrast and spatial frequency affected these components differently. These effects were consistent with the hypothesis that one component is peripherally located and the other more central. It was argued that persistence at both levels is due to the prolonged nature of sustained cell responses with high frequency gratings stimulating mechanisms with longer response durations than the mechanisms stimulated by low frequency gratings. It was also proposed that response duration may be influenced by the amplitude or latency of transient mechanisms but neither flicker adaptation nor reaction time experiments supported this. It was noted that the conditions which elevate persistence such as the use of higher spatial frequencies oblique orientations and lower contrast levels would all be expected to produce weaker neural stimulation. It appears therefore that for weaker neural stimulation the response duration is lengthened as if in compensation."