Fig. 20. The 4-2 bracketing strategy to determine threshold. Thestimulus intensity is varied so that threshold is crossed twice, first using 4-dB steps and then 2-dB steps. In this example, the initial stimulus presented was seen. The stimulus intensity was decreased by 4 dB. The second stimulus was also seen, so the intensity again was decreased by 4 dB. The third stimulus crossed the threshold (first crossing) and was not seen. The stimulus intensity was increased by 2 dB. The fourth stimulus was not seen, so the intensity was increased by 2 dB. The fifth stimulus crossed the threshold (second crossing) and was seen. Threshold is either the intensity of the last seen stimulus (Humphrey Field Analyzer) or the average of the last seen and unseen stimulus (Octopus). The profile of the hill of vision is represented by the threshold at each location.