Current distance relationships estimated using the refractory interaction technique with equal stimulation intensities applied to both electrodes. A. Current distance constants estimated using different interelectrode distances (mean ± s.d. across three randomized populations). B. Current-distance curves estimated using only data from the 400 µm interelectrode distance to estimate k (dot), using data from 100, 200, 300 and 400 µm interelectrode distances to estimate both I0 and k (dash), or using data from only the 300 and 400 µm interelectrode distances to estimate both I0 and k (dash-dot), and the optimal CDR obtained from the lower boundary (bold).