Spatial summation for single line and multi-line motion stimuli

Purpose. To investigate spatial summation for motion displacement of line stimuli as a function of line length, line number and line separation. Method: A video display under PC control was used to present horizontal line stimuli of either a single E line or triple lines separated by 8, 14, 27 or 54 min arc. Two experienced normal subjects were tested at -27,3 eccentricity with peripheral refraction corrected. Five presentations of each of seven motion displacements were made between 0-24 min arc. Stimulus presentations consisted of three oscillations, duration of 200 msec per cycle. Contrast was maintained constant at 84%. Each parameter was tested three times, frequency of seeing curves (FOS) were obtained and 50% seen threshold calculated by probit analysis. Results: Mean thresholds (min arc) are summarized by chart opposite. Conclusions: Equivalent displacement thresholds were found for equivalent lines whether single or distributed amongst three shorter lines, indicating spatial summation. This principal may be used to predict motion displacement thresholds when testine, within these target dimensions.