The effect of immediate background size on target detection

Several studies show that the surround conditions of an on-axis target have a significant effect on target visibility at the threshold between the target and the surround. In complex visual fields such as roadways in urban areas, where a target signal-to-noise ratio is often low, the surround conditions of targets may significantly impair a driver's ability to detect the targets. This study investigated how the immediate background size of an off-axis target influences a driver's detection of the target at a mesopic light level under different conditions. The experiment used a target (subtended 2 ? 2 degrees) framed by a square-shaped immediate background surrounding the target, and framed by a larger square-shaped far background surrounding the immediate background. The experiment varied the luminance contrast of the target to the immediate background and the size of the immediate background, but kept the luminance of the far background constant at 0.009cd/m(2). The target was located at 15 degrees off-axis and was presented to six subjects under each condition. The reaction times and number of missed trials of detecting targets were measured to evaluate the subjects' ability to detect a change in the luminance contrast. The results suggested that, if the width of the immediate background framing the target was narrower than the size of the target (2 degrees), the immediate background impaired the subjects' peripheral detection of targets at the threshold. When the width of the immediate background was wider than that of the target, the reaction time was constant regardless of the width of the immediate background. As these results show, conventional luminance contrast of a target to its background cannot take into account the spatial effect of a complex luminance distribution around a target on the detection of the target. To quantitatively investigate how the immediate background influences the target detection, this study attempted to adopt an image filtering method developed by Nakamura that can analyze how strong and how frequent luminance changes exist over a specific area and its surrounds. The results of the analysis suggested that this image filtering method could explain the mechanism of target detection under complex background conditions.