Stimulus configuration.
Various fog conditions. Four uppermost panels: full intensity fog with placard in the far position. Fog colors shown are, at upper left, 'red', along the positive LM half-axis; at upper right, 'blue - green', along the negative LM half-axis; at middle left, 'purple', along the positive S half-axis, and at middle right, 'yellow - green', along the negative S half-axis. The two lowermost panels show, at left, full-intensity gray fog with the placard in the near position; and at right, half-intensity fog with the placard in the far position.