Previous research using the chain of compound letters as letter strings in artificial grammar learning (AGL) suggested that visual saliency known as global precedence influenced the extent of learning. In this study the luminance of letter strings in the learning phase was manipulated to investigate the effect of visual input on AGL regardless of top down attention control. As a result, participants assigned to the low luminance condition were not able to learn any grammar even though they could percept letter strings in the learning phase. This finding suggested that AGL is influenced by the visual saliency from outer environment independently of top down attention control. The results implied that AGL mechanism as adaptive system is affected both by the top down selective attention to acquire covariance sensitively and by the bottom up visual saliency from the complex environment rather than automatic processing system.