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A prevailing hypothesis of visual perceptual learning (VPL) is that for a feature to be learned, focused attention must be directed to that feature. Along these lines, top-down, task-related factors are also required for such learning to take place. The first aim of this thesis was to challenge these ideas by the use of a procedure in which human participants, who were deprived of food and water, passively viewed visual stimuli while receiving occasional drops of water as rewards. Visual orientation stimuli, which were temporally paired with the liquid-rewards, were viewed monocularly and rendered imperceptible by continuously flashing contour rich patterns to the other eye. Results show that VPL can occur not only as a result of task-involvements but also as a result of stimulus-reward pairing in the absence of any task and without awareness of the stimulus presentation. Thus, a question arises as to whether reward-driven exposure-based VPL (REVPL) occurs in the same way as reward-driven task-based VPL (RTVPL). The second aim of this thesis was to investigate whether there are different properties in REVPL and RTVPL by combining VPL and a training procedure employing classical or operant conditioning. Results show different learning patterns between REVPL and RTVPL, suggesting that the mechanism of REVPL is not the same as that of RTVPL. Furthermore, neither REVPL nor RTVPL followed the rules of conditioning. These findings indicate that the underlying mechanism between VPL and conditioning is different. It was shown that VPL could occur without awareness of the stimulus presentation. However, there has been little research on the influence of weak or invisible stimuli. In the third aim, the incident probability of two alternative orientations were manipulated to examine how human observers' choices are influenced by past and present changes in probability. Results show that perceptual decision-making can be influenced to a greater degree by past incidence of weak signals than by stronger signals. In summary, VPL can arise through stimulus-reward pairing in the absence of a task and without awareness of the stimulus presentation. Furthermore, weak or invisible stimuli have great effects on visual learning and perceptual decision-making.