We investigated the perception of auditory patterns of two neighboring empty time intervals, t1 and t2 in this order. In RPPW 2003, we reported that bilateral assimilation took place in these auditory patterns. The PSEs of t1 and t2 approached each other. It seemed that these auditory patterns yielded a nearly 1:1 ratio perceptually when the difference between the physical durations of t1 and t2 was small, despite the change in physical temporal ratio between t1 and t2. However, we were uncertain as to whether or not these auditory patterns were actually perceived as 1:1 ratios. In the present study, we confirmed that the category of 1:1 ratio was formed by bilateral assimilation and unilateral assimilation (time-shrinking) when the difference between two neighboring empty time intervals was small. We conducted two experiments. First, we measured similarities of two auditory patterns and analyzed the result by cluster analysis. The auditory patterns which physically varied in a rhythmic manner were divided into three clusters and one cluster consisted of the auditory patterns in which bilateral and unilateral assimilations takes place. Second, we presented the auditory patterns, with a total duration of 180, 360, 540, or 720 ms, in such a way that the difference between t1 and t2 changed systematically. We asked the participants whether the subjective durations of t1 and t2 were the same or not. When the total durations were 180, 360, and 540 ms, the range of ratios in which participants perceived the neighboring durations as equal was almost the same as the range established in the previous experiments. When the total duration was 720 ms, a different tendency appeared. It was shown that bilateral and unilateral assimilations indeed contributed to the formation of the 1:1 category in the short total durations.