Two designs are proposed which manipulate both the placement of the trace interval and its duration in CER trace conditioning. It is hypothesized that to account for data already reported we need to conceptualize the CS in CER trace conditioning as representative of both CS plus and CS minus. Further, the extent to which the CS takes on either one of these functions is determined in part by the ratio of the CS segment to the trace segment. Using this method to determine the "CS unit", it is proposed that these designs will show that non contiguous traces can acquire more suppression than tracer contiguous with the UCS if the ratio of CS to trace is appropriate for the formation of a perceptual unit.