[Examination of rats' serial learning process with a wild card test in a modified Hill maze]

The present study examined rats' learning process of three-item series task in a modified Hill maze, using a subset test and a wild card test. In the first phase, four rats were trained with three item series composed of three simultaneously presented barriers (items A, B, C). They learned to get over the barriers in a prescribed order (A-B-C) reliably. In the next phase, three subsets of items (AB, BC, AC) were presented as prove trials. All rats responded to the subsets in a serial order consistent with the original series. In the final phase, rats were trained to produce "wild card" series (W-B-C, A-W-C, A-B-W) in addition to the original series. With training, rats mastered to substitute the wild card item (W) for the omitted original items. These results suggested that rats learned the series without using item association learning or response chaining.