We have developed a new method for studying list learning in animals and humans. We have used different variants of the task to examine list learning in rats, mice, and humans. This method was created as a functional analogue of human pattern learning tasks that requires subjects to press buttons in an array in the proper sequential order. Instead of buttons, our procedure requires subjects to respond in a circular array in a predetermined order. This method holds several advantages over other methods of studying list learning. This procedure has been found to be easily learned without lengthy pre-training. The data gathered with this procedure provide 1) a measure of correct response rates, 2) a measure of incorrect responses and the locations of these responses, and 3) a measure of response latency on a trial-by-trial basis. Subjects can receive numerous patterns each day without interruption between patterns.

The procedures used for mice and rats use an octagonal or hexagonal operant chamber equipped with a retractable lever or a nose-poke receptacle with an indicator lamp mounted on each wall. In the typical experiment, all levers/lights are presented/illuminated at the beginning of each trial, and the subject was allowed to respond to any of the manipuli. If the correct response is generated, all levers/lights excluding the correct lever/lights are removed/turned off and the subject is not reinforced until the correct response was performed. Upon responding correctly, all of the levers/lights are removed/turned off for an inter-trial interval ranging from 1 to 10 seconds and are then reinserted/re-illuminated to indicate the beginning of the next trial. An analogous procedure is used to examine list learning in humans, which requires subjects to respond to a circular array of circles on a computer screen. The response indicators are in a circular array. Thus, the patterns generated for the subjects to learn can be continuous, that is, composed of the endless combination of the eight choices to form patterns of any length and complexity.

We have examined mouse, rat, and human list acquisition of patterns ranging from 12 to 48 items in length. Organization of the items within the list can also be easily examined. We have required rats and humans to learn lists with organizations ranging from a very simple pattern, which could be expressed by two levels of rules, to patterns whose organization could be expressed by four or more levels of rules.

This procedure has also been used to examine other aspects of list learning such as: 1) the effects of the placement of phrasing cues which are either consistent or inconsistent with the structure of the list in rats and mice, 2) the effects of phrasing cues of differing modalities in mice, 3) the sensitivity of subjects to violations of list structure in rats, and 4) subjects abilities to "chunk" from non-adjacent serial positions in structured lists in rats. The procedure has also been used to examine the effects of numerous drugs on sequential learning. We have used variants of this procedure to examine the effects of MK-801, atropine, haloperidol, and clozapine on organized list learning and retention in rats.

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