Working memory is defined as the capability to maintain during a brief period of time the representation of a stimulus that is no longer present and to perform mental operations on such representation. This test technically measures of short term spatial memory, which is part of the larger aforementioned construct of working memory. That is because the animal is required to maintain the representation of the stimulus but not required to mentally operate the representation. Notwithstanding, in scientific literature “working memory” and “short term memory” are often used interchangeably.
During the first phase of a trial, the sample phase, one of two retractable levers is protracted into the chamber. The subject must press the lever and, as a result, the lever will be removed from the chamber. The animal is often then required to go to the pellet dispenser placed within the opposite wall of the chamber. This arguably guarantees that the animal is not using non-mnemonic strategies (i.e. motor) which confound the results and, in addition, it forces the animal to start from similar positions upon the initiation of the test phase. Prior to the introduction of the test phase a short delay is introduced (i.e. 6 seconds). During the delay, which can be fixed or variable, the position of the lever that was activated (and subsequently retracted) during the sample face has to be held in memory. After the delay both levers are protracted and it is at this stage when the animal is to recall which lever it pressed and respond accordingly. The correct response depends on whether it is a DMTP or a DNMTP task. In a DMTP task the animal is required to press the same lever that it pressed during the sample phase. To the contrary, in order to obtain the reward, in a DNMTP task the rodent has to press the lever that it did not activate during the sample phase. The percentage of correct responses typically declines with the increase in the time delay that separates the sample and test phases. This memory decline is known as the “forgetting curve” and it susceptible to drugs or lesions.
