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An antagonist is any substance that fits into a receptor site on the post-synaptic neuron, inhibiting the neuron.  This means that the neuron will not fire and therefore a behaviour will not happen.  Acetylcholine is a neurotransmitter that plays a role in the formation of memories. It is not possible for researchers to directly observe the role of neurotransmitters, so one of the ways that they attempt to understand the role of different neurotransmitters is by giving animals or human participants an antagonist.  They want to see what happens when acetylcholine receptors are blocked and the neurons are "not allowed" to fire.

An example of an antagonist commonly used in memory research is scopolamine. Rasmusson and Dadar (1979) gave participants a scopolamine tablet and asked them to either learn a series of numbers or to solve a maze task. In a second condition, participants were given a placebo as a control.  In the number series condition, verbal declarative memory was being tested.  In the maze task, spatial memory was being tested. The researchers found that there was no significant difference between the placebo group and the AcH antagonist group in the number series condition.  It appears that AcH may not play a key role in the encoding of declarative memories. However, participants in the scopolamine group made more errors and took longer to solve the maze.  This study is supported also by animal research - such as the study by Rogers and Kesner - that found that scopolamine inhibits a rat's ability to remember a maze. It also makes sense as Maguire found that taxi drivers had more grey matter in their posterior hippocampus - an area of the brain high in AcH receptor sites.

By using an antagonist, blocking AcH receptor sites, researchers are better able to understand the role of the neurotransmitter in the formation of memory.

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