Do you remember when…?

The process of learning ultimately leads to the formation of memory. There are two types of memory, explicit and implicit. Explicit memory is the memory of facts such as names, dates and places. Whereas implicit memory is the memory of skills and behaviours without conscious thought, such as riding a bike. Much of our understanding of the learning process comes from animal studies.

These studies highlight an important feature of learning: Learning involves association of two stimuli. The first learning model dates back to the 1900s - the classical conditioning of Pavlov’s dogs that led to the formation of an implicit memory. Classical conditioning involves the pairing of two stimuli- one stimulus, called the conditioning stimulus (image 1a) would normally evoke a physiological response. The conditioning stimulus is repeatedly paired with a neutral stimulus (image 1b). After conditioning the neutral stimulus evokes the physiological response in the absence of the conditioning stimulus (image 1c). In Pavlov’s paradigm the conditioning stimulus was food that made the dogs salivate (image 2a).

The repeated pairing of a bell just before the presentation of food made the dogs salivate in response to the bell alone (image 2b). What is the mechanism of this learning? The association of the stimuli occurs at the level of synapses. You have learned about synaptic plasticity where strong firing triggers molecular changes that make communication at the synapse more efficient . In Pavlov’s experiment, pairing of the food and sound stimuli provide the strong trigger necessary for plastic changes at synapses. While Pavlov’s experiment is a model for implicit memory, explicit memory can also be studied in the laboratory. A commonly used model is the Morris Water maze, which assesses spatial memory in rodents.

For the Morris Water maze, rats are introduced into a milky pool of water with a hidden submerged platform. The pool is placed in a room with distinctive visual features. The rats learn to use these features to escape the water and find the platform (image 3). With multiple trials, the rats take less time to find the platform. This is because they have memorised its location (image 4). However, when a drug that inhibits neurotransmitter receptors is given to the rats, they cannot memorise the position of the platform. Conversely, stimulation of these neurotransmitter receptors can improve learning. These experiments show that even a behaviour as complex as spatial learning is understood to a certain extent at the molecular level and can be manipulated.