2.1 Synaptic Modifications
Non-associative plasticity
Habituation & Sensitization
Non-associative learning is when a behaviour towards the same stimulus changes over time, without any link to outcome or any other stimuli. In non-associative learning, the properties of a single stimulus are learned. This type of learning is based on the frequency of the occurrence of the stimulus. There are two main types of non-associative learning: habituation and sensitization. Habituation occurs when there is a decreased behavioral response to a harmless stimulus that is repeatedly presented (Fig 1). Sensitization, on the other hand, occurs when there is an increased response to various stimuli following the presentation of a noxious stimulus.
An everyday example of this is when watching fireworks on a holiday. When you hear the first firework you may get startled, but as you hear more and more fireworks, you are no longer startled as you get habituated to the sound. After your startle response has weakened, if your friend were to pinch you, the next time you hear a firework, your startle response would be restored and you would show the same level of startle as you did to the first firework you heard. The pinch has caused you to become sensitized to an unrelated stimuli, the sound of the firework. (Kandel textbook)
Habituation & Dishabituation
In a neural circuit, a stimulus can act on sensory neurons in an organism and subsequently elicit a response in the form of behavioural output (Fig 2). Habituation occurs when repeated presentations of the same stimulus cause an output response that becomes weaker each presentation (Fig 3). The first response’s magnitude can be considered the baseline level of response, and each response thereafter is weaker than the baseline response. Dishabition occurs when the output response returns back to the previous baseline level, usually after a stimulus free time interval (Fig 3). Both habituation and dishabituation have been extensively studied in the sea slug Aplysia californica.
Habituation experiments
The gill withdrawal reflex (GWR) in the aplysia is a well-studied example of habituation. This organism was chosen due to its large neurons and relatively simple nervous system. From an evolutionary perspective, the gill withdrawal reflex is a defensive reflex that causes the delicate siphon and gill to become retracted when the animal is disturbed, thereby protecting it from potential threats (Fig 4).
The circuit involved is relatively simple. There is a siphon, which is used to expel seawater, that when a tactile stimulus comes into contact with, elicits the gill-withdrawal reflex. Mechanoreceptors in the abdominal ganglion innervate the siphon skin. These excitatory sensory neurons form synapses with the motor neurons that innervate the gill as well as on both inhibitory and excitatory interneurons that also synapse on the motor neurons. Repeated tactile stimulation of the siphon leads to a reduced response in the GWR (Fi