Content for part 1:
• some species specific examples of exposure learning, before moving on to more
general phenomena that also have application to humans.
• My first two examples both depend on experience during a critical period of
development.
Learning Birdsong using exposure learning
Peter Marler pioneered research on bird song learning.
The male white crown sparrow is a good example.
- They learn their song over a critical period early in life.
- If they miss this period - an impoverished song is produced.
- For example: Birds in isolation experiments are raised without access to intact
adult song (i.e., no template) and will subsequently show deficiencies in their
own song upon maturation.
- The song does nonetheless contain valid elements of intact adult song.
- Young birds reared in the presence of taped song will learn and present that song,
even if the tape came from another species.
- For example: A Song sparrow raised with a Swamp sparrow tape will
experience little difficulty in learning the Swamp sparrow song.
Learning imprinting through exposure learning
Bateson and Horn have done a lot of work on the neural mechanisms underlying this
phenomenon.
Imprinting simply involves showing the chick the stimulus. The stimulus can be a model of
an adult bird, but it can also be something as abstract as a rotating red cube!
- Then you compare how fast the chick will run towards it compared to a non-exposed
stimulus.
- The chicks preference for a given stimulus is measured via the speed with which it
runs in the wheel.
Findings:
- They prefer the imprinted stimulus.
- It’s clear that they learn something about the stimulus they are exposed to during this
critical period.
- Horn and Bateson were able to show that this corresponded with changes at the
synapses of neurons in the avian equivalent of the hippocampus.
Aplysia Research on habituation and sensitization
, PSY2304 Biological Basis of Behaviour
Our next examples are of basic exposure learning in a simple organism - Aplysia - the marine
sea snail. Nevertheless these phenomena occur in most animals and all mammals, including
us!
Habituation and Sensitization
We know that a response to a repeated stimulus will decline over time, which is called
habituation. It can easily be observed in humans e.g: response to a repeated loud noise will
decrease. Habituation is often studied using what is called the startle response in the rat to a
noise (it moves its head from side to side and moves around) or the orienting response to a
light (it rears up to inspect the light source).
A less well known phenomena is sensitization, which is where repeated exposure can cause
an increase in response over time. Again this is easily observed in humans, the most common
example being the increased reaction to a mildly painful stimulus (e.g. shock). This has
important implications for any demonstration of conditioning as we will see.
Benefit of studying Aplysia
The Aplysia has an exposed siphon and mantle on its underneath, and when it is stimulated,
then they contract along with the gill. This form of preparation allows us to directly
manipulate neurons and so study basic learning processes The research reported here was
performed in Eric Kandel’s laboratory.
Habituation in Aplysia
Findings:
An initial response to weak stimulation that is not rewarding
or aversive gradually declines over repeated presentations.
- Aplysia initially responds to a gentle touch on the
siphon by withdrawing both gill and siphon, but
after many trials this response all but disappears.
The connection strength between the sensory neuron that
synapses on the motor neurons for the siphon and the gill
gradually weakened as the snail habituated.
Sensitzation in Aplysia
An initial response to weak stimulation is this time strengthened, but this is not conditioning
per se.
Aplysia initially responds weakly to a gentle touch on the siphon by withdrawing both gill
and siphon as we have said, but after an aversive shock to the tail (that’s not paired with the
sensory stimulation) this response becomes more vigorous.
The connection between Siphon sensory neuron synapses on motor neurons for siphon and
gill. strengthens during course of sensitization because of facilitation by the interneuron. The
interneuron is activated by the shock to the tail, hence why it leads to sensitization.
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