Learning & Memory: Anatomy
Fall 2002

An experimental search for brain areas involved in learning (i.e., the search for the engram) began in the 1920s. Karl Lashley did a series of experiments to determine the location of the engram. Lashley made lesions in different parts of the rat cortex and examined the effect on the acquisition and retention of running a maze (Figure 19.3). Although cortical lesions disrupted learning, there was no specific location that abolished learning in the maze. Lashley concluded that memories were distributed throughout the cortex.
 

In addition to being distributed throughout the  brain, there are also different types of learning. For example:

Fact learning is called Declarative memory. Includes:
  Short Term Memory (STM) which is temporary and of limited capacity
Long Term Memory (LTM) which is long lasting and seems to have unlimited capacity


Skill learning is called Procedural memory

   There are also other very specialized types of learning such as Conditioned Taste Aversions. Taste aversions are similar to classical conditioning in that a normally innocuous stimulus, food (CS), is paired with illness (UR). Subsequent presentation of the food causes an aversive reaction. Conditioned taste aversion differs from classical conditioning in that the delay between CS-US pairing can be several hours (classical conditioning requires close CS-US pairing).
 

Much of what is known about the neuroanatomy of memory is from patients suffering from brain damage. For example, H.M. is a very famous patient who has severe anterograde amnesia (an excellent description of H.M.'s deficits can be found on pages 528-530 of the textbook). In 1953, H.M. had temporal lobe surgery for severe epilepsy (Figure 19.6). Both the hippocampus and overlaying rhinal cortex were removed. The hippocampus and rhinal cortex are important for declarative learning, but not other types of learning. Examination of different types of learning reveals:

Short term memory is OK. H.M. could hold information in his head long enough to carry on a conversation
Long term memory was OK. H.M. could remember stories from his childhood.
Procedural memory was OK. H.M. could learn new motor skills.
Conditioned taste aversion was OK. H.M. will avoid foods that made him sick.

H.M.'s only problem is that he can not form new declarative memories. That is, he can not shift information from short-term to long-term memory (i.e., he has anterograde amnesia). What does this indicate about the role of the hippocampus in learning?
 
 

 Brain structures that contribute to different types of learning:

1. The hippocampus and rhinal cortex appear to contribute to declarative memory by shifting information from short-term to long-term memory. The circuit through which this happens appears to include (in order):

Sensory cortex projects to
Rhinal cortex which projects to
Hippocampus which projects via the fornix to
Mammillary bodies (located in the hypothalamus) which projects to
Anterior thalamus which projects to
Cingulate gyrus which projects back to the rhinal cortex.  


Korsakoff's syndrome disrupts this circuit by killing neurons in the mammillary bodies. Korsakoff's syndrome is characterized by anterograde amnesia and partial retrograde amnesia. During the early stages confabulations often accompany these memory problems. Korsakoffís syndrome is caused by a thiamine deficiency resulting from poor eating habits in alcoholics.
 

 2. The cerebellum contributes to procedural learning (i.e., motor learning).
 

3. The prefrontal cortex contributes to working memory. Working memory is used to make decisions and makes use of both short-term and long-term memory. The prefrontal cortex is especially large in humans. Damage to the prefrontal cortex produces very specific and interesting deficits (see page 541 and Figure 19.17).
 
 

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