Archive for the ‘Learning & Memory’ Category

History’s Top Brain Computation Insights: Day 13

Saturday, April 14th, 2007

A small man whose proportions represent the amount of cortical space dedicated to each body part13) Larger cortical space is correlated with greater representational resolution; memories are stored in cortex (Penfield – 1957)

Prior to performing surgery, Wilder Penfield electrically stimulated epileptic patients' brains while they were awake. He found the motor and somatosensory strips along the central sulcus, just as was found in dogs by Fitsch & Hitzig (see previous post). The amount of cortex dedicated to a particular part of the body was proportional not to that body part's size, but to its fine control (for motor cortex) or its sensitivity (for somatosensory cortex).

Thus, the hands and lips have very large cortical spaces relative to their size, while the back has a very small cortical space relative to its size. The graphical representation of this (see above) is called the 'homunculus', or 'little man'.

Penfield also found that stimulating portions of the temporal lobe caused the patients to vividly recall old memories, suggesting that memories are transfered from the archicortical hippocampus into neocortical temporal lobes over time.

Implication:  The mind, largely governed by reward-seeking behavior, is implemented in an electro-chemical organ with distributed and modular function consisting of excitatory and inhibitory neurons communicating via ion-induced action potentials over convergent and divergent synaptic connections strengthened by correlated activity. The cortex, a part of that organ whose spatial dedication determines representational resolution, is involved in perception (e.g., touch: parietal lobe, vision: occipital lobe), action (e.g., frontal lobe), and memory (e.g., temporal lobe).

 

[This post is part of a series chronicling history's top brain computation insights (see the first of the series for a detailed description). See the history category archive to see all of the entries.]

-MC

History’s Top Brain Computation Insights: Day 12

Friday, April 13th, 2007

Transverse section showing a superior view of the hippocampus of each hemisphere12) Hippocampus is necessary for episodic memory formation (Milner – 1953)

Patient H.M. had terrible epilepsy originating in the medial temporal lobes. His neurosurgeon decided to take out the source of the epilepsy: the hippocampus. Surprisingly, after the operation H.M. could no longer form new long-term memories. He could remember things for short time periods if he wasn't distracted (i.e., he had near normal working memory). Also, he could learn new sensory-motor skills, though he could not recall how he learned them.

Patient H.M. is still alive today, and has no new episodic memories since the early 1950s. He still thinks he's in his twenties, and meets his doctors anew each day.

Some have compared his experience with those dramatized in the movie Memento.

Implication:  The mind, largely governed by reward-seeking behavior, is implemented in an electro-chemical organ with distributed and modular function consisting of excitatory and inhibitory neurons communicating via ion-induced action potentials over convergent and divergent synaptic connections strengthened by correlated activity. A part of that organ, the medial temporal lobe, is essential for memory formation.

[This post is part of a series chronicling history's top brain computation insights (see the first of the series for a detailed description)]

-MC

History’s Top Brain Computation Insights: Day 10

Wednesday, April 11th, 2007

Hebbian reverbatory cell assembly 10) The Hebbian learning rule: 'Neurons that fire together wire together' [plus corollaries] (Hebb – 1949)

D. O. Hebb's most famous idea, that neurons with correlated activity increase their synaptic connection strength, was based on the more general concept of association of correlated ideas by philosopher David Hume (1739) and others. Hebb expanded on this by postulating the 'cell assembly', in which networks of neurons representing features associate to form distributed chains of percepts, actions, and/or concepts.

Hebb, who was a student of Lashley (see previous post), followed in the tradition of distributed processing (discounting localizationist views).

The above figure illustrates Hebb's most original hypothesis (which is yet to be proven): The reverbatory cell assembly formed via correlated activity. Hebb theorized that increasing connection strength due to correlated activity would cause chains of association to form, some of which could maintain subsequent activation for some period of time as a form of short term memory (due to autoassociation).

Implication: The mind, largely governed by reward-seeking behavior, is implemented in an electro-chemical organ with distributed and modular function consisting of excitatory and inhibitory neurons communicating via convergent and divergent synaptic connections strengthened by correlated activity.

[This post is part of a series chronicling history's top brain computation insights (see the first of the series for a detailed description)]

-MC