Fuster and Alexander. (1971) ‘Neuronal Activity Related to Short-Term Memory’, notes

by Corry Shores
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Fuster and Alexander

Neuronal Activity Related to Short-Term Memory

Brief summary:

This research suggests that our brains retain visual information, as sensory memory, by keeping parts of the brain active even after the stimulus is removed from sight and reaction to it is delayed.

Notes:

Monkeys look at 2 identical wooden blocks set to the right and left. The blocks are in an enclosure but there are doors under the blocks to allow the monkey to grab them [or things placed on them, or things placed in a well below things placed on the block]. The monkey’s vision is first obstructed, then the blind is lifted, allowing the monkey to see a piece of apple [available through one of the doors]. Then the blind is put back, which ends the ‘cue’ phase. Then there is a delay period, after which the doors are unlocked and the blind is lifted again. If the animal choses the right object [placed on the block, near which the apple piece once was], then it receives a reward. The monkeys were trained for 15 second delays. Then electrodes were inserted into their brains to measure activity.

What they found was that

Almost all the units investigated (57 in MD [nucleaus medialis dorsalis of the thalamus], 110 in prefrontal cortex) showed rather irregular patterns of spontaneous firing while the animal was at rest during inter-trial periods. The MD units generally displayed higher frequencies of firing than the cortical units did and, similar to units in other thalamic nuclei, a tendency to discharge in periodic groups or bursts of action potentials. [p653 BC]

In the course of delayed response trials the majority of units (58 percent of those in MD, 65 percent in prefrontal cortex) increased their spike activity to levels higher than those prevalent in intertrial periods. Some units exhibited a higher discharge rate during cue presentation, others during the delay, and still others during both cue and delay periods (Figs.1 and 2)

[From p653 of this article. As we can see in figure 1, there was more activity in the delay period than in the 20 second spontaneous firing segments. This is shown even more clearly in figure 3 below, from page 653]

The magnitude of the activation varied widely between different units, some reaching discharge levels more than tenfold higher than the spontaneous discharge level. Increased firing was in some units preceded by an inhibitory phase covering the beginning or the entirety of the cue presentation period. This inhibition was most conspicuous in units showing maximum dishcarge during the delay (Fig. 3). In some delay-activated units the increased firing | persisted throughout the delay period, slowly and irregularly declining toward baseline in the course of it. Figure 3 shows a unit activated for almost the entire duratino of delays longer than 1 minute.
[pp.653-654]

It is during the transition from cue to delay that apparently the greatest number of prefrontal units discharge at firing levels higher than the inter-trial baseline. […]

We believe that the excitatory reactions of neurons in MD and granular frontal cortex during delayed response trials are specifically related to the focusing of attention by the animal on information that is being or has been placed in temporary memory storage for prospective utilization.

[p654, boldfaces mine]

Fuster, J.M., G.E. Alexander. Neuron Activity Related to Short-Term memory. Science, New Series, Vol. 173 No.3997 (Aug. 13, 1971), pp.652-654. 
http://www.sciencemag.org/content/173/3997/652.abstract
http://www.cns.nyu.edu/~wendy/class/2006sp/reading9/Fuster_1971.pdf