22 Apr 2014

Funahashi et al. (1989) ‘Mnemonic coding of visual space in the monkey's dorsolateral prefrontal cortex,’ notes


by Corry Shores
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Funahashi S, Bruce CJ, Goldman-Rakic PS.

“Mnemonic coding of visual space in the monkey's dorsolateral prefrontal cortex”



Brief summary:

This experiment suggests that we visually remember something because neurons used in our vision continue their activity even after the stimulus goes away.

 

Notes:

The authors are testing the activity of neurons used for vision during periods when the test subject needs to ‘remember’ what they saw, in absence of that stimulus. In their introduction they write:

Studies of single neuron activity in monkeys during performance of delayed-response tasks have also provided strong evidence of a prefrontal contribution to memory (4, 5, 15, 16,20,22, 37,4 1, 42, 46, 50-53). Although many types of neural activity have been found in the prefrontal cortex, neurons that show sustained activity during the delay period are particularly relevant to the issue of mnemonic processing. Usually these neurons increase their discharge rates following the brief cue presentation and continue firing tonically during the delay period until the response is executed (5, 15, 20, 22, 52, 53).
[p.331Bc]

The authors recorded activity from single neurons of monkeys. The monkeys were shown visual stimuli, and their eye moments were tracked. There is a fixation target, a small white spot in the middle of the screen. And there were peripheral visual cues, small filled white squares. The procedure went like this:

(1) 5s interval

(2) the fixation target appears at the center of the screen

(3) the monkey needed to be fixated for 0.75s (the fixation period)

(4) after those 0.75s, the peripheral visual cue was presented for 0.5s (the cue period) at one of many peripheral locations. The monkey was only rewarded if they maintained fixation in the center during the cue period and also during a subsequent delay period.

(5) delay period (monkey must stay fixated at center) of either 1.5, 3 or 6 (normally 3) seconds long.

(6) response period, within the next 0.5s after the delay, monkey must make saccadic eye movement to the cue’s location (to get reward). [332AB]

Our concern here lies with the neuronal activity. The measured the ‘discharge rate’ (p.334Aa) of 319 neurons in the prefrontal cortex (336Ba)

image

[Image from Funahashi et al., p.332.]

[The above diagrams show that during the delay period, when the monkey needs to visually ‘remember’ the cue’s location, there is heightened neuron activity.]

Findings: They find that of the 288 neurons (in the prefrontal cortex, within and surrounding the principal sulcus (PS)) “170 exhibited task-related activity during at least one phase of this task and, of these, 87 showed significant excitation or inhibition of activity during the delay period relative to activity during the intertrial interval,” and “For 50 of the 87 PS neurons, activity during the delay period was significantly elevated above the neuron’s spontaneous rate for at least one cue location.”

[331Bd]

S. Funahashi , C. J. Bruce , P. S. Goldman-Rakic. Mnemonic coding of visual space in the monkey's dorsolateral prefrontal cortex. Journal of NeurophysiologyPublished 1 February 1989Vol. 61no. 331-349.
http://www.ncbi.nlm.nih.gov/pubmed/2918358

 

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