Neural coding: Difference between revisions

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=== Population coding ===

Population coding is a method to represent stimuli by using the joint activities of a number of neurons. In population coding, each neuron has a distribution of responses over some set of inputs, and the responses of many neurons may be combined to determine some value about the inputs. From the theoretical point of view, population coding is one of a few mathematically well-formulated problems in neuroscience. It grasps the essential features of neural coding and yet is simple enough for theoretic analysis.<ref name="Wu">{{cite journal |vauthors=Wu S, Amari S, Nakahara H |title=Population coding and decoding in a neural field: a computational study |journal=Neural Comput |volume=14 |issue=5 |pages=999–1026 |date=May 2002 |pmid=11972905 |doi=10.1162/089976602753633367 |s2cid=1122223 }}</ref> Experimental studies have revealed that this coding paradigm is widely used in the sensorsensory and motor areas of the brain.

For example, in the visual area [[Medial temporal lobe|medial temporal]] (MT), neurons are tuned to the moving direction of object motion.<ref name="Maunsell">{{cite journal |vauthors=Maunsell JH, Van Essen DC |title=Functional properties of neurons in middle temporal visual area of the macaque monkey. I. Selectivity for stimulus direction, speed, and orientation |journal=J. Neurophysiol. |volume=49 |issue=5 |pages=1127–47 |date=May 1983 |pmid=6864242 |doi=10.1152/jn.1983.49.5.1127 |s2cid=8708245 }}</ref> In response to an object moving in a particular direction, many neurons in MT fire with a noise-corrupted and [[Normal distribution|bell-shaped]] activity pattern across the population. The moving direction of the object is retrieved from the population activity, to be immune from the fluctuation existing in a single neuron's signal. When monkeys are trained to move a joystick towards a lit target, a single neuron will fire for multiple target directions. However it fires the fastest for one direction and more slowly depending on how close the target was to the neuron's "preferred" direction.<ref>{{Cite web|url=http://homepage.psy.utexas.edu/homepage/class/psy394U/hayhoe/IntroSensoryMotorSystems/week6/Ch38.pdf|title=Intro to Sensory Motor Systems Ch. 38 page 766|access-date=2014-02-03|archive-date=2012-05-11|archive-url=https://web.archive.org/web/20120511112450/http://homepage.psy.utexas.edu/homepage/class/psy394U/hayhoe/IntroSensoryMotorSystems/week6/Ch38.pdf|url-status=dead}}</ref><ref>Science. 1986 Sep 26;233(4771):1416-9</ref> If each neuron represents movement in its preferred direction, and the vector sum of all neurons is calculated (each neuron has a firing rate and a preferred direction), the sum points in the direction of motion. In this manner, the population of neurons codes the signal for the motion.{{citation needed|date=November 2013}} This particular population code is referred to as [[population vector]] coding.

Place-time population codes, termed the averaged-localized-synchronized-response (ALSR) code, have been derived for neural representation of auditory acoustic stimuli. This exploits both the place or tuning within the auditory nerve, as well as the phase-locking within each nerve fiber auditory nerve. The first ALSR representation was for steady-state vowels;<ref>{{cite journal|last1=Sachs|first1=Murray B.|last2=Young|first2=Eric D.|title=Representation of steady-state vowels in the temporal aspects of the discharge patterns of populations of auditory-nerve fibers|journal= The Journal of the Acoustical Society of America|date=November 1979|volume=66|issue=5|pages=1381–1403|doi=10.1121/1.383532|pmid=500976|bibcode=1979ASAJ...66.1381Y}}</ref> ALSR representations of pitch and formant frequencies in complex, non-steady state stimuli were later demonstrated for voiced-pitch,<ref>{{cite journal|last1=Miller|first1=M.I.|last2=Sachs|first2=M.B.|title=Representation of voice pitch in discharge patterns of auditory-nerve fibers|journal=Hearing Research|date=June 1984|volume=14|issue=3|pages=257–279|pmid=6480513|doi=10.1016/0378-5955(84)90054-6|s2cid=4704044}}</ref> and formant representations in consonant-vowel syllables.<ref>{{cite journal|last1=Miller|first1=M.I.|last2=Sachs|first2=M.B.|title=Representation of stop consonants in the discharge patterns of auditory-nerve fibrers|journal= The Journal of the Acoustical Society of America|date=1983|volume=74|issue=2|pages=502–517|doi=10.1121/1.389816|pmid=6619427|bibcode=1983ASAJ...74..502M}}</ref>