Biomedical Engineering Reference
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This makes the whisker system a popular preparation for studying the operation of
cortical columns.
13.3.3
Whisker physiology
Most neurons in the whisker system can be easily activated by delivering a rapid
tap
to the whiskers - but this response is largely invariant to the steady-state position
of the whiskers. The standard experimental protocol for studying the system is to
deliver low frequency step-like deflections to each individual whisker, often using
a piezoelectric wafer. Such studies revealed that the anatomical mapping between
whiskers and cortical barrel-columns has a simple physiological counterpart [35, 43].
Deflection of the whisker anatomically associated with a given barrel-column - the
principal whisker
(PW) - typically elicits a robust response. Deflection of the sur-
rounding whiskers (SW) can often also evoke a response. Although individual neu-
rons in a given barrel-column might respond well to particular SWs, different neu-
rons prefer different SWs so that the average SW response is considerably weaker
than the PW response. With standard, extracellular recording, the average response
elicited by deflecting a neuron's PW is about 1 spike per trial - that for immedi-
ately neighbouring SWs about 0.5 spikes per trial, that for distant SWs even less [3].
Responses in barrel cortex are therefore low enough for the series expansion to be
applicable.
In fact, these data probably reflect a subpopulation of particularly active cortical
neurons. Recent studies using the whole cell patch method (where activity is not
used as a criterion for selecting cells) have reported substantial numbers of neurons
firing at rates 0.1 spikes per deflection or less under standard
in vivo
conditions (M.
Brecht, personal communication).
13.4
Coding in the whisker system
13.4.1
Introduction
Classical whisker physiological has shown that the primary somatosensory cortex
contains a topographic map for the location of a whisker on the rat's snout. Since de-
flection of a whisker evokes responses for neurons both in the topographically match-
ing barrel-column as well as in surrounding barrel-columns [3, 24], whisker location
seems to be a
population code
. Due to the relatively sparse nature of the neuronal
response and to the modular organisation of the whisker system, this seemed to us a
good opportunity for getting thorough insight into the nature of a cortical population
code.
We asked two basic questions. (1) The role of spike timing: is whisker location
coded simply by the number of spikes that occur over 100s of milliseconds (spike
count coding), or, is the millisecond-precision spike timing crucial (spike time cod-
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