Neural network model of L4.

<p><b>(a)</b> Diagram of the recurrent model of L4 network. <b>(b)</b> Spike shape of VPM (schematic), L4E and L4I neurons. <b>(c)</b> Temporal dynamics of individual EPSPs for the different synaptic connections (T = VPM; I = L4 FS; E = L4E). The convention is that that the first letter corresponds to the post-synaptic neuron and the second letter to the presynaptic neuron. <b>(d)</b> Thalamic generating function <i>F</i>_T (<a href="http://www.ploscompbiol.org/article/info:doi/10.1371/journal.pcbi.1005576#pcbi.1005576.e003" target="_blank">Eq 1</a>). The panels on the right show the same figure in a magnified scale. For simplicity, we assume that all T neurons have the same preferred phase.</p

Summary of experimental results in the object-localization task [37].

<p><b>(a)</b> Top, schematic illustrates measurement of whisker position (azimuthal angle <i>θ</i>), instances of touch and an example trace of whisker position. Protraction corresponds to positive changes in <i>θ</i>. <b>(b)</b> Schematics of the thalamocortical circuit and relevant cell-types. <b>(c)</b> Spike rate aligned to transitions from non-whisking to whisking (adapted from panel 5e in [<a href="http://www.ploscompbiol.org/article/info:doi/10.1371/journal.pcbi.1005576#pcbi.1005576.ref037" target="_blank">37</a>]). <b>(d)</b> Average spike rate as a function of whisking amplitude. <b>(e)</b> Average population response aligned to touch (adapted from panel 5c in [<a href="http://www.ploscompbiol.org/article/info:doi/10.1371/journal.pcbi.1005576#pcbi.1005576.ref037" target="_blank">37</a>]). Data and figures corresponding to previously reported datasets [<a href="http://www.ploscompbiol.org/article/info:doi/10.1371/journal.pcbi.1005576#pcbi.1005576.ref036" target="_blank">36</a>, <a href="http://www.ploscompbiol.org/article/info:doi/10.1371/journal.pcbi.1005576#pcbi.1005576.ref037" target="_blank">37</a>].</p

Simulated light activation of halorhodopsin expressed in L4I-Hr⁺ neurons.

<p>Simulations with <i>f</i>_halo = 0.5 reveals a reduction in the whisking suppression and an enhancement of touch responses by L4E neurons. <b>(a)</b> Halorhodopsin activation in L4I-Hr⁺ causes an average increase in response of L4E during whisking and no touch, with a wide distribution of halorhodopsin—induced modifications. <b>(b)</b> Most L4I-Hr⁺ neurons reduce their activity during whisking while L4I-Hr^- neurons increase it. <b>(c)</b> Increase in the touch responses in L4E neurons during suppression of L4I-Hr⁺. <b>(d)</b> Increase in the touch responses in L4I neurons. The increase in touch responses is only seen in Hr⁺ cells. <b>(e</b>,<b>f)</b> Population PSTH of L4E (<b>e</b>) and L4I (<b>f</b>) neurons with and without L4I-Hr⁺ activity suppression. <b>(g)</b> Reduction of L4I-Hr⁺ activity diminishes the whisking suppression effect in L4E neurons. Black line: T neurons; solid grey line: L4E neurons without halorhodopsin activation; dashed grey line: L4E neurons during halorhodopsin activation. <b>(h)</b> Reduction of L4I-Hr⁺ activity diminishes the whisking response in L4I-Hr⁺ neurons. Solid red line: L4I-Hr⁺ neurons without halorhodopsin activation; dashed red line: L4I-Hr⁺ neurons during halorhodopsin activation. Dashed blue line: L4I-Hr^- neurons during halorhodopsin activation.</p

Network parameters in the reference parameter set: - synaptic delay, <i>K</i>_<i>αβ</i>—average number of presynaptic inputs, <i>g</i>_<i>αβ</i>—synaptic conductance, <i>V</i>_extr—the extremal value of the unitary synaptic membrane potential change.

<p>The corresponding experimental values for <i>K</i>_<i>αβ</i> and <i>V</i>_extr are written in the two right columns. Those values are taken from the following references: a—[<a href="http://www.ploscompbiol.org/article/info:doi/10.1371/journal.pcbi.1005576#pcbi.1005576.ref053" target="_blank">53</a>], b—[<a href="http://www.ploscompbiol.org/article/info:doi/10.1371/journal.pcbi.1005576#pcbi.1005576.ref056" target="_blank">56</a>], c—[<a href="http://www.ploscompbiol.org/article/info:doi/10.1371/journal.pcbi.1005576#pcbi.1005576.ref034" target="_blank">34</a>], d- [<a href="http://www.ploscompbiol.org/article/info:doi/10.1371/journal.pcbi.1005576#pcbi.1005576.ref028" target="_blank">28</a>], e–[<a href="http://www.ploscompbiol.org/article/info:doi/10.1371/journal.pcbi.1005576#pcbi.1005576.ref057" target="_blank">57</a>], f—[<a href="http://www.ploscompbiol.org/article/info:doi/10.1371/journal.pcbi.1005576#pcbi.1005576.ref027" target="_blank">27</a>].</p

Effect of varying thalamocortical conductances on the function of L4E neurons.

<p>Symblols and lines are as in <a href="http://www.ploscompbiol.org/article/info:doi/10.1371/journal.pcbi.1005576#pcbi.1005576.g009" target="_blank">Fig 9</a>. <b>(a)</b> Changing <i>g</i>_ET. <b>(b)</b> ν_E vs. <i>A</i>_T during whisker movements only. <b>(c)</b> R_E vs. <i>C</i>_T. <b>(d)</b> Changing <i>g</i>_IT. <b>(e-f)</b> Same as <b>b-c</b>.</p

A neural network model of L4 explains suppression of whisker movement signals in L4 excitatory neurons.

<p>The colors black, grey and red denote T, L4E and L4I neuronal populations respectively. <b>(a)</b> Example L4E and L4I membrane potential during simulated whisking (green). <b>(b)</b> The population- and time average spike rates ν_E and ν_I of the L4E and L4I neurons respectively as function of the thalamic input <i>A</i>_T in the absence of touch. L4I neurons follow linearly the thalamic input while L4E neurons increase only weakly with <i>A</i>_T beyond firing threshold. Inset, zoom in. <b>(c)</b> Membrane potential for an example neuron during whisking and touch (black dots). <b>(d)</b> Population PSTH aligned to touch onset. Inset, zoom in.</p

Effects of varying intracortical recurrent excitatory conductances <i>g</i>_EE on the function of L4E neurons.

<p><b>(a)</b> The circuit with changing g_EE emphasized in green. <b>(b)</b> ν_E vs. <i>A</i>_T during whisker movements only, for 11 values of <i>g</i>_EE from 0 (light green) to 0.4 mS/cm², that is twice the reference parameter value (dark green). Recurrent excitation <i>g</i>_EE increases ν_E while not affecting the slope of the ν_E-<i>A</i>_T curve far from spiking threshold substantially. <b>(c)</b> <i>R</i>_T, <i>R</i>_E and <i>R</i>_I as functions of <i>g</i>_EE. Other parameters: <i>A</i>_T = 14 spikes/s, <i>C</i>_T = 0.6. <b>(d)</b> PSTH aligned to touch onset for L4E and without recurrent excitation (<i>g</i>_EE = 0 mS/cm²). <b>(e)</b> Same as <b>C</b> for L4I. <b>(f-g)</b> Same as <b>c</b>-<b>d</b> for <i>g</i>_EE = 0.2 mS/cm². <b>(h-i)</b>: Same as <b>c</b>-<b>d</b> for <i>g</i>_EE = 0.35 mS/cm². Beyond <i>~g</i>_EE = 0.4 mS/cm² the network exhibits runaway excitation.</p

Touch response in function of synaptic delay, AMPA receptors’ time constant and parameters defining thalamic input.

<p>In each panel, responses to touch in L4E and L4I neurons (<i>R</i>_E and <i>R</i>_I, in grey and red respectively) are plotted, as well as the thalamic response in black. Spikes per touch were counted up to 25 ms after touch onset, and baseline computed by counting spikes 25 ms before touch is subtracted. Responses to touch are plotted as functions of <b>(a)</b> I-to-E synaptic delay , <b>(b)</b> the AMPA receptor time constant <i>t</i>_AMPA, <b>(c)</b> the thalamic response to touch, <i>C</i>_T, and <b>(d)</b> the thalamic spike rate <i>A</i>_T during whisker movements without touch.</p

Summary of effect of photoinhibition of L4 FS neurons on L4 neurons.

<p><b>(a)</b> Schematic of the Hr⁺ experiment. A subset of L4 FS neurons do not express Hr (Hr^- neurons). <b>(b)</b> Photostimulation of Hr⁺ neurons decreases the activity in L4 FS neurons and increases the activity in L4 excitatory neurons. Black: without photostimulation; orange: with photostimulation. <b>(c)</b> Response of L4 excitatory neurons during whisker movements (adapted from panel 8d in [<a href="http://www.ploscompbiol.org/article/info:doi/10.1371/journal.pcbi.1005576#pcbi.1005576.ref037" target="_blank">37</a>]). <b>(d)</b> Response to touch of L4 excitatory neurons. The black circle denotes the mean values (adapted from panel 8e in [<a href="http://www.ploscompbiol.org/article/info:doi/10.1371/journal.pcbi.1005576#pcbi.1005576.ref037" target="_blank">37</a>]).</p

Example neurons recorded in VPM during whisker-based object localization.

<p><b>(a)</b> Chronic silicon probe recordings in VPM. <b>(b</b>,<b>c)</b> Two example trials from the same neuron showing increases in activity with whisker movements without touch. Green line, whisker azimuthal angle. Black ticks, spikes. <b>(d</b>,<b>e)</b> Two example trials showing increases in spike rate after touch. <b>(f)</b> Touch rasters (black dots; sorted by last touch in a trial). The magenta line shows when the pole was moved within reach of the whiskers. The green line represents the last touch in each trial. <b>(g)</b> Whisker movement amplitude. Red rectangle, epoch of high whisker movement amplitude and high spike rate (same as in <b>h</b>). Orange rectangle, epoch of low amplitude and low spike rate (same as in <b>h</b>). <b>(h)</b> Spike raster for an example neuron in VPM (blue dots, spikes). The black arrow in <b>g-h</b> indicates onset of whisker twitching [<a href="http://www.ploscompbiol.org/article/info:doi/10.1371/journal.pcbi.1005576#pcbi.1005576.ref052" target="_blank">52</a>]. The twitching is triggered by an auditory signal generated by the brief activation of a shutter. <b>(i)</b> Peri-stimulus-time-histogram (PSTH) showing response for touch. <b>(j)</b> Spike rate as a function of whisker movement amplitude. <b>(k-o)</b> Same as <b>f-j</b> for another example VPM neuron with low baseline spike rate. <b>(p-t)</b> Same as <b>f-j</b> for another example VPM neuron that does not show modulation with whisking amplitude.</p