Dynamic Encoding of Natural Luminance Sequences by LGN Bursts

In the lateral geniculate nucleus (LGN) of the thalamus, visual stimulation produces two distinct types of responses known as tonic and burst. Due to the dynamics of the T-type Ca (2+) channels involved in burst generation, the type of response evoked by a particular stimulus depends on the resting membrane potential, which is controlled by a network of modulatory connections from other brain areas. In this study, we use simulated responses to natural scene movies to describe how modulatory and stimulus-driven changes in LGN membrane potential interact to determine the luminance sequences that trigger burst responses. We find that at low resting potentials, when the T channels are de-inactivated and bursts are relatively frequent, an excitatory stimulus transient alone is sufficient to evoke a burst. However, to evoke a burst at high resting potentials, when the T channels are inactivated and bursts are relatively rare, prolonged inhibitory stimulation followed by an excitatory transient is required. We also observe evidence of these effects in vivo, where analysis of experimental recordings demonstrates that the luminance sequences that trigger bursts can vary dramatically with the overall burst percentage of the response. To characterize the functional consequences of the effects of resting potential on burst generation, we simulate LGN responses to different luminance sequences at a range of resting potentials with and without a mechanism for generating bursts. Using analysis based on signal detection theory, we show that bursts enhance detection of specific luminance sequences, ranging from the onset of excitatory sequences at low resting potentials to the offset of inhibitory sequences at high resting potentials. These results suggest a dynamic role for burst responses during visual processing that may change according to behavioral state

CORRELATION BETWEEN LEUKOCYTE TELOMERE LENGTH AND DRUG ELUTING STENT STRUT COVERAGE BY OPTICAL COHERENCE TOMOGRAPHY

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Sequence Detection at Different Resting Potentials

<p>(A) The temporal profile of the excitatory sequence and the area under the ROC curves for the IFB and IF models in the excitatory sequence detection task at different resting potentials with stimulus SNR = 1/2.</p> <p>(B) The temporal profile of the inhibitory sequence and the ROC areas for the IFB and IF models in the task involving the detection of the offset of inhibitory sequences at different resting potentials with stimulus SNR = 1/2.</p> <p>(C) The temporal profile of the biphasic sequence and the ROC areas for the IFB and IF models in the biphasic sequence detection task at different resting potentials with stimulus SNR = 1/2.</p> <p>(D) The ROC areas for the IFB and IF models in the biphasic sequence detection task at different overall firing rates with stimulus SNR = 1/2 and <i>V_T </i> = −60 mV. The mean firing rate of the models was varied by changing the gain of the filter relating stimulus intensity to membrane potential. </p

Detection of the Offset of Inhibitory Luminance Sequences

<p>(A) LGN responses to a noisy stimulus in which an inhibitory sequence randomly appeared were simulated. The stimulus was classified as <i>S</i>₀ (black) or <i>S</i>₁ (red) depending on whether or not each interval contained the excitatory transient of the sequence. A typical realization of the stimulus with SNR = 1/2 and sequence duration = 128 ms is shown (intensity averaged over all pixels in RF center). The black line indicates the actual stimulus and the gray line indicates the underlying sequence. </p> <p>(B) Voltage traces of the IFB and IF responses to the stimulus shown in (A) at two different resting potentials, <i>V_R </i> = −67 mV (top) and <i>V_R </i> = −50 mV (bottom), with <i>V_T </i> = −60 mV. The interval in the response that corresponds to condition <i>S</i>₁ is shaded (response was shifted for presentation to remove latency between stimulus and response). The spike threshold ( <i>V_Θ, </i> green), burst de-inactivation potential and threshold ( <i>V_T, </i> red), and resting potential ( <i>V_R, </i> blue) are shown. </p> <p>(C) The probability distributions of the firing rate of the IFB and IF models during the <i>S</i>₀ (black) and <i>S</i>₁ (red) stimulus conditions at <i>V_R </i> = −67 mV (top) and <i>V_R </i> = −50 mV (bottom). </p> <p>(D) ROC curves for the IFB and IF models at <i>V_R </i> = −67 mV (top) and <i>V_R </i> = −50 mV (bottom). The area under the ROC curve is indicated. </p

The Luminance Sequences That Trigger Burst Events

<p>(A) The BTAs calculated from simulated responses to a two-minute segment of natural scene movie at different resting potentials. The resting potential of the model and BP of the response corresponding to each BTA are indicated. Full spatiotemporal BTAs were calculated, and the plots show the intensity of the BTA averaged over all pixels in the RF center. Each BTA was scaled so that the integral of its absolute value was 1.</p> <p>(B) A plot of E/I ratio of the BTA versus resting potential for simulated responses to natural scene movies. E/I ratio was calculated as the ratio of the areas of the excitatory and inhibitory components of the BTA (see inset).</p> <p>(C) The BTAs calculated from the experimental responses of three LGN Y cells recorded at different times during a single experiment to natural scene movies (average of nine different two-minute segments). The BP of each response is indicated. Spatiotemporal BTAs were averaged and scaled as in (A).</p> <p>(D) A scatter plot of E/I ratio of the BTA versus BP for a sample of 27 LGN cells (11 X cells, 16 Y cells). E/I ratio was calculated as described in (B).</p> <p>(E) The normalized BTAs for three LGN Y cells. BTAs were normalized for the temporal correlations in the natural scene movies by spectral normalization (see <a href="http://www.plosbiology.org/article/info:doi/10.1371/journal.pbio.0040209#s4" target="_blank">Materials and Methods</a>). The non-normalized BTAs corresponding to each normalized BTAs are shown in gray (same BTAs as in (C)). </p> <p>(F) A scatter plot of E/I ratio of the normalized BTA versus BP for a sample of 27 LGN cells. E/I ratio was calculated as described in (B).</p