Identifying Falsifiable Predictions of the Divisive Normalization Model of V1 Neurons

The divisive normalization model (DNM, Heeger, 1992) accounts successfully for a wide range of phenomena observed in single-cell physiological recordings from neurons in primary visual cortex (V1). The DNM has adjustable parameters to accommodate the diversity of V1 neurons, and is quite flexible. At the same time, in order to be falsifiable, the model must be rigid enough to rule out some possible data patterns. In this study, we discuss whether the DNM predicts any physiological result of the V1 neurons based on mathematical analysis and computational simulations. We identified some falsifiable predictions of the DNM. The main idea is that, while the parameters can vary flexibly across neurons, they must be fixed for a given individual neuron. This introduces constraints when this single neuron is probed with a judiciously chosen suite of stimuli. For example, the parameter governing the maintained discharge (base firing rate) is associated with three characteristic observable patterns: (A) the existence of inhibitory regions in the receptive fields of simple cells in V1, (B) the super-saturation effect in the contrast sensitivity curves, and (C) the narrowing/widening of the spatial-frequency tuning curves when the stimulus contrast decreases. Based on this fact, it is predicted that the simple cells can be categorized into two groups: one shows A, B, and widening (C) and the other one shows not-A, not-B, and narrowing (C). We will also discuss roles of other DNM parameters for emulating the V1 neurons in physiological experiments

Electron-spin-resonance studies of \u3csup\u3e12\u3c/sup\u3eCH\u3csub\u3e3\u3c/sub\u3eF\u3csup\u3e+\u3c/sup\u3e, \u3csup\u3e13\u3c/sup\u3eCH\u3csub\u3e3\u3c/sub\u3eF\u3csup\u3e+\u3c/sup\u3e, and \u3csup\u3e12\u3c/sup\u3eCH\u3csub\u3e2\u3c/sub\u3eDF\u3csup\u3e+\u3c/sup\u3e in neon matrices at 4 K: Comparison with theoretical calculations

Various isotopic forms of the methyl fluoride cation 12CH3F+, 13CH3F+, and 12CH2DF+ have been generated by photoionization at 16.8 eV and separately by electron bombardment at 50 eV. The first electron-€spin-€resonance (ESR) results are reported for this radical cation which was isolated in neon matrices at 4 K. The measured Atensors or nuclear hyperfine parameters were compared with the results obtained from various computational approaches. Surprising observations were the large amounts of spin density on the methyl group, especially the hydrogen atoms, and the extreme differences in the deuterated spectra compared to the nondeuterated case. The presence of a single D atom apparently acts to prevent dynamic Jahn-Teller averaging which makes the methyl hydrogens equivalent on the ESR time scale. Such a dramatic Jahn-Teller effect has been previously observed for the similar methane cations CH+ 4 and CH2D+ 2. The magnetic parameters for CH2DF+ in neon at 4 K are g X =2.0032(5), g Y =2.0106(8), and g Z =2.0120(5); for H: A X -€‰=-€‰483(1), A Y =476(1), and A Z =483(1) MHz; for D: -€-A X -€-=5.0(3), -€-A Y -€-\u3c3, and -€-A Z -€-=7.1(3) MHz; for 19F : A X =965(1), A Y =-ˆ’130(2), and A Z =-ˆ’166(1) MHz. For CH3F+, the gtensor and 19F Atensor were similar to those above but the H atoms were equivalent with values of A X =317(1), A Y =323(2), and A Z =312 MHz

Frictional Properties of Simulated Chlorite Gouge at Hydrothermal Conditions : Implications for Subduction Megathrusts

Chlorite is abundant at hypocentral depths in subduction zones and is likely to play a key role in controlling megathrust slow slip and catastrophic rupture. However, no data exist on the frictional properties of chlorite(-rich) fault rocks under the hydrothermal conditions relevant for the subduction seismogenic zone. We report results from experiments conducted under such conditions, using chlorite powders prepared from single crystal clinochlore (Mg-chlorite), as well as limited experiments using a stack of single crystal sheets. Shear experiments were carried out at effective normal stresses (σn) of 100 to 400 MPa, pore fluid pressures (Pf) of 50 to 220 MPa, and at temperatures (T) of 22 to 600 °C, using stepped displacement rates (v) from 0.3 to 100 μm/s. The gouges are characterized by a coefficient of friction (μ) of 0.2–0.3 at T ≤ 400 °C and 0.3–0.4 at 500–600 °C, while (a-b) values showed positive values for nearly all conditions tested, except at 300 °C. Microstructures of gouges sheared at T ≤ 300 °C show evidence for widespread comminution, compared with a lower porosity at 600 °C. Experiments using a stack of single crystal sheets showed μ ≤ 0.008 at low displacements (<3 mm) followed by hardening, while microstructures are suggestive of slip along (001), folding and tear of cleavage planes, and gouge production. Our results have important implications for the mechanisms controlling megathrust fault slip under greenschist facies conditions in a subduction zone and shed new light on the strain accommodation mechanisms within sheared gouges versus single crystals composed of phyllosilicates

Frictional Properties of Simulated Chlorite Gouge at Hydrothermal Conditions: Implications for Subduction Megathrusts

Chlorite is abundant at hypocentral depths in subduction zones and is likely to play a key role in controlling megathrust slow slip and catastrophic rupture. However, no data exist on the frictional properties of chlorite(-rich) fault rocks under the hydrothermal conditions relevant for the subduction seismogenic zone. We report results from experiments conducted under such conditions, using chlorite powders prepared from single crystal clinochlore (Mg-chlorite), as well as limited experiments using a stack of single crystal sheets. Shear experiments were carried out at effective normal stresses (σn) of 100 to 400 MPa, pore fluid pressures (Pf) of 50 to 220 MPa, and at temperatures (T) of 22 to 600 °C, using stepped displacement rates (v) from 0.3 to 100 μm/s. The gouges are characterized by a coefficient of friction (μ) of 0.2–0.3 at T ≤ 400 °C and 0.3–0.4 at 500–600 °C, while (a-b) values showed positive values for nearly all conditions tested, except at 300 °C. Microstructures of gouges sheared at T ≤ 300 °C show evidence for widespread comminution, compared with a lower porosity at 600 °C. Experiments using a stack of single crystal sheets showed μ ≤ 0.008 at low displacements (<3 mm) followed by hardening, while microstructures are suggestive of slip along (001), folding and tear of cleavage planes, and gouge production. Our results have important implications for the mechanisms controlling megathrust fault slip under greenschist facies conditions in a subduction zone and shed new light on the strain accommodation mechanisms within sheared gouges versus single crystals composed of phyllosilicates

Earthquakes in the mantle? Insights from rock magnetism of pseudotachylytes

Ultramafic pseudotachylytes have been regarded as earthquake fossils formed at mantle depths (i.e., >30 km). Here we show that pseudotachylytes hosted by ultramafic rocks from three localities have distinct magnetic properties. Fresh host-peridotites contain only small amounts of coarse-grained magnetite. In contrast, the ultramafic pseudotachylytes contain variable amounts of significantly finer magnetite that formed coseismically through melting. Among each locality, magnetite abundance in the pseudotachylytes ranges over several orders of magnitude (4~2000 ppm), and magnetic grain size varies considerably (from single domain to multidomain). Because the host-peridotites are compositionally similar, the pseudotachylyte magnetic properties are interpreted to primarily reflect the physical and cooling conditions prevailing during seismic slip. Further, the examination of laboratory- produced ultramafic pseudotachylytes shows that quenching does not produce superfine magnetite. We hypothesize that the magnetic properties of ultramafic pseudotachylytes are controlled by fO2 and in consequence vary systematically with depth of formation. Therefore these properties can be used to assess if the ruptures producing the earthquakes that these pseudotachylytes represent nucleated at actual mantle depths or at shallow depths during exhumation of mantle rocks. This research was originally published in Journal of Geophysical Research. Solid Earth. © 2017 Wile

Earthquakes in the Mantle? Insights From Rock Magnetism of Pseudotachylytes

Ultramafic pseudotachylytes have been regarded as earthquake fossils formed at mantle depths (i.e., >30 km). Here we show that pseudotachylytes hosted by ultramafic rocks from three localities have distinct magnetic properties. Fresh host peridotites contain only small amounts of coarse-grained magnetite. In contrast, the ultramafic pseudotachylytes contain variable amounts of significantly finer magnetite that formed coseismically through melting. Among each locality, magnetite abundance in the pseudotachylytes ranges over several orders of magnitude (4\u20132,000 ppm), and magnetic grain size varies considerably (from single domain to multidomain). Because the host peridotites are compositionally similar, the pseudotachylyte magnetic properties are interpreted to primarily reflect the physical and cooling conditions prevailing during seismic slip. Further, the examination of laboratory-produced ultramafic pseudotachylytes shows that quenching does not produce superfine magnetite. We hypothesize that the magnetic properties of ultramafic pseudotachylytes are controlled by fO2 and in consequence vary systematically with depth of formation. Therefore, these properties can be used to assess if the ruptures producing the earthquakes that these pseudotachylytes represent nucleated at actual mantle depths or at shallow depths during exhumation of mantle rocks