title: Nonlinear Effects in Three-minute Oscillations of the Solar Chromosphere. II. Measurement of Nonlinearity Parameters at Different Atmospheric Levels

authors: Chae, Jongchul

subjects: Science & Technology

abstract: Recent theoretical studies suggest that the nonlinearity of three-minute velocity oscillations at each atmospheric level can be quantified by the two independent parameters—the steepening parameter and the velocity amplitude parameter. For the first time, we measured these two parameters at different atmospheric levels by analyzing a set of spectral lines formed at different heights of sunspots ranging from the temperature minimum to the transition region. The spectral data were taken by the Fast Imaging Solar Spectrograph of the Goode Solar Telescope, and by the Interface Region Imaging Spectrograph. As a result, from the wavelet power spectra of the velocity oscillations at different heights, we clearly identified the growth of the second harmonic oscillations associated with the steepening of the velocity oscillation, indicating that higher-frequency oscillations of periods of 1.2 to 1.5 minutes originate from the nonlinearity of the three-minute oscillations in the upper chromosphere. We also found that the variation of the measured nonlinearity parameters is consistent with the theoretical expectation that the nonlinearity of the three-minute oscillations increases with height, and shock waves form in the upper chromosphere. There are, however, discrepancies as well between theory and observations, suggesting the need to improve both theory and the measurement technique.

text: Nonlinear Effects in Three-minute Oscillations of the Solar Chromosphere. II. Measurement of Nonlinearity Parameters at Different Atmospheric Levels Recent theoretical studies suggest that the nonlinearity of three-minute velocity oscillations at each atmospheric level can be quantified by the two independent parameters—the steepening parameter and the velocity amplitude parameter. For the first time, we measured these two parameters at different atmospheric levels by analyzing a set of spectral lines formed at different heights of sunspots ranging from the temperature minimum to the transition region. The spectral data were taken by the Fast Imaging Solar Spectrograph of the Goode Solar Telescope, and by the Interface Region Imaging Spectrograph. As a result, from the wavelet power spectra of the velocity oscillations at different heights, we clearly identified the growth of the second harmonic oscillations associated with the steepening of the velocity oscillation, indicating that higher-frequency oscillations of periods of 1.2 to 1.5 minutes originate from the nonlinearity of the three-minute oscillations in the upper chromosphere. We also found that the variation of the measured nonlinearity parameters is consistent with the theoretical expectation that the nonlinearity of the three-minute oscillations increases with height, and shock waves form in the upper chromosphere. There are, however, discrepancies as well between theory and observations, suggesting the need to improve both theory and the measurement technique.

year: 2018

title: A contrast-sensitive, redundancy reduction mechanism acting on MT neurons can explain global motion direction biases without the need for Bayesian priors

authors: Perrone, John A.

subjects: Bayesian observer model

abstract: Introduction: The perceived global direction of moving objects can be influenced by the contrast of the object (Weiss, Simoncelli & Adelson, Nature Neuroscience, 2002). There is currently no detailed, neural-based explanation for how this could occur. Weiss et al., proposed an ideal Bayesian observer model that included a physiologically unspecified ‘low speed’ prior. We have recently developed a new velocity code for extracting image velocity from small groups of MT neurons (Perrone & Krauzlis, VSS 2011). The code includes a stage of local spatial inhibition between MT neurons designed to reduce the amount of redundant signals passed onto the global motion integration stage (MST). The inhibition is made dependent upon the contrast of the stimulus by exploiting the fact that some MT neurons change their speed tuning (V shifts to ∼ .5V) at low contrast (Krekelberg et al., J. Neurosci., 2006). An inhibitory signal based on the difference in output from two such MT units tuned to V and 2V will be high at high contrast, but will switch off at low contrast when the input speed is V.
Methods: We tested our velocity code using high- and low-contrast rhombus stimuli that have been shown to produce contrast dependent biases in global motion estimates (Weiss, et al., 2002).
Results: The model produced contrast dependent estimates of global motion direction that match the perceptual results. Low contrast rhomboids tended to produce estimates orthogonal to the major axis of the rhombus whereas high contrast rhomboids produced estimates closer to the veridical direction. Our simulations indicate that at low contrast the spatial inhibitory mechanisms between MT neurons may be switching off causing an increase in the density of responses signalling motion orthogonal to the edges.
Conclusion: Our model provides a biologically plausible mechanism by which the Weiss et al., Bayesian prior could be implemented.

text: A contrast-sensitive, redundancy reduction mechanism acting on MT neurons can explain global motion direction biases without the need for Bayesian priors Introduction: The perceived global direction of moving objects can be influenced by the contrast of the object (Weiss, Simoncelli & Adelson, Nature Neuroscience, 2002). There is currently no detailed, neural-based explanation for how this could occur. Weiss et al., proposed an ideal Bayesian observer model that included a physiologically unspecified ‘low speed’ prior. We have recently developed a new velocity code for extracting image velocity from small groups of MT neurons (Perrone & Krauzlis, VSS 2011). The code includes a stage of local spatial inhibition between MT neurons designed to reduce the amount of redundant signals passed onto the global motion integration stage (MST). The inhibition is made dependent upon the contrast of the stimulus by exploiting the fact that some MT neurons change their speed tuning (V shifts to ∼ .5V) at low contrast (Krekelberg et al., J. Neurosci., 2006). An inhibitory signal based on the difference in output from two such MT units tuned to V and 2V will be high at high contrast, but will switch off at low contrast when the input speed is V.
Methods: We tested our velocity code using high- and low-contrast rhombus stimuli that have been shown to produce contrast dependent biases in global motion estimates (Weiss, et al., 2002).
Results: The model produced contrast dependent estimates of global motion direction that match the perceptual results. Low contrast rhomboids tended to produce estimates orthogonal to the major axis of the rhombus whereas high contrast rhomboids produced estimates closer to the veridical direction. Our simulations indicate that at low contrast the spatial inhibitory mechanisms between MT neurons may be switching off causing an increase in the density of responses signalling motion orthogonal to the edges.
Conclusion: Our model provides a biologically plausible mechanism by which the Weiss et al., Bayesian prior could be implemented.

year: 2012

title: Te Whakatōhea Maurua

authors: Maxwell, Te Kahautu

abstract: Te Whakatōhea Maurua is a Whakaeke item that was composed to commemorate the rangatira of Te Whakatōhea that lost their lives in the colonial wars of the mid nineteenth century at the hands of the Te Arawa people who were loyalists to the Crown. This composition was composed also to bury the hatchet between Te Whakatōhea and Te Arawa. This composition was performed by Ōpōtiki Mai Tawhiti at the Te Matatini World Championship Kapa Haka Competition 2013

text: Te Whakatōhea Maurua Te Whakatōhea Maurua is a Whakaeke item that was composed to commemorate the rangatira of Te Whakatōhea that lost their lives in the colonial wars of the mid nineteenth century at the hands of the Te Arawa people who were loyalists to the Crown. This composition was composed also to bury the hatchet between Te Whakatōhea and Te Arawa. This composition was performed by Ōpōtiki Mai Tawhiti at the Te Matatini World Championship Kapa Haka Competition 2013

year: 2013