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: Lie theory and separation of variables. 6. The equation iUt + ∆2U = 0

authors: Boyer, C.P.

subjects: Mathematics

abstract: This paper constitutes a detailed study of the nine−parameter symmetry group of the time−dependent free particle Schrödinger equation in two space dimensions. It is shown that this equation separates in exactly 26 coordinate systems and that each system corresponds to an orbit consisting of a commuting pair of first− and second−order symmetry operators. The study yields a unified treatment of the (attractive and repulsive) harmonic oscillator, linear potential and free particle Hamiltonians in a time−dependent formalism. Use of representation theory for the symmetry group permits simple derivations of addition and expansion theorems relating various solutions of the Schrödinger equation, many of which are new.

text: Lie theory and separation of variables. 6. The equation iUt + ∆2U = 0 This paper constitutes a detailed study of the nine−parameter symmetry group of the time−dependent free particle Schrödinger equation in two space dimensions. It is shown that this equation separates in exactly 26 coordinate systems and that each system corresponds to an orbit consisting of a commuting pair of first− and second−order symmetry operators. The study yields a unified treatment of the (attractive and repulsive) harmonic oscillator, linear potential and free particle Hamiltonians in a time−dependent formalism. Use of representation theory for the symmetry group permits simple derivations of addition and expansion theorems relating various solutions of the Schrödinger equation, many of which are new.

year: 1975-03

title: Ethnicity and deliberate self-injury: A review of the literature

authors: Wilson, Cate

subjects: Maori

abstract: Deliberate self-injury is a significant social problem affecting youth in New Zealand. Rates of
hospitalisation for youth (aged 15 to 19) from deliberate self-injury approximate 225 per
100,000. It appears that the rates for Maori and women are significantly higher. From 1987 to
1993, an average of 488 Maori women per 100 000 population have been hospitalised each
year (Ministry of Health: Manatu Hauora, 1996). This paper draws upon both local and
international literature to examine factors underlying this ethnic disparity.
There is a wealth of literature examining risk factors underlying suicidal behaviour as a whole.
Deliberate self-injury is usually assumed to be an adjunct of youth suicide; prevention
strategies are conflated. This paper argues that this assumption is untenable, and in particular,
that prevention strategies designed for youth suicide are problematic in terms of deliberate selfinjury.
While prevention strategies are based upon studies that do not differentiate between
these groups, results will be compromised.

text: Ethnicity and deliberate self-injury: A review of the literature Deliberate self-injury is a significant social problem affecting youth in New Zealand. Rates of
hospitalisation for youth (aged 15 to 19) from deliberate self-injury approximate 225 per
100,000. It appears that the rates for Maori and women are significantly higher. From 1987 to
1993, an average of 488 Maori women per 100 000 population have been hospitalised each
year (Ministry of Health: Manatu Hauora, 1996). This paper draws upon both local and
international literature to examine factors underlying this ethnic disparity.
There is a wealth of literature examining risk factors underlying suicidal behaviour as a whole.
Deliberate self-injury is usually assumed to be an adjunct of youth suicide; prevention
strategies are conflated. This paper argues that this assumption is untenable, and in particular,
that prevention strategies designed for youth suicide are problematic in terms of deliberate selfinjury.
While prevention strategies are based upon studies that do not differentiate between
these groups, results will be compromised.

year: 1999