title: Light attenuation characteristics of glacially-fed lakes

authors: Rose, Kevin

abstract: Transparency is a fundamental characteristic of aquatic ecosystems and is highly responsive to changes in climate and land use. The transparency of glacially-fed lakes may be a particularly sensitive sentinel characteristic of these changes. However, little is known about the relative contributions of glacial flour versus other factors affecting light attenuation in these lakes. We sampled 18 glacially-fed lakes in Chile, New Zealand, and the U.S. and Canadian Rocky Mountains to characterize how dissolved absorption, algal biomass (approximated by chlorophyll a), water, and glacial flour contributed to attenuation of ultraviolet radiation (UVR) and photosynthetically active radiation (PAR, 400–700 nm). Variation in attenuation across lakes was related to turbidity, which we used as a proxy for the concentration of glacial flour. Turbidity-specific diffuse attenuation coefficients increased with decreasing wavelength and distance from glaciers. Regional differences in turbidity-specific diffuse attenuation coefficients were observed in short UVR wavelengths (305 and 320 nm) but not at longer UVR wavelengths (380 nm) or PAR. Dissolved absorption coefficients, which are closely correlated with diffuse attenuation coefficients in most non-glacially-fed lakes, represented only about one quarter of diffuse attenuation coefficients in study lakes here, whereas glacial flour contributed about two thirds across UVR and PAR. Understanding the optical characteristics of substances that regulate light attenuation in glacially-fed lakes will help elucidate the signals that these systems provide of broader environmental changes and forecast the effects of climate change on these aquatic ecosystems.

text: Light attenuation characteristics of glacially-fed lakes Transparency is a fundamental characteristic of aquatic ecosystems and is highly responsive to changes in climate and land use. The transparency of glacially-fed lakes may be a particularly sensitive sentinel characteristic of these changes. However, little is known about the relative contributions of glacial flour versus other factors affecting light attenuation in these lakes. We sampled 18 glacially-fed lakes in Chile, New Zealand, and the U.S. and Canadian Rocky Mountains to characterize how dissolved absorption, algal biomass (approximated by chlorophyll a), water, and glacial flour contributed to attenuation of ultraviolet radiation (UVR) and photosynthetically active radiation (PAR, 400–700 nm). Variation in attenuation across lakes was related to turbidity, which we used as a proxy for the concentration of glacial flour. Turbidity-specific diffuse attenuation coefficients increased with decreasing wavelength and distance from glaciers. Regional differences in turbidity-specific diffuse attenuation coefficients were observed in short UVR wavelengths (305 and 320 nm) but not at longer UVR wavelengths (380 nm) or PAR. Dissolved absorption coefficients, which are closely correlated with diffuse attenuation coefficients in most non-glacially-fed lakes, represented only about one quarter of diffuse attenuation coefficients in study lakes here, whereas glacial flour contributed about two thirds across UVR and PAR. Understanding the optical characteristics of substances that regulate light attenuation in glacially-fed lakes will help elucidate the signals that these systems provide of broader environmental changes and forecast the effects of climate change on these aquatic ecosystems.

year: 2014

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: Street-level bureaucracy and occupational therapy practices in Aotearoa New Zealand. A genealogical critique.

authors: Silcock, Elizabeth Mary

subjects: power

abstract: As a human science that is also part of administering and supporting bureaucracy, occupational therapy functions in multi-layered networks of power  that support many different social structures. Occupational therapists’ work often involves operationalising health, education and social services at the street-level, positioning them between the aspirations of both policy and social activism and material life outcomes. Through a genealogical critique, this thesis problematises the practice of occupational therapy through a Foucauldian lens that views everything as dangerous. By viewing occupational therapy practice as dangerous, the power relations at the street-level of the mundane and taken-for-granted have been problematised as part of much larger social structures of power and discourse. 
Several Foucauldian tools were deployed in order to ensure that a focus on occupational therapy practices was maintained; three distinct research methods were utilised to access a range of heterogeneous genealogical information.  Firstly, an historical archival analysis of the four iterations of the Competencies for Registration for occupational therapists document (1990 to the current day) produced by the regulatory authority, the Occupational Therapy Board of New Zealand, was carried out.  Secondly, practice observations of occupational therapists working in different practice settings were analysed as three individual case studies. Concurrently with these two methods, a critical action method using a research blog was developed to create a medium for public discussion of practice through an open engagement with the profession. The combination of these three methods enabled access to a range of elements for analysis.  Many of these elements inter-linked, overlapped and connected to historically contingent networks of dominant power that have formed through the processes of colonisation, the establishment of the health system and normalised medical practices, and economic forms of governance. The nuances of how occupational therapy is practised at the street-level offered opportunities to contest the dominance of these networks. In all three case studies there was an unspoken understanding and a normalisation of materiality as an accepted and significant element of how the profession practised.  The material regimes of knowledge and material practices in these unspoken norms were highly productive and able to contest other lines of power.  Along with this alternate network of power, the most recent version of the Competencies for Registration offers a further opportunity for occupational therapists to contest dominant power relations in a way that could capitalise on the secure position the profession has developed through its material network of power.  Opportunities for alternate pathways of practice which privilege Te Ao Māori and Te Tiriti o Waitangi are a possibility in the legally sanctioned Competencies. The close connection of occupational therapy practices to real life material outcomes for other people, position the street-level doing of occupational therapists at a juncture where action and decision making can re-configure power relations. Re-thinking practice in terms of what the doing of occupational therapy does in relation to networks of power, is an opportunity for practitioners to enact policy at the street-level in ways that use their ability to influence power from the bottom up whilst being alert to the dangers presented by doing so.

text: Street-level bureaucracy and occupational therapy practices in Aotearoa New Zealand. A genealogical critique. As a human science that is also part of administering and supporting bureaucracy, occupational therapy functions in multi-layered networks of power  that support many different social structures. Occupational therapists’ work often involves operationalising health, education and social services at the street-level, positioning them between the aspirations of both policy and social activism and material life outcomes. Through a genealogical critique, this thesis problematises the practice of occupational therapy through a Foucauldian lens that views everything as dangerous. By viewing occupational therapy practice as dangerous, the power relations at the street-level of the mundane and taken-for-granted have been problematised as part of much larger social structures of power and discourse. 
Several Foucauldian tools were deployed in order to ensure that a focus on occupational therapy practices was maintained; three distinct research methods were utilised to access a range of heterogeneous genealogical information.  Firstly, an historical archival analysis of the four iterations of the Competencies for Registration for occupational therapists document (1990 to the current day) produced by the regulatory authority, the Occupational Therapy Board of New Zealand, was carried out.  Secondly, practice observations of occupational therapists working in different practice settings were analysed as three individual case studies. Concurrently with these two methods, a critical action method using a research blog was developed to create a medium for public discussion of practice through an open engagement with the profession. The combination of these three methods enabled access to a range of elements for analysis.  Many of these elements inter-linked, overlapped and connected to historically contingent networks of dominant power that have formed through the processes of colonisation, the establishment of the health system and normalised medical practices, and economic forms of governance. The nuances of how occupational therapy is practised at the street-level offered opportunities to contest the dominance of these networks. In all three case studies there was an unspoken understanding and a normalisation of materiality as an accepted and significant element of how the profession practised.  The material regimes of knowledge and material practices in these unspoken norms were highly productive and able to contest other lines of power.  Along with this alternate network of power, the most recent version of the Competencies for Registration offers a further opportunity for occupational therapists to contest dominant power relations in a way that could capitalise on the secure position the profession has developed through its material network of power.  Opportunities for alternate pathways of practice which privilege Te Ao Māori and Te Tiriti o Waitangi are a possibility in the legally sanctioned Competencies. The close connection of occupational therapy practices to real life material outcomes for other people, position the street-level doing of occupational therapists at a juncture where action and decision making can re-configure power relations. Re-thinking practice in terms of what the doing of occupational therapy does in relation to networks of power, is an opportunity for practitioners to enact policy at the street-level in ways that use their ability to influence power from the bottom up whilst being alert to the dangers presented by doing so.

year: 2018