2011 Postgraduate Research Competition - UNSW Science - The ...

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Using hole transitions in highly charged ions as a
test for varying fundamental constants
Andrew Ong, Victor Flambaum and Julian Berengut
School of Physics
Abstract
The ongoing search for a cosmological variation of fundamental constants has prompted
independent laboratory corroboration of what might be one of the most counter-intuitive
theories yet - that the fundamental constants that govern our universe actually vary, itself an
important and necessary for grand unifying theories of nature to be formulated.
Cosmological searches have focused on the spatial variation of these constants, therefore if
such is true then since our solar system is moving through a region of changing constants, in
time we should be able to detect this variation using clocks in laboratories as well. Atomic
clocks are the most precise clocks available, and we have discovered that using hole
transitions in highly charged ions as a basis for atomic clocks, combined with specific level
crossings, a factor of 13 improvement in sensitivity can be yielded over the best current
experimental results. We believe that this level of precision will most likely be sufficient in
detecting a spatial variation of fundamental constants in a laboratory conclusively.
Abstract
Adaptation improves neural coding efficiency
despite increasing correlations in variability
Mehdi Adibi and Ehsan Arabzadeh
School of Psychology
Prolonged exposure of cortical neurons to sensory stimuli results in a change in their response
function. This phenomenon, known as sensory adaptation, is a common feature across
sensory modalities. Here, we characterise adaptation in rat whisker system. This is an ideal
model system because of its functional efficiency and its well-studied anatomical
organisation. Every whisker is represented by a cluster of neurons in S1, called a barrel. Barrels
preserve the arrangement of whiskers on the snout. Using an electrode array, we
extracellularly recorded the activity of barrel cortex neurons in anaesthetised rats and
characterised the response of neurons in 3 states of adaptation. The stimuli comprised of
vibrations ranging 0-33µm while the adapters had magnitudes of 0, 6µm or 12µm. The
neuronal responses, as a function of test stimulus amplitude, were well-fit by a sigmoid curve.
Simultaneous recordings from multiple neurons (5-25 units) in one session allowed us to
quantify the trial-by-trial correlations across neurons (in the form of signal- and noisecorrelations).
The noise correlation was in the direction of the signal correlation which led to a
decreased discriminability of the population by as much as 14% compared to an
independent population. The adapters produced a systematic lateral shift in the population
response function. The magnitude of the rightward shift was proportional to the amplitude of
the adapter. Receiver operating characteristics (ROC) and information theoretic analyses
revealed that adaptation enhances the discrimination performances most prominently at
amplitudes greater than the adapter.
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