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Browsing by Subject "Mesoscale"

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    Mesoscope: A miniaturized head-mounted device for whole cortex mesoscale activity mapping in freely behaving mice
    (2020-08) Surinach, Daniel
    The advent of genetically encoded calcium indicators, along with surgical preparations such as thinned skulls or refractive index matched skulls, have enabled mesoscale cortical activity imaging in head-fixed mice. Such imaging studies have revealed complex patterns of coordinated activity across the cortical surface during spontaneous behaviors, goal-directed behavior, locomotion, motor learning and perceptual decision making in head-fixed animals. However, neural activity during free, unrestrained behavior significantly differs from that recorded in head-fixed animals. Furthermore, freely moving animals exhibit a repertoire of behaviors that is vastly increased from head-fixed animals, and therefore the ability to perform mesoscale imaging of the cortex in freely behaving mice needs further investigation. Here we present the "Mesoscope", a miniature, head-mountable imaging device compatible with transparent polymer skulls recently developed by our group. With an 8x10 mm field of view, the Mesoscope can image most of the mouse dorsal cortex with a resolution ranging from 39.37 to 55.68 micrometers. The Mesoscope weighs 3.8 g (<15% of a mouse's body weight) and incorporates a magnetic interlocking mechanism that allows quick fixation (<1 s of temporary restraint) to a 3D-printed morphologically realistic transparent polymer skull (See-Shell) implanted on an awake mouse. Open-field behavior tests indicate neither the See-Shell implantation nor the addition of Mesoscope significantly inhibits locomotion. The Mesoscope employs an array of blue LEDs to illuminate the cortical surface uniformly, with up to 31 mW of light power. An additional green LED provides illumination for reflectance imaging of intrinsic optical signals arising from changes in cortical blood volume and oxygenation following neural activity. The LEDs are powered using a custom-designed printed circuit board that allows for fast LED switching on a microsecond scale. We have used the Mesoscope to record neural activity across the dorsal cortex in mice during in a variety of different behaviors described as follows: spontaneous neural activity under light isoflurane anesthesia, sensory evoked neural responses under light isoflurane anesthesia, spontaneous neural activity during free awake behavior, spontaneous neural activity during awake social behavior, and spontaneous neural activity during sleep. The Mesoscope employs compact optical and electrical technologies that can perform mesoscale imaging across the dorsal cortex in freely moving and behaving mice and will potentially open new avenues of scientific inquiry.
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    Non-Equilibrium Two-State Switching in Mesoscale, Ferromagnetic Particles
    (2019-07) Delles, James
    There has been much theoretical study attempting to expand upon the Arrhenius law, $f=f_o exp(U/kT)$, which describes the switching rate in thermally activated, two-state systems, but few experiments to verify it. This is especially true for ferromagnetic particles. Most of the previous experiments performed attempting to study the Arrhenius law focus on the effect the Boltzmann factor, exp(U/kT), has on the switching rate since it dominates any measurement due to its exponential dependence on temperature. This has made it difficult to probe the underlying physics of the prefactor in front of the exponential. Using square, ferromagnetic particles of sizes 250 nm x 250 nm x 10 nm and 210 nm x 210 nm x 10 nm, controlling the barrier height using an applied field, and measuring the average dwell times in each individual state has allowed us to focus on these prefactors. Our measured prefactors vary by twenty five orders of magnitude, and they are smaller than those predicted by previous theories for particles of this size. They become so small as to reach unphysically short timescales. We attribute these unexpectedly small prefactors to our magnetic particles being multidomain and undergoing transitions before the particles have time to reach thermal equilibrium. We show that our particles have a higher probability of transitioning the less time they have been in a state which we attribute to the magnetization spending most of its time near the barrier allowing faster transitions.