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"Libby, Alexandra"
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Venice in the age of Canaletto
\"Canaletto's landscapes are arguably, even today, the most familiar artistic products of eighteenth-century Venice For those with the requisite means, no visit to the city would be considered complete without the purchase of at least one of the artist's views, which captured the city's topography and urban activity with apparent verisimilitude. ... View painting, not a favored genre during the preceding several centuries, gained considerable popularity in the 1700s. Its ascendency corresponded directly to increased foreign travel and in particular to the aristocratic Englishmen who, having embarked on the Grand Tour - an itinerary which necessarily included Venice - sought mementos of their travels. ... However, for all Canaletto's popularity and his ability to capture the fabric of Venice at its most appealing and evocative, his work is curiously devoid of the rich coloring, sensuality, and exuberance of most Venetian art of the period. To help elucidate the complicated forces that shaped Canaletto and the city of Venice during his age, this catalogue offers a range of essays.\"--preface.
BOOK
Rotational dynamics reduce interference between sensory and memory representations
Cognition depends on integrating sensory percepts with the memory of recent stimuli. However, the distributed nature of neural coding can lead to interference between sensory and memory representations. Here, we show that the brain mitigates such interference by rotating sensory representations into orthogonal memory representations over time. To study how sensory inputs and memories are represented, we recorded neurons from the auditory cortex of mice as they implicitly learned sequences of sounds. We found that the neural population represented sensory inputs and the memory of recent stimuli in two orthogonal dimensions. The transformation of sensory information into a memory was facilitated by a combination of ‘stable’ neurons, which maintained their selectivity over time, and ‘switching’ neurons, which inverted their selectivity over time. Together, these neural responses rotated the population representation, transforming sensory inputs into memory. Theoretical modeling showed that this rotational dynamic is an efficient mechanism for generating orthogonal representations, thereby protecting memories from sensory interference.
During implicit learning, the authors find that sensory representations in mouse auditory cortex evolve over time, rotating into orthogonal memory representations. This allows short-term memories to avoid interference from new sensory inputs.
Journal Article
Little beasts : art, wonder, and the natural world
Art played a pivotal role in the development of natural history during the sixteenth and seventeenth centuries. European colonial expansion enabled naturalists to study previously unknown insects, animals, and other beestjes - 'little beasts' - from around the globe. 'Little Beasts' explores how artists such as Joris Hoefnagel and Jan van Kessel helped deepen and spread knowledge of these creatures with highly detailed and playful works that inspired generations of printmakers, painters, decorative artists, and naturalists.
BOOK
Multiplexed subspaces route neural activity across brain-wide networks
Cognition is flexible, allowing behavior to change on a moment-by-moment basis. Such flexibility relies on the brain’s ability to route information through different networks of brain regions to perform different cognitive computations. However, the mechanisms that determine which network of regions is active are not well understood. Here, we combined cortex-wide calcium imaging with high-density electrophysiological recordings in eight cortical and subcortical regions of mice to understand the interactions between regions. We found different dimensions within the population activity of each region were functionally connected with different cortex-wide ‘subspace networks’ of regions. These subspace networks were multiplexed; each region was functionally connected with multiple independent, yet overlapping, subspace networks. The subspace network that was active changed from moment-to-moment. These changes were associated with changes in the geometric relationship between the neural response within a region and the subspace dimensions: when neural responses were aligned with (i.e., projected along) a subspace dimension, neural activity was increased in the associated regions. Together, our results suggest that changing the geometry of neural representations within a brain region may allow the brain to flexibly engage different brain-wide networks, thereby supporting cognitive flexibility.
How the brain flexibly engages different networks of regions to perform different cognitive processes remains unknown. Here, the authors show changing the geometry of a neural representation to align with different subspaces within a brain region can engage different brainwide networks.
Journal Article
Innovation and Identity in Cornelis Gijsbrechts' \A Hanging Wall Pouch\
For decades it was believed that the life and career of the Flemish trompe l'œil master and court painter to the kings of Denmark, Cornelius Norbertus Gijsbrechts ended in 1675. Having left the Danish court around 1672 he is said to have made his way to Stockholm and then Breslau (present day Wrocław, Poland), where he then vanished into obscurity. However, recently transcribed inscriptions on A Hanging Wall Pouch at the National Gallery of Art, Washington, tell an entirely different story. According to trompe l'œil documents tucked throughout the illusionistic pouch, Gijsbrechts executed the work in 1677 and he did so in the service of the second prince of Auersperg and duke of Silesia-Münsterberg, Johann Ferdinand von Auersperg. This new information not only extends Gijsbrechts' life two years longer than previously believed (and makes A Hanging Wall Pouch his last known work), but it also revises the assessment of his late career and offers insights into two of the work's most striking features: the painting is cut in the shape of the objects it represents and also has collaged to it a real mirror – techniques Gijsbrechts only ever used while working for the kings of Denmark. Situated in relation to Gijsbrechts' Danish works, A Hanging Wall Pouch sheds light on the nature of Gijsbrechts' production, the intimate relationship that existed between painter and patron, and the ways in which Gijsbrechts selected certain kinds of illusionism to craft his own artistic identity.
Journal Article
Geometric and Dynamic Neural Codes of Sensory, Memory and Prediction
Our brains use memories and predictions to improve how we process information and make decisions. When we interact with our environment, we are receiving sensory inputs, storing memories, making predictions and choosing behaviors. Thus, the brain needs mechanisms to represent, organize and combine all these forms of information. This thesis focuses on the neural dynamics that represent and combine sensory and memory information, which are essential components of the sequential and predictive neural codes that support behavior. In Chapter 2, we use data from mouse auditory cortex to show how neural population codes support sequential sensory processing, by facilitating predictive coding, while also storing short-term memories. We show that specific geometries in the population code can align sensory representations to make predictions, while others separate sensory and memory representations to avoid interference. Using experimental evidence and computational modeling, we show that the brain’s uses an efficient, rotational mechanism to orthogonalize the sensory and memory representations, which keeps them separate and avoids overwriting. In Chapter 3, we study how the brain controls the neural dynamics of sensory, memory and sequential representations across time. With an inhibition stabilized firing rate model, we show that oscillations allow the brain to sample among ‘dynamical regimes’ or to switch across externally sensory driven dynamics and internally memory generated dynamics, such as predictions or sequences. Additionally, we show that coherent oscillations can couple dynamics across networks. For example, two networks that show pattern completion and associative dynamics when alone, produce sequential dynamics when coupled. Taken together these projects illustrate two mechanisms that control neural dynamics. First, the geometry of neural population responses can align or separate information. Second, oscillations in inhibition can combine multiple dynamics to facilitate behaviorally relevant computations.
Dissertation
Comparing Vermeer’s painting techniques in Woman Holding a Balance and A Lady Writing using chemical imaging spectroscopy
Chemical imaging of Johannes Vermeer’s
Woman Holding a Balance
and
A Lady Writing
has augmented understanding of Vermeer’s painting process. When combined with previous magnified examination of the paint surface and limited sample analysis, reflectance and X-ray fluorescence imaging spectroscopies allowed for visualizing the texture of Vermeer’s underpaint, an early stage that approximated the colors of the final image. The presence of brushmarked underpaint suggests Vermeer worked quickly during this initial painting stage—a shift from previous scholarly understanding, based on surface appearance, that Vermeer worked slowly and smoothly throughout his painting process. This paper also presents pigment maps for surface paint and underpaint in the flesh tones of the women’s faces, their jackets, and the tablecloths. The results address how Vermeer achieved subtle color effects by adding or layering specific pigments. Finally, this paper further characterizes the painted sketch and discusses the challenges in imaging this sub-surface design layer.
Journal Article
Acupuncture-Evoked Response in Somatosensory and Prefrontal Cortices Predicts Immediate Pain Reduction in Carpal Tunnel Syndrome
The linkage between brain response to acupuncture and subsequent analgesia remains poorly understood. Our aim was to evaluate this linkage in chronic pain patients with carpal tunnel syndrome (CTS). Brain response to electroacupuncture (EA) was evaluated with functional MRI. Subjects were randomized to 3 groups: (1) EA applied at local acupoints on the affected wrist (PC-7 to TW-5), (2) EA at distal acupoints (contralateral ankle, SP-6 to LV-4), and (3) sham EA at nonacupoint locations on the affected wrist. Symptom ratings were evaluated prior to and following the scan. Subjects in the local and distal groups reported reduced pain. Verum EA produced greater reduction of paresthesia compared to sham. Compared to sham EA, local EA produced greater activation in insula and S2 and greater deactivation in ipsilateral S1, while distal EA produced greater activation in S2 and deactivation in posterior cingulate cortex. Brain response to distal EA in prefrontal cortex (PFC) and brain response to verum EA in S1, SMA, and PFC were correlated with pain reduction following stimulation. Thus, while greater activation to verum acupuncture in these regions may predict subsequent analgesia, PFC activation may specifically mediate reduced pain when stimulating distal acupoints.
Journal Article
Frequency-Dependent Relationship Between Resting-State Functional Magnetic Resonance Imaging Signal Power and Head Motion Is Localized Within Distributed Association Networks
Recent studies have highlighted the importance of analyzing spectral power in resting-state functional magnetic resonance imaging (rs-fMRI) data. Significant modulation of power has been ascribed to the performance of cognitive tasks and has been ascribed clinical significance. However, the role of confounding factors such as head motion on spectral power is not fully understood. Specifically, the spatial distribution of frequency-dependent associations between rs-fMRI power and motion is unknown. We utilized a large rs-fMRI dataset (n=1000) to quantify the influence of head motion on spectral power in different frequency bands. We (1) performed regression analyses across the entire sample and (2) computed difference maps between high- and low-motion groups, more consistent with common experimental designs, and both analyses gave similar results. Greater head motion led to reduced spectral power at lower frequencies (0.007–0.05 Hz), but increased power at higher frequencies (0.12–0.167 Hz). Importantly, our whole-brain voxel-wise analysis showed that brain areas in distributed association networks (e.g., default mode and frontoparietal control networks) were most susceptible to head motion. These results were consistent with or without global signal regression (GSR). Additionally, without GSR, we noted a positive correlation with low-frequency power in the pre- and postcentral gyrus (S1/M1), mid-cingulate cortex, and insula and a negative correlation with mid-frequency (0.05–0.12 Hz) power in S1/M1, visual, and lateral temporal cortices. Hence, head motion significantly affects rs-fMRI power and great care must be taken when assigning a diagnostic marker for clinical populations known to present with greater head motion.
Journal Article