38 research outputs found
Coverage, Continuity and Visual Cortical Architecture
The primary visual cortex of many mammals contains a continuous
representation of visual space, with a roughly repetitive aperiodic map of
orientation preferences superimposed. It was recently found that orientation
preference maps (OPMs) obey statistical laws which are apparently invariant
among species widely separated in eutherian evolution. Here, we examine whether
one of the most prominent models for the optimization of cortical maps, the
elastic net (EN) model, can reproduce this common design. The EN model
generates representations which optimally trade of stimulus space coverage and
map continuity. While this model has been used in numerous studies, no
analytical results about the precise layout of the predicted OPMs have been
obtained so far. We present a mathematical approach to analytically calculate
the cortical representations predicted by the EN model for the joint mapping of
stimulus position and orientation. We find that in all previously studied
regimes, predicted OPM layouts are perfectly periodic. An unbiased search
through the EN parameter space identifies a novel regime of aperiodic OPMs with
pinwheel densities lower than found in experiments. In an extreme limit,
aperiodic OPMs quantitatively resembling experimental observations emerge.
Stabilization of these layouts results from strong nonlocal interactions rather
than from a coverage-continuity-compromise. Our results demonstrate that
optimization models for stimulus representations dominated by nonlocal
suppressive interactions are in principle capable of correctly predicting the
common OPM design. They question that visual cortical feature representations
can be explained by a coverage-continuity-compromise.Comment: 100 pages, including an Appendix, 21 + 7 figure
The Osteology of the Basal Archosauromorph Tasmaniosaurus triassicus from the Lower Triassic of Tasmania, Australia
Proterosuchidae are the most taxonomically diverse archosauromorph reptiles sampled in the immediate aftermath of the Permo-Triassic mass extinction and represent the earliest radiation of Archosauriformes (archosaurs and closely related species). Proterosuchids are potentially represented by approximately 15 nominal species collected from South Africa, China, Russia, Australia and India, but the taxonomic content of the group is currently in a state of flux because of the poor anatomic and systematic information available for several of its putative members. Here, the putative proterosuchid Tasmaniosaurus triassicus from the Lower Triassic of Hobart, Tasmania (Australia),is redescribed. The holotype and currently only known specimen includes cranial and postcranial remains and the revision of this material sheds new light on the anatomy of the animal, including new data on the cranial endocast. Several bones are re-identified or reinterpreted, contrasting with the descriptions of previous authors. The new information provided here shows that Tasmaniosaurus closely resembles the South African proterosuchid Proterosuchus, but it differed in the presence of, for example, a slightly downturned premaxilla, a shorter anterior process of maxilla, and a diamond-shaped anterior end of interclavicle. Previous claims for the presence of gut contents in the holotype of Tasmaniosaurus are considered ambiguous. The description of the cranial endocast of Tasmaniosaurus provides for the first time information about the anatomy of this region in proterosuchids. The cranial endocast preserves possibly part of the vomero-nasal (= Jacobson's) system laterally to the olfactory bulbs. Previous claims of the absence of the vomero-nasal organs in archosaurs, which is suggested by the extant phylogenetic bracket, are questioned because its absence in both clades of extant archosaurs seems to be directly related with the independent acquisition of a non-ground living mode of life
Cortisol, cognition and the ageing prefrontal cortex
The structural and functional decline of the ageing human brain varies by brain
region, cognitive function and individual. The underlying biological mechanisms are
poorly understood. One potentially important mechanism is exposure to
glucocorticoids (GCs; cortisol in humans); GC production is increasingly varied with
age in humans, and chronic exposure to high levels is hypothesised to result in
cognitive decline via cerebral remodelling. However, studies of GC exposure in
humans are scarce and methodological differences confound cross-study comparison.
Furthermore, there has been little focus on the effects of GCs on the frontal lobes and
key white matter tracts in the ageing brain. This thesis therefore examines
relationships among cortisol levels, structural brain measures and cognitive
performance in 90 healthy, elderly community-dwelling males from the Lothian
Birth Cohort 1936. Salivary cortisol samples characterised diurnal (morning and
evening) and reactive profiles (before and after a cognitive test battery). Structural
variables comprised Diffusion Tensor Imaging measures of major brain tracts and a
novel manual parcellation method for the frontal lobes. The latter was based on a
systematic review of current manual methods in the context of putative function and
cytoarchitecture. Manual frontal lobe brain parcellation conferred greater spatial and
volumetric accuracy when compared to both single- and multi-atlas parcellation at
the lobar level. Cognitive ability was assessed via tests of general cognitive ability,
and neuropsychological tests thought to show differential sensitivity to the integrity
of frontal lobe sub-regions. The majority of, but not all frontal lobe test scores shared
considerable overlap with general cognitive ability, and cognitive scores correlated
most consistently with the volumes of the anterior cingulate. This is discussed in
light of the diverse connective profile of the cingulate and a need to integrate
information over more diffuse cognitive networks according to proposed de-differentiation
or compensation in ageing. Individuals with higher morning, evening
or pre-test cortisol levels showed consistently negative relationships with specific
regional volumes and tract integrity. Participants whose cortisol levels increased
between the start and end of cognitive testing showed selectively larger regional
volumes and lower tract diffusivity (correlation magnitudes <.44). The significant
relationships between cortisol levels and cognition indicated that flatter diurnal
slopes or higher pre-test levels related to poorer test performance. In contrast, higher
levels in the morning generally correlated with better scores (correlation magnitudes
<.25). Interpretation of all findings was moderated by sensitivity to type I error,
given the large number of comparisons conducted. Though there were limited
candidates for mediation analysis, cortisol-function relationships were partially
mediated by tract integrity (but not sub-regional frontal volumes) for memory and
post-error slowing. This thesis offers a novel perspective on the complex interplay
among glucocorticoids, cognition and the structure of the ageing brain. The findings
suggest some role for cortisol exposure in determining age-related decline in
complex cognition, mediated via brain structure
Temazepam triggers the release of vasopressin into the rat hypothalamic paraventricular nucleus: novel insight into benzodiazepine action on hypothalamic-pituitary-adrenocortical system activity during stress
We investigated the influence of a representative classical benzodiazepine on the regulation of the hypothalamic-pituitary-adrenocortical (HPA) axis activity both under basal conditions and stress. Adult male Wistar rats were intravenously administered with temazepam (0.5, 1, and 3 mg/kg body weight) and plasma concentrations of corticotropin (ACTH) and vasopressin (AVP) were measured in blood samples collected via chronically implanted jugular venous catheters. Simultaneously, the release of AVP within the hypothalamic paraventricular nucleus (PVN) was monitored via microdialysis. Plasma AVP levels remained unaffected by the different treatment conditions. Temazepam blunted the stressor exposure-induced secretion of ACTH in a dose-dependent manner. Concurrently, and also in a dose-dependent manner temazepam enhanced the intra-PVN release of AVP, known to originate from magnocellular neurons of the hypothalamic neurohypophyseal system. Furthermore, temazepam did not affect the in vitro secretion of ACTH from the adenohypophyseal cells. Taken together, the results of this study suggest that temazepam modulates the central nervous regulation of the HPA axis by altering intra-PVN AVP release. An increasingly released AVP of magnocellular origin seems to provide a negative tonus on ACTH secretion most probably via inhibiting the release of ACTH secretagogues from the median eminence into hypophyseal portal blood