When stimuli are presented over headphones, they are typically perceived as internalized; i.e., they appear to emanate from inside the head. Sounds presented in the free-field tend to be ...externalized, i.e., perceived to be emanating from a source in the world. This phenomenon is frequently attributed to reverberation and to the spectral characteristics of the sounds: those sounds whose spectrum and reverberation matches that of free-field signals arriving at the ear canal tend to be more frequently externalized. Another factor, however, is that the virtual location of signals presented over headphones moves in perfect concert with any movements of the head, whereas the location of free-field signals moves in opposition to head movements. The effects of head movement have not been systematically disentangled from reverberation and/or spectral cues, so we measured the degree to which movements contribute to externalization.
We performed two experiments: 1) Using motion tracking and free-field loudspeaker presentation, we presented signals that moved in their spatial location to match listeners' head movements. 2) Using motion tracking and binaural room impulse responses, we presented filtered signals over headphones that appeared to remain static relative to the world. The results from experiment 1 showed that free-field signals from the front that move with the head are less likely to be externalized (23%) than those that remain fixed (63%). Experiment 2 showed that virtual signals whose position was fixed relative to the world are more likely to be externalized (65%) than those fixed relative to the head (20%), regardless of the fidelity of the individual impulse responses.
Head movements play a significant role in the externalization of sound sources. These findings imply tight integration between binaural cues and self motion cues and underscore the importance of self motion for spatial auditory perception.
The ‘Great Ordovician Biodiversification Event’ (GOBE) saw a spectacular increase in marine biodiversity at all taxonomic levels largely within the phyla established much earlier during the so-called ...‘Cambrian Explosion’. The diversification was probably the result of a combination of several geological and biological processes and the positive feedbacks resulting from them. The present paper reviews the palaeoecological dimension of the GOBE. It involved major increases in α, ß and γ biodiversity largely associated with the rise of the Paleozoic Evolutionary Fauna dominated by suspension feeders and involving a greater occupation of ecospace and more complex ecological structures in the Ecological Evolutionary Units P1 and P2. In the benthos, these include more complex food webs than those of the Cambrian, greater tiering, especially above the sediment–water interface, and the development of guild structures indicating increased competition between taxa for particular resources. The Ordovician is characterized by a profound change in reef composition, with a switch from microbial-dominated reefs in the Early and Middle Ordovician to metazoan-dominated reefs in the Late Ordovician. Increases in complexity of deep-water trace fossil assemblages began in the Early Ordovician and mark the increasing exploitation in that environment and the development of hardgrounds permitted bioerosion and encrusting strategies together with the appearance of cryptic communities.
Within the water column, the GOBE involved a significant increase in the diversity of the phytoplankton and the development of a diverse zooplankton (including planktotrophic larvae from a range of invertebrate clades). This revolution in the plankton enabled the establishment of a diverse fauna of pelagic vertebrates, molluscs and arthropods and promoted the rise of suspension feeders in the benthos. An escalation amongst predators and thus community evolution may also have been a major driver of biodiversification.
A new species of bubble-headed trilobite, Staurocephalus oliveae sp. nov., is described from 250 specimens collected from a recently discovered locality in the Sholeshook Limestone Formation (upper ...Katian, Upper Ordovician; Ashgill, Cautleyan Zone 1 equivalent) in the village of Llanddowror, Carmarthenshire. Although no fully articulated specimens were found, the material is the most abundant and well-preserved of any Ordovician Staurocephalus species known from Britain and Ireland, and enables the complicated 3D structure of its cephalon to be reconstructed. Staurocephalus oliveae sp. nov. is the earliest representative of the genus recorded in Avalonia, following its presumed migration into the area from Laurentia or Baltica. Recognition of this Cautleyan species resurrects the potential usefulness of the long established, widely distributed, Staurocephalus clavifrons as a latest Katian (= Rawtheyan) indicator. Key words: Trilobita, Phacopida, Encrinuridae, Staurocephalus, Late Ordovician, biostratigraphy, Wales, Great Britain.
The signal-to-noise ratio (SNR) benefit of hearing aid directional microphones is dependent on the angle of the listener relative to the target, something that can change drastically and dynamically ...in a typical group conversation. When a new target signal is significantly off-axis, directional microphones lead to slower target orientation, more complex movements, and more reversals. This raises the question of whether there is an optimal design for directional microphones. In principle an ideal microphone would provide the user with sufficient directionality to help with speech understanding, but not attenuate off-axis signals so strongly that orienting to new signals was difficult or impossible. We investigated the latter part of this question. In order to measure the minimal monitoring SNR for reliable orientation to off-axis signals, we measured head-orienting behaviour towards targets of varying SNRs and locations for listeners with mild to moderate bilateral symmetrical hearing loss. Listeners were required to turn and face a female talker in background noise and movements were tracked using a head-mounted crown and infrared system that recorded yaw in a ring of loudspeakers. The target appeared randomly at ± 45, 90 or 135° from the start point. The results showed that as the target SNR decreased from 0 dB to −18 dB, first movement duration and initial misorientation count increased, then fixation error, and finally reversals increased. Increasing the target angle increased movement duration at all SNRs, decreased reversals (above −12 dB target SNR), and had little to no effect on initial misorientations. These results suggest that listeners experience some difficulty orienting towards sources as the target SNR drops below −6 dB, and that if one intends to make a directional microphone that is usable in a moving conversation, then off-axis attenuation should be no more than 12 dB.
•Investigated the minimum signal-to-noise ratio (SNR) required to localize a target.•Head movement to targets at varying SNRs and locations was measured.•Orienting towards a new off-axis target became difficult below −6 dB SNR.•An ideal directional microphone should not attenuate off-axis sources by > 12 dB.
Sound sources at the same angle in front or behind a two-microphone array (e.g., bilateral hearing aids) produce the same time delay and two estimates for the direction of arrival: A front-back ...confusion. The auditory system can resolve this issue using head movements. To resolve front-back confusion for hearing-aid algorithms, head movement was measured using an inertial sensor. Successive time-delay estimates between the microphones are shifted clockwise and counterclockwise by the head movement between estimates and aggregated in two histograms. The histogram with the largest peak after multiple estimates predicted the correct hemifield for the source, eliminating the front-back confusions.
Lenses within the schizochroal eyes of phacopine trilobites are made principally of calcite, and characterization of them using light microscopy and high‐resolution electron imaging and diffraction ...has revealed an array of microstructural arrangements that suggest a common original pattern across the suborder. The low convexity lenses of Odontochile hausmanni and Dalmanites sp. contain calcite fibres termed trabeculae. The c axis of trabecular calcite lies parallel to the lens axis, and adjacent trabeculae are distinguished by small differences in their a axis orientations. Despite the common alignment, the boundaries between trabeculae cross‐cut the c axis as they fan out towards the lens base. Trabeculae are absent from the lens immediately beneath the visual surface, and instead, a radial fringe is present and is composed of micrometre‐thick sheets of calcite whose c axes are oriented at a low angle to the visual surface. High convexity lenses are more common than those of lower convexity among the species studied, and they have a much thicker radial fringe. Beneath this fringe, all of the lens calcite is oriented with its c axis parallel to the lens axis and it lacks trabeculae. We propose that both the high and low convexity lenses formed by rapid growth of calcite from a surface that migrated inwards from the cornea, and they may have had an amorphous calcium carbonate precursor. The trabeculae and radial fringes are unlikely to have had any beneficial effect on the transmission or focusing of light, but rather are the outcomes of an elegant solution to the problem of how to construct a biconvex lens from a crystalline solid.
Stable isotope ratios of whole rock carbonates and faunas from three low latitude Upper Ordovician sections demonstrate a coherent pattern of shifting subtropical and tropical water masses and ...associated climate belts. We suggest that tropical water beneath the Inter-tropical Convergence Zone (δ
18O
=
−
10‰ to −
15‰) was isotopically light reflecting high sea-surface temperatures (SST) and reduced sea-surface salinity (SSS). Subtropical water in both the northern and southern hemispheres is characterised by
δ
18O values from −
4 to −
6‰. Our interpretation is consistent with the published results from physical–chemical modeling of Ordovician seawater composition and climate models. Deriving absolute temperature and salinity values from Ordovician
δ
18O
carb data is problematic as they could reflect SST, SSS, ice volume and potentially a variable diagenetic overprint, and currently no independent constraints exist for any of these variables.
The ITCZ was positioned at ~−
10°N of the Equator during the early Katian consistent with the likely position of the region of peak sea-surface temperature (SST). During the late Katian Boda Event the ITCZ moved across the Equator to ~−
10°S. By analogy with the Pliocene this resulted in a symmetrical distribution of sea-surface currents about the Equator. This led to a net increase in heat flow towards the poles and lower latitudinal temperature gradients, potentially explaining the migration of warm water faunas to higher southern latitudes during the Boda Event. During the Hirnantian the ITCZ shifted north of the Equator, resulting in a reduced poleward heat transport. ITCZ position has been proposed as a likely forcing mechanism of Neogene glaciation and the evidence from the Ordovician suggests a discrete palaeo-ITCZ whose position was controlled by solar insolation and latitudinal temperature gradients linked to ice volume changes and Hadley Cell dynamics.
We hypothesize that the position of the ITCZ was sensitive to the growing Hirnantian ice sheet during the Late Ordovician. We propose that latitudinal movement of the ITCZ during the Late Ordovician resulted in a major atmosphere–ocean circulation re-organization. Higher
pCO
2 estimates for the Ordovician caused more extreme changes in this circulation that were the likely primary cause of biotic mass extinction. If the monsoon is considered an off-equator ITCZ then we provide direct evidence of an Ordovician monsoonal climate.
The trilobite fauna of the upper Ordovician (middle Katian) Pyle Mountain Argillite comprises a mixture of abundant mesopelagic cyclopygids and other pelagic taxa and a benthic fauna dominated by ...trilobites lacking eyes. Such faunas were widespread in deep water environments around Gondwana and terranes derived from that continent throughout Ordovician time but this is the only known record of such a fauna from North America and thus from Laurentia. It probably reflects a major sea level rise (the ‘Linearis drowning events’) as does the development of coeval cyclopygid-dominated deep water trilobite faunas in terranes that were marginal to Laurentia and are now preserved in Ireland and Scotland. The Pyle Mountain Argillite trilobite fauna occurs with a deep water Foliomena brachiopod fauna and comprises 22 species. Pelagic trilobites (mostly cyclopygids) constitute 36% of the preserved sclerites, and 45% of the fauna is the remains of trilobites lacking eyes, including one new species, Dindymene whittingtoni sp. nov. Three species of cyclopygid are present, belonging in Cyclopyge, Symphysops and Microparia (Heterocyclopyge). Cyclopygids are widely thought to have been stratified in the water column in life and thus their taxonomic diversity reflects the relative depths of the sea-beds on which their remains accumulated. A tabulation of middle and upper Katian cyclopygid-bearing faunas from several palaeoplates and terranes arranged on the basis of increasing numbers of cyclopygid genera allows an assessment of the relative depth ranges of the associated benthic taxa. The Pyle Mountain Argillite fauna lies towards the deeper end of this depth spectrum.
: The interpretation of the lenses of schizochroal trilobite eyes as aplanatic doublets by Clarkson and Levi‐Setti over 30 years ago has been widely accepted. However, the means of achieving a ...difference in refractive index across the interface between the two parts of each lens to overcome spherical aberration has remained a matter of speculation and lately it has been argued that the doublet structure itself is no more than a diagenetic artefact. Recent advances in technologies for imaging, chemical analysis and crystallographic characterization of minerals at high spatial resolutions have enabled a re‐examination of the structure of calcite lenses at an unprecedented level of detail. The lenses in the eyes of the specimen of Dalmanites sp. used in the original formulation of the aplanatic doublet hypothesis are shown to have undergone diagenetic alteration, but its products reflect original differences in mineral chemistry between the upper lens unit and lower intralensar bowl. The turbidity of the bowl and of the core within the upper part of the lens are the result of their greater microporosity and abundance of microdolomite inclusions, both of which were products of diagenetic replacement of original magnesian calcite in these areas. Such a difference in magnesium concentration in the original calcite has long been postulated as one of the ways by which the interface between these lens units could have produced an aberration‐free image and the present study provides the first direct evidence of such a chemical contrast, thus confirming the doublet hypothesis.
Previous research suggests that the endogenous synthesis of gamma-aminobutyrate (GABA), a naturally occurring inhibitory neurotransmitter, serves as a plant defense mechanism against invertebrate ...pests. Here, we tested the hypothesis that elevated GABA levels in engineered tobacco confer resistance to the northern root nematode (Meloidogyne hapla). This nematode species was chosen because of its sedentary nature and economic importance in Canada. We derived nine phenotypically normal, homozygous lines of transgenic tobacco (Nicotiana tabacum L.), which contain one or two copies of a full-length, chimeric tobacco glutamate decarboxylase (GAD) cDNA or a mutant version that lacks the autoinhibitory calmodulin-binding domain, under the control of a chimeric octopine synthase/mannopine synthase promoter. Regardless of experimental protocol, uninfected transgenic lines consistently contained higher GABA concentrations than wild-type controls. Growth chamber trials revealed that 9–12 weeks after inoculation of tobacco transplants with the northern root-knot nematode, mature plants of five lines possessed significantly fewer egg masses on the root surface when the data were expressed on both root and root fresh weight bases. Therefore, it can be concluded that constitutive transgenic expression of GAD conferred resistance against the root-knot nematode in phenotypically normal tobacco plants, probably via a GABA-based mechanism.
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