Latent-image formation in tabular AgBr grains: simulation studies

A simulation programme based on the nucleation-and-growth model of latent-image formation was used to study how trap depth and trap density at various tabular grain thicknesses affected quantum sensitivity and reciprocity failure. Using a 1.2x0.2 µm model ‘grain’, the unsensitized case was simulated with 0.05 eV traps located on both the face and the core of the grain, the latter to simulate the effect of twin planes on latent-image location. The trap densities were adjusted to achieve a higher internal speed than surface speed, as seen experimentally with the emulsion used to validate the simulation. To simulate the effects of chemical sensitization, these parameters were held fixed while edge traps of depths 0.2–0.6 eV were added at various trap densities for grain thicknesses of 50, 100 and 200 nm. All but the lowest trap densities at 0.2 eV changed the situation to complete or almost complete edge domination for latent-image location. Maximum efficiencies for latent-image formation were six to eight absorbed photons/grain for a 0.01 s exposure, although the trap density had to be decreased as the trap depth increased to achieve these maximum values. A decrease in efficiency with decreasing thickness as well as with decreasing diameter was seen for the lower trap depth values. These grain diameter and thickness effects disappeared for simulations using a 10^–6 s exposure, indicating that the decreasing efficiency at 0.01 s was due to differences in the onset of low-irradiance reciprocity failure. Reciprocity failure was simulated for chosen trap depth/density combinations. These data were compared with experimental reciprocity failure data to help validate the model. Reasonable agreement was obtained for trap depths in the range of those deduced from the experimental phase of the project. However, uncertainties regarding other parameters that affect the position of the reciprocity failure curve with respect to exposure time must be reduced before this agreement can be considered a validation of the model (Refer to PDF file for exact formulas)

Matlab application for fitting progress curves to the Equilibrium Model

The general procedures for carrying out the necessary rate determinations required for accurate determination of the Equilibrium Model parameters, and fitting this data to the mathematical model to generate the parameters, are described in "Peterson, M.E., Daniel, R.M., Danson, M.J. & Eisenthal, R. (2007) The dependence of enzyme activity on temperature: determination and validation of parameters. Biochemical Journal, 402, 331-337". It should be borne in mind that the Equilibrium Model equation contains exponentials of exponentials – quite small deviations from ideal behaviour, or a failure to obtain true Vmax values, may lead to difficulty in obtaining reliable Equilibrium Model parameters

It's not what you play, it's how you play it: timbre affects perception of emotion in music.

Salient sensory experiences often have a strong emotional tone, but the neuropsychological relations between perceptual characteristics of sensory objects and the affective information they convey remain poorly defined. Here we addressed the relationship between sound identity and emotional information using music. In two experiments, we investigated whether perception of emotions is influenced by altering the musical instrument on which the music is played, independently of other musical features. In the first experiment, 40 novel melodies each representing one of four emotions (happiness, sadness, fear, or anger) were each recorded on four different instruments (an electronic synthesizer, a piano, a violin, and a trumpet), controlling for melody, tempo, and loudness between instruments. Healthy participants (23 young adults aged 18-30 years, 24 older adults aged 58-75 years) were asked to select which emotion they thought each musical stimulus represented in a four-alternative forced-choice task. Using a generalized linear mixed model we found a significant interaction between instrument and emotion judgement with a similar pattern in young and older adults (p < .0001 for each age group). The effect was not attributable to musical expertise. In the second experiment using the same melodies and experimental design, the interaction between timbre and perceived emotion was replicated (p < .05) in another group of young adults for novel synthetic timbres designed to incorporate timbral cues to particular emotions. Our findings show that timbre (instrument identity) independently affects the perception of emotions in music after controlling for other acoustic, cognitive, and performance factors

The cognitive organization of music knowledge: a clinical analysis

Despite much recent interest in the clinical neuroscience of music processing, the cognitive organization of music as a domain of non-verbal knowledge has been little studied. Here we addressed this issue systematically in two expert musicians with clinical diagnoses of semantic dementia and Alzheimer’s disease, in comparison with a control group of healthy expert musicians. In a series of neuropsychological experiments, we investigated associative knowledge of musical compositions (musical objects), musical emotions, musical instruments (musical sources) and music notation (musical symbols). These aspects of music knowledge were assessed in relation to musical perceptual abilities and extra-musical neuropsychological functions. The patient with semantic dementia showed relatively preserved recognition of musical compositions and musical symbols despite severely impaired recognition of musical emotions and musical instruments from sound. In contrast, the patient with Alzheimer’s disease showed impaired recognition of compositions, with somewhat better recognition of composer and musical era, and impaired comprehension of musical symbols, but normal recognition of musical emotions and musical instruments from sound. The findings suggest that music knowledge is fractionated, and superordinate musical knowledge is relatively more robust than knowledge of particular music. We propose that music constitutes a distinct domain of non-verbal knowledge but shares certain cognitive organizational features with other brain knowledge systems. Within the domain of music knowledge, dissociable cognitive mechanisms process knowledge derived from physical sources and the knowledge of abstract musical entities

Mechanisms of voice processing in dementia

Perception of nonverbal vocal information is essential in our daily lives. Patients with degenerative dementias commonly have difficulty with such aspects of vocal communication; however voice processing has seldom been studied in these diseases. This thesis comprises a series of linked studies of voice processing in canonical dementias: Alzheimer’s disease, behavioural variant frontotemporal dementia, semantic dementia and progressive nonfluent aphasia. A series of neuropsychological tests were developed to examine perceptual and semantic stages of voice processing and to assess two aspects of accent processing: comprehension of foreign accented speech and recognition of regional and foreign accents; patient performance was referenced to healthy control subjects. Neuroanatomical associations of voice processing performance were assessed using voxel based morphometry. Following a symptom-led approach, a syndrome of progressive associative phonagnosia was characterised in two detailed case studies. Following a disease-led approach, this work was extended systematically to cohorts of patients representing the target diseases and assessing voice processing in relation to other aspects of person recognition (faces and names). This work provided evidence for separable profiles of voice processing impairment in different diseases: associative deficits were particularly severe in semantic dementia, whilst perceptual deficits showed relative specificity for Alzheimer’s disease. Neuroanatomical associations were identified for voice recognition in the right temporal pole and anterior fusiform gyrus, and for voice discrimination in the right inferior parietal lobe. The final phase of this work addressed the neuropsychological and neuroanatomical basis of accent processing, as an important dimension of nonverbal vocal analysis that is not dependent on voice identity. This work provides evidence for impaired processing of accents in progressive nonfluent aphasia and Alzheimer’s with neuroanatomical associations in the anterior and superior temporal lobe. The thesis contributes new information about voice processing in the degenerative dementias and furthers our understanding of the mechanisms of human voice analysis

What can a mean-field model tell us about the dynamics of the cortex?

In this chapter we examine the dynamical behavior of a spatially homogeneous two-dimensional model of the cortex that incorporates membrane potential, synaptic flux rates and long- and short-range synaptic input, in two spatial dimensions, using parameter sets broadly realistic of humans and rats. When synaptic dynamics are included, the steady states may not be stable. The bifurcation structure for the spatially symmetric case is explored, identifying the positions of saddle–node and sub- and supercritical Hopf instabilities. We go beyond consideration of small-amplitude perturbations to look at nonlinear dynamics. Spatially-symmetric (breathing mode) limit cycles are described, as well as the response to spatially-localized impulses. When close to Hopf and saddle–node bifurcations, such impulses can cause traveling waves with similarities to the slow oscillation of slow-wave sleep. Spiral waves can also be induced. We compare model dynamics with the known behavior of the cortex during natural and anesthetic-induced sleep, commenting on the physiological significance of the limit cycles and impulse responses