Sixty years on: the price of assembling military gas masks in 1940

BACKGROUND

Between 1940 and 1944 military gas masks with filter pads containing 20% crocidolite were assembled in a Nottingham factory.

METHODS

Records supplied by the late Professor Stephen Jones were of 1154 persons, mainly women, who had worked in the factory during this period; they included many deaths from mesothelioma. A systematic effort was therefore made to establish causes of death for the whole cohort.

RESULTS

Of 640 employees with full name and sex recorded, 567 (89%) were traced. Of these, 491 had died, including 65 from mesothelioma, though only 54 were certified as such. After exclusion of these 54, standardised mortality ratios were significantly raised for respiratory cancer (SMR 2.5) and carcinomatosis (SMR 3.2). The pattern of mortality in the remaining 514 employees without full identification was similar, but a low tracing rate (40%) did not justify their further analysis. The first death from mesothelioma was in 1963 (22 years after first exposure) and the last in 1994, whereas a further 5.0 cases would have been expected between 1996 and 2003 (p = 0.0065).

CONCLUSION

These findings in a cohort followed over 60 years after brief exposure to crocidolite confirm a high and specific risk of mesothelioma (28% peritoneal) and perhaps of lung cancer some 20-50 years later. The statistically significant absence of further mesothelioma cases during the past eight years suggests that crocidolite, though durable, is slowly removed.

Distribution, habitat and conservation status of the eastern pygmy-possum Cercartetus nanus in Victoria

We review the distribution, habitat and conservation status of the eastern pygmy-possum (Cercartetus nanus) in Victoria. Data on the habitat occurrences and rates of detection were gleaned from 133 published and unpublished fauna surveys conducted from 1968 to 2003 in Gippsland; northern Victoria; the Melbourne area and south-western region. C. nanus was reported from a broad range of vegetation communities, which predominantly included a dense mid-storey of shrubs rich in nectar-producing species such as those from the families Proteaceae and Myrtaceae. Survey effort using a range of methods was immense across surveys: 305,676 Elliott/cage trap-nights, 49,582 pitfall trap-nights, 18,331 predator remains analysed, 4424 spotlight hours, and 7346 hair-sampling devices deployed, 1005 trees stagwatched, and 5878 checks of installed nest-boxes. The surveys produced 434 records of C. nanus, with Elliott/cage trapping, pitfall trapping and analysis of predator remains responsible for the vast majority of records (93%). These data and those from the Atlas of Victorian Wildlife indicate that although C. nanus has a widespread distribution in Victoria, it is rarely observed or trapped in fauna surveys. Only 11 (8%) of the surveys we reviewed detected >10 individuals. C. nanus is likely to be sensitive to several recognised threatening processes in Victoria (e.g., feral predators, high frequency fire, feral honeybees). There is also evidence of range declines in several regions, which suggests that the species is vulnerable to extinction. Therefore, we recommend that it be nominated as a threatened species in Victoria.

Detection of the eastern pygmy-possum Cercartetus nanus (Marsupialia: Burramyidae) at Barren Grounds Nature Reserve, New South Wales

THE eastern pygmy-possum (Cercartetus nanus) has an extensive distribution, from south-eastern Queensland to south-eastern South Australia, and also into Tasmania (Strahan 1995). Despite this it is rarely detected in fauna surveys (Bowen and Goldingay 2000). This rarity in detection suggested that the species may be characterised by small and isolated populations, and therefore vulnerable to extinction. Consequently, it became listed as a 'Vulnerable' species in New South Wales (NSW) in 2001. Unless resolved, the low rate of detection of C. nanus will continue to hinder the acquisition of basic ecological information that is needed to more clearly define its conservation status and that is fundamental to the development of a recovery plan. An extensive body of survey data for NSW involving C. nanus has been reviewed by Bowen and Goldingay (2000). Among a range of survey methods aimed at detecting this species, trapping within flowering banksias and checking installed nest-boxes had the highest rates of detection. Indeed, one study in northern NSW captured 98 individuals over a 3- year period from within nest-boxes (Bladon et al. 2002). All other studies detected fewer than 15 C. nanus. It is clear that further research is required to investigate the effectiveness of a range of detection methods.

Early allergen exposure, skin prick responses, and atopic wheeze at age 5 in English children: a cohort study

BACKGROUND

For many years it has been assumed that the risk of childhood respiratory allergies is related to allergen exposures in early life. There are, however, few prospective data in support. We aimed to examine this relationship in a representative cohort of children born in Ashford, Kent (UK).

METHODS

625 children (94% of those eligible) were followed from birth to the age of 5.5 years at which time 552 underwent skin prick testing to extracts of house dust mite and cat fur allergens. Maternal reports of wheeze in the last year were collected by interview. These outcomes were related to individual domestic concentrations of Der p 1 and Fel d I allergens estimated from dust collection at the age of 8 weeks.

RESULTS

10% of children were sensitised to house dust mite or cat at age 5.5 years; 7% had atopic wheeze. No significant relationships between allergen exposure and either sensitisation or wheeze were found but, on examination, the exposure-response relationships for both allergens and for each outcome rose steeply at low levels of exposure and were attenuated at high levels of exposure. These patterns were modified by paternal atopy and by birth order.

CONCLUSIONS

There are no linear relationships between early allergen exposure and the induction of childhood respiratory allergy; rather, the risks of IgE sensitisation and asthma rise at very low levels of exposure and are attenuated thereafter. These patterns are influenced by parental atopy and birth order. These findings suggest important gene-environment interactions in the development of atopy and asthma and imply that reductions in domestic allergen exposure alone are unlikely to have a major impact in decreasing the incidence of these diseases in childhood.

A polymorphic variation in the interleukin 1A gene increases brain microglial cell activity in Alzheimer's disease

OBJECTIVE

To investigate the impact of possession of the -889 C/T polymorphism of the interleukin 1A gene (IL-1A) and the -511 C/T polymorphism of the interleukin 1B gene (IL-1B) on the extent of neuroinflammation in the brain in Alzheimer's disease (AD), as demonstrated by the degree of microglial cell activity associated with each IL-1A and IL-1B genotype.

METHOD

Microglial cell activity within the frontal cortex was determined in 68 patients with necropsy confirmed AD by image analysis as the percentage area of tissue occupied by ferritin immunostained material (microglial cell load). IL-1A, IL-1B, and apolipoprotein E (APOE) genotyping were performed by polymerase chain reaction on DNA extracted from frontal cortex or cerebellum.

RESULTS

The microglial cell load was 31% greater in patients with IL-1A T allele, 62% greater with IL-1A TT genotype, but 108% greater with IL-1A TT genotype in combination with APOE epsilon4 allele. No effects on microglial cell load occurred with polymorphisms in IL-1B, or APOE alone.

CONCLUSIONS

Polymorphisms within IL-1A influence the degree of brain microglial cell activation, especially in bearers of APOE epsilon4 allele, reinforcing the importance of neuroinflammatory processes in the pathogenesis of AD, and supporting the rationale for treating the disease with inflammation modulating drugs.

Association between chronic fatigue syndrome and the corticosteroid-binding globulin gene ALA SER224 polymorphism

Chronic fatigue syndrome (CFS) is characterized by idiopathic fatigue of greater than 6 months' duration with postexertional exacerbation and many other symptoms. A trend toward relative hypocortisolism is described in CFS. Twin and family studies indicate a substantial genetic etiologic component to CFS. Recently, severe corticosteroid-binding globulin (CBG) gene mutations have been associated with CFS in isolated kindreds. Human leukocyte elastase, an enzyme important in CBG catabolism at inflammatory sites, is reported to be elevated in CFS. We hypothesized that CBG gene polymorphisms may act as a genetic risk factor for CFS. A total of 248 patients with CFS defined by Centers for Disease Control criteria, and 248 controls were recruited. Sequencing and restriction enzyme testing of the CBG gene coding region allowed detection of severe CBG gene mutations and a common exon 3 polymorphism (c.825G-->T, Ala-Ser224). Plasma CBG levels were measured in 125 CFS patients and 198 controls by radioimmunoassay. Total and free (calculated and measured) cortisol levels were ascertained in single samples between 8-10 a.m. The age of onset (mid 30s) and gender ratio (2.2:1, female:male) of the patients were similar to those reported in U.S. epidemiologic studies. A trend toward a preponderance of serine224 homozygosity among the CFS patients was noted, compared with controls (chi2 = 5.31, P = 0.07). Immunoreactive-CBG (IR-CBG) levels were higher in Serine/Alanine (Ser/Ala) than Ala/Ala subjects and higher again in Ser/Ser subjects, this effect was strongest in controls; Ser/Ser: 46.1+/-1.8 (n = 31, P = 0.03) vs. Ser/Ala: 42.4+/-1.0 (n = 56, P = 0.05) vs. Ala/Ala: 40.8+/-1.7 microg/mL (n = 21). Despite higher CBG levels, there was a nonsignificant trend toward lower total and free plasma cortisol in serine allele positive patients, total cortisol: Ser/Ser: 13.3+/-1.4 (n = 34) vs. Ser/Ala: 14.0+/-0.7 (n = 66) vs. Ala/Ala: 15.4+/-1.0 (n = 23). Homozygosity for the serine allele of the CBG gene may predispose to CFS, perhaps due to an effect on hypothalamic-pituitary-adrenal axis function related to altered CBG-cortisol transport function or immune-cortisol interactions.

New pregnancies and loss of allergy

BACKGROUND

Inverse associations between allergic disease and sibship have been consistently described and are frequently explained by purported lower rates of early infection among children from small families. Alternative explanations include the possibility that pregnancy itself determines maternal atopic status.

OBJECTIVE

To test the hypothesis that atopy defined by skin prick test (SPT) declines with increasing numbers of pregnancies.

METHODS

At enrollment to a birth cohort, mothers were skin prick tested to three common allergens. Seven years later these women underwent a second SPT and provided information on their reproductive histories. At both visits, information on allergic disease was also sought.

RESULTS

Twenty five (15%) women who were initially atopic were no longer so at the second visit; loss of hayfever symptoms was reported by 33 (29%) women. Women with higher numbers of intervening pregnancies were more likely to 'lose' their atopy (P=0.05) and symptoms of hayfever (P=0.02); this was not true for asthma. The findings could not be accounted for by maternal age.

CONCLUSION

Successive pregnancies may in part determine a mother's atopic state. Since maternal atopy is a risk factor for childhood atopic disease, this process may affect the atopic state of successive children. These findings suggest an alternative explanation for the sibship effect in allergic disease.

Can early infection explain the sibling effect in adult atopy?

Atopy is strongly and inversely related to family size, a pattern which is plausibly assumed to reflect a protective effect of early infection. The current study tested this hypothesis by case-referent analysis of an adult cohort in the UK. The study established that atopy, defined by prick tests to common aeroallergens, was less common among those from larger families after adjustment for potentially confounding factors. In particular, a higher number of brothers appeared to offer protection. The current authors attempted to explain this distribution by examining contemporary family-doctor records of early childhood infections; and by a number of other indirect indices of early-life "hygiene". The sibling effect was unexplained by evidence of infection with either hepatitis A or Helicobacter pylori, or by counts of infections or antibiotic prescriptions in early life. There was a significant and independent negative association between the number of gastrointestinal infections before the age of 5 yrs and the odds of atopy. Dog ownership and home moving in early life also displayed potentially protective associations. Although the current study replicates the finding that atopy is inversely associated with family size this could not be explained by documentary or serological evidence of early infection. The findings support the suggestion that the "sibling effect" in atopy may not simply reflect protection by early infection.

A polymorphism in the angiotensin 1-converting enzyme gene is associated with damage to cerebral cortical white matter in Alzheimer's disease

The impact of the insertion (I)/deletion (D) (I/D) polymorphism in the angiotensin 1-converting enzyme (ACE) gene on the extent of white matter myelin loss (ML) was investigated in four regions of the cerebral cortex in an autopsy-confirmed series of 93 patients with Alzheimer's disease (AD). The possible influence of APO E epsilon4 allele acting in concert with ACE D allele was assessed. The extent of ML did not differ between D/D, I/D and I/I genotype groups when data from all four brain regions were combined. However, separate analysis showed that the frontal and temporal cortex tended to be affected more severely by ML in patients with D/D genotype compared to those with I/D and I/I genotypes. Stratification according to APO E epsilon4 allele revealed a greater overall ML in patients bearing at least one copy of ACE D allele and one APO E epsilon4 allele, especially in individuals homozygous for both. The APO E epsilon4 allele may therefore act synergistically in patients with AD (and other subjects) bearing ACE D/D genotype to increase the risk of ML, perhaps through transient ischaemic episodes consequent upon poor cardiac output associated with coronary atherosclerosis in patients with the APO E epsilon4 allele.

Genetic variation of the APOE promoter and outcome after head injury

The APOE-epsilon4 allele is associated with risk for Alzheimer's disease (AD) and poorer outcome after head injury. Several studies show that polymorphisms in the promoter that influence APOE expression also increase risk for AD. The authors' data from a study of 92 patients are consistent with a possible influence of the G-219T promoter polymorphism on outcome after head injury. The group with unfavorable outcome had a genotype frequency distribution similar to that found in AD.

A community-based survey of the koala Phascolarctos cinereus in the Lismore region of north-eastern New South Wales

A community-based survey was undertaken in the Lismore Local Government Area (LGA) of north-eastern New South Wales to provide a basis for the development of a Shire-wide koala management plan. A questionnaire and maps were distributed to identify community attitudes towards P. cinereus conservation and management, as well as to document locations of sightings. There were 1121 surveys returned from 23,751 distributed (4.7% returned) across 18,000 ratepayers (6.2% response). Respondents indicated the frequency with which P. cinereus were seen in different suburbs, whether they had young or were sick, and provided 840 map-based records. Ten percent of respondents saw P. cinereus on at least a weekly basis, highlighting the importance of this LGA for the conservation of this species. More than 80% of respondents considered that roving dogs, land clearing, road traffic and housing development were serious threats to long-term P. cinereus survival. More than 90% of respondents supported restrictions on dogs, tree-planting programs, as well as planning activities to protect P. cinereus habitat while 85% approved of protection zones to control development within P. cinereus habitat. These results if representative of the entire community suggest strong support for the development of conservation options for P. cinereus. The study also confirms the usefulness of conducting such community-wide surveys for conspicuous threatened species.

The angiotensin 1-converting enzyme insertion (I)/deletion (D) polymorphism does not influence the extent of amyloid or tau pathology in patients with sporadic Alzheimer's disease

An insertion (I)/deletion (D) polymorphism in the angiotensin 1-converting enzyme (ACE) gene has, in some studies, been associated with increased risk for Alzheimer's disease (AD), and functionally the enzyme has been implicated in the degradation of amyloid beta protein (Abeta). We have investigated the frequency of the I/D polymorphism in a clinic-based and autopsy-confirmed series of cases of AD, and investigated what impact the I/D polymorphism in ACE gene might have on the extent of Abeta and tau pathology in the frontal cortex in the autopsy-confirmed series. We found no differences in I/D allele or genotype frequencies between the clinic-based and autopsy-confirmed AD cases, or between the pooled clinic-based and autopsy-confirmed AD cases and a series of normal control subjects. Moreover, Abeta (Abeta(40) and Abeta(42)) load, tau load or extent of amyloid angiopathy did not differ between D/D, I/D and I/I genotype groups, though Abeta(42) load tended to be higher in bearers of I/I genotype (compared to D/D genotype). Neither age at onset nor duration of illness differed according to genotype. We conclude therefore that the frequency of ACE I-allele is not increased in AD and, in autopsy-confirmed AD cases, possession of the ACE I allele has no impact upon the pathology of AD, at least in terms of the amount of Abeta or tau deposited in the brain.

Contribution of APOE promoter polymorphisms to Alzheimer's disease risk

OBJECTIVE

To determine whether the effects of APOE promoter polymorphisms on AD are independent of the APOE-epsilon4 allele.

BACKGROUND

Recently, the -491 A-->T and -219 G-->T polymorphisms located in the APOE promoter have been suggested to be risk factors for AD. However, the effects of these polymorphisms have not always been reproduced in case-control studies, possibly because of the strong linkage disequilibrium existing at this locus or the characteristics of the populations studied.

METHODS

Data collection was performed from six independent samples (1,732 patients with AD and 1,926 control subjects) genotyped for APOE exon 4 and the two APOE promoter polymorphisms. The risks associated with the APOE polymorphisms for developing AD were estimated using logistic regression procedures and calculation of odds ratios with 95% CI adjusted by age, sex, and collection center. Independence of the APOE promoter polymorphisms was tested by stratification for APOE-epsilon4 and tertile design was used for age stratification.

RESULTS

The independence of the -491 AA genotype was observed in the whole sample whereas the independence of the -219 TT genotype was observed only in the oldest population.

CONCLUSION

The -491 and -219 APOE promoter polymorphisms incur risk for AD in addition to risk associated with the APOE-epsilon4 allele, with age accentuating the effect of the -219 TT genotype. Because these polymorphisms appear to influence apoE levels, these results suggest that APOE expression is an important determinant of AD pathogenesis.

Chemistry for peptide and protein PEGylation

Poly(ethylene glycol) (PEG) is a highly investigated polymer for the covalent modification of biological macromolecules and surfaces for many pharmaceutical and biotechnical applications. In the modification of biological macromolecules, peptides and proteins are of extreme importance. Reasons for PEGylation (i.e. the covalent attachment of PEG) of peptides and proteins are numerous and include shielding of antigenic and immunogenic epitopes, shielding receptor-mediated uptake by the reticuloendothelial system (RES), and preventing recognition and degradation by proteolytic enzymes. PEG conjugation also increases the apparent size of the polypeptide, thus reducing the renal filtration and altering biodistribution. An important aspect of PEGylation is the incorporation of various PEG functional groups that are used to attach the PEG to the peptide or protein. In this paper, we review PEG chemistry and methods of preparation with a particular focus on new (second-generation) PEG derivatives, reversible conjugation and PEG structures.

Are interleukin-1 gene polymorphisms risk factors or disease modifiers in AD?

Polymorphisms in the interleukin-1 genes, IL-1A and IL-1B, have been associated with AD, but not in all studies. The authors genotyped the IL-1A(-889) and IL-1B(-511) polymorphisms in large independent cohorts of 503 control individuals and 395 patients with AD, and a further 100 with brain Abeta load. No evidence was found of risk for AD with these variants, nor of an effect on age at onset. However, an impact of IL-1B(-511) on Abeta(40) load (p < 0.05) was detected.

A polymorphism within intron 11 of the tau gene is not increased in frequency in patients with sporadic Alzheimer's disease, nor does it influence the extent of tau pathology in the brain

There are numerous polymorphisms within the tau gene but these are in complete linkage disequilibrium and exist as two common extended haplotypes H1 and H2. We have investigated the frequency of these haplotypes in 83 cases of sporadic Alzheimer's disease (AD) using the +34 polymorphism in intron 11 of the tau gene as a marker of H1 and H2 haplotypes. The total amount of hyperphosphorylated tau protein (tau load), present as neurofibrillary tangles, neuropil threads or plaque neurites, was quantified in the frontal cortex of these patients and related to tau haplotype. We found no increase in H1H1 haplotype in this autopsy population of cases with AD compared to published control data. Stratification of cases for apolipoprotein E (APO E) genotype showed a slight, but not statistically significant, overrepresentation of epsilon 4 allele amongst bearers of H2 haplotype. There were no overall differences in tau load between haplotype groups though cases within each haplotype group bearing APO E epsilon 4 allele had a significantly higher tau load than those without epsilon 4 allele. Neither age at onset or duration of illness differed according to tau haplotype. We conclude that the frequency of tau gene H1 haplotype is not elevated in AD and possession of this has no impact upon the amount of tau pathology in AD.

Pegylation: a novel process for modifying pharmacokinetics

The use of liposomal carriers and the modification of therapeutic molecules through the attachment of poly(ethylene glycol) [PEG] moieties ('pegylation') are the most common approaches for enhancing the delivery of parenteral agents. Although 'classical' liposomes (i.e. phospholipid bilayer vehicles) have been effective in decreasing the clearance of encapsulated agents and in passively targeting specific tissues, they are associated with considerable limitations. Pegylation may be an effective method of delivering therapeutic proteins and modifying their pharmacokinetic properties, in turn modifying pharmacodynamics, via a mechanism dependent on altered binding properties of the native protein. Pegylation reduces renal clearance and, for some products, results in a more sustained absorption after subcutaneous administration as well as restricted distribution. These pharmacokinetic changes may result in more constant and sustained plasma concentrations, which can lead to increases in clinical effectiveness when the desired effects are concentration-dependent. Maintaining drug concentrations at or near a target concentration for an extended period of time is often clinically advantageous, and is particularly useful in antiviral therapy, since constant antiviral pressure should prevent replication and may thereby suppress the emergence of resistant variants. Additionally, PEG modification may decrease adverse effects caused by the large variations in peak-to-trough plasma drug concentrations associated with frequent administration and by the immunogenicity of unmodified proteins. Pegylated proteins may have reduced immunogenicity because PEG-induced steric hindrance can prevent immune recognition. Two PEG-modified proteins are currently approved by the US Food and Drug Administration; several others, including cytokines such as interferon-alpha (IFNalpha), growth factors and free radical scavengers, are under development. Careful assessment of various pegylated IFNalpha products suggests that pegylated molecules can be differentiated on the basis of their pharmacokinetic properties and related changes in pharmacodynamics. Because the size, geometry and attachment site of the PEG moiety play a crucial role in determining these properties, therapeutically optimised agents must be designed on a protein-by-protein basis.

Genetic fieldwork for hereditary prostate cancer studies

The success of a genetic family study depends on the recruitment of a sufficient number of unaffected family members. We present our experiences from interviews performed in two family studies, a genetic family study of prostate cancer (PC) and a medical, anthropological, qualitative study. In the genetic family study, 949 PC patients were contacted, and 29% responded. Response rates were higher (44%) among subjects contacted by health providers participating in the study, compared to only 18% of those contacted by letter. Thirty-six pedigrees were ascertained. On average, each family had 3.3 affected relatives. Average age at time of diagnosis was 61.9 years in the probands. 58% of the families reported additional cancers. Breast cancer was reported in 12 families; colon cancer was the second most reported cancer, followed by lung, stomach, and throat cancers. Beliefs about the inheritance of PC were explored with 20 participants. The parental origin of the proband's PC in each family did not significantly affect participants' beliefs about the inheritance of PC. 95% agreed that PC could be inherited from a father to a son. Participants thought that a mother (n = 12) or daughter of a patient (n = 11) could not give PC to their sons. This misperception of the inheritance of PC can result in (1) an underreporting of PC cases in a kindred, and (2) healthy men underestimating their risk of developing PC when the disease runs in the mother's family. Thus health educators and genetic counselors might consider these findings when teaching patients and their relatives about hereditary PC.

Source regions for atmospheric aerosol measured at Barrow, Alaska

Aerosol data consisting of condensation nuclei (CN) counts, black carbon (BC) mass concentration, and aerosol light scattering coefficient at the wavelength of 450 nm (SC) measured at Barrow, AK, from 1986 to 1997 have been analyzed. BC and SC show an annual cycle with the Arctic haze maxima in the winter and spring and the minima in the summer. The CN time series shows two maxima in March and August. Potential source contribution function (PSCF) that combines the aerosol data with air parcel backward trajectories was applied to identify potential source areas and the preferred pathways that give rise to the observed high aerosol concentrations at Barrow. Ten-day isentropic back trajectories arriving twice daily at 500 and 1500 m above sea level were calculated for the period from 1986 to 1997. The PSCF analyses were performed based on the 80th percentile criterion values for the 2- and 24-h averages of the measured aerosol parameters. There was a good correspondence between PSCF maps for the 2- and 24-h averages, indicating that 1-day aerosol sampling in the Arctic adequately represents the aerosol source areas. In winter, the high PSCF values for BC and SC are related to industrial source areas in Eurasia. The trajectory domain in winter and spring is larger than in summer, reflecting weaker transport in summer. No high PSCF areas for BC and SC can be observed in summer. The result is related to the poor transport into the Arctic plus the strong removal of aerosol by precipitation in summer. In contrast to the BC and SC maps, the CN plot for summer shows high PSCF areas in the North Pacific Ocean. High CN values appearto be mostly connected with the long-range transport from Eurasia in winter and spring and with the reduced sulfur compound emission from biogenic activities in the ocean in the summer. PSCF analysis was found to be effective in identifying potential aerosol source areas.

Quantitative dosing of surfaces with fluorescent molecules: characterization of fractional monolayer coverages by counting single molecules

Quantitative deposition of dye molecules onto a substrate has been achieved at very low surface concentrations, in the range of 5 x 10(-8) - 1 x 10(-6) monolayer, using the technique of controlled substrate withdrawal from solution. These small surface populations were determined with high (>96%) efficiency by single-molecule counting using an epi-illumination, fluorescence microscope with charge-coupled device detector. The fluorescence imaging resolution (3sigma) is 0.78 microm; over a uniform excitation area of 67 x 67 microm2, a large number (>7,500) of spatially resolved channels are available for counting individual molecules. At low coverages, the number density of fluorescence spots on the surface agrees with the expected surface concentration of molecules, based on the concentration of dye in solution and the solution film thickness predicted from theory. When the surface density of molecules is high enough that fluorescence spot overlap is likely to occur within the optical resolution of the instrument, the observed fewer number of spots can be corrected for overlap through a site occupation model based on Poisson statistics.

The effects of dot density and motion coherence on perceptual fading of a target in noise

A peripherally presented target embedded in dynamic texture perceptually disappears (or 'fills-in') after around 10 s of steady fixation. This phenomenon was investigated for a target containing moving dots. The effects of manipulating the coherence of the motion within the target and the density of dots across the whole screen were explored. Coherence thresholds for the detection of a target at different dot densities were recorded for comparison. Fading occurred faster as either motion coherence or dot density was reduced. Coherence thresholds for target detection were unaffected by manipulations of dot density. There appeared to be no relationship between the stimulus exposure time needed for fading and the coherence threshold for detection of a target. The results suggest that the time taken for a target to fade is not a simple function of its motion detection threshold.

Detecting binocular 3D motion in static 3D noise: no effect of viewing distance

Relative binocular disparity cannot tell us the absolute 3D shape of an object, nor the 3D trajectory of its motion, unless the visual system has independent access to how far away the object is at any moment. Indeed, as the viewing distance is changed, the same disparate retinal motions will correspond to very different real 3D trajectories. In this paper we were interested in whether binocular 3D motion detection is affected by viewing distance. A visual search task was used, in which the observer is asked to detect a target dot, moving in 3D, amidst 3D stationary distractor dots. We found that distance does not affect detection performance. Motion-in-depth is consistently harder to detect than the equivalent lateral motion, for all viewing distances. For a constant retinal motion with both lateral and motion-in-depth components, detection performance is constant despite variations in viewing distance that produce large changes in the direction of the 3D trajectory. We conclude that binocular 3D motion detection relies on retinal, not absolute, visual signals.