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N Graham

Publications and source records attributed to N Graham.

At least 19 recordsLinked to original sources
PubMed1992

Nonlinear processes in spatial-frequency channel models of perceived texture segregation: effects of sign and amount of contrast.

Observers rated the degree of segregation between two textures, each composed of the same two element types but in differing arrangements (a checkerboard arrangement in the middle region of the pattern and a striped arrangement in the top and bottom regions). The two element types in a given pattern were either both solid squares or both center-surround elements. In center-surround elements the average luminance equaled the background luminance. The two elements types were identical in size but differed in sign and/or amount of contrast. Discrepancies between the observers' ratings of perceived segregation and the predictions of simple (linear) spatial-frequency and orientation channels models of texture segregation suggested adding nonlinear processes to the model. Complex channels (a rectification-type nonlinearity between two linear-filtering stages) can explain why some patterns made of center-surround elements segregate even though there is little energy at the spatial frequencies that differentiate the two textures. Complex channels cannot, however, explain the very poor segregation of "same-sign-of-contrast" patterns (where the luminances of the two element types were both far above or both far below the background). This second result might arise from a local nonlinearity preceding the channels and might be ascribed to retinal light adaptation except that it occurs at contrasts less than or equal to 25%! Alternatively, it might arise from normalization, which may result from intracortical inhibition. Some preliminary quantitative predictions were computed from two models, one incorporating complex channels and an early local nonlinearity, the other complex channels and normalization. With suitable choices of parameters, either model could account for the results.

Contrast Sensitivity↗
PubMed1992

Quantal noise and decision rules in dynamic models of light adaptation.

To evaluate some of the consequences of including probabilistic processes (e.g. quantal noise) in a computable model of light-adaptation dynamics, we considered the behavior of a general class of models. These models contain four stages: (1) early noise; (2) a deterministic filtering and gain-changing stage; (3) late noise; (4) a decision rule that is either an ideal (signal-known-exactly) detector or a peak-trough detector. With the ideal detector and without late noise, the observer's sensitivity as a function of mean luminance and temporal frequency is not affected by the filtering and gain-changing stage. Consequently, if the early noise is entirely quantal fluctuations, sensitivity will always be a square-root function of mean luminance and a uniform (flat) function of temporal frequency. This latter prediction is contradicted by all known data; either the ideal-detector is the wrong decision rule or sensitivity is almost always limited by sources of noise other than quantal fluctuations. With the peak-trough detector, however, with or without late noise, the observer's sensitivity as a function of temporal frequency does reflect the sensitivity of the low-level filtering and gain-changing stage. Late noise is needed, however, if the observer's sensitivity as a function of mean luminance is to go through both a square-root and a Weber region. Comparing these conclusions to similar work on the spatial frequency dimension highlights differences between the spatial and temporal frequency domains. Finally, on the basis of these analyses and evidence from the literature, we question whether quantal fluctuations limit visual sensitivity under any condition.

Adaptation, Ocular↗
PubMed1992

Modeling the dynamics of light adaptation: the merging of two traditions.

Light adaptation has been studied using both aperiodic and periodic stimuli. Two well-documented phenomena are described: the background-onset effect (from an aperiodic-stimulus tradition) and high-temporal-frequency linearity (from the periodic-stimulus tradition). These phenomena have been explained within two different theoretical frameworks. Here we briefly review those frameworks. We then show that the models developed to predict the phenomenon from one tradition cannot predict the phenomenon from the other tradition, but that the models from the two traditions can be merged into a class of models that predicts both phenomena.

Adaptation, Ocular↗
PubMed1992

Non-linear processes in perceived region segregation: orientation selectivity of complex channels.

Models incorporating linear spatial-frequency- and orientation-selective channels explain many aspects of visual texture segregation. The inability of such models to fully explain texture segregation results, indicates that non-linear processes are also involved. One non-linearity that has been suggested is complex channels consisting of two stages of linear filtering separated by a rectification-type non-linearity (much like cortical complex cells). Here we further demonstrate the usefulness of complex channels in explaining texture segregation results and investigate the orientation-selectivity of the first stage of such complex channels. Our results suggest that the first stage is much more selective for orientation than are lateral geniculate nucleus cells, but that the first-stage orientation bandwidth is rather wide with some interaction occurring between perpendicular orientations.

Form Perception↗
PubMed1991

Lightness differences and the perceived segregation of regions and populations.

A striking finding reported by Beck, Sutter, and Ivry (1987) was that, in textures composed of regions differentiated by the arrangement (checks and stripes) of two texture elements (light and dark squares), a large lightness difference between the squares could fail to yield segregation between the regions, whereas a smaller lightness difference could sometimes yield strong segregation. In the experiments reported here, we compared the segregation of striped and checked arrangements of light and dark squares into regions with the segregation of two randomly interspersed populations of light and dark squares into subpopulations. Perceived lightnesses are the same for a given set of squares, whether they are arranged in regions or in intermixed populations. Perceived population segregation is approximately a single-valued function of the lightness differences of the squares, but perceived region segregation is not. The reason for the difference between population segregation and region segregation may be that region segregation is mediated by detectors' having large oriented receptive fields (large bar detectors) that are sensitive to the fundamental spatial frequency and orientation of the texture region as defined by the arrangement of the squares (Beck et al., 1987; Sutter, Beck, & Graham, 1989). These detectors cannot be responsible for population segregation, because the light and dark squares are distributed randomly throughout these patterns and therefore do not define a consistent arrangement of any particular spatial frequency or orientation. The light and dark squares in the population patterns fall equally on excitatory and inhibitory regions of large bar detectors. A plausible explanation for population segregation is to suppose that the segregation is the result of similarity grouping of the light and dark squares.

Contrast Sensitivity↗
PubMed1990

Cytomegalovirus infections in pediatric liver transplantation.

From 1986 to 1989, 26 consecutive pediatric liver transplant recipients were followed up at The Hospital for Sick Children, Toronto, Canada. The patients were reviewed to assess the incidence of infection with cytomegalovirus, the severity of disease, and the relationship of recipient and donor serostatus to cytomegalovirus disease. Overall, the incidence of infection was 54% (14/26). Over 90% of patients who were seropositive or whose donors were seropositive developed evidence of cytomegalovirus infection after transplantation. Forty-three percent (6/14) of those with cytomegalovirus infections developed severe, fatal cytomegalovirus disease, despite treatment with immunoglobulins and ganciclovir (Syntex Laboratories Inc, Palo Alto, Calif) or foscarnet sodium (Astra, Pharmaceutical Products Inc, Westborough, Mass). Of all posttransplant deaths, two thirds were associated with severe cytomegalovirus infection. Cytomegalovirus-related deaths occurred in three of four seronegative patients with seropositive donors and three of six seropositive patients with seropositive donors. Therefore, a seropositive donor appeared to be a major risk factor for severe cytomegalovirus disease.

Adolescent↗
PubMed1990

The Gospel Oak Study: prevalence rates of dementia, depression and activity limitation among elderly residents in inner London.

The steps to setting up a population register of elderly residents are described. Based upon this, 87.2% of the elderly residents of an inner-city electoral area were screened for memory disorder, depression and activity limitation using the Short CARE. Contact with medical and social agencies was also recorded; 4.7% were classed as cases by the dementia diagnostic scale, sufferers being older and not living alone. Of the residents 15.9% were classed as depressed, this state being more prevalent in those not currently married. The depressed were, in contrast to the demented residents, likely to be in recent contact with hospital and general practitioner. Thirty-two per cent of the population showed impairment in daily activity, these individuals were usually older, not married and receiving hospital care. Sleep disorder and complaint of many somatic symptoms were associated with a diagnosis of depression. In contrast, most respondents with a subjective complaint of memory disorder, which was common in this population, were neither suffering from depression nor dementia. With this accurate sampling frame and a good response rate, the prevalence rate of clinical depression must be seen as disturbingly high. The prevalence rate of dementia approximated to that of other surveys. This study also indicated that general practitioners' lists may be inaccurate and that non-responders to first approaches for interview, although similar in demographic features to those responding, may contain among them many suffering from dementia.

Activities of Daily Living↗
PubMed1989

Liver transplantation in children: the initial Toronto experience.

The Hospital for Sick Children's initial 2-year experience with pediatric liver transplantation is reviewed. Patients are divided into high- and low-risk groups according to certain criteria. The high-risk group includes patients under 10 kg in weight, those with extrahepatic biliary atresia (EHBA), those with portal vein atresia or thrombosis, and those in hepatic coma. All others were considered low risk. Twenty-nine patients were assessed for transplantation: 18 were transplanted and 6 (21% of total referred) died while on the waiting list. Eighteen patients received 23 transplants. Of the 18 recipients, nine had EHBA, four had fulminant hepatic failure, two had tyrosinemia, one had glycogen storage disease, one had Indian childhood cirrhosis, and one had idiopathic cirrhosis. Seven of the 13 patients in the high-risk group survived (55% survival) with 1 to 23 month follow-up. Survival was significantly higher (80%) in the low-risk group (P less than 0.05). Four patients were retransplanted and two survived. Early deaths occurred from prolonged warm ischemia, recurrent portal vein thrombosis, and brain death in a patient who had been transplanted in hepatic coma. Late deaths occurred from cytomegalovirus (CMV) disease (2 patients), acute rejection (1 patient), and myocardial infarction (1 patient). The incidence of primary nonfunction was 4.3% (1 of 23) and of arterial thrombosis was 13% (3 of 23). Survival in patients transplanted for EHBA (67%) was slightly higher than it was for the rest of the group, although not as good as it was in the low-risk group.(ABSTRACT TRUNCATED AT 250 WORDS)

Biliary Atresia↗
PubMed1989

Contrast and spatial variables in texture segregation: testing a simple spatial-frequency channels model.

Observers were shown patterns composed of two textures in which each texture contained two types of elements. The elements were arranged in a striped pattern in the top and bottom regions and in a checked pattern in the center region. Observers rated the degree to which the three regions were seen as distinct. When the elements were squares or lines, perceived segregation resulting from differences in element size could be canceled by differences in element contrast. Minimal perceived segregation occurred when the products of the area and the contrast (areal contrasts) of the elements were equal. This dependence of perceived segregation on the areal contrasts of the elements is consistent with a simple model based on the hypothesis that the perceived segregation of the regions is a function of their differential stimulation of spatial-frequency channels. Two aspects of the data were not consistent with quantitative predictions of the model. First, as the size difference between the large and small elements increased, the ratings at the point of minimum perceived segregation increased. Second, some effects of changing the fundamental frequency of the textures were not predicted by the model. These discrepancies may be explained by a more complex model in which a rectification or similar nonlinearity occurs between two stages of orientation- and spatial-frequency-selective linear filters.

Adult↗
PubMed1988

Psychiatric illness in elderly residents of Part III homes in one London Borough: prognosis and review.

The psychiatric morbidity among 390 (89%) residents of the 12 Part III homes managed by the London Borough of Camden has been assessed using a standard interview, for comparison with data obtained from residents in a previous survey. Fewer residents were now assessed as having no evidence of dementia or depression, the proportion with some degree of dementia having risen in the 3.6-years follow-up period. Of the original residents, 64% were no longer present by the time of the second survey; the great majority were likely to have died. Residents who had previously been assessed as severely demented or depressed were over-represented in this group. Of the residents who remained, only 17% of those previously depressed had recovered. This evidence suggests that Part III homes are continuing to amass a large number of residents suffering from dementia and depression. The need for adequate psychogeriatric services to Part III homes remains pressing.

Aged↗
PubMed1978

Heterogeneity of oxygen delivery in normoxic and hypoxic states: a fluorometer study.

An on-line, real-time histogram display of heterogeneity of oxygen delivery to perfused and in situ organs is afforded by a flying-spot fluorometer that provides excitation for either oxidized flavoprotein of the mitochondrial space or reduced pyridine nucleotide of mitochondrial and cytosolic spaces. Emission from the two fluorochromes is acquired at 10(4) to 10(5) data points/s and histograms of the fluorescence intensity versus the number of occurrences of that intensity are displayed at 1--10 times per second. The histograms show alterations of the intensity and of the degree of heterogeneity of the redox states of perfused heart with model coronary occlusion, of perfused and in situ rat liver, and of rat and gerbil models of stroke. The percentage change of oxygen delivery to the intracellular space can be calculated from the areas under the histogram.

Brain↗