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Biomedical subjects

F Wilkinson

Publications and source records attributed to F Wilkinson.

At least 19 recordsLinked to original sources
PubMed1992

Distractor ratio and grouping processes in visual conjunction search.

According to feature integration theory, conjunction search is conducted via a serial self-terminating search. However, effects attributed to search processes operating on the entire display may actually reflect search restricted to elements defined by a single feature. In experiment 1 this question is addressed in a reaction-time (RT) paradigm by varying distractor ratios within an array of fixed size. For trials in which the target was present in the array, RT functions were roughly symmetric, the shortest RTs being for extreme distractor ratios, and the longest RTs being for arrays in which there were an equal number of each distractor type. This result is superficially consistent with Zohary and Hochstein's interpretation that subjects search for only one distractor type and are able to switch search strategy from trial to trial. However, negative-trial data from experiment 1 case doubt on this interpretation. In experiment 2 the possible role of 'pop out' and of distractor grouping in visual conjunction search is investigated. Results of experiment 2 suggest that grouping may play a more important role than does distractor ratio, and point to the importance of the spatial layout of the target and of the distractor elements in visual conjunction search. Results of experiment 2 also provide clear evidence that groups of spatially adjacent homogeneous elements may be processed as a unit.

Attention↗
PubMed1991

The role of the corpus callosum and extra striate visual areas in stereoacuity in macaque monkeys.

Stereoacuity was measured in six normal male rhesus monkeys by requiring them to indicate the closer of two vertical line targets. Their stereoacuity ranged from 13-23 arc sec and was as good as that of human observers tested in the same way. Sectioning the splenium of the corpus callosum, either before or after removing the foveal representation in visual area 2, had no effect on stereoacuity thresholds, indicating that the splenium is not essential for the detection of small retinal disparities at the visual mid-line. In contrast, removal of the foveal representation in V2 permanently and markedly elevated stereoacuity thresholds, almost to the level observed in two monkeys after removal of the representation in striate cortex of the central 5 degrees of the retina. Posterior infero-temporal ablation also impaired stereoacuity, though less strikingly. In these two monkeys, the effect of subsequent damage to the rostral superior colliculi was examined. One animal was unimpaired. In the other, in which the pre-tectum was damaged, the stereoacuity threshold was substantially raised, most plausibly as a consequence of imperfect binocular fixation.

Animals↗
PubMed1990

The development of vernier acuity in the cat.

The development of vernier acuity was assessed in nine kittens using offsets in a series of square-wave gratings as stimulus targets. The kittens were tested on a jumping stand in a two-choice simultaneous discrimination paradigm, and daily thresholds were tracked with a modified staircase procedure. Vernier acuity improved gradually from initial values of 13-57' arc and reached asymptote at 1.5-5.4' arc by 70-90 days of age. The developmental function for vernier acuity is discussed in relation to physiologic development of the kitten visual system and is related to published data on the development of stereoacuity and spatial resolution in the same species.

Animals↗
PubMed1986

Vernier acuity in the cat: its relation to hyperacuity.

Vernier thresholds were measured behaviourally in five cats using offsets in gratings of two spatial frequencies and in single lines. Thresholds ranged from 2.2 to 6.7', and no threshold differences were found across stimuli. These results are discussed in relationships with published spatial resolution data and are related to the optical and neural characteristics of the cat's visual system. It is concluded that if vernier acuity is a hyperacuity in cats, the improvement in grain is not of the same magnitude as it is in humans.

Animals↗
PubMed1986

Some effects of early monocular deprivation in the Mongolian gerbil.

The effects of short- and long-term early monocular deprivation were investigated in 4 Mongolian gerbils using interocular differences in grating acuity as a measure of deprivation-induced impairment. All gerbils showed acuity deficits in the initially deprived eye, ranging in magnitude from 0.24-1.40 octaves. One month of monocular deprivation from eye opening proved sufficient to induce a permanent acuity impairment.

Animals↗
PubMed1986

Eye and brain growth in the Mongolian gerbil (Meriones unguiculatus).

Eye and brain growth were evaluated by measures of weight and surface dimensions at 11 ages spanning the period from birth to 150 days of age in the Mongolian gerbil (Meriones unguiculatus). Brain weight was found to increase very rapidly until day 18, followed by a more gradual increase to asymptote by 70 days of age. Eye growth follows overall body growth more closely than brain growth. Eye shape was found to change from ovoid to spherical over the first 70 days of life. The possible relationship between eye growth and retinal specialization is considered, and brain growth is compared across several rodent species.

Animals↗
PubMed1986

Visual texture segmentation in cats.

The segmentation of visual images on the basis of textural discontinuity was studied in cats using two behavioural paradigms. The first task (Experiment 1) required the animals to detect the presence of a target square of one texture embedded in a background of a second texture. The second task (Experiment 2) required the cats to discriminate between two forms (square and triangle) both of which were defined by texture-texture boundaries. Both detection and discrimination tasks were presented in a transfer paradigm in which the animals first learned a luminance-based form of the same problem. The results indicate that cats are able to segment a visual array on the basis of textural discontinuity in the absence of global luminance differences between figure and ground, and that they are able to use the contour information provided by texture-texture boundaries as a basis for form discrimination.

Animals↗
PubMed1986

Ultrastructure of ultraviolet radiation-induced hairless mouse skin carcinogenesis, with special reference to the epidermal-dermal junction.

The ultrastructure of ultraviolet (UV)-induced skin pathology was studied in mice to complement previously reported gross and light microscopic findings, and to assess further the usefulness of the animal model for study of sunlight associated epidermal tumours in man. Hairless albino (HRA/Skh-1) mice were exposed to a minimal erythemal dose from a filtered light source emitting both UVA and UVB, approximating solar emission. Samples of normal and hyperplastic skin, pedunculated papillomas, carcinomas in situ and invasive squamous cell carcinomas were processed for transmission electron microscopy once their identity was confirmed by light microscopic examination. Keratinocyte pleomorphism became more marked and cell to cell contact diminished as malignancy developed. For papillomas, carcinomas in situ and invasive squamous cell carcinomas, there was a progressive disruption of the epidermal junction which became marked upon frank invasion. Most of the differences between the various categories of pathological change, therefore, were not absolute but rather of degree, supporting the notion that invasive squamous cell carcinoma represents an end stage for malignancy which may arise de novo, directly from hyperplastic skin, or proceed from other tumour types. The similarity in structure of the mouse tumours to comparable tumours in man supports the usefulness of the animal model and suggests that the results have implications for sunlight associated tumours in man.

Animals↗
PubMed1984

The development of visual acuity in the Mongolian gerbil (Meriones unguiculatus).

The development of visual acuity for vertically oriented gratings was followed in gerbil pups from 34 days of age until adulthood using a jumping stand procedure. Acuity was found to improve gradually from an initial level of about 0.7 cycles/degree to asymptotic values of 1.2-1.6 cycles/degree by 70 days of age. Acuity did not vary as a function of viewing distance over the range tested (17.5-46 cm). Practice was found to play some role in the acuity changes seen but cannot alone explain the results. The contributions of other performance variables and of neural development are considered.

Animals↗
PubMed1975

Follow-up of emergency ambulance calls in Nottingham: implications for coronary ambulance servie.

Information about patients in ambulance service records has been linked to that in the patients' hospital records in an attempt to make the most efficient use of a special ambulance service for patients suspected of having heart attacks. During one week 248 emergency (999) calls for an ambulance were made by the public in the city of Nottingham. The quality of information given to the ambulance centre was poor, and all four patients eventually found to have had a myocardial infarction were described as having collapsed. A further study of patients who were also described as having collapsed has led to a system which allows an ambulance controller to send a "coronary ambulance" only in answer to those emergency calls where there is a reasonable possibility that the patient has had a heart attack.

Ambulances↗
PubMed1974

The effect of early astigmatism on the visual resolution of gratings.

1. Orientational differences in visual resolution were measured at a number of different luminance levels on two subjects with high astigmatism that had remained optically uncorrected until the age of 10. Because of their astigmatism both of these subjects see vertical contours more clearly than horizontal contours with the unaided eye.2. The measurements were made using sinusoidal gratings generated on the face of an oscilloscope with the refractive error carefully corrected with lenses and with the gratings viewed through 3 mm artificial pupils.3. Visual resolution was found to be much better for vertical than for horizontal gratings for both these subjects under these conditions. The difference between the contrast sensitivities for vertical and horizontal gratings was even evident with gratings having spatial frequencies as low as 1 c/deg, but became progressively more pronounced at higher spatial frequencies. In one of the subjects the visual acuity (the cut-off spatial frequency) for horizontal gratings was more than 3/4 of an octave lower than that for vertical gratings.4. This is very different from the results obtained from normal subjects who typically show only a slight reduction in contrast sensitivity for oblique gratings but resolve vertical and horizontal gratings equally well.5. The quantitative differences between the contrast sensitivities for vertical and horizontal gratings of both high and low spatial frequencies cannot be accounted for by either errors of focus in one meridian or by the presence of meridional aniseikonia.6. In order to completely eliminate any optical explanations for these findings measurements of contrast sensitivity were made using sinusoidal interference fringes formed directly on the retina, thereby bypassing the eye's optics. Since the orientational differences in resolution persisted with this method it must be concluded that they are of neural origin.7. By analogy with the effects on cortical physiology that follow early selective visual deprivation in cats and monkeys, it is argued that these orientational differences in resolution are a consequence of changes induced in the neural organization of the astigmat's visual system by the distorted visual input provided by the uncorrected astigmatism early in life. It is furthermore argued that the smaller orientational differences in resolution observed in normal eyes might similarly be induced by certain asymmetries in the early visual input.

Adult↗