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Biomedical subjects

F Wilkinson

Publications and source records attributed to F Wilkinson.

At least 19 recordsLinked to original sources
PubMed1995

The late onset of visual texture segmentation in kittens.

Texture segmentation was studied developmentally in kittens using a two-alternative forced choice jumping stand paradigm. None of 15 kittens tested solved a texture segmentation task based on orientation contrast prior to 83 days of age, despite their rapid acquisition age, despite their rapid acquisition of an analogous luminance-based image segmentation problem. However, three kittens showed rapid acquisition of the texture segmentation task when the textures were composed of non-oriented elements (dots and annuli), reaching criterion performance by 52-59 days of age. A control experiment demonstrated that kittens can discriminate between vertical and horizontal gratings comparable in line width to the oriented texture elements as early as 53 days of age. The surprisingly late appearance of orientation-based texture segmentation is considered in the context of current models of texture segmentation and is compared to recent reports of a similar finding in human infants [3,54].

Aging↗
PubMed1995

Acuities for textures and gratings in kittens assessed by preferential looking.

A forced-choice preferential looking paradigm, similar to that developed for human infants, was used to assess visual thresholds in kittens between 21 and 45 days of age. In agreement with the earlier work of Sireteanu [19,30], the technique was found to be successful for grating acuity measurement. Acuity for square wave gratings (vs. matched greys) increased from 0.12 cycles/degrees at 21 days to 1.6 cycles/degrees at 40 days of age. Comparable results were obtained at three viewing distances: 20, 40, and 60 cm, indicating that accommodation is not a limiting factor in spatial resolution over this range of distances. Kittens also showed preferences for texture arrays with strong oriented components and 'texture acuities' were found to be comparable to acuity for square wave gratings. A consistent orientation anisotropy favouring horizontal line elements was seen in very young kittens but disappeared by 5 weeks of age. Preferences for texture arrays composed of dots were much weaker and thresholds could not be determined in all cases. The results are discussed in the context of the Banks and Ginsberg [2] model of infant preferences.

Aging↗
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↗
PubMed1983

The long-term effects of removal of sensorimotor cortex in infant and adult rhesus monkeys.

A comparison is made between the long-term effects of the unilateral removal of sensorimotor cortex in infant and adult rhesus monkeys. Both infants and adults recovered to a remarkable extent. They walked, climbed and jumped with ease. However, neither infant nor adult monkeys could grip food by using thumb and forefinger independently of the other fingers. It was demonstrated in the adults that there was a permanent impairment in the use not only of the fingers but also the wrist and forearm. The results do not support the claim made by Kennard (1942) that infants recover more completely than adults from the effects of brain lesions. An analysis of the relevant evidence suggests that compensation occurs only when the animal is very immature at the time of operation. The brain is much more mature in a neonatal monkey than a rat or hamster. True compensation can probably only occur in monkeys if the lesion is made well before birth.

Animals↗
PubMed1980

Young kittens can learn complex visual pattern discriminations.

Kittens begin to display visually elicited responses shortly after eye opening. The pupillary reflex, optokinetic response, visual placing and avoidance of the deep side of the visual cliff all appear between the second and fifth weeks of life, and evidence of visual learning in homing behaviour has been described at the same early age. However, an attempt to elicit pattern preferences in kittens of this age (usually considered the easiest way to demonstrate discriminatory behaviour in immature organisms) was unsuccessful, evidently because such kittens show little spontaneous visual interest. This is surprising in view of electrophysiological findings which emphasise the importance of the fourth to sixth weeks in the development of cortical feature-analysing mechanisms; perhaps the stimulus-seeking methodology of Dodwell et al. is anappropriate for the visual modality at such an early age. Recently, visual acuity measurements have been reported for kittens as young as 30 d of age using a contour versus no-contour discrimination on a modified Lashley jumping stand. With this technique we have now trained kittens of under 50 d of age to perform more complex pattern discriminations.

Age Factors↗
PubMed1980

A pulse radiolysis study of zinc(II)-insulin.

Reactions of eaq with zinc(II)-insulin at pH 6.6 and 9.0 yielded relatively low disulphide anion absorptions, suggesting eaq reacts at other sites than S-S. A similar conclusion was reached for the reation of CO2 where an even lower yield of disulphide anion was found. However, here the disulphide anion yield increased with 'prepulsing'. Simultaneously the rate constant decreased, implying that a more reactive site was 'cleaned up'. While no reaction of Br2 with insulin was observed, both OH and Cl2 reacted rapidly and predominantly at the tyrosine residues. The second order rate constants, calculated in terms of insulin monomer concentrations, are reported for eaq, CO2 and Cl2. The transient spectra qualitatively support evidence regarding the accessibility of S-S bonds and tryosine residues in the various forms of insulin as predicted from earlier studies.

Amino Acids↗