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Biomedical subjects

D E Mitchell

Publications and source records attributed to D E Mitchell.

At least 37 records · Page 2Linked to original sources
PubMed1990

Vernier acuity with opposite-contrast stimuli.

Vernier acuity has usually been tested with stimuli of the same contrast polarity (SC). This traditional vernier acuity was compared to that obtained with stimuli of opposite-contrast (OC) in which one target was brighter than the background and the other was darker. For both bar and dot targets vernier acuity with OC stimuli was about half as good as with SC stimuli. There were large individual differences in the size of the disadvantage with OC stimuli, although thresholds remained within the hyperacuity range. There were also individually-differing biases to see a dark vernier stimulus on one or the other side of a bright stimulus. Differences between OC and SC vernier acuities persisted over a wide range of interstimulus spacings, widths, and contrasts. At extremes of these spatial manipulations acuities became similar, but only because SC acuities were degraded to the level of OC acuities. Subjects showed little improvement in OC vernier acuity, even after 50,000 trials. It is concluded that finest judgements of spatial position arise in a level of the visual system at which light and dark stimuli are treated independently.

Discrimination Learning↗
PubMed1989

Fertility after ectopic pregnancy.

In the period July 1983 to March 1985, 264 women had surgery for ectopic pregnancy at Grady Memorial Hospital; 76 had postoperative hysterosalpingograms. Of these, 55 (76.4%) women were followed up for 3 to 41 months (mean, 23.8) to determine subsequent fertility. During the follow-up period, 30 pregnancies occurred among the 55 patients; 24 were intrauterine and 6 were repeat ectopic pregnancies. In the surgical group of 39 patients with salpingectomy, 60.8% of those desiring pregnancy achieved an intrauterine pregnancy. Of the 12 patients with salpingostomy, the three who desired pregnancy achieved it (100%). In the tubal abortion group, the two women desiring pregnancy conceived (100%). There were six repeat ectopic pregnancies (10.9%). Of the ectopic pregnancies, one occurred in the salpingectomy group (2.6%), four in the salpingostomy group (33.3%), and one in the tubal abortion group (25%). Five of the six ectopic gestations were found in the contralateral fallopian tube. Hysterosalpingographic evidence of contralateral tubal patency was a good prognostic indicator for subsequent intrauterine pregnancy. By contrast, one half of study patients with findings suggesting tubal occlusion still achieved an intrauterine pregnancy.

Adult↗
PubMed1989

Normal and abnormal visual development in kittens: insights into the mechanisms that underlie visual perceptual development in humans.

For many years the mechanisms that underlie normal and abnormal development of visual perception in human infants have been explored in anatomical and physiological studies on two species of frontal-eyed mammals, namely, cats and, to a lesser extent, monkeys. The unstated assumption underlying the investigations on cats is that despite substantial differences in the organization of the visual pathways of cats and humans, as well as quantitative differences in their perceptual abilities, principles of development established in the former also apply to humans. This review examines the extent to which this assumption may be valid. Following a review of certain anatomical peculiarities of the cat visual system, several of the differences as well as the parallels between the perceptual abilities of cats and humans are summarized. The latter similarities, as well as the larger number of parallels between the two species that can be drawn during development, attest to the validity of the choice of the cat for study of the mechanisms that underlie human visual development.

Animals↗
PubMed1989

Hysteroscopic removal of an interstitial ectopic gestation. A case report.

A 31-year-old woman with plauteauing quantitative serial beta-human chorionic gonadotropin levels, suspect pelvic sonography and the absence of products of conception on uterine curettage was suspected of having an interstitial ectopic pregnancy. Confirmation was by endoscopy. Cornual uterine wedge resection was avoided with laparoscopically guided hysteroscopic removal of the products of conception.

Adult↗
PubMed1988

Stachyose Synthesis in Source Leaf Tissues of the CAM Plant Xerosicyos danguyi H. Humb.

Leaf tissues from Xerosicyos danguyi H. Humb., a succulent member of the Cucurbitaceae, were found to possess both galactinol synthase activity and the capacity for photosynthetic production of stachyose, the phloem transport oligosaccharide common to other nonsucculent cucurbits. The amounts of stachyose isolated from leaf tissues, and the extractable activity of galactinol synthase, were somewhat higher in leaf tissues obtained from plants operating in the Crassulacean acid metabolism (CAM) mode (well watered plants) compared to leaf tissues from plants operating in the CAM-idling mode (water-stressed plants). In contrast, in leaf discs, the photosynthetic incorporation of label into stachyose following pulse labeling with (14)CO(2) was similar for stressed and for nonstressed tissues. Stachyose could be extracted from, and was synthesized photosynthetically by, leaf discs which contained no vascular tissues, indicating that synthesis of stachyose can occur in photosynthetic mesophyll cells of Xerosicyos.

Journal Article↗
PubMed1988

The extent of visual recovery from early monocular or binocular visual deprivation in kittens.

1. The rate and extent of recovery of vision was studied in the deprived eye of kittens that had been monocularly deprived from near birth for periods that lasted from 6 weeks to 18 months. Recovery was measured in the two situations, where either both eyes were open following the initial deprivation (binocular recovery), or else the non-deprived eye was occluded so as to force the animal to employ its deprived eye (reverse occlusion). 2. Measurements were made of the visual acuity of the deprived eye for gratings at frequent intervals during recovery by means of a simple behavioural technique. 3. The acuity that the deprived eye eventually attained declined with increasing length of deprivation in a manner that could be approximated by a simple exponential decay. Only minimal visual recovery was observed in animals deprived beyond 1 year of age; only two of six animals recovered sufficient vision to enable measurement of visual acuity. In general, animals that were reverse occluded recovered better vision than did those that had both eyes open during recovery. 4. The recovery of vision in the deprived eye of monocularly deprived kittens was compared to that observed following equivalent periods of two forms of binocular deprivation, namely dark-rearing and binocular eyelid suture. 5. The recovery from the two forms of binocular deprivation was quite different. Whereas the extent of recovery from dark-rearing was considerably greater than that observed after equivalent periods of monocular deprivation, the recovery of a limited sample of cats that were binocularly deprived by eyelid suture was worse. 6. These findings suggest that some plasticity remains in the visual pathways for a longer time than indicated by experiments that examine the physiological effects of monocular deprivation on various visual cortical structures.

Animals↗
PubMed1987

Hysterosalpingographic evaluation of tubal patency after ectopic pregnancy.

In the period 1983 to 1985, hysterosalpingography was performed in 76 women after laparotomy to manage ectopic gestations. Clinical characteristics of these women, including their operative findings, were then correlated with the follow-up hysterosalpingography. Study participants were classified into five groups according to surgical management of the ectopic gestation. In each of the five groups, a substantial percentage of those women with a nonpatent fallopian tube (contralateral to the tube bearing the ectopic gestation) on hysterosalpingography had a normal-appearing fallopian tube at laparotomy. Approximately 50% of women with a history of previous pelvic inflammatory disease were found to have nonpatent contralateral fallopian tubes on follow-up hysterosalpingography.

Adolescent↗
PubMed1987

Reduced visual acuity in both eyes of monocularly deprived kittens following a short or long period of reverse occlusion.

While there can be substantial recovery of vision during reverse occlusion in the deprived eye of kittens that have been monocularly deprived from birth, in many situations this visual improvement does not appear to be retained following the introduction of binocular visual input. This study examines the consequences of periods of reverse occlusion whose onset and duration would be thought to maximize the opportunity for good and permanent recovery of vision in the initially deprived eye. Twenty kittens were monocularly deprived from near birth to either 4, 5, or 6 weeks of age and then reverse occluded for periods that were either very brief (9-18 d) or very long (9-12 weeks). In the former situation, binocular visual experience was introduced when the visual cortex was most susceptible to environmental modification, which would be thought to maximize the opportunity for the eventual development of good vision in both eyes. On the other hand, the long periods of reverse occlusion might be expected to lead to a permanent recovery of vision in only the initially deprived eye, since binocular visual input would have been introduced at a time when the cortex was no longer very plastic. Surprisingly, the end result of both these extreme regimens of reverse occlusion was the same, namely, a severe bilateral amblyopia in which the acuity of both eyes was permanently reduced to levels that were as low as 1/10, but more typically between 1/3 and 1/4 of normal values. Contrast sensitivity functions measured on 2 of the cats that received a brief period of reverse occlusion revealed deficits in contrast sensitivity of about an order of magnitude at all spatial frequencies. These observations have important implications for the nature of the physiological and anatomical changes that occur in the visual cortex during reverse occlusion.

Animals↗
PubMed1987

An evaluation of the Gore-Tex surgical membrane for the prevention of postoperative peritoneal adhesions.

The Gore-Tex surgical membrane, expanded polytetrafluoroethylene, has been used as a pericardial graft with minimal adhesion formation reported. The purpose of this study was to assess its efficacy as a barrier method for diminishing postoperative peritoneal adhesions in an animal model. Fifteen New Zealand white rabbits underwent laparotomy, with scrape and cut lesions created bilaterally on the uterine body and horns, respectively. On one side, the lesions were covered with the graft using 7-0 Gore-Tex suture; the contralateral side served as an internal control. After four weeks, the adhesions were graded and mean adhesion scores were calculated. The Gore-Tex score was 4.1 times higher than the control for scrape lesions and 1.9 times higher for cut lesions, but the difference was not statistically significant in either case. Overall, the Gore-Tex mean adhesion score was 2.3 times higher than the control, a statistically significant difference. The Gore-Tex surgical membrane did not appear to be an effective adjuvant for postoperative adhesion prophylaxis in this animal model.

Animals↗
PubMed1984

Immediate and long-term effects on visual acuity of surgically induced strabismus in kittens.

In order to isolate some of the factors responsible for strabismic amblyopia as well as to ascertain the time course of its development, frequent measurements were made of the visual acuity of the two eyes of kittens following imposition of surgically induced strabismus. Following behavioural training on a jumping stand, strabismus was induced in all but one animal by simple section of either the lateral (esotropia) or medial (exotropia) rectus muscle of one eye. The one exception was a kitten on which esotropia was induced by another common but more radical surgical procedure that involved removal of the body of both the lateral rectus and superior oblique muscles of one eye. There was a surprising difference between the immediate consequences for vision of section of the lateral and medial rectus muscles that were reflected by equally large differences in the magnitude of the amblyopia that developed eventually in the two situations. Following section of the lateral rectus muscle, there was an immediate reduction in the visual acuity of the operated eye of as much as 2 octaves after which vision returned to normal levels over 4 to 8 days. The acuity of the two eyes remained comparable for a few days after which the vision of the operated eye began to decline once more, signalling the onset of amblyopia some 10 to 12 days following imposition of strabismus. In contrast to the severity of these effects, the effects observed following section of the medial rectus were both mild and transitory. Furthermore, whereas all kittens that were rendered esotropic early developed amblyopia, none of the kittens that were made exotropic at the same age did so. Together, these results suggest that factors associated with the immediate consequences of the surgical procedure employed to produce a misalignment of the visual axes may contribute to the severity of the effects of surgically induced strabismus.

Age Factors↗
PubMed1984

The role of beta-endorphins and catechol estrogens on the hypothalamic-pituitary axis in female athletes.

Competitive swimmers were followed over a 2-year period when they trained at different levels of exercise which coincided with distinct changes in their menstrual history. Oligomenorrhea was identified in 5 of 13 of these athletes when they swam approximately 100,000 yards per week. Weight and percentage of body fat were not significantly different between the period of oligomenorrhea and regular menstrual function (P = 0.24). Mean and median levels of luteinizing hormone, follicle-stimulating hormone, prolactin, and 17beta-estradiol were decreased and catechol estrogens and beta-endorphins were increased in serum during the strenuous, when compared with the moderate, training period. The serum levels of the steroid and protein hormones were similar to those of normal cycling, nonexercising control subjects during moderate exercise (60,000 yards per week). The significant differences between beta-endorphins and catechol estrogens during periods of strenuous exercise suggest an explanation for oligomenorrhea in female athletes. These hormonal changes result in hypothalamic anovulation, which appears to be reversible, because the hormone levels and menstrual cycles return to normal when the exercise is reduced.

Adolescent↗
PubMed1984

Labile nature of the visual recovery promoted by reverse occlusion in monocularly deprived kittens.

Kittens were monocularly deprived by closing one eye at the time of natural eye opening for periods that ranged from 4 to 14 weeks. This eye was then opened, and the other eye was closed for an approximately equal period of time. During this period of reverse occlusion, the vision of the initially deprived eye improved from apparent blindness to a level of good visual acuity. Surprisingly, however, this recovery was largely eliminated in only 2 weeks once the initially nondeprived eye was opened to restore visual input to both eyes. This finding has important implications for the nature of the mechanism(s) responsible for the dramatic physiological effects of monocular occlusion on the visual cortex. It may also help to elucidate recent observations on patching therapy in human amblyopia.

Amblyopia↗
PubMed1984

The permanence of the visual recovery that follows reverse occlusion of monocularly deprived kittens.

While the behavioral and physiologic effects of an early period of monocular deprivation can be extremely severe, they are not necessarily irreversible. Considerable recovery can occur if visual input is restored to the deprived eye sufficiently early, particularly if the nondeprived eye is occluded at the same time (reverse occlusion). This study examines the permanence of the visual recovery promoted by a period of reverse occlusion in kittens that were monocularly deprived from near birth for periods ranging from 3 to 18 weeks. During the period of reverse occlusion, the vision of the initially deprived eye improved from apparent blindness to good levels of acuity. However, upon restoring visual input to the formerly nondeprived eye a surprisingly rapid and reciprocal change occurred in the visual acuity of both eyes. Much of the substantial gain in the vision of the initially deprived eye that occurred during reverse occlusion was lost within 3 weeks, while at the same time the vision of the initially nondeprived eye improved substantially. Nevertheless, in many animals the acuity of the initially nondeprived eye did not recover to levels it had reached prior to reverse occlusion. These results hold important implications for the nature of the mechanisms responsible for the dramatic physiologic effects of monocular occlusion and reverse occlusion on the visual cortex. The results also may help elucidate recent observations on patching therapy in human amblyopia.

Animals↗
PubMed1983

The effects of short-term occlusion therapy on reversal of the anatomical and physiological effects of monocular deprivation in the lateral geniculate nucleus and visual cortex of kittens.

The relative efficacy of distributed versus massed reverse occlusion therapy in promoting recovery from the anatomical and physiological effects of monocular deprivation was studied in two experiments performed on kittens raised with one eye occluded from eye-opening until 5 weeks of age. The first experiment explored the effects of different periods (ranging from 0.5 to 4 h) of reverse occlusion imposed daily for 20 days. The second, involving a fixed period of reverse occlusion (20 h), examined recovery with respect to the distribution of that period over a varied number of brief daily sessions. Recovery was assessed in terms of changes in cortical ocular dominance and lateral geniculate cell morphology. Although recovery of both cortical ocular dominance and geniculate cell morphology showed the same overall progression with increasing periods of reverse occlusion, changes were apparent in the lateral geniculate nucleus before changes were evident in cortical ocular dominance. A given period of reverse occlusion was found to be far more effective in promoting recovery when distributed over a number of different exposure sessions than when massed together in just one or two sessions. The data suggest that there is a maximal rate of cortical recovery which is achieved with surprisingly brief daily periods of forced visual exposure of the initially deprived eye.

Aging↗
PubMed1983

The effect of minimal occlusion therapy on binocular visual functions in amblyopia.

The binocular visual functions of amblyopic children were studied during treatment involving brief weekly periods of occlusion of the unaffected eye while the child performed demanding visuomotor tasks against either a background of rotating gratings or a stationary uniform gray stimulus. The gains in stereoacuity were quite significant and in most cases more obvious than the rather small gains in letter visual acuity. On initial presentation only 21 of the 60 patients showed evidence of stereopsis and of these only seven possessed a stereoacuity of 100 secs or better. Following six treatment sessions the number of patients that demonstrated stereopsis increased to 36 of whom 17 possessed reasonably good stereoacuity (100 secs or better). However, there was no difference in the degree of improvement exhibited by those patients that viewed rotating grating patterns during treatment and others from the control group that viewed the uniform gray stimulus. Thus, there was no evidence that any of the visual gains were enhanced or promoted by active visual stimulation of the amblyopic eye with rotating gratings during the brief periods of occlusion of the unaffected eye. Finally, a comparison of the scores of the children on various stereo-tests suggest that tests comprised of small figure elements that are present in high density may be best for screening purposes. On the other hand, for quantifying the stereoacuity of children known to possess abnormal binocular vision it may be more appropriate to employ tests that use large figure elements that provide strong fusion cues.

Adolescent↗
PubMed1983

Contrast thresholds for sine gratings of children with amblyopia.

Contrast threshold functions were measured on 26 amblyopic children before and after minimal occlusion therapy. On initial presentation the relative contrast sensitivity deficit the amblyopic eye was in every case much less than that predicted from the deficit in visual acuity for letters. In fact, in seven of the children the contrast sensitivity functions for the amblyopic and fellow nonamblyopic eye were indistinguishable despite the expression of substantial amblyopia on letter charts. Only four children exhibited a substantial contrast sensitive loss in the amblyopic eye with a cut-off spatial frequency below 30 cycles/degree. The majority of children who showed deficits in contrast sensitivity did so only at medium and high spatial frequencies. On the basis of these findings it appears that measurements of contrast thresholds for single sinusoidal gratings do not probe fully the deficits of spatial resolution in amblyopia. Finally, among the few children who exhibited a sizeable initial deficit, only two showed obvious improvement in contrast sensitivity in response to minimal occlusion therapy.

Adolescent↗
PubMed1982

Normality of spatial resolution of retinal ganglion cells in cats with strabismic amblyopia.

1. A convergent or divergent strabismus was induced surgically in eight kittens and a cyclotropia of about 90 deg in two additional kittens. 2. Behavioural measurements were made of the visual acuity of each eye for square-wave gratings. All eight animals that were so tested displayed a reduction of acuity in one eye relative to the other of 1.3-2.5 octaves. 3. The activity of retinal ganglion cells was recorded within the amblyopic eye of six cats, three with a convergent strabismus, two with a divergent strabismus and one with a cyclotropia. Measurements were made of the spatial resolution with 215 on-centre cells for horizontal and vertical gratings. 4. In contrast to other reports, we found the spatial resolution of ganglion cells in the amblyopic eye of the strabismic animals to be comparable to those of normal cats at all retinal eccentricities. In particular there was no evidence for a loss of resolution in the vicinity of the area centralis. 5. Measurement of the cross-sectional area of cells in the lateral geniculate nucleus (l.g.n.) revealed no evidence of cell shrinkage in laminae receiving a projection from the amblyopic eye. 6. Together, these findings lead to the conclusion that the neural deficit responsible for the strabismic amblyopia in these animals did not lie in the retina but rather at more central levels of the visual pathway.

Action Potentials↗