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Biomedical subjects

D H Kelly

Publications and source records attributed to D H Kelly.

At least 55 records · Page 3Linked to original sources
PubMed1982

Motion and vision. IV. Isotropic and anisotropic spatial responses.

When the thresholds for periodic spatial patterns containing two or more differently oriented components (e.g., crossed gratings) are measured under normal, unstabilized conditions, each component seems to be detected almost independently of the others if their angular orientations are sufficiently different. This psychophysical behavior has been attributed to anisotropic or orientation-tuned units in the visual cortex. Here we report that when the image of such a multicomponent pattern is stabilized on the retina, the independent-detection behavior vanishes. Under stabilized-image conditions, the contrast sensitivity is governed by the maximum local contrast at the retina. The number and relative contrast of individual components, even orthogonal ones, behave almost additively in making up the threshold contrast. We confirmed this conclusion with a variety of patterns that give orientation-tuning effects in unstabilized viewing. Controlled image motion (resembling the effect of the natural drifts of the eye) restores the independent-detection behavior in every case, as do other forms of temporal modulation (e.g., flicker or flash presentations). We infer (1) that orientation-tuned units in man do not respond to unchanging stimuli--they cannot function unless the pattern on the retina is temporally modulated, and (2) in the absence of temporal modulation, spatial patterns are detected by isotropic units of relatively low sensitivity.

Biomedical Engineering↗
PubMed1982

Eliminating transient artifacts in stabilized-image contrast thresholds.

In the measurements of contrast thresholds for stationary, stabilized images, the choice of the psychophysical procedure may profoundly affect the results by constraining the time course of the stimulus presentation. With some procedures, the subject is unable to ignore transients; as a result, he necessarily responds to onset and offset transients rather than to the intended stimulus. Here we report significant effects of (a) the rapidly of onset and (b) the duration of the stimulus on thresholds for stabilized sinusoidal-grating patterns; these effects occur with both forced-choice and standard yes/no procedures. No value of either parameter (a) or (b) can eliminate such presentation effects when these procedures are used. However, we have devised a new, continuous-presentation procedure that yields results that do not depend on these parameters. Contrast thresholds for stabilized and for unstabilized stimuli measured by this procedure differ by 1 log unit or more, in good agreement with the results obtained by careful use of the method of adjustment.

Differential Threshold↗
PubMed1981

Disappearance of stabilized chromatic gratings.

When the image of a stationary, sinusoidal luminance grating is stabilized on the retina of a human subject, he becomes unable to detect this stimulus at contrasts that are readily visible in normal, unstabilized vision. At much higher contrasts, such stabilized gratings can still be seen over most of the normal range of spatial frequencies, although the threshold contrast may be increased by as much as 20 or 30 times. When the analogous experiment is performed with an isoluminance chromatic grating, however, there is no contrast that can restore the visibility of the stabilized grating; the threshold elevations for stabilized chromatic gratings are too great to measure. Saturated red/green gratings fade out and disappear at 100 percent contrast (even where this is 45 times the unstabilized threshold), and they do not reappear as long as stabilization is maintained. Without some kind of temporal variation of the proximal stimulus, the opponent-color pathways apparently do not respond to spatial patterns.

Color↗
PubMed1981

Ventilatory chemoreceptor response in parents of children at risk for sudden infant death syndrome.

We postulated that parents of infants who sustain near-death episodes associated with defective chemical regulation of breathing might share a similar defect. We, therefore, measured the ventilatory responses to progressive hypoxia and hypercapnia individually in eight sets of parents of infants who had sustained at least one near-death episode (apnea, cyanosis, pallor, limpness, and responsive only to mouth-to-mouth resuscitation); each infant had a ventilatory response to CO2 which was more than 2 S.D. below the mean normal. Ventilatory function measured by vital capacity forced expiratory volume 1.0 and flow curves was normal in each group. Responses to hypercapnia and hypoxemia in both fathers and mothers were similar to 11 pairs of controls. Ventilation during CO2 rebreathing normalized for surface area increased 0.87 liter/min/mm Hg in fathers, 0.94 in controls, 0.87 in mothers, and 0.75 in controls. Ventilation during progressive hypoxemia increased 88 liter/min/1 divided by mm Hg in fathers, 92 in controls, 86 in mothers, and 101 in controls. None of these differences was significant.

Adult↗
PubMed1981

Nonlinear visual responses to flickering sinusoidal gratings.

Over a range of high temporal and low spatial frequencies, counterphase flickering gratings evoke the so-called frequency-doubling illusion, in which the apparent brightness of the grating varies at twice its real spatial frequency. The form of the nonlinearity that causes this second-harmonic distortion of the visual response was determined by a cancellation technique. The harmonic distortion can be measured as a function of amplitude (or contrast) by adding to the flickering grating a real, nonflickering, double-frequency component with the amplitude and phase required to cancel the illusory second harmonic. Harmonic distortion curves obtained in this way imply that the nonlinearity is of the form /s/p, where s is the stimulus pattern (without its dc component) and p is close to 0.6. If p = 1, or if the absolute value is not taken, this expression predicts distortion curves that differ significantly from the experimental results. Hence neither rectification nor compression alone is sufficient to account for the second-harmonic distortion; both are required.

Flicker Fusion↗
PubMed1981

Contrast gain measurements and the transient/sustained.

We measure threshold for a vertical test grating superimposed on a fixed-contrast horizontal background grating of the same spatial and temporal frequency. The rate of change of this threshold with increasing contrast of the background grating is a measure of the contrast gain of the responding mechanism. Large slopes (high contrast gains) occur when spatial frequency is low and temporal frequency is high; small slopes (low contrast gains) occur when both spatial and temporal frequencies are low and when spatial frequency is high. This division of the spatiotemporal frequency domain into low- and high-gain regions is consistent with the transient/sustained dichotomy found in previous psychophysical studies. Furthermore, our results suggest that the mechanism responsible for detecting low spatial frequencies has a gain characteristic similar to that of cat retina Y cells and that the mechanism responsible for detecting high spatial frequencies has a gain characteristic similar to that of cat retina X cells, as found by Shapley and Victor [J. Physiol. (London) 285, 275-298 (1978)].

Humans↗
PubMed1981

Episodic complete airway obstruction in infants.

Four infants with episodic silent complete obstructed apnea are described. All had central sleep apnea, three had hypoventilation, and none had obstructive apnea observed on polygraphic recordings. Two infants were siblings of a sudden infant death syndrome victim and one was a sibling of a "near miss" infant. One infant died as a result of a prolonged episode that began while she was awake. In two infants apnea has been controlled with theophylline administration and in the third infant resolution of the episode was temporally related to atropine administration.

Airway Obstruction↗
PubMed1981

Treatment of apnea and excessive periodic breathing in the full-term infant.

Twenty-two full-term infants, aged 0 to 6 weeks, with a history of unexplained apnea and respiratory abnormalities on pneumogram recordings, were treated with theophylline (average dose 7.5 mg/kg/day and average serum level 11 micrograms/ml). Subsequent recordings showed a significant decrease in the amount of periodic breathing (14.3% vs 0.7%) and apnea 10 to 14.9 seconds (12.8 vs 1.0/100 min) when compared to the initial pneumogram. It is concluded that theophylline therapy in this group of infants will result in a reduction of apnea and periodic breathing.

Apnea↗
PubMed1980

Spatiotemporal characteristics of visual mechanisms: excitatory-inhibitory model.

The stabilized spatiotemporal threshold response surface can be modeled as the linear difference between the threshold response surfaces of two mechanisms, each of which is simply the product of a spatial and temporal frequency response curve. With no free parameters, the resulting model is shown to be a good fit to available data.

Form Perception↗
PubMed1980

Motion and vision. III. Stabilized pattern adaptation.

It has been suggested that local variations of retinal sensitivity may be responsible for elevating the threshold in pattern-adaptation experiments of the Blakemore-Campbell type. Subjects are unable to scan high-contrast gratings uniformly enough to eliminate this possibility. To control this effect, we performed grating-adaptation experiments under stabilized-image conditions, while both adapting and test targets were moved at retinal velocities determined by the experimenter. By means of an afterimage technique, we also measured the strength of the retinal sensitivity mask that forms under these conditions. Varying the spatial frequency and velocity of the adapting stimulus, we inferred the spatial and temporal properties of the principal mechanism that contributes to the afterimage. We found that the Blakemore-Campbell effect persists at adapting velocities that are fast enough to rule out local variations of retinal sensitivity. More surprisingly, even the clearly visible afterimages that occur at a retinal velocity of 0.1 deg/s seem to have no effect on pattern adaptation. (Sensitivity masking can raise the adapted threshold, but only at adapting velocities slower than normal eye movements). By manipulating the image velocity, we were able to shift the spatial frequencies of some threshold-elevation curves, but these shifts were not great enough to suggest that velocity tuning plays important role in pattern adaptation.

Adaptation, Ocular↗
PubMed1980

Parents' perceptions of the psychological and social impact of home monitoring.

Self-assessments of 133 parents' (74 families) feelings, perceptions, reactions to stresses, and satisfactions during a period of electronic home monitoring are reported. Data were collected during structured interviews by students in a graduate social work program. Although extreme anxiety was prevalent initially, only 27.4% of the parents felt they were very anxious beyond the first month. Social life was restricted in 55.7% but job attendance was seldom affected. Only four parents felt very irritated by the increased demands of the monitored baby. The majority (72.9%) said that the monitor made them feel more comfortable with their baby. Only 14.2% felt that their marriage relationship worsened during the period of monitoring; two couples separated. Most supportive to parents were their spouses, least supportive were friends and relatives. With availability of a psychosocial support system, electronic home monitoring of infants can be conducted by parents without constant and extreme anxiety and, in their judgment, can even be a satisfying experience.

Anxiety↗
PubMed1980

Astrocytoma in an infant with prolonged apnea.

A 4-week-old infant experienced prolonged central sleep apnea requiring resuscitation. At 6 months of age he developed episodic obstructive apnea (diagnosed as laryngospasm by direct laryngoscopy) with an abnormal EEG and a normal computed tomography (CT) scan, and at 14 months of age he developed hemiparesis due to a gemistocytic astrocytoma grade III-IV. Following removal of the mass, he has had occasional seizure activity, but no further episodes of obstructive apnea.

Astrocytoma↗