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Biomedical subjects

V C Smith

Publications and source records attributed to V C Smith.

At least 73 records · Page 4Linked to original sources
PubMed1989

Foveal cone thresholds.

The method of constant stimuli was used to estimate the psychometric functions for detection of one or two flashes when two light pulses were presented. The test stimulus consisted of two simultaneous 0.5 msec, 1' pulses separated by 17'. Observers reported seeing 0, 1 or 2 flashes. A computer-controlled direct-view apparatus allowed sampling of slightly different foveal locations on each trial. The data were analyzed assuming a binomial probability for sampling of L and M cones and Poisson distributed quantal fluctuation. Under these assumptions, the measurements imply that detection requires a minimum of 5-7 quanta absorbed per cone, and that the effective number of cones illuminated by the 1', 0.5 msec pulse is two. The estimated L/M cone ratio was 1.6 for one observer and 4.0 for the other; each observer's ratio was in general agreement with the value estimated independently by heterochromatic flicker photometry.

Adult↗
PubMed1989

Heterochromatic modulation photometry.

Heterochromatic modulation photometry is a method for obtaining equiluminance for a pair of heterochromatic lights presented in temporal alternation. A series of fixed standard luminance/test luminance ratios are presented, and at each ratio the modulation depth of the pair is reduced in tandem until the observer reports that flicker disappears. The data can be described by a luminance contrast template that appears V shaped when plotted on log-log coordinates. In the fitting of individual data, a free vertical scaling factor reflects the observer's sensitivity to luminance modulation and a free horizontal scaling factor reflects the observer's similarity in spectral sensitivity to the CIE standard observer. Data for red/green flicker photometric matches demonstrate the technique. Heterochromatic modulation photometry offers several advantages over flicker photometry: (1) a single fixed perceptual transition occurs on each trial series, namely, the transition from flicker to steady, and (2) luminance matches can be obtained at fixed frequencies at a number of luminance levels. The same procedure can be applied to the measurement of the minimally distinct border and to the identification of tritan pairs (stimulus pairs that differ only in their stimulation of short-wavelength-sensitive cones).

Adult↗
PubMed1989

Marfan's syndrome, pregnancy, and the cardiac surgeon.

Marfan's syndrome in a pregnant woman appears to be particularly hazardous because of the increased risk of aortic rupture. The first reported case report of replacement of the ascending aorta in a pregnant woman with a successful outcome for mother and fetus is presented. The literature is reviewed with regard to Marfan's syndrome and pregnancy as well as to open heart surgery during pregnancy.

Adult↗
PubMed1989

Retinal densitometry in acute posterior multifocal placoid pigment epitheliopathy.

Cone photopigment kinetics were investigated by retinal densitometry in six patients with acute posterior multifocal placoid pigment epitheliopathy (APMPPE). Our goal was to document the course of cone impairment during a period of at least 12 months following the onset of the disease process. During the active stage, we found that the amount of pigment measurable by densitometry (the density difference) was reduced and that the time constant of pigment regeneration was unmeasurable. Following resolution of the fundus lesions, the densitometric parameters gradually improved in eight of ten eyes. In patients who maintained foveal fixation (five eyes), the density difference and the time constant of pigment regeneration improved simultaneously, though with individual differences in time course. Photopigment kinetics returned to normal in all these patients but the density difference remained lower than normal in four of the five eyes. In patients with parafoveal fixation (five eyes), photopigment kinetics were slow, possibly reflecting mixed cone and rod contributions. With time, the density difference improved to a level comparable to that measured in normal observers at a similar retinal location, but the photopigment regeneration time constant remained abnormal or unmeasurable. Recovery was variable, with one eye changing from parafoveal to foveal fixation, and no improvement noted in two eyes with extreme parafoveal fixation. Our findings demonstrate a large interpatient variation associated with APMPPE.

Color Perception↗
PubMed1989

Chest wall hamartoma of infancy.

Chest wall hamartoma is a rare intrathoracic mesenchymal tumor occurring in infants and usually present from birth. The tumor arises from the ribs and is a histologically benign focal overgrowth of normal skeletal elements. Infants often present with respiratory problems. We report a case of a 5-month-old infant with a chest wall hamartoma who presented with a fever of unknown origin.

Hamartoma↗
PubMed1989

The effect of background luminance on cone sensitivity functions.

Implementations of the Wald-Marré technique have employed fixed luminance backgrounds to isolate cone sensitivity mechanisms. We evaluated the effect of varying the adaptation level on the relative isolation of the different cone types. For MWS and LWS cone isolation we used a 15 Hz flickering test light to isolate the achromatic channel, and we modelled the resulting spectral sensitivity functions as a linear sum of LWS and MWS input. We found only mild improvement in relative cone isolation with increasing adaptation level. The LWS and MWS cone mechanisms showed decreasing sensitivity with adaptation level and reached limiting Weber behavior above 1000 Td. SWS cones were isolated with a 2.4 Hz flickering light. SWS cone isolation improved with adapting level, reaching a plateau above 1000 Td. The SWS cone mechanism showed decreasing sensitivity with adaptation level but did not reach a limiting Weber region. Our data indicate that the use of fixed high adaptation levels has different effects on the cone mechanisms. Absolute sensitivity loss for LWS or MWS mechanism will not be revealed. LWS and MWS thresholds will appear normal unless there is an adaptation abnormality. On the other hand, the SWS cone thresholds would be sensitive to both absolute and increment sensitivity loss. More than one adaptation condition is needed to separate different types of sensitivity loss characteristic of eye disease.

Adaptation, Physiological↗
PubMed1988

X-linked congenital stationary night blindness. Review and report of a family with hyperopia.

X-linked congenital stationary night blindness is almost always associated with myopia. We have reviewed all previously reported pedigrees and have found only two with patients without myopia. A recently proposed classification of night blindness includes a complete type associated with myopia and an incomplete type in which both hyperopia and myopia were found. Complete and incomplete types did not occur within the same pedigree. We report on a family in which three of the five affected members had hyperopia and could be classified as the incomplete type and in which a fourth member with myopia was more consistent with the complete type. The lack of myopia in three members of our pedigree can be explained by two hypotheses: crossing over of the night blindness and myopic genes on the X-chromosome, or an autosomal dominant hyperopic gene that masks the myopic gene. The data from our family support the first of these two hypotheses.

Adult↗
PubMed1988

Effects of chromatic adaptation on phase-dependent sensitivity to heterochromatic flicker.

Temporal modulation sensitivity was measured as a function of the relative phase of two equiluminous chromatic sources (564 and 625 nm) for temporal frequencies from 6 to 20 Hz. The difference between 180 degrees and the phase of least sensitivity was computed as the measured phase shift. A 2 degree test field was superimposed upon 8 degrees chromatic adapting fields with luminances from 100 to 3000 Td and chromaticities of 500, 600, and 650 nm. For each adapting field, the 564-nm source was set to 175 Td, and the 625-nm source was matched to it with heterochromatic flicker photometry (giving an effective mean luminance of 350 Td). The 650-nm adapting fields produced large changes in photometric setting but only small changes in the measured phase shift. The 600- and 500-nm adapting fields produced smaller changes in photometric setting but larger changes in the measured phase shift. In general, increased adapting luminance resulted in an increase in the measured phase shift for 600-nm adaptation and a decrease in the measured phase shift for 500-nm adaptation.

Adaptation, Physiological↗
PubMed1988

Color matching and the Stiles-Crawford effect in observers with early age-related macular changes.

We studied the color-match-area effect and the Stiles-Crawford effect in 10 observers with age-related macular changes. Observers were graded on a scale of I to IV according to the Sarks classification, which correlates fundus appearance and visual acuity with the severity of postmortem histological changes in Bruch's membrane. Observers in group II showed subtle abnormalities of color matching; those in groups III and IV showed more-severe abnormalities. The Stiles-Crawford effect was abnormal in 9 of 10 eyes tested. Only one observer showed a Stiles-Crawford effect that had a well-defined peak near the center of the pupil and a near-normal bandpass. The results suggest that early age-related changes visible in the fundus can be revealed by psychophysical tests of photoreceptor architecture.

Aged↗
PubMed1988

Bovine pericardial patch fibrosis requiring reoperation.

Glutaraldehyde-fixed bovine pericardium (GFBP) has multiple uses. Its use as a vascular patch material is under investigation. A case report of severe fibrosis causing early reoperation of a GFBP patch used to repair coarctation of the aorta is presented.

Aortic Coarctation↗
PubMed1987

Appearance of steadily viewed lights.

An interocular matching technique was used to investigate the variation of chromaticity and brightness following steady viewing of a chromatic test light (identical adapting and testing color). Adaptation times were of sufficient duration to ensure stable matches. Following chromatic adaptation we found changes in hue, saturation and brightness. The spectral colors appeared desaturated. The hue shift for the spectral region 546 to 570 nm was towards green and for 586 to 670 nm was towards red. The brightness decrease, independent of chromaticity, was 0.8 log unit at 150 td and 0.3 log unit at 8 td. Data were analyzed within a two-process framework of brightness. Bezold-Brücke effect measurements showed chromaticity shifts in the same direction as for dimming caused by continuous adaptation. Changes in saturation were also observed but were usually in the opposite direction from those found for adaptation.

Adaptation, Ocular↗
PubMed1987

Temporal modulation sensitivity and pulse-detection thresholds for chromatic and luminance perturbations.

We studied temporal processing of chromatic and luminance perturbations of a 600-nm field, measuring both modulation sensitivity (sinusoidal frequencies from 0.25 to 40 Hz) and pulse-detection thresholds (pulse durations from 5 to 2560 msec) for mean luminances of 0.9 to 900 Td and field sizes of 0.5 degrees to 8 degrees. Chromatic stimuli were produced by antiphase modulation of lights matched by heterochromatic flicker photometry. Both mean luminance and field size affected sensitivity, and the magnitude of field-size effects increased with mean luminance. We derived both luminance and chromatic impulse response functions for each set of experimental conditions, using the modulation-sensitivity data. At high mean luminances and large field sizes the chromatic impulse response functions are complex, suggesting contributions from both chromatic and luminance mechanisms. Pulse-detection data were fitted by a peak detector model based on these impulse response functions.

Color↗
PubMed1987

Effects of temporal frequency on phase-dependent sensitivity to heterochromatic flicker.

Modulation sensitivity was measured as a function of the relative phase of two equiluminous chromatic sources (564 and 625 nm) for temporal frequencies ranging from 1 to 40 Hz. Measurements were made at a retinal illuminance of 900 Td, with a 2 degree field metameric to approximately 600 nm. For most frequencies, the phase of least sensitivity was other than 180 degrees, indicating the existence of a luminance response to antiphase modulation. Additional data sets examined the effects of field size (0.5 degrees-8 degrees) and mean chromaticity (583-610 nm). Variation in field size, chromaticity, and photometric setting had little effect on the phase of least sensitivity. The data were fitted by using vector summation between luminance and chromatic responses; the modulation-sensitivity functions that resulted from this analysis agreed with those of earlier studies.

Color Perception↗
PubMed1987

Clinical electroretinography for short wavelength sensitive cones.

We measured electroretinograms (ERGs) for the isolated short-wavelength-sensitive (SWS) cones using a substitution technique. The stimulus was a 5 Hz alternation of 460 nm and 565 nm or 490 nm and 565 nm light of equivalent photopic luminous efficiency. We used a 571 nm narrow-band adaptation field of 7000 td to improve SWS cone isolation and to suppress rod activity. The resulting SWS cone ERG amplitudes were 10-30 microV with latencies of 60-80 msec. A rapid clinical protocol to assess the radiance response function of the SWS cone ERG is described.

Adolescent↗
PubMed1986

Eye disease and color defects.

The groundwork for understanding color defects in eye disease was established by the end of the nineteenth century. Thereafter the field was neglected as scientists concentrated on studies of normal color vision and congenital color vision defects. Spurred by the development of the Farnsworth 100 hue-test, interest was renewed in the 1950s. The past 25 years have seen an explosion of interest in color defects in eye disease. The International Research Group on Color Vision Deficiencies has played an important role in this activity. The development of new clinical tests and instruments as well as refinement of laboratory techniques are among the important developments.

Color Perception Tests↗
PubMed1986

Phase-dependent sensitivity to heterochromatic flicker.

We measured modulation sensitivity to a pair of equally luminous sinusoidally modulated lights (568 and 630 nm) as a function of their relative phase. Measurements were made for 2, 3, 6, and 12 Hz at a retinal illuminance of 100 Td. The data indicated that two processes were active and their outputs combined by a vector summation rule. There was a phase shift of -18 degrees to -20 degrees (630 nm leads 568 nm) at 6 Hz, no phase shift at 12 Hz, an equivocal shift at 2 Hz, and an indeterminate shift at 3 Hz. At frequencies where a phase shift was observed, our analysis indicated that the phase shift affected sensitivities measured at all relative phase settings. These results are inconsistent with models postulating equal contributions of long-wavelength- and middle-wavelength-receptors to centers and surrounds of processes responsible for the detection of luminance flicker.

Color Perception↗