Biomedical subjects

J C Kotulak

Publications and source records attributed to J C Kotulak.

15 recordsLinked to original sources

PubMed1995

Red-green opponent channel mediation of control of human ocular accommodation.

1. It has been hypothesized, but not verified empirically, that the control of human ocular accommodation is mediated by either the red-green or yellow-blue colour channels. Our goal was to determine experimentally whether the red-green channel by itself could influence the accommodative response. 2. To find out, we isolated the red-green channel through chromatic bandpass filtering and measured accommodation under dynamic and static conditions. The effect of this filtering was to modulate the red-green channel without disturbing either the yellow-blue or luminance channels. 3. Accommodative gain (ratio of response to stimulus amplitude) declined monotonically with decreasing bandwidth under dynamic conditions. Because the outputs of both the luminance and yellow-blue colour channels did not vary with bandwidth, the only explanation is that the red-green opponent process was responsible for the effect. 4. Under static conditions, however, accommodation was independent of bandwidth. This may be attributable to the decreased sensitivity to chromatic contrast that occurs at low temporal frequencies.

Accommodation, Ocular↗

PubMed1995

The effect of perceived distance on accommodation under binocular steady-state conditions.

Spatiotopic cues, such as perceived distance, have little effect on accommodation unless blur has been reduced or eliminated. We investigated the effect of perceived distance on accommodation under binocular steady-state conditions, about which little is known. Blur was reduced but not eliminated by using a stimulus with a moderately low luminance. Accommodation was measured under two conditions: (1) when cues from perceived distance, blur, and convergence were aligned; and (2) when perceived distance was opposed by both blur and convergence. We found a significant difference in accommodation between the two conditions, which we attribute to perceived distance.

Accommodation, Ocular↗

PubMed1995

Optical compensation for night myopia based on dark focus and CA/C ratio.

PURPOSE: To determine whether individual differences in dark focus and convergence accommodation to convergence (CA/C) ratio can be used to prescribe the best optical correction for night myopia. METHODS: The best correction for night myopia was obtained by measuring visual acuity and contrast sensitivity across a range of lens powers and luminances. Dark focus was measured with an infrared optometer, and CA/C ratio was measured with an infrared optometer and eyetracker. Only young subjects were used (mean age = 25.4 years). RESULTS: Optimal lens power was significantly correlated with dark focus, regardless of CA/C ratio. However, the slope of the regression line relating lens power to dark focus was steeper for subjects with CA/C ratios less than 0.4 diopters/meter angle (D/MA, n = 7) than for subjects with CA/C ratios greater than 0.4 D/MA (n = 9). The mean CA/C ratio for the entire sample (n = 16) was 0.59 D/MA. The mean optimal lens power and dark focus were -0.79 and 0.74 D, respectively, for the low CA/C group, and -0.60 and 0.91 D, respectively, for the high CA/C group. CONCLUSIONS: Visual performance in night myopia can be optimized by taking into account intersubject differences in dark focus and CA/C ratio. Best visual performance was found with a lens roughly equaling the full dark focus for subjects with low CA/C ratios and half the dark focus for subjects with high CA/C ratios.

Accommodation, Ocular↗

PubMed1994

The effect of knowledge of object distance on accommodation during instrument viewing.

We present data in which instrument accommodation was measured while knowledge of object distance was varied. The accommodative feedback loop was 'semiopen'--an intermediate state between the closed-loop and open-loop conditions of previous experiments. The semiopen-loop situation mimicked the degraded-image conditions which are frequently encountered during instrument viewing. The results show that for some subjects knowledge of object distance is a more powerful cue for instrument accommodation than is the optical distance of the object; however, for the majority of subjects this is not the case. We also found that subjects whose accommodation is influenced by knowledge of object distance tend to have a more proximal dark focus than those whose accommodation is independent of knowledge of object distance. We propose that the Mandelbaum effect, in which involuntary accommodation occurs when a transparency is superimposed between the observer and the object of regard, could account for the accommodative behavior of all subjects. However, the Mandelbaum effect would have to be interpreted more broadly than before. In the broader interpretation, the transparency could be cognitive (ie known distance) rather than physical.

Accommodation, Ocular↗

PubMed1994

Relationship among accommodation, focus, and resolution with optical instruments.

We studied instrument myopia as an overall effect consisting of interactions among the observer's accommodation and resolution during instrument viewing and the manner in which the instrument is focused. Previous investigations of instrument myopia generally have been limited to only one of these variables. Our results are consistent with the dark-focus-bias theory of accommodation, which holds that accommodation tends to seek its resting point, or dark focus, whether or not the observer is viewing through an optical instrument. The amount of accommodation measured during instrument viewing was found to be dependent on instrument design features that modulate the bias of accommodation for the dark focus.

Accommodation, Ocular↗

PubMed1994

Focus adjustment effects on visual acuity and oculomotor balance with aviator night vision displays.

Sixteen U.S. Army aviators, who were given training on focus adjustment technique with aviator night vision goggles (NVG), showed an improvement in visual acuity with focus adjustment compared to a fixed infinity focus control. The long-term effect of focus adjustment on vision was not measured; however, adjustment accuracy was found to be generally within acceptable limits based on computer modeling and available physiologic data. Fixed focus eyepieces that are set to a low minus power may partially compensate for instrument myopia, but they may not optimize visual acuity to the extent that adjustable focus eyepieces do. Eyepiece adjustment proficiency with present night vision devices can be improved through training that emphasizes focusing to the least possible minus dioptric power. Future night vision displays can minimize focus misadjustment by providing a tactile zero marking, a limited dioptric adjustment range, and a focusing knob capable of finer adjustment than is available with current NVG's.

Aerospace Medicine↗

PubMed1992

The effects of pyridostigmine bromide on visual performance.

The effects of pyridostigmine bromide (PB) on selected visual functions were measured on four healthy aviator candidates. Following a pretreatment day during which baseline measurements were completed, subjects were administered currently recommended doses (30 mg, t.i.d.) of PB for 3 d during which their visual functions were assessed using a repeated measures design. Spatial resolution ability was evaluated with high and low contrast visual acuity charts and contrast sensitivity charts at three luminance levels. Dark adaptation was evaluated by measuring visual thresholds for 40 min after a standardized retinal photopigment bleach. Also, refractive error and several oculomotor functions (lateral phoria, fusional vergence, accommodative amplitude, and pupil size) were measured. On days that the subjects ingested PB, only refractive error and pupil diameter were significantly different, and these only minimally. We conclude that the use of PB at doctrinal doses will not significantly compromise an aviator's visual ability.

Accommodation, Ocular↗

PubMed1987

The effects of optical vergence, contrast, and luminance on the accommodative response to spatially bandpass filtered targets.

The steady-state monocular accommodative response was measured using a new type of stimulus, a spatially bandpass filtered luminance distribution known as a difference of Gaussian or DOG. The independent variables were: spatial frequency, optical vergence, contrast, and mean luminance. High-contrast DOGs of varying peak spatial frequencies were presented monocularly over a range of target optical vergences. In addition, DOGs were presented at a fixed dioptric vergence (1) over a range of contrasts with a constant mean luminance, and (2) over a range of mean luminances with a constant contrast. The magnitude of the accommodative error was found to depend on the total stimulus condition, i.e. the particular level of each independent variable. Changes in retinal-image contrast from the background (zero-defocus) level, which were brought about by the errors of accommodation under the various stimulus conditions, were mathematically determined and were used to calculate the "accommodative (contrast discrimination) Weber fractions". Comparisons between accommodative and psychophysical contrast discrimination data revealed that there are limits to the degree of shared processing of contrast discrimination information between accommodation and visual perception.

Accommodation, Ocular↗

PubMed1986

A computational model of the error detector of human visual accommodation.

A mathematical model is proposed for the error detector of the human visual accommodative system. The model supposes that the accommodative error detector derives both the direction and the magnitude of the accommodative error from naturally-occurring oscillations of the lens and their effects on retinal-image contrast. Differential operators take the first derivatives of two time varying functions: lens power and retinal-image contrast. Directional information is obtained by comparing the signs of these two derivatives and magnitude information is obtained by comparing their amplitudes.

Accommodation, Ocular↗

PubMed1986

Dynamic interactions between accommodation and convergence are velocity sensitive.

Aftereffects of accommodation and vergence occur following approximately 1 min of adaptation to lenses and prisms respectively. This observation can be interpreted to mean that accommodation and vergence responses have phasic and tonic components. We have examined the role that these proposed subcomponents play in mutual interactions between accommodation and vergence. Both accommodative vergence (AV) and vergence accommodation (VA) were unresponsive to low temporal frequency variations (less than 0.1 Hz) in defocus and disparity respectively. However, both AV and VA were responsive to higher temporal frequency stimuli (up to 0.5 Hz). When negative feedback to the stimulated system was cancelled electronically, both AV and VA become responsive to low temporal frequency stimuli. The ratio or gain of accommodative vergence/accommodation (AC/A) and vergence accommodation/vergence (CA/C) increased nonlinearly with stimulus amplitude. Vergence aftereffects resulted from stimulation of AV and accommodative aftereffects resulted from stimulation of VA. These results are interpreted to mean there could be a complementary relationship between the amplitude of the AC/A ratio and proposed tonic adaptation of accommodation, and between the amplitude of the CA/C ratio and proposed tonic adaptation of vergence. A low saturation limit or stimulus window for tonic adaptation may account for the amplitude dependent nonlinearities of the AC/A and CA/C ratios.

Accommodation, Ocular↗

PubMed1986

The accommodative response to subthreshold blur and to perceptual fading during the Troxler phenomenon.

A study is reported which shows that accommodation can be stimulated by a blur stimulus which is below the threshold for visual perception. It is also shown that perceptual fading of the target, caused by stabilization of the retinal image (Troxler phenomenon), can eliminate the accommodative response causing it to default to its resting level. The first finding suggests a way in which the visual system can filter the percept of blur out of our conscious awareness and still effectively utilize the blur as a steady-state error for the accommodative control system. The second finding is consistent with a locus for the Troxler phenomenon in the early afferent part of the visual pathway, ie the retinal ganglion cells.

Accommodation, Ocular↗

PubMed1986

Temporal variations in accommodation during steady-state conditions.

The amplitude of the naturally occurring 2-Hz lens fluctuations at levels of accommodative response ranging from 1 to 4 D were measured in absolute terms (i.e., in diopters) and compared with the overall variability of accommodation (in the time domain) over the same range of accommodative output levels. The amplitude of the 2-Hz component was found to be positively correlated with the mean level of the accommodative response. It was as small as 0.02 D (mean to peak) when the mean response level was 1 D, and it was no larger than 0.1 D (mean to peak) when the mean response level was 4 D. Furthermore, the overall variability of accommodation (in the time domain) was also found to be positively correlated with the mean response level, having its minimum when the accommodative response level was least rather than at the resting focus of accommodation (when neurologic activity is least). This suggests that the observed increase in accommodative noise with dc (response) level is more likely attributed to plant noise than to neurologic (controller) noise.

Accommodation, Ocular↗

PubMed1986

Adaptation of tonic accommodation reduces accommodative lag and is masked in darkness.

Tonic aftereffects of accommodation were investigated under three open loop conditions; Maxwellian view (pin hole pupil), empty field, and darkness. Duration of accommodative aftereffects, following 5 sec of adaptation, ranged within several seconds for all three open loop conditions. After adapting for 1 min to a 2-D stimulus, the duration of accommodative aftereffects increased by approximately an order of magnitude for the Maxwellian view and empty field conditions; however, they increased modestly for the dark condition. Tonic aftereffects of accommodation were also stimulated by convergence with base out prism or divergence with base in prism while the accommodative loop was opened. The ratio of open-loop aftereffect amplitude/closed-loop accommodative response to lenses ranged from 0.95 to 0.48, and it became smaller when the accommodative stimulus approached the amplitude of accommodation. The rapid decay of tonic aftereffects of accommodation in darkness was reversable when a visible stimulus was presented while the accommodative loop was opened by Maxwellian view or empty field. Hence, darkness only masked aftereffects that were manifest primarily when there was light stimulation of the retina. The amplitudes of accommodative aftereffects were reciprocally related to the amplitude of the lag of accommodation. During a 2-min adaptation period, accommodative response increased gradually for 30-60 sec as it approached the amplitude of the accommodative stimulus. This resulted in a reduction of accommodative error or lag. Opening the accommodative loop before or after the lag was reduced resulted in a small or large aftereffect of accommodation respectively. Accommodative aftereffects appear to be related to accommodative lag as prism adaptation is related to fixation disparity.

Accommodation, Ocular↗

PubMed1986

The dissociability of accommodation from vergence in the dark.

A special case was set up in which accommodation and vergence were totally deprived of their stimuli, in order to determine whether their responses remained coordinated or whether they became dissociated. A dynamic, infrared optometer and infrared, eye-movement sensors were used simultaneously to monitor accommodation and vergence respectively in the dark. Responses were recorded continuously over an extended time course, and the two systems were found to behave essentially independently of each other in darkness. However, a statistical cross-correlation analysis revealed that, despite the readily apparent dissociation of the two responses in the time domain, there was a correlation between them in the frequency domain. The time-domain dissociation suggests that the tonic controllers of both motor systems are positioned downstream from the crosslinks that join them (accommodative vergence and vergence accommodation). The frequency-domain correlation suggests that the crosslinks are not completely dormant in the dark, but that they carry low-amplitude noise, at a more or less fixed temporal frequency, from one motor system to the other.

Accommodation, Ocular↗

PubMed1980

Age-related changes in the cornea.

The optometrist observes and must distinguish between physiologic and pathologic changes in the cornea. This paper deals with normal, physiologic age-related corneal changes. Age-related changes are those that occur over the entire life span, not merely in senescence. They include changes in corneal curvature, toricity, diameter, asphericity, thickness, tactile sensitivity, and a variety of senescent changes. The senescent changes of the cornea include a reduction in epithelial luster, dellen, microcystic dystrophy, the Hudson-Stahli line, crocodile shagreen, the white limbus girdle of Vogt, an increase in stromal relucency, cornea farinata, arcus senilis, Hassall-Henle warts, guttata, a reduction in endothelial cell count and Krukenberg's spindle.

Adolescent↗