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Biomedical subjects

S M Ebenholtz

Publications and source records attributed to S M Ebenholtz.

At least 19 recordsLinked to original sources
PubMed1995

Absence of adaptive plasticity after voluntary vergence and accommodation.

Subjects maintained their eyes crossed (verged) for a period of 8 min in darkness with monitoring provided by an infrared video system. Changes in resting vergence (RV) and resting focus (RF) were examined. Results showed: (i) visual stimulation was not necessary for adaptation of either RV or RF, but (ii) these purely motor effects were significantly smaller and more dissipative than those attributable to visually driven adaptation, and (iii) voluntary vergence amplitude was negatively correlated with pupil size. Assuming that voluntary vergence is driven by accommodation, then the voluntary signal must enter the oculomotor control system prior to the cross links between channels, but beyond the site of the visually driven adaptive elements.

Accommodation, Ocular↗
PubMed1995

Effects of optical pitch on oculomotor control and the perception of target elevation.

In two experiments, we used an ISCAN infrared video system to examine the influence of a pitched visual array on gaze elevation and on judgments of visually perceived eye level. In Experiment 1, subjects attempted to direct their gaze to a relaxed or to a horizontal orientation while they were seated in a room whose walls were pitched at various angles with respect to gravity. Gaze elevation was biased in the direction in which the room was pitched. In Experiment 2, subjects looked into a small box that was pitched at various angles while they attempted simply to direct their gaze alone, or to direct their gaze and place a visual target at their apparent horizon. Both gaze elevation and target settings varied systematically with the pitch orientation of the box. Our results suggest that under these conditions, an optostatic response, of which the subject is unaware, is responsible for the changes in both gaze elevation and judgments of target elevation.

Adolescent↗
PubMed1994

The possible role of nystagmus in motion sickness: a hypothesis.

An alternative hypothesis to that provided by conflict theory is formulated to account for motion sickness. The new approach is predicted on the oculocardiac reflex (i.e., bradycardia produced by extra-ocular muscle traction) (18) and empirical evidence that retrobulbar anesthesia significantly lowers the incidence of emesis after strabismus surgery (14). Eye muscle traction is presumed to elicit afferent signals that ultimately stimulate the vagus nerve (15). The same neuromotor sequence is presumed to occur during reflexive eye movements under vestibular control and during more complicated combinations of reflexive and voluntary eye movements. It is proposed that the blocking of afferent signals from extra-ocular muscle in an otherwise intact vestibulo-ocular system will eliminate the signs and symptoms of motion sickness normally produced in a provocative environment.

Eye Movements↗
PubMed1992

Accommodative hysteresis as a function of target--dark focus separation.

The concept of dark focus (DF) as fulcrum for accommodative effort was examined by inducing adaptation of tonic accommodation with the target at varying dioptric distances from the DF. Mean pre- to post-changes in DF at the initial post fixation test essentially were linear (r2 = 0.975), with zero intercept, with respect to the target-DF separation, as were the data of most individual subjects. Target-DF distance thus is a major determinant of adaptation of tonic accommodation. These results are consistent with the interpretation of DF as a zero reference point or fulcrum about which accommodative effort varies.

Accommodation, Ocular↗
PubMed1991

Accommodative hysteresis. Fundamental asymmetry in decay rate after near and far focusing.

The resting level of accommodation, or dark focus (DF), was compared before and after subjects maintained 8-min of clear focus on targets either at the far point (FP) of accommodation or at an equivalent dioptric distance on the near side of the DF. Although both conditions showed significant shifts in tonic accommodation in the direction of their respective fixation targets, only after near fixation was the shift maintained in darkness for 24-min postfixation. After FP viewing the aftereffect decayed in darkness with a significant linear trend (P less than 0.05). Thus, maintaining focus on a near target is more likely to induce an enduring adaptive shift in tonic accommodation than is far-target viewing, for reasons associated with the systems controlling far and near accommodation, respectively. Additional analyses provided further evidence that the degree of separation of the target from the DF is a highly significant factor controlling individual differences in initial adaptation levels.

Accommodation, Ocular↗
PubMed1989

Effects of peripheral circular contours on dynamic spatial orientation.

The rod-and-frame effect (RFE) was investigated with the use of a frame that oscillated about an axis at its center at five different frequencies, ranging from .013 to .213 Hz. The resultant RFE shifted continuously with the roll motion of the frame, and it was significantly larger at the lowest frequency (.013 Hz) than under comparable static conditions. The dynamic RFE was lowest at the higher oscillation frequencies. Oscillatory roll vection--apparent self-motion--was reported by 3 of the 9 subjects when the frame was oscillating at its highest frequency (.213 Hz). The subjects yielded large increases in the RFE during the sessions with reports of vection. Surrounding the kinetic frame with a circular contour eliminated all reports of vection and significantly interacted with frequency to reduce the RFE--but only at low frequencies. The reduction amounted to 21.2% averaged over all 9 subjects at the three lowest frequencies. A surrounding contour, therefore, suppressed low-frequency kinetic visual orientation information that might otherwise have produced larger changes in apparent self-orientation and perceived vertical. Vection-sensitive subjects differed from nonvection subjects by exhibiting (1) a high-frequency fall-off in real-motion gain, (2) a high-frequency enhancement in illusory-motion gain, and (3) only a small and nonsignificant increase in illusory-movement phase lag with increases in frequency.

Adolescent↗
PubMed1988

Long-term endurance of adaptive shifts in tonic accommodation.

After an 8 min inducing period of sustained focusing on a target at the near point (NP) of accommodation, tonic accommodation was monitored at 8 min intervals over a 48 min period in darkness. A laser optometer provided evidence for asymptotically increasing tonic levels. Accordingly, eyes closed in darkness (EC) was not a condition for the relaxation or dissipation of enhanced tonic levels. Furthermore, refixating the NP target for 6 min, 24 min after the initial focusing period, provided no significant enhancement of tonic accommodation over EC. On the other hand, focusing at far point (FP) or walking within a lighted hallway (WH) significantly reduced the tonic after-effect to pre-adaptative levels, but only transiently. Tonic after-effects induced by NP focusing thus appear to be remarkably resistant either to long-term attenuation or enhancement. A model based on the decay of accumulated sympathetic and parasympathetic neurotransmitters was developed to account for the contrasting fact that focusing at near targets for periods up to about 2-3 min as well as sustained focusing at far targets tends to yield complete decay of tonic after-effects.

Accommodation, Ocular↗
PubMed1987

Accommodative hysteresis: influence on closed loop measures of far point and near point.

The study examined accommodative hysteresis under closed loop viewing conditions. After sustained focus for 8 min at either the far-point (FP) or near-point (NP) of accommodation, subjects exhibited hysteresis-like shifts in tonic accommodation, ie, dark focus (DF), in the direction of the fixation target. Measurement of NP and FP before and after the focusing period showed that only the position of the NP was significantly influenced, in a direction matching that of the DF. The NP shifted outward by 0.37 diopters (D) and inward by 0.62 D after sustained FP and NP focusing respectively. Control conditions in which subjects focused on a target at the dioptric distance of the DF or were exposed to an 8 min dark interval showed no changes in either NP, FP or DF. The ready susceptibility of the NP to hysteresis effects suggests a potential contributing role of these phenomena to near-work-induced myopia by way of increased accommodative amplitudes with attendant increases in scleral and/or choroidal stress.

Accommodation, Ocular↗
PubMed1985

Accommodative hysteresis: relation to resting focus.

Seventeen subjects sustained focus for up to 8 min on targets placed either at the nearpoint (NP) or farpoint (FP) of accommodation. This was repeated in separate sessions from three to six times at each target distance and pre- to postexposure changes in dark focus (DF), i.e., accommodative hysteresis, were recorded. For eight subjects the probability (P) of exhibiting hysteresis effects was greater than or equal to 0.50 on both types of sessions. Five additional subjects showed P(NP) greater than or equal to 0.66 and P(FP) less than or equal to 0.33, whereas 4 subjects showed a reversed preference with P(NP) less than or equal to 0.17 and P(FP) greater than or equal to 0.83. Thus, strong individual differences emerged with respect to susceptibility to accommodative hysteresis. Linear regression analyses were performed on the individual data of the 13 subjects in condition NP and 12 in condition FP who exhibited hysteresis effects on greater than or equal to 50% of their sessions. Relative magnitudes of accommodative hysteresis resulting either from sustained NP or FP focusing were negatively related to the DF level measured directly before the period of sustained focus. Thus, for example, a relatively near initial DF correlated with a small inward shift of the DF after NP focusing, but with a large outward shift after FP focusing. Only in the FP condition was there evidence for a positive regression of hysteresis effects on the dioptric distance separating the sustained focus target and the initial DF level, viz., the less the separation, the smaller the hysteresis effect.(ABSTRACT TRUNCATED AT 250 WORDS)

Accommodation, Ocular↗
PubMed1983

Accommodative hysteresis: a precursor for induced myopia?

After an 8-min fixation period, the dark focus of accommodation of 12 emmetropes either increased by -0.34 diopters (D) or relaxed by 0.21D depending upon whether the fixation target was at the near point or far point, respectively. No significant shifts occurred after fixation at the distance of the subject's dark focus. The obtained shifts had long time-constants as measured after 16 min in darkness. Decay was much slower after near point (tau = 123.1 min) rather than far point (tau = 14.4 min) fixation.

Accommodation, Ocular↗
PubMed1983

Peripheral circular contours inhibit the visual orientation control system.

Peripheral visual stimulation with a tilted luminous frame has long been known to influence both body orientation and the apparent tilt of objects. Peripheral visual inputs along with vestibular and somaesthetic components thus constitute a multi-channel perceptual orientation control system. Surrounding the visual induction pattern with a luminous circle reduced the effectiveness of the visual channel to about 23% of maximum, whereas inscribing a circle within the luminous frame was without effect. This finding suggests a simple method for dealing with undesirable visual-vestibular interactions and an explanation for some instances of disorientation.

Female↗
PubMed1982

Directional changes in the vestibular ocular response as a result of adaptation to optical tilt.

Before and after exposure to clockwise optical tilt, subjects' compensatory eye movements were recorded, as the head was oscillated around a vertical axis by the experimenter. An analysis of the horizontal and vertical components of eye movements showed a change in eye movement path in the clockwise direction. This corresponded to similar changes in visual spatial orientation judgments indicative of perceptual adaptation. It was suggested that long-term visual feedback serves to restore the compensatory nature of the eye movements which was altered by the introduction of the prism transformation.

Adaptation, Ocular↗
PubMed1982

Effects of tilt adaptation on the direction of voluntary saccades.

Records were taken of the horizontal and vertical amplitudes of eye moments of subjects instructed to move their eyes back and forth from 12 to 6 and from 9 to 3 o'clock without targets. These records were used to compute the angles of the eye-moment paths, and corresponding paths were compared before and after exposure in a hallway to a prism-induced clockwise tilt of 30 degrees. Perceived orientation was also measured, by having the subjects set a luminous line in the dark to the orientations indicated above. Both tasks yielded significant preexposure-postexposure changes in the direction of tilt, such that after exposure the line was set at a tilt and eye movements were made at an angle clockwise with respect to the preexposure orientation. A control group exposed to 0 degrees tilt showed no change on either task. Thus, tilt adaptation is capable of altering the direction of volitional eye movements.

Adaptation, Ocular↗