Biomedical subjects

E W Graf

Publications and source records attributed to E W Graf.

3 recordsLinked to original sources

PubMed2001

Plasticity of convergence-dependent variations of cyclovergence with vertical gaze.

Binocular alignment of foveal images is facilitated by cross-couplings of vergence eye movements with distance and direction of gaze. These couplings reduce horizontal, vertical and cyclodisparities at the fovea without using feedback from retinal image disparity. Horizontal vergence is coupled with accommodation. Vertical vergence that aligns tertiary targets in asymmetric convergence is thought to be coupled with convergence and horizontal gaze. Cyclovergence aligns the horizontal retinal meridians during gaze elevation in symmetrical convergence and is coupled with convergence and vertical gaze. The latter vergence-dependent changes of cyclovergence have been described in terms of the orientation of Listing's plane and have been referred to as the binocular extension of Listing's law. Can these couplings be modified? Plasticity has been demonstrated previously for two of the three dimensions of vergence (horizontal and vertical). The current study demonstrates that convergence-dependent changes of the orientation of Listing's plane can be adapted to either exaggerate or to reduce the cyclovergence that normally facilitates alignment of the horizontal meridians of the retinas with one another during gaze elevation in symmetrical convergence. The adaptability of cyclovergence demonstrates a neural mechanism that, in conjunction with the passive forces determined by biomechanical properties of the orbit, could play an active role in implementing Listing's extended law and provide a means for calibrating binocular eye alignment in three dimensions.

Accommodation, Ocular↗

PubMed2001

Adaptation of torsional eye alignment in relation to smooth pursuit and saccades.

The long-term fusion of vertical or horizontal disparities by vergence eye movements is known to evoke persistent changes in vertical and horizontal eye alignment. Adaptive changes in response to torsional disparities have not been well studied. Torsional eye position was measured binocularly with a video system before and after 90 min training periods in which subjects attempted to fuse cyclodisparities. Subjects trained with either a single cyclodisparity presented at a single vertical eye position or with cyclodisparities that varied smoothly from an incyclodisparity to an excyclodisparity as a function of either vertical or horizontal eye position. All five subjects showed persistent changes in binocular torsional eye alignment following both types of training. Incyclodisparities were more easily fused during training and the training aftereffect was greater in that direction. The training aftereffect was observed in relation to both saccades and smooth pursuit under both open-loop and closed-loop viewing conditions. During saccades, the dynamics of the cyclovergence training aftereffect more closely resembled the dynamics of cyclofusional movements than the dynamics of the saccades with which they were associated.

Adaptation, Physiological↗

PubMed2001

Fatigue reduces tonic accommodation.

Ocular accommodation adopts a mean baseline response level of approximately 1.0 D in the absence of blur feedback (open-loop state). This baseline or tonic accommodation (TA) can be elevated following a sustained monocular accommodative response to a dioptric stimulus (lens adaptation) that exceeds the baseline open-loop level of TA. The accommodative response to the lens persists in the open-loop state (accommodative hysteresis), and eventually decays to a stable end-point. Interestingly, if the baseline TA is high, the monocularly adapted accommodative state can decay to an end-point that is below the initial pre-adapted baseline level of the TA (counter-adaptive response) (McBrien, N.A. and Millodot, M., (1988). Differences in adaptation of TA with refractive state. Invest. Ophthalmol. Vis. Sci., 29, 460-469). We have investigated the possible contribution of accommodation fatigue to the counter-adaptive change in baseline TA following sustained accommodation. Two fatigue procedures were used while viewing a target at 66 or 33 cm. In a monocular condition, accommodation was stimulated for 3 min with lens values alternating from -1.5 to +1.5 D at a rate of 0.25 Hz. In the binocular condition, convergence was stimulated with alternating prism values from 9 prism diopters (PD) base-out to 9 PD base-in. Both monocular and binocular tasks resulted in a significant reduction of TA. These results suggest that previously reported reductions of baseline TA following sustained monocular accommodation or binocular convergence could have resulted from fatigue of the accommodative system. Accommodative fatigue could be responsible for the lower values of TA observed in counter-adaptive responses to sustained accommodative or convergence effort.

Accommodation, Ocular↗