Egocentric and allocentric alignment tasks are affected by otolith input

Gravicentric visual alignments become less precise when the head is roll-tilted relative to gravity, which is most likely due to decreasing otolith sensitivity. To align a luminous line with the perceived gravity vector (gravicentric task) or the perceived body-longitudinal axis (egocentric task), the roll orientation of the line on the retina and the torsional position of the eyes relative to the head must be integrated to obtain the line orientation relative to the head. Whether otolith input contributes to egocentric tasks and whether the modulation of variability is restricted to vision-dependent paradigms is unknown. In nine subjects we compared precision and accuracy of gravicentric and egocentric alignments in various roll positions (upright, 45°, and 75° right-ear down) using a luminous line (visual paradigm) in darkness. Trial-to-trial variability doubled for both egocentric and gravicentric alignments when roll-tilted. Two mechanisms might explain the roll-angle–dependent modulation in egocentric tasks: 1) Modulating variability in estimated ocular torsion, which reflects the roll-dependent precision of otolith signals, affects the precision of estimating the line orientation relative to the head; this hypothesis predicts that variability modulation is restricted to vision-dependent alignments. 2) Estimated body-longitudinal reflects the roll-dependent variability of perceived earth-vertical. Gravicentric cues are thereby integrated regardless of the task's reference frame. To test the two hypotheses the visual paradigm was repeated using a rod instead (haptic paradigm). As with the visual paradigm, precision significantly decreased with increasing head roll for both tasks. These findings propose that the CNS integrates input coded in a gravicentric frame to solve egocentric tasks. In analogy to gravicentric tasks, where trial-to-trial variability is mainly influenced by the properties of the otolith afferents, egocentric tasks may also integrate otolith input. Such a shared mechanism for both paradigms and frames of reference is supported by the significantly correlated trial-to-trial variabilities.

Effects of Tilt Adaptation on the Direction of Voluntary Saccades

Records were taken of the horizontal and vertical amplitudes of eye movements 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-movement paths, and corresponding paths were compared before and after exposure in a hallway to a prism-induced clockwise tilt of 30°. 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° tilt showed no change on either task. Thus, tilt adaptation is capable of altering the direction of volitional eye movements.

Bender Gestalt Test Distortions in Hemiplegia

Drawings of Bender Gestalt Test figures by 16 left hemiplegics and 22 right hemiplegics were compared with those of 11 unilateral amputees who drew the figures with both their right hand and their left hand. The LH group showed systematic tilt in their drawings in a counterclockwise direction in both the vertical and horizontal planes as well as disturbances in the integration of vertical and horizontal planes. When the hemiplegic groups were combined, deviations on the BGT were found to be associated with deviations in judging a luminous line in a dark room. The findings suggest a rationale for some kinds of disturbances on the BGT. In addition, the LH group had significantly more rotations, omissions, and additions.

On the formation of multiple images in the normal eye

It is well known that a small bright object for which the eye is not accommodated often presents a multiform appearance, the number of separate images perceived varying in different cases from about six to fifteen. In Helmholtz’s ‘Physiological Optics,’ drawings are given illustrative of the phenomena exhibited by a luminous point when the conjugate focus is situated a little in front of or a little behind the retina. A narrow luminous line such as that formed when a spectroscope slit is held before a flame or other bright background may become similarly multiplied.

3. On some Phenomena of Indistinct Vision

If an exceedingly small luminous object be placed in front of the eye, but out of the range of distinct vision, it appears to be a roundish bright disc traversed by various markings, and most of these markings are found to be permanent for the same eye, but different for the two eyes. The subject of the present paper is a modification of this appearance.Instead of a luminous point let us take a very narrow luminous line, such as a small slit in the window-shutter, or the reflection of the light of a fire from a polished metallic rod, and let us put this out of focus by means of a pair of convex spectacles. On regarding the luminous line, without any attempt to adjust the eye to distinct vision, we perceive a long luminous band, the breadth of which varies with the distance of the line. When the luminous line is brought into focus, the apparent breadth becomes zero; it increases as the line is moved beyond or within the distance for distinct vision.