Depth Perception and Grasp in Central Field Loss

Abstract

Purpose: We set out to determine whether individuals with central field loss benefit from using two eyes to perform a grasping task. Specifically, we tested the hypothesis that this advantage is correlated with coarse stereopsis, in addition to binocular summation indices of visual acuity, contrast sensitivity, and binocular visual field.

Methods: Sixteen participants with macular degeneration and nine age-matched controls placed pegs on a pegboard, while their eye and hand movements were recorded. Importantly, the pegboard was placed near eye height, to minimize the contribution of monocular cues to peg position. All participants performed this task binocularly and monocularly. Before the experiment, we performed microperimetry to determine the profile of field loss in each eye and the locations of eccentric fixation (if applicable). In addition, we measured both acuity and contrast sensitivity monocularly and binocularly, and stereopsis by using both a RanDot test and a custom stereo test.

Results: Peg-placement time was significantly shorter and participants made significantly fewer errors with binocular than with monocular viewing in both the patient and control groups. Among participants with measurable stereopsis, binocular advantage in peg-placement time was significantly correlated with stereoacuity (ρ = -0.78; P = 0.003). In patients without measurable stereopsis, the binocular advantage was related significantly to the overlap in the scotoma between the two eyes (ρ = -0.81; P = 0.032).

Conclusions: The high correlation between grasp performance and stereoacuity indicates that coarse stereopsis may benefit tasks of daily living for individuals with central field loss.

Figure 1

Pegboard setup. (A) Pegboard viewed from above. Black dot shows where peg is placed at the start of each trial. Red and green dots indicate calibration marks for the eye tracker. (B) Pegboard as seen by observer. The observer was seated so that eye position was 10 cm above the pegboard. (C) Participant reaching for the peg. The participant's head is in a chin and forehead rest. At the start of the trial, the participant lifts his hand from the table, picks up the peg and places it in the appropriate hole. Sensors on the thumb and index finger measure grasp and hand position in three dimensions. A table-mounted eye tracker placed under the pegboard measures binocular eye movements.

Figure 2

(A, B) Microperimetry for the right and left eye for P8. The red and green dots mark locations of unseen and seen flashes. The yellow crosses mark the fixation loci during perimetry. (C) Cross-sectional topography map allowing visualization of the foveal pit. (D) Binocular scotoma estimated by overlaying the two eyes' scotoma images, aligned on the fovea (yellow cross). The green crosses show the center of the bivariate contour ellipse of the fixation locus measured during a 10-second fixation assessment, for each eye separately (using the Ghahghaei and Walker method).

Figure 3

(A) Hand velocity data from one observer on a single trial as a function of time during peg placement. Peak velocity and the velocity minima associated with peg pick-up and peg drop-off are clearly marked. Pick-up time is the duration from peak velocity to peg pick-up, and peg placement time is the duration from peg pick-up to peg drop-off. (B) Comparison of hand velocity under monocular and binocular viewing conditions, while placing the same block in the same location.

Figure 4

Average performance indices in the peg-placement task for controls and patients, under monocular (black) and binocular (gray) viewing conditions. (A–D) Maximum grip aperture, pick-up time, peg-placement time, and errors, respectively. The line within the box marks the median, and the upper and lower bounds of the box mark the upper and lower quartiles, respectively. The whiskers show the range of the data, excluding outliers (see text). Significance levels are marked with asterisks: *P < 0.05, **P < 0.01, ***P < 0.005 (see Table 2).

Figure 5

Average performance indices in the peg-placement task for participants with and without stereo, under monocular (black) and binocular (gray) viewing conditions. (A–D) Maximum grip aperture, pick-up time, peg-placement time, and errors, respectively. The error bars indicate the range of the data, excluding outliers. Significance levels are marked with asterisks: *P < 0.05, **P < 0.01, ***P < 0.005 (see Table 3).

Figure 6

The binocular-to-monocular ratio of peg-placement time versus the reciprocal of stereoacuity. Patient data are marked with a red circle. Patients and control subjects have data that are well described by the same regression line.

Figure 7

The binocular-to-monocular ratio of peg-placement time versus the binocular-to-monocular scotoma ratio for patients without stereo (A) and with stereo (B).