Mapping the neglected space: gradients of detection revealed by virtual reality

Abstract

Background: Spatial neglect affects perception along different dimensions. However, there is limited availability of 3-dimensional (3D) methods that fully map out a patient's volume of deficit, although this could guide clinical management.

Objective: To test whether patients with neglect exhibit simple contralesional versus complex perceptual deficits and whether deficits are best described using Cartesian (rectangular) or polar coordinates.

Methods: Seventeen right-hemisphere persons with stroke (8 with a history of neglect) and 9 healthy controls were exposed to a 3D virtual environment. Targets placed in a dense array appeared one at a time in various locations.

Results: When tested using rectangular array of targets, subjects in the neglect group exhibited complex asymmetries across several dimensions in both reaction time and target detection rates. Paper-and-pencil tests only detected neglect in 4 of 8 of these patients. When tested using polar array of targets, 2 patients who initially appeared to perform poorly in both left and near space only showed a simple left-side asymmetry that depended almost entirely on the angle from the sagittal plane. A third patient exhibited left neglect irrespective of the arrangements of targets used. An idealized model with pure dependence on the polar angle demonstrated how such deficits could be misconstrued as near neglect if one uses a rectangular array.

Conclusions: Such deficits may be poorly detected by paper-and-pencil tests and even by computerized tests that use regular screens. Assessments that incorporate 3D arrangements of targets enable precise mapping of deficient areas and detect subtle forms of neglect whose identification may be relevant to treatment strategies.

Figure 1.

Apparatus and 3-dimensional (3D) view of stimuli and results in experiment 1. (A) Screenshot of an individual performing in the VRROOM (Virtual Reality and Robotics Optical Operations Machine) system. (B) Spatial arrangement of all possible virtual targets appearing within the 3D virtual environment. Note that targets appeared one at a time within the scene during the experiment. The visual targets were static. (C) Mean reaction times and (D) percentage of correct detection are represented as spheres for a representative healthy subject and a neglect patient (left and right panels, respectively). The sphere diameters are proportional to the mean reaction time value and the percentage of correct detection for each target location, (ie, larger spheres represent longer mean RT and higher percent of correct detection).

Figure 2.

Data collapsed across horizontal, radial, and vertical spatial dimensions (left, middle, and right panels, respectively). (A) Mean reaction times in seconds (±SE) and (B) percentage of correct detection (±SE). Data are presented separately for 8 neglect patients and as mean values for 9 control stroke patients and 9 healthy controls.

Figure 3.

An illustration of stimuli and results in experiment 2. (A) The relationship between the target location (black circle) and the angle in the field of view with respect to the subject’s midline (the angle from the sagittal plane). Note how the target located closer to the subject’s body lay on a wider angle. (B) An illustration of a subject sitting in front of the VRROOM (Virtual Reality and Robotics Optical Operations Machine) system, and the spatial arrangement of all possible virtual targets appearing within the 3-dimensional (3D) scene (using a polar grid of targets). (C) Mean reaction times and (D) percentage of correct detection are represented as 2D spheres (diameters are proportional to the calculated value as in Figure 1) for patient N+ 1. (E) Mean reaction times and (F) percentage of correct detection calculated for patient N+ 7.

Figure 4.

Predicted reaction times for each target location in the “theta-patient model,” presented according to θ, r, x, and y using (A) Cartesian grid (experiment 1) and (B) polar grid (experiment 2). Note that upper right panel (for x-coordinate) differs in scale from the corresponding panel in A.