GB2375821A

GB2375821A - Automatic vision testing system for children

Info

Publication number: GB2375821A
Application number: GB0110386A
Authority: GB
Inventors: Peter Hamilton Galloway
Original assignee: Individual
Current assignee: Individual
Priority date: 2001-04-27
Filing date: 2001-04-27
Publication date: 2002-11-27
Also published as: GB0110386D0

Abstract

A visual acuity testing system includes a computer 1 with a high-resolution display 2 linked to a video camera 3. Vertical gratings of varying width are presented either side of a central target image or video, coincident with an iso-luminant image opposite the grating. A digital video camera adjacent to the display is employed to track eye position using software algorithms that relate eye movement to the position of the vertical grating. Software analysis adjusts for head movement, blinks and corneal reflexes. The system eliminates the need to lift acuity cards manually, and provides an automated and objective method of vision assessment in pre-verbal children and those unable to communicate in standard visual acuity tests.

Description

AUTOMATED VISION TESTING SYSTEM FOR CHILDREN Technical field This invention relates to a vision-testing system and in particular to a grating visual acuity testing device for pre-verbal children, using a method that automatically calculates a visual acuity level by monitoring eye movement simultaneous to visual acuity target presentation.

Background Vision in very young children is difficult to assess, particularly when they are unable to talk. This particularly applies to children under the age of two. The gold standard for assessing vision in this circumstance is termed'forced choice preferential looking', which involves presenting a series of cards a fixed distance in front of the infant or child.

Each card has a small central observation hole behind which an observer monitors eye movement. On one side of the card the infant may view a pattern (known as a grating) consisting of vertical lines of a specific width and separation within a fixed sized frame. On the same face of the card, but on the other side of the observation hole, there is a greycoloured iso-luminant picture with a frame of identical size to the opposite grating. Such an image is required to be equally luminant to the grating stimulus when viewed from a distance so that a subject does not merely avert gaze to a brighter object without resolving detail. Visually aware subjects prefer to look at images with vertical gratings rather than the bland appearance of an iso-luminant image. An eye movement towards the correct image becomes more significant when the subject looks in such a direction correctly on repeated testing, and the probability of this may be quantified.

The problem with this test is that different observers may achieve varying results, and the attention of the child will vary according to the behaviour of the observer. A standardised method of capturing the attention of the child is required, with an automatic observation system.

An object of this invention is to provide an objective means of assessing visual acuity in children, avoiding subjective observer response by automating the assessment process.

Accordingly, this invention describes a system of measuring eyesight using a video camera, a computer with a high-resolution graphics display showing linear gratings, and software algorithms to detect eye movement by processing image data captured on video. This system is directed towards pre-verbal children, or those unable to communicate to perform standard vision tests.

The invention described here does not test visual acuity by evoking nystagmus as described in Patent GB2129963. The gratings displayed are static. Further this vision testing system is not portable, and does not link to a phoropter as described in US patent US4861156 and is not in anyway designed to measure refractive state.

Preferential looking gratings presented using a computer have already been developed (Brovarone et al, Documenta Ophthalmologica 83 : 299-305, 1993), however the present invention describes a fully automated system with a standard grating display using target video and a video camera to track eye movement.

Summary The present invention is an automated vision testing system and method which significantly adds to the efficiency and objectivity of testing the visual acuity of a child. No verbal response is required from the subject being tested. The testing device includes a computer system, a graphics display for displaying gratings and image or video targets, and an attached video camera.

Computer-generated vertical gratings of different spatial frequency are displayed on one side of a central fixation target or video, simultaneous to an iso-luminant target on the other side of the same target or video. The central fixation target is designed to capture and sustain attention towards the graphical display. A video camera linked to the computer permits eye movement analysis with optimised edge-detection software algorithms. The key to this automated device is computer-control, eliminating inter-observer variability, thus better standardising the procedure. The system removes variability in human assessment that affects other manual and computerised methods currently available. The degree of certainty in any sequence of responses is recorded and related to indices of reliability based on head and eye movement.

Brief description of the drawings FIGURE I A is a front view of the vision testing device according to an embodiment of the present invention; FIGURE IB is a side view showing a vision testing device and subject positioning according to an embodiment of the present invention ; FIGURE 2A is a front view showing the vision testing display with linear grating according to an embodiment of the present invention; FIGURE 2B is a front view showing the vision testing display with linear grating of different spatial frequency according to an embodiment of the present invention; and FIGURE 3 is a diagram showing the timeline scheme for the vision testing device according to an embodiment of the invention.

Detailed description of the invention The following description illustrates the general principles of the invention, and does not apply any limitation on the scope of the invention.

Referring to FIG. 1A and FIG. IB, a typical examination begins with positioning the child 11 the desired distance XY from the computer graphic display 2. This distance XY is measured and the computer software drives the presentation of the stimuli 4 at the appropriate spatial frequency. A video camera 3 linked to the computer 1 permits eye movement analysis with optimised edge-detection software algorithms. The video camera 3 is ideally positioned immediately above the front of the graphical display 2. As with standard preferential-looking techniques an occluding patch is positioned over the left or right eye unless binocular acuity is to be assessed. Background illumination is fixed at a standard level.

Computer-generated vertical gratings 4 of different spatial frequency are displayed either side of a central fixation target or video 5, simultaneous to an iso-luminant target 6 on the other side of the same target or video 5. The key to this automated device is computercontrol, eliminating inter-observer variability, and this better standardises the procedure. The system removes variability in human assessment that affects manual and other methods currently available. Several different vertical gratings are displayed in a pseudo-random order at predetermined points during the video presentation and eye movement is recorded at each point, before and after stimulus presentation (FIG. 3).

A high-resolution graphical display may be a monitor (19 inch or 21 inch) or a digital projector, which offers a larger display to attract attention. The viewing distances must be varied accordingly to offer the same spatial frequency.

A web-camera with sufficient resolution (typically 640 X 480 pixels) and sufficient frame acquisition rate (typically 25 frames per second) may be used. Alternatively any video input may be employed, with software appropriately adjusted to correct for any variable timing of simultaneous presentation of video target display and video acquisition.

Various video sequences are employed to achieve maximum sustained concentration from the child. Usually this is a cartoon based on large brightly-drawn characters, such that the images within the video are more easily seen than the largest grating target. Video sequences are standardised so that the different sequences shown do not alter vision assessment. The audio source to accompany the video may be placed directly behind the graphical display for optimal performance. This invention is of particular benefit to deaf children however as no audio is required and audio may be turned off within the software application.

Error-detection for the system includes features to monitor the relative head size from baseline as an indication of distance XY from graphical display 2. The software provides an alert when head position is not optimal and outside the field of view of the camera. In addition, head movements within the field of view of the video camera are monitored and linked to eye tracking software analysis, which must measure eye movement in relation to head movement. When a grating target is presented a typical response from the subject is not just a simple sustained alteration of gaze towards the new target, but repeated flickers of eye movement between the grating 4 and the central target 5. The eye tracking software therefore

is required not merely to measure the position of the eye but make an overall assessment about the relative position of the eye.

The system adjusts for factors that might influence the result. For example infants wearing glasses require an alternative algorithm for measuring eye movement as a more complex analysis is required. Similarly video frames capturing blinks are automatically eliminated from the analysis. Furthermore the system can make an assessment of vision in children with nystagmus, a condition where the eyes involuntarily flick from side to side several times a second.

This approach of vision testing offers improved retest reliability through capturing the sustained attention of the child. A record of the results of each test may be printed out via an attached printer. The results display the maximum grating resolution, an equivalent estimate of LogMAR acuity, the responses for each target presentation, reliability indices including head movement and blink. Measures of concentration including time of sustained front-gaze

to video target without gratings are also included. These results are displayed alongside any Z=l previous assessment.

Claims

CLAIMS 1. A system of measuring eyesight in persons unable to communicate verbally, using a video camera, a computer with a high-resolution graphics display showing linear gratings, and

software algorithms to detect eye movement by processing image data captured on video. zn
2. A system as stated in claim 1 where computer-generated images are presented either side of a graphics display, one image being more visually attractive than the other.
3. A system as stated in claim 1, which grades the visibility of the more attractive image by varying the spatial frequency of the gratings, and monitors for eye movement in the direction of the preferred image.
4. A system as stated in claim 1, which uses a video camera to record the anatomical features of the eye and eyelids so as to determine the position of the eye in relation to the displayed target.
5. A system as stated in claim 1, which presents the images in a logical sequential manner and determines the statistical probability of correctness, and the reliability of response.
6. A system as stated in claim 1, where the software utilises edge detection, binary contrast enhancement, or individual or combined pixel analysis to determine the position of the eye relative to the previous frame.
7. A system as stated in claim 1, which uses computer software to generate an image on the graphics display, forming a central fixation target image or video sequence.
8. A system substantially as herein described and illustrated as in the accompanying drawings.