WO2012141576A1

WO2012141576A1 - A method for investigating visual field defects

Info

Publication number: WO2012141576A1
Application number: PCT/NL2011/050260
Authority: WO
Inventors: Johannes VAN DER STEEN; Johannes Jacob Mient Pel
Original assignee: Erasmus University Medical Center
Current assignee: Erasmus University Medical Center
Priority date: 2011-04-15
Filing date: 2011-04-15
Publication date: 2012-10-18
Anticipated expiration: 2013-10-15

Abstract

The invention relates to a method for investigating visual field defects of a patient's eye comprising the steps of providing a temporal sequence of distributed visual stimuli; tracing a displacement of the patient's eye in response to the said sequence; calculating visual latency per visual stimulus present in the said sequence. The invention further relates to a system for enabling investigating the visual field defects and a computer program product.

Description

Title: A method for investigating visual field defects FIELD

The invention relates to a method for investigating visual field defects.

The invention further relates to a system for enabling investigating visual field defects.

The invention still further relates to a computer program for enabling investigating visual field defects.

BACKGROUND

Vision plays a prominent role in obtaining information from an environment. Humans continuously update visual information by making orienting eye movements. Eye movements can be voluntary, as during reading, or reflexive in response to a novel feature.

The first step in visual information processing is that the image of the visual world perceived is inverted and focussed by the lens and cornea of the eye onto the retina. The retina is densely covered with photoreceptors that are intrinsically connected via a retinal neural network that locally processes and converts the incoming light into neural signals. These signals converge in the brain via ganglion cells nerve where it leaves the eye. The integrity of the whole system of photoreceptors and neural networks of optic nerve fibres is crucial for processing contrast, colour and motion.

Impairments caused by either retinal (day/night sight, colour blindness) or optical nerve trauma (glaucoma, hemianopsia) have a large impact on the perception of the visual world. A major problem is that loss of function in retina and/or optic nerve develops gradually over the course of many years. Glaucoma is such an irreversible neurodegenerative disease that leads to visual field damage. Glaucoma develops unnoticed for a long time because the brain fills in the eventual gaps of the missing parts in the visual field.

Worldwide, glaucoma is the second leading cause of blindness. A major cause for concern is that about 80 million people worldwide will be affected with glaucoma by the year 2020. Population-based studies have shown that more than 90% of glaucoma cases in developing countries remain undiagnosed. These numbers are alarmingly high, although even in Western countries 40-60% of the population remains undiagnosed until late stage of the disease. The progression of glaucoma can be slowed or halted when treatment is started at early stages of the disease, but the first symptoms are virtually undetectable without a detailed eye exam.

An embodiment of a method for carrying out a detailed eye exam is known from US 2010/0195051. The method is adapted to use a suitable display for presenting a visual stimulus and an eye tracker for tracking movement of at least one patient's eye to determine its position in three dimensions. By tracking the position of the patient's eye in response to a given visual stimulus on the display, the eye's visual field angle may be determined.

It is a disadvantage of the known method that it may be not always accurate. In addition, it is a disadvantage of the known method that it may be not suitable for determining impediments in the visual field for persons having no symptoms of eye pathology.

SUMMARY OF THE INVENTION

It is an object of the invention to provide an improved method for investigating visual field defects in an eye of a patient. In particular, it is an object of the invention to provide a method capable of investigating visual field defects for a patient having no symptoms of eye pathology.

To the end the method according to the invention comprises the steps of:

- providing a temporal sequence of distributed visual stimuli; tracing a displacement of the patient's eye in response to the said sequence;

calculating visual latency per visual stimulus present in the said sequence.

It is found that by assessing a clinically relevant parameter of visual latency to a given stimulus accurate determination of visual field defects is enabled. The visual latency is defined as a saccadic reaction time to a new appearing visual stimulus that may differ in contrast.

Preferably, stimuli are presented on a display screen, while a suitable monitoring device, such as a per se known eye tracker measures the gaze direction of the eye of the patient. If a shift in gaze corresponds to the location of a presented stimulus, the system indicates that a stimulus has been seen by the patient.

It will be appreciated that the test does not require voluntary effort of the patient. Because eye movement responses are reflexive, this test is intuitive to the patient. It also allows precise measurement of eye movement reaction time or visual latency, which is found to be a reliable feature in glaucoma diagnosis.

For example, the patient may be provided in front of a suitable electronic display, whereby his head is fixed. Then a number of different stimuli is provided in a visual field and the displacement of the eye is tracked and recorded. Visual latency is then calculated based on the measured velocity of the eye when following a visual stimulus. More details on calculation of the visual latency will be given with reference to Figure 2a, 2b and 2c.

Preferably, the visual stimuli are provided in a randomized way in the selected visual field. More preferably, the contrast of the visual stimuli is randomized as well. It is found to be practical to select a visual field of +- 30 degrees in horizontal direction and +- 20 degrees in vertical direction from the eye's centre. In an embodiment of a method according to the invention data on the displacement of the patient's eye are used for determining the visual latency.

This is found to be practical for minimizing signal noise, such as minor arbitrary eye movements, on the outcome of the procedure.

In a further embodiment of the method according to the invention the visual stimuli are classified in accordance with a comparison between an actual measured eye velocity and a threshold value set for the measured eye velocity.

It is found to be feasible to classify the given stimuli a-posteriori as

'seen' or 'not seen" based on analysis of the collected visual latency data. This embodiment will be discussed in more detail with reference to Figure 2b.

Next, it is found to be advantageous to determine a relation between the calculated visual latency per stimulus and the coordinates of the stimulus. Such analysis enables determination of the blind spots in the visual field even for the symptom-less patient. Preferably, a graph is generated representing the relation between the calculated visual latency per stimulus and the coordinates of the stimulus.

The system for enabling investigating visual field defects is accordance with the invention comprises:

a display arranged for providing a temporal sequence of distributed visual stimuli;

a monitoring unit adapted for tracing a displacement of the patient's eye in response to the said sequence;

- a processor for calculating visual latency per visual stimulus present in the said sequence.

Further advantageous embodiments of the system according to the invention are given in the dependent claims.

A computer program product according to the invention comprises instructions for causing a processor to carry out the steps of the method as is discussed with reference to the foregoing. Those skilled in the art of IT will readily appreciate which particular instructions may be used for embodying the computer program product according to the invention.

These and other aspects of the invention will be discussed in more detail with reference to drawings, wherein like reference numerals refer to like elements. It will be appreciated that the drawings are presents for illustrative purposes and may not be used for limiting the scope of the appended claims.

BRIEF DESCRIPTION OF THE DRAWINGS

Figure 1 presents in a schematic way an embodiment of a sequence of stimuli as presented to the patient.

Figure 2a presents an embodiment of a gaze curve in relation to a peripheral stimulus.

Figure 2b presents an embodiment of a velocity curve corresponding to the gaze curve of Figure la.

Figure 2c presents an embodiment of an actual displacement of the eye according to the gaze curve of Figure 1.

Figure 3 presents an embodiment of a visual contrast map representing a blind-spot.

Figure 4 presents an embodiment of a visual latency map having a blind spot.

Figure 5 presents in a schematic way an embodiment of a system according to an aspect of the invention. DETAILED DESCRIPTION OF THE DRAWINGS

Figure 1 presents in a schematic way an embodiment of a sequence of stimuli as presented to the patient. For the purpose of providing temporal sequence of suitable visual images an electronic display 2 may be used. The electronic display 2 may be a commonly used computer monitor or any other suitable display. Preferably, the user is made conscious of a central point 4 on the display which is selected in correspondence with an AP-PA axis running through the centre of the patient's eye. Usually, the patient will be requested to fix his eye at the central point 4 before gazing towards a stimulus of the type 3a, 3b, ... 3n when such stimulus will be provided. The user may also be requested to return his eye back to the central point upon a certain dwell time fixed on the visual stimulus.

It will be appreciated that visual stimuli 3a, 3b, 3n are provided in sequence in a random way. Randomization of the position of the stimuli over the surface area of the display 2 is advantageous to prevent user's anticipation.

The stimuli 3a, 3b, 3n are of different contrast which respect to the background colour (usually grey or black) of the display 2. Preferably, the stimuli 31, 3b, 3n have shades of grey. The visual stimuli may be provided at a frequency of 1 per 2 seconds, however, this frequency may be varied to prevent the patient from anticipating a stimulus leading to false

measurements.

Figure 2a presents an embodiment of a gaze curve in relation to a peripheral stimulus. The characteristic curve presented in Figure 2a is generated by correlating the position of the patient's eye by a suitable eye tracking module and by setting it off with respect to time lapsed. It will be appreciated that the state of the art eye tracking systems are suitable for implementing the invention. Usually, the eye tracking modules are based on infrared cameras adapted to track a position of the patient's pupil.

Because the ordinate coordinate zero corresponds to the central point on the display, the initial dwelling D2 corresponds to the user glancing at the central point. When a peripheral stimulus is provided, the user's eye will detect it and a reflexive movement towards a second dwell position Dl will be carried out. The transition portion G corresponds to the actual gaze transition of the patient's eye.

Figure 2b presents an embodiment of a velocity curve corresponding to the gaze curve of Figure la. It will be appreciated that the curve of Figure 2b is obtained by differentiating the curve shown in Figure 1 over time. It is found to be advantageous to define a threshold, corresponding to noise seen in the data. Because the graph depicted in the Figure 2b represents the actual eye velocity, the threshold T may be referred to as a threshold eye velocity. In the particular example the threshold eye velocity is about 20 degrees/second. Accordingly, for an eye movement caused by a seen point on a display, a characteristic peak will occur. The visual latency VL is defined as time calculated from the moment of presenting a visual stimulus to the eye until the eye deliberately moves towards the visual stimulus. It is found to be practical to set a suitable window characterizing the saccade (see vertical lines in the Figure 2b). The visual latency will be calculated then as time

corresponding to the start saccade.

Accordingly, the velocity curve as is depicted in Figure 2c may be used as a characteristic of the "seen" event. The peripheral visual stimulus may be scored as "seen". Accordingly, the measurements falling within the vertical frame schematically given by the vertical lines correspond to an event which is seen.

The visual field integrity can be on-line or off-line reconstructed from the locations of visual target labelled as "seen". Figure 3 shows an embodiment of the visual field integrity graph which is obtained on the basis of the minimum contrast of the "seen" targets to which an eye movement was made.

Figure 2c presents an embodiment of an actual displacement of the eye according to the gaze curve of Figure 1. In this figure the gaze direction corresponding to a peripheral stimulus is illustrated. Accordingly, the region C corresponds to a first dwelling, which may be pre-determined at the centre of the display. However, it will be appreciated that any reference eye dwelling may be defined.

When a visual stimulus S is provided, the patient reacts to it in accordance to a reflex and a gaze movement is being made over the trajectory G. In accordance with an aspect of the invention, the coordinates of the "seen" visual stimuli are being recorded for generating a visual field integrity graph, given in Figure 3.

Figure 3 presents an embodiment of a visual contrast map representing a blind-spot. Preferably, the visual field integrity map is scaled in accordance with the angular dimensions of the electronic display, discussed with reference to Figure 1. In the resulting map 40 two black areas Bl and B2 may be identified, which correspond to the areas of reduced responsiveness of the patient.

Figure 4 presents an embodiment of a visual latency map having a blind spot. This plot is obtained by analyzing the results of the Figure 3 together with the visual latency measurements data obtained for all stimuli presented to the patient. However, it will be appreciated that the visual latency is a function which is dependent on the viewing angle, increasing towards the larger deviation angles. Therefore, the measured visual latency data may be corrected for the natural reduction of latency towards the periphery. Such correction may be carried out based on data collected for healthy population, preferably per age group. Alternatively, the measured data may be corrected using the individual patient's profile, which may be deduced from the actual patient's data, such as presented in the Figure 3, for example. Accordingly, an area corresponding to a substantially reduced or absent visual latency (area 41) may be identified. By combining information about contrast sensitivity and visual latency a decision may be drawn about the quality of the visual field of the patient. It will be appreciated that during the measurements according to the invention, the visual stimuli may be provided having a plurality of contrasts and intensities per measurement point. Threshold intensity is defined as a count of seen stimuli having the lowest contrast.

Preferably, stimuli having at least four different contrast values per measurement point are generated during the procedure of investigating the visual field defects according to the method of the invention. Figure 5 presents in a schematic way an embodiment of a system according to an aspect of the invention. The system 50 according to the invention comprises a support 56 for installing the patient's head 52 in an affixed position in front of en electronic display 57. The display 57 is arranged to that a central portion thereof is positioned on an AP-PA axis A running through the centre of the patient's eye 54.

The system 50 according to the invention is adapted to investigate visual field defects per eye. For this purpose the system 50 comprises a suitable eye tracking module 59 adapted to monitor the eye movement and to record corresponding data. The eye tracker 59 may be controlled by a computer 60.

In accordance with an aspect of the invention, the computer 60 comprises a processor 61 arranged for calculating visual latency per visual stimulus present on the display 57 and forming part of a suitable sequence. The processor 61 may also be arranged to control the display for generating stimuli with varied contrast. The processor 61 may be further adapted to collect data from the eye tracker and to combine this data with the data on the coordinates of the generated visual stimuli for calculating visual latency. Based on the visual latency the stimuli may be classified by the processor as being "seen" or "not seen". For the "seen" stimuli the processor may be further adapted to generate a map correlating a position of the stimuli and the visual latency. Preferably, such mapping is carried out for visual stimuli having lowest contrast. It will be appreciated, however, that different embodiments of the system according to the invention may be envisaged. For example, the display 57 and the eye tracker 59 may be part of a virtual reality system, which may be embodied as a wearable helmet. It is found that a wearable helmet may be advantageous as the head movement with respect to the display is substantially avoided.

The operation of the system 50 may be controlled by a computer program product 62 which may be configured to comprise instructions for causing the processor 61 to carry out the steps of providing a temporal sequence of distributed visual stimuli; tracing data representative of a displacement of the patient's eye in response to the said sequence; calculating visual latency per visual stimulus present in the said sequence. The computer program product may further comprise instructions for causing the processor to modulate the visual stimuli, for example to vary the contrast of the stimuli and the time between the stimuli. The computer program product 62 may be further arranged to comprise instructions for causing the processor 61 to determine parameters as discussed with reference to Figures 2a, 2b, 2c and 3.

It will be appreciated that while specific embodiments of the invention have been described above, that the invention may be practiced otherwise than as described. In addition, isolated features discussed with reference to different figures may be combined.

Claims

A method for investigating visual field defects of a patient's eye comprising the steps of:

providing a temporal sequence of distributed visual stimuli; tracing a displacement of the patient's eye in response to the said sequence;

calculating visual latency per visual stimulus present in the said sequence.

The method according to claim 1, wherein visual latency is calculated based on the measured velocity of the eye when following a visual stimulus.

The method according to claim 1 or 2, wherein data on the displacement of the patient's eye are used for determining the visual latency.

The method according to claim 3, wherein the visual stimuli are classified in accordance with a comparison between an actual measured eye velocity and a threshold value set for the measured eye velocity.

The method according to any one of the preceding claims wherein the visual stimuli are provided with varied contrast.

The method according to any one of the preceding claims, wherein the visual stimuli are provided in a visual field having angular dimensions of +- 40 degrees with respect to a centre of the patient's eye.

The method according to any one of the preceding claims, wherein a relation is determined between the calculated visual latency per stimulus and the coordinates of the stimulus.

The method according to claim 7, further comprising the step of generating a graph representing the relation between the calculated visual latency per stimulus and the coordinates of the stimulus.

A system for enabling investigating visual field defects of a patient's eye comprising:

a processor for calculating visual latency per visual stimulus present in the said sequence.

The system according to claim 9, wherein the processor is further arranged to control the display for displaying the said sequence.

The system according to claim 10, wherein the visual stimuli are generated having different contrast.

The system according to any one of the preceding claims 9 - 11, wherein monitoring unit is arranged to calculate a velocity of the patient's eye occurring in response to a visual stimulus, the processor being further arranged to calculate visual latency based on the measured velocity.

The system according to claim 12, wherein the processor is further arranged to determine a relation between the calculated visual latency per stimulus and the coordinates of the stimulus.

The system according to any one of the preceding claims 9 - 13, further comprising a display unit for feeding-back the said relation as a two-dimensional graph.

A computer program product comprising instructions for causing a processor to carry out the steps of the method as claimed in any one of the preceding claims 1 - 8.