WO2002069788A1

WO2002069788A1 - Characterisation of eye fixation control anomalies, particulary dyslexia

Info

Publication number: WO2002069788A1
Application number: PCT/GB2002/000823
Authority: WO
Inventors: Donald Nigel Jarrett
Original assignee: Qinetiq Ltd
Current assignee: Qinetiq Ltd
Priority date: 2001-03-02
Filing date: 2002-03-01
Publication date: 2002-09-12
Anticipated expiration: 2003-09-02
Also published as: GB0105222D0

Abstract

A device suitable for use in the characterisation and mitigation of eye fixation control problems, particularly dyslexia, comprises means for measuring small changes in eye pointing direction and for generating signals which are indicative of such changes, filter means applied to said signals and capable of determining from said signals periods of eye fixation, and means, which are driven by said signals only during a period of eye fixation, for dynamically shifting the direction of visual stimuli as perceived by the eye, in response to a measured change in the eye pointing direction. In a preferred embodiment the dynamic shifting means comprise a pair of mirrors which give a vertically offset but otherwise normal view of the visual stimuli, one of said mirrors being movable so as to effect the required dynamic shift. Preferably two devices are used, one for each eye, and are arranged to resemble spectacles or a pair of binoculars and to allow the wearing of corrective spectacle lenses and/or compatibility with ophtalmic lens testing.

Description

CHARACTERISATION OF EYE FIXATION CONTROL ANOMALIES , PARTICULARY DYSLEXIA

The present invention relates to a device for use in the detection, characterisation and mitigation of eye fixation control problems and to use of the device in the diagnosis of eye fixation control problems, particularly dyslexia and to methods of diagnosing, characterising and correcting such problems.

Within the context of this specification the word "comprises" is taken to mean "includes, among other things". It is not intended to be construed as "consists of only".

A theory advanced to explain why some individuals experience reading difficulties postulates that these individuals suffer from mildly impaired development of a particular sort of neurone in the brain that gives rise to involuntary wobble of the eye while fixating on reading material [J Stein "The neurobiology of reading difficulties" in Prostaglandins, Leukotrienes and Fatty Acids (2000) 63 (V₂), 109-116, Harcourt Publishers Ltd].

This impairment of fixation control or wobble has been found to have a magnitude of about 1 °, which corresponds to about four or five letters in normal typescript, and it is this that tends to transpose the apparent order of letters - the most common expression of the condition. The wobble of the left eye is not correlated with that of the right eye, so reading with binocular vision is worse than reading with one eye blocked.

It has previously been found that masking, or diffusing the vision of one eye, alleviates the condition for some individuals. However, this has not been successful for all individuals and it suffers from the problem that sight in one eye is restricted. Assuming the theory is correct, there is a need for a simple, robust, and accurate way of measuring eye pointing direction while the individual is reading in order to check whether the individual has an involuntary wobble and also to characterise any wobble which takes place.

Corneal reflex eye trackers are known, for example in the following documents: "Method and Apparatus for Locating Image Features," U.S. Patent 5,231,674, July 1993. - Cleveland, D; "Focus Control System," U.S. Patent 4,974,010, Nov 1990. - Cleveland, D; "Eye Tracker," U.S. Patent 3,712,716, Jan 1973. - Cornsweet, T. N.; "Eye Movement Detector," U.S. Patent 4,836,670, June 1989. - Hutchinson, T. E.; "Control Apparatus Sensitive to Eye Movement," U.S. Patent 3,462,604, August 1969. - Mason, K. A. In addition, scleral coil measurement has been suggested using a contact lens with coils embedded therein and Helmholtz coils producing an excitatory magnetic field.

In addition, apparatus for passive and active image stabilisation are known to exist. Known passive methods for image stabilisation include conventional camcorder image stabilisation and image stabilisation in hand held binoculars. Known active methods include using electronics to stabilise an image.

However, it has not been suggested that impaired eye fixation control, such as may be involved in the phenomenon of dyslexia, could be mitigated by tracking eye movement and stabilising an image in concert with that movement

A difficulty that has existed with corneal reflex eye trackers, is that small movements in the, optics relative to a subject's head can cause a shift of the corneal reflex, and hence cause a change in tracker output. However, given the necessary sensitivity of eye movement measurement, some head shift relative to the optics is inevitable. Therefore, there has been a need to distinguish between eye movement and head/tracker shift. Remarkably it has now been found that it is possible to use real time measurements (i.e. active control) of eye wobble to control what is seen, using an adjustable mirror. Involuntary movement must be isolated from voluntary movement of the eye and this may be achieved by means of a filter which is capable of discriminating between features of movement such as frequency, velocity, acceleration and magnitude.

Consequently, in a first aspect the present invention provides a device suitable for use in the characterisation and mitigation of eye fixation control problems, comprising means for measuring small changes in eye pointing direction and for generating signals which are indicative of such changes, filter means applied to said signals and capable of determining from said signals periods of eye fixation and means, which are applied to said signals only during a period of eye fixation, for dynamically shifting the perceived direction of visual stimuli in response to a measured change in the eye pointing direction.

Preferably an embodiment of the eye pointing direction measuring means ("eye tracker") measures eye movements over a range of about 10°, which corresponds to a corneal reflex shift of about 1.0mm, preferably to an accuracy of about 0.2°.

Preferably, resolution is about 0.1°, or about 0.01mm shift of the corneal reflex. Sensitivity is therefore most important, and this depends primarily upon the optical magnification of the eye image formed on the surface of the position sensing detector which is used in the eye tracker (conventionally a photodiode) in relation to the sensing area. The optical system in the eye tracker should be dimensioned sufficient to collect as much light as possible to generate sufficient signal, and phase sensitive detection maximises signal to noise ratio.

The use of a corneal reflex tracker requires that it is helpful to hold the head still relative to the measurement system, because any head shift would also displace the corneal reflex. However, advantageously, small head movements are tolerated providing they are within the measurement range and are slow relative to eye wobble. Advantageously, eye and head movements should be distinguishable by virtue of their velocities.

The eye tracker is based on a refinement of a well-known optical technique that senses the movements of the corneal-reflex, the small spot caused by reflection of light from the convex cornea. The spot moves by about 1mm for an eye rotation of 10°. In addition, preferably, the eye tracker is based on the well-known 'corneal reflex' principle in which the displacement of the spot image produced by the reflection of invisible infra-red light from the convex cornea is almost linearly related to changes in eye pointing direction.

Preferably the overall magnification achieved is about 5:1. Preferably the eye tracker optical system includes a focussing lens and a filter which eliminates light outside the illumination spectrum, thus preferably including only about 50nm centred on about 900nm.

Preferably, an embodiment of the device comprises at least one channel, preferably two channels, one for each eye, and each of which comprises a pair of mirrors that give a vertically off-set but otherwise normal view of the target visual stimuli.

Preferably a first mirror is fixed and a second mirror is lightweight and is mounted on an adjustable platform that can be set to remove any horizontal offset introduced by the mirrors.

Preferably, stabilising the line of sight is achieved by introducing small dynamic angular tilts to the lightweight mirror, in response to the signals from the eye tracker. Preferably, the platform is moved by actuators constructed by adapting small loudspeaker units.

Intentional eye movements, for instance when a mature reader is reading text, are a series of short fixations (of about 300msecs duration) separated by one or two rapid saccadic jumps that can be as small as 2° and as large as 40°. During the saccadic jumps the eye rotation reaches a speed of about 500 -1000 sec, and the acceleration/ and deceleration are also fairly high, in the order of 1000 to 5000 °/sec . During fixations a subjects eyes are generally controlled to point accurately at the feature of interest, but to counter eye drift the muscles are triggered to make small corrective micro-saccades. These are usually less than V2⁰.

Preferably, the filter is programmed to analyse the signals produced by the eye tracker and to apply a series of tests that take account of the eye velocity and acceleration for an individual and divide the signal into 'jumping' and 'fixating' periods. To do this, the filter calculates the angular magnitude, frequency and velocity of eye movements from the measured eye pointing direction and uses this data to distinguish between periods of fixation, periods when the eye is performing a saccadic eye movement and periods when the device shifts relative to the subject's head.

In a preferred mode of operation, during identified fixation periods, the statistical distributions of the calculated angular magnitude, frequency and velocity of the eye pointing direction are computed and may then be used to characterise eye fixation control and in particular to distinguish relatively rapid wobble from the slower shifts attributable to head movement. Although the filters are described herein as separate horizontal and vertical elements, the computations are based upon shared data. Also, preferably data is shared between the left and right eye channels to characterise the synchronism of the saccadic jumps, and the correlation between the left and right eye wobble during fixations. Preferably, the filter unit also comprises a recorder which records this data.

Preferably, the eye movement signals, with noise, head motion and saccadic jumps removed, drive the dynamic shifting means.

Preferably, an embodiment of a device according to the invention comprises electro-optics which are optimised to meet the measurement requirements described above. In addition, preferably, the device comprises some flexibility which provides the advantage of minimising both the manufacturing difficulties and the cost of components. Preferably the device is compact and robust which provides the advantage of facilitating making the system portable, rather than 'transportable'. Preferably the device comprises a battery pack and it is battery-powered.

Preferably, the device includes means for recording eye movement. Preferably this comprises an electronic recording apparatus. This provides the advantage that recordings of eye movement signals obtained from a subject having significant eye fixation difficulties such as a marked wobble, can be played back into the stabiliser. This enables individuals with well-stabilised vision to experience the disturbance or wobble.

In a second aspect the invention provides use of a device according to a first aspect of the invention in the characterisation of disturbances to eye fixation, for example, eye wobble movements.

In a third aspect the invention provides use of a device according to a first aspect of the invention in the diagnosis of eye fixation control problems such as are suggested to be a causal factor in dyslexia.

In a fourth aspect the invention provides apparatus comprising a pair of devices according to a first aspect of the invention in which the devices therein are arranged to co-operate with two eyes independently such that a line of sight from each eye of a subj ect passes through a separate channel. The apparatus is preferably made to resemble a pair of spectacles or binoculars and is so arranged as to allow the wearing of corrective spectacle lenses and be usable by very young children. Given that testing for refractive errors is done by opticians who ask the individual to identify the characters in fine text (or, for pre-readers, identify small easily identified features in pictures), the advantages conferred by the device of the present invention in enabling the measured eye pointing direction to be used to reduce significantly, or to remove, the deleterious effects of any fixation control difficulties experienced by an individual, will be very important as it will allow the individual to then have the sensation of fixating stably and be better able to judge the effect of alternative test lenses on acuity.

In a preferred embodiment the apparatus is arranged so that the correlation between the measured eye pointing directions of the two eyes and/or the correlation between the calculated angular velocities of the two eyes are computed and outputted for use as additional parameters to characterise eye fixation control.

In a fifth aspect the invention provides a method of characterising eye fixation control problems such as dyslexia which comprises use of a device according to a first aspect of the invention or of apparatus according to a fourth aspect with a subject to be investigated, to generate data relating to changes in eye pointing direction for said subject while the subject is fixating on the visual stimuli, outputting said data from the device or apparatus and analysing said data in order to detect anomalies in eye fixation behaviour.

In a preferred embodiment the data comprises the angular magnitude, frequency and velocity of changes in eye pointing direction.

In a sixth aspect the invention provides a method of correcting or mitigating eye fixation control problems which¹ comprises use of a device according to a first aspect of the invention or of apparatus according to a fourth aspect of the invention with a subject having significant eye fixation problems.

Additional features and advantages of the present invention are described in, and will be apparent from, the description of the presently preferred embodiments which are set out below with reference to the drawing in which:

Figure 1 shows a schematic for one channel of a system to measure eye wobble and stabilise vision.

For the purposes of clarity and a concise description features are described herein as part of the same or separate embodiments, however it will be appreciated that the scope of the invention may include embodiments having combinations of all or some of the features described.

As seen in Figure 1, the stabiliser operates by dynamically displacing the corners of a small, lightweight, front-silvered, square mirror from a centre-sprung datum position using a set of four solenoid actuators. (This can be visualised by imagining facing a square mirror that has been rotated through 45° so that the corners point up, down, left and right. A horizontal deflection of the reflected image would be induced by driving the left and right corners in opposite directions, say forward and backward respectively. Nertical deflections would be produced by similarly opposing movements of the top and bottom corners, and such opposed movements would keep the mirror centre stationary.) The actuators are driven as opposed pairs and mounted on a stiff base that could be adjusted to align the reading target within the small field of view. Since the angular image shift would be double the mirror tilt, a 30mm diagonal mirror would need to have a corner deflect by about 0.25mm to shift the image by 1.5°. It is possible to produce such shifts relatively cheaply by adapting miniature acoustic coil-driven transducers. These are mounted on a base plate in a compact diamond formation, and a lightweight mirror is bonded to the centres of the cones in a flexible way that allows a small tilt without strain. The device comprises at least one channel, more preferably two channels - one for each eye of a subject. Preferably the channels are identical and each channel preferably comprises each of the three elements :-

i) a sensitive eye tracker that does not disturb the subject's ability to see over a small field of view of about 10°, and measures the line of sight over the field of view;

ii) a filter that can analyse the measurements to distinguish between (1) intentional eye movement, (2) the postulated unintentional eye wobble, (3) artefacts such as head shift;

iii) a stabiliser that is driven by only the filtered unintentional eye wobble signals to adjust, the apparent direction of something that the subject can read or examine within the field of view.

The device may also include a movement recorder which enables recording of eye movement and a movement player which enables recorded eye movement to be played back.

The device is assembled as a robust, compact, portable unit which provides the advantages of:-

allowing ophthalmic test lenses to be placed close to the eyes, of a subject to enable the subject to judge their effect on visual acuity,

recording the eye movement signals,

allowing playback of the extracted unintentional wobble from the recording to stimulate the stabiliser. Preferably, the apertures that a subject looks into are adjustable to suit their inter-eye distance (as in a pair of binoculars), and either have soft, stiff, face-shaped cups that can be held against the brow, cheeks and bridge of the nose to hold the apertures as still as possible relative to the face or for a pair of spectacles.

Preferably, the device is provided in a number of alternative sizes needed to accommodate a size range from small children to large adults.

Advantageously the device is capable of :

i) characterising an impairment of fixation control (wobble) of at least one, more preferably two eyes in terms of the angular extent, direction and frequency for each eye, and the correlation between two eyes,

ii) enabling the subject to look at a variety of test material suited to the age of the subject (e.g. text, pictures, a computer screen) at a wide range of distances, and experience the effect of stabilised vision,

iii) enabling a tester to insert test lenses in a subject's line of sight so that a subject can assess the effect of the test lenses on acuity without a confounding wobble, iv) enabling a tester to experience a characteristic wobble of a subject.

As shown in figure 1, elements of an embodiment of the device are as follows :-

The eye tracker

The eye tracker must measure eye movements over a range of about 10°, to an accuracy of about 0.2° and a resolution of about 0.1°. The scheme suggested is a refinement of the well-known optical technique that senses the movements of the corneal-reflex, the small spot caused by reflection of light from the convex cornea. The spot moves by about 1mm for an eye rotation of 10°.

Figure 1 shows that the viewer looks at the target stimuli through beamsplitter A (as well as the two reflecting surfaces in the stabiliser S and T). The beamsplitter A is transparent to the subject, but it is coated with a layer that reflects infra-red light from the source C, so that the front of the eye is illuminated with invisible light. It is this light that is reflected from the cornea and appears to originate from the small 'corneal-reflex' spot.

The means for detecting an optical spot may be a quadrant detector or a position sensing photodiode. In an embodiment where it is a quadrant detector, the lower beamsplitter is replaced by an angle-deflecting mirror - much like that in the image stabiliser. The angle through which the mirror is deflected is that which is necessary to bring the corneal reflex spot onto the centre of the quadrant, which would supply the direction signals to close the control loop. The mirror deflections are proportional to eye rotation.

The rest of the optical elements in the Figure 1 embodiment include a camera that focuses the reflection onto a position-sensing photo-diode F. This generates separate horizontal and vertical signals directly related to the position of the focussed spot on the diode surface, and is reasonably insensitive to fluctuations in the strength of the light or sharpness of focus. The overall magnification would need to be about 5:1. Lens D represents the focussing lens, Filter E eliminates light outside the illumination spectrum, say 50nm around 900nm.

Beamsplitter B is included so that the camera looks at the eye along the same path as the illumination, and therefore also collects the light that enters the pupil, is back-scattered from the retina and exits via the pupil. The arrangement in the upper left of the figure shows an alternative off-axis illumination that elirninates beamsplitter B and the collection of light backscattered from the retina.

The electronic elements of the eye tracker (G, H, J and K) take the horizontal and vertical spot position analogue voltages from the position-sensing photodiode and amplify them to produce a stable pair of signals. Gain and datum adjustments are available to centre and calibrate each dimension so that the output signals have a fixed relationship with look angle. In practice the operator of the equipment would make the adjustment by asking the subject to look at a set of fixed points in the field of view.

The filter unit

The function of the filters L and M is to distinguish between (1) intentional eye movement, (2) the postulated unintentional eye wobble, and (3) small head movements. Digital processing is assumed.

Intentional eye movements, for instance when reading text, are a series of short fixations (of about 300msecs duration) separated by one or two rapid saccadic jumps that can be as small as 2° and as large as 40°. During the saccadic jumps the eye rotation reaches a speed of about 500 -1000 7sec, and the acceleration/ and deceleration are also fairly high, in the order of 1000 to 5000 7sec². During fixations the eyes are controlled to point accurately at the feature of interest, but to counter eye drift the muscles are triggered to make small corrective micro-saccades. These are usually less than Vz°.

The filter is programmed to analyse the signals produced by the tracker and apply a series of tests that divide the signal into 'jumping' and 'fixating' periods. During the fixating periods the angular magnitude, frequency and velocity of the eye movements are measured and the distributions computed. These parameters are used to characterise the wobble and distinguish the relatively rapid wobble from the slower ι α

shifts attributable to head movement. Although L and M are shown as separate elements in the figure, the computations are based upon shared data. Also data are shared between the left and right eye channels to characterise the synchronism of the saccadic jumps, and the correlation between the left and right eye wobble during fixations. The filter also records the data.

The eye movement signals, with noise, head motion and saccadic jumps removed, drive the stabiliser.

The stabiliser

The stabilisers for the left and right channels consists of pair of mirrors S and T that give a vertically off-set but otherwise normal binocular view of the target. Mirrors S are mounted on adjustable platforms N that can be set to align the target direction for the two eyes.

Stabilising the line of sight is done by introducing small dynamic angular tilts to the lightweight mirror S, driven by the outputs from the filter via amplifiers Ν and P. The actuators R may be constructed by adapting small loudspeaker units.

It should be understood that various changes and modifications to the presently preferred embodiments described herein will be apparent to those skilled in the art. Such changes and modifications can be made without departing from the spirit and scope of the present invention and without diminishing its attendant advantages. It is therefore intended that such changes and modifications are covered by the appended claims.

Claims

1. A device suitable for use in the characterisation and mitigation of eye fixation control difficulties, comprising means for measuring small changes in eye pointing direction and for generating signals which are indicative of such changes, filter means applied to said signals and capable of determining from said signals periods of eye fixation, and means, which are driven by said signals only during a period of eye fixation, for dynamically shifting the perceived direction of visual stimuli in response to a measured change in the eye pointing direction.

2. A device as claimed in claim 1 wherein the means for dynamically shifting the perceived direction of visual stimuli in response to a detected change in the eye pointing direction comprises a pair of mirrors which give a vertically off-set but otherwise normal view of the visual stimuli, one of said mirrors being movable so as to effect the required dynamic shift.

3. A device as claimed in claim 2 wherein the first of the pair of mirrors is fixed and the second mirror is lightweight and is mounted on an adjustable platform that can be set to remove any horizontal offset introduced by the mirrors.

4. A device as claimed in claim 3 wherein stabilisation of the line of sight is achieved by introducing small dynamic angular tilts to the lightweight mirror, in response to the signals from the eye pointing direction measuring means.

5. A device as claimed in claim 3 or claim 4 wherein the platform is moved by actuators which are constructed by adapting small loudspeaker units.

6. A device according to any of claims 1 to 5 wherein the angular magnitude, frequency and velocity of eye movements are calculated from the measured eye pointing direction and used by the filter means to distinguish between periods of i t

fixation, periods when the eye is performing a saccadic eye movement and periods when the device shifts relative to the head.

7. A device according to any of claims 1 to 6 wherein during identified fixation periods, the statistical distributions of the calculated angular magnitude, frequency and velocity of the changes in eye pointing direction are computed and outputted for use in characterisation of eye fixation control.

8. A device according to any preceding claim and further comprising electro-optics which are optimised to meet the requirements for measurement of changes in eye pointing direction.

9. A device according to any preceding claim and further comprising a battery pack and which is battery powered.

10. A device according to any preceding claim which further includes means for recording eye movement.

11. A device according to any preceding claim which comprises electronic recording apparatus which enables recordings of eye movement signals obtained from a subject having significant eye fixation difficulties to be played back into the dynamic shifting means to thereby enable individuals with well-stabilised vision to experience the disturbances in eye fixation.

12. Apparatus comprising a pair of devices according to any of claims 1 to 11, ' arranged to co-operate with two eyes independently.

13. Apparatus according to claim 12 in which the correlation between the measured eye pointing directions of the two eyes and/or the correlation between the calculated angular velocities of the two eyes are computed and outputted for use as additional parameters to characterise eye fixation control.

14. Apparatus as claimed in claim 12 or claim 13, which is arranged to allow the wearing of corrective spectacle lenses and/or compatibility with ophthalmic lens testing.

15. Apparatus as claimed in any of claims 12 to 14 which is arranged to resemble a pair of spectacles or binoculars.

16. Use of a device according to any of claims 1 to 11 in the characterisation of disturbances to eye fixation.

17. Use of a device^" according to any of claims 1 to 11 in the correction of eye fixation control difficulties.

18. Use of a device according to any one of claims 1 to 11 in the diagnosis of eye fixation control problems.

19. Use of a device according to claim 18 where the eye fixation control problem is dyslexia.

20. A method of characterising eye fixation control problems which comprises use of a device according to any one of claims 1 to 11 or of apparatus according to any of claims 12 to 15 with a subject to be investigated, to. generate data relating to changes in eye pointing direction for said subject while the subject is fixating on visual stimuli, outputting said data from the device or apparatus and analysing said data in order to detect anomalies in eye fixation behaviour.

21. A method as claimed in claim 20 wherein the data comprises the angular magnitude, frequency and velocity of changes in eye pointing direction.

22. A method of correcting or mitigating eye fixation control problems which comprises use of a device according to any one of claims 1 to 11 or of apparatus according to any of claims 12 to 15 with a subject having significant eye fixation problems.

23. A method according to any of claims 20, 21 or 22, wherein the eye fixation problem is dyslexia.

24. A device or apparatus substantially as shown or described herein with reference to the accompanying drawings.