Automated visual assessment system with steady state visual evoked potential stimulator and product detector

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BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an automated visual assessment system suitable for performing a variety of vision tests and, more particularly, to an evoked potential vision testing system which performs spherical and aspherical refractometry, contrast sensitivity, color sensitivity, acuity, transient pattern and flash evoked potential tests for visual pathway disfunction which is low in cost, does not intimidate a patient and provides a highly accurate easy-to-use vision testing system.

2. Description of the Related Art

Typical vision testing systems in an eye doctor's office include a plurality of separate testing systems which each provide a different test. These individual tests take at least 10 minutes each, resulting in at least an hour in a doctor's office for a comprehensive vision test. For example, eye refraction is generally measured by placing lenses of different refractive power in front of the patient's eye and allowing the patient to indicate whether or not the visual target has improved in focus, a process that can take up to 30 minutes, while color vision testing is typically performed using flash cards having images of different colors which the patient is asked to detect. These vision testing systems, even though each is a separate device all have a single common feature, all require that the patient verbally cooperate to indicate the results of the tests.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide an automated visual assessment system suitable for a commercial environment.

It is another object of the present invention to provide a compact low cost visual testing system that will allow refractometry, contrast, color and other vision tests using the same machine and without requiring that the patient respond verbally to the tests.

It is a further object of the present invention to provide a vision testing system that does not intimidate the patient and which can conduct a test in a very short period of time (before the patient gets tired and irritable).

It is also an object of the present invention to improve the efficiency of an eye doctor's office by reducing test time.

It is an additional object of the present invention to provide a simple, low cost stimulus source for visual testing or stimulation.

It is an object of the present invention to provide a stimulus source that allows a wide range of duty cycles and frequencies for an alternating or complementing stimulus pattern.

It is still another object of the present invention to provide a stimulus generator that will produce variable color vision targets as well as variable spatial frequency, intensity and contrast targets.

It is also an object of the present invention to provide a simple mechanism for changing image contrast in a predictable manner while maintaining constant luminance.

It is a further object of the present invention to provide a frequency locked evoked potential detector capable of detecting the brain wave patterns produced by a patient when viewing a steady state evoked potential stimulator.

It is an object of the present invention to provide a detector capable of detecting steady state evoked potentials produced in the human brain when caused by visual, aural or somatic steady state stimuli.

It is also an object of the present invention to provide a product detector that is capable of changing the detection frequency by merely changing a reference clock frequency.

It is an object of the present invention to isolate a patient's brain wave response to a visual stimulus from surrounding environmental and artificial noise.

It is another object of the present invention to provide a system capable of detecting the object of interest to a viewer.

It is a further object of the present invention to provide a system that can detect which of several different displays a viewer is watching.

The above objects are accomplished by an automated visual assessment system that includes a computer driving a reversing checkerboard steady state stimulus generator to stimulate a patient through a lense system that is controlled by the computer. The system may also include a reference image that prevents the patient's eye from adapting to any change in the stimulus. The lense system and stimulus generator allow the following tests to be conducted: spherical refractometry, aspherical refractometry, contrast threshold, color vision, acuity, transient pattern evoked potential and flash evoked potential. The evoked potentials produced by the patient are amplified, filtered and used to determine an average evoked potential. The amplitude of this potential is used to produce a response wave for the visual function of the patient for the test being conducted. Each test results in at least one response curve for the patient which can be reviewed by a doctor. The response curves are then used for determining vision problems and/or vision corrections.

The steady state visual evoked potential stimulator is a device by which a rapidly complementing or flashing pattern can be presented to the patient. This stimulator allows the contrast of the image to be varied without varying luminance and allows operation in a true bicolor and multicolor mode. The stimulus generator includes a precision patterned stimulus mirror which is used to produce the complementing pattern by shining light through and reflecting light off of the patterned mirror using highly controllable color light sources. A wash source is used to vary the contrast of the mirror pattern using a rotating polarizer so that a contrast vision test can be performed.

The present invention also includes a steady state frequency locked evoked potential product detector for monitoring the steady state evoked potentials created in the patient's brain by the stimulus generator. The product detector detects the level of the steady state evoked potential signals even in the presence of substantial background noise and extraneous electroencephalographic signals. The product detector includes filters which isolate the patient's evoked potentials, a modulator which detects the response using the stimulus source frequency and a demodulator that determines the amplitude of the response. The detector can be used to monitor the evoked potential caused by visual, aural or somatic steady state stimuli.

These together with other objects and advantages which will be subsequently apparent, reside in the details of construction and operation as more fully hereinafter described and claimed, reference being had to the accompanying drawings forming a part hereof, wherein like numerals refer to like parts throughout.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1, including 1A and 1B, is a system block diagram of an automated visual assessment system in accordance with the present invention;

FIG. 2 depicts the details of one embodiment of the stimulus controller 18 of FIG. 1;

FIG. 3 is a first embodiment of the stimulus generator 20 of FIG. 1 using continuous incandescent or fluorescent light sources 104, 106 and 130;

FIGS. 4A-4C depict contrast for various polarizations;

FIG. 5 depicts a second embodiment of the stimulus generator using tricolored light emitting diode arrays or fluorescent lamps 140 and 142 as the light source;

FIG. 6, including 6A and 6B, is a circuit diagram for the stimulus controller 18 of FIG. 1 when the stimulus generator of FIG. 5 is used;

FIG. 7 is a detailed version of the system optics when the stimulus generator of FIG. 3 is used;

FIG. 8 is a detailed diagram of the system optics according to the present invention when the stimulus generator of FIG. 5 is used;

FIG. 9 is a detailed diagram of the product detector 58 of FIG. 1.

FIG. 10 depicts evoked potential amplitude during a spherical refractometry test;

FIG. 11 depicts evoked potential amplitude during an aspherical refractometry test;

FIG. 12 depicts evoked potential amplitude during a contrast threshold test;

FIG. 13 depicts a curve of contrast threshold versus spatial frequency or a contrast sensitivity;

FIG. 14 depicts evoked potential amplitude during a color vision test;

FIGS. 15A-15C depict the initialization procedures performed by the control processor 12, supervisor microcomputer 10 and programmable analog to digital converter 14 of FIG. 1;

FIGS. 16A-16C depict the procedures performed by the control processor 12, supervisor microcomputer 10 and programmable analog to digital converter 14 during a spherical refractometry test;

FIGS. 17A-17C depict the procedures performed by the control processor 12, supervisor microcomputer 10 and programmable analog to digital converter 14 of FIG. 1 during an astigmatic axis determination test;

FIGS. 18A-18C depict the procedures performed by the control processor 12, supervision microcomputer 10 and programmable analog to digital converter 14 during an astigmatic refractive power determination test;

FIGS. 19A-19C depict the procedures performed by the control processor 12, supervisor microcomputer 10 and programmable analog to digital converter 14 of FIG. 1 during an acuity test;

FIGS. 20A-20C depict the procedures performed by the control processor 12, supervisor microcomputer 10 and programmable analog to digital converter 14 of FIG. 1 during a contrast sensitivity test;

FIGS. 21A-21C depict the procedures performed by the control processor 12, supervisor microcomputer 10 and programmable analog to digital converter 14 of FIG. 1 during a color vision test;

FIGS. 22A-22C depict the procedures performed by the control processor 12, supervisor microcomputer 10 and programmable analog to digital converter 14 of FIG. 1 during a flash or transient test; and

FIG. 23 depicts a system which will allow determination of which object an observer is viewing.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

An automated visual assessment system in accordance with the present invention presents a 6 Hz reversing checkerboard pattern to a patient who views the continuously varying pattern through optic components required for a particular test. An electroencephalographic (EEG) brain wave electrode or monitor picks up the resulting evoked potential which is then amplified to a useful level. When performing some tests, the amplitude of the sinusoidal varying wave pattern is detected and during other tests, the absolute magnitude of the amplified evoked potential EEG is detected. A time varying digital value of the level of the evoked potential is interpreted in accordance with the test being performed.

Any test begins with a supervisor microcomputer 10 (FIG. 1) such as an IBM PC/XT initializing a control processor 12, such as a COMMODORE 64, and a programmable analog digital converter 14 such as a 2801 D/A board available from Data Translation of Massachusetts. The test control processor 12 positions the appropriate lenses, slits and polarizers necessary for the test through a digital input/output unit 16 such as an MW611 digital input/output board manufactured by Micro West Electronics. The control processor 12 also produces a 2.4 kilohertz control signal for driving stimulus controller 18. The stimulus controller 18 controls reversing or complementing checkerboard stimulus generator 20 to produce a visually reversing checkerboard pattern. The alternating checkerboard visual stimulus passes through the optical system to the patient 22. The optical system includes selectable and controllable lenses and filters for performing the various vision tests.

In the first embodiment, the optical system includes a counterpolarized light leak 24 and light leak driver 26, a rotatable polarizer 28 and polarizer driver 30. A rotatable slit 32 and a slit driver and selector 34, a zoom lense system 36 and zoom drivers 38, an image plane aperture 40 which defines the field of view as the zoom lense system 36 changes the apparent size of the image pattern followed by a far field lense 42 controlled by far/near selector 43. Image focus is controlled by a variable focal length lense 44 controlled by test lense driver 46. Because the human eye attempts to adapt to out-of-focus images, a television 48 is used to project an image through a far field lense 50 producing an image at infinity which is combined by an image combining half-reflecting mirror 52 with the visual stimulus from the stimulus generator and projected to the patient 22. The patient 22 produces evoked potentials in response to the stimulus and the potentials are picked up by self-preparing electrodes mounted in a head set cap, as described in U.S. application Ser. No. 822,725 mentioned in the cross-references to related applications section. The electrodes are held at positions to confront the patient's occipital lobe. In an adult, the first or lowest electrode should be positioned just above the inion, the second electrode should be positioned behind one ear and the third electrode should be positioned behind the other ear.

The evoked potentials produced by the patient and picked up by the self-preparing electrodes are conducted over an optimized shielded cable wiring system as described U.S. application Ser. No. 727,060 mentioned in the cross-references to related application section. The evoked potentials having a magnitude of from 1-10 microvolts are transmitted to a low noise, high gain shielded amplifier 54 as described in U.S. application Ser. No. 727,056.

The evoked potential signals produced by amplifier 54 are switched by switch 56 to product detector 58 or directly to the programmable analog to digital converter 14. The product detector 58 detects the amplitude of the steady state evoked potentials produced by the patient even in the presence of large scale background noise and extraneous EEG signals. The amplitude modulated steady state evoked potential produced by the brain during certain tests includes a very strong fundamental signal at the frequency of the reversal rate of the stimulus, along with undesired harmonics, muscle movement noise and environmental noise. The product detector 58 produces the amplitude of the steady state alternating evoked potential signal free from harmonics and relatively free from noise. The programmable analog to digital converter 14 samples either the amplitude of the steady state evoked potentials from product detector 58 or the evoked potentials themselves at a number of points over a short period of time. The supervisor microcomputer 10 retrieves the samples from the A/D converter 14 and produces various test curves, combined test curves and computes test results.

FIG. 2 illustrates the stimulus controller 18 when a stimulus generator having a continuous incandescent or fluorescent light source is used. A square wave generator 60 in the controller processor 12, which is included in a COMMODORE 64, produces a 2.4 KHz square wave signal which is divided by divider 62 down to a 3 Hz square wave signal. The divider 62 converts the 2.4 KHz signal down to the 3 Hz signal by dividing by 800. The 3 Hz signal is applied through an inverter 64 to an A path liquid crystal shutter 100 and directly to a B path liquid crystal shutter 102. The 3 Hz waveform is used to open and close the shutters 100 and 102 to produce a resultant 6 Hz stimulus transition frequency when tests requiring the alternating stimulus are being performed. In other types of tests, such as in the flash evoked potential test, the shutters are directly controlled by control processor 12. In a first embodiment of the stimulus generator 20, as illustrated in FIG. 3, light sources 104 and 106 are white light sources such as Westinghouse fluorescent bulbs called ULTRALUME bulbs which provide equal luminance (line response characteristics) in the three portions of the spectrum to which the human eye is sensitive. The complementing or alternating pattern produced by the generator 20 is generated by alternately opening liquid crystal shutters 100 and 102 causing light to shine along equal length paths A or B, respectively. The preferred shutters 100 and 102 are available from Tektronix in Beaverton, Oreg. as Liquid Crystal Color Shutters. The shutters 100 and 102 are available with a refresh and control circuit that refreshes the current state of the shutter so that it does not decay to the other state. The refresh rate must be adjusted so that the maximum transmission through each shutter is obtained when the shutter is in an open or transmissive state. The shutters 100 and 102 polarize the light passing therethrough vertically. When path A is active, light from source 104 passes through shutter 100, a matte transmission filter 108, a neutral density filter 110 and another matte filter 112. The three filters 108-112 serve to diffuse the light evenly over the light path area, so that all parts of the path are equally illuminated. The filters 108-112 are obtainable from most optical supply houses. The neutral density filter 110 is a photographic neutral density filter available from Kodak.

The diffused light traversing path A bounces off the reverse side of the beam splitter 114 and then off the mirrored portions of a checkerboard pattern mirror 116 having opaque portions 118 and 120. A suitable beam splitter is obtainable from most optical supply houses. The details of construction of the patterned mirror 116 are discussed in the Precision Patterned Mirror application listed in the Cross-References to Related Applications section. The reflected pattern from mirror 116 passes back through the beam splitter 114 to the patient 22. The light reflecting off the mirrored portions of mirror 116 appear to the patient as white while the non-mirrored portions are black whenever path A is active.

When path B is active, light from source 106 passes through shutter 102 matte filter 122, neutral density filter 124 and matte filter 126 which are the same type filters as 108-112 discussed above. The light is then reflected off beam splitter 128 and passes through the non-mirrored portions of patterned mirror 116 and through beam splitter 114 to the patient 22. The mirrored coating of the patterned mirror blocks light from path B while the non-mirrored portions of the mirror 116 pass light. As a result, the mirrored portions appear black and the non-mirrored portions appear white when light path B is activated.

To obtain equal luminance from path A and B, the light source for one of the paths is turned on and the light intensity of the output to the polarizer 28 is measured using a light meter. The second light path is then turned on and the intensity is again measured. The neutral density filters 110 and 124 are then changed to different densities until the paths have equal luminance.

As the two light paths are alternately activated using two square waves at