Abstract:
Apparatus for evoking and sensing ophthalmic physiological signals in an eye, the apparatus comprising: an elongated tubular light pipe having a longitudinal axis, a distal end and a proximal end, the distal end terminating in a spheroid recess; an active electrode having a distal end and a proximal end, the active electrode being mounted to the elongated tubular light pipe and extending proximally along the elongated tubular light pipe so that the distal end of the active electrode terminates at the spheroid recess at the distal end of the elongated tubular light pipe; and a reference electrode having a distal end and a proximal end, the reference electrode being mounted to the elongated tubular light pipe and extending proximally along the elongated tubular light pipe so that the distal end of the reference electrode terminates at the spheroid recess at the distal end of the elongated tubular light pipe; wherein the distal end of the active electrode is located closer to the longitudinal axis of the elongated tubular light pipe than the distal end of the reference electrode.

Description:
REFERENCE TO PENDING PRIOR PATENT APPLICATION 
       [0001]    This patent application claims benefit of pending prior U.S. Provisional Patent Application Ser. No. 62/160,503, filed May 12, 2015 by Diagnosys LLC and Bruce Doran et al. for COMBINED STIMULATOR AND BIPOLAR ELECTRODE ASSEMBLY FOR MOUSE ELECTRORETINOGRAPHY (ERG) (Attorney&#39;s Docket No. DIAGNOSYS- 1  PROV), which patent application is hereby incorporated herein by reference. 
     
    
     FIELD OF THE INVENTION 
       [0002]    This invention relates generally to apparatus and methods for the assessment of electrophysiological signals, and more particularly to apparatus and methods for the assessment of ophthalmic physiological signals. 
       BACKGROUND OF THE INVENTION 
       [0003]    Full-field ophthalmic electrophysiology generally involves flashing a light from a large “bowl” into the eye of the subject, and then measuring the response from the retina of the subject using electrodes, i.e., an active electrode which contacts the eye of the subject and other electrodes (reference and ground electrodes) which contact other portions of the subject. This procedure is sometimes referred to as electroretinography (ERG). 
         [0004]    Clinically, the hardest part of performing ophthalmic electrophysiology is properly connecting the electrodes to the subject and, more particularly, properly connecting the active electrode to the eye of the subject. 
         [0005]    In some cases the ophthalmic electrophysiology must be conducted on humans. In other cases the ophthalmic electrophysiology must be conducted on small rodents of the sort commonly used in laboratory experiments, e.g., mice and rats (for the purposes of the present invention, such animals will generally be referred to herein as “mice”, however, it should be appreciated that such term is meant to be exemplary and not limiting). It will be appreciated that conducting electrophysiology on mice can present issues which may be different from the issues which might arise when conducting electrophysiology on humans. 
         [0006]    In present configurations for performing ophthalmic electrophysiology on mice, e.g., with an ERG dome such as that offered by Diagnosys LLC of Lowell, Mass., the anesthetized mouse is placed on a heated platform that maintains its body temperature during the test. At least three electrodes must be attached to the mouse: (i) a ground electrode; (ii) a reference electrode; and (iii) a corneal (active) electrode. In best current practice, all three electrodes are made out of platinum or silver/silver chloride and consist of two needles and a wire. One of the needles is used as a ground electrode and is easy to attach to the mouse because its position is not critical—anywhere in the haunch or tail of the mouse will do. Placement of the other two electrodes (i.e., the reference and active electrodes) requires much more care. The remaining needle electrode is the reference electrode. It must be inserted very precisely into the mouse, either at the midline of the scalp, in the mouth, or in the cheek. Mispositioning of the reference electrode will cause imbalances in the readings between the two eyes of the mouse. The last electrode, the wire electrode, is the corneal (active) electrode. It too must be placed in just the right position on the eye in order to avoid biasing the recording: too close to the center of the eye and the wire will block light; too far to the periphery of the eye and the wire will record lower voltages than if placed nearer to the center of the eye. If both eyes of the animal are to be tested, a second corneal wire must be placed in a homologous position to the first corneal wire. An added complication is that, usually, all this must be done in a room only dimly illuminated by deep red light. 
         [0007]    After the three electrodes have been placed on the mouse, the ERG dome is either moved into position over the mouse or the platform supporting the mouse is moved into the dome. Either movement may disturb the electrodes placed on the mouse, which would then require that the electrodes be repositioned. Since the mouse is hidden by the dome, it sometimes wakes up and escapes under cover of darkness. 
         [0008]      FIG. 1  shows the current Diagnosys mouse ERG dome platform in its open position. 
         [0009]      FIG. 2  shows the same Diagnosys mouse ERG dome platform in its closed position. 
         [0010]    It will be appreciated that conducting ophthalmic electrophysiology on a mouse is time-consuming and requires personnel with special skills. For this reason, ophthalmic electrophysiology is sometimes not performed on mice even where the results of performing ophthalmic electrophysiology could be beneficial. By way of example but not limitation, NIH has an impending campaign to phenotype more than 300,000 mutated mice. Among other things, the mice are being tested for deficits analogous to human eye disease. Although some of these deficits can only be detected using ophthalmic electrophysiology, electrophysiology was initially excluded from the testing protocols because existing techniques for performing ophthalmic electrophysiology on mice are too time-consuming and require personnel with rare skills. 
         [0011]    Ophthalmic electrophysiology would be significantly easier to perform on mice if there were a way to rapidly and automatically position the active and reference electrodes on the mouse. There is an existing device (a “contact lens bipolar corneal electrode”) that does this effectively for humans, but in its present state the contact lens bipolar corneal electrode is not practical for widespread use with mice. 
         [0012]    More particularly, a contact lens bipolar corneal electrode consists of a lid-retracting speculum with a reference electrode embedded in its outer circumference. A contact lens ringed by the corneal electrode is suspended by a spring from the inner part of the speculum. Since both active and reference electrodes are built into the device, the two electrodes occupy the same position on every eye (which is easily adjusted during manufacture to be at the correct position on the eye of the subject). As a result, the contact lens bipolar corneal electrode provides highly reliable positioning of the active and reference electrodes, and hence provides highly reliable results. A further advantage of the contact lens bipolar corneal electrode is that both electrodes (active and reference) touch the tear film, making excellent electrical contact with the subject without special preparation. 
         [0013]      FIG. 3  shows a human contact lens bipolar corneal electrode which was introduced by Diagnosys in  1986 . 
         [0014]      FIG. 4  shows another human contact lens bipolar corneal electrode sold by Hansen Ophthalmic Development Laboratories of Coralville, Iowa (hereinafter “Hansen Labs”). 
         [0015]    As noted above, human contact lens bipolar corneal electrodes work effectively, but mouse contact lens bipolar corneal electrodes are impractical for widespread use with mice. More particularly, a mouse contact lens bipolar corneal electrode is available from Hansen Labs, but the mouse contact lens bipolar corneal electrode is impractically delicate, expensive, and hard to make. The basic problem with the mouse contact lens bipolar corneal electrode sold by Hansen Labs is that the manufacturer does not know how its customers are going to use the lens—they may have an application that needs the animal to view an image—and so the manufacturer has to start by wrapping a corneal electrode around an optically “good”, zero-power mouse contact lens, and this is a challenging task. 
         [0016]    Another problem with mouse contact lens bipolar corneal electrodes is that, if anything, they slow the testing process down rather than speed it up. The mouse contact lens bipolar corneal electrodes are so delicate and sensitive that they require great care and skill in order to place them properly on the eye of the mouse—by way of example but not limitation, it is very easy to accidentally cover the mouse contact lens bipolar corneal electrodes with saline solution which shorts them out, and they often break during handling. In any case, mouse contact lens bipolar corneal electrodes are so hard to make that they are usually now offered only in monopolar versions, which means that the problem of placing the reference electrode on the mouse is still left to the user. The only real advantage of current mouse contact lens bipolar corneal electrodes over current wire electrodes is that the mouse contact lens bipolar corneal electrodes cover the cornea and prevent the formation of cataracts in the mouse due to drying. 
         [0017]      FIG. 5  shows the mouse contact lens bipolar corneal electrode sold by Hansen Labs. 
         [0018]    Thus there is a need for a new and improved approach for quickly and easily performing ophthalmic electrophysiology on mice. 
       SUMMARY OF THE INVENTION 
       [0019]    The present invention comprises the provision and use of a new and improved method and apparatus for quickly and easily performing ophthalmic electrophysiology on mice. 
         [0020]    In one form of the present invention, there is provided apparatus for evoking and sensing ophthalmic physiological signals in an eye, the apparatus comprising: 
         [0021]    an elongated tubular light pipe having a longitudinal axis, a distal end and a proximal end, the distal end terminating in a spheroid recess; 
         [0022]    an active electrode having a distal end and a proximal end, the active electrode being mounted to the elongated tubular light pipe and extending proximally along the elongated tubular light pipe so that the distal end of the active electrode terminates at the spheroid recess at the distal end of the elongated tubular light pipe; and 
         [0023]    a reference electrode having a distal end and a proximal end, the reference electrode being mounted to the elongated tubular light pipe and extending proximally along the elongated tubular light pipe so that the distal end of the reference electrode terminates at the spheroid recess at the distal end of the elongated tubular light pipe; 
         [0024]    wherein the distal end of the active electrode is located closer to the longitudinal axis of the elongated tubular light pipe than the distal end of the reference electrode. 
         [0025]    In another form of the present invention, there is provided a method for evoking and sensing ophthalmic physiological signals in an eye, the method comprising: 
         [0026]    providing apparatus comprising:
       an elongated tubular light pipe having a longitudinal axis, a distal end and a proximal end, the distal end terminating in a spheroid recess;   an active electrode having a distal end and a proximal end, the active electrode being mounted to the elongated tubular light pipe and extending proximally along the elongated tubular light pipe so that the distal end of the active electrode terminates at the spheroid recess at the distal end of the elongated tubular light pipe; and   a reference electrode having a distal end and a proximal end, the reference electrode being mounted to the elongated tubular light pipe and extending proximally along the elongated tubular light pipe so that the distal end of the reference electrode terminates at the spheroid recess at the distal end of the elongated tubular light pipe;   wherein the distal end of the active electrode is located closer to the longitudinal axis of the elongated tubular light pipe than the distal end of the reference electrode;       
 
         [0031]    positioning the elongated tubular light pipe against the eye of a test subject; and 
         [0032]    introducing light into the proximal end of the elongated tubular light pipe. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0033]    These and other objects and features of the present invention will be more fully disclosed or rendered obvious by the following detailed description of the preferred embodiments of the invention, which is to be considered together with the accompanying drawings wherein like numbers refer to like parts, and further wherein: 
           [0034]      FIGS. 1 and 2  are schematic views of a prior art rodent table for the ColorDome Stimulator of Diagnosys LLC; 
           [0035]      FIG. 3  is a schematic view of a prior art GoldLens Corneal Electrode; 
           [0036]      FIG. 4  are schematic views showing prior art Burian speculum type electrodes and prior art cotton wick electrodes; 
           [0037]      FIG. 5  is a schematic view showing a prior art mouse ERG electrode; 
           [0038]      FIGS. 6-12  are schematic views showing novel apparatus formed in accordance with the present invention for evoking and sensing ophthalmic physiological signals in an eye; 
           [0039]      FIG. 13  is a schematic view showing an alternative form of the apparatus shown in  FIGS. 6-12 ; 
           [0040]      FIG. 14  is a schematic view showing another alternative form of the apparatus shown in  FIGS. 6-12 ; and 
           [0041]      FIGS. 15-17  are schematic views showing exemplary novel apparatus formed in accordance with the present invention for evoking and sensing ophthalmic physiological signals in an eye. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0042]    The present invention provides a new and improved approach for quickly and easily performing ophthalmic electrophysiology on mice. 
         [0043]    More particularly, and looking now at  FIGS. 6-11 , there is shown a combined stimulator and bipolar electrode assembly  5  formed in accordance with the present invention. Combined stimulator and bipolar electrode assembly  5  generally comprises a housing  10 , a light pipe subassembly  15  and a light source subassembly  20 . 
         [0044]    Housing  10  preferably comprises a main body  22  having a cavity  25  formed therein, and a side arm  30  extending at an angle (e.g., 125 degrees) to the longitudinal axis of main body  22 . Side arm  30  includes a cavity  35  formed therein, and a magnetic mount  40  (preferably in the form of a steel ball) secured to side arm  30 . 
         [0045]    Light pipe subassembly  15  is disposed partially within, and protrudes from, cavity  25  of main body  22 . Light pipe subassembly  15  generally comprises a light pipe  45  formed out of a light-transmissive material (e.g., Plexiglass) and having a distal end  50  and a proximal end  55 . Light pipe  45  has an elongated configuration, and may be cylindrical (e.g., substantially straight with a substantially circular cross-section), or non-linear pseudo-cylindrical (e.g., bent or curved with a substantially circular cross-section), or light pipe  45  may have another acceptable configuration. Distal end  50  of light pipe  45  has a spheroid recess  60  formed therein. The radius of curvature of spheroid recess  60  is preferably similar to the radius of curvature of the eye of a mouse, so that the distal end  50  of light pipe  45  can be seated against the outside surface of the eye of a mouse. Light pipe  45  also comprises a pair of slots  65 A,  65 B formed in the outer surface of light pipe  45 . In one preferred form of the invention, slots  65 A,  65 B are diametrically opposed to one another. The distal end of slot  65 A has a greater depth than the remainder of slot  65 A, so that the distal end of slot  65 A approaches (but preferably does not reach) the center of spheroid recess  60 . Preferably at least the distal portion of slot  65 A outboard of wire  70 A is filled with an appropriate material (e.g., a light-transmissive, non-conductive, waterproof material) so as to eliminate air gaps between light pipe  45  and the eye of the mouse. A platinum (or silver or gold, etc.) wire  70 A, which serves as the active electrode for combined stimulator and bipolar electrode assembly  5 , is disposed in slot  65 A. Note that the distal end of platinum wire  70 A follows the floor of slot  65 A so that the distal end of platinum wire  70 A approaches the center of spheroid recess  60 . The distal end of platinum wire  70 A communicates with spheroid recess  60 . A platinum (or silver or gold, etc.) wire  70 B, which serves as the reference electrode for combined stimulator and bipolar electrode assembly  5 , is disposed in slot  65 B. The distal end of platinum wire  70 B also communicates with spheroid recess  60 . Preferably at least the distal portion of slot  65 B outboard of wire  70 B is filled with an appropriate material (e.g., a light-transmissive, non-conductive, waterproof material) so as to eliminate air gaps between light pipe  45  and the eye of the mouse. Note that the distance between the distal end of platinum wire  70 A (which will act as the active electrode) and the distal end of platinum wire  70 B (which will act as the reference electrode) is substantially equal to the distance between a portion of the eye which exhibits an evoked physiological signal and a portion of the eye which exhibits a lesser evoked physiological signal (or, preferably, does not exhibit an evoked physiological signal), e.g., the distance between the cornea and the perimeter of the eye. The intermediate portions of platinum wires  70 A,  70 B may be held to the body of light pipe  45  with shrink bands  75 . The proximal end  55  of light pipe  45  is disposed in cavity  25  of main body  20 , and the proximal ends of platinum wires  70 A,  70 B are passed through cavity  35  of side arm  30  so that they can be brought out the proximal end  80  of side arm  30  for connection to appropriate amplification (e.g., by a differential amplifier) and processing electronics (not shown) for ERG signal processing. 
         [0046]    Light source subassembly  20  is disposed within cavity  25  of main body  20 . Light source subassembly  20  generally comprises LEDs  85  for generating light, and any appropriate optics (not shown) required to transmit the light generated by LEDs  85  into the proximal end  55  of light pipe  45 , whereupon the light will travel down the length of light pipe  45  to the distal end  50  of light pipe  45 . A power line  90  provides power to LEDs  85 . Preferably a wire mesh  95  (or similar element) is provided distal to LEDs  85  and proximal to platinum wires  70 A,  70 B so as to provide electromagnetic interference (EMI) shielding between LEDs  85  and platinum wires  70 A,  70 B. 
         [0047]    It will be appreciated that, on account of the foregoing construction, combined stimulator and bipolar electrode assembly  5  can be supported via its magnetic mount  40  for use with an ERG mouse platform, with the proximal ends of platinum wires  70 A,  70 B being connected to appropriate amplification and processing electronics for ERG signal processing, and with power line  90  being connected to an appropriate source of power. When a mouse is to be tested, the mouse is placed on the ERG mouse platform, a ground electrode (not shown) is attached to the mouse, and then housing  10  can be moved so as to bring the distal end  50  of light pipe  45  into contact with the eye of the mouse. This action will position the distal end of platinum wire  70 A (i.e., the active electrode) at the appropriate position on the eye of the mouse, and will simultaneously position the distal end of platinum wire  70 B (i.e., the reference electrode) at another appropriate position on the eye of the mouse. When LEDs  85  are thereafter energized, the light from LEDs  85  passes down light pipe  45  and into the eye of the mouse, whereby to stimulate the eye of the mouse. Platinum wires  70 A (i.e., the active electrode) and  70 B (i.e., the reference electrode) pick up the electrophysiological response of the eye of the mouse as electrical signals, and these electrical signals are passed along platinum wires  70 A,  70 B to appropriate amplification and processing electronics for ERG signal processing. 
         [0048]    Thus it will be seen that with the combined stimulator and bipolar electrode assembly  5  of the present invention, the assembly simultaneously provides (i) the stimulator needed for conducting ophthalmic electrophysiology on a mouse (i.e., LEDs  85  and light pipe  45 ), (ii) the bipolar electrode needed for conducting ophthalmic electrophysiology on a mouse (i.e., platinum wires  70 A,  70 B supported by light pipe  45 ), and (iii) the support structure (e.g., magnetic mount  40 ) for holding the bipolar electrode securely against the eye during testing. 
         [0049]    Significantly, mounting platinum wires  70 A,  70 B to the light pipe  45  provides a robust mechanical support for the platinum wires, making it possible to quickly, easily and precisely position the active electrode (i.e., platinum wire  70 A) and the reference electrode (i.e., platinum wire  70 B) on the eye of the mouse. At the same time, the small acceptance angle of light pipe  45  restricts the light reaching the eye of the mouse to that generated by LEDs  85 , which eliminates the normal need for a large Ganzfeld to conduct ophthalmic electrophysiology. Note that LEDs  85  may be a three-color RGB system, although UV could also be used and would be desirable in mice. In one preferred form of the invention, appropriate electronic drivers are provided to drive RGB LEDs  85  accurately enough to form precisely-defined metameric colors. If desired, and looking now at  FIG. 12 , light pipe  45  may comprise a main body  45 A and an end diffuser  45 B. End diffuser  45 B can, advantageously, help provide full retinal illumination. More particularly, end diffuser  45 B acts to broaden the angle at which light exits main body  45 A of light pipe  45  and enters the eye of the mouse, and ensures that light exiting the light pipe is distributed equally to all parts of the retina of the mouse. The diffusing material of end diffuser  45 B is preferably of non-uniform thickness, i.e., it is made thinner at the edges to compensate for the lower flux density occurring at the perimeter of the light pipe. Furthermore, if desired, reference electrode  70 B may be “doubled over” so as to increase the surface area contact of reference electrode  70 B with the eye of the mouse. And, if desired, and looking now at  FIG. 13 , a conductive foil (or conductive film)  100  may be provided at distal end  50  of light pipe  45 , with conductive foil (or conductive film)  100  electrically connected to reference electrode  70 B so as to increase the surface area contact of reference electrode  70 B with the eye of the mouse. 
         [0050]    In some cases, it can be helpful to provide the user with “red light” illumination to help the user set the combined stimulator and bipolar electrode assembly  5  against the eye of the mouse. To this end, if desired, and looking now at  FIG. 14 , a light-transmissive sleeve  105  may be disposed coaxially about light pipe  45 , with light-transmissive sleeve  105  acting as an additional light pipe for delivering red light to the distal end of light pipe  45 . More particularly, in this form of the invention, when red light is introduced into the proximal end of light-transmissive sleeve  105 , a ring of red light will be provided at the distal end of light-transmissive sleeve  105 , whereby to provide a rim of red illuminating light about the distal perimeter of light pipe  45 . 
         [0051]    The combined stimulator and bipolar electrode assembly  5  of the present invention can be set up not only more accurately, but also much more quickly, than the present state-of-the-art, even by relatively unskilled personnel. After positioning the mouse on the heated table described above and inserting the ground electrode (e.g., in the haunch or tail of the animal), the combined stimulator and bipolar electrode assembly  5  is simply brought into contact with the eye of the mouse by moving housing  10  (which causes magnetic mount  40 , e.g., a steel ball, to roll within a magnetic cup, e.g., a magnetic ball holder (see  FIG. 1  above, which shows a magnetic ball holder of the sort which may be used), and then the test is ready to run. A second device can be used simultaneously on the fellow eye (i.e., the other eye of the mouse) if desired. This eliminates several minutes fumbling in near darkness to carefully adjust the electrodes and position the Ganzfeld. Additionally, since light pipe subassembly  15  is held in position against the eye by an external mechanical mount (i.e., magnetic mount  40 ) and is not supported by the eye per se, it is not necessary to use particular care to position combined stimulator and bipolar electrode assembly  5  precisely against structurally robust eye tissue. Furthermore, since light pipe subassembly  15  has no accessible distal surface once it is seated against the eye, it is substantially impossible to obscure the light path from light pipe subassembly  15  into the eye by the use of excessive saline. 
         [0052]    Testing of the combined stimulator and bipolar electrode assembly  5  on mice has yielded excellent results. It produces expected waveforms with very little noise, although the overall amplitude of the waveforms is small. 
         [0053]    In addition to the foregoing, some investigators have used an active electrode in one eye, and a reference electrode in the other eye. This technique still involves accurate placement of two corneal wires (extremely challenging with prior art electrodes), but the fellow eye makes an excellent impedance-matched reference. However, with this approach, care must be taken to avoid light crosstalk between the eyes—the reference eye must not receive any stimulus light. 
         [0054]    Using the combined stimulator and bipolar electrode assembly  5  of the present invention solves both problems (i.e., accurate placement of electrode and avoiding light crosstalk between the eyes). More particularly, in one form of the invention, the corneal electrode  70 A of, for example, the right eye is plugged into the active side of the differential amplifier, and the corneal electrode  70 A of the left eye into the reference side of the differential amplifier. The electrodes in each eye are automatically correctly positioned. The eyes are then stimulated one at a time using the light source subassemblies  20  of the combined stimulator and bipolar electrode assemblies  5 , and there is no optical crosstalk because of the light pipe configuration (i.e., the positioning of a light pipe on an eye of the mouse limits the light reaching that eye of the mouse to only the light transmitted by that light pipe). When the right eye is being driven, the signal is normally polarized, and when the left eye is being driven, the signal is inverted. Alternatively, both eyes of the mouse could be simultaneously stimulated using light source subassemblies  20  of the combined stimulator and bipolar electrode assemblies  5 , and the differential between the two corneal electrodes  70 A may be measured so as to identify differences in eye function. 
         [0055]    Alternatively, the reference electrodes  70 B may be used in place of the corneal electrodes  70 A. In this form of the invention, the reference electrode  70 B of, for example, the right eye is plugged into the active side of the differential amplifier, and the reference electrode  70 B of the left eye is plugged into the reference side of the differential amplifier. The electrodes in each eye are automatically correctly positioned. The eyes are then stimulated one at a time using the light source subassemblies  20  of the combined stimulator and bipolar electrode assemblies  5 , and there is no optical crosstalk because of the light pipe configuration (i.e., the positioning of a light pipe on an eye of the mouse limits the light reaching that eye of the mouse to only the light transmitted by that light pipe). When the right eye is being driven, the signal is correctly polarized, and when the left eye is being driven, the signal is inverted. Alternatively, both eyes of the mouse may be simultaneously stimulated using light source subassemblies  20  of the combined stimulator and bipolar electrode assemblies  5 , and the differential between the two reference electrodes  70 B may be measured so as to identify differences in eye function. 
         [0056]    In one preferred form of the invention, and looking now at  FIGS. 15-17 , platinum wire  70 A can be omitted and platinum wire  70 B can be provided with a conductive foil (or conductive film)  100 . When configured in this manner, the present invention essentially comprises a combined stimulator and monopolar electrode assembly. This form of the invention can be advantageous where combined stimulator and monopolar electrode assemblies are positioned against both eyes of the mouse (for stimulating one eye at a time or for simultaneously stimulating both eyes at the same time). 
         [0057]    The robustness of the electrical and optical connections that the new combined stimulator and bipolar electrode assembly  5  makes with the mouse has been dramatically demonstrated during testing. Toward the end of testing, the mice may wake up and begin to move. With conventional setups, the first movement of the awakening mouse breaks corneal contact and the testing is over. With the combined stimulator and bipolar electrode assembly  5  of the present invention, contact with the awakening mouse was successfully maintained even though the mouse was moving and testing continued with good results until the mouse literally walked away. 
         [0058]    In the foregoing disclosure, platinum wire  70 A (i.e., the active electrode) is disposed within slot  65 A which extends along an outer surface of light pipe  45 , and platinum wire  70 B (i.e., the reference electrode) is disposed within slot  65 B which extends along an outer surface of light pipe  45 . However, if desired, slot  65 A could be replaced with a bore extending longitudinally through light pipe  45  and platinum wire  70 A (i.e., the active electrode) may be disposed within this longitudinal bore, and/or slot  65 B could be replaced with another bore extending longitudinally through light pipe  45  and platinum wire  70 B (i.e., the reference electrode) may be disposed within this other longitudinal bore. In such a construction, the longitudinal bore receiving platinum wire  70 A (i.e., the active electrode) is disposed closer to the longitudinal axis of light pipe  45  than the longitudinal bore receiving platinum wire  70 B (i.e., the reference electrode). 
       Modifications Of The Preferred Embodiments 
       [0059]    It should be understood that many additional changes in the details, materials, steps and arrangements of parts, which have been herein described and illustrated in order to explain the nature of the present invention, may be made by those skilled in the art while still remaining within the principles and scope of the invention.