Eye Movement Measurement in the MRI

3/12/04 Biomedical Engineering 201

Team Members: Joshua Anders Betsy Appel Bryan Baxter Alyssa Walsworth

Client: Luis Populin, Ph.D. Department of Anatomy University of Wisconsin - Madison

Advisor: Justin Williams, Ph.D. Department of Biomedical Engineering University of Wisconsin - Madison Introduction

Neuroscience is a branch of the sciences that deals specifically with the anatomy, biochemistry, physiology, or molecular biology of nerves and nerves tissue with relation to behavior and learning. Today, in research labs all across the country, neural studies are being conducted to relate motion, behavior, reaction, and other mechanical, biological processes demonstrated by all forms of life to neurological occurrences in the anatomical systems of the body.

One may question the biological rationale behind such research. As neurology is a study relating to behavior and learning, neurological research can give us insight about the thought processes occurring in the brain. Furthermore, this study can be coupled with a physical or mental process and a functional magnetic resonance imaging machine

(fMRI machine) to examine the activity in the brain of a subject while performing the specified task. Dr. Populin, a member of the anatomy department at the University of

Wisconsin – Madison, wishes to study the neural mechanism behind the spatial visual attention in the cerebral cortex of macaque monkeys through functional magnetic resonance imaging.

The basic set up involved in Dr. Populin’s procedural portion of study is initiated by a macaque monkey viewing a projection screen. The monkey is then exposed to a preliminary sources of red light projected onto the screen. Almost immediately after, a second red dot appears on the projection screen, in the line of sight of the monkey. At this point, the subject may either switch its attention to the second dot or maintain its focus on the initial point. A rewarded is granted to the monkey based on whether its visual attention performed the requested command. Given this procedure, a device

2 capable of detection the line of sight of the monkey is needed as eye movements relate to attention.

To date, different procedures have been used in isolation to gather information related to the results Dr. Populin seeks. Anesthetized monkeys have been examined in the fMRI. However, this approach is obviously flawed if used in conjunction with Dr.

Populin’s experimenting as the anesthetics do not allowed for the subject involvement that is required for testing behavior, specifically spatial visual attention.

With these procedures and constraints in mind, the end goal of this design is to create an eye detection device that can be used in conjunction with the fMRI.

Problem Statement

The client requests a detection device to measure horizontal eye movements of a macaque monkey in a Magnetic Resonance Imaging (MRI) environment.

Background Information

Eye Movements

Eye movements in most mammals operate in the same manner. All vertebrates have six extraocular muscles attached to the eye to aid in movement (Delgado-García,

1999). There are two types of eye movements common to primates: saccadic eye movements and smooth eye movements. Saccadic eye movements shift the line of sight due to angular displacement caused by rapid jumps a few times every second. An example of such movements is when a person reads a book, as shown in Figure 1.

Saccadic movement depends on both eye and head movement. The second type of movement, smooth eye movement, keeps the gaze on a specific object within its visual field between saccadic eye movements. A motion signal must be present to initiate this

3 type of movement (Kowler, 1990). Smooth eye movements allow the eye to rotate at the same velocity as the object it is focused on (Gazzaniga, 1999). The horizontal eye movements of the monkeys that will be tested using this detection device will be recording saccadic eye movements.

Figure 1: This graph shows the change in the angular position of the eye (saccadic movement) as time progresses while a person reads a book. Short, rapid jumps are made as the eye follows the page from the left to the right. A sharp decrease in angular position occurs as the eye moves back from the right to the left to start reading the next line.

Magnetic Resonance Imaging (MRI)

Magnetic Resonance Imaging is an imaging technique that serves as an important diagnostic tool in hospitals everywhere. As an MRI machine scans the body, it builds a map of tissues, either 2- or 3-dimensionally, that can be made into a 2- or 3-dimensional image of the body. An MRI scanner is composed of three different types of magnets: resistive, permanent, and super-conducting magnets. The strengths of these magnets are measured in tesla. The strong magnetic field produced by these magnets aligns the hydrogen atoms in the body. A radio frequency pulse specific to hydrogen causes the hydrogen atoms to absorb energy and spin at a certain frequency, creating a resonance.

4 MRI machines are equipped with coils that apply this radio frequency pulse. Different sized and shaped coils are used for different areas of the body. The radio frequency pulse is slowed down, allowing atoms to release energy and return to their natural alignment.

The coil picks up the change in energy and sends signals to a computer system that will transform the signals into an image. The stronger the magnetic field, the sharper, more defined the image. An MRI can image any plane of the body, axially, coronally, or sagitally.

Functional Magnetic Resonance Imaging (fMRI) is a more specific neuroimaging technique used to detect local changes in brain activity. Anatomical scans are done first, using normal MRI, in order to determine the position for a functional scan (Buckner and

Logan, 2001). As brain activity in a region increases, blood flow and oxygen levels also increase in this region. fMRI detects these changes in the oxygen content of different regions of the brain through a decrease in levels of the paramagnetic molecule deoxyhemoglobin. The signal used to pick up this change is referred to as the BOLD contrast, or the Blood Oxygenation Level Dependent contrast. (Buckner and Logan,

2001). The signals are processed and an image of brain activity is created, as shown in

Figure 2.

Figure 2: This image is an example of the image that an fMRI creates by mapping the active regions of the brain during human saccadic eye movement. The active brain regions are shown in red.

5 Research involving fMRI locates imaging measurements and visual areas simultaneously in order to compare activity in single brain cells (fMRI measurement methods, 2003 MIT press). This imaging technique will be used for a similar purpose with our device. The fMRI scanner that will be used with our device is located at the

Waisman Center and operates with a magnetic strength of 3 tesla.

Literature Search

Experimental research requiring the measurement of eye movement has been performed for the last 40 years. Within the last ten years, Magnetic Resonance Imaging has been used in conjunction with eye movement measurements to study the brain areas functioning when eye movements are made. This has led to the development and subsequent improvement of eye trackers for use in the MRI environment. Currently, there is a lack of published papers documenting non-human eye movement detection devices suitable for the MRI environment.

A variety of methods have been used outside of the MRI, but some of these are not so promising for use within. Electrooculography measures the voltage difference between the cornea and the retina but utilizes electrodes that are not compatible with the

MRI and is extremely invasive and time consuming to position these at the beginning of every recording session. Another method is utilizing sclera search coils which are implanted within the muscle layer of the eye (Judge et al., 1980). These consist of coils of wire through which current is induced when the eye are moved in relation to a search coil located around the head of the subject. The search coil generates magnetic fields in two planes to measure both horizontal and vertical eye movement. This technique is generally thought to be unusable in the MRI due to the strong magnetic fields generated

6 by the MRI coil rendering those generated by a field coil unreadable and from the interference of the MRI imaging by the field coil. The field coil would also interfere with the MRI.

The use of infrared detecting photodiodes with an infrared source, usually an

LED, has been used in humans to detect eye movement. Reulen used a plate consisting of 9 infrared LEDs and the same number of phototransistors to measure the reflection of the infrared light off of the sclera, the white part of the eye, and off of the pupil (Reulen et al., 1988). Voltage from each phototransistor is measured and compared. With eye movement, the voltage of the 3 transistors on one side will decrease due to absorption by the pupil, and voltage of the 3 transistors on the other side will increase as increasing scleral area will reflect more of the infrared light. The initial experiment was performed in a normal, non-MRI environment. Kimmig used the method developed by Reulen but adapted it to the MRI environment (Kimmig et al., 1999). Fiber optic cables were used to carry the infrared light to and from the subject’s eye. The radiation is created outside of the MRI room’s Faraday cage and carried in through these cables. The reflected light is carried out of the room by the cables and is converted to the electric current by photodiodes located outside the room. This output is filtered and conditioned by analog circuits and then passed through an ADC to a computer.

A third method of eye measurement detection consists of a utilizing an infrared camera to view the eye with infrared radiation directed onto the eye using fiber optic cable. The source light is generated outside of the Faraday cage, similar to the photodiode method, and directed onto the eye via a fiber optic cable. A camera designed to capture infrared light is positioned within the MRI room and, in the case of humans,

7 captures the image of the eye from a mirror positioned above the head. Software is used to find the darkest point of the image and this point is designated the pupil.

There are multiple devices marketed for use with humans based on these outlined concepts. Sensimotoric Instruments produces the human long range eyetracker, infrared camera, used by Gitelman (Gitelman et al., 2000), as well as a short range camera device.

A device utilizing a camera and infrared source mounted on glasses has previously been patented (Bullwinkel, 2000). The tracker used by Kimmig is not available commercially and has not been patented. Considering these patents and devices, there is nothing currently on the market that would be fit the desires of the client.

Design Requirements

The purpose of the eye scanner is to measure horizontal saccadic eye movements of a monkey in an MRI. The client requires that the scanner not harm the monkey in any way. Also, the scanner must not block the monkey’s view of the stimuli.

Since the scanner will be functioning in an MRI, it will be exposed to magnetic fields and radio waves. It cannot contain any ferromagnetic material or electrical components. It should measure the horizontal eye movements from only one eye, since both eyes move identically in all mammals. Also, the device cannot block the monkey’s view of the projection screen, which will be presenting the visual stimuli approximately

10 feet away from the monkey.

Due to the monkey’s training, it must be quickly rewarded for correct behavior.

Therefore, the device must transfer the recordings within 0.5 seconds of the action (~60

Hz). In order to receive clear signals and precise measurements, the device cannot make any additional noise.

8 The device should function for several hours a day with a life span of approximately 10 years. The temperature of the environment will remain at approximately room temperature (25° C). The device should be compatible with the coils used in the MRI. The shape and form should coincide with the dimensions of the machine that the designed will be placed, not exceeding the size of the bore. Parts of the device will need to be taken out or moved in order to allow other operations in the MRI.

Since the components should be easily transported, they should not exceed a weight of 30 lbs.

For a more detailed description of the client’s design requirements, see Appendix

2 for the Product Design Specifications.

Design Alternatives

In generating different design ideas, it is important to ensure that the design requirements are considered. After research and much brainstorming, the design process is initiated. Ideas are evaluated according to the design requirements and the top three designs are selected. From these three designs, a chosen design is selected.

Design 1: Magnetic Eye Coils

A design for measuring horizontal and vertical eye location in monkeys and cats that is currently in use today involves the use of implanted magnetic search coils. Prior to the experimental set up, a surgical procedure is required to implant the device into the muscular layer of one of the eyes of the subject. The monkey is anaesthetized with a light dose of Nembutal, with the eye that is being operated on positioned facing the surgeon. A 360 degree incision is made at the limbus, the junction between the cornea and the sclera, into the muscular layer with a scalpel. The incision allows for the

9 insertion of the search coil. After closure, the monkey may take anywhere from 2 to 7 days for recovery (Judge et al., 1980).

The search coil piece itself is made up of 3 interwoven turns of copper wire coated in Teflon. Each turn has an approximate diameter of 16 mm. The final turn of the wire is twisted spirally about the previously two turns to maintain the coil structure integrity and eliminate invalid induced voltages (Judge et al., 1980).

When the experiment is ready to proceed, the monkey, with the newly implanted search coil is placed in a horizontal magnetic field induced by a second coil surrounding the body of the monkey. This magnetic field is complimented by the vertical produced by the implanted magnetic search coils. A 90 degree angle is formed between the two perpendicular fields. This makes calculations quiet easy as the voltage induced by the implanted search coil is proportional to the sine of the angle between the search coil and the magnetic field formed by the exterior coil (sinθ=1 when θ=90º). This calculation, known as phase sensitive detection as the currents are 90 degrees out of phase of one another, determines the horizontal and vertical components of the eyes location (Judge et al., 1980).

In general terms, when the eye moves a current is induced in the implanted search coil. This current is then output and conditioned by a circuit located outside the faraday cage, or MRI room. Finally, computer software is used to determine the current and position of the eye (Judge et al., 1980).

A design involving magnetic search coil implantation has two major advantages.

One, the precision and accuracy of the reading will be increased as the device is not located a certain distance d away from the subject, but rather within the muscular layer of

10 the eye. Secondly, this system has an established history of success. The laboratory of

Sensorimotor Research and Clinical Branch in Bethesda, Maryland has successfully implanted the device into eight different monkeys. Other advantages include a limitless detection range and a clear line of sight for the monkey between the fMRI machine and the projection screen.

However, in the case of the search coil design, the disadvantages far outweigh the advantages. A surgical procedure would be involved. Not only would this procedure require a recovery process, but it may also alter the monkey behavior. In considering alternative designs relating to living things, one must always remember to keep the subjects safety as the top priority. Secondly, since communication with a primate is limited due to obvious language barriers, it is necessary to maintain a stable environment and a uniform daily schedule in order to assure accurate, repeatable, and consistent results. A slight change may alter the monkey’s behavior during the experiment, thus requiring more training of the monkey to follow procedure. Lastly, due to magnetic constraints, the implanted search coil device is yet to be tested successfully in an fMRI setting.

Design 2: Infrared Camera Figure 3: Diagram of implanted magnetic search coils (Skalar Medical BV) 11 A long-range infrared camera system is another possible design alternative. One such system is manufactured by Applied Science Laboratories. In this system, as shown in Figure 4, the module that contains the infrared camera and illuminator can be placed up to 16 feet away from the subject, preventing interference with the fMRI. The power for the module is supplied by cables that run through a filtered connection (Applied

Science Laboratores).

Figure 4: Applied Science Laboratories Model 504 LRO optics module

As the subject is viewing a stimulus on the rear projection screen, the illuminator is projecting infrared light onto the eye. The infrared camera inside the module records horizontal and vertical eye movement, as seen in Figure 5, Appendix 1 (Applied Science

Laboratories).

The video signal captured by the infrared camera is converted into an optical signal, which is transmitted to computers outside the MRI room via fiber optic cables.

Computer software then analyzes the data and converts the signal into x and y components to determine the line of sight (Applied Science Laboratories). However, errors can result from infrared reflective light effects, video distortion and variations in eye curvature (Richards, 1990).

12 The infrared camera system is non-invasive, so the subject would not have to go through surgery. Only infrared light will be shined on the eye. The electronics in some cameras may make them incompatible with the MRI environment. Even though cameras are readily available for purchase, generally they are out of a reasonable price range.

Design 3: Photodiodes (Chosen Design)

The photodiode system utilizes infrared radiation reflected off of the eye to determine the eye position. This design is based on previous devices successfully used in humans (Kimmig et al., 1999; Reulen et al., 1988) both outside and inside the MRI environment. The system consists of two LEDs, four fiber optic cables, two photodiodes, and multiple circuit elements to measure the horizontal movement of one eye.

Two LEDs emitting infrared radiation of a 940nm wavelength are located on a circuit inside of the MRI control room. Two fiber optic cables, one for each LED, transmit the maximum amount of generated light possible and are positioned to reduce the attenuation and reflection due to a medium change from the air to the glass of the fiber optic cables. The cables are run under the floor of the control room to the MRI scanning room. The other ends of the cables, located inside the MRI room, are attached to the monkey restraining chair and directed at the subject’s selected eye. The cables are located above the eye, one on either side of the pupil when the subject is looking directly forward, and the cables direct the transmitted radiation at the eye.

The intensity of infrared radiation reflected off of the eye is based on the section of the eye the radiation is incident upon. The sclera will reflect more of the infrared radiation than the iris or pupil. The pupil will absorb the greatest amount, followed by the surrounding iris. When the eye moves, the amount of light reflected from each source

13 changes, one has an increase in reflection, the other has a decrease. When eye movement is to the subject’s right, from center, the light source on the subjects left will have an increase in reflection and the light source on the right will have a decrease. Calibration of eye movement will be required for accurate measurements.

The radiation reflected from the eye will be captured by two fiber optic cables paralleling those that the radiation was transmitted from. These cables will be located below the eye on either side of the pupil. The receiving fiber optic cables will carry the optical signals into the control room and bring the radiation in contact with the photodiodes. The photodiodes used have maximum photosensitivity at 940 nm. These generate a voltage based on the intensity of incident radiation, converting light signals to electrical signals. The voltage signal from each photodiode is sent through a buffer and then both signals through an operational amplifier to generate a differential voltage with needed amplification. A bandpass filter is used to further condition the signal and then the signal is output to an oscilloscope to display horizontal position based upon this voltage.

The photodiode method has several advantages based upon the operating environment and our design specifications. The system is fully compatible with the fMRI function as fiber optic cables, which do not interfere with the scanning, are the only material within the MRI room. There is little invasiveness inherent in our design; infrared radiation on the eye is the extent of the device’s interaction with the subject. Our spending is limited by the money our client and our department are willing to provide.

This device will not exceed our budget, the only expensive parts being the four sections of fiber optic cable. This system will provide the needed information, eye position with

14 desired accuracy, .25°, in the desired amount of time. A minor disadvantage is the

calibration time before each session to standardize output to eye movement.

Design Comparison

After developing an understanding for our three design alternatives, they must be

analyzed for their advantages and disadvantages with the end object being to select a final

design that will be pursued for the remainder of the project. A system must be devised in

order to sort the merits and fallbacks of each design, considering the most important

differences between the alternative designs. With this in mind, a design matrix is

constructed and used to evaluate the three options (magnetic eye coils, infrared camera,

and photodiodes). In particular, their invasiveness, expense, MRI machine compatibility,

and responsiveness are contrasted. Each category of each design receives a value

between 0 and 5, 0 being the lowest and 5 being the highest, corresponding to its

performance in the specified area. After the design matrix is completed, the design with

the highest cumulative score will be the pursued design. The design matrix used in the

procedure described can be found as Table 1.

Table 1: Design Matrix

Future Work

Magnetic Eye Coils Infrared Camera Photodiodes Invasiveness 0 5 4 Expense 2 1 5 MRI Compatibility 1 3 5 Responsiveness 5 4 5 Total 8 13 19

The final design chosen is both feasible and functional, meeting the client’s

design requirements. The final design and two alternatives will be presented to the client

15 to receive his input on the team’s work. After meeting with him, we will incorporate his feedback into the final design. Further research needs to be done to determine possible materials and parts that are needed to complete a functional prototype. We need to consider the cost of materials and our ability to build the device with the resources available to the team. We will consult specialized professors in the field of fiber optics for their specific design advice. Our end goal is to create a functional prototype by the end of the semester.

16 References

Applied Science Laboratories (ASL): www.a-s-l.com

Buckner, R.L. and Logan J.M. 2001. Functional Neuroimaging Methods: PET and fMRI. In R. Cabeza and A. Kingstone (eds.) Handbook of Functional Neuroimaging of Cognition. Boston, MA: MIT Press.

Bullwinkel, P.E., 2000. Fiber optic eye-tracking system utilizing out-of-band light source. US Patent, 6,079,829

Delgado-García, J.M. 1999. Oculomotor System. Nature: Encyclopedia of Life Sciences. Nature Publishing Group.

Gazzaniga, M.S. (ed.). 1999. fMRI measurement methods. The New Cognitive Neurosciences. Boston, MA: MIT Press.

Gitelman, D.R., Parrish, T.B., LaBar, K.S., Marsel Mesulam, M. 1999. Rapid Communication: Real-time monitoring of eye movements using infrared video oculography during functional magnetic resonance imaging of the frontal eye fields. NeuroImage. 11: 58-65.

Judge, S.J., Richmond, B.J., Chu, F.C. 1980. Implantation of magnetic search coils for measurement of eye position: An improved method. Vision Research. 20: 535 538.

Kimmig, H., Greenlee, M.W., Huethe, F., Mergner, T. 1999. MR-Eyetracker: a new method for eye movement recording in functional magnetic resonance imaging. Exp Brain Res. 126: 443-449.

Kowler, E. (ed.). 1990. Eye Movements and Their Role in Visual and Cognitive Processes. Reviews of Oculomotor Research. 4: 1-70.

Reulen, J.P.H., Marcus, J.T., Koop, D., de Vries, F.R., Tiesinga, G., Boshuizen, K., Bos, J.E. 1988. Med. & Biol. Eng. & Comput. 26: 20-26.

Richards, J.E. 1990. Eye position prospectus for measuring eye movements. http://jerlab.psych.sc.edu/pdf/eyeposition.pdf

Skalar Medical BV. www.skalar.nl.

17 Appendix 1

Head coil

Eye camera & IR illuminator

Figure 5: Diagram of the infrared camera system as seen from the side of the MRI. Arrows show light paths for the module and the line of sight for the subject (adapted from Gitelman et al., 1999)

18 Appendix 2

19 Appendix 3

Product Design Specification 2/22/04

Team Members: Josh Anders, Betsy Appel, Bryan Baxter, Alyssa Walsworth

Function: A device is needed to measure the saccadic eye movements of a monkey in the MRI. The monkey's head will be restrained. The device should not block the monkey's line of sight to the stimulus displayed on a screen directly in front of it. All materials used in the device must be compatible with the MRI operating environment.

Client Requirements: l Device must not discomfort or harm subject in any way. l Device must not block the subject's view of the stimulus. l Device must measure horizontal eye movement only.

Design Requirements:

1. Physical and Operational Characteristics

a. Performance Requirements: This device needs to function in an MRI scanner, and therefore cannot be made of anything ferromagnetic or electrical. The system must measure the horizontal eye movements of a monkey while a functional MRI scan measures the physiological recording of multiple cells of a monkey. Because both eyes move identically, the device needs to record movements from only one eye. The monkey will be unrestrained, sitting in a monkey chair and behaving normally. The monkey needs to be able to see a projection screen, so its field of view can not be obstructed. The device cannot exceed the size of the bore in the MRI scanner. In order to receive clear signals, the device cannot make any additional noise.

b. Safety: The monkey can not be harmed in anyway through the use of this system. Because of its use with live animal research, the safety of this device will need to be demonstrated.

c. Accuracy and Reliability: This machine must accurately record the horizontal eye movements of the monkey. The speed at which these recordings are transferred must be faster than 0.5 seconds so that the monkey can be quickly rewarded for the correct action performed (~60Hz). The signals received must be clearly transmitted for the most precise measurements.

d. Life in Service: The device must endure usage for several hours a day for 10 years.

e. Shelf Life: The device should function for approximately 10 years in a research laboratory setting. f. Operating Environment: The eye scanning system will be in an MRI environment, so it will be exposed to magnetic fields and radio waves. The temperature will remain at approximately 25°C.

20 g. Ergonomics: The eye scanner will be placed a few inches away from the monkey’s eye, preferably off to the side to prevent blocking the view. In front of the monkey, about 10 feet away, will be the projection screen, which the monkey will look at for visual stimuli.

h. Size: The system should be compatible with the coils used in the MRI. Parts of the system will need to be taken out or moved in order to allow other operations in the MRI. The size of the room which the MRI is located is approximately 20ft x 20ft, with the MRI taking up a majority of the space.

i. Weight: The system should be easily transported, as certain components will need to be switched out. Under 30lbs is preferable.

j. Materials: As our design involves an MRI machine, ferrous materials will not be allowed in the construct of our prototype.

k. Aesthetics, Appearance, and Finish: No specifications were requested by our client regarding color or texture. The shape and form of the device should coincide with the dimensions of the machine that the design will be placed.

2. Production Characterisitics

a. Quantity: One eye motion detection device unit is requested by our client

b. Target Product Cost: The Biomedical Engineering Department of the University of Wisconsin – Madison has supplied us with a minimal budget. Our client has suggested allotting funds for the construction process of the prototype. Ideally, the design would be within the range of funds given by the department and our client.

3. Miscellaneous

a. Standards and Specifications: ANSI safety requirements for incidence of infrared light on the eye must be followed. Approval for use in research experiment will be acquired by client.

b. Customer: Client prefers an optical system for eye movement measurement. System should be attached to subject restraining device if possible.

c. Patient-related concerns: Device must not harm subject or cause discomfort in any way. Use of this device should not stress the monkey in any way.

d. Competition: Similar devices exists for use on humans in the MRI. Device is integrated into glasses and captures movements using infrared light reflection technique and fiber optic cable. Capturing device is connected to fiber optic cable and computer for analysis. Other commercially marketed devices use mirrors and an infrared camera in conjunction with computer analyzing systems to measure eye movements in humans. Companies marketing these products include SensiMotoric Instruments, Skalar, and Applied Science Laboratories. No devices for use with non-human primate subjects were found.

21 Appendix 3

Ethical Issues

Throughout the design process, ethics must always be considered. The main ethical concern with the designing of the eye movement detection device involves the safety of the monkey. The monkey exposed to the device must be properly handled and not harmed by any experimentation. The monkey should not be put in pain or under any unusual stress during the use of the device. The device’s safety must be proven prior to use with the monkeys. Also, operators must be prepared and able to prove its safety to inspectors at any time during future use. This issue was considered in choosing the final design for the device. Evaluations were conducted on each of the three designs based on their invasiveness in relation to the monkey. For this reason, the implanted search coils were immediately eliminated.

Another concern during the design process involves intellectual property rights.

There are many types of devices manufactured for use with humans that accomplish what is needed by the client, including varied versions of our three designs. Although no current device exists for use with monkeys, we must be cautious not to unethically use someone else’s ideas as our own.

22