Direction of Mouse Head Position and Orientation

Midsemester Report

BME 200/300

Department of Biomedical Engineering

University of Wisconsin – Madison

Client: Matthew I. Banks, Ph.D.

Professor of Department of Anesthesiology

UW-Madison

Advisor:John G. Webster, Ph.D

Professor of Department of Biomedical Engineering

UW-Madison

Group Members: Yao Lu, Andrea Zelisko

October 10, 2003

Problem Statement

Our purpose is to develop a device to help study how the auditory system of laboratory mice responds to acoustic stimuli. The device will determine the position and angle of the mouse’s head relative to the speaker emitting the sounds. The device should fit ergonomically with the animal and lab setting, and should not interfere with recording of brain waves.

Motivation

The client, Prof. Matthew Banks, studies the inhibitory receptor, the GABAA receptor, is related to various cognitive activities in the brain. For his research, he wants to study the auditory system of mice and how GABAA receptors affect the perception and cognition of sound stimuli. The research requires recording ofthe brain neural activity in response to a specific sound stimulus as well as the position and orientation of the mouse’s head in relation to the location of the sound. This project focuses on developing a device that measures the location as well as orientation of the mouse head.

Background

Auditory System Overview

The auditory system provides an important source of information about the external environment. Sound stimuli are perceived by the brain, and then the informationtriggers learning, higher level cognitive behaviors, and responsive behaviors. The system consists of the peripheral auditory system which transmits the sound waves and then converts the physical energy of sound waves into neural signals. Then the neural signals are then sent and processed in the central auditory system which consists of brain parts such as the brain stem and the auditory cortex(Troost and Waller, 1998) (Figure 1). The neurons in the brain are all interconnected to form neural pathways, and sensory information such as sound is sent through certain pathways for processing.

Figure 1. The entry of sound into the auditory system. The sounds waves is transformed into neural signals in the peripheral auditory system. Then the neural signals are processed and perceived in the central auditory system located in central nervous system.

Perception of Sound

Different sounds vary in characteristics such as the wave frequency and amplitude as well as the difference in time required tostrike the two ears. The above three factors are crucial information the brain uses to interpret the pitch, tone, and location of the sound. Once the brain translates the sound, it then uses its cognitive capabilities such as memory to understand and interpret the sound. Thus one’s response to the elicited by the specific sound reflects the brain’s perception and cognition. In other words, differences in sound characteristics will result in different neural responses (Griffths et al., 1998).

GABAA Receptors

The client’s main study focus is on how GABAA receptors are involved in the auditory system. The GABAA receptors bind with gamma-amino butyric acid (GABA), the main inhibitory neurotransmitter in the mammalian central nervous system. When the receptor is bound to a GABA, neural excitation is inhibited (Birnir). The inhibitive activity of GABAA receptors has a very important part in controlling central neural responses, especially of the auditory system. Thus many current research deals with how various drugs such as anesthetics or barbiturates affect or alter the receptor activity, and thus the cognition and perception of the individual (Troost and Waller, 1998).

Current Technologies

The current technologies on the markets include 4 main categories of position and orientation sensors: mechanical, electromagnetic, optical, and acoustic. However, the are not any products that are specifically used for small animals. The marketed devices are for tracking of large objects such as movement of a limb (Baratoff and Blankensteen).

Client Requirements

The client has several requirements for the final device. The device must be small enough to fit on the mouse’s head and weigh less than one gram. The device should measure the position and the orientation of the mouse’s head in respect to the sound stimulus location. The device itself and the data must not interfere with the recording of brain waves. The client also prefers the device to be detachable, to require minimal calibration, and easy to operate. The development of the device should cost less than the allotted budge of $5,000. Finally, the device should be user friendly, and not pose any dangers, whether electrical or toxic, on the mice and researchers.

sDesign Alternatives

Electromagnetic Tracking

The basic idea behind electromagnetic tracking is having a magnetic field source located on the object to be tracked and a sensor at a fixed point that detects the magnetic field (Sensing in VR). The source consists of three small wire coils that are perpendicular to each other and they emit magnetic waves that vary inversely with distance from the magnet. The source is secured on top of the mouse’s head, and a magnetic field sensor is positioned at a fixed position and the distance between the sensor and the sound source is also known (Figure 2). The sensor detects the magnetic field and gives an output as analog voltage signals, and then the signals are amplified and processed to find the position and orientation of the head. As the mouse’s head changes in its location and also it’s orientation in space, the magnetic field with respect to the sensor changes also. The sensor records the differences, and the change in location and orientation can be found.

Figure 2. The basic setup of the electromagnetic sensing device. The source unit is placed on the mouse’s head, then the magnetic field from the source is detected by a sensor, and signal detected is subsequently processed into analog voltage signals.

One advantage with the electromagnetic sensing system is that the data obtained is very accurate, the error being less than 0.1in for position and 0.1 degree for orientation. Also, unlike other tracking devices such as optical and acoustic systems, electromagnetic sensing system is independent of line-of-sight observations. However, the system only works in an environment free of metallic objects because metal alter the magnetic field from the source (Sensing in VR). The lab environment contains many metallic components, including electrodes in the mouse brain. Therefore unless the insulation is excellent, the data from this system is likely to be distorted.

Acoustic Ultrasound tracking

Many of the problems and inaccuracies with other types of tracking devices (mechanical, optical, or electromagnetic) can be avoided and overcome by using an acoustic tracking system. An ultrasound tracking system measures the distance between a transmitter and receiver; this method is classified as “direct measurement” in time-of-flight tracking (Auer et al). In the orientation tracking of a mouse’s head, two transmitters must be used, positioned either a known distance apart on the head of the

mouse or on the amplifier. These transmitters will individually produce a pulse (at different frequencies) which will then be detected by two sets of three receivers located around the holding cell for the experiment. These receivers will individually record the x, y, and z coordinates of each transmitter. Each set of receivers will be connected to a Whitening filter, amplifier, and analogue to digital converters (ADC) before being processed by a digital signal processor and connected to a PC (Alusi et al) (Figure 3).

Figure 3. Schematic of the acoustic sensing system. The two transmitters, T1 and T2, emit different sound frequencies that are picked up by the corresponding

receivers. The signals are then filtered, amplified, and then processed.

This design is considerable because there are no requirements such as a constant line of sight as in optical tracking devices. Also, the container for the experiment is placed into a sound proof chamber and so the complications normally arising with ultrasound tracking (such as humidity, pressure, air circulation, noise, etc.) will not be a factor in the experiment. Much of past ultrasound research has been completed for 2D ultrasound use. The acoustic system has an accuracy of approximately 1mm (Simon). There have been many new innovations with 3D ultrasound. This would be a disadvantage because there would have to be a conversion in techniques from 2D to 3D. Another disadvantage of using this design would be all the complications needed to create data in a usable and readable way.

Optical Tracking

Another design alternative is to use optical tracking devices such as cameras and infrared LEDs (Light Emitting Diodes) to measure head location and orientation. There are two basic set ups that can be constructed. In one setup, one or more cameras are mounted on the object to be tracked and a set of LEDs are situated above the head in known locations. A camera located on the head of a mouse would not be feasible due to the size constraints. Instead, the other setup, with a few LEDs placed in fixed and known positions on the head and a camera mounted on the frame, would be more reasonable due to the small size of a mouse’s head. In this specific design, four LEDs can be arranged in a pattern on the mouse’s head and would be monitored by a camera that is located in a concrete position. The LEDs would be pulsed one at time and the positions of the flashes on the camera, together with the known relationship between the LEDs, is enough information to compute the position and orientation of the head. A disadvantage of this system is that since the infrared LEDs are active and need to emit energy on their own, they will not fit the size constraints required. This can be modified by using the same set up, but instead of using active LEDs, passive ones will be used along with an external infrared source. A reflecting pattern is arranged on the mouse’s head and it is then illuminated by an external infrared source to make is visible to the camera (Baratoff and Blanksteen). This system will be used to continuously monitor the mouse head (Figure 4). The camera footage from the experiment can be analyzed with various computer softwares.

Figure 4. The block diagram of how the optical sensing system functions. The

external infrared light source allows the passive LEDs on mouse head to reflect

light. The reflected light is captured by a camera for further processing.

A positive aspect of this design is that optical tracking has a high update rate and significantly short lags. Ambient light can negatively affect the optical tracking, however in the closed set up of the experiment, this will not be a concern. Other advantages are that the LEDs are lightweight and the system is the accurate to the 0.1mm (Simon). A disadvantage to this design is that there always needs to be a clear line of sight to the object being tracked and if this line is blocked or disrupted the performance of the tracking is degraded.

Evaluation of Design

To decide on the best design to solve the specified problem of the client, a design matrix was developed to compare the three design alternatives (Table 1). The matrix is set up so that device requirements such as amount of sources that interfere the device, ease of manufacture, accuracy, weight, and amount of interference with brain wave recording. The three design alternatives are rated for each of the six categories using a scale of one to three, with one being the worst and three being the best for the purposes and requirements of this project. The scores are then added together, and the design that is best suited for the project requirements has the highest final score.

Design Matrix

Tracking Devices / Acoustic
(Ultrasound) / Optical / Electromagnetic
Interference Sources / 3 / 2 / 1
Ease to Manufacture / 1 / 3 / 2
Accuracy / 2 / 3 / 1
Weight / 1 / 3 / 2
Interference with Data / 2 / 3 / 1
Overall Score / 9 / 13 / 7

Table 1. The design matrix which evaluates the acoustic, optical, and electromagnetic design alternatives based on how well they meet device requirements.

Proposed Design:

The final design was chosen to be the optical system due to the overall quality. It has a higher rating than the acoustic and electromagnetic designs. The optical system is the most accurate, lightweight, and easiest to construct. The light waves will not interfere with experimental sound stimuli as the acoustic system may do, and the output data has a less chance of interference as the electromagnetic system will be by the numerous metallic objects. The light waves also will not interfere at all with the brain wave recordings.

Conclusions and Future Work

A main potential problem with the optical design is interference with the line of sight. This problem can be easily be solved by making sure the camera is not blocked because there are not many components inside the mouse container

After finally choosing one design that will be focused on, much more preparation must be accomplished before any manufacturing will occur. Extensive research must be completed on the specific design that will be agreed upon in the large group setting. Since there is a large market of various LEDs and cameras, investigation on deciding upon the best components suited for the purposes of the client. The next step is to begin assembly of the final device and then to test the device.

References

Alusi, G., Hadjiprocopis A., Linney, A., Wright, A. “Three Dimensional Tracking with Ultrasound for Virtual Reality Applications in Surgery.” Accessed: September 25, 2003. URL:

Auer, V., Bonfim, M.J.C., Lamar M.V., Maes M.M., Wanderley M.M. “3D Positioning Acquisition System with Application in Real-Time Processing.” Accessed: October 1, 2003. URL:

Barafoff, G., Blanksteen, S. “Tracking Devices.” Accessed: September 29, 2003. URL:

Birnir, B. ”GABAA Receptors: Structure, Function and Pharmacology”. Accessed

September 17th, 2003. URL:

Griffths, D.T., et al. “Right Parietal Lobe is Involved in the Perception of Sound

Movement in Humans”. Nature Neuroscience 1, pp74-79. May 1998.

Reyes, S. “The Auditory Central System”. Accessed September 19th, 2003. URL:

Simon, D.A. “Intra-Operative Position Sensing and Tracking Devices”. Accessed:

September 20th, 2003. URL:

Troost, B.T, Waller, M.A. Diagnostic Principals in Neuro-Otology: The Auditory System.

In: Rosenberg RN, PleasureDE, eds. Comprehensive Neurology (2nd Ed), Wiley and Sons, Inc., New York, New York. 1998:611-623.

“Honeywell Magnetic Position Sensors”. Accessed September 25th, 2003. URL:

“Sensing in VR”. Accessed September 21th, 2003. URL

Appendix

Product Design Specifications

Date: October 6th, 2003

Title: Mouse Head Positioning Device

Group members: Melissa Haehn, Yao Lu, Meghan Olson, Ben Sprague, Heather Waldeck, and Andrea Zelisko.

Problem Statement: Our purpose is to develop a device to help study how the auditory system of laboratory mice responds toacoustic stimuli. The device will determine the position and angle of the mouse’s head relative to the speaker emitting the sounds. The device should fit ergonomically with the animal and lab setting, and should not interfere with recording of brain waves.

Client Requirements:

  • Less than 1g and fits on mouse head
  • Detachable
  • Data on position and angle of mouse head relative to sound
  • No interference with experimental data
  • Data in analog voltage format

Design requirements:

1. Physical and Operational Characteristics

a. Performance requirements:

  • Signal emitting device must either be positioned on mouse head or on preamplifier.
  • Batteries or electrical power source
  • Minimal calibration for each trial
  • Data output is analog voltage signal
  • Voltage signals should either be less than 1µV or greater than 20KHz
  • Must not interfere with the sound stimuli presented to the mice.

b. Safety:

  • Device should be made from non-toxic and non-corrosive materials.
  • should not pose any electrical dangers

c. Accuracy and Reliability:

  • orientation within quadrants (at a minimal)

d. Life in Service:

  • Each trial lasts 2 hours
  • Mouse tested 5 days per week

e. Shelf Life:

  • Sterile environment
  • Room temperature
  • Standard lab condition
  • Must last for minimal of 5 years with normal experimental use.

f. Operating Environment:

  • Sound proof chamber
  • Standard lab environment
  • Close proximity to mice
  • Room temperature
  • Professor and associates will handle device / researchers

g. Ergonomics:

  • Device easy to attach
  • different frequencies than what is tested at
  • able to withstand handling by researchers
  • Requires an easy set up (flip of a switch)

h. Size: Establish restrictions on the size of the product, including maximum size, portability, space available, access for maintenance, etc.

  • small enough to fit on pre-amplifier (3 x 2 x 1mm) or on the mouse’s head.
  • Preferably be detached and reattached at each experimental trial

i. Weight:

  • Less than 1g.

j. Materials:

  • Non-corrosive and non-toxic materials that is not harmful to the researchers and mice.
  • Material should not deteriorate under normal usage circumstances.

k. Aesthetics, Appearance, and Finish: Color, shape, form, texture of finish should be specified where possible (get opinions from as many sources as possible).

  • Professional appearance
  • non-hazardous edges or wires

2.Production Characteristics

a. Quantity: number of units needed

  • one for each mouse to be tested
  • or preferably one for preamplifier

b. Target Product Cost: manufacturing costs; costs as compared to existing or like products