Vanderbilt University
School of Engineering
Department of Biomedical Engineering
Multisensory EnvironmentforNeurophysiological Monitoring
Team Members:
Gregory Apker
Vern Huang
Nazriq Lamien
Andrew Lin
Emma Sirajudin
Advisor:
Daniel Polley, Ph.D.
Mark Wallace, Ph.D.
Date of Submission:
April 24, 2007
Abstract:
Dr. Polley and Dr. Wallace are trying to explore the behavioral significance of physiological plasticity during awake recordings. However, they require a refinement of the sensory stimuli to improve its accuracy in location and time to more closely simulate a naturalistic environment, eliciting more realistic responses. Thus, the goal of our project is to create the hardware and software necessary to make these improved multisensory environments. This will include improving or updating rat backpack hardware, a wireless auditory transmitter-receiver system, and force plate calibration while also involving ground up development of a somatosensory stimulator. The software consists of three parts, which are visual, auditory and somatosensory; each of these parts must be able to collect and sort incoming data. Software requirements also included developing Pre-Pulse Inhibition software to be used in conjunction with the force plate. The development of the wireless auditory transmitter-receiver produced an experiment-ready model using Bluetooth technologies. The somatosensory stimulator system developed used a piezoelectric bending actuator with an 800μm deflection and accounted for and incorporated all of the necessary system parameters required for the design of a somatosensory tactile stimulator. In order to determine frequency ranges of hearing, an analysis of Forward Masking data was required. Tuning curves, the visual representation of frequency thresholds, are complex enough to have to be delineated subjectively by a person. The sensory software developed was also made into a flowchart diagram and serves as a guide to the researchers in the lab so that they can understand how the software works a little better. Another purpose of this analysis is to guide other programmers if they are interested to create software similar to this.
1. Introduction:
Many studies have shown the existence of large-scale plasticity in the visual, somatosensory, and auditory cortices of the brain. In addition, other research has focused on achieving a better grasp of multisensory interactions.
However, these areas of neurophysiological monitoring have a great deal of room for improvement. It is important to conduct these studies because they drive our understanding of pathologies and perception.
The labs of Dr. Polley and Dr. Wallace are trying to address the limitations of these past studies by adding new functionality to their multisensory environments. Awake recording capabilities will allow the exploration of the behavioral significance of physiological plasticity, compared to its simple characterization through anaesthetized recordings. Furthermore, the refinement of sensory stimuli through its accuracy in location and time will more closely simulate a naturalistic environment, eliciting more realistic responses.
The goal of our project is to create the tools, involving both hardware and software, necessary to make these improved multisensory environments. The development of multisensory environment hardware for use in neuronal response characterization in rats and cats involves several requirements. The hardware needed to produce visual, auditory, and somatosensory stimulation, which can be modulated in location and intensity and can be integrated to receive as well as deliver information. The development of environment-specific software for closed loop control of the environment also entails a few requirements. The software needed to allow user-defined stimulation parameters, initiate coordinated sensory stimulation, and record, associate, and organize the system output.
Our goals were outlined as such:
• Update and repair of existing hardware components
• Increase usability/durability of system components
• Install environment hardware and architecture
• Implementation of wireless transmitter-receiver system
• Manage frequency range, spectral fidelity, and channel crosstalk impact
• Ground-up development of somatosensory stimulator system
• Design stimuli delivery and mounting apparatus
• Offer precision and versatility in application without confounding artifacts
• Integrate designed components
• Modification of existing in-lab software
• Increase functionality of Pre-Pulse Inhibition protocol
• Convert software from Visual Basic to Matlab
• Development of visual and auditory software
• Control delivery of sensory stimuli in time
• Collect, sort, and analyze data
2. Methods:
2.1 Wireless transmitter-receiver
Two wireless transmitter-receiver systems were under consideration for use in the multisensory environment experiments. One system was a custom PCB design, which was quite expensive and still in prototype form, while the other system was a Bluetooth solution. An analysis of the wireless Bluetooth transmitter-receiver system involved a focus on frequency range, spectral fidelity, and the impact of crosstalk between the right and left channels of the earphones.
2.2 Ear-speaker interface
Working with live rats in the multisensory environment, ear molds needed to be produced in order to make well-fitted earbuds for the semi-permanent attachment of Etymotic 6i Isolator™ earphones to the ears of rats. Two materials, Cerebond Skull Fixture Adhesive and Jet Denture Repair mold, were analyzed with respect to their functionality as ear molds using the following characteristics: their density and weight, the strength of their bond to metal, the weakness of their bond to tissue, their malleability, and the ability to create a canal in them.
In the analysis of their density and weight, molds of identical volume were created by filling tubing that was 0.5 inches in length with the Cerebond and Jet materials. The density and weight of the materials were important for the future experiments on the rats because it would be preferable that the ear buds do not hinder the rats or make them uncomfortable to a significant degree.
To test the other characteristics, actual ear molds using rat ears were made using the Cerebond and Jet materials. This process involved burning two holes in the ears, placing two small screws in the holes, and letting the molding material set to the shape of the ear. Combined with washers, the screws were to be used to hold the earbud in place during live experiments.
2.3 Rat backpack hardware
Investigation into the successful combination of the component parts involved in the rat backpack hardware system was conducted through the perfection of the design of certain parts to prevent potential problems and to ensure its overall proper function in future experimental applications. First of all, research into braided electrical shielding material was conducted in order to obtain material that would protect the earphones from electromagnetic interference (EMI) produced by the surrounding environment. After the appropriate electrical shielding product was found, ordered, and delivered, designs for the electrical shielding of the rat backpack hardware system were illustrated.
The electrical shielding comprised three sections: (1) the rat backpack, (2) the combined earphone wires, and (3) the separated earphone wires. The shielding for sections (2) and (3) was accomplished with a combination of the braided expandable shielding material and cable ties. The electrical shielding for section (3) did not adequately cover the transducer in the earphone. Therefore, efforts to cover the transducer portion of the earphones with a thin sheet of copper were pursued. A copper sheet in the form of a cross with a hole in the middle was designed to wrap the earphone, effectively covering the transducer. The dimensions of the cross involved a 10 mm strip, an 8 mm strip, and a 3 mm hole. Once the copper shielding was folded over the earphone, super glue was applied to the edges to hold it in place. Since the braided electrical shielding rested over the copper shielding, a lightweight surgical tape was wrapped around the earphone to keep all of the stray ends together and to ensure contact between the two types of electrical shielding.
2.4 Simulation of hearing attenuation
An alternative solution to physically stitching shut the ear canal in rats in order to limit their hearing ability during early development was needed. Tight-fitting ear plugs were a potential solution that was explored. The length, the top diameter, and the bottom diameter were adjustable variables in the process of creating model ear plugs that fitted easily and snugly into the rat’s ear. Alterations to E-A-R ear plugs provided decent models. An assessment of the various ear plug designs was necessary to determine the effectiveness of “deafening” by judging the seal provided in the ear and finding the acceptable amount of material fitted in the ear canal among other things. The final design had a cone-like shape with a length of 12 mm, a top diameter of 8 mm, and a bottom diameter of 2 mm.
2.5 Force plate for PPI
In order to determine a relationship between a change in force and a change in voltage for the force plate transducer, a pulley system needed to be devised. The pulley system needed to meet the following characteristics: manual operation, minimization of friction, and uniform lifting of the mass. Materials were purchased in order to create a model of the pulley system: two different sizes of eye screws, a block of wood, and fishing line. Using a power drill, a pilot hole was made in the block of wood to insert the eye screw. After testing with masses up to 200 g attached to the fishing line, it was determined that the smaller eye screw is capable of supporting the required weights and creates more uniform lifting of the mass from a surface compared to the larger eye screw.
This model was approved and needed to be implemented into the actual multisensory environment. The force plate transducer inside the sound chamber was attached to a square piece of metal, which was secured with four screws at the corners. However, with the existing hardware in place, the pulley system was not able to be employed over the middle of the force plate. In order to achieve consistent calibration for future experiments, the location of the force plate needed to be standardized. Rotation and translation of the metal square were possible solutions that were explored. The location of the force plate was standardized to a position where the square piece of metal extended 1.5 cm past the edge of the opening of the sound chamber. Once the location of the force plate was standardized, an eye screw was affixed to the ceiling of the sound chamber, which already contained some small holes. One of the holes needed to be widened in order to adequately secure the eye screw.
2.6 Tactile stimulator
Development of a tactile stimulator for somatosensory activation was dictated by a number of performance requirements. In all current experimental protocols, the feline subject is placed in a fixed, consistent position to prevent the confounding input of movement. However, the ability to activate the somatosensory system from a variety of positions requires that the system be placed and functional in a number of orientations. That is, the system must be as reliable if it rotated 90 or 180 degrees in any plane as it is un-rotated. Not unrelated to this need for the system to be versatile in position is the need for it to also be small and portable. Portable for the reason stated above, easily placed and moved to a number of locations around the test animal. It must be small for a number of reasons, the most important of which being that lab space is limited and space is a commodity which is to be preserved if at all possible. Also, if the system is too large, it limits where it can be placed and used because of the potential of producing confounding artifacts in the recorded data. If the system enters the animal’s visual field, or if it produces too broad of a tactile stimulation, then it will result in corrupted/non-specific neuron recordings. Also, the system must be silent. Any perceivable noise will generate auditory neuronal stimulation in the animal, contaminating the data. A stimulation that is too forceful could also produce misleading data from the unintended firing of different nerve endings with the skin. Therefore, the touching force generated by the system must contain all of the following attributes: Gentle, controllable, and accurate/consistent.
Another important consideration in designing the system is its integration into the protocol software. Because of the precise sequencing required during multi-sensory experimentation, the activation of the tactile stimulus must be able to be driven by an electric signal initiated by the software. Therefore, the system is constrained to something which can be automated, accepting electrical signal as an impetus for activation. Furthermore, because timing is such an important parameter of the stimulus activation, the final system must be adequately responsive to rapidly changing input signal.
From the automating software, the somatosensory stimulator system will receive a +/- 5 volts input voltage. It is expected that an in-line amplifier will result in a +/- 60 volt RMS signal reaching the device. A trapezoidal wave is used for the input signal to produce a tap-and-hold tactile stimulation. While a square-wave would also produce this effect, the gentler acceleration of a trapezoidal input will protect the equipment from excessive wear while also allowing for a more controlled motion of the stimulator system. To further increase these benefits of the trapezoidal wave, the edges of the wave form will be synthetically rounded before reaching the system.
2.7 Multisensory protocol software
The software is an important feature of the experiment as it is the only way for the user to communicate with the hardware. The main purpose of the software is to record the activity of the neurons when experiments are being held and have them analyzed. Data recording, time intervals, and input commands are just some of the functions of the software. The software is also where the user inputs parameters of the desired stimulus and have the results recorded.
The software consists of three parts, which are visual, auditory and somatosensory. All of these three stimuli have different ways of recording data and are created to perform the experiments while recording data efficiently. The software is also designed to decrease minor errors or information during experiments therefore every single function in the software is created so that no single detail is left out.
The visual part of the software has been tested and was proven to be working perfectly well. Firstly, the user will input parameters such as the location and the type of object to be projected onto the screen. When the different locations are projected under the user's time intervals, the brain activity of the animal being tested are recorded and are analyzed using the options provided by the software.