Galvanic Vestibular Stimulation
In this series of tutorials I would like to give to cover a set of basic stimulation techniques that can be used in cybernetics for the human-machine integration.
Galvanic Vestibular Stimulation (GVS from now on) is a simple, safe and specific way to elicit vestibular reflexes. The vestibular systems is responsible for a fundamental function of our body: encoding head orientation and keep balance.
You might have experience some vertigo in the past, if you didn't I can tell you that having an infection of the vestibular ear is a very nasty sensation as you experience nausea for every small movement, resulting in most of the times vomit etc.
GVS was used for over a century as a means to discover and then look at the function of the vestibular system. The first scientist who discovered it was Johann Purkyne in his 1820 dissertation, reported that a galvanic current flowing through the head upset the balance and equilibrium.
After that Volta in 1790, playing with his newly invented battery almost electrocuted himself with the same technique experiencing "the sensation of an explosion inside the head, spinning and boiling of matter".
This was probably due to the use of a 30V Zn/Ag pile and we will see later that 30 V are over the safe limit of stimulation.
More recently with the work of Fitzpatrick the mechanisms behind the GVS effect are being unveiled. There are basically several ways to elicit the response:
- bilater bipolar GVS
- bilateral monopolar GVS
- unilater monopolar GVS
First we need to know more about the vestibular system, I have used material available online (ScienceDirect) to write a compact introduction so that the reader doesn't have to google all the time.
If somebody thinks I have used some copyrighted material please do let me know.
Known as balance organs of the inner ear, the vestibular organs serve this complex motor function at a largely subconscious level, but their role does not stop with balance. They contribute to a surprising range of brain functions, from the highest levels of consciousness to the most automatic reflexes. The value of the vestibular sensory system to brain functions such as perception of self and non-self motion, spatial orientation, navigation, voluntary movement, oculomotor control, and autonomic control, comes from their unique and complete description of head motion and orientation in three dimensions.
Two different vestibular organs, the otolith organs and semicircular canals, sense different types of acceleration. Two otolith sensors, the utricle and saccule, sense linear acceleration. Three semicircular canals, the anterior, posterior and horizontal canals, sense rotational movements. These two complimentary signals are necessary for the brain to understand the range of physical situations that we experience, probably the most fundamental of which is to work out which way is up.
Which way is up?
All terrestrial and aquatic animals need to know which way is up and, therefore, which way gravity acts, so it is not surprising that special graviceptive systems appear early in evolutionary history. A sense of the force of gravity and which way is up is with us at all times. This internal construction is based on multiple sensory sources, important among which are the vestibular organs. It provides our brains with a deep and special understanding of how the force of gravity moves things, from the fall of our body, as we lift a foot to take a step, to the fall of a ball during a game of cricket. In all of these situations, the brain predicts the trajectory of fall with startling accuracy.
The significance of this internal representation for predicting motion in gravity and its link to the vestibular system was recently shown by Indovina and colleagues. They displayed a ball moving in a visual scene that had strong cues to the up direction. Observers accurately predicted the flight and timing of the ball when the gravitational field was conventionally aligned with the visual scene. Yet when the gravitational field was reversed so that it acted upwards, observers made large prediction errors, even though the ball was subject to exactly the same acceleration. Furthermore, functional magnetic resonance imaging revealed that certain areas of the cerebral cortex were more active when the gravitational field was consistent with the visual scene than when it was reversed. These included areas that receive strong vestibular signals, pointing to a vestibular contribution to our internal representation of gravity.
The equivalence problem
The otolith organs seem ideally suited to sense the direction of gravity and signal directly which way is up. The otoliths are essentially masses supported on hair cells. Tilting the head causes the cilia to bend under the sideways component of the gravitational force on the masses. This modulates the firing of the sensory nerves connected to the hair cells. Different hair cells respond to bending in different directions so that the total signal from all hair cells defines the direction of gravity with respect to the skull. There is, however, a serious problem that Einstein explained with his equivalence principle: the effect of a gravitational field on a mass is indistinguishable from the effect of linear acceleration.
For example, consider the otolith organs of a person sitting in a bus. When the head is accelerated forward as the bus pulls away from the bus stop, the inertia of the otolith masses causes them to be left behind and to bend the cilia of the hair cells backward. This is also exactly what happens when the front of the bus tilts upwards on starting to climb a hill. Thus, the otolith sensors send the same signal for two different physical situations; linear acceleration and tilt in the gravitational field. On their own, the otolith organs cannot signal unequivocally which way is up.
Why distinguish tilt and acceleration?
To operate beyond ourselves and navigate the environment, we need to create a stable internal representation of external, earth-referenced space in which the position and movement of objects is independent of our own. A navigation system needs to be able to distinguish the two physical situations of tilt and linear acceleration to build an accurate internal map of our movements. Evidence for this is seen at the processing level of place cells in the hippocampus. These neurons, which code for spatial location as part of a navigation system, depend on vestibular information for their function.
Control of eye movement is probably the most overt example of how the vestibular system creates a stable representation of external space, in this case visual space. The vestibular system profoundly influences eye movements via the vestibulo-ocular reflex to stabilize the visual image on the retina in the face of head motion. When we fix our gaze on an object and our head moves, the vestibular organs detect that movement and produce a counter movement of the eyes to maintain the retinal image. Clockwise head tilt requires the eyes to rotate anticlockwise. Leftward acceleration requires a rightward horizontal shift of the eyes. These two situations, which stimulate the otolith organs identically, must be distinguished to generate appropriate eye movements.
The angular trick
It appears that the brain goes some way toward solving the equivalence problem by simultaneously listening to the messages from the other vestibular organs, the semicircular canals. Like the otolith organs, the hair cells of the semicircular canals respond when their cilia are bent. The difference is that when the head rotates in the plane of a canal, the enclosed fluid is left behind and exerts a pressure that deflects the hair cells. Thus, they respond specifically to angular acceleration of the head and not to gravity or linear acceleration.
Now when the head tilts, say to the right, the brain receives two vestibular signals. The otolith organs signal the static head-tilt, which could equally represent leftward linear acceleration, but the semicircular canals report the transient head-rotation. If the otolith signal results from leftward linear acceleration, the semicircular canals report nothing. Angelaki and colleagues have shown recently, by recording from neurons in the cerebellum in the vestibular nuclei of the brainstem, that the brain can tell apart tilt and linear acceleration by combining the otolithic and semicircular canal signals.
Bilateral bipolar GVS
Now that we know more about the vestibular system we need to find a way to stimulate it in a useful manner without implanting electrodes in our inner ear.
We only want to stick some electrodes on our skin avoiding any surgery: the GVS basically bypasses the transduction mechanism of the hair cells by exciting all the afferents
with a DC current stimulation between the mastoid processes.
It's a brute force stimulation approach where all the afferents are excited So what Volta discovered is that the net effect of such a general excitation is a strong body swing.
Results were summarized by Fritzpatrick the expert in this field with this nice figure.
With only 0.5mA, GVS produces a trunk roll and head at 1-2 deg/s2 ! With 1 mA, body angular roatation was 2-3 deg/s2 .
Fritpatrick developed a model of the semicircular canals and otolith organs to predict the effect of the GVS on the linear and angular acceleration vector ...
but enough talking let's experience ourselves !
How to build a simple GVS stimulator
As a preliminary prototype we are going to use a simple stimulator built with:
- 9V battery source
- H-bridge for switching voltage polarity
- potentiometer for adjusting current flow
This is a minimalist setup BUT is not SAFE as we are assuming that:
- skin's impedance is constant for the duration of the stimulation
- we are lucky and there will be no short-circuits :-)
For a short experiment this is not a big problem but if you plan to have a long session the skin will change its impedance for many reasons including sweating, abrasion etc.
A simplified electrical model of the electrodes, skin, tissue system is illustrated here:
- The boundary of electrodes and the skin is of special importance, because that is where the flow of electrons from the stimulator is transduced into the ion flow of the tissue. At the boundary of every metal-electrolyte interface there is a potential difference which is called cell potential. The model also contains a series RC suggested by Warburg, and the faradic leakage resistance Rf accounting for DC characteristics of the model
- The skin can be modeled by a serial resistance Rs and a parallel Rp,Cp. Rp can be practically eliminated by removing the outermost layer of skin, the stratum corneum.
- The deep tissue can be modeled by the resistive Rt, bulk tissue resistance. Because of the complexity of the model and the lots of contributing nonlinear physical factors it is very difficult to estimate the total impedance between the two electrodes, but for practical applications the impedance should be in the magnitude of 1 kOhm. However if the skin is not prepared, the total impedance can be several times higher.
The circuit we are going to use in the first tutorial (we will use a current feedback in the next one) is in Figure.
The main components of the circuit are:
- 9 Volt battery supply
- a H-bridge composed of 2x NPN irf7105 and PNP irf7105p mosfet transistors
- a potentiometer of 100 kOhm
- 2 guard resistors of 150 Ohm
- 2 opto isolators
If the electrodes are in the short-circuit condition and the potentiometer is at 0 Ohm a maximum current of about 2.4 mA will flow.
If your skin-electrode conductance is at about 400 Ohm and you keep the potentiometer at 0 Ohm there will be a current flow of about 1 mA.
Believe me if I tell you that with electro gel electrodes your skin resistance will never reach 400 Ohm so is better ALWAYS to keep the potentiometer at 100kOhm and decrease gradually to achieve the desired effect.
An LT Spice (free software) simulation circuit is available in the repository so that you can see what is going to happen if you change the parameters.
The opto isolators are not included as LT Spice complain about some floating point resistance (bah couldn't find any solution for that) so I have changed the opto couplers with a simple voltage source.
I have also included the library model for the irf7105 so make sure you place the corresponding lib in the LT Spice folder.
First we need to locate our mastoid bones as in Figure, if you touch the area below your ears is the bony spike.
Second we need to find some proper Gel electrodes, because I work with EEG I have used the Maxensor Ag/Agcl/Solid adhesive pre-gelled.
I also generally use some Electro Gel to moisture my skin and increase conductivity for a better "sensation" ghghghg.
I don't want to have a wired GVS but a wireless GVS! So I took my Jeenodes and decide to use a master-slave configuration.
The master jeenode is a basically a remote controller (like the one in your TV) which has 2 buttons: left for direct polarization and right for reverse polarization.
The master will decode the button press and send 3 possible actions to the slave:
- CENTER: disable the H-bridge no polarization
- LEFT: enable the H-bridge with a forward polarization
- RIGHT: enable the H-bridge with a reverse polarization
The slave jeenode will then execute the command by activating the proper opto inputs.
All the documentation and code is available here:
If you want to support my research or you want to get the circuit already up and running,
I encourage you to buy my pcb in my shop.
The jeenode hardware looks like this:
How to use it
It is very important to follow this simple algorithm:
- check any short circuits
- apply the electrodes to your skin
- set the potentiometer at 100 KOhm
- try left and right control with closed eyes
- if you feel nothing decrease the potentiometer and start from 3
- if you start to feel a pull on the left or right direction
- stand up and try to walk straight
- give the remote to some friend and have fun
- do not use voltages bigger than 9 V
- do not fiddle with the resistances unless you know what are you doing
- do not short circuit the leads
- use the stimulation for a short period of time
- do not apply on animals or kids
Gvs Video Tutorial
In the next tutorial we are going to build a current controlled stimulator.
Last Updated (Sunday, 28 November 2010 17:46)