This visual-to-tactile conversion system for forehead skin was just presented at the SIGGRAPH 2006 in Boston.
Developed by EyePlusPlus, Inc. of Japan and Tachi Laboratory in the University of Tokyo, the system takes into account the fact that forehead skin is supposedly very thin, hence the distance to tactile receptors.
Nerve Selective Stimulation
There are several types of mechano-receptors inside our skin. If we could independently stimulate each type of receptor’s nerves, we could reconstruct complex tactile sensations by combining them. It is similar to visual color construction method using RGB. Therefore, we call it “Tactile Primary Color Approach”. We found out that by appropriately designing electrical potential distribution using multiple surface electrodes, we could selectively stimulate each type of receptors. In FRS, Meissner corpuscle that responds to low frequency vibration is selectively stimulated. This is why you feel stable vibratory sensation while wearing FRS.
In conventional electrical stimulation, coaxial electrode is used. It is composed of central stimulating electrode and surrounding ground electrode. In this case, the size of the outer electrode limits electrode density. In FRS, a matrix of electrodes is used. Each electrode alternates between anode and ground to form virtual coaxial electrode, enabling denser alignment.
Though sensory substitution through electrical stimulation was proposed since 1960s, using forehead as a stimulation area is quite a new approach. However, forehead stimulation is quite reasonable. It is easy to put on and take off, while coordinate system transformation in our brain should be much easier than other parts of the body.
Visual image captured by the camera is converted to tactile information through two processes. The first is a spatial outline extraction to enhance edges. The second is a temporal band-pass filtering to enhance time-varying information. These are actually what the retina does. FRS imitates pre-process of the real visual system to facilitate image cognition.
From SIGGRAPH website, details of the inside of the system:
Compared to current portable electronic devices, most of the proposed “portable” welfare devices are not portable in reality. They are bulky and heavy, and they have a limited operation time. By using electrical stimulation, the Forehead Retina System partially solves these problems. Nevertheless, driving 512 electrodes with more than 300 volts is quite a difficult task, and it normally requires a large circuit space. The system uses a high-voltage switching integrated circuit, which is normally used to drive micro-machines such as digital micro-mirror devices. With fast switching, current pulses are allocated to appropriate electrodes. This approach enables a very large volume of stimulation and system portability at the same time.