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Risultati finali
On the visuo-tactile sensory substitution side of this project, 2 main categories of results with well trained subjects were demonstrated:
1) First of all, at a behavioural level very satisfactory capacities of fine discrimination of forms can be obtained, especially with the high miniaturized version of the TVSS (the TDU: Tongue Display Unit) that associates a high density of tactors and natural sensors in the mouth. Thus, ''visual'' acuity of blind persons perceiving information through the Tongue Display Unit has been quantified, using a standard Ophtalmologic test (Snellen Chart). Without any training, blindfolded sighted adults and blind subjects averaged 1/43 (whereas Dobelle Institute (2000) obtained, with a well trained blind subject, an acuity of 1/60 with an invasive device implanted in the occipital cortex). At the end of a training period, subjects had doubled their acuity to 1/22.
Moreover, intensive training with the TVSS, with adapted and progressive levels of difficulty, allowed both groups (blind subjects or control blindfolded sighted subjects) to perceive specific visual information such as depth, perspective or parallax. In fact, sensory substitution constitutes the only way for blind people to reach this kind of information located at the distant, extra-corporal, space. Even with a small density of tactors (as with the PTD version of the TVSS), appropriate training allowed to successful navigation through a 3D space, by radio-controlling a robot in a maze, exclusively using the substitutive visuo-tactile information. Such a result has a potential social and economic impact as it points out the possibilities of facilitating the navigation of blind people in the real world. Furthermore, it suggests also that it is possible to implement video-games-like experiences for the blind. Last, but not least, in an extension research program of the current SenSub project, we showed two therapeutic possibilities:
a) to use visuo-tactile devices (TVSS) as soon as few months of life in babies. This result suggests that the TVSS can be a useful therapeutic tool to avoid developmental disorders usually observed in blind children; and
b) to use visuo-tactile devices (TVSS) to restore balance with eyes closed, including with people with severe vestibular damage.
2) At the cerebral level, visual pattern of brain activation in areas usually responsible for depth and motion processing from visual input were obtained after intensive training, although a tactile entry was used. Thus, functional imagery suggests that visual information can be processed by tactile modality when adequate transduction is given by means of the TVSS. Furthermore, it was possible to demonstrate that not only pneumatic TVSS (PTD), specially designed with non-magnetic material can be used in the fMRI: the electronic miniaturized version of the TDU (array of 3x3 cm, with144 tactors, explored with the tongue) can also be safely used in the fMRI environment. This result widely opens new scientific investigation perspectives.
To sum up, sensory substitution constitutes a scientific and practical contribution to the scientific community as well as to daily life of people with disabilities. It seems to be a heuristic way to investigate brain plasticity and perceptual learning on one hand and, on the other hand, to help giving access to some kind of specific visual information to blind people or to sighted people in restricted visual environment (such as fireman in a foggy environment, diving in trouble water, etc.). Furthermore, the perspective of therapeutic and leisure applications, will undoubtedly have a social and economic impact.
On the visual-auditory sensory substitution side of this project, two main categories of results with well-trained subjects were demonstrated: new insights were provided into the behavioural abilities of the human blind as well as into the cerebral substrates of sensory substitution
First of all, at a behavioural level, the auditory substitution of vision provides very satisfactory and new abilities to the early blind (EB) user. When compared with previous sensory substitution devices using the auditory channel, there is a substantial advantage offered by the prosthesis of substitution of vision by audition (PSVA) used in this project : PSVA allows simultaneously the localization and the recognition of a given object outside the apprehensive space. This combination of perceptual gain has been demonstrated to be a prerequisite before improving significantly the spatial processing abilities by EB subjects. In the project, very conclusive results were obtained both in pattern recognition and in object localization using PSVA, allowing formal hope that EB subjects can build a harmonious synthesis between substitute and natural information coding the space.
For pattern recognition, as few as four learning sessions, each lasting about one hour, are sufficient to master the PSVA code. An explicit, interactive and reinforcing learning was provided by verbal feedback during these learning sessions.
On average, pattern recognition using PSVA averaged 50% before the training, which is similar to random. After learning the mean scores increased to 70%, indicating a significant progression (Z= -2.667, p less than 0.01). In addition, both EB and blindfolded sighted control (SC) subjects trained with this procedure acquired a sufficient level of expertise in mastering the code of PSVA for the recognition of basic components of the patterns (more than 80% correct answers).
The sensory substitution systems (TVSS/PTD and PSVA) are likely to allow early blind (EB) users to experience new perceptual phenomena like depth perception and induction of visual illusions. Behavioural results from this phase were carried out with subjects who were trained to the code but still naïve for depth perception to provide an estimation of spontaneous perceptual abilities with PSVA and were further carried out after specific training of the subjects to depth perception, to provide an estimation of their new cognitive abilities. The results indicate that blindfolded SC subjects are able to localise distant objects with the PSVA with a significantly more accurate efficiency than EB volunteers at the starting point of the study. This difference of precision in the localisation of the object seems more notable for the ordinate (distance estimation or depth perception) than for the abscissa. This suggests that depth perception with the PSVA is mainly based on visual perceptual processes depending on previous visual experience.
However, after three 30-minutes-training sessions, EB subjects demonstrate an equivalent level of performance (accuracy in the localisation of objects in the depth direction) to the one of blindfolded SC subjects. This indicates that EB subjects are able to acquire quite rapidly the rules of the visual depth perception and to use them adequately with the PSVA. In addition, the undertaken neuropsychological experiments also demonstrated that visual illusions can be induced using the PSVA prosthesis.
The results show that only blindfolded SC subjects are affected by the horizontal-vertical and Ponzo illusions on a similar way as in normal vision. EB subjects are also sensible to the illusions provided by auditory substitution but a different way. This highlights the importance of previous visual experience in the perception of visual illusions and indicates the existence of common processes governing the perception with the PSVA and the normal visual perception.
At the cerebral level, a recruitment of parieto-occipital cortex and visual association areas was demonstrated in EB and blindfolded SC subjects using the PSVA, indicating that high-level visual-spatial information can be processed using sensory substitution by audition as the input modality. After a specific training to the depth perception, training related changes are found in these activated brain areas. The activation maps related to depth perception demonstrate parietal activation (dorsal stream) and also temporal activation (ventral stream) in blindfolded SC subjects only.
This indicates that sighted controls engaged an image generation process during the depth perception task, recruiting the ventral stream, whereas the EB subjects used only spatial strategies throughout the study.
The final aim was to test whether the brain is able to create a multisensory perception of space in EB subjects. The obtained results favour this hypothesis, although this processing in EB acts on a different way than using vision and other integrated senses in SC subjects.
In this workpackage of the SENSUB project a novel type of pneumatically driven graphical tactile display (PTD) was developed, built and put into operation in fMRI and PET brain imaging experiments. The special challenge of the project was to operate the PTD in high B-fields and to avoid any artefacts in the recorded images in particular when using it in the fMRI.
The display consists of 64 pneumatically driven tactile elements (taxels) which are arranged in a 8x8 matrix distributed in an area of 80 x 80 mm. A single taxel is built up by an actuator connected to a pressurized pipe. The pressure pushes a piston rod with a pass of 10 mm. Contact to the skin is established via a spherical plastic cap. The effective force applied to the skin can be varied between 1.3 and 2.7 N. Static and vibrating pattern can be applied. The maximum vibration frequency is 6 Hz. The latency of the stimulus with respect to the electrical switching of the pressure valves is 50 ms. The actual display unit is connected via 6 m long pressure piped to a Electro-Pneumatical Interface which received trigger signal from the fMRI imager and pattern files from a control computer both via a CAN-bus protocol. The device has been delivered with a very flexible software package to monitor and control the pattern generation of the device. Pattern can be generated from manual input, from pre-defined sequence files or from video camera input.
Safety of the PTD is ensured by the use of fully non-ferrromagnetic materials. The human subjects are electrically decoupled from the switching valved which consume a power of 30 W at an operation voltage of 24 V. Since the acoustical noise of the valves is not negligible the valva system is located about 6 m away from the display and is shielded by a foam box. The height of the display including the pressure tubes is 8 cm, which allows convenient mounting in the narrow fMRI solenoidal tubes.
Two PTD systems have been built and used in the project. They are fully operational and have been used in fMRI and PET experiments. No problems with image artefact have been observed. A technical publication of the device as well as scientific results based on brain activations with the PTD are available.
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