Abstract
This study aims at understanding the effect of multiple stimulations of a human optic nerve. Phosphenes (visual sensations) were obtained by electrical stimulation of a four contacts spiral cuff electrode previously implanted around the right optic nerve of a completely blind retinitis pigmentosa patient.
When a stimulation was made through several contacts or repeated, the patient perceived several phosphenes whereas when the stimulation was made through only one contact, the patient usually perceived only one phosphene. Moreover, when comparing different types of multiple stimulations (synchronous, interlaced and sequential), the most efficient was the synchronous stimulation.
1.
Introduction
Electrical stimulations of a human right optic nerve have been done in the frame of the 'Optimization of a Visual Implantable Prosthesis' or OPTIVIP project which is in the continuity of the MIVIP project [1,2, 4& 5].
When the optic nerve was stimulated through a four contact spiral cuff electrode, the patient perceived visual sensations in her visual field. Those visual sensations, produced without any light, are called phosphenes.
Moreover a model, based on physiological parameters of the optic nerve, has been validated. This model predicts the localization, size and luminosity of phosphenes for single contact stimulations [3].
In the MIVIP project, the nerve stimulations were usually made through only one contact, which produced generally one phosphene. One may wonder if, when several contacts were to be activated in concordance, the patient would perceive multiple phosphenes gathered in a single perception. Therefore, in the present study, we investigated the effect of multi-contact stimulations.
2.
Material and method
In the MIVIP project, a blind volunteer affected by retinitis pigmentosa was recruited to be intracranially
implanted with a four contacts self-sizing spiral cuff electrode.
The
stimulations were composed of biphasic pulses with a charge recovery duration
five times longer than the first phase of the pulse. Moreover, the shape of the
pulses was rectangular and there were no pre-pulses. The charge per phase was
always under 100 nC/phase with a contact area of 0.2mm² [4].
The
volunteer described the phosphenes she perceived by pointing the center of a Plexiglas
hemisphere with her left index finger and she localized phosphenes with her right
hand. We determined if she perceived one or more phosphenes mostly on basis of her
descriptions. For example, when she described
two vertical bars at a distance one from the other, we considered that
these were two different phosphenes.
More
than 1200 descriptions of stimulations were recorded and processed. Different parameters
varied: number of pulses, pulse duration, frequency and the activated contact. Either
a single contact could be activated, or 2, 3 or 4 within the same stimulation. Moreover,
there were three types of summation: synchronous, interlaced and sequential. For
each of these condition, the nerve was stimulated through the different
involved contacts with exactly the same pulse duration, number and frequency. Pulse
duration were: 21.3, 42.6, 63.9 or 85.2 µs; pulse number were: 1, 2 or 3; pulse
frequency was: none, 60, 80 or 100 Hz.
A
stimulation was synchronous if the nerve was stimulated through 2, 3 or 4 contacts
exactly at the same time. In the interlaced scheme, the stimulation were
initiated from the various involved contacts at various delays within the stimulation
period. For technical reasons, four contacts interlaced stimulation were not
allowed. In a sequential stimulation, after the completion of the first pulse
train, a second train started after a delay equal to the period of the train
frequency.
3.
Results
We computed a two way analysis of variance with 'the number of contacts stimulating the nerve' and the ‘type of summation' as independent variables and the number of phosphenes as dependant variable. The different conditions for the variable ‘number of contacts’ were : 2, 3 et 4. And those for the ‘type of summation’ were: synchronous, interlaced and sequential. We didn’t take into account the single contact stimulation because the aim here was to compare the three type of multiple stimulations. The results are thus based on 896 descriptions of stimulations.
We obtained a main effect of the number of contacts [F(2,888)=96.897;p<0.0001] and of the type of summation [F(2,888)=69.026;p<0.0001]. We also observed a significant interaction between these two variables [F(3,888) = 6.693;p<0.0001]. All these results means, as figure 1 emphasizes, that the more the contacts stimulating the nerve, the more the synchronous stimulation elicited multiple phosphenes.
Moreover, we also computed pairwise comparisons which showed that all averages for the different number of contacts stimulating the nerve were significantly different (p<0.001). For example, stimulating three contacts produced more phosphenes than stimulating two one. For the type of summation, the pairwise comparisons were significant between synchronous summation and the two others (p<0.0001) but not between interlaced and sequential stimulations except for the difference between alternate and sequential stimulations. This means that, as suggested in figure 1, the synchronous stimulation produced more phosphenes than interlaced or sequential stimulations.
Fig.
1: Number of phosphenes as a function of stimulation characteristics
4.
Conclusion
Our
results clearly show that the synchronous summation produces more phosphenes than
the two others and that stimulating several contacts improves stimulation efficacy.
However, the number of phosphenes must be interpreted cautiously for different
reasons. This is the volunteer who described one or more phosphenes what is quite
subjective. For example, she sometimes described two phosphenes for stimulations
through one contact. An other example is that it is possible that two phosphenes
very close become one large phosphene but she described several times two
phosphenes very close.
The
most important thing in this experiment is that it is demonstrated that it is
possible to produce several phosphenes within the same perception and that the
synchronous summation on a maximum of contacts is probably a good way to do
this.
Acknowledgment
We thank C.
Archambeau for his useful help. This work was supported by the Commission of
the European Union (OPTIVIP project, Information Society Technology - RTD: IST
# 2000-25145) and the FRSM Grant # 3.4590.02..
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