Claude Veraart

Visual sensations produced by optic nerve stimulation using an implanted self-sizing spiral cuff electrode

A blind volunteer with retinitis pigmentosa was chronically implanted with a self-sizing spiral cuff electrode around an optic nerve. Electrical stimuli applied to the nerve produced localized visual sensations that were broadly distributed throughout the visual field and could be varied by changing the stimulating conditions. These results demonstrate the potential for constructing a visual prosthesis, based on electrical stimulation of the optic nerve, for blind subjects who have intact retinal ganglion cells.

Selective control of muscle activation with a multipolar nerve cuff electrode

Acute experiments were performed on adult cats to study selective activation of medial gastrocnemius, soleus, tibialis anterior, and extensor digitorium longus with a cuff electrode. A spiral nerve cuff containing twelve dot electrodes was implanted around the sciatic nerve, and evoked muscle twitch forces were recorded in six experiments. Spatially isolated dot electrodes in four geometries (monopolar, longitudinal tripolar, tripolar with four common anodes, and two parallel tripoles) were combined with transverse field steering current(s) from an anode(s) located 180 degrees around from the cathode(s) to activate different regions of the nerve trunk. A selectivity index was used to construct recruitment curves for a muscle with the optimal degree of selectivity. Physiological responses were correlated with the anatomical structure of the sciatic nerve by identifying the nerve fascicles innervating the four muscles, and by determining the relative positions of the electrodes and the nerve fascicles. The results indicated that the use of transverse field steering current improved selectivity. The relative performance of the various electrode arrangements is discussed.<<ETX>>

High metabolic activity in the visual cortex of early blind human subjects

Glucose metabolism has been studied in the visual cortex of early blind human subjects. In the forebrain of these subjects, regional glucose utilization was the highest in the striate and prestriate cortical areas. Furthermore, this activity was higher than in blindfolded sighted subjects, whether at rest or during an auditory or tactile task. These observations raise the question of the functionality of the blinds visual cortex.

A real-time experimental prototype for enhancement of vision rehabilitation using auditory substitution

The rehabilitation of blindness, using noninvasive methods, requires sensory substitution. A theoretical frame for sensory substitution has been proposed (C. Veraart, 1989) which consists of a model of the deprived sensory system connected to an inverse model of the substitutive sensory system. This paper addresses the feasibility of this conceptual model in the case of auditory substitution, and its implementation as a rough model of the retina connected to an inverse linear model of the cochlea. The authors have developed an experimental prototype. It aims at allowing optimization of the sensory substitution process. This prototype is based on a personal computer which is connected to a miniature head-fixed video camera and to headphones. A visual scene is captured. Image processing achieves edge detection and graded resolution. Each picture element (pixel) of the processed image is assigned a sinusoidal tone; weighted summation of these sinewaves builds up a complex auditory signal which is transduced by the headphones. On-line selection of various parameters and real-time functioning of the device allow optimization of parameters during psychophysical experimentations. Assessment of this implementation has been initiated, and has so far demonstrated prototype usefulness for pattern recognition. An integrated circuit of this system is to be developed.

Auditory motion perception activates visual motion areas in early blind subjects.

We have previously shown that some visual motion areas can be specifically recruited by auditory motion processing in blindfolded sighted subjects [Poirier, C., Collignon, O., De Volder, A.G., Renier, L., Vanlierde, A., Tranduy, D., Scheiber, C., 2005. Specific activation of V5 brain area by auditory motion processing: an fMRI study. Brain Res. Cogn. Brain Res. 25, 650-658]. The present fMRI study investigated whether auditory motion processing may recruit the same brain areas in early blind subjects. The task consisted of simultaneously determining both the nature of a sound stimulus (pure tone or complex sound) and the presence or absence of its movement. When a movement was present, blind subjects had to identify its direction. Auditory motion processing, as compared to static sound processing, activated the brain network of auditory and visual motion processing classically observed in sighted subjects. Accordingly, brain areas previously considered as specific to visual motion processing could be specifically recruited in blind people by motion stimuli presented through the auditory modality. This indicates that the occipital cortex of blind people could be organized in a modular way, as in sighted people. The similarity of these results with those we previously observed in sighted subjects suggests that occipital recruitment in blind people could be mediated by the same anatomical connections as in sighted subjects.

Occipital activation by pattern recognition in the early blind using auditory substitution for vision.

This PET study aimed at investigating the neural structures involved in pattern recognition in early blind subjects using sensory substitution equipment (SSE). Six early blind and six blindfolded sighted subjects were studied during three auditory processing tasks: a detection task with noise stimuli, a detection task with familiar sounds, and a pattern recognition task using the SSE. The results showed a differential activation pattern with the SSE as a function of the visual experience: in addition to the regions involved in the recognition process in sighted control subjects, occipital areas of early blind subjects were also activated. The occipital activation was more important when the early blind subjects used SSE than during the other auditory tasks. These results suggest that activity of the extrastriate visual cortex of early blind subjects can be modulated and bring additional evidence that early visual deprivation leads to cross-modal cerebral reorganization.

Position, size and luminosity of phosphenes generated by direct optic nerve stimulation.

Pulses of low intensity current, delivered through a cuff electrode chronically implanted around the optic nerve of a blind retinitis pigmentosa patient generate visual sensations. These phosphenes are obtained at lower thresholds for a train of stimuli than for single pulses, which suggests the existence of a spatial and temporal integrating mechanism. The perceptions are much smaller than those predicted from model simulations. A set of equations are derived which show the effect of pulse current, duration, number and frequency on the position, size and, to some extent, luminosity of the resulting phosphenes.

Brain energy metabolism in early blind subjects: neural activity in the visual cortex.

As an attempt to better understand the metabolic basis for the previously reported increases in glucose metabolism in the visual cortex of congenitally blind subjects, cerebral blood flow, oxygen consumption and glucose utilization were investigated with multitracer positron emission tomography. Measurements were carried out in three subjects who became blind early in life and in three age-matched blindfolded controls. Regional analysis of cerebral blood flow, metabolic rates for oxygen and glucose utilization revealed that these parameters were relatively higher in the visual cortex in case of early blindness (109.7 +/- 2.4%; 114.3 +/- 1.5%; 118.0 +/- 5.5%, respectively) than in controls (98.1 +/- 3.9%; 108.6 +/- 3.6%; 105.2 +/- 4.8%). There were slight differences, albeit statistically not significant, between early blind and control subjects in terms of oxygen-to-glucose metabolic ratios. The relatively preserved stoichiometry in the visual areas of blind subjects points to the lack of variation in the yield of glucose oxidation in this cortex. Those observations suggest that the high level of energy metabolism disclosed in early blind visual cortex is related to neural activity.

Creating a meaningful visual perception in blind volunteers by optic nerve stimulation

A blind volunteer, suffering from retinitis pigmentosa, has been chronically implanted with an optic nerve visual prosthesis. Vision rehabilitation with this volunteer has concentrated on the development of a stimulation strategy according to which video camera images are converted into stimulation pulses. The aim is to convey as much information as possible about the visual scene within the limits of the devices capabilities. Pattern recognition tasks were used to assess the effectiveness of the stimulation strategy. The results demonstrate how even a relatively basic algorithm can efficiently convey useful information regarding the visual scene. By increasing the number of phosphenes used in the algorithm, better performance is observed but a longer training period is required. After a learning period, the volunteer achieved a pattern recognition score of 85% at 54 s on average per pattern. After nine evaluation sessions, when using a stimulation strategy exploiting all available phosphenes, no saturation effect has yet been observed.

Improved selective and divided spatial attention in early blind subjects

Spatial attention paradigms using auditory or tactile stimulation were used to explore neural and behavioral reorganization in early blind subjects. Although it is commonly assumed that blind subjects outperform sighted subjects in such tasks, the empirical data to confirm this remain controversial. Moreover, previous studies have often confounded factors of sensory acuity with those of attention. In the present work, we compared the performance of individually matched early blind and sighted subjects during auditory and tactile tasks. These consisted of sensory acuity tests, simple reaction time task as well as selective and divided spatial attention tasks. Based on sensory measurements, we made sure that the reliability and salience of auditory and tactile information were identical between the two populations to estimate attentional performance independently of sensory influence. Results showed no difference between groups in either sensory sensitivity or simple reaction time task for both modalities. However, blind subjects displayed shorter reaction times than sighted subjects in both tactile and auditory selective spatial attention tasks and also in bimodal divided spatial attention tasks. The present study thus demonstrates an enhanced attentional performance in early blind subjects which is independent of sensory influence. These supra-normal abilities could be related to quantitative and qualitative changes in the way early visually deprived subjects process non-visual spatial information.