Processing 30.4-3

of Biological Signals

TOPOGRAPHIC MAPPING THE VISUAL EVOKED POTENTIAL AFI'ER SOURCE DERIVATION

ABSTRACT

S.del V.Carrefio-Rimaudo and kF.C.Infantosi

Biomedical Engineering Program, COPPE/UFRJ P.O.Box 685 10-21945-Rio de Janeiro-Brazil

Mapping Visual Evoked Potentials (VEP) after source derivation has been implemented in order t o facilitate the interpretation of VEP signals and aid in identifying the distribution of the sources that generate them. The signals have been obtained from normal adults stimulated with full-field and half-field pattern reversal checkerboards. After amplifying, digitizing and off-line coherently averaging the 16-channel-responses, source derivation was applied. This was implemented based on the finite difference technique, considering inter-electrode distances. It was observed that the spatial spreading of the signals was reduced leading to improved spatial selectivity, indicating the locations of the sources of the VEP and suggesting the existence of electrical dipoles.

INTRODUCTION

Visual Evoked Potentials, stimulated by pattern reversal checkerboard, have become a standard tool in the study of the visual system. Clinically, it is indicated in a range of diseases (such as degenerative, vascular and metabolic) of the visual pathways and cortex [3]. Non- invasive diagnostic methods, which relate spatial distribution of function rather then just anatomic structures, have proven important for the investigation of cortical diseases . Electrical brain response to sensory stimuli provides an inexpensive and simple option. To this end, the spatial distribution of the VEP at a given instant of time after stimulation may be used to form an I' image 'I, of electrical activity. However, these images are affected by the choice of reference electrode and by the different conducting layers between cortex and scalp. The latter causes spreading of the electrical field over the scalp surface, blurring the maps. Source Derivation is one method to compensate for this [ 11, which deemphasize tangential field components, emphasizing sources immediately below the electrode. Additionally, the dependence on the reference electrode is reduced. This method has been applied to EEG data, confirming the expected improvement in spatial selectivity of topograms [l].

Source Derivation has now also been applied to VEP in order to better estimate the electrical activity below each electrode, and through subsequent mapping identify more clearly the field sources in the cortical regions.

MATERIALSANDMETHODS

Six normal adults (age range 25-33) of either sex have been examined. The stimulus was a 16 x 16 checkerboard (visual angle of each element 44'), reversed at a frequency of 2Hz. For the full-field, 200 responses were collected, and 250 each for the half-fields. The electrode system used was that proposed by Bodis-Wollner [2] with 16 electrodes concentrated above the visual cortex (Broadman's - areas 17, 18 and 19). All recording channels were simultaneously sampled (120 Hz, 10 bits), stored and coherent averaged off-line in an IBM/PC compatible microcomputer. The averages were then source derived based on the finite difference technique, and the implementation considering the inter-electrode distances [l]. Topograms are formed by spatially interpolating the VEP (using the four nearest neighbour algorithm [4]) at a user selectable latency [5].

RESULTS AND CONCLUSION

The source derived signals were quantitatively evaluated by correlation in order to measure the independence between data from adjacent electrodes. It was observed that, in general, correlation decreased, as desired. In some cases the correlation became negative, which may indicate phase inversion.

Qualitative analysis was carried out by visual inspection of the topograms comparing maps before and after source derivation. Typical results are shown in the figures below. With full-field checkerboard stimuli, the topogram after source derivation shows that the distribution of activity becomes restricted to the parietal posterior regions of both hemispheres, reducing the original spatial spread. Stimulating in the left visual half-field, the expected complex NP was observed ipsilaterally and PN contra- laterally. After source derivation, phase inversion may be observed in the parietal region of the right hemisphere (R34, RP2). Furthermore, the signal in the inferior occipito- parietal region (Z5, R17) was emphasized, especially the second peak. This effect is more clearly depicted in the topographic maps constructed at the latency of either peak. Again, the spatial spread of fields is reduced by source derivation. Similar observations were made when stimulating in the right visual half-field.

Annual International Conference of the IEEE Engineering in Medicine and Biology Society, Vol. 13, No. 1, 1991 0395 CH3068-4/91/0000-0395 SO1 .OO 0 1991 IEEE

LPT T3 L P 1 2 5 0 L34 LP2 L17 235

RPI R34 RP2 R17 T4 RPT

:: ::: :: Z##M... -0.64 2 .34

JJU

LPT T3 L P i 250 L34 LP2 L17 z35 220 25 R P i R 3 4 RP2 R13 T4 RPT

. . . . 2.:. 2..% x x X I .... .... st s i I I I 0 .82 -0 .82

JJU

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Figure 1: The 16 channel VEP (full-field checkerboard stimuli) and the topograms corresponding to the latency of 128 ms (vertical line): i) before and ii) after source derivation.

LPT T 3 LPI ZJ 0 L3 4 LP2 L17 235 22 0 25 R P I R3 4 RP2 R I 7 T 4 RPT

-3. ii U

Figure 2: The 16 channel VEP (left half-field stimulation) and the topograms after source derivation at latencies of (i) 96 ms and (ii) 160 ms.

It may be concluded that source derivation appears to place the sources of the VEP in the regions where they are expected from the anatomo-physiology. Furthermore, it suggests that the fields are generated by various electrical dipoles within in the visual area. It therefore appears that the improved spatial seletivity achieved through source derivation may be useful in clinical monitoring the development of multiple sclerosis and other degenerative diseases.

ACKNOWLEDGEMENTS

This work was partially supported by CNPq (Brazilian Research Council), CAPES (Brazilian Ministry of Education) and FAPERJ (Research Council of the State of Rio de Janeiro).

REFERENCES

[ 11 Almeida, A.C.G. and Infantosi, A.F.C., Topographic Brain Mapping of EEG Signals After Source Derivation, Proc. 12th Ann. Int. Conf. IEEE Eng. Med. Biol., Vol. 4, pp. 1598-1599: 1990.

[2] Bodis-Wollner, I., Early Negative VEP Component, Abstracts 12th Int. Congr. EEG & Clin. Neurophys., Rio de Janeiro, Brazil: 1990.

[3] Mauguiere, F. and Fisher, C., Les Potentiels Evoquh dans les Affections Neurologiques, Encyclop. Medico- Chirurgicale, Vol. 17031 (BlO), pp. 1-16: 1982.

[4] Babiloni, F., Cracas, S., Johnson, P., Salinari, S . and Urbano, A., Computerized Mapping System of Cerebral Evoked Potentials, Comput. and Biomed. Res., Vol. 23, pp. 165-178: 1990.

[5] Carreno-Rimaudo, S.del V., Mapeamento do Potencial Evocado Visual Utilizando a T6cnica de DerivacBo da Fonte, MSc Dissertation, Biomedical Engineering Program, COPPEWFRJ, Rio de Janeiro, 1991.

Address of the authors: S. del V. Carreno-Rimaudo, A.F.C. Infantosi COPPEWFRJ - Programa de Engenharia Biomtdica Caixa Postal 68.510 21945 - Rio de Janeiro - RJ BRAZIL FAX: 55-21-290.6626 Phone: 55-21-230.5108

0396 Annual International Conference of the IEEE Engineering in Medicine and Biology Society, Vol. 13, No. 1, 1991 CH3068-4/91/0000-0396 $01.00 0 1991 IEEE