EFFECT OF RATE OF STIMULATION ON TVEP: A COMPARATIVE STUDY USING MAGNITUDE AND PHASE SPECTRAL ANALYSIS

Academic Open Internet Journal

ISSN 1311-4360

www.acadjournal.com

Volume 17, 2006

R.Sivakumar

Assistant Professor, Sri Krishna College of Engg. and Tech.,

Kuniamuthur, coimbatore, Tamilnadu, india.

+91 422 2678001,sivarkumar@mailcity.com

Transient Visual Evoked Potential (TVEP) is an important diagnostic test for specific ophthalmologic and neurological disorders. Normally the procedure time to perform TVEP takes about 30 to 60 minutes. In order to reduce the procedure time, we use to go for higher rate to stimulation. In this paper we present the comparative study on effect of rate of stimulation on TVEP using Magnitude and Phase spectral analysis. It has been shown that both the measures having the positive correlation with the rate of stimulation

Keywords: Transient Visual Evoked Potential, Ophthalmologic and neurological disorders, Rate of stimulation, Magnitude, Phase, Spectral analysis.

INTRODUCTION

The Transient Visual Evoked Potential (TVEP) is an important diagnostic test for specific ophthalmological and neurological disorders [1-3]. VEP recordings are obtained in a simple and non-invasive way. The precision of clinical interpretation depends on the amount of information available. This requires long periods of stimulation. TVEP investigation focused on the dominant peaks N75 P100 N135, a negative deflection followed by a positive and then a negative deflection. The amplitude and latencies of these peaks are measured directly from the signal. Quantification of these latency changes can contribute to the detection of possible abnormalities. This requires the precise definition of the starting and the end points. Latency measure depends on the point at which the latency is calculated.

The clinical use of TVEP is mainly based on the peak amplitude and latency of P100. In general, the stimulation rate used for the recording is 1Hz. But it requires minimum of 50-60 trials for calculating P100 value. Usually, it takes minimum of 30 minutes for completing the entire procedure and the fatigue condition affects the procedure. Higher rate of stimulation is preferred to reduce the procedure time and also to reduce fatigue levels in patients. But at higher rate of stimulation, identification of P100 peaks becomes very difficult and also there is an increase in the latency compared to that of the latency obtained using the 1Hz stimulation rate. All the previous studies confirmed that the as the rate of stimulation increases there will be a change in the latency [4,5] and none of the studies have shown the correlation between the rate of stimulation and the change in TVEP P100 latency. Our previous analysis has shown the effect of rate of stimulation on spectral components and latency [6,7]. In this paper, first we present the comparative analysis on the effect of rate of stimulation on transient TVEP using magnitude and phase spectral analysis.

MATERIALS AND METHOD

Experiments were carried with 70 normal subjects (19 – 32 years old, 25 females and 45 males). Patients have been chosen such a way that all subjects have the latency value exactly 100msec.TVEP was performed in a specially equipped electro diagnostic procedure room (darkened, sound attenuated room). Initially, the patient was made to sit comfortably approximately 1 meter away from the pattern-shift screen. Subjects were placed in front of a black and white checkerboard pattern displayed on a video monitor. The checks alternate black/white to white/black at a rate of approximately twice per second. Every time the pattern alternates, the patient's visual system generates an electrical response that was detected and was recorded by surface electrodes, which were placed on the scalp overlaying the occipital and parietal regions with reference electrodes in the ear. The patient was asked to focus his gaze onto the center of the screen. Each eye was tested separately (monocular testing). Only the rate of stimulation is changed. Other parameters remained constant (brightness, contrast, stimulation pattern size etc.,). The response of each stimulation rate was done for 30 cycles and stored and similarly respondents repeated for remaining rate of stimulation also (2Hz, 3Hz, 4Hz, 5Hz and 6Hz stimulation).

The TVEP waveform was sampled at 1024Hz (as per IFCN Guidelines issued by Nuwer et al 1994). The spectral components of the each pre-stimulus waveform and post-stimulus averaged waveform were identified by Welch’s averaged periodogram method with a frequency resolution of 1Hz. Beyond 10Hz no major spectral components were obtained for of all the subjects, so the values on the frequency band 1 - 10 Hz were normalized according to the maximum value in that band. Specifically, the first two dominant peaks were extracted from the spectral plot along with the corresponding magnitude and frequency values. A program to extract the magnitude and frequency of the first two dominant spectral components values has been developed. The relations between the rate of stimulation and P100 latency, rate of stimulation and spectral components have also been identified.

Periodicity measure has been applied to TVEP at different rate of stimulation. The periodicity was calculated by summing the energy at highest amplitude frequency and its multiples and comparing the quantity to energy at remaining frequencies. We found the periodicity measure P_fof a signal f as a normalized difference of the sum of the power spectrum values at the highest amplitude frequency and its multiples and the sum of power spectrum values at the frequency half way between them i.e.,

P_f= (å F_iw - å F_(iw+w/2)) / ( å F_iw + å F_iw+w/2)

Where F is the energy spectrum of the signal f and w is the frequency corresponding to the highest amplitude in the energy spectrum.

RESULTS

Primary results indicate that the spectral components have been observed in the frequency range of 2Hz - 6Hz values. For 1 Hz stimulation rate, the peak frequency has been observed at 2Hz, for 2Hz stimulation rate, the peak frequency has been observed at 3Hz. For 3Hz, 4Hz and 5Hz stimulation rate, the peak frequency has been observed at 4Hz, 5Hz and 6Hz respectively. Beyond the 4Hz stimulation rate, it has been found that the spectral component value exactly at the stimulation rate. It has been found that as the rate of stimulation increases the spectral component moves towards the higher frequencies (table 1).

The important finding of our result is that there are distinct differences at the periodicity at different stimulation rates. For 1Hz stimulation rate the periodicity measure is 0.0596, for 2Hz stimulation rate the periodicity measure is 0.6999 and for 3Hz stimulation we found that the periodicity measure is 2.4768. It has been found that as the rate of stimulation is increased, the value of the periodicity measure also increases (Table 2). Thus the magnitude spectral components and phase spectral periodicity are found to have a positive correlation with the rate of stimulation. Figures 1 shows the normal subject TVEP waveform. Figures 2-3 show the corresponding magnitude and phase spectrum.

Figure 1 Transient VEP Waveform

Figure 2 TVEP Magnitude Spectrum

Figure 3 TVEP and corresponding phase spectrum

Table 1 Dominant spectral components value vs. Rate of stimulation

S.No	Rate of stimulation	Spectral component values
1.	1Hz	2Hz
2.	2Hz	3Hz
3.	3Hz	4Hz
4.	4Hz	5Hz
5.	5Hz	5Hz
6.	6Hz	6Hz

Table 2 Phase periodicity measure vs. Rate of stimulation

S. No.	Rate of stimulation	Periodicity Measure
1	1HZ	0.0596
2	2 HZ	0.6999
3	3 HZ	2.4768

CONCLUSION

In the earlier studies, many methods have been used for TVEP P100 latency analysis and those methods have adopted only time domain analysis. In time domain analysis, at higher rate of stimulation, it is very difficult to identify the exact P100 values due to the irregular peaks. As the rate of stimulation increases, the visual system will not get time to come back to the original state before the next stimulation. Because of this reason, the responses will overlap, thus making it difficult to measure the exact P100 value. At the higher rate of stimulation, the TVEP is almost similar to sinusoidal waveform [5] and it is called SSVEP. Many researchers have been utilized the frequency domain analysis for SSVEP [8-11]. In this study, TVEP waveforms have been analyzed at different rate of stimulation. Up to 4Hz stimulation, the P100 latency value has been clearly observed and after 4Hz stimulation P100 peak completely disappears from the waveform and almost the waveform becomes sinusoidal. Beyond the 4Hz stimulation rate, it has been shown that the spectral component value observed exactly at the stimulation rate. This result exactly coincides with the previous SSVEP spectral components analysis results [12,13]. Using the both magnitude and phase values the effect of rate of stimulation on latency could identified more precisely.

REFERENCES

1. AbdelMageed M.A.S. and Assem H.M. (2002), ‘Electroretinography and visual evoked potential in children with IDDM’, Proceedings of ISLRR Vision 2002 Conference abstracts, No. P150.

2. Lauritzen L., Jorgensen M.H. and Michaelsen K.F. (2004), ‘Test-retest reliability of swept visual evoked potential measurements of infants visual acuity and contrast sensitivity’, Pediatric Research, Vol. 55, pp. 701-708.

3. Suttle C.M. and Turner A.M. (2004), ‘Transient pattern Visual Evoked Potentials in children with Down’s syndrome’, Ophthalmic and Physiological Optics, Vol. 24, No. 2, pp. 91.

4. Misra J.K. and Kalith (1999) ‘Clinical Neurophysiology’, I.Churchill Livingstone Pvt Ltd, New Delhi .

5. Heravian S.J., Douthwaite W.A. and Jenkins T.C.A. (1999) ‘Acuity predictions from visual evoked potential to checkerboard pattern reversal stimuli: the effect of reversal rate’, Clin. Exp. Optom., Vol. 82, No. 6, pp. 244-249.

6. Sivakumar R. and Ravindran G. (2002) ‘Effect of rate of stimulation on pattern shift visual evoked potential spectral components’, Proc. Of IEEE EMBS UK & RI Postgraduate Conference on Biomedical Engineering and Medical Physics, Aston University, Aston, U.K., pp. 15.

7. Sivakumar R. and Ravindran G. (2004) ‘Analysis of Effect of Rate of Stimulation on Transient VEP Latency’ Proc. Int. Conf. on Bio-Medical Electronics and Telecommunication (BET-04), Andhra University, Visakhapatnam, pp. 311-314.

8. Tobimatsu S. and Kato M.M. (1996) ‘The effect of binocular stimulation on each component of transient and steady-state VEPs’, Electroencephalogr Clin Neurophysiol., Vol. 100 No.3, pp. 177-83.

9. Nakayama M. (1994) ‘Transient and steady-state electroretinograms and visual evoked potentials to pattern and uniform-field stimulation in humans’, Fukuoka Igaku Zasshi., Vol. 85, No. 7, pp. 225-234.

10. Suttle C.M. (2001) ‘Visual acuity assessment in infants and young children’, Clinical and Experimental Optometry, Vol. 84, No. 6, pp. 337-345.

11. Marrelli A., Tozzi E., Porto C., Cimini N., Aloisi P. and Valenti M. (2001) ‘Spectral analysis of visual potentials evoked by pattern-reversal checkerboard in juvenile patients with headache’, Headache, Vol. 41, No. 8, pp. 792-797.

12. Tobimatsu S., Kurita-Tashima S., Nakayama-Hiromatsu M. and Kato M. (1993) ‘Effect of spatial frequency on transient and steady-state VEPs: stimulation with checkerboard, square-wave grating and sinusoidal grating patterns’, J Neurol Sci., Vol. 118, No.1, pp.17-24.

13. Johansson B. and Jakobsson P. (2000) ‘Fourier analysis of steady-state visual evoked potentials in subjects with normal and defective stereo vision’, Documenta Ophthalmologica, Vol. 101, No. 3, pp. 233-246.

Technical College - Bourgas,