Martti Juhola

MEDIAN FILTERING IS APPROPRIATE TO SIGNALS OF SACCADIC EYE MOVEMENTS

In this paper we show nonlinear standard and hybrid median filters are appropriate to the filtering of signals of saccadic eye movements. Although these filters effectively attenuate noise, they do not harm the parameters of saccades, particularly the maximum velocity which is an important indicator of certain diseases and disorders in man. The filters have been tested by simulation as well as actual eye movement signals recorded electro-oculographically and photoelectrically.

A syntactic method for analysis of saccadic eye movements

Abstract A syntactic method is presented which takes advantage of a context free grammar and an LR ( k ) parser in order to recognize saccadic eye movements from digitalized eye position signals. The method functions in linear time and may be applied in real time systems. Saccades, which are voluntary eye movements, are measured in several areas of medicine for diagnosis and research. After the analog-digital conversion the signal is transformed into a symbol string. A valid saccade beginning is first looked for and the complete saccade is then recognized by applying the parser. An example of the application of the method is demonstrated on photoelectrically recorded data.

Effect of sampling frequencies on computation of the maximum velocity of saccadic eye movements

The maximum velocity of saccades is widely used in the clinical assessment of topographical diagnoses. Several methodological factors affect the maximum velocity results. The sampling frequency, the resolution of the analog-digital converter, and filtering of the signal are the most important factors. The sampling frequency should preferabel be higher than 300 Hz. Frequencies below 200 Hz severily deform the velocity profile. The resolution of the analog-digital converter should be 10 bits or more. A theoretical model was constructed for maximum velocity computation. A case study of electro-oculographic and photoelectric recordings confirmed the theoretical model.

A syntactic method for analysis of nystagmus and smooth pursuit eye movements

Eye movements are studied in neurophysiology, neurology, ophthalmology, and otology both clinically and in research. In this article, a syntactic method for recognition of horizontal nystagmus and smooth pursuit eye movements is presented. Eye movement signals, which are recorded, for example, electro-oculographically, are transformed into symbol strings of context free grammars. These symbol strings are fed to an LR(k) parser, which detects eye movements as sentences of the formal languages produced by these LR(k) grammars. Since LR(k) grammars have been used, the time required by the whole recognition method is directly proportional to the number of symbols in an input string.

Detection of saccadic eye movements using a non-recursive adaptive digital filter

Saccadic eye movements provide important information about the neuron system at several levels. In recent years computer analysis of saccades has been adapted for use in clinical work. The most common detection methods do not always function without the users control and aid. In the present paper a digital filter is described for the detection of saccades. This non-recursive filter unscrambles saccade data which has been collected during the execution of an algorithm. The method is suitable for use with microcomputers. The filter is adaptive. Two concise experiments using the method are described.

Visual stimulator using laser beam for eye-movement studies.

References AHLSTROM, M. L. and TOMPKINS, W. J. (1985) Digital filter for real-time ECG signal processing using microprocessors. IEEE Trans., BME-32, 708-713. LEVKOV, C., MICHOV, G., IVANOV, R. and DASKALOV, I. K. (1984) Subtraction of 50 Hz interference from the electrocardiogram. Med. & Biol. Eng. & Comput., 22, 371-373. LYNN, P. A. (1975) Frequency sampling filters with integer multipliers. In Introduction to digital filtering. BOGNER, R. E. and CONSTANTIN1DES, A. G. (Eds.), John Wiley & Sons, Chap. 9, I60-169, Russian translation. LYNN, P. A. (1977) Online digital filters for biological signals: some fast designs for a small computer. Med. & Biol. Eng. & Comput., 15, 534-540. MALIK, N. R. (1980) Microcomputer realisation of Lynns fast digital-filtering designs. Ibid., 18, 638-642. MALIK, N. R. and GANG HUANG (1988) Integer filters for image processing. Ibid., 26, 62-67. OPPENI-mIM, A. V. and SCHAFER, R. W. (1975) Digital signal processing. Prentice Hall, Engelwood Cliffs, New Jersey, USA. PRINCIPE, J. C. and SM]Tn, J. R. (1986) Design and implementation of linear phase FIR filters for biological signal processing. IEEE Trans., BME-33, 550-559. TnAKOR, N. V. and MOREAU, D. (1987) Design and analysis of quantised coefficient digital filters: application to biomedical signal processing with microprocessors. Med. & Biol. Eng. & Comput., 25, 18-25.

Postural Control in Blinds and in Usher's Syndrome

The postural control mechanisms were evaluated in 10 blinds and in 10 subjects with Ushers syndrome. The results were compared with 27 age matched healthy volunteers. In visual conditions the subjects with Ushers syndrome performed worse, but in nonvisual condition they performed equally well as the controls. The results were the same irrespective of whether the tests were performed on a rigid surface or foam rubber covered surface. The blinds and controls performed equally well on the bare surface, but the blinds performed significantly better on the foam rubber covered surface. The results indicate that in subjects with Ushers syndrome the vestibulo-cochlear degeneration is well compensated but the postural stability aggravated due to retinal degeneration. Blind subjects have a better postural control than their seeing referents, but the difference is only evident in situations in which the postural control is hampered by surface perturbation.

A syntactic analysis method for sinusoidal tracking eye movements

A syntactic method for the recognition of sinusoidal eye movements is presented. The method is based on a piecewise transformation of an eye movement signal to strings of a formal language and use of a formal grammar which generates the formal language. Gain and phase parameters of sinusoidal eye movements between a stimulus and its response are computed after the recognition of eye movements and after removing corrective saccades from the transformed signal. The method is used at an otoneurological laboratory.

Effect of sampling frequencies on the velocity of slow and fast phases of nystagmus

A computer analysis of slow and fast phase of velocity of nystagmus is presented. Several methodological factors are known to affect the computation of velocities, such as sampling frequencies, cutoff frequencies of the differentiation method, and resolution in bits of analog-digital conversion. A simulated signal without noise was first applied in nystagmus data in order to study maximum velocities at the theoretical level. Electro-oculographic postrotational nystagmus movements were then examined to test theoretical results. It was shown that the sampling frequency ought to be 400 Hz at least, the cutoff frequency about 70 Hz, and the resolution of analog-digital conversion 12 bits or higher.

A microcomputer system for clinical signal analysis of vestibular laboratory

A vestibular laboratory system was implemented for clinical testing and medical examination of patients with balance disorders. Programs for analysis of postural control and eye movements were made for a microcomputer. The microcomputer system can be used by a nurse without technical education. The system has now been used for two years at the laboratory.