Analogue analyser for on-line eye movement detection

Electroencephalography and Clinical Neurophysiology, 1 9 7 9 , 4 6 : 1 1 0 - - 1 1 3

110

© E l s e v i e r / N o r t h - H o l l a n d Scientific Publishers, Ltd.

Technical contribution ANALOGUE

ANALYSER

FOR ON-LINE EYE MOVEMENT DETECTION l

P A O L O I N C H I N G O L O , L E O P O L D O BON a n d R U G G E R O C O R A Z Z A 2

Institute of Human Physiology, University of Trieste, Trieste (Italy) ( A c c e p t e d for p u b l i c a t i o n : A u g u s t 28, 1 9 7 8 )

In eye m o v e m e n t studies a n d p a r t i c u l a r l y in studies a t t e m p t i n g to c o r r e l a t e individual eye m o v e m e n t s w i t h single u n i t a c t i v i t y ( A r d u i n i e t al. 1 9 7 4 ; A r d u i n i a n d Corazza 1 9 7 7 ; B o n et al. 1 9 7 7 ) , we very o f t e n n e e d to collect t h e m o v e m e n t s in classes pres e n t i n g similar c h a r a c t e r i s t i c s o f v e l o c i t y , d u r a t i o n , a m p l i t u d e , etc. T h e a n a l o g u e m e t h o d s o f eye m o v e m e n t d e t e c t i o n so far available can o n l y process o n e c o m p o n e n t of t h e m o v e m e n t . We have t h e r e f o r e d e v e l o p e d a s i m p l e a n a l o g u e s y s t e m o f single m o v e m e n t d e t e c t i o n , b a s e d o n t h e d i s c r i m i n a t i o n of v e l o c i t y a n d d i r e c t i o n f r o m the whole movement pattern. In this way t w o t y p e s of m o v e m e n t c o u l d be collected: ' a c t u a l ' a n d ' p r o j e c t e d ' m o v e m e n t s , t h e first r e p r e s e n t i n g the real t r a j e c t o r i e s of t h e eye balls b e t w e e n t w o f i x a t i o n p o i n t s , a n d t h e s e c o n d , t h e proj e c t i o n o f t h e real t r a j e c t o r i e s o n a h o r i z o n t a l or vertical axis. Since, as is k n o w n , t h e eyes m o v e w i t h o n l y t w o degrees of f r e e d o m ( N a k a y a m a 1 9 7 5 ) we can a s s u m e t h a t every globe p o s i t i o n in space is d e s c r i b e d b y t h e vector P defined by the two orthogonal coordinates Y ( i n c l i n a t i o n or vertical c o m p o n e n t ) a n d X (declinat i o n or h o r i z o n t a l c o m p o n e n t ) or b y t h e t w o p o l a r c o o r d i n a t e s R (ray v e c t o r ) a n d O p ( a n o m a l y ) , w h e r e : Y = R sin 0p

V x a n d V y ( h o r i z o n t a l a n d vertical c o m p o n e n t s o f t h e eye v e l o c i t y ) or b y t h e t w o polar c o o r d i n a t e s V ( m o d u l u s of t h e v e l o c i t y ) a n d Ov ( a n o m a l y o f t h e velocity, i.e. t h e i n s t a n t a n e o u s o r i e n t a t i o n of t h e eye movement), where : Vx = V cos 0 v Vy = V sin 0 v

V =x/~x 2 + V y 2

(2)

0v = a r c t g ( V y / V x ) N o w , a c c o r d i n g to a preset v e l o c i t y t h r e s h o l d V0, we define t h e o n s e t (tb) a n d t h e e n d (te) o f a n a c t u a l movement when : V(ti) < V0

for t i < t b

V ( t b ) = V0 V ( t k ) > V0

for t b < t k < t e

(3)

V(te) = V0 V ( t i) < V0

fort i> te

F u r t h e r m o r e we c o m p u t e t h e l e n g t h o f t h e a c t u a l m o v e m e n t L b y m e a n s o f t h e d e f i n e d integral o f t h e v e l o c i t y in t i m e :

X = R cos ~p R = X/~

+ y2

te

(1) L = /

0p = a r c t g ( Y / X ) T h e t i m e d e r i v a t i o n o f t h e P v e c t o r gives t h e v e l o c i t y v e c t o r V~, d e f i n e d b y t h e t w o o r t h o g o n a l c o o r d i n a t e s

1 This research was s u p p o r t e d by G r a n t No. 75.00484.04 from Consiglio N a z i o n a l e delle Ricerche. 2 P r e s e n t address: I n s t i t u t e o f H u m a n P h y s i o l o g y , U n i v e r s i t y o f F e r r a r a , F e r r a r a , Italy.

V(t) dt

(4)

tb and, b y s u b s t i t u t i n g in (3) a n d in (4) Vx or V y to V, we can define t h e o n s e t a n d t h e e n d of t h e p r o j e c t e d m o v e m e n t a n d its l e n g t h Lx or Ly. M o r e o v e r , all t h e u p w a r d m o v e m e n t s a n d t h e eye d i s p l a c e m e n t s to t h e right c a n b e d e t e c t e d very s i m p l y b y c o m p u t i n g t h e a b o v e Ly a n d Lx values for Vy = 0 or, respectively, V x = 0. By s u b s t i t u t i n g for V y a n d Vx t h e o p p o s i t e values - - V y a n d - - V x , all t h e d o w n w a r d m o v e m e n t s a n d all t h e d i s p l a c e m e n t s t o t h e left can b e determined.

EYE MOVEMENT ANALYSER

111 T w o derivative circuits, mainly c o m p o s e d of 4 operational amplifiers (A1, A2, A3 and A4 in Fig. 1A) and performing the first derivative of the input with noise integration, generate the two velocity components Vx and Vy f r o m X and Y. To amplify the lower f r e q u e n c y c o m p o n e n t s o f the signal w h e n the eye m o v e m e n t s have low velocity, thus increasing the signal-to-noise ratio, the analyser provides, through $1, 3 derivation time constants with 3 different levels of noise integration (Fig. 1A): R1C1 = ~d = 0.5--1--2 sec; RIC2 = ~'i = 0 . 0 0 5 - - 0 . 0 2 - 0.1 sec. The A1 and A2 o u t p u t s supply - - V x and - - V y , whereas the A3 and A4 inverting amplifiers supply Vx and Vy. The velocity modulus V is calculated as ~ / V x 2 + Vy 2 by means o f the 4352 T e l e d y n e Phylbrick vector o p e r a t o r circuit (Fig. 1A). The d e t e c t o r of single m o v e m e n t s p e r f o r m i n g l(t), Ix(t) or ly(t) functions has t w o c o m p o n e n t s . (1) An amplitude discriminator (Fig. 1B) fed at the input by one of the following signals chosen by the o p e r a t o r through $2: V, Vx, V y , - - V x , - - V y . The amplitude discriminator has been obtained with the A5 operational amplifier, in which the reference voltage can be preselected through $3 f r o m 12 values in calibrated steps. This reference voltage represents the threshold velocity V0, Vx0 or Vy0. The A5 o u t p u t is 0 for values at the input less than the reference voltage, whereas it b e c o m e s negative for input values higher than the reference voltage. F o r this reason the A5 o u t p u t remains negative for the whole d u r a t i o n of every d e t e c t e d m o v e m e n t . If the chosen signal at the

Finally, if we consider the f u n c t i o n :

t l(t)= f

tb

V(t) dt for tb < t < t e

(5)

which is 0 for t = t b and L for t = te, we obtain the eye run at every instant f r o m the beginning to the end of the m o v e m e n t . Moreover, the shape of the graphic representation of this f u n c t i o n can give s o m e i n f o r m a t i o n a b o u t acceleration of the m o v e m e n t and, therefore, a b o u t the kind of m o v e m e n t . Thus, we can c o m p a r e the temporal e v o l u t i o n l(t) of t w o or m o r e m o v e m e n t s as well as their length L and their duration (t e - - t b ) . The same comparisons can be carried o u t for the p r o j e c t e d m o v e m e n t s and the related f u n c t i o n s lx(t) and ly(t).

Circuit description F o l l o w i n g the above assumptions an analogue analyser has been designed using, basically, operational amplifiers. All the above functions are generated by the analyser in real t i m e and are visualized on an 8-channel paper polygraph. The input to the analyser is represented by the two o r t h o g o n a l coordinates of the eye position P, i.e. X and Y, measured in our e x p e r i m e n t s using the t e c h n i q u e of the scleral search coil in a magnetic field ( R o b i n s o n 1963). It should be noted that the analyser can successfully process even eye m o v e m e n t s r e c o r d e d by electrooculography, providing low-pass filtering (18 d B / o c t a v e ) with cut-off f r e q u e n c y at 100 Hz.

A

R1 C2

ilB

~

i

C i:

~

Vx

"

'""

ix(t)

st

R

~

Vector i Operato r

cD o66 W -Vy e

It"

A5

lOOk +15 o

y

vv

":

~

47,~s

....

Fig. 1. Simplified circuit diagram of the analyser. A: generator of eye m o v e m e n t velocity parameters. B: a m p l i t u d e discriminator. C: integration circuit. The diagram does not show the drift and f r e q u e n c y compensations, because of their c o n v e n t i o n a l i t y (Widlar 1969; D o b k i n 1970).

112

P. I N C H I N G O L O ET AL.

input is V, all the ocular m o v e m e n t s with velocity higher than the preset value V0 can be d e t e c t e d by the analyser. If the chosen signal at the input is, instead, Vx, only those displacements to the right at velocities higher than the preset value Vx0 can be c o m p u t e d . Hence, for Vx0 = 0 the analyser can detect every m o v e m e n t e x e c u t e d to the right. On the o t h e r hand, w h e n the analyser is fed by o p p o s i t e signals (--Vx) at the input, it operates the d e t e c t i o n of o n l y the left-running m o v e m e n t s . Obviously, the same operations can be p e r f o r m e d w h e n the signals at the input are Vy and - - V y ; in this case the upward movements will substitute the displacements to the right and the d o w n w a r d m o v e m e n t s those to the left. (2) An integrator circuit, p e r f o r m i n g the l(t), lx(t) or ly(t) functions, set and reset through the A5 o u t p u t and fed into the input by the velocity signal previously selected for the a m p l i t u d e discrimination (Fig. 1C). The integrator circuit is formed by the A6 operational amplifier, working as an inverted integrator whose feedback is set to 0 by the switch SW driven by the A5 output. Finally, the A7 operational amplifier restarts the correct polarity of the o u t p u t signal. Operatively, p e r f o r m a n c e of the single movem e n t d e t e c t o r can be described by the f u n c t i o n : t

l(t)=

1/7/'V(t)dt

Ix_~

w'--[]

I v~ I lx,

LI

I lx I

Vv

.

,/

!

t

(6)

tb with ~ = R3C 3 = 1 sec, w h e n V is the preselected velocity signal. Analogous functions describe lx(t) and ly(t), w h e n the preset velocity signals are Vx and Vy.

Results A typical analysis of ocular m o v e m e n t s p e r f o r m e d by the above-described analyser is shown in Fig. 2, where records 1 and 5 represent the horizontal (X) and the vertical (Y) c o m p o n e n t of the eye m o v e m e n t patterns r e c o r d e d in a cat. Records 2 and 6 show the horizontal (Vx) and the vertical (Vy) velocity c o m p o nents, w h e n the derivative t i m e c o n s t a n t is 1 sec with an integration c o n s t a n t o f 20 msec. Records 3 and 4 display the horizontal c o m p o n e n t s of the eye movem e n t patterns respectively to the right (lxr) and to the left (lXl) , records 7 and 8 the upward (JYu) and the d o w n w a r d (lYd) vertical c o m p o n e n t s and records 9 and 10 the velocity m o d u l u s (V) and the m o v e m e n t s (l) c o m p o s i n g the patterns. F o r identification of the m o v e m e n t s a threshold of 10°/sec was used throughout. R e c o r d 11 represents a detail at higher speed of record 10 in order to show the different patterns of the f u n c t i o n l(t) for different types o f m o v e m e n t . By simple inspection of the records s h o w n in

Fig. 2. Strip-chart recordings of f u n c t i o n s generated by the analyser. R e c o r d 11 is a detail at higher speed of record 10 between the two arrows. Vertical bars: 5 ° for records 1, 3, 4, 5, 7, 8, 10, 11; 25°/sec for records 2, 6, 9. T i m e calibration (horizontal bar): 1 sec for record 11 and 5 sec for records 1--10. X and Y are raw tracings obtained by means of the search coil recording t e c h n i q u e (EOG recordings lend themselves equally well to this kind of analysis).

Fig. 2 the m o v e m e n t s can easily be pooled into classes on the basis of their direction and velocity. F u r t h e r m o r e , several physical parameters of the individual m o v e m e n t s , such as duration, a m p l i t u d e and velocity, can easily be measured and, through two additional sample-hold circuits and a r a m p generator, the amplitude-velocity and the a m p l i t u d e - d u r a t i o n relationships could automatically be visualized on a CRT.

Summary A system o f single m o v e m e n t d e t e c t i o n is o b t a i n e d by discriminating the velocities and orientations of eye ball displacements. The o r t h o g o n a l c o m p o n e n t s

EYE M O V E M E N T A N A L Y S E R

113

of eye velocity (Vx and Vy) are generated f r o m the horizontal (X) and the vertical (Y) c o m p o n e n t s of the m o v e m e n t s by means of two derivative circuits and the velocity m o d u l u s (V) is generated through a vector o p e r a t o r f r o m Vx and Vy. The onset and end of individual m o v e m e n t s are d e t e c t e d on the basis of a preo selected V, Vx or Vy threshold and, for every movem e n t thus identified, the integral f u n c t i o n of V(t), Vx(t) or Vy(t), characteristic of the t y p e of movem e n t , is generated in such a way that its final value represents ball displacement.

R6sum~

Analyseur analogique permettant la ddtection ligne' du mouvement oculaire

'en

Dans ce travail on d~crit un appareit p e r m e t t a n t l'~tud'e du m o u v e m e n t oculaire individuel par disc r i m i n a t i o n de sa vitesse et de sa direction. Les composantes orthogonales de la vitesse du m o u v e m e n t (Vx et Vy) s o n t d6duites des c o m p o s a n t e s horizontales (X) et verticales (Y) du m o u v e m e n t par d e u x circuits d~rivateurs et le m o d u l e (V) de la vitesse est o b t e n u grace ~ un op6rateur vectoriel ~ partir de Vx et Vy. Le d6but et la fin de chaque m o u v e m e n t sont d6tect~s ~ partir de valeurs liminaires pr6d~termin~es de V, de Vx ou de Vy. Pour chaque m o u v e m e n t ainsi identifi~, on o b t i e n t la f o n c t i o n int6grale de V(t), Vx(t) ou Vy(t) caract~ristique du m o u v e m e n t considerS. Sa valeur finale repr6sente le d 6 p l a c e m e n t du globe oculaire.

The authors wish to thank Mr. Fabio Chiarelli for his helpful technical c o n t r i b u t i o n .

References Arduini, A. and Corazza, R. A c t i v i t y of neurons in the deep layers of the cat superior colliculus correlated with slow eye m o v e m e n t s . Pflilgers Arch. ges. Physiol., 1977, 369: 161--165. Arduini, A., Corazza, R. and Marzollo, P. Relationships of the neuronal activity of the superior colliculus to the eye m o v e m e n t s in the cat. Brain Res., 1974, 73: 473--481. Bon, L., Corazza, R. and Inchingolo, P. Neuronal activity correlated with eye m o v e m e n t in the cat's p r e t e c t u m . Neurosci. Lett., 1977, 5: 69--73. Dobkin, R.C. Op. A m p . Circuit collection. Nat. Semic o n d u c t o r , 1970, AN-31: 1--20. N a k a y a m a , K. C o o r d i n a t i o n of extraocular muscles. In: G. Lennerstrand and P. Bach-y-Rita (Eds.), Basic Mechanisms of Ocular Motility and their Clinical Implications. Pergamon Press, New York, 1975: 193--207. R o b i n s o n , D.A. A m e t h o d of measuring eye movem e n t using a scleral search coil in a magnetic field. I E E E Trans. bio-med. Electr. Engng, 1963, 10: 137--143. Widlar, R.J. IC Op. A m p . beats F E T s on input current. Nat. S e m i c o n d u c t o r , 1969, AN-29: 1--20.