DIORITA

C L A IR E C. G IA N N A ,' S E V E R IN H E IM B R A N D ,' T A D A S H I N A K A M U R A '' an d M IC H A E L A. G R E S T Y '

F rom the ' M R C N e u ro -o to lo g y unit, IIM B U , N a tio n a l H o sp ita l, Queen Scjuare, London, U K , a n d -D e p a rtm en t o f O to la ryn g o lo g y, Yagarnata U n iversity S c h o o l o f M edicin e, Japan

Gianna CC, Heimbrand S , Nakamura T, Gresty MA. Thresholds f o r p e rc e p tio n o f la te ra l m otion in n o rm a l su b jects a n d

p a tie n ts with b ila te ra l loss o f vestibu lar fu n ctio n . Acta Otolaryngol (Stockh) 1995; Suppl 520: 343 346.

Thresholds for detection o f direction o f whole-body lateral linear acceleration were determined for normal (N ) and labyrinthine defective ( LD) subjects. Thresholds for 67% correct detection o f direction o f acceleration steps for 5 LDs (mean 5.65 cm /s-, peak gradient = 25 cm/s'^) were not significantly different from 8 N s (mean 4.84 cm /s-, peak gradi­ ent = 22 cm/s^). High inter-subject variability was found both among the 7 N s and 3 LDs for detection o f parabolic accelerations with some individuals being unable to detect their motion direction. Mean N s thresholds were 15.2 cm /s- for a ramp with gradient o f acceleration = 2.8 c m /s \ 26.4 cm /s- for a ramp with gradient = 7.9 cm/s^ and 20.2 cm /s- for a parabola with second derivative = 1.52 cm/s"*. Thresholds for LDs were respectively 19.1 cm /s-, 32 cm /s- and 26.7 cm/s-. The lower thresholds for acceleration steps demonstrate the important effect o f acceleration gradient on motion detection. For all stimuli, thresholds for some LDs could be in the range o f N s showing that somatosensory signals can play a significant role in detecting lateral acceleration. K ey w ords: m o tio n p e rc e p tio n , lin ear a cceleration , o to lith , vestibu lar loss, so m a to sen so rv.

IN T R O D U C T IO N

W alsh ( 1) show ed th a t L D s w ere nearly 10 lim es less sensitive th a n N s in d etec tin g sinusoidal lateral linear ac ce le ra tio n , an d co n c lu d e d th a t N s relied on signals fro m the v estib u la r system to perfo rm the task. M o re recently, studies o f F e rn a n d e z et al. (2) have d e m o n ­ stra te d the existence o f b o th “ re g u la r” an d “ irre g u ­ la r ” o to lith units resp o n d in g respectively to ac ce le ra­ tio n an d its g rad ien t. T h e p resent study exam ined the influence o f acceleratio n g ra d ie n t on th resh o ld s o f lin e ar m o tio n p erc ep tio n by d eterm in in g th resh o ld s o f d etec tio n for c o n s ta n t, lin ear an d p ara b o lic accel­ era tio n s. C o m p a riso n o f th resh o ld s in N s an d LD s en a b le d us to ev a lu a te the p o te n tia l o f each accelera­ tio n profile for d etec tin g v estib u la r d ysfunction.

M A T E R IA L A N D M E T H O D S

S ubjects w ere seated , w h o le-b o d y restrain ed , on a lin e ar m o to r-p o w e re d bogie w hich accelerated them a lo n g the in te r-a u ra l axis on a precision levelled trac k . T hey were isolated from w ind efleet by a c u rta in s u rro u n d in g the ch air, an d a u d ito ry a n d vi­ b ra tio n cues to d irec tio n o f m o tio n w ere excluded or m a sk ed w ith w hite noise. T h e stim ulus w as generated by a c o m p u te r w hich also recorded the velocity feed­ b ack fro m the ch a ir, a n d th e p osition o f a joystick co n tro lle d by the subjects.

E xperim ent I: acceleration steps

W e tested 8 N s (ag e ran g e 24 to 50 years, m ean 38) a n d 5 L D s (ag e ran g e 31 to 64 years, m ean 41; 2

(T 1995 Scandinavian University Press. ISSN 0365-5237

id io p a th ic , 2 m eningitic, 1 b ila te ra l ac o u stic n e u ri­ n o m a , 3 w ere also deal). S ubjects w ere in stru c ted to signal th e ir perceived m o tio n d irec tio n using a direc- tio n ally c o n g ru e n t joy stick .

Stim uli: F ifty stim uli w ere d ispensed in a ra n d o m le ftw a rd -rig h tw a rd o rd e r (25 stim uli for each d irec­ tion). T h e stim u lu s was an ac celeratio n step from rest o f v a ria b le a m p litu d e an d d u ra tio n o f 2.3 to 3.7 s, delivered in to ta l d ark n e ss. T his w as follow ed by a s h o rt p erio d o f deceleratio n an d re tu rn to the initial p o sitio n in the light, w hich gave the subjects feedback on the w ay they h ad m oved.

Procedure'. T h e th resh o ld fo r p erc ep tio n o f m o tio n d irec tio n was defined as the acce le ra tio n w hich w ould yield a p ro b a b ility o f co rrec t d etec tio n equal to 0.67. T he stim u lu s a m p litu d e s w ere d eterm in e d by an a d a p tiv e psychophysical p ro c e d u re (3) w hich c o n ­ tro lled th e ev o lu tio n o f the test so th a t a m ax im u m n u m b e r o f stim uli w as dispensed close to the th re sh o ld level. F o r th resh o ld d e te rm in a tio n , a p sy­ c h o m e tric fu n ctio n o f the fo rm P (a ) = l/(1 + exp( —(a — m ))/s) [w here P (a ) is the p ro p o rtio n o f c o rrec t resp o n ses a t an acce le ra tio n level a, m is the m id p o in t o f th e p sy ch o m etric fu n ctio n , an d s its spread] w as fitted to the d a ta using least sq u ares e stim a tio n . T h re sh o ld w as d eterm in e d as the abscissa o f the p o in t w hose o rd in a te w as 0.67 (F ig . 1).

E xperim ent 2: parabolic and linear accelerations

T o m inim ize the elTect o f the je rk c o m p o n e n t o f m o ­ tio n on d ete c tio n th re sh o ld , fu rth e r ex p erim en ts w ere c o n d u c te d using lin ear an d p a ra b o lic accelerations.

344 C. C. Gianna et al.

4.6 7.6

AoceloaüoD levd (cm/s^)

10,6

Fig. I. Average data from 8 Ns and 5 LDs. I) Curves: Percentage of correct detections vs acceleration levels: O data from Ns, --- logistic function associated; * data

from L D S , logistic function associated. 2) Bars: Per­

centage of stimuli vs acceleration levels for: ^ Ns; B LDs. Note 1: 70"/i of the stimuli for Ns and 67% for LDs were dispensed between 3.9 and 6.5 cm/s- therefore surrounding threshold values which validates the adaptative procedure employed. Note 2: as the adaptative procedure delivered the maximum number of stimuli for every other accelera­ tion, only results corresponding to these levels are plotted. Percentage o f stimuli at one acceleration level a = % at level a + 0.5* {'Yii at (a — 1 ) -h'Xi at ( a -t- 1 )).

S ubjects w ere 7 N s (age; 23 to 50 years, m ean 35) an d 3 L D s (age: 31 to 40 years, m ean 35). Every L D and 4 o f the N s to o k p a rt in ex p erim en t 1, 6 m o n th s b efore exp erim en t 2. S ubjects w ere req u ired to in d i­ cate their perceived d irec tio n o f m o tio n w ith a jo y ­

stick as so o n as possible a n d w ere allow ed to c o rre c t th eir responses.

Stim uli: T h e lin e ar stim uli ( Fig. 2) co m p rise d a low er a c ce le ra tio n ram p (S lo w R ) w hich a tta in e d 28.4 cm /s- in 9.9 s w ith a g ra d ie n t o f 2.8 cm/s'* an d a higher acce le ra tio n ram p (F a s tR ) w hich a tta in e d 56.2 cm /s- in 7.1 s w ith a g ra d ie n t o f 7.9 c m / s \ T h e p a ra b o lic w aveform (P a r) p ro d u ce d an ac ce le ra tio n o f 60.4 cm /s^ in 8.9 s w ith a g rad ie n t rising fro m 0 to

13.5 c m / s \ th u s in te rsec tin g th e g ra d ie n ts o f th e 2 lin ear acceleratio n s. H igh levels o f ac ce le ra tio n w ere g en e rate d as p relim in ary ex p e rim en ts sho w ed th a t stim uli w ith a slow o n set an d a final v alue low er th a n 20 cm /s- did n o t en a b le th e subjects to d etec t th eir m o tio n directio n . T h e test consisted o f 30 stim uli ra n d o m ly ord ered : 5 fo r each c o m b in a tio n o f ac ce le r­ atio n profile an d directio n . S ubjects w ere given p eri­ odic rests to aid c o n c e n tra tio n . In a d d itio n a “ m in im u m rea ctio n tim e ” w as e stim a ted fo r each subject from his responses to d irec tio n a lly u n a m b ig u ­ ous high ac ce le ra tio n steps to w hich he m a d e sure an d p ro m p t responses.

M eth od o f analy.ds: T h re sh o ld was defined as the ac celeratio n level yielding 67% o f co rrec t d etec tio n s o f m o tio n d irec tio n . R ig h tw ard an d leftw ard trials were c o m b in ed to o b ta in 10 responses to each stim u ­ lus profile. F o r each acce le ra tio n profile, the “ m in i­ m um re a ctio n tim e” w as su b tra c te d from the latency o f the response to estim a te the ac tu a l tim e a t w hich d irec tio n a lity w as detected. T h e tim es for co rrec t responses w ere then o rd ered an d the ac celeratio n levels co rre sp o n d in g to the 6 ''’ an d 7"’ longest la te n ­ cies w ere calcu lated . T he th re sh o ld was tak en as 2/3 betw een these aecelerations.

VELOCITY F a s tR Par S low R ACCELERATION F astR P a r S lo w R

Fig. 2. Linear (SlowR and FastR) and parabolic (Par) accelerations. Top curves: velocity feedbacks from the chair.

Bottom curves: corresponding accelerations.

R E S U L T S

E.xperinient I

C o rre la tio n s betw een p red ic ted , P (l) , an d observed values w ere in the ran g e r = 0.73/0.99 fo r N s (tu ea n = 0.92) an d 0.8/0.99 fo r L D s (m ea n = 0.91) v alid atin g the logistic fu n ctio n . T he m ean th resh o ld for 67% co rrect d etectio n o f directio n fo r N s w as 4.84 cm /s- (ra n g e 2.88 to 6.28 cm /s") an d fo r L D s 6.85 cm /s-; 4.85 an d 6.59 cm /s- (id io p a th ic ), 5.63 an d

12.30 cm /s- (m en in g itic), 4.97 cm /s- (n eu rec to m ie s o f aco u stic n eu rin o m a). O n la ter testing the p a tie n t w ith

12.3 cm /s- low ered his th re sh o ld to 6.2 cm /s- (new m ean th re sh o ld fo r L D s = 5.65 cm /s-) show ing th a t he w as able to develop his ta len ts for d etectin g m o ­ tion. T h e a d a p ta tiv e p ro c e d u re ensured th a t 70% o f the stim uli fo r N s an d 67% fo r L D s w ere d ispensed at ac ce le ra tio n s betw een 4.1 c m /s - - 6 .7 cm /s- an d th e re ­ fore closely b rac k etin g th re sh o ld values. T h re sh o ld s for N s w ere c o m p a ra b le to th o se estim ated by Ben­ son et al., w ho used single cycle sinusoids: 5.7 cm /s-

P ercep tio n th resh olds o f la te r a l m o tio n 345

T ab le I. Thresholds, response latencies and percentages

parabolic accelerations

o f correct detectum for norm al subjects, during linear and

Thresholds Latency

Correct

Mean & range (cm/s-) Mean & range (s) detections

FastR 6 subjects 26.4 (13.6/43.6) 2.8 (L4/4.2) 90'%, SlowR 15.2 (9.9/20.9) 4.5 (2.9/6.3) 82'%, FastR 4 subjects 23.7 (13.6/32.1) 2.6 (L4/3.6) 92'%, Par 20.2 (14.2/29.9) 3.9 (3.3/4.9) 82'%, SlowR 3 subjects 12.9 (9.9/16.6) 3.8 (2.9/5.1) 83'%, Par 16.9 (14.2/20.3) 3.6 (3.3/4.0) 86'%,

(4) an d 3.7 cm /s- (5). T h e fact th a t m ean th resh o ld s fo r LD s an d N s were n o t significantly dilTerent and th a t som e L D s could detect ac ce le ra tio n s equal to N s ’ m ean th re sh o ld show s th a t so m a to se n so ry signals can be as sensitive as v estib u la r in p u ts to detect d irec tio n o f acce le ra tio n steps.

Experintent 2

Thresholds f o r tiorm al subjects: O nly 3 N s ind icated at least 70'%, o f co rrect resp o n ses at each acceleratio n profile. F o r S low R , on e subject an d for P ar, 3 s u b ­ je cts failed to give the m in im u m o f 7 co rrec t re­ sponses req u ired to d eterm in e thresh o ld s. T h e m ean p ercen tag e o f co rrec t resp o n ses o f the 7 subjects were: 77.1%, S low R ; 90'%,, F a stR ; an d 71.4%, P ar, in d ic at­ ing th a t m o tio n d irec tio n w as m ore difficult to esti­ m a te d u rin g P ar th a n d u rin g linear accelerations. T he in tra -su b je c t c o m p a riso n s (T a b le 1) are given for subjects w ho co u ld p erfo rm b o th tasks.

FastR: the high p erc en ta g e o f co rrect responses suggests th a t subjects w ere a b o v e th resh o ld w hen in­ d ic atin g th e ir m o tio n d irec tio n . T h erefo re the true th re sh o ld s m ight have been low er. T he d istrib u tio n o f

th re sh o ld s w as bim o d al: 3 betw een 13.6 and

20.21 cm /s- an d 4 betw een 30.45 to 43.6 cm /s- (all Ns c o n sid ered ). T h e w ide ran g e an d the bim o d al d istri­