AM 67-7 

ADAPTATION TO VESTIBU.LAR DISORIENTATION 

VI. Eye-Movement and Subjective Turning Responses to 

Varied Durations of Angular Acceleration 

Approved by 

~~ J. RoBERT DILLE, M.D. 

CHIEF, CIVIL AEROMEDICAL 

INSTITUTE 

Fred E. Guedry, Jr., Ph.D. 

William E. Collins, Ph.D. 

May 1967 

Released by 

~AM> P. v. SIEGEL, M.D. 

FEDERAL AIR SURGEON 

FEDERAL AVIATION ADMINISTRATION 

Office of Aviation Medicine 
ACKNOWLEDGMENTS 

Dr. Guedry is Chief of Experimental Psychology, Research Department, 

USN Aerospace Medical Institute, Pensacola, Florida; Dr. Collins is Chief, 

Psychology Laboratory, Civil Aeromedical Institute. 

This study was conducted at the Civil Aeromedical Institute in Oklahoma City with the technical assistance of Billy P. Updegraff, Ruth Ann 

Mertens, and Kathryn Quattlebaum. 

Qualified requesters may obtain Aviation Medical Reports from Defense Documentation 

Center. The general public may purchase from Clearinghouse for Federal Scientific 

and Technical Information, U.S. Dept. of Commerce, Springfield, Va. 22151 
ADAPTATION TO VESTIBULAR DISORIENTATION 

VI. Eye-Movt(ment and Subjective Turning Responses to 

Varied Durations of Angular Acceleration 

I. Introduction 

Ocular nystagmic responses elicited by angular 

acceleration have been found to be different in 

cat and in man.6 With prolonged stimuli (36 

sec), a rise and decline of nystagmus during 

stimulation was obtained from cats for both 

lateral- and vertical-canal stimulation. Human 

subjects did not exhibit a comparable decline 

during prolonged stimulation. Nystagmic reactions of the cat thus resembled the subjective 

(rather than the nystagmic) reactions of human 

subjects. 9,10,12,19,20 

At least part of the difference noted above in 

regard to primary nystagmic reactions appears 

related to the ·prominent secondary nystagmus 

which characterizes responses of the cat to angular accelerations. Secondary nystagmus is opposite in direction to the primary response, and 

it may reflect a process which develops during, 

and opposes continuation of, the ·primary reaction. 

The present investigation represents an extension of the earlier study.6 A range of stimulus 

durations was used ~to clarify relations between 

stimulus dumtion, the rate of decline of primary 

nystagmus, and the intensity of secondary nystagmus, and to compare further vestibular processes in cat and man. 

II. Methodology 

A. Cats 

Apparatus. Rotational stimulation was provided in a light-proof room with the Huffman 

Rotation Device.7 Angular accelerations and 

decelerations were 4° jsec2 separated by 54 sec of 

constant velocity. Animals were tested in pairs 

(with their heads at the center of rotation) by 

means of a set of tiers.8 

Restraint. Restraint was effected by the method 

of Henriksson, Fernandez, and Kohutt 5 and in 

the manner described elsewhere.6 

1 

Recording. An Offner Type R Dynograph recorded horizontal components of eye movements 

from needle electrodes inserted by the outer 

canthi. A 3-sec time constant ·was used in 

amplification. 

B. Human Subjects 

Apparatus. A Stille-Werner RS-3 rotator was 

programmed to provide accelerations and decelerations of 4° jsec2 separated by 2 min of constant velocity. 

Recording. An Offner Type T polygraph (time 

constant : 3 sec) recorded horizontal components 

of eye movements from surface electrodes taped 

by the outer canthi. 

III. Procedure 

Each of 12 cats and 12 human subjects received 6 rotatory trials comprising 4° jsec2 accelerations and decelerations for 1.2, 3, 9, 15, 21, 

and 30 sec. Rotation was always counterclockwise and the order of presentation of the stimulus durations was counterbalanced among pairs 

of subjects as indicated in Table 1. 

TABLE 1. Order of presentation of stimulus durations 

(in seconds) . Durations varied from 1.2 to 30 sec. 

All angular accelerations were 4 o ;sec• and rotation 

was always counterclockwise. Only the lateral 

canals were stimulated. 

Human 

Subjects 

Pz & Ch 

Dy&Do 

Sa & Br 

Dt & Da 

Te & Ro 

Ve& Pa 

Oats 1 

108 & 109 1.2 

110 & 111 3.0 

112 & 113 9.0 

114 & 115 15.0 

116 & 117 21.0 

118 & 119 30.0 

2 

3.0 

9.0 

15.0 

21.0 

30.0 

1.2 

8 

9.0 

15.0 

21.0 

30.0 

1.2 

3.0 

TriaZ8 

4 5 6 

15.0 21.0 30.0 

21.0 30.0 1.2 

30.0 1.2 3.0 

1.2 3.0 9.0 

3.0 9.0 15.0 

9.0 15.0 21.0 

None of the subjects had been used in previous 

vestibular experiments. Human subjects (6 men 

and 6 women) were instructed to signal onset 

and cessation of their rotatory experiences by 

means of a signal key. 
Scoring. Slow-phase displacement of ~the eyes 

w.as scored by measuring the peak to base-line 

distance for each beat of nystagmus and summing these values for 3-sec intervals. Time 

measurements were also made from the end of 

each stimulus (a) to the end of the primary 

response and (b) to the start of the secondary 

nystagmus. The number of beats of primary 

nystagmus which followed stimulus termination 

was tabulated. 

IV. Results and Discussion 

A. Cats 

Measures of both response time and the number 

of beats of primary nystagmus following stimulus termination .appear in Table 2. Post-stimulus 

responses of greatest duration and magnitude 

occurred when stimulus durations were between 

3 and 15 sec with the maximum post-stimulus 

primary response occurring, in general, with 

stimuli of 9 sec duration. 

90 

80 

70 

60 

Cl) 

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~ 

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~ 

0 

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< 

1-

Cl) 

>-

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0.. 

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~ 

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30 

20 

10 

PRIMARY NYSTAGMUS 

X.... 

// 'X 

,x ·~ x"' / ,-. 

/~~.< \ \\ !/ '· \ 

·--· 30 SEC 

x--x 21 SEC 

o--o 15 SEC 

9 SEC 

·-· 3 SEC 

1.2 SEC 

STIMULUS DURATION 

.\ \ .\ \, .\ /\6\ \ \ 

I \ \ \ • 

' A \ 0 \ 

'·~ ·,,. "0 ...... ~.&--·-- -' ~ ............ 

--9 ·-. ....,_. 

3 6 9 12 15 18 21 24 27 30 33 36 39 42 45 48 51 54 

AVERAGE FOR 12 CATS 

3 6 9 12 15 18 21 24 27 30 33 36 39 42 45 48 51 54 

TIME IN SECONDS 

FIGURE 1. Time-course plots of slow-phase eye displacement per 3-sec interval for 12 cats exposed to 6 stimulus 

durations of 4° ;sec2 angular accelerations. For the 21 and 30 sec durations, responses reach a peak and 

begin to decline during the stimulus. For the two shortest durations, the peak response occurs after stimulus termination. Peak magnitude of the secondary reactions appears related to stimulus duration. 

2 
TABLE 2. Measures of primary nystagmus following 

the termination of each rotatory stimulus for cats. 

Each response value is a mean of responses to an 

acceleration and a deceleration stimulus. Stimuli 

were 4° ;sec2 for 1.2, 3, 9, 15, 21, or 30 seconds. 

Cat 

108 

109 

110 

. 111 

112 

113 

114 

115 

116 

117 

118 

119 

M= 

Time From End of Stimulus to End of Primary 

Nystagmus (Sec) 

1.'2 

10.9 

5.9 

18.5 

7.1 

3.0 

2.8 

8.2 

10.2 

11.5 

7.6 

8.9 

10.4 

8.8 

3.0 9.0 

12.4 

8.3 

7.3 

11.1 

12.4 

11.8 

11.6 

8.0 

12.1 

3.0 

12.3 

10.7 

10.1 

14.6 

8.2 

9.2 

8.8 

11.6 

9.8 

9.3 

10.2 

14.2 

8.2 

13.3 

7.2 

10.4 

15.0 

10.6 

5.4 

12.0 

6.5 

15.7 

9.4 

8.0 

'21.0 

8.1 

4.0 

5.0 

4.3 

12.2 

6.0 

13.6 

10.6 13.1 

15.3 12.4 

9.3 6.2 

13.3 12.5 

5.0 7.2 

10.1 8.7 

30.0 

6.0 

1.6 

2.3 

7.2 

10.8 

3.8 

6.8 

9.0 

11.3 

5.0 

12.1 

2.5 

6.5 

Time From End of Stimulus to Start of Secondary 

Nystagmus (Sec) 

108 

109 

110 

111 

112 

113 

114 

115 

116 

117 

118 

119 

M= 

15.6 

7.9 

15.9 

15.0 

15.7 

12.5 

17.3 

13.6 

11.2 

9.3 

15.2 

13.6 

15.4 

16.9 

6.6 

16.8 

14.6 

18.3 

12.7 

19.5 

16.9 

18.0 

16.3 

14.9 

15.6 

20.8 

13.5 

12.9 

12.6 

17.5 

14.1 

14.7 

14.6 

21.3 

14.8 

15.3 

11.8 

15.3 

15.6 14.5 13.2 

9.9 6.5 6.2 

6.0 8.4 6.7 

10.8 7.5 10.6 

24.4 20.4 18.1 

11.6 9.0 7.5 

25.4 15.4 15.6 

16.7 15.0 14.0 

17.5 15.4 16.1 

12.6 13.0 15.6 

15.5 16.4 15.7 

11.4 9.5 7.1 

14.8 12.6 12.2 

Beats of Primary Nystagmus After End of Stimulus 

108 

109 

110 

111 

112 

113 

114 

115 

116 

117 

118 

119 

M= 

2.0 

1.5 

3.0 

2.5 

2.5 

2.0 

5.5 

4.5 

5.5 

4.0 

12.0 

4.0 

4.1 

12.0 

7.0 

3.0 

7.0 

8.0 

7.5 

8.5 

7.0 

8.0 

5.0 

15.5 

8.0 

8.0 

19.0 

9.0 

6.5 

8.5 

14.0 

9.5 

12.0 

8.5 

16.0 

17.0 

5.5 

-----,"'~ 

11.4 

12.5 11.5 6.5 

6.5 2.5 1.0 

6.5 4.0 3.0 

7.0 4.0 2.0 

18.5 16.5 9.5 

9.0 5.0 3.0 

7.5 17.0 5.5 

9.5 16.5 10.5 

18.0 9.5 18.5 

8.5 5.0 12.0 

19.5 19.0 15.0 

4.5 7.0 1.5 

10.6 9.8 7.3 

3 

TABLE 3. Measures of primary nystagmus following 

the termination of each rotatory stimulus for human 

subjects. Each response value is a mean of responses to an acceleration and a deceleration stimulus. Stimuli were 4° /sec2 for 1.2, 3, 9, 15, 21, or 

30 seconds. 

Time From End of Stimulus to End of Primary 

Nystagmus (Sec) 

Subject 

Pa 

Da 

Br 

Do 

Ch 

Ro 

Ve 

Te 

Dt 

Sa 

Pz 

Dy 

M= 

1.'2 

22.0 

12.6 

18.3 

13.1 

13.6 

10.9 

31.8 

21.3 

7.3 

17.6 

16.9 

3.0 

11.0 

25.2 

28.8 

19.6 

25.1 

41.3 

62.2 

20.9 

42.0 

22.3 

29.1 

46.3 

31.2 

9.0 

17.3 

42.4 

20.5 

38.9 

40.0 

28.4 

44.0 

42.6 

44.4 

55.6 

57.9 

42.4 

39.5 

15.0 21.0 

32.0 16.8 

54.1 64.6 

25.7 37.5 

51.3 36.3 

33.0 24.5 

34.9 44.2 

41.3 38.4 

26.7 33.1 

48.4 40.1 

44.1 32.8 

40.3 39.2 

45.6 33.7 

30.0 

22.3 

45.3 

29.6 

39.8 

23.9 

40.5 

37.9 

27.2 

35.7 

28.8 

28.5 

29.2 

39.8 36.8 32.4 

Time From End of Stimulus to Start of Secondary 

Nystagmus (Sec) 

Pa 

Da 

Br 

Do 

Ch 

Ro 

Ve 

Te 

Dt 

Sa 

Pz 

Dy 

M= 

31.8 

21.3 

* 

11.4 

40.2 

* 

30.6 

72.7 

48.8 

23.0 

44.4 

57.7 

50.4 

46.8 

26.1 

67.3 

42.8 28.3 27.1 

41.3 51.4 45.4 

46.1 40.2 38.1 

17.1 34.1 35.9 

58.8 40.1 35.7 

32.2 32.9 28.8 

41.9 39.2 35.3 

50.0 33.7 31.9 

41.3 37.5 37.2 

Beats of Primary Nystagmus After End of Stimulus 

Pa 

Da 

Br 

Do 

Ch 

Ro 

Ve 

Te 

Dt 

Sa 

Pz 

Dy 

M= 

13.0 13.0 

10.0 25.0 

27.0 

9.0 13.0 

7.5 29.5 

8.5 24.5 

4.5 30.0 

23.0 

12.0 39.5 

20.0 

4.0 

15.0 

10.4 

31.5 

12.0 

20.5 

24.0 

19.5 

66.5 

11.5 

39.5 

47.0 

43.0 

41.0 

53.0 

55.5 

78.5 

34.5 

47.0 

44.7 

36.5 39.0 27.5 

82.5 94.0 81.0 

29.0 33.0 29.5 

47.0 42.5 39.0 

45.5 42.5 37.0 

50.5 70.0 62.5 

44.0 37.0 32.5 

46.0 51.5 38.5 

63.5 59.5 55.5 

73.0 52.5 54.0 

34,0 28.5 24.0 

51.0 46.5 43.5 

50.2 49.7 43.7 

*Too few scores on which to base a mean. 
100 PRIMARY NYSTAGMUS /•\ 

90 ;: \ ·-· 30 SEC 

x--x 21 SEC 80 

/1£ \ • o-o Ill SEC 

•; ' \ ·--· 9 SEC 10 

;;; f)~-o \ ·-· 3 SEC 

"' . \ ' 1.2 SEC "' 0: 

<!I 60 0 ' • . "'-

... STIMULUS DURATION 

e 

., 50 I ,,, \ \_ ',, \ \ :> I \ '\ ' 

!. \ ~ '. ::1! 

<!I 40 ' \ c 

,_ ·, ~ \ . ., 

)o 

30 '·,,._ . \,, \ z 

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3 6 9 12 Ill 18 21 24 27 30 .33 36 39 42 45 48 51 54 ll7 60 63 66 69 12 75 78 81 84 87 90 93 96 

20 SECONDARY NYSTAGMUS STIMULUS: 4°/ SECt 

TIME IN SECONOS 

FIGURE 2. Time-course plots of slow-phase displacement per 3-sec interval for 12 human subjects exposed to 6 

stimulus durations of 4° ;sec• angular accelerations. No decline of nystagmus during stimulation is evitdent for any duration (compare with Figure 1). Note that the peak response for the three shortest durations occurs after stimulus termination. No secondary responses were evident for the 1..2 and 3 sec stimuli 

and not an· subjects showed secondary nystagmus for the longer stimuli. 

The slow-phase velocity of primary and secondary nystagmic responses was plot·ted for each 

of the 6 stimulus durations (Figure 1). For 

the 21 and 30 sec stimuli, there was •a slight decline in response during constant angular acceleration. Consistent with data from cats in 

the earlier experiment,6 it appears that the nystagmic response reaches maximum intensity between 15 and 20 sec and declines after this, even 

though the stimulus is of constant magnitude 

and direction. 

The peak magnitude of secondary nystagmus 

appears directly related to the duration of the 

angular accelemtion. Secondary responses were 

obtained from all but one cat for the 6 stimulusduration conditions; the exception (cat 112) 

gave no secondary nystagmus following the 1.2 

second stimulus. 

B. Human Subjects 

Figure 2 shows little or no decline in human 

nystagmus during constant angular •acceleration 

4 

irrespective of stimulus duration. This is consistent with human results of earlier studies,5•

6 •

14 

but it is in contrast with results from cats in 

which nystagmus declined after about 20 sec of 

constant angular acceleration. 

With brief stimuli ( 1.2 sec ·and 3 sec), it appears tha·t in both man and -eat the slow-phase 

velocity of nystagmus continues to increase 

("overshoots") after the stimulus terminates 

(Figures 1 and 2), whereas, with prolonged 

stimuli, slow-phase velocity of nystagmus declines immediately after (in humans) or before 

(in cats) the stimulus terminates (Figure 1). 

As stimulus duration is increased up to about 

15 seconds, the dumtion of the post-stimulus 

primary nystagmus increases. With longer 

stimuli, the duration of the post-stimulus primary response declines. In this respect human 

primary nystagmus corresponds fairly well in 

its temporal characteristics with the human subjective response. This is shown in Figure 3 
r 

40 

(f) 

0 

z 

0 

·---· ~ PRIMARY NYSTAGMUS 

u ·~M HUMANS 35 

w 

(f) 

z 

z 

0 

~ 0::: 

:::) 

0 

30 

25 

20 

• 

6.---6.----6. SUBJECTIVE RESPONSES 

fl.--fl. PRESENT STUDY 

w 

(f) 

z 

0 

£L 

(f) 

• ~6. ~---~----~----~ .... ·-_-----!). ,' -.. ~ 

6--6 GUEDRY, CRAMER, 

6 KOELLA, 1958 

15 , -- / -- w 

0::: ~ -------- 10 ,o------0------o---- --- -~ (f) 

:::) 

_.J 

:::) 

~ 

1--

(f) 

PRIMARY 

FROM CATS 

o 0 

NYSTAGMUS ---------0 

I 

1--

(f) 

0 

£L 

5 

5 10 15 20 25 30 35 40 45 50 

TIME IN SECONDS 

FIGURE 3. Duration of the post-stimulus subjective and nystagmic reactions obtained in this study are compared with subjective data from an earlier study. The abcissa represents stimulus duration. Functions depicted for the two sets of subjective data and for primary (slow-phase displacement) nystagmus from cats are 

in close agreement. 

where the mean subjective turning scores from 

the present study are presented along with subjective data obtained in a previous study. In 

this earlier study/ 3 subjects had been trained in 

making subjective reports, whereas the present 

subjects had no ·preliminary practice. The rise 

and decline in duration of human nystagmus 

and subjective response as stimulus duration increases is also matched fairly well by the change 

in temporal characteristics of nystagmus in the 

cat, ,also shown in Figure 3. Reference to Tables 

2 and 3 shows that, in both man and cat, time 

elapsed from stimulus •termination to onset of 

secondary nystagmus increases and declines in a 

manner which ·approximately parallels duration 

of the primary reaction. 

5 

Secondary nystagmus was not evident in the 

recordings of any of the human subjoots for the 

1.2 and 3 sec stimuli, and several subjects gave 

no secondary response to the 30 sec stimulus. 

However, frequency of occurrence of secondary 

nystagmus increased with stimulus duration, and 

this may be interpreted as evidence for a relationship b~tween stimulus duration and intensity 

of secondary response in humans similar to, but 

more variable than, that observed in the cat. In 

comparison with secondary nystagmus of the 

cat, the secondary nystagmus in man has a later 

onset, lower aver:age intensity (relative •to man's 

primary reaction), and seems to be more subject 

to individual differences. 

Tables 2 and 3 suggest that the number of 
beats of secondary nystagmus is more closely 

rela;ted to the duration of primary post-stimulus 

nystagmus than to .the duration of the stimulus. 

This is in contrast to the maximum slow-phase 

velocity of secondary nystagmus which seems 

related to the stimulus duration, at least in the 

cat (see Figure 1). The decline in number of 

secondary beats with stimuli longer than 15 sec 

may signify an encroachment of the secondary 

reaction on the primary reaction. 

Primary nystagmus in the ca;t is shorter in 

duration and has a lower beat-frequency than 

that of man for the range of stimuli investigated. 

There is ·also a pronounced difference between 

man and ca;t in regard to the intensity-ratio of 

secondary to primary nystagmus, the ratio being 

higher for the ca;t. 

A log plot of nystagmus slow-phase velocity 

with respect to time also shows the ca;t to have 

30 

25 

20 

>-

1-

u 

15 

0 

....J 

LLJ~ 10 

>c 

(/) ;:: ::::>~ 

(!) ~-= • 5 ~ lrl 

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>-' zrt> .... LLJ Q) (/)a.. 

<{ u I Q) 

a_ (f) 

:: ·~ ...... 0 

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g 

'\, \ ~'. HUMAN I 0-0 30 SEC 

e--e 9 SEC 

., \ CAT {A-A 30 SEC 

~ '., 0"-o &--& 9 SEC 

\ ' \ ~ ,,~ . . ,, •, \ 

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\ ' \ \ 0" 

A\ I \\ \ 

\ \_ \ ~ \ 

I \ t.\ \ 0 \ \ •, ~ \ 

\ \ •, 

' 

3 6 9 12 15 18 21 24 27 30 33 36 39 42 

TIME IN SECONDS 

FIGURE 4. Log plots of slow-phase eye velocity following two stimulated durations for cat and human 

subjects. Data represent nystagmus measured from 

the point of stimulus termination and thus depict 

the rates of response decay. Decay is more rapid 

in cats and the longer stimulus duration produces 

more rapid declines. 

6 

a substantially different r:ate of decline of nystagmus (lower time constant) than man for 

comparable stimuli (see Figure 4). It also appears that the slope of nystagmus decline for both 

man and ca;t changes as a function of stimulus 

duration, the. longer stimuli producing poststimulus nystagmus with a higher rate of decay 

(lower time constant) . 

V. General Discussion 

Because neurophysiological data from cats are 

sometimes applied to the explanation of reactions 

in man, it is important to compare ·the same 

response variables in men and cats exp<>sed to 

identical vestibular stimuli. N ysbagmus in cats 

declined during prolonged constant angular acceleration in spite of efforts to maintain alertness, whereas nystagmus in alert men did not 

decline during prolonged angular acceleration; 

the rate of decline of primary vestibular . nystagmus following each stimulus was greater in. 

ca;ts than in man; the ratio of secondary to primary nystagmus was higher in cat than in man. 

If it is assumed that secondary nystagmus 

reflects a process which develops during the primary reaction and opposes its continuation, then 

all of the differences between man and cat may 

be due to this secondary process in cat having a 

more direot control of nystagmus than does its 

counterpart in man. The characteristics of subjective data reported by man resembled the 

altera•tions of nystagmus in cats during and after 

prolonged angular accelerations,6 and in some 

respects the nystagmus of man deviates from 

both the nystagmus of cat and the subjective 

responses of man during and ·after these unusual 

stimuli. It appears that the hypothesized secondary process exerts a control on man's sensory 

experience, i.e., the perceived angular velocity, 

comparable to the control of nystagmus in the 

ca;t. This is not ·to say that the nystagmus of 

man is completely without the modulating influence of this secondary process because the rate 

of decline of both post-stimulus primary nystagmus and the occurrence of secondary nystagmus were influenced in man by the duration 

of the stimulus. However, the control is apparently less consistent and, hence, probably less 

direct for the range of stimuli used in the present 

experiment. (Subsequent experiments have illustrated a pronounced secondary nystagmus in 
man when ·prolonged high-magnitude angular 

accelerations are used.) 

It is parsimonious and reasonable to assume 

that a single process accounts for (a) the decline 

in response during prolonged angular acceleration, (b) the increased response decline following 

prolonged acceleration, and (c) the increased 

secondary response following prolonged acceleration. The possibility that the cupula endolymph 

system is under-critically damped, contrary to 

the common supposition, would explain a secondary response, but this seems at variance with 

much of the evidence for the over-damped Torsion Pendulum analogy of van Egmond, Groen, 

and J ongkees21 and could not explain findings 

(a) and (b). To explain these results on the 

baJSis of cupula deflection, it would be necessary 

to assume that the stiffness of the cupula increases during prolonged constant angular acceleration. Van Egmond, et al21 have proposed 

that events within the end organ (either bioelectric, biochemical or plastic changes within the 

cupula) could account for secondary responses, 

and this possibility cannot be ruled out. Lowenstein16 has reported that increasing cupula deflection in e1asmobranch initiates neural activity 

"in one after the other previously silent unitf:?" 

which often adapt themselves rapidly. Adrian1 

and recently Melvill Jones,H recording from the 

vestibular nuclei in cats, found little evidence for 

rapid adaptation. However, CappeJB has noted 

that slow declines in some units recorded in the 

vestibular nuclei of cats correspond temporally 

to declines in human subjective data. The possibility that there is a diminished sensory inflow 

during prolonged cupula deflection in cats 

remams open. 

In man, one aspect of the vestibular reaction 

(subjective velocity) declines while another aspect (nystagmus) does not decline during prolonged acceleration.14 From this, it does not 

seem reasonable to attribute the decline of the 

one response to a suppression of sensory inflow. 

(Some reservations are necessary in this interpretation because average curves of different 

groups of subjects form the basis of the conclusion. Subjective and nystagmic data from the 

same subjects should be compared and studied 

for the presence of correlation.) Aschan and 

Bergstedt2 have evidence which implicates the 

central nervous system in secondary responses. 

Prolonged primary responses induced by uni7 

lateral caloric stimulation should provide an 

opportunity for adaptive changes within the 

cupula; yet Aschan and Bergstedt2 reported 

little or no secondary nystagmus with unilat.eral 

caloric stimuli, whereas bilateral hotjcold caloric 

stimuli and rotational stimuli, yielding primary 

responses of equivalent length, produced secondary responses. 

Although secondary nystagmus (and apparently associated response modulation) in man 

and cat are probably attributable to the central 

nervous system, it remains quite possible that 

some of the differences between man and cat are 

attributable to differences in ·the properties of 

endorgans; e.g., the shorter primary nystagmic 

response in cats (irrespective of stimulus duration) is probably attributable to a greater cupula 

spring-action in cat. Response parameters of 

the central nervous system may be conditioned 

or inherently matched to the response parameters 

of the cupula-endolymph system so that the 

shaping of responses to unusual stimuli may be 

similar in form but on a different time base in 

different animals. 

The conditions under which the "secondary 

processes" have been demonstrated, viz., prolonged constant angular acceleration, are seldom, 

if ever, encountered in natural movement. Even 

a single, brief, unidirectional angular acceleration followed by constant velocity does not occur in natural movement and, as noted in the 

present study, intensity of nystagmus continues 

to increase briefly beyond the termination of 

short unidirectional stimuli. In natural movement, any brief angular acceleration is immediately followed by angular acceleration of 

opposite sign which returns the cupula toward 

its position of static equilibrium. Hence, in the 

case of either brief unidirectional stimuli or 

prolonged unidirectional stimuli, the vestibular 

reactions fail to follow the theoretical11 cupula 

deflection. However, this does not necessarily 

signify either an inadequate response· system 

within the range of natural movements or ·a 

gross error in theoretical cupula mechanics. 'l'he 

departure from expected results signifies a range 

of unnatural stimuli which is not accurately 

followed due to either inaccurate sensory detection or unfaithful central following of the input, 

or both. Because ·the natural periods of movements of various animals are different,18 it is 

quite possible that the ranges of accurate sensory 
representation of movement, due to central and 

peripheral differences, will differ slightly in 

different animals. 

The functional significance of the secondary 

process is not established. Some pathological 

conditions undoubtedly yield a central imbalance 

of spontaneous input from the two ears, and the 

"secondary process" may serve to readjust the 

point of homeostatic balance. Some complex 

motions of the head and body may terminate 

with minor residual cupula deflections, and this 

could require minor shifts in the point of balance between the two ears, which would be accomplished ·too slowly by the elasticity of the 

cupula. Tolerance to an increased level of vestibular stimulation encountered in land, sea, and 

air travel may require a suppression at some 

level of vestibular inflow, and it is possible that 

this secondary process serves this function. It 

has been shown that standard test stimuli administered after an angular ·acceleration are 

influenced in proportion to the duration of the 

preceding angular acceleration,13 and it may be 

assumed that this finding is another manifestation of •the "secondary process." Moreover, with 

repetitive angular accelerations, the peaks of 

both primary and secondary nystagmus in cats 

8 

diminish and shift toward earlier occurrences.4 •

6 

This suggests that the secondary process encroaches more and more upon the primary reaction and thus limits the magnitude and duration 

of the primary reaction with repetitive stimulation. In humans, there are large individual 

differences in secondary nystagmus. If the secondary reaction is a manifestation of an adaptive process which serves to limit the primary 

reaction, it may •prove to be an indicator, among 

people with c9mparable histories of motion exposure, of individual differences in ability to 

habituate to repetitive vestibular stimulation. 

VI. Summary 

A direct relationship between duration of acceleration and (a) decline of response during 

acceleration, (b) rate of decline of response after 

acceleration, and (c) magnitude of secondary 

reaction, is regarded as an indica.tion of a central ·process which limits a prolonged vestibular 

primary reaction. The process is manifested by 

its influence on relatively basic reflex reactions 

(nystagmus) in the cat, and is more prominently 

manifested in man by its influence on sensory 

perception. 
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9 

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