Chapter 19 



MOTION SICKNESS: PHYSICAL CONSIDERATIONS REGARDING 



ITS ETIOLOGY 



MANUEL F. MORALES 

 University of Chicago 



Introduction 



It is evident that certain features of the 

 motion sickness problem are easier to de- 

 scribe physically than are others. For 

 instance, even with present-day methods 

 we can expect to obtain a fairly good knowl- 

 edge of the time course of the forces exerted 

 by various craft on their passengers. We 

 shall show below that the afferent neural 

 response which these forces evoke from the 

 receptors is also understandable in physical 

 terms. When we come to study the en- 

 suing behavior of the central nervous sys- 

 tem, however, we are rapidly forced back to 

 purely qualitative descriptions, and indeed 

 this part of the chapter will add nothing 

 to what has already been said regarding 

 central phenomena. This disparity in levels 

 of information makes presentation difficult, 

 and calls for special caution on the part of 

 the reader. Specifically, he must not jump 

 to the conclusion that the biophysical ap- 

 proach is acceptable in studying receptor 

 phenomena but futile in studying central 

 phenomena; still worse would it be to infer 

 from the larger space allotted to the de- 

 scription of receptors that motion sickness 

 is a purely peripheral problem. Finally, 

 he should exercise tolerance in demanding 

 immediate practical results, for the progress 

 reviewed below is much more by way of 

 understanding than of solving the motion 

 sickness problem. 



The Motion of Naval Vessels 



The motion of a vessel can be regarded as 

 a superposition of oscillations with respect 

 to certain coordinates, e.g., the vertical dis- 



tance from the center of mass to a horizon- 

 tal reference plane, the angle which a mast 

 makes with the vertical, etc. These oscilla- 

 tions are executed in response to periodic 

 forces applied on the vessel by the waves. 

 If one knew the mathematical form of the 

 applied forces and the dynamic constants of 

 the vessel (its geometry, distribution of 

 masses, etc.) it should be possible to deduce 

 the observable oscillations. As with many 

 such problems, however, solution by con- 

 ventional methods is hopeless (31, 22), and 

 one takes refuge in approximations which 

 are frequently of the crudest sort. For 

 the purposes of estimating the forces which 

 a vessel exerts on its passengers it has been 

 the custom to use the following semi- 

 empirical method. By comparatively sim- 

 ple means (e.g., a bubble gauge inclinometer 

 and a stopwatch) one may measure the aver- 

 age periods and amplitudes of the oscilla- 

 tions. Assuming that these oscillations are 

 simple harmonic, one may then readily inter- 

 polate to obtain the entire time course of 

 the oscillations, and also calculate velocities 

 and accelerations at any instant. Thus, 

 if t stands for time, and if the observed aver- 

 age amplitude and period of an oscillation 

 of the coordinate x are xo and T respectively, 

 one assumes that 



X = - {2Tr/TYx, 



(1) 



where the superscript dot refers here and 

 elsewhere to differentiation with respect to 

 time. From this it follows that the coor- 

 dinate is given at any time by 



X = Xq sin i2ir/T)t 



(2) 



399 



