PHYSIOLOGICAL FACTORS IN MOTION SICKNESS 



391 



ceivable that the sense organs could be but 

 faithful transmitters of the changes in en- 

 vironmental acceleration and that the ap- 

 pearance of motion sickness depends upon 

 the relative facilitation or inhibition of stim- 

 uli leading to the excitation of various 

 centers in the brain. 



In motion sicloiess there is a decrease in 

 the rhythmic contractions of the stomach 

 and intestine, i.e., decreased gastric tone and 

 peristalsis (75). However, there are no 

 characteristics of the gastrointestinal tract 

 which are related to susceptibility to motion 

 sickness. In a detailed study of persons 

 with various degrees of motion sickness sus- 

 ceptibility, there were found no anatomical 

 or functional aspects of the gastrointestinal 

 tract which were associated with susceptibil- 

 ity to motion sickness (75). It would ap- 

 pear that responses of the gastrointestinal 

 tract in motion sickness are not significantly 

 influenced by individual differences in the 

 characteristics of these organs. It is likely 

 that conflicting results in the past have been 

 due to the high residual incidence of gastro- 

 intestinal defects in men chronically seasick 

 for a protracted period (100). Such findings 

 could result from continuous seasickness 

 rather than existing as a predisposing con- 

 dition, since negative results were obtained 

 from examinations of 10 such men after 

 several months of shore duty (19). 



Studies of the brain as related to motion 

 sickness have been of two types, (a) electro- 

 encephalograph records of humans, and (b) 

 extirpation experiments on animals. Ab- 

 normal brain waves were reported to occur 

 with slightly greater frequency in very sus- 

 ceptible persons (79). Several subsequent 

 studies, however, have indicated little or no 

 relation between cortical brain activity in 

 the electroencephalograph and susceptibility 

 to seasickness (19, 60, 85). In one of these 

 studies, the response to hyperventilation was 

 also checked and no relation to motion sick- 

 ness was found (60). 



The integrity of portions of the cerebellum 

 appears necessary for the appearance of mo- 



tion sickness although much, if not all, of the 

 cortex of the cerebrum may be removed with- 

 out eliminating motion sickness in dogs (12, 

 13). Susceptible dogs subjected to surgical re- 

 movals of the cerebellum indicated that a 

 limited area (nodulus and uvula) of this 

 structure is critical for the production of 

 motion sickness. This result was not merely 

 due to interference with the vomiting mech- 

 anism, since the dogs could be stimulated to 

 vomiting with an emetic drug, apomorphine. 

 Many of the detailed physiological obser- 

 vations have failed to indicate significant 

 differences between susceptible and non-sus- 

 ceptible persons (8, 48, 85). Negative find- 

 ings for such factors as cardiovascular re- 

 sponses, blood chemistry, blood gases, 

 respiratory rate, and general medical evalua- 

 tion are important as they point to motion 

 sickness as a more specific response than 

 formerly realized. 



Indications for Future Research 



Although present knowledge of motion 

 sickness can be used to reduce the magnitude 

 of the problem, there are many areas needing 

 research. When the basic question is asked, 

 "Why do people become motion sick?", the 

 inadequancy of present knowledge becomes 

 apparent. It is a little surprising that few 

 theories of the mechanisms involved have 

 been advanced in recent years. Even teleo- 

 logical speculation about motion sickness is 

 very unsatisfying, for it is difficult to per- 

 ceive the reasonableness of motion sickness 

 as a response. Control of motion sickness 

 will be handicapped until we can state with 

 certainty, for example, the action of accelera- 

 tion upon particular sensory receptors, the 

 nature of the afferent nerve impulses, the 

 neural pathways in the central nervous sys- 

 tem, and have a more complete knowledge 

 of the facilitating and inhibiting influences 

 upon the processes. 



Continuation of research on the action po- 

 tentials of the sensory nerves of the vestibu- 

 lar apparatus should be fruitful. Such 

 studies as the recording of sensory impulses 



