A communications medium, the late Marshall McLuhan wrote, "is not something neutral - it does something to people. It takes hold of them. It massages them, it bumps them around."

Perhaps no single event in the history of mass communications better illustrates McLuhan's thesis than Orson Welles' radio production of "The War of the Worlds."The updated version of H.G. Wells' tale of a Martian invasion of Earth was aired 50 years ago, on Oct. 30, 1938, creating instant panic from coast to coast. While no masterpiece of literary adaptation, Welles' "War" is often cited as a paradigm of radio's power of suggestion.

To impart a sense of immediacy to his script, Welles set the opening action, such as it was, in a hotel ballroom where an orchestra was playing. An announcer then broke in with "a special bulletin from the Intercontinental Radio News." It and several succeeding newsbreaks described a series of explosions on Mars, followed by the landing of a "huge flaming object" on a farm near the tiny hamlet of Grovers Mill, N.J.

Reporting live from the scene, newsman "Carl Phillips" told the nationwide radio audience: "Ladies and gentlemen, this is the most terrifying thing I have ever witnessed. . . . Wait a minute, someone's crawling. Someone or . . . something. I can see peering out of that black hole two luminous disks . . . are they eyes? . . . The mouth is kind of V-shaped with saliva dripping from its rimless lips that seem to quiver and pulsate."

Read with the benefit of half a century's hindsight, passages like this seem so overwrought as to defy credibility. Moreover, the program had begun with an announcement that "Orson Welles and the Mercury Theatre on the Air in `The War of the Worlds' by H.G. Wells" would occupy the next hour.

Despite this disclaimer, many thousands of Americans assumed that the "bulletins" and invading Martians were real.

Distraught listeners swamped newspapers with telephone calls, rushed to interrupt church services with the news that the world was coming to an end, and fled their homes in search of refuge.

The New York Times received 875 calls, and 15 people were treated for shock in Newark, N.J. Reports of similar reactions trickled in from other parts of the country.

One reason so many people were taken in by the broadcast was that the format chosen by Welles closely resembled that of a typical radio news program of the time. In addition, most Americans were still conditioned to accept at face value what they read in newspapers or heard on radio news programs.

Still another contributing factor was the ominous state of the world in 1938. From Spain, H.V. Kaltenborn had broadcast the sounds of war to an American radio audience for the first time in 1936. From Vienna, Edward R. Murrow had described the German march into the city in early 1938. And during the 18 days preceding Hitler's invasion of Czechoslovakia in September 1938, Murrow and William L. Shirer had reported on the deepening crisis in Europe.

Given these circumstances, the feverish response to "The War of the Worlds" becomes easier to understand. But radio's unique capacity for inspiring listeners to form vivid mental images of the disembodied words coming their way no doubt played a part, too.