Thursday, 10 February 2022 05:31

Neurology and Jiggle Analysis

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Milicent Cranor reexamines some of the historical research using neurology and jiggle analysis to assess the blurred images in the Zapruder film and discusses the complexities of the auditory startle reflex. [Note: This article, posted long ago on a defunct website, is still relevant.]

How long does it take the muscles of the average person to contract in reflexive response to an unexpected loud noise?

Reasonably precise information on this reaction, known as the “auditory startle reflex,” is presented below. It is vital to “jiggle analyses,” studies of blurred images on the Zapruder film of the Kennedy assassination, based on the assumption that the blurs are caused by the sounds of shots making the photographer jerk.

Under ideal conditions, jiggle analysis could suggest answers to some questions: When were audible shots fired? Did Zapruder react to all of them equally? If not, which shot elicited the greatest reaction and why? Is the time interval between the apparent impact on the victim and the blur on the film too long—or too short—to work with the official story?

There are no answers in this report, but I present it in response to the number of researchers who have asked me to find at least some serviceable neurological information.

I also present the findings of a very original researcher—Gene Case—whose inspired work led to an insight into the nervous system of the camera that is at least as interesting as the nervous system of Zapruder. These findings, published years ago in The Fourth Decade, deserve more attention.[1]

But first, a few basics. The speed of Zapruder’s camera was 18.3 frames per second—or 54.6 milliseconds from one frame to another. The speed of sound is 1100 feet per second (fps). The muzzle velocity of a Mannlicher-Carcano is close to 2200 fps.  A Carcano bullet from the alleged sniper’s nest would strike Kennedy before the sound of the muzzle blast reached Zapruder. What happens next?

Auditory Stimulus Response Times in Milliseconds (m/s)

The following figures come from a study by Brown et al, published in the British journal, Brain.[2] The authors tested the latency period (time it takes to respond) of the auditory startle reflex in 12 healthy volunteers ranging in age from 18 to 80 years. While relaxing in a chair, the subjects were randomly treated about every 20 minutes to a tone burst of 124 decibels, the equivalent BANG! of a car backfire 20 feet away. The average latency period of the relevant muscle groups in milliseconds:

Neck: 58 m/s (range 40–136 m/s)

Paraspinal muscles: 60 m/s (range: 48–120 m/s)

Forearm Flexors: 82 m/s (range: 60–200 m/s)

Forearm Extensors: 73 m/s (range 62–173 m/s)

Thumb: 99 m/s (range 75–179 m/s)

Back of Hand: 99 m/s (range 72–176 m/s)

The authors concluded:

The most generalized startle response to the standard sound stimulus employed consisted of eye closure, grimacing, neck flexion, trunk flexion, slight abduction of the arms, flexion of the elbows, and pronation of the forearms.

There was considerable variation in the degree to which this response was expressed, and in some subjects only eye closure and flexion of the neck were apparent.

Accelerated Reaction Time

Aniss et al found that the startle response "is more easily elicited in a state of muscular contraction."[3] (In rare instances, the opposite occurs, i.e., muscle contraction can inhibit the startle response.) The muscles of Zapruder’s neck, trunk, arms, and hands would all have been in a state of contraction, so one might suppose he was well-primed to jump at the sound of shots—if they were loud enough compared with the ambient noise in Dealey Plaza.

Jacqueline Kennedy’s Important Observation

Habituation—the process of becoming so accustomed to a stimulus that it loses its effect—can take place rapidly.[4] The reflex in neck muscle is the last to habituate, according to a 1951 study still being cited.[5] The subjects of the Brown study were hit with tone bursts while relaxing in an otherwise quiet environment—and even they habituated to some extent, within two to six trials.

Dealey Plaza was already filled with the noise of motorcycles before the shooting began, which could lead to habituation on the part of some witnesses at least. Jacqueline Kennedy makes this clear:

You know, there is always noise in a motorcade and there are always motorcycles besides us, a lot of them backfiring. I guess there was a noise, but it didn’t seem like any different noise really because there is so much noise, motorcycles and things. But then suddenly Governor Connally was yelling…[6]

She GUESSED there was a noise? At the time of this shot, the first, she was closer to the alleged source of the noise than Zapruder had been. On the other hand, she was also closer to the motorcycles.

Two for the Price of One

While ambient noise can habituate a witness to the sound of shots that are not much louder, there is something that could even prevent one from hearing a shot altogether: the sound of a shot fired immediately before. Muscles supporting the eardrum contract defensively, making one temporarily deaf.

Thus, two shots can sound like one, creating only one startle reflex, if any, depending on the location of the sniper in relation to the photographer.  

(Elsewhere, see description of how one shot can sound like two.)

Impact of Bullet, Impact of Sound

Luis Alvarez, the Nobel Laureate known mostly for “jet effect” also performed a jiggle analysis.[7] One big concern of his involved the amount of time between the impact of the bullet, and the impact of the sound.

The expected neuromuscular reaction occurs about one-quarter to one-third of a second later, as shown by the large accelerations near 318. (I’ll adopt five frames as Mr. Zapruder’s experimentally determined reaction time.)[8]

Alvarez did not quote any authoritative source for this claim about the latency of the “expected” neuromuscular reaction, and his explanation for its length is inappropriate:

For those readers who are surprised that the neuromuscular response time is so long, let me recall a common ‘parlor trick’: A bets B that if A drops a vertically held dollar bill without any warning, B cannot stop its fall by pinching his fingers together, if his fingers are poised, ready to clamp together, at the bottom edge of the bill. The fact that the bill can almost never be stopped (unless A gives a precursor signal with his fingers) indicates that a nervous system ‘or hair trigger’ takes more than one-sixth of a second (3.1 frames) to respond to an optical stimulus.[9]

Alvarez was apparently correct about the speed of this particular kind of reaction. Tests performed on baseball hero Babe Ruth showed he took 140 milliseconds to twitch at the sight of a ball on its way—as opposed to the average person’s best, 150 milliseconds.[10]

But why compare (a) an involuntary response to an auditory stimulus with (b) a voluntary response to an optical stimulus?

Zapruder’s “reaction time”—assuming he was normal and assuming the sound was loud enough compared with the ambient noise—would be much quicker than Alvarez has claimed, according to neurologists. There is another problem with Alvarez’ analysis, as shown by this statement:

The human nervous system cannot transmit signals fast enough for the angular acceleration between frames 312 and 313 to have been caused by Mr. Zapruder’s muscles reacting to impulses from a brain that had been startled by the shot that killed the President.[11]

Gene Case, mentioned earlier, noted that Zapruder’s nervous system, no matter how fast, could hardly be expected to react to a sound that had not yet arrived!

But why the blur at Z–313? Since it was too soon to have been due to a startle reflex (unless frames are missing between 312 and 313), Alvarez found another explanation, inspired by the observation of another physicist, Enrico Fermi, in a very different context:

Fermi has almost instantly measured the explosive yield of the first atomic bomb by observing how small pieces of paper which he ‘dribbled’ from his hand were suddenly moved away from ‘ground zero’ by the shock wave.[12]

Alvarez concluded the blur was “caused directly by shock wave pressure on the camera body.” But, as Case noted, the speed of sound is again relevant since it takes time (1.1 Zapruder frames) for the shock wave to reach Zapruder.

Case also doubted a shock wave from a bullet could move a three-pound camera at any distance. He bought a Carcano, drove to a quarry with a friend and fired bullets past materials of different weights hanging freely on a stick. His results were conclusive:

The cardboard, the tinfoil and the strings were unimpressed. The shock wave from a Mannlicher Carcano bullet passing three feet away does not flutter cardboard, tinfoil or string, much less the body of a movie camera (three pounds) 75 feet away.

“Dr. Luis Alvarez, Nobel laureate, winner of the National Medal of Science, the Medal of Merit and the Einstein Medal, was blowing it out his ass.”[13]

Bullets Fired Behind Zapruder?

Case then tried something that lead to a rather exciting discovery. When he fired bullets past a CAMERA—and from NEARBY—he created a blur:

Alvarez could have been right about the cause—a shock wave—but wrong about the nature of the ‘interaction.’ The ‘interaction’ could be a vibration in the shutter mechanism or elsewhere in the workings of the camera. Firing a rifle past a VHS camcorder, I was able to record the image of the shock wave of a passing bullet. It is an extreme undulation of the picture which lasts three video frames—3/30ths of a second. Of course, an 8mm film movie camera is a very different mechanism. But vibration of the shutter in Zapruder’s camera, or of the film itself, is a plausible explanation for this triple imaging.[14]

A shock wave [manifest on film] at 313 could only have come from behind Zapruder.[15]

Another Startle or Shock Wave at Z–318?

Complicating jiggle analysis is the fact that Zapruder said he heard only two shots: the head shot, and one immediately before it which appeared to cause Kennedy to “lean over.” Zapruder either did not hear, or consciously register, the first shot. As I have previously documented in the newsletter Probe,[16] several witnesses heard only one—or even no shot—before the fatal one, then they heard a flurry.

Edited excerpt from my Probe article:

Charles Brehm. Saw head wounded on the “second” shot, heard a third. (22H837)

Mr. and Mrs. John Connally. Both heard last shot only after lying down in the   seat, with Mrs. Connally’s head next to his. (4H133,147)

Chief Curry. Heard a shot after Motorcycle Officer Chaney rode up to tell him   what was happening. (4H161) The Nix film shows that Chaney was still behind   the limousine several frames after the headshot.

Sheriff Decker. Heard first shot when a “spray of water” come Kennedy; heard one more. (9H458)

James Foster. Saw head wounded on “second” shot; heard a third. (CD897)

Clint Hill. Heard shot, saw head wounded, while briefly “mounted” on the limousine the first time. Apparently unknown to Hill, the head was already  wounded about 1.5 seconds earlier. (2H144)

Jean Hill. Said she wrapped up Moorman’s first Polaroid photo and put it in her pocket before she heard any shots. (6H206) At the time, she is still in view, about four seconds after the second shot, the photo is still in her hand.

Emmett Hudson. Saw head wounded on “second” shot; heard a third while on the ground. (7H560)  (Nix film shows him on the ground after head wounded.)

Mary Moorman. Heard a shot for the first time as she took a Polaroid photo of Kennedy being hit in the head. She heard two or three more. (19H487)

Royce Skelton. Heard a shot after seeing Kennedy react to headshot. (19H496)

Mrs. Philip Willis. She said the head was wounded on the “second” shot; then heard a third. (CD 1245)


As Mrs. Kennedy put it, “I guess there was a noise.”

[1] Case G. Scientific Slumming with Luis Alvarez. The Fourth Decade, 1996; 3(2):32–42.

[2] Brown P, Rothwell JC, Thompson PD, Britton TC, Day BL, and Marsden CD. New observations on the normal auditory startle reflex in man. Brain 1991; 114:1891–1902.

[3] Aniss AM, Sachdev PS, and Chee K. Effect of voluntary muscle contraction on the startle response to auditory response. Electromyography and Clinical Neurophysiology, 1998; 38:285–293.

[4] Valls-Sole J, Valldeoriola, Tolosa E, Nobbe F. Habituation of the auditory startle reaction is reduced during preparation for execution of a motor task in normal human subjects. Brain Research 1997; 751:155–159.

[5] Jones FP, Kennedy JL. An electromyographic technique for recording the startle pattern. Journal of Psychology, 1951; 32:63–68.

[6] Kennedy, J. 5 WCH 180.

[7] Alvarez L. A physicist examines the Kennedy assassination film. American Journal of Physics, 1976; 44(9):813–827.

[8] Ibid.

[9] Ibid.

[10] Fuchs AH. Psychology and the Babe. Journal of the History of the Behavioral Sciences, 1998; 34(2):153–165.

[11] Alvarez.

[12] Ibid.

[13] Case.

[14] Ibid.

[15] Ibid.

[16] Cranor M.  Probe 1999 6(6):6–13.

Last modified on Friday, 11 February 2022 22:57
Milicent Cranor

Milicent Cranor is currently a senior editor at She has been a creative editor at E.P. Dutton (fiction, non fiction); comedy ghostwriter; co-author of numerous peer-reviewed articles for medical journals; editor of consequential legal and scientific documents; former member of the American Mensa Society. Milicent was a frequent contributor to Probe.

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