The Scearce/Roselle 'study' is a so-called analysis of a handful of people in the Z-film (ignoring the hundreds of other people shown who apparently don't count).
It's supposed to be an analysis of startled reactions to the sound of a shot but, as we shall see, very few of the reactions are actually startled.
In reality, this is nothing more than projecting an interpretation on to a few people turning their heads.
But don't take my word for it.
Here is the heart of the 'study':

The tables describe the 'reactions' of certain people beginning at various z-frames. All Roselle and Scearce had to do was compare these reactions with the testimonies of these people to very quickly discover that they were simply projecting their own interpretations onto the Z-film.
For instance, Jackie Kennedy said she wasn't even aware of a first shot until John Connally started screaming like "stuck pig". So how Roselle and Scearce can interpret her movements as being a reaction to the loud sound of a shot is truly baffling. She wasn't even aware of the sound of the first shot.
John Connally testified that JFK was hit by the first shot. This clearly does not happen when Roselle and Scearce propose Connally is reacting to the first shot as we see JFK smiling and waving after this moment. How easy is it to debunk this 'study'?
Why didn't they compare their subjective interpretations of what they thought they were seeing in the Z-film to the actual testimony of the people they were focusing on?? It's really difficult to understand why they didn't do such a basic thing if this was supposed to be a genuine, scientifically based study.
Roy Kellerman.According to the study, Kellerman begins to react to the first shot around z148
"Begins leaning over and looking behind/down to the right"This is what Kellerman had to say about the first shot in his WC testimony:
"As we turned off Houston onto Elm and made the short little dip to the left going down grade, as I said, we were away from buildings, and were there was a sign on the side of the road which I don't recall what it was or what it said, but we no more than passed that and you are out in the open, and there is a report like a firecracker, pop. And I turned my head to the right because whatever this noise was I was sure that it came from the right and perhaps into the rear..."They were "away from the buildings", presumably meaning the TSBD building and there was a sign they had just passed and "you are out in the open".
The sign in question was the Thorntons Freeway sign.
The Tyler frame below shows the position of the presidential limo at z148. The Thornton sign is in the red circle.
The presidential limo in which Kellerman was riding has clearly not passed the Thornton sign. I'll leave it up to the reader to decide if it's "away from the buildings".

Does Kellerman's testimony match up with the findings of the study?
Not really.
George HickeyAccording to the study, Hickey begins to react around z143/z144:
"Begins leaning over to the left looking down in the direction of the rear tire or ground"This is what Hickey actually had to say about the first shot:
"I heard what seemed to me that a firecracker exploded to the right and rear. I stood partially up and turned to the rear to see if I could observe anything."In the study, Roselle and Scearce believe Hickey's reaction to the first shot is to look down to the left. In reality, his reaction is completely opposite - he turns to the right rear.
In fact, there is photographic evidence of Hickey's reaction to the first shot:
In Altgens 6 we see Agents Landis, Ready and Hickey looking over their right shoulders towards the TSBD, presumably in response to the sound of gunfire:

Landis - "I heard what sounded like the report of a high-powered rifle from behind me, over my right shoulder...", "My first glance was at the President, as I was practically looking in his direction anyway...", "I immediately returned my gaze, over my right shoulder."
Ready - "I heard what appeared to be fire crackers going off from my position. I immediately turned to my right rear trying to locate the source but was not able to determine the exact location."
Hickey - "I heard what seemed to me that a firecracker exploded to the right and rear. I stood partially up and turned to the rear to see if I could observe anything. "
Each agent describes their immediate reactions to hearing the first shot, turning to look over their right shoulders looking towards where they felt the sound came from. This is exactly what we see in Altgens 6. However, when we take a closer look at Zapruder we see no reaction from them (Hickey looks briefly over the side of the car but then returns to his original position). The partial footage of the Z-film below focuses on the follow-up car. It runs from z133 to z207. At no point do we see any reaction from the agents mentioned above. These trained agents may not be reacting to the sound of a shot, yet we are supposed to believe, according to Scearce and Roselle, that little Rosemary Willis is reacting to the sound of a shot:

A first shot as early as Roselle and Scearce are proposing is a nonsense and it doesn't take too much effort to demonstrate that.