Day of the Dead

[Sgt Pepper]

In George A Romero's Day of the Dead, zombies are studied in a lab.
The lab features 3 crosses; zombies are leashed onto the middle cross:

 

[Richard Stanley]

Is there any indication why the crosses are in the lab, or why multiple zombies (one at a time?) are tied to the middle cross? Is the cross supposed to 'resurrect' them a normal state?

 

[Sgt Pepper]

No indication given for why crosses are in the lab nor for why the zombies (one at a time) are tied to the middle cross. I think the scientist is trying to return them to a "normal" state… something like that.
The cross does not resurrect them. They're just chained to the one in the middle.
One zombie (Bub) shows more intelligence then the others: