In the following question, there are six sentences marked S1, S6, P, Q, R, S. The positions of S1 and S6 are fixed. You are required to choose one of the four alternatives which would be the most logical sequence of the sentence in the passage.

S1. Visual recognition involves storing and retrieving of memories.
P. Psychologists of the Gastalt school maintain that objects are recognized as a whole in a parallel procedure.
Q. Neural activity, triggered by the eye, forms an image in the brain’s memory system that constitutes an internal representation on the viewed object.
R. Controversy surrounds the question of whether recognition is a single one-step procedure or a serial step-by-step one.
S. When an object is encountered again, it is matched with its internal recognition and thereby recognized.
S6. The internal representation is matched with the retinal image in single operation.