Replacing key subject words with pronouns (I, we, they, it, he, she, his, her, their)

An alternative to directly repeating the subject word or using a synonym is to refer to the subject with a pronoun.

Pronouns were used to reference the key subject in the model paragraph:

Information is received from the environment by a range of sensory registers that reside in sensory memory. The sensory receptors are modality specific: a register exists for each sensory modality such as vision, auditory and olfactory. These have the ability to transform stimuli from the environment into physiological representations, which can be understood and retained in the memory system. They therefore have a key role to play in human cognitive processing. Sensory registers also play a role in the larger system of memory.

Which pronoun?