Hi Chloe, both sentences make little sense by themselves.
I've tried to put some examples of sentences that make sense and I will explain more about 'that' for you. Please note I am not an English teacher :)
-Highly familiar stimuli produce weaker activity than novel stimuli.
-Highly familiar stimuli produce weaker activity than novel stimuli, as discovered by Dr. Chloe.
-It has been discovered that highly familiar stimuli produce weaker activity than novel stimuli.
-That highly familiar stimuli, 'Stimuli X', has been found to produce weaker activity than novel stimuli.
"That" can be used in a few ways, but I think the one you have written in your sentence doesn't make sense because there is no additional context. It would make sense in a conversation, because 'that' can be used in relation to objects near a listener:
Mr. Mark: "This highly familiar stimuli is amazing! It produces so much strong activity!"
Dr. Chloe: "Sorry Mark, but that highly familiar stimuli produce weaker activity than novel stimuli."
A teacher will be able to explain things more technically but I hope this helps for now.