Write here any additional comment you might have (this includes minor concerns such as typos and structure of the manuscript)
In the introduction clarify the distinction between previous findings in human and animal models. It would be important to mention if there is any evidence that supports the fact that older humans have harder time understanding fast speech. You might want to mention that the way that the startle response circuit is wired in humans is different from the rat. 
In the introduction I wonder if you need to go into such details to understand the circuit in order to motivate the study. I would focus on stating the problem, what's known about the problem, what's you hypothesis, and how you are going to test it. Conclude the intro with a short paragraph about your findings. Leave the description of the methods to the method session.