Would you take a look at the following?
The movements of another persons eyes are critical for us to understand what it is they are paying attention to. We are extraordinarily good at understanding peoples gaze direction and eye movements.
I think "that" should be put before "they" in "what it is they are paying attention to".
Is it quite common to leave "that" out in this type of sentence structure? :blink:
Thank you very much in advance for your help on this.
Your example sentence is not an expletive-it structure of the type "it is <adjective> that", wherein 'that' is required (e.g., It is understandable that he left), but speakers will indeed leave it out.