I have heard (I can't remember where I heard this unfortunately) that some types of captchas don't verify that the user is human by having them click on the correct images but rather examine the users pattern of mouse movements prior to, or while, completing the captcha puzzle. Or at least, I have heard it claimed that mouse movements are used in addition to the completion of the puzzle to determine if the user is human. I would like some evidence that this is, or is not, the case.
There is a system developed by Google, called "No CAPTCHA" or "reCAPTCHA". With that system you do not need to click on pictures with cars, traffic lights or distorted text. Instead there is only a box to tick. It works by tracking mouse movements. Google claims it is very secure, as it checks to see if the movements look like a human. If it's not sure, it reverts to the old picture mechanism.
For more details, see http://qnimate.com/how-does-googles-no-captcha-recaptcha-work/