Keyboard_response items
The keyboard_response item is the primary way to collect keypress responses.
Options
	- Correct response is the name of the expected response. Usually 
	you will set the correct_response variable elsewhere (e.g., in a loop item)
	and you can leave this field empty.
- Allowed responses is a semicolon-separated list of keys that are
	accepted, e.g., 'a;b;/'.
- Timeout contains a maximum response time in milliseconds or
	'infinite' for no timeout.
- Flush pending keypresses indicates that old, pending responses
	should be discarded.
Variables set by keyboard_response
	- response is the name of the response key.
- response_time is the time between the start of the response
	interval and the response.
- correct is 1 if the response matches the correct_response and 0
	otherwise.
- avg_rt or average_response_time is the average response
	time since the last feedback item.
- acc or accuracy is the average percentage of correct
	responses since the last feedback item.
Names of keys
Keys are generally identified by their character and/ or their description
(depending on which is applicable). For example, the '/' key is identified as
'slash' and '/'. The 'a' is just identified as 'a'. The left arrow is just
identified as 'left'. For a full list of available key names, click on the 'List
available keys' button.