Timer event in viztask

[GiudiceLab]

I am trying to detect when a timer has expired in order to capture the time it takes a person to respond to a change. Using viztask, my code looks something like

# Start a timer
viz.starttimer(0, 1, viz.PERPETUAL)

# define our changing function
def changeThing():
        # change the thing
        thing = thing + change

# Register the timer callback
viz.callback(viz.TIMER_EVENT, changeThing)

# Define the experimental loop
def experiment():
	# Initialize trial number
	trial = 1
	# Wait for spacebar to begin experiment
	yield viztask.waitKeyDown(' ')
	while trial <= numTrials:
		# Update the instructions
		msgbox.message('Trial ' + str(trial))
		# Wait for the timer
                yield viztask.waitEvent( viz.TIMER_EVENT )
		# Wait for response
		yield viztask.waitKeyDown(' ')
		# Increment the trial number
		trial = trial + 1

# Schedule the experiment
viztask.schedule( experiment() )

which I thought would work, but never gets past the waitEvent. I have tried using a timer callback and vizact.ontimer, but neither works to move the script forward (although the function that is called with the timer works fine). Am I doing something wrong, or is this not possible to do? Or do I need a different approach? This was supposed to be a simple example for class, and I don't want to cover custom events yet. Any help is appreciated.
Thanks!