[farshizzo]

Are you using the viztask module to run your trials? Either way, here is some code that shows how you can detect every third trial:

import viz
import viztask
viz.go()

NUM_TRIALS = 10

def runTrial():

	yield viztask.waitTime(1.0)



def runExperiment():

	for i in range(NUM_TRIALS):

		if i % 3 == 2:
			print 'Special event for every third trial'

		yield runTrial()

		print 'Finished trial',i+1

viztask.schedule( runExperiment() )