What is the nature of neural representation? How does the activity of neurons in the brain reflect the external stimuli and internal states of the animal?

Even the simplest of behaviors utilizes large populations of neurons. Thus it seems unlikely that single neurons will exhibit clear and simple relationships to external stimuli or internal states. Our ability to record the detailed activity of single neurons, however, has led to a bias in searching for precisely these types of neurons. Recent advances in recording from larger populations of neurons in an unbiased manner reveal that complex response properties seem to be the rule, and simple neurons the exception.