Which transformations of stimuli are the most disruptive to facial recognition?

In the literature on face recognition, there is a considerable volume of data indicating that transforming a face between a familiarization procedure and recognition testing impairs performance. As many different types of transformations have been shown to produce this effect, it seems tempting to assume that any type of transformation can lead to this performance decrement. However, to date, there have been no systematic attempts to identify which transformations produce the most impairment in facial recognition. Therefore, an experiment was conducted in which subjects viewed facial stimuli during the familiarization phase and were then required to perform a memory test with either transformed or unaltered stimuli. The transformations examined in this experiment were a disguise (the addition of sunglasses), lateral reversal, and inversion. Results suggest that inversion produced the largest decrease in recognition, and the addition of a disguise or lateral reversal of the stimuli produced similar, but smaller, detrimental effects.