Rats Freezing to a Tone (2 of 2) (image) University of Puerto Rico, Medical Sciences Campus Share Print E-Mail Caption Laboratory rats listening to a tone that had been paired with shock. Control rats (top) froze with fear but rats injected with brain-derived neurotrophic factor (bottom) were not afraid, continuing to press a bar for food. Credit Dr. Gregory Quirk, University of Puerto Rico Usage Restrictions None Share Print E-Mail Disclaimer: AAAS and EurekAlert! are not responsible for the accuracy of news releases posted to EurekAlert! by contributing institutions or for the use of any information through the EurekAlert system.