Rats Freezing to a Tone (1 of 2) (video) University of Puerto Rico, Medical Sciences Campus Share Print E-Mail Loading video... Caption Laboratory rats listening to a tone (indicated by light on bottom right) that had been paired with shock. Control rats froze with fear but rats injected with brain-derived neurotrophic factor (BDNF) were not afraid, continuing to press a bar for food. Credit Dr. Gregory Quirk, Univ. 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.