Neonatal amygdala or hippocampus lesions influence responsiveness to objects

Abstract

Medial temporal lobe brain structures, such as the amygdala, play an important role in the normal perception and generation of emotional behavior. Little research, however, has assessed the role of such structures across the neurodevelopmental trajectory. We assessed emotional behavioral responses of rhesus macaques that received bilateral ibotenic acid lesions of the amygdala or hippocampus at 2 weeks of age and sham-operated controls. At 9 and 18 months of age, animals interacted with novel objects that varied in visual complexity as a means of varying emotional salience. All animals behaved differently in the presence of visually simple, as compared to complex, objects, suggesting that they were sensitive to variation in emotional salience. Across both experiments, amygdala-lesioned animals appeared to be less behaviorally inhibited insofar as they explored all objects most readily. Interestingly, hippocampus-lesioned animals' propensity for exploration mirrored that of control animals in some contexts but that of amygdala-lesioned animals in other contexts. At 18 months of age, both amygdala-lesioned and hippocampus-lesioned animals were judged to be less fearful than controls during the testing procedure. Implications for understanding the neurobiology of emotional behavior are discussed.

Figure 1

Objects used for Experiments 1 and 2. Note: Examples of novel objects used in Experiment 1 (a) and Experiment 2 (b). Example of simple (c) and complex (d) animal-like objects used in Experiments 1 and 2. Examples of objects presented either stationary or in motion. For the motion condition, the “Barbie Disco Ball” (e) spun while flashing lights, while the truck (f) drive across the object platform.

Figure 2

Experiment 2: Temperament Assessments. Note: Mean temperament assessment across object series. Panel 1: Simple/Complex object series. Panel 2: Stationary/Mobile object series. A-IBO: Amygdala-lesioned animals; H-IBO: Hippocampus-lesioned animals; CON: Control animals