Laboratory studies have related specific neural systems to the influence of emotion on memory. Cross-species investigations have shown that emotional arousal causes neurohormonal changes, which engage the amygdala. The amygdala modulates the encoding, storage, and retrieval of episodic memory. These memories are later retrieved with an enhanced recollective experience, similar to the recollection of flashbulb memories. The amygdala, therefore, may be important in the encoding and retrieval of memories for emotional public events. Since the role of the amygdala in memory is associated with increased arousal induced by the emotional event, factors that influence arousal should also influence the nature of these memories. The constancy of flashbulb memories over time varies based on the individual factors related to the arousal response, such as emotional engagement and personal involvement with the shocking event. The strength of amygdala activation at retrieval has been shown to correlate with an enhanced recollective experience for emotional scenes, even when accuracy is not enhanced. Memory storage is increased by endocrine responses to shocking events; the more shocking an individual finds an event, the more likely a vivd flashbulb memory will develop.