Figure 2. Associative memory cells and their working principle in memory, cognition and emotion.

There are four stages of associative learning and memory, associated signals’ acquisition, exogenous information integration and storage, endogenous information integration and storage as well as information retrieval by behavioral presentation. Associative memory cells (AMC) are classified into primary AMCs in sensory cortices that integrate and memorize exogenous information and secondary AMCs in cognition-, emotion- and behavior-related brain areas that memorize endogenous information during associated cognition and emotion events. Cross-modal associative memory cells are recruited by mutual innervations among sensory cortices or between cognition- and emotion-relevant brain regions. Intramodal associative memory cells are recruited by mutual innervations among the neurons in a single-modality sensory cortex, cognition brain area or emotion brain area. In addition to activations by innate input and new mutual innervations from co-activated brain regions to integrate and encode associated signals, associative memory cells are activated by the arousal system including the ascending reticular activating pathway in the brain stem and thalamus as well as the ascending activating pathways from the cholinergic nuclei, midbrain raphe nuclei and locus coeruleus that release acetylcholine (ACh), serotonin (5-HT) and norepinephrine (NE), respectively, which can maintain well wakefulness, permit normal consciousness as well as grant specific alertness and attention. Primary and secondary associative memory cells innervate and activate memory-output cells in the motor cortex for memory presentations by languages, gestures and countenance. Emotional reactions are often accompanied by the activities of autonomic nerves and hypothalamic hormones. Emotion-related brain areas include the amygdala, nucleus accumbens (NAc), ventral tegmental area (VTA) and so on that are involved in emotional reactions. Cognition-related brain areas, such as the prefrontal cortex, work for cognitive processes. The upregulations of AMC number and activity strength can facilitate memory to be impressive, or vice versa. The functional downregulation of motion-related brain areas leads to the inability of memory retrieval and presentation.