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Abstract
Episodic memory or the ability to store context-rich information about everyday events depends on the hippocampal formation (entorhinal cortex, subiculum, presubiculum, parasubiculum, hippocampus proper, and dentate gyrus). A substantial amount of behavioral-lesion and anatomical studies have contributed to our understanding of the organization of how visual stimuli are retained in episodic memory. However, whether auditory memory is organized similarly is still unclear. One hypothesis is that, like the “visual ventral stream” for which the connections of the inferior temporal gyrus with the perirhinal cortex are necessary for visual recognition in monkeys, direct connections between the auditory association areas of the superior temporal gyrus and the hippocampal formation and with the parahippocampal region (temporal pole, perhirinal, and posterior parahippocampal cortices) might also underlie recognition memory for sounds. Alternatively, the anatomical organization of memory could be different in audition. This alternative “indirect stream” hypothesis posits that, unlike the visual association cortex, the majority of auditory information makes one or more synapses in intermediate, polymodal areas, where they may integrate information from other sensory modalities, before reaching the medial temporal memory system. This review considers anatomical studies that can support either one or both hypotheses – focusing on anatomical studies on the primate brain, primarily in macaque monkeys, that have reported not only direct auditory association connections with medial temporal areas, but, importantly, also possible indirect pathways for auditory information to reach the medial temporal lobe memory system.
Highlights
Episodic memory or the ability to store context-rich information about everyday events depends on the hippocampal formation
In contrast to the effortful nature of skill learning, encoding of episodic memory is most often automatic or incidental, whereas retrieval requires conscious effort. This memory system allows us to process and remember information received via different sensory modalities, including the auditory domain
Auditory afferents to the perirhinal and posterior parahippocampal cortices are very meager compared to visual input. This is in line with the second hypothesis that the major route, by which the auditory information reaches the medial temporal cortex is via indirect connections, the dorsal temporal pole (TP) possibly being one of the major indirect pathway (Figure 2)
Summary
Episodic memory or the ability to store context-rich information about everyday events depends on the hippocampal formation (entorhinal cortex, subiculum, presubiculum, parasubiculum, hippocampus proper, and dentate gyrus). The anatomical organization of memory could be different in audition This alternative “indirect stream” hypothesis posits that, unlike the visual association cortex, the majority of auditory information makes one or more synapses in intermediate, polymodal areas, where they may integrate information from other sensory modalities, before reaching the medial temporal memory system. In contrast to the effortful nature of skill learning, encoding of episodic memory is most often automatic or incidental, whereas retrieval requires conscious effort. This memory system allows us to process and remember information received via different sensory modalities, including the auditory domain. We can remember, for example, our past holidays, the flavor of our grandmother’s cake, the voice of our friends when they phone us, or the sound of our own car
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