Hippocampus Needed to Recognize Image Sequences

Article: Brain’s “memory center” is needed to recognize image sequences, but not single sights
Source: Picower Institute for Learning and Memory at MIT, via ScienceDaily
Published: July 26, 2021

While the mammalian brain stores images in the visual cortex, new research shows that the ability to recognize a sequence of images depends on the hippocampus, a brain structure associated with memory. The hippocampus essentially influences how the images are stored in the cortex if they have a sequential relationship, but does not affect the encoding of simple visual stimuli in the cortex. In their experiments, the researchers trained mice with two forms of visual recognition memory: (1) stimulus selective response plasticity (SRP), which involves learning to recognize a non-rewarding, non-threatening single visual stimulus after it has been presented over and over, eventually leading to disinterest, and (2) visual sequence plasticity, which involves learning to recognize and predict a sequence of images. Visual sequence plasticity also evokes an elevated electrical response if the stimulus is novel. This response is much greater than what is observed if the same stimuli are presented in reverse order or at a different speed. To test the role of the hippocampus, the researchers chemically removed large portions of the structure in mice and examined the tell-tale electrical response each kind of recognition memory should evoke. Mice with or without a hippocampus performed equally well in learning SRP, suggesting that the hippocampus was not needed to form that kind of memory. However, mice lacking an intact hippocampus did not perform well in visual sequence plasticity, showing no elevated electrical response to the sequences, no ability to recognize them in reverse or when delayed and no inclination to "fill in the blank" when one was missing. The experiments reveal there is a "division of labor" for many different forms of memory, in this case between simple recognition of images and the more complex recognition of image sequence, only the latter of which involves the hippocampus. Because SRP and visual sequence plasticity involve different brain circuits, the researchers next plan to explore whether those differences can help to diagnose neurodegenerative diseases such as dementia.

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Finnie PSB, Komorowski RW, Bear MF, et al. "The spatiotemporal organization of experience dictates hippocampal involvement in primary visual cortical plasticity." Current Biology.  26 July 2021. https://doi.org/10.1016/j.cub.2021.06.079