Brain's 'memory center' needed to recognize image sequences but not
single sights
Date:
July 26, 2021
Source:
Picower Institute at MIT
Summary:
The visual cortex stores and remembers individual images, but
when they are grouped into a sequence, mice can't recognize that
without guidance from the hippocampus, according to a new study.
FULL STORY ==========================================================================
A new MIT study of how a mammalian brain remembers what it sees shows
that while individual images are stored in the visual cortex, the ability
to recognize a sequence of sights critically depends on guidance from
the hippocampus, a deeper structure strongly associated with memory but shrouded in mystery about exactly how.
==========================================================================
By suggesting that the hippocampus isn't needed for basic storage of
images so much as identifying the chronological relationship they
may have, the new research published in Current Biology can bring neuroscientists closer to understanding how the brain coordinates
long-term visual memory across key regions.
"This offers the opportunity to actually understand, in a very concrete
way, how the hippocampus contributes to memory storage in the cortex,"
said senior author Mark Bear, Picower Professor of Neuroscience in The
Picower Institute for Learning and Memory and MIT's Department of Brain
and Cognitive Sciences.
Essentially, the hippocampus acts to influence how images are stored
in the cortex if they have a sequential relationship, said lead author
Peter Finnie, a former postdoc in Bear's lab.
"The exciting part of this is that the visual cortex seems to be involved
in encoding both very simple visual stimuli and also temporal sequences
of them, and yet the hippocampus is selectively involved in how that
sequence is stored," Finnie said.
To have hippocampus and have not To make their findings, the researchers including former postdoc Rob Komorowski trained mice with two forms
of visual recognition memory discovered in Bear's lab. The first
form of memory, called stimulus selective response plasticity (SRP)
involves learning to recognize a non-rewarding, non-threatening single
visual stimulus after it has been presented over and over. As learning
occurs, visual cortex neurons produce an increasingly strong electrical response and the mouse ceases paying attention to the once novel,
but now decidedly uninteresting, image. The second form of memory,
visual sequence plasticity, involves learning to recognize and predict
a sequence of images. Here, too, the once novel but now familiar and
innocuous sequence comes to evoke an elevated electrical response,
and it is much greater than what is observed if the same stimuli are
presented in reverse order or at a different speed.
==========================================================================
In prior studies Bear's lab has shown that the images in each form of
memory are stored in the visual cortex, and are even specific to which
eye beheld them if only one did.
But the researchers were curious about whether and how the hippocampus
might contribute to these forms of memory and cortical plasticity. After
all, like some other forms of memory that depend on the hippocampus,
SRP only takes hold after a period of "consolidation," for instance
overnight during sleep. To test whether there is a role for hippocampus,
they chemically removed large portions of the structure in a group of
mice and looked for differences between groups in the tell-tale electrical response each kind of recognition memory should evoke.
Mice with or without a hippocampus performed equally well in learning
SRP (not only measured electrophysiologically but also behaviorally), suggesting that the hippocampus was not needed for that form of memory. It appears to arise, and even consolidate, entirely within the visual cortex.
Visual sequence plasticity, however, did not occur without an intact hippocampus, the researchers found. Mice without the structure showed
no elevated electrical response to the sequences when tested, no ability
to recognize them in reverse or when delayed and no inclination to "fill
in the blank" when one was missing. It was as if the visual sequence --
and even each image in the sequence -- was not familiar.
"Together these findings are consistent with a specific role for the hippocampus in predictive response generation during exposure to familiar temporal patterns of visual stimulation," the authors wrote.
==========================================================================
New finding from a classic approach The experiments follow in a long
tradition of attempting to understand the hippocampus by assessing
what happens when it's damaged. For decades, neuroscientists at MIT
and elsewhere were able to learn from a man known as "H.M.," who had
undergone hippocampal removal to relieve epileptic seizures.
His memory of his past before the surgery remained intact but he exhibited
an inability to form "declarative" memories of new experiences such as
meeting someone, or performing an activity. Over time, however, scientists realized that he could be trained to learn motor tasks better, even though
he wouldn't remember the training itself. The experiments helped to reveal
that for many different forms of memory there is a "division of labor"
among regions of the brain that may or may not include the hippocampus.
The new study, Bear and Finnie said, produces a clear distinction through
the division of labor in visual memory between simple recognition of
images and the more complex task of recognizing of sequence structure.
"It's a nice dividing line," Bear said. "It's the same region of the
brain, the same method of an animal looking at images on a screen. All
we are changing is the temporal structure of the stimulus." Alzheimer's assessment? Previous research in the lab showed that SRP and visual
sequence plasticity arise via different molecular mechanisms. SRP can
be disrupted by blocking receptors for the neurotransmitter glutamate
on involved neurons while sequence plasticity depends on receptors for acetylcholine.
The next question Bear wants to address, therefore, is whether an
acetylcholine producing circuit links the hippocampus to the visual cortex
to accomplish sequence learning. Neurons that release acetylcholine in the cortex happen to be among the earliest disrupted in Alzheimer's disease.
If the circuit for sequence learning indeed runs through those neurons,
Bear speculated, then assessing people for differences in SRP and
sequence learning could become a way to diagnose early onset of dementia progression.
The National Eye Institute of the National Institutes of Health and the
JPB Foundation funded the research.
========================================================================== Story Source: Materials provided by Picower_Institute_at_MIT. Note:
Content may be edited for style and length.
========================================================================== Journal Reference:
1. Peter S.B. Finnie, Robert W. Komorowski, Mark F. Bear. The
spatiotemporal
organization of experience dictates hippocampal involvement in
primary visual cortical plasticity. Current Biology, 2021; DOI:
10.1016/ j.cub.2021.06.079 ==========================================================================
Link to news story:
https://www.sciencedaily.com/releases/2021/07/210726113942.htm
--- up 11 weeks, 3 days, 22 hours, 45 minutes
* Origin: -=> Castle Rock BBS <=- Now Husky HPT Powered! (1:317/3)