Why is the hippocampus considered to be a cortical structure but not the amygdala?

Why is the hippocampus considered to be a cortical structure but not the amygdala?

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I'm having some trouble understanding the anatomical differences that classify the hippocampus as a cortical structure but not the amygdala. I have included the screenshot of a diagram from Gray's Clinical Neuroanatomy.

Medial aspect of the left cerebral hemisphere demonstrating some limbic structures (yellow). The anterior nuclear group of the thalamus is coloured orange, and the rest of the thalamus is magenta. The approximate position of the brain stem is outlined in a heavy interrupted line.

You can't distinguish cortical from deep structures from a picture like the one you reference in Gray's Clinical Neuroanatomy. The hippocampus is a cortical structure because its cell bodies are in the outer layers of the cerebrum. The amygdala is a deep structure because its cell bodies are in nuclei. I'm not sure if your textbook has stained sections, but if it does, that should help clarify the difference between the two structures.

In this sagittal section you can see both. Notice the hippocampus (at both ends) follows the bumps and grooves of the cortex where the amygdala is a circular/almond shaped cluster of cells.

What Is the Hippocampus?

Shaheen Lakhan, MD, PhD, is an award-winning physician-scientist and clinical development specialist.

The hippocampus plays a critical role in the formation, organization, and storage of new memories as well as connecting certain sensations and emotions to these memories. Have you ever noticed how a particular scent might trigger a strong memory? It is the hippocampus that plays a role in this connection.

What Is the Hippocampus?

The hippocampus is a small, curved formation in the brain that plays an important role in the limbic system. The hippocampus is involved in the formation of new memories and is also associated with learning and emotions.  

What happens in the brain with anxiety?

Once the amygdala flags incoming information as a threat—or, due to hyperreactivity, jumps to that conclusion even in the absence of threat—it sends out an alarm, notifying many other areas of the brain to prepare for defensive action. It acts as if your life may be at stake. The hypothalamus relays the signal neurally and hormonally, setting off the stress response. Heart rate increases. Blood pressure rises. Breathing quickens. Areas in the brainstem switch on, pitching you into a state of high alertness and vigilance. The hippocampus, home of memory, draws on past experience to try to put the nature of the threat into context. The prefrontal cortex, which receives all the information to create a coherent interpretation of events and to orchestrate an appropriate behavioral response, can dampen or amplify the sense of threat and degree of distress. In the anxious brain— whether through overexcitability of the stress response system, the activity of various neurochemicals, impairments in nerve circuitry, or inactivation of specific cell populations in the prefrontal cortex—the amygdala essentially overpowers the prefrontal cortex.


The amygdala’s name refers to its almond-like shape. Located right next to the hippocampus, the left and right amygdalae play a central role in our emotional responses, including feelings like pleasure, fear, anxiety and anger. The amygdala also attaches emotional content to our memories, and so plays an important role in determining how robustly those memories are stored. Memories that have strong emotional meaning tend to stick.

The amygdala doesn't just modify the strength and emotional content of memories it also plays a key role in forming new memories specifically related to fear. Fearful memories are able to be formed after only a few repetitions. This makes ‘fear learning’ a popular way to investigate the mechanisms of memory formation, consolidation and recall.

QBI researchers are working on mapping the neural connections that underpin learning and memory formation in the amygdala. Suppressing or stimulating activity in the amygdala can influence the body’s automatic fear response, which kicks in when something unpleasant happens, such as a startling noise. Through this research, QBI scientists have identified receptors in the amygdala that could help to develop new types of anti-anxiety drugs.

Recently QBI researchers have confirmed that new neurons are made in the amygdala.


There also appear to be specific neurotransmitters involved with the process of memory, such as epinephrine, dopamine, serotonin, glutamate, and acetylcholine (Myhrer, 2003). There continues to be discussion and debate among researchers as to which neurotransmitter plays which specific role (Blockland, 1996). Although we don’t yet know which role each neurotransmitter plays in memory, we do know that communication among neurons via neurotransmitters is critical for developing new memories. Repeated activity by neurons leads to increased neurotransmitters in the synapses and more efficient and more synaptic connections. This is how memory consolidation occurs.

It is also believed that strong emotions trigger the formation of strong memories, and weaker emotional experiences form weaker memories this is called arousal theory (Christianson, 1992). For example, strong emotional experiences can trigger the release of neurotransmitters, as well as hormones, which strengthen memory therefore, our memory for an emotional event is usually better than our memory for a non-emotional event. When humans and animals are stressed, the brain secretes more of the neurotransmitter glutamate, which helps them remember the stressful event (McGaugh, 2003). This is clearly evidenced by what is known as the flashbulb memory phenomenon.

A flashbulb memory is an exceptionally clear recollection of an important event ([link]). Where were you when you first heard about the 9/11 terrorist attacks? Most likely you can remember where you were and what you were doing. In fact, a Pew Research Center (2011) survey found that for those Americans who were age 8 or older at the time of the event, 97% can recall the moment they learned of this event, even a decade after it happened.

Most people can remember where they were when they first heard about the 9/11 terrorist attacks. This is an example of a flashbulb memory: a record of an atypical and unusual event that has very strong emotional associations. (credit: Michael Foran)


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The limbic system

Many regions fundamental to mood are buried deep in the most primordial parts of the brain that is, they are thought to have been among the first to develop in the human species. This is probably because mood is evolutionarily important.

Being glum can be advantageous and has been shown to sharpen our eye for detail, for instance. But, overall, the brain seems geared towards maintaining a mildly positive frame of mind. Being in a good mood makes us more likely to seek new experiences, be creative, plan ahead, procreate and adapt to changing conditions.

The limbic system is the major primordial brain network underpinning mood. It’s a network of regions that work together to process and make sense of the world.

If you feel great, the hippocampus might guide you to walk down a path fringed with daffodils. from

Neurotransmitters, such as serotonin and dopamine, are used as chemical messengers to send signals across the network. Brain regions receive these signals, which results in us recognising objects and situations, assigning them an emotional value to guide behaviour and making split-second risk/reward assessments.

The limbic system sits under the cerebrum (the largest and newest part of the brain) and is made up of structures such as the hypothalamus, hippocampus and the amygdala.

The almond-shaped amygdala attaches emotional significance to events and memories. It came to the attention of emotion researchers in 1939 when monkeys whose amygdalae were removed showed bizarre patterns of behaviour. They became fearless, hypersexual and either devoid of emotion or irrationally aggressive.

Dubbed Kluver-Bucy Syndrome, it is rare in humans, but has been observed in people with amygdala damage incurred, for instance, after a bout of brain inflammation.

The hippocampus, meanwhile, reminds us which courses of action are congruent with our mood. For instance, if you feel great you might like to walk down a path fringed with daffodils. If you feel crap, you may instead be drawn to that bar that spins melancholy albums by The Smiths.

The hippocampus has been shown to be shrunken in people with chronic depression. This may account for common features of the condition, such as vague or non-specific recall of personal memories.

The limbic system also regulates biological functions in line with our mood, such as accelerated heart rate and sweating triggered by feeling flustered. Being so old, however, the limbic system is rather primitive. In day-to-day life it’s controlled by some newer networks that co-ordinate how we think and act, so our behaviour is conducive to achieving longer-term goals, rather than always going wherever the mood takes us.

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General Function of the Hippocampus

The hippocampus is a part of the limbic system of the brain that is associated with memories, emotions, and motivation. The Limbic system includes (but is not limited to) the hippocampus, cingulate cortex, amygdala, entorhinal cortex, and the olfactory cortex. And the Limbic system is located right above the brain stem and below the cerebral cortex lining the edge of the cerebral cortex. Of course, the hippocampus of the Limbic system is highly involved with the production of new memories.

Patient H.M. Shows the Importance of the Hippocampus

A good example that shows the involvement of the hippocampus with our memories concerns the peculiar case of a man named Henry Molaison, also known as patient H.M. Patient H.M. suffered from chronic epilepsy, which is thought to have been caused by a head injury sustained from a bike accident at age 7. During childhood, the person’s brain neurons are extremely excitable to aid in rapid learning and brain development. But in exchange, the developing brain is extremely vulnerable to injuries. Such that a brain injury at a young age can result in permanent damage & complications. Like epilepsy.

An old school graduation photo of Patient H.M. before he went under the scalpel to have his brain picked out to cure his epilepsy.

So in order to attempt to cure his epilepsy, doctors chose to have Patient H.M. undergo bilateral medial temporal lobectomy. Which means to cut out the middle part of the temporal lobe from both hemispheres of the brain. Which included cutting out most of Henry Molaison’s anterior hippocampi. Note that this procedure resulted in the remaining hippocampal tissue becoming entirely useless and atrophied because the entire entorhinal cortex was destroyed during the operation. The entorhinal cortex is a brain structure that delivers the major amount of sensory input to the hippocampus. And when a part of the brain lacks stimulation, it atrophies.

Stimulation develops the brain. And the opposite is also true.

After the surgery, Henry Molaison developed severe anterograde amnesia. Which means that he could no longer form new explicit memories. Explicit memories include personal experiences and factual information. But he could still learn or get better at certain visual-motor skills like drawing.

And so what does it mean that Patient H.M. couldn’t form new memories? Well, essentially Patient H.M. was stuck in the past because he wasn’t able to remember information long-term. So that every day that passed him was the same day to him- according to his memory.

To conclude, Patient H.M. has taught us that the hippocampus and other associated structures of the temporal lobe are required for the formation of new long-term explicit memories, including episodic memories of personal experiences, and semantic memories pertaining to factual information & language.

Additionally, you may note that the hippocampus is needed for response inhibition and the formation of spatial memories.

The response inhibition functions of the hippocampus was discovered by scientists who observed animals who suffered damage to the hippocampus became hyperactive. Secondly, animals with hippocampal damage have a hard time learning to inhibit responses previously taught, especially if the response requires remaining quiet.

What Happens When a Neurosurgeon Removes Your Hippocampus

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Image: © Shubhangi Ganeshrao Kene/Science Photo Library/Corbis

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Until the past few decades, neuroscientists had one way to plumb the human brain: wait for disaster to strike people and, if the victims pulled through, see how their minds worked differently afterward.

These poor men and women endured strokes, seizures, saber gashes, botched surgeries, and accidents so horrific that their survivals seemed little short of miracles. To say these people “survived,” though, doesn’t quite capture the truth. Their bodies survived, but their minds didn’t quite their minds were warped into something new.

Some people lost all fear of death others started lying incessantly a few became pedophiles. But however startling, in one way these transformations proved predictable, since people with the same deficit tended to have damage in the same area of the brain—offering vital clues about what those areas did.

There are a thousand and one such stories in neuroscience. These tales expand our notions of what the brain is capable of, and show that when one part of the mind shuts down, something new and unpredictable and sometimes even beautiful roars to life.

In the early 1930s a bicyclist in Connecticut struck a boy, who tumbled and cracked his skull. He started having seizures. Each lasted around forty seconds, during which time his mouth flopped open, his eyes slipped shut, and his arms and legs crossed and uncrossed as if curled by an invisible puppeteer. He suffered his first grand mal on his fifteenth birthday. So at an age when most people are struggling to find an identity, he was saddled with one he didn’t want: the kid who shook, who bit his tongue, who slumped over and blacked out and pissed himself.

The Tale of the Dueling Neurosurgeons.

Finally the desperate young man — soon immortalized as H.M. — decided to try surgery. He started seeing Dr. William Scoville around 1943. A noted daredevil — before a medical conference in Spain once, he’d stripped off his jacket and mixed it up with the toros in the bullring — Scoville liked risky surgeries, too, and had jumped onto the American lobotomy bandwagon early.

But he disliked the drastic changes in his patients’ personalities, so he began experimenting with “fractional” lobotomies, which destroyed less tissue. Over the years he basically worked his way around the brain, carving out this piece or that and checking the results, until he finally reached the hippocampus. Because it was part of the limbic system, scientists at the time believed that the hippocampus helped process emotions, but its exact function remained unknown.

In the early 1950s he started removing the hippocampi (you have one in each hemisphere) from a few psychotics. Although it was hard to be sure in people with such disturbed minds, they seemed to suffer no ill effects, and two women in particular showed a marked reduction in seizures. Unfortunately Scoville neglected to do careful followup tests until November 1953 — after he’d convinced H.M. to try the surgery.

Scoville peeled back the scalp, then used a hand crank and a drill saw from a local hardware store to remove a bottle cap’s worth of bone from above each eye.

H.M.’s operation took place on September 1, 1953. Scoville peeled back his patient’s scalp, then used a hand crank and one-dollar drill saw from a local hardware store to remove a bottle cap’s worth of bone from above each eye. As cerebrospinal fluid drained away, the brain settled down in its cavity, giving Scoville more room to work. With what looked like an elongated shoehorn, he nudged aside H.M.’s frontal and temporal lobes and peered inside.

The hippocampus sits at ear level and has the rough shape and diameter of a curled thumb. Scoville grabbed a long metal tube and began cutting and sucking out tissue gram by gram he eventually removed three inches’ worth of hippocampus on each side. (Two nubs of hippocampal tissue remained behind, but because Scoville also removed the connections between those nubs and other parts of the brain, the nubs were useless, like unplugged computers.) For good measure, Scoville removed H.M.’s amygdalae and other nearby structures as well. Given how deeply all these structures are embedded in the brain, only a neurosurgeon could have destroyed them with such precision.

By many measures, the operation succeeded. The seizures all but disappeared (two attacks per year at most) and when the fog of epilepsy lifted, his IQ jumped from 104 to 117. Just one problem: His memory was shot.

Aside from a few small islands of recollection — like the fact that Dr. Scoville had operated on him — an entire decade’s worth of memories from before the surgery had vanished. Equally terrible, he couldn’t form new memories. Names escaped him now, as did the day of the week. He repeated the same comments over and over, verbatim, and while he might remember directions to the bathroom long enough to get there, he always had to ask again later. He’d even consume multiple lunches or breakfasts if no one stopped him, as if his appetite had no memory, either. His mind had become a sieve.

He’d even consume multiple lunches or breakfasts if no one stopped him, as if his appetite had no memory, either. His mind had become a sieve.

In light of modern knowledge, H.M.’s deficit makes sense. Memory formation involves several steps. First, neurons in the cortex jot down what our sensory neurons see and feel and hear. This ability to record first impressions still worked in H.M. But like messages scrawled on the beach, these impressions erode quickly.

It’s the next step, involving neurons in the hippocampus, that makes memories last. These neurons produce special proteins that encourage axon bulbs to swell in size. As a result, the axons can stream more neurotransmitter bubbles toward their neighbors. This in turn strengthens the synapse connections between those neurons before the memory decays.

Over months and years — provided the first impression was strong enough, or we think about the event from time to time — the hippocampus then transfers the memory to the cortex for permanent storage. In short, the hippocampus orchestrates both the recording and the storage of memories, and without it, this “memory consolidation” cannot occur.

After his memory vanished, H.M. lost his job and had no choice but to keep living with his parents. He spoke in a monotone now and had no interest in sex, but otherwise seemed normal. He took a part-time job packing rubber balloons into plastic bags, and did odd chores around the house. (Although his parents had to remind him where they kept the lawn mower every single time, he could actually mow just fine, since he could see what grass he hadn’t cut.)

He whiled away most days peacefully, either doing crossword puzzles — working through the clues methodically, in order — or flopping in front of the television and watching either Sunday Mass or the old movies that, to him, would never become classics. It was like early retirement, except for the days a Ph.D student named Brenda Milner arrived to test him.

Her battery of tests confirmed Scoville’s basic observations pretty quickly: H.M. had little memory of the past and no ability to form new memories going forward. This was already a big advance — proof that some parts of the brain, namely the hippocampus, contribute more to forming and storing memories than other parts. And what Milner discovered next redefined what “memory” even meant.

She gave him a piece of paper with two five- pointed stars on it, one nested inside the other: The outer star was about six inches wide, and there was a half- inch or so gap between them. The test required H.M. to trace a third star between the two with a pencil. The catch was, he couldn’t see the stars directly: Milner had shielded the diagram, and he had to look at them in a mirror instead. Left was right, right was left, and every natural instinct about where to move his pencil was wrong. Anyone taking this mirror test for the first time makes a mess — the pencil line looks like an EKG — and H.M. proved no exception.

Somehow, though, H.M. got better. He didn’t remember any of the 30 training sessions Milner ran him through. But his unconscious motor centers did remember, and after three days he could trace the star in the mirror fluently. He even commented near the end, “This is funny . I would have thought it would be rather difficult, but it seems I’ve done pretty well.”

This distinction between procedural and declarative memories (sometimes called “knowing how” versus “knowing that”) now undergirds all memory research.

Milner remembers the star test as a eureka. Before this, neuroscientists thought of memory as monolithic: the brain stored memories all over, and all memory was essentially the same. But Milner had now teased apart two distinct types of memory. There’s declarative memory, which allows people to remember names, dates, facts this is what most of us mean by “memory.” But there’s also procedural memory — unconscious memories of how to pedal a bicycle or sign your name.

Tracing the stars proved that H.M., despite his amnesia, could form new procedural memories. Procedural memories must therefore rely on distinct structures within the brain. This distinction between procedural and declarative memories (sometimes called “knowing how” versus “knowing that”) now undergirds all memory research.

Scientists also discovered that time worked differently for H.M. Up to about 20 seconds, he reckoned time as accurately as any normal person. After that, things veered wildly. Five minutes lasted, subjectively, just 40 seconds for him one hour lasted three minutes one day 15 minutes. This implies that the brain uses two different timekeepers — one for the short term and one for everything beyond 20 seconds, with only the latter suffering damage in H.M. Eventually more than one hundred neuroscientists examined H.M., making his probably the most studied mind in history.

In 1980, after H.M.’s father died and his mother got too sick to care for him, he moved into a nursing home. He got pretty portly after too many forgotten second helpings of cake and pudding. But overall he was a fairly normal patient and lived a (mostly) placid life.

He loafed through the nontesting days reading poems or gun magazines, watching trains rumble by, and petting the dogs, cats, and rabbits the facility owned. When he dreamed at night, he often dreamed of hills — not of struggling up them, but cresting them and being at the top.

Watch the video: Ιππόκαμπος στο Ενυδρείο τού Ηρακλείου Κρήτης Seahorse at the Creta Aquarium,Greece (February 2023).