Memoria e consolidación celular

In 1885, Hermann Ebbinghaus conducted systematic experimental research on human memory, showing that some records have short durations while others can last days, months or years; the amount of repetition can influence the length of memory.

In 1890, William James distinguished between what he called primary memory (information that is the focus of current attention) and secondary memory (information that persists even after it has left consciousness).

In the early 20th century, Ramón y Cajal described the connections between neurons in layers II and III of the entorhinal cortex to the hippocampus. Based on these connections, Cajal speculated that the functions of the entorhinal cortex and hippocampus should be closely related to each other and possibly associated with memory.

In the 1920s, the debate over memory focused on whether it was located in a specific brain region or distributed throughout the brain. Early experimental results were contradictory, leading Lashley to conduct a series of experiments using rats mazes and observing the effect of lesions on different parts of their cortex.

Lashley proposed the principles of "mass action" and "equipotentiality," which suggest that memories are not localized in specific brain regions, but rather are widely distributed throughout the cortical areas, which act holistically; the volume of a lesion is more important than the specific area where it occurs.

In the mid-20th century, there was still a holistic view of memory and its interaction with intellectual and perceptual functions. The minority view held that memories are formed by the potentiation of interactions between neurons in a neural network, and that various mental capacities are the result of the activity of cortical regions with specific functions.

In the second half of the 20th century, research revealed that memory has a neural, cellular, and molecular substrate, which involves synaptic plasticity and interactions between neural networks.

Hebb, a disciple of Lashley but also influenced by Cajal's ideas, proposed that memories are stored as neural assemblies, or "engrams," with the potentiation of their synapses.

Penfield's investigations into the effects of electrical stimulation on specific cortical areas revealed that sensations and images could be evoked in temporal and parietal areas, suggesting that they might be stored in memory.

Kandel and others' investigations into the cellular basis of learning and memory in the Aplysia model, which received a Nobel Prize in Physiology or Medicine in 2000, showed that memory involved interactive networks of neuronal populations in various brain regions.

Scoville and Milner's publication of a study on 10 cases of "memory loss after bilateral hippocampal damage" in 1957 was a major turning point in the understanding of the neural basis of memory. Patients underwent surgery for epilepsy or psychiatric diseases, resulting in bilateral removal of brain areas in various regions, including the hippocampus and surrounding areas in the medial temporal lobe.

These patients suffered from irreversible memory loss (amnesia), particularly affecting memories of the years preceding the lesion and the inability to consolidate new information into long-term declarative memory. While some cognitive and intellectual skills were preserved, the memory loss affected all sensory modalities. However, memory for recent events, working memory, and non-declarative memory were still intact.

Cases of amnesia due to various forms of brain damage (surgery, infections, tumors, or physical or psychological trauma) in humans (cases H.M., K.F., E.P., C.W., N.A.).

Amnesia amosa, or H.M.'s case, revealed specific memory deficits and the disconnection between the medial-temporal lobe (MLP) and the medial-cerebellar peduncle (MCP). H.M. was affected in the MLP declarative memory, while K.F. showed a selective MCP impairment.

The MLP exhibited disconnections between declarative (explicit) memory, which presented amnesia anterograde, and non-declarative (implicit) memory.

The loss of recent memory after bilateral hippocampal lesions was first reported in the Scoville and Milner (1957) study. Similar cases include EP, who suffered Herpes Simplex virus infection and lost the ability to consolidate new information in the MLP, and CW, known as the "man with 7 seconds of memory."

Amnesia can affect specific types of memory or aspects of memory processing. Each functional deficit is associated with a distinct brain lesion. For instance, damage to the left hemisphere can result in selective memory verbal deficits, while damage to the hippocampus can lead to non-verbal memory deficits.

Human brain injury studies, as well as subsequent human and animal studies, have demonstrated fundamental principles of memory organization in the brain. These include: (1) the localization of memory in specific brain regions, (2) the ability to study memory independently of other cognitive functions, (3) the role of the medial temporal lobe in the establishment of declarative memory for facts and events, (4) the foundation for more recent studies focusing on defining the neuroanatomical basis of different types and systems of memory in humans and non-human animals, and (5) the existence of distinct types and systems of memory supported by different distributed brain networks.

Memory systems are organized hierarchically, with posterior regions serving sensory memories and anterior regions serving executive memories. Both systems organize from simple and concrete to complex, abstract, and global.

Neurons or neuronal populations can belong to multiple processing networks, optimizing their functionality. Neuronal populations within each functional network also communicate with neurons of the same node and with neurons of other nodes in the network. This extensive interconnection is organized hierarchically.

The memory system is divided into two main categories: (1) short-term memory, or working memory, is the type or system that maintains information for seconds to minutes while it is being processed, with a limited capacity, and (2) long-term memory, which is not only a passive repository of information but also the collection of processes involved in controlling, regulating, and maintaining relevant information for ongoing tasks, and essential for complex cognitive operations.

The sensory memory can be considered a form of short-term memory or part of the attentional system. For instance, imagine watching the final 3 kilometers of a Tour de France stage on TV, where your favorite rider is escaping, and a group is chasing him closely. We are fully concentrated on the details of the race and emotionally invested in the outcome. Suddenly, our partner enters the room and says something we don't pay attention to and ends with, "…and you never listen to what I say." Our attention shifts to our partner, and we quickly respond, "Yes, I heard that, and, yes, turn on the baby monitor." The phrase our partner said, "turn on the baby monitor," which we didn't attend to before, seems to echo in our head and can be quickly retrieved by focusing our attention on it. This type of memory is called sensory memory, a transient store of incoming information that allows us to use it during a short period of time (from milliseconds to seconds), also known as echoic memory for sounds and iconic memory for visual stimuli. The persistence of sensory memory in humans has been demonstrated psychophysically, using the mismatch negativity (MMN) component of evoked potentials, which is an index of sensory memory and has allowed determining the duration of echoic memory (up to 9 or 10 seconds) and iconic memory (between 300 and 500 ms), and the location of the sensory memory store in sensory cortices (auditory, visual, etc.).

Working memory or operative memory is more than just a passive and transient store of information; it is the collection of processes responsible for controlling, regulating, and maintaining relevant information for ongoing tasks, serving complex cognitive operations.