Memorising the passing of time

a team from the cnrs, working in ptnership with scis from the netherlands, has managed to identify neurons that can encode the passing of time. an primordial advance n'our cogging of so-called episodic memory.

in human cogg, episodic memory tis conscious memory that allos us to recall past events, or wha’ we refer to as “recollections”. it answers the ?s “wha’”, “with whom”, “where” and “when”, and thus places these recollections within a temporality.  tis the neurobiological foundations of this episodic memory that the team atta brain and cogg research centre (cerco) investigated, in collaboration w'da netherlands institute for neurosci. the scis focused in pticular onna folloing ?: does the human brain contain neurons that process time-rel8d information, like those found inna hippocampus of rodents? 

the hippocampus deciphered

the hippocampus is a brain region that belongs to the limbic system, the structure that manages most human behaviours and emotions. tis also involved inna formation of long-term memory, na spatial memory used for orientation. tis actually 1-odda 1st structures to be impacted by alzheimer’s disease, thus explaining the memory and orientation disorders affecting these patients. 

zone of the hippocampus (in yello).

the 1st link tween it and memory was established inna mid-20th century when a neurosurgeon and a professor of neurology removed the hippocampus from an individual sufferation from severe epileψ-. however, folloing the surgical procedure, the patient was affected by a new disorder: selective amnesia. he was no longer able to create new memories and had lost the most recent. 

neurobiological studies conducted in rodents subsequently showed that some neurons inna hippocampus deal specifically with time-rel8d information and pticularly with changes to situation over time. these neurons function throughout a given action, thus encoding its duration. other studies also established that rodents with lesions affecting the hippocampus were unable to succeed in completing tasks that required them to remember the order in which they had smelt pticular odours. 

cogging human temporal memory

memorising past events ⊢ establishing a temporal connection tween the specific actions of an experience. the brain must thus accurately represent the order of events as a function of the time that has elapsed. the neurons that analyse time inna hippocampus may thus play a crucial role in this temporal organisation of human memory. 

in order to ‘ve a better insite inna'da involvement of the human hippocampus in temporal memorisation, the neurobiologist leila reddy and her colleagues atta cerco carried out two experiments at amsterdam university med center. they called on patients with severe epileψ- and in whose brains drs had implanted electrodes in preparation for a neurosurgical procedure. “to carry out these experiments, we needed to record data from isol8d neurons inna human brain, which requires electrodes to ‘ve been implanted inna brain to record neuronal activity,” the researcher explains. “this is a very invasive procedure which, for obvious ethical reasons, we ‘d never ‘ve conducted in humans js'4 the purposes of our study. the subjects we selected were ⊢ patients in whom electrodes had previously been implanted for strictly med reasons.”

during two nearly similar experiments carried out on ≠ patients, pticipants had to memorise a series of images presented during 1.5 2nd, swell as the order o'their appearance. the sequence was repeated 60 times.

during the 1st experiment, pticipants were asked to watch a screen projecting a series of images they had to memorise, together w'da order o'their appearance. the 2nd experiment, performed in ≠ patients, was identical except that tween two series of images, the research team inserted a black screen for a short period. the aim was to determine which neurons were activated individually during these intervals devoid of temporal information. the specific activities of 429 neurons inna hippocampus were thus recorded during the 1st trial, as opposed to 96 neurons during the 2nd. 

the results confirmed the presence of so-called “temporal” neurons inna human hippocampus. during the two experimental sessions, the activity of round 30% of the hippocampal neurons was modul8d by time during the learning period of the sequence (128 out of 429 neurons in experiment 1 and 26 out of 96 during experiment 2). the 2nd test showed that these neurons ‘d also encode time inna absence of temporal information. while 13 temporal neurons were 1-ly activated when presented with an image (learning phase), 3 continued to be active when the black screen was shown. thus the № and type of temporal neurons mobilised varied according to the demands of the task. 

hippocampal neurons ⊢ possess ponderable flexibility in terms o'their operating mode. “we do indeed know that some of these neurons ray'vel to encode other dimensions of our actions, s'as place or time, etc., b'we do not yet know how they operate. this remains an open ? which mite be answered by studying a larger № of neurons, notably in a healthy brain,” notes reddy. “nevertheless, the discovery of temporal neurons inna human brain constitutes a major advance n'our cogging of the perception of time.” 

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