The removal of the hippocampal formation from both temporal lobes can have a severe and permanent impact on recent memory. While short-term and long-term memories remain intact, the ability to establish new long-term memories is lost. As a result, patients with this amnesia can recall events that occurred before their surgery but struggle to remember new events, even after repeated exposure. They often need to be reintroduced to people they meet after the surgery. However, they can still learn certain tasks because procedural memory, which involves associational and motor skills, is retained. Interestingly, these patients may show improvement and perform better on complex tasks like mirror writing, despite denying any previous experience with the task. The specific cerebral structures involved in procedural memory are not yet defined.

Plasticity refers to the ability of the central nervous system (CNS) to change its connectivity. This change can occur in various contexts, including learning, memory, damage, and development. CNS damage can lead to remodeling of neural pathways, which can then alter behavior. The greatest plasticity is observed in the developing brain. Studies of the hippocampus and the adjacent entorhinal cortex have revealed the existence of "place cells" that become active when a subject enters a specific place in a familiar environment. Although these place cells are numerous, they do not follow a specific order that resembles a topographic map. Place cells appear in very young animals as soon as they are able to explore. Recent research has also identified "grid cells" that respond to specific locations and are arranged in a hexagonal pattern in the posterior entorhinal cortex, resembling a map of the environmental space. While this cognitive map remains fixed in any given context, changes to the environment or relocation to a new test environment lead to the generation of a new and appropriate map of grid cells.

It is worth noting that the entorhinal cortex is a major source of input to the hippocampus. Studies on London's taxicab drivers, who must possess detailed knowledge of the city's streets and efficient routes before obtaining a license, have shown that the posterior hippocampus in experienced drivers is larger than that in beginners or the general population. This finding suggests that the loss of spatial memory skills may contribute to the common complaint of getting lost in individuals with amnesia.

Although some degree of plasticity persists in the adult brain, as seen in responses to brain lesions, sensory deprivation, and experience, the capacity for developmental plasticity may be highest during a critical period. For instance, it is possible to modify connections formed in the visual pathways during development by preventing input from one eye, but only within a specific critical period early in life. In animals deprived of visual input, the visual connections become abnormal, and restoring normal input after the critical period does not reverse the abnormality or restore functional vision from the deprived eye. In contrast, similar visual deprivation later in life does not lead to abnormal connections. These plastic changes observed in experiments may involve competition for synaptic connections, where less functional connections are pruned away. Research has shown a correlation between a specific gene variant that influences synaptic pruning efficacy and the likelihood of developing schizophrenia.
The temporal lobes appear to be particularly important for memory because bilateral removal of the hippocampal formation can severely and permanently disrupt recent memory. Short-term and long-term memories are unaffected, but new long-term memories can no longer be established. Thus patients with such amnesia remember events before their surgery but fail to recall new events, even with multiple exposures, and must be reintroduced repeatedly to people they meet after the surgery. This loss of declarative memory involves the conscious recall of personal events, places, and general history. Such patients, however, can still learn some tasks because they retain procedural memory, which involves associational and motor skills. If such patients are given a complex task to perform (e.g., mirror writing), they not only improve during the first training session but also perform better on subsequent days despite their denial of having any earlier experience with that task. The cerebral structures involved in procedural memory are not yet defined.
Plasticity most commonly refers to the ability of the CNS to change its connectivity. Such changes can occur in various contexts, including learning and memory (see previous discussion), damage, and development. Damage to the CNS can induce remodeling of neural pathways and thereby alter behavior. Plasticity is greatest in the developing brain, but
Cellularstudiesofthehippocampusandtheentorhinalcortex(whichisadjacentandparalleltothehippocampus)havedemonstratedtheexistenceof“placecells”thatfirewhenthesubjectentersaspecificplaceinatestenvironment.Althoughtherearemanyplacecells,theyarenotdistributedinanorderlymannerthatwouldresembleatopographicmap.Placecellsappearinveryyounganimalsassoonastheyareabletoexplore.Morerecentstudiesrevealthepresenceof“gridcells,”whichalsorespondtospecificsitesbutaredistributedinhexagonalarraysthatresemblesanorderlymapoftheenvironmentalspaceintheposteriorentorhinalcortex.Althoughthiscognitivemapisfixedforanycontext,changestotheenvironmentorremovalofthepersontoanewtestenvironmentcausesthegenerationofanewandappropriatemapofgridcells.
Becausetheentorhinalcortexisamajorsourceofinputtothehippocampus,itisinterestingthatstudiesofLondon’staxidrivers—whomustdemonstrateanextremelydetailedknowledgeofthecity’sstreetsandmostefficientroutesbeforebeinglicensed—indicatethattheposteriorhippocampusintrainedandexperienceddriversislargerthanthatinbeginnersorthegeneralpopulation.“Gettinglost,”acommoncomplaintassociatedwithamnesia,maybeduetothelossofspatialmemoryskills.
some degree of plasticity remains in the adult brain, as evidenced by responses to certain manipulations, such as lesions of the brain, sensory deprivation, or even experience.
The capability for developmental plasticity may be maximal for some neural systems at a time referred to as the critical period. For example, it is possible to alter some connections formed in the visual pathways during their development by preventing one eye from providing input, but only during a specific critical period early in development. In such visually deprived animals, the visual connections become abnormal (
Fig. 10.10 ), and restoration of normal visual input after the critical period does not undo the abnormality, nor does it restore functional vision from the deprived eye. In contrast, similar visual deprivation later in life does not result in abnormal connections. The plastic changes seen in such experiments may reflect a competition for synaptic connections, whereby the less functional connections are pruned away. Research has shown a correlation between the form of a gene that modulates the efficacy of synaptic pruning and the probability of schizophrenia.