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.