Different Brain Areas and Memory Function

Memory is closely tied to Intelligence, since information encoding and manipulation must be one of the key features of Intelligence.  Changes in the way the brain processes information have been noticed by researchers.  One of the areas of interest is the prefrontal cortex. 

Changes in brain activity have been observed in many MRI studies involving different age groups.  The areas with the most notable differences are all within the prefrontal cortex.  A study conducted by Rypma et al. (2001) used the functional magnetic resonance imaging technique to observe brain activation when participants were given a variety of tasks designed to tax working memory at different levels. 

Different aspects of the memory system seems to be affected differentially by aging.  The phonological loop seems not to be very affected by the process of aging.  This is evidenced by the fact that digit span is constant across all age groups.  Elderly adults do not lose much capacity in verbal memory.  The central executive is impaired at a greater level than the two slave systems in the elderly.

The changes in brain activity observed by Rypma et al.  (2001) are believed to be compensatory mechanisms by the brains of the elderly fashioned to counteract the effects of reduced activity in other regions. 

One of the problems with research of this type is that age-related changes in activation of brain regions do not always equate to different levels of performance between young and old participants.  This is why researchers such as Cabeza et al (1997) have put forth the idea that increased activation in prefrontal cortical regions during episodic retrieval may be a result of functional compensation. 

The Rypma et al (2001) study examined the ventrolateral prefrontal cortex, the dorsolateral prefrontal cortex, and the rostrolateral prefrontal cortex as the prime potential prefrontal cortical regions for differential activation.  When the high cognitive load of this study was imposed, a shift in brain region activity was observed.  The older adults showed an increased activation of their rostrolateral prefrontal cortex when compared to the younger adults.  The younger adults, however, had more activation in the dorsolateral prefrontal cortex. 

    Ventrolateral Prefrontal Cortex

Parts of the ventral prefrontal cortex are thought to be crucial parts of the phonological loop.  Specifically, BA 44, 45, and 47 are thought to be involved.  These three regions were found to be activated at identical levels in young and old participants.

    Dorsolateral Prefrontal Cortex

This brain region is thought to be involved with the maintenance and manipulation of stored semantic information.  It is significantly more active in younger participants.  It is believed to therefore be involved especially with the maintenance and manipulation of supracapacity information.  The fact that older participants performed as well as younger ones hints at the idea that perhaps older participants employ more mnemonic processes (a central executive process) to compensate for the reduction in more automatic supracapacity semantic storage.

    Rostrolateral Prefrontal Cortex

This area, BA 10, was found to be more active in the older participants.  The precise function of this brain region is as yet unclear.  It could be that it represents a compensatory Working Memory process.  It could also be that it is involved in the storage of emotional states during semantic encoding.  Activation of this BA has been found during the viewing of emotional stimuli (Lane et al. 1997, and Paradiso et al. 1997) 

The change observed in older adults with normal memory capability is thought to be related to a redistribution of task load.  "Cognitive Load" is  the amount of mental processes active at a time.  A high cognitive load signifies that too many mental processes are going on at once.  Too much "cognitive load" on a person will inhibit their abilities at various concentration and memory tasks, and generally impede any sort of mental process.  Older adults may be using slightly different mechanisms for encoding of memory than younger adults.