Cerebral Changes Result from Training

The first demonstration that the brain can be altered by training or differential experience was made by an interdisciplinary team (Figure 1). They found that either formal training or informal experience in varied environments leads to measurable changes in the neurochemistry and neuroanatomy of the rodent brain (E. L. Bennett et al., 1964; Renner and Rosenzweig, 1987; M. R. Rosenzweig, 1984; M. R. Rosenzweig et al., 1961).

Figure 1 Pioneer Investigators of the Effects of Training and Differential Experience on Brain Chemistry and Anatomy  Pictured from left to right are neurochemist Edward L. Bennett; neuroanatomist Marian C. Diamond; biological psychologists David Krech and Mark R. Rosenzweig. (Photograph taken around 1965.)

In some experiments, animals were given different opportunities for informal learning. For example, littermate rats of the same sex were assigned by a random procedure to various laboratory environments. The following three environments were the most common:

  1. Standard condition (SC). Three animals were kept in a standard laboratory cage and provided with food and water (see textbook Figure 17.17a). This is the typical environment for laboratory animals.
  2. Impoverished (or isolated) condition (IC). A single animal was housed in an SC-sized cage (see textbook Figure 17.17b).
  3. Enriched condition (EC). A group of 10 to 12 animals was kept in a large cage containing a variety of stimulus objects, which were changed daily (see textbook Figure 17.17c). This environment is considered enriched because it provides greater opportunities for informal learning than does the SC.

In the initial experiments, animals in the enriched condition (EC) were found to have developed significantly greater activity of the enzyme acetylcholinesterase (AChE) in the cerebral cortex than their IC littermates had. (Recall that AChE breaks down the synaptic transmitter ACh and clears the synapse for renewed stimulation.) Control experiments showed that this effect could not be attributed to either greater handling of the EC animals or greater locomotor activity in the EC situation (M. R. Rosenzweig et al., 1961). Scrutiny of the data then revealed that the experimental groups differed not only in total enzymatic activity but also in weight of the cortical samples: the EC animals had developed a significantly heavier cerebral cortex than their IC littermates had (M. R. Rosenzweig et al., 1962).

This result was a real surprise because, since the beginning of the twentieth century, brain weight had been considered a very stable characteristic and not subject to environmental influences. The differences in brain weight were extremely reliable, although small. Moreover, these differences were not distributed uniformly throughout the cerebral cortex. They were largest in the occipital cortex and smallest in the adjacent somatosensory cortex. Later experiments demonstrated that shorter periods could similarly produce cerebral changes and that brains of adult rats also responded to differential experience.

The differences in cortical weights among groups were caused by differences in cortical thickness: animals exposed to the EC environment developed slightly but significantly thicker cerebral cortices than their SC or IC littermates had (M. C. Diamond, 1967; M. C. Diamond et al., 1964). More-refined neuroanatomical measurements were soon undertaken on pyramidal cells in the occipital cortex, including sizes of cell bodies, counts of dendritic spines, measurements of dendritic branching, and measurements of the size of synaptic contacts (M. R. Rosenzweig et al., 1972). Each of these measurements showed significant effects of differential experience, as we will see shortly.

Enriched experience has beneficial effects on brain anatomy, neurochemistry, and behavior

Experience in the EC environment promotes better learning and problem solving in a variety of tests. An enriched environment alters the expression of a large number of genes, many of which can be related to neuronal structure, synaptic plasticity, and transmission; some of these genes may play important roles in learning and memory (Rampon, Tang, et al., 2000). Enriched experience also aids recovery from or compensation for a variety of conditions, including malnutrition, thyroid insufficiency, and brain damage (Galani et al., 1997; Hamm et al., 1996; Johansson and Ohlsson, 1996; Rampon, Tang, et al., 2000; Will et al., 1977). An extensive review shows that, for recovery from brain injury in animals, environmental enrichment is more effective than either formal training or physical exercise (Will et al., 2004). In some cases a combination of transplanting fetal cells and providing enriched experience is significantly more effective in restoring function after brain damage than either treatment is alone (Kelche et al., 1995). As we will see later in this chapter, enriched experience also appears to protect against age-related declines in memory, both in laboratory animals and in humans.

Because enriched experience has such widespread beneficial effects on brain anatomy, neurochemistry, and behavior, it is likely that some experiments done with animals raised in standard, restricted housing have been confounded by suboptimal neural development of their subjects. It has been shown that enrichment doesn’t have negative consequences for the reproducibility of experiments, thereby dispelling one of the few arguments against EC housing that has been mustered (Wölfer et al., 2004).

Learning can produce new synaptic connections

The idea that enriched conditions could affect the number of synapses in the brain was slow to gain acceptance. In 1965, Nobel-winning neurophysiologist John C. Eccles (1903–1997) remained firm in his belief that learning and memory storage involve “growth just of bigger and better synapses that are already there, not growth of new connections” (p. 97). Not until the 1970s did experiments with laboratory rats assigned to enriched or impoverished environments provide evidence that learning can produce new synaptic connections.

When dendritic spines in the cerebral cortex were counted, the number of spines per unit of length of dendrite was found to be significantly greater in EC than in IC animals (Globus et al., 1973). Psychologist William Greenough also placed laboratory rats in SC, EC, and IC environments, and he quantified dendritic branching by the methods described in textbook Figure 17.18. The dendritic branching developed by EC animals was significantly greater than that of IC animals (Greenough and Volkmar, 1973; Volkmar and Greenough, 1972). The SC values fell between the IC and EC values and tended to be closer to the IC values. With enriched experience, each cell did not send its dendrites out farther, but instead tended to fill its allotted volume more densely with branches. These results, together with the dendritic spine counts, indicate that EC animals develop new synapses and more-elaborate information-processing circuits.

Several neuroanatomical studies confirm that dendritic morphology is constantly changing. Electrical activity of neurons promotes the growth of fine extensions from dendrites. These extensions, called filopodia (singular filopodium), occur not only during development, as mentioned in Chapter 7, but throughout the life span. They may become dendritic spines if they make contact with an axon (Maletic-Savatic et al., 1999; S. J. Smith, 1999), or form new dendritic branches for additional synapses (M. Fischer et al., 2000).

The size of existing synaptic contacts also changes as a result of differential experience. The mean length of the postsynaptic thickening in synapses of the occipital cortex is significantly greater in EC rats than in their IC littermates (M. C. Diamond et al., 1975; Greenough and Volkmar, 1973). Such increases in the size and number of synaptic contacts may increase the certainty of synaptic transmission in the circuits where changes occur. The fact that these changes, related to long-term memory, are found in the cerebral cortex is consistent with the hypothesis that much long-term memory is stored in the cortex, whereas information is processed for memory storage in other brain regions, such as the hippocampus, depending on the attributes of the particular memory (see Chapter 17).

EC and IC environments affect both brain values and problem-solving behavior. Similar effects on brain measurements have been found in several species of mammals: mice, gerbils, ground squirrels, cats, and monkeys (Renner and Rosenzweig, 1987); effects of differential experience on brain measurements have also been found in birds, fishes, and other vertebrate species (Rampon and Tsien, 2000; Van Praag et al., 2000).

A few studies indicate similar plasticity of the human brain in response to experience. For example, we saw in Chapter 11 that the hand area of the motor cortex becomes larger in musicians, presumably because of their extensive practice. In addition, the occipital cortex of people who are blind becomes sensitive to auditory stimuli, and transcranial magnetic stimulation of the occipital cortex can disrupt Braille reading, indicating that cross-modal neural reorganization can take place in the mature human brain (Kujala et al., 2000). In another example, 100 children were assigned to a 2-year enriched nursery school program at ages 3 to 5 while others received normal educational experience (Raine et al., 2001). When the children were tested at age 11, with skin conductance and electroencephalographic measures of arousal and attention, the children with early environmental enrichment showed increases in orienting and arousal.

Thus the cerebral effects of experience that were surprising when first reported for rats in the early 1960s are now seen to occur widely in the animal kingdom—from flies to philosophers (Mohammed, 2001). The finding that measurable changes can be induced in the brain by experience, even in adult animals, was one of several factors that led increasing numbers of investigators to ask in more detail how the brain reacts to training and how new information can be stored by the nervous system.

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