Biology 202

2006 Third Web Paper

On Serendip

In my earlier explorations of memory (1), (2), I came across an interesting distinction between procedural and declarative memory and gap in my research (memory development from conception to adulthood). The process of neurological development suggests that declarative memories are complex procedural memories located, morphologically and evolutionarily, near the emergence of the I-function.

Procedural memories are memories formed when repeated signals reinforce synapses (the connections between neurons). These memories explain central pattern generators not present at birth. Although a procedural memory can be as simple as a connection between two nerve cells in the fingertip, CPG's are a good example of complex, centralized procedural memories. Procedural memories generate learned output, sometimes in response to specific inputs and other times to create coordinated "motor symphony" outputs for a given command.

Declarative memories are nervous system changes that allow us to recall and narrate past experiences. Academic theories, historical facts, the relationships between family members, and word meanings are also declarative memories. While repeated actions forming procedural memories in already present synapses is relatively easy to conceptualize, a network of neurons to record the experience of your 10th birthday does not exist at birth. Also teaching coordinated outputs seems at first glance simpler than reinforcing inputs.

Data from cricket studies help with some of this. Male crickets that are hybrids of two species produce a hybrid song. Female crickets do not chirp. Still, the hybrid females are more attracted to the hybrid song than the song of either parent species. Similar genetic characteristics produce song generators and song receptors that are hybridized in the same way. (3) If receiving and producing systems of neurons respond to the same genes, it is not difficult to imagine that memories would form in similar ways, and might even employ some of the same neuron networks.

The interneuron configuration that can produce procedural memories should produce declarative memories in a similar way; the reception of information from one's environment under certain situations (repetition if one is studying times tables, and heightened emotions during important events) can cause certain receiving neurons to fuse together, forming a memory of sensory information instead of muscular output. Furthermore, once sensory signals are inside the brain, they are not, in themselves, in any way different from signals starting within the brain. All the sensory inputs that form a memory could be joined in a pattern generator that fires the right sensory neurons for us to perceive, again in the brain, remembered perceptions. This makes declarative memories seem not that different from procedural ones, but there is an important difference, and that difference is the presence of the I-function.

The declarative-procedural division is also described as an explicit-implicit divide, because unlike procedural (implicit) memories, declarative (explicit) memories form and are recalled along side the I-function. We learn to walk not by thinking and figuring out how to walk, but through attempts that subconsciously pattern and train our motor neurons. We are conscious of what happens around us and within our brains at our tenth birthday parties and when we remember it years later. We consciously learn and use vocabulary in a second language.

How does this framework compare to the developmental evidence, which according to "ontogeny recapitulates phylogeny" parallels evolution? Does it indicate that declarative memories are a step up from procedural ones, using the same basic biological properties and the addition of the I-function? While possibly more complex, declarative memories form among the excess of "blank" nerves toward the front of the brain that can connect in a myriad of ways based on the person's experiences. These synapses have more flexibility because they are not directly motor or sensory neurons or interneurons necessary for survival.

Before birth, human embryonic nervous systems develop from a ridge of cells on the back that form the neural tube. The neural tube eventually branches out to all parts of the body and bulges at the head to form the brain. The neural cells multiply and divide, producing most of the necessary cells before birth. Then, the individual cells position themselves and begin to form the connections necessary for survival. (4) Development begins and finishes first at the tail end of the embryo and lastly in the head. Within six months, all the spinal and motor neurons in the body and toward the base of the brain are functional, and the baby can survive outside the womb. This tail to head order of development reoccurs within the brain as well, with the brainstem mostly developed by six months but the cerebral cortex not finished until fifteen to twenty years after birth. (5)

Following birth, the sensory neurons must be programmed through experience so that the baby can properly see and hear. Through childhood the brain continues to develop more and more based on experience, moving from the base toward the highest and most frontal parts of the brain and forming procedural pattern generators for complex movements. Children learn to distinguish words and faces by first establishing procedural auditory and visual processors and then through declarative memory. While most of the neurons for life are present at birth, the newborn brain is only about a quarter of the adult brain size. Neurons grow, branching and fusing together, mylenation occurs, accelerating signals between neurons, and unused cells die, leaving only useful and efficient networks. (4), (5)

After early childhood, the next set of major changes occurs during adolescence. In addition to hormones and physical body development due to puberty, teenagers are still developing the frontal parts of their brains. The number of certain receptors and the gray matter (amount of neurons) in the frontal lobes peak in early adolescence and then decline as connections solidify, unused neurons die, and white matter (nervous system cells that speed up signal transmission) increase (6). The frontal regions of the brain that change the most during the second decade of life are used by adults for planning, judgment, and goal directed behavior (7).

While impossible to prove, declarative memories do appear in age groups (and species) with I-function abilities. These memories appear without structural changes to the nervous system so they are probably not structurally or chemically different. Also these processes appear to occur in the brain regions closest to the forehead, the most recently evolved and latest to develop. Thus declarative memories probably evolved when the biological framework from procedural memories continued to be used in the more recently evolved and less restrained frontal areas of the brain where the I-function emerges.

Sources

1)Memory Loss and Recovery, Erin Schifeling

2)The Biological Basis for Memory Manufacture, Erin Schifeling

3) Bentley, D. and Hoy, R. "The neurobiology of cricket song." Scientific American. August, 1974. In class lecture notes, January 23

4)Brain Development, Chudler, Eric H.

5)Brain Development: Frequently Asked Questions

6)Alcohol and the Adolescent Brain, White, Aaron M.

7)Inside the Teenage Brain


| Course Home | Serendip Home |