Memory Development
For many years, based on the findings of developmental nonhuman primate research, it was assumed that the earliest of preverbal infants’ behavioral learning was reflective of implicit perceptual motor learning and memory expression. However, recent delayed recognition tests suggest otherwise. Research that monitors intrinsically reinforcing motor learning after one day of training suggests that a three month old infants’ abilities at learning to intentionally kick a crib mobile during one single fifteen minute session and the ability for cue dependent and independent recall is reflective of explicit rather than implicit processes (for a review, see Rovee-Collier, 1997). Furthermore these training sessions were contextually dependent, i.e. infants did not generalize learned kicking behavior to a new mobile in a new crib setting (Rovee-Collier, 1990). Findings that three month old infants can remember previously learned context-dependent tasks coupled with those reflecting respective object size sensitivity and insensitivity in total, suggest that both explicit and implicit learning and their respective “memory systems develop in parallel rather than hierarchically during the first year of life” (Gerhardstein, Adler, & Rovee-Collier, 2000, p. 132).
Moreover Yakolev & Lecours (1967) research findings on ontogenetic myelin development suggested a hierarchal temporally sequenced developmental pattern for CNS and brain development, with the magnitude of prenatal and earliest of postnatal neuronal growth initiating in the spinal column, brain stem, and subcortical structures and then later spreading to neocortical regions. According to Patricia Goldman-Rakic and colleagues these findings contrast with findings in the nonhuman primate, which suggest a concurrent and concomitant manner (Goldman-Rakic, Bourgeois, & Rakic, 1997). They also challenged the hierarchal growth pattern hypothesis in response to stark ambiguities and exceptions. They cited that the corticospinal motor system is among the last brain regions to myelinate. They also noted that the sensory association area never really fully myelinates like other brain regions (including the prefrontal cortex, which is presumed to be one of the last brain region to myelinate). Furthermore the corpus callosal axons connecting to sensory association areas myelinate a good deal less than those in the sensory association areas. In response to these discrepancies Goldman-Rakic and colleagues have concluded that human primate brain development likely matches the pattern of the nonhuman primate. According to their analysis of the research in the nonhuman primate, postnatal synaptogenesis and neurogenesis, occurs “contemporaneously” in many different neocortical regions (Rakic, Bougeois, Eckenhoff, et al., 1986). However, there seems to be a timetable for neurotransmitter receptor expression, i.e. for dopamine, serotonin, and noradrenaline receptors in different laminae throughout the whole brain (Lidow, Goldman-Rakic, & Rakic, 1991). Moreover cells in laminae VI and V in the visual cortex develop earlier than neurons in layers II and I, which project through the corpus callosum or by commissures (Trevarthen, 1986). These findings are supported by a cross-sectional positron emission tomography (P.E.T.) study that measured local cerebral glucose metabolism (lCMRGlc) rates (Chugani, Phelps, & Mazziotta, 1987). The reported data demonstrated annualized interregional glucose utilization patterns with striking similarities. Cerebral glucose utilization patterns stabilized at ranges of 17.59-27.15 levels during the first year, 19.50-32.39 levels in the second year, and 30.74-61.01 levels in the third year, etc. However daily (and probably weekly and monthly) variability was noted to likely exist. This was supported by the authors’ referencing one five day old subject, who demonstrated greater glucose utilization in the thalamus and basal ganglia when compared with other brain regions. The annualized data though showed similar regional activity throughout the brain. The existing research seems to suggest a “contemporaneous” brain developmental pattern but local growth variability (probably relating to local neurotransmitter expression) the laminae in discrete regions of the brain at any point in time.
Within this whole brain growth construct a within region analysis demonstrated some variability in overall growth rates in the whole brain (Rabinowicz, 1986). For instance in the human the ages from 15-24 months are characterized by growth that allows all regions and layers to reach similar states of maturation. Another time period, 6-8 years, is characterized by remodeling of the cortex, i.e. its thickness, number of neurons and neuronal dendrites. Growth spurts in all regions, possibly evidence of remodeling, also seem to occur between 10-12 years and finally at 18 years. As noted in subsequent sections these growth spurts may be reflective of neural growth supporting aspects relating to developmental milestones relating to sense of self and theory of mind. Again brain growth is likely characterized by “contemporaneous” growth, with variations in growth in certain laminae or in whole brain growth at certain key ages.
References
Chugani, H.T., Phelps, M.E., & Mazziotta, J.C. (1987) Positron emission tomographic study of human brain functional development. Annals of Neurology, 22, 487-497.
Gerhardstein, P., Adler, S.A., & Rovee-Collier, C. (2000). A dissociation in infants’ memory for stimulus size: evidence for the early development of multiple memory systems. Developmental Psychobiology, 36(2), 123-135.
Goldman-Rakic, P.S., Bourgeois, J.P., & Rakic, P. (1997). Synaptic stustrate of cognitive development life-span analysis of synaptogenesis in the prefrontal cortex of the nonhuman primate. In: N.A. Krasnegor, G.R. Lyon, P.S. Goldman-Rakic (Eds.) Development of the prefrontal cortex, (pp. 27-47), Baltimore, MD: Brookes.
Lidow, M.S., Goldman-Rakic, P.S., Rakic, P. (1991). Synchronized overproduction of neurotransmitter receptors in diverse regions of the primate cerebral cortex. Proceedings of National Academy of Sciences, U.S.A., 88, 10218-10221.
Rabinowicz, T. (1986). The differentiated maturation of the cerebral cortex. In: F. Falkner, & Tanner, J.M. (Eds.) Human Growth, (pp. 385-410), New York: Plenum Press.
Rakic, P., Bourgeois, J.P., Eckenhoff, M.F., Zecevic, N., Goldman-Rakic (1986). Concurrent overproduction of synapses in diverse regions of the primate cerebral cortex. Science, 232, 232-235.
Rovee-Collier, C. (1990). The “memory system” of prelinguistic infants. Annals of New York Academy of Sciences, 608, 517-536.
Rovee-Collier, C. (1997). Dissociations in infant memory: rethinking the development of implicit and explicit memory. Psychological Review, 104(3), 467-498.
Trevarthen, C.B. (1986). Neuroembryology and the development of perceptual mechanisms. In: In: F. Falkner, & Tanner, J.M. (Eds.) Human Growth, (pp. 301-383), New York: Plenum Press.
Yakovlev, P.I., & LeCours, A.R. (1967). The myelogenetic cycles of regional maturation of the brain. In: A. Minkowsky (Ed.), Regional development of the brain in early life (pp. 3-70). Oxford, UK: Blackwell Scientific.