Neuroimaging techniques can augment
current methods of inspecting
the effect of prenatal alcohol exposure
on cognitive functions
The goal of the current proposal is to study how moderate prenatal alcohol
affects learning in general and learning about alcohol at various postnatal ages.
A limited review of current knowledge on neurobiological and behavioral effects
of prenatal alcohol exposure will be followed by methods and specific hypotheses
of the current proposal. This paper will conclude with a discussion of what
implications this line of research has for prevention and treatment of individuals
with prenatal alcohol exposure.
Much research has been done on fetal alcohol syndrome (FAS) before but
especially since its recognition as such in the late sixties and early seventies (No
authors listed, 2000). FAS is diagnosed when children are born with growth
deficiencies, craniofacial abnormalities and central nervous system dysfunction
manifested by structural and functional (cognitive deficits) abnormalities. In
addition to children with FAS are children with similar symptoms but little or no
growth- or facial-abnormalities. These children have been labeled with what is
known as an alcohol-related neurodevelopmental disorder (ARND). Children
without the full criteria for FAS are said to have PEA or prenatal exposure to
alcohol, which results in FAE or fetal alcohol effects. Recently, in an attempt to
curb the apparently endless expansion of labels, the umbrella term fetal alcohol
spectrum disorders (FASD) has been applied to refer to the innumerable
combinations of physical and neuropsychological effects of prenatal alcohol
exposure (Riley & McGee, 2005). The variations commonly seen in the
phenotypes (both neuroanatomical and behavioral) of FASD that prompted so
many labels stem from differences in amount and pattern of maternal alcohol
intake and perhaps most importantly the gestational age at exposure. Other
mediating factors are the mother’s age, nutrition, use of other drugs, and the
child’s genetic composition.
In addition to the well-known facial phenotype of many individuals with
FASD are gross brain structure abnormalities. Of these is microencephaly, or
small brain, a consistent finding in FASD individuals that is usually inferred from
microcephaly (small head relative to body size). Initially using autopsied
individuals and later with magnetic resonance imaging (MRI) it was determined
that this microencephaly is not a global defect but rather results from specific
damage. Most consistently found is a reduction in size of the parietal lobes and
the junction at the parietal and temporal lobes. Reduced size of the frontal lobes
(specifically the left ventral frontal lobe) has also been noted. Finally, the right
temporal lobe is usually larger than the left temporal lobe in controls but this
asymmetry was reduced in FASD subjects (Riley & McGee, 2005). Analyses of
brain structure abnormalities have overwhelmingly found white matter to be
specifically affected (reduced) with a concomitant increase in gray matter.
In addition to these gross reductions in size, specific structural deficits
have also been seen in the corpus callosum, cerebellum, hippocampus and
basal ganglia. In severe cases, the corpus callosum is completely absent
(agenesis) but in most cases it is thinned. This thinning tends to occur in the
rostral- and caudal-most parts. Additionally, the corpus callosum is displaced in
three dimensional space compared to control children (Mattson, Schoenfeld, &
Riley, 2001). Another structure reduced in size subsequent to prenatal alcohol
exposure is the cerebellum, particularly in the anterior vermis. Death of the
Purkinje cells of the cerebellum may be responsible for this reduced volume as
these output cells are especially sensitive to the effects of alcohol. FAS children
also show a greater than normal (right is larger) asymmetry in the hippocampus
(Mattson, Schoenfeld, & Riley, 2001). The caudate nucleus is part of the basal
ganglia and also implicated in spatial memory (as well as higher cognitive
functions). This structure is reduced in volume after prenatal exposure to
General deficits in intellectual functioning as well as specific impairments
including learning and memory, language, attention, reaction time, executive
functions, motor skills, visuospatial functioning and social-emotional functioning
have been documented subsequent to prenatal alcohol exposure (No authors
listed, 2000; Riley & McGee, 2005). Most individuals with FASD are not mentally
retarded although lower IQ scores are commonly seen (Riley & McGee, 2005).
Verbal learning is one area that consistent deficits are seen though non-verbal
learning has also been studied. However, memory (especially implicit) in
individuals with FASD is relatively spared (No authors listed, 2000). Attentional
deficits make FASD subjects likely to get a diagnosis of ADHD. The attentional
difficulties seen in FASD, however, are more specific. The impulsivity is more
consistently found in children with an ADHD diagnosis. During tests of reaction
times, children exposed to alcohol had slower premotor and motor reaction times
than control children (Riley & McGee, 2005). In addition to, and perhaps
because of, these cognitive dysfunctions, people with FASD have a higher
incidence of delinquency, criminality, and mental health disorders (e.g.,
depression and suicide) (Kelly, Day, & Streissguth, 2000).
The CNS structures most susceptible to in-utero alcohol exposure
(mentioned previously) have all been implied in the behaviors (e.g., attention,
learning and memory, movement, executive functions) affected by prenatal
alcohol exposure. For example, structural deficits in the corpus callosum are
related to functional deficits in verbal learning (Mattson, Schoenfeld, & Riley,
2001; Riley & McGee, 2005). Also, deficits in eye-blink conditioning are likely
rooted in cerebellar damage (Stanton & Goodlett, 1998). The link between
structural and behavioral abnormalities is indirect and weak without functional
neuroimaging techniques in a behaving individual. Furthermore, since many of
these techniques are invasive and difficult for a child to complete (i.e., they entail
not moving) not many studies have been conducted.
Nevertheless, positron emission topography (PET) was used with FAS
adults and adolescents at rest and found reduced activity in the caudate nucleus
and thalamus. As of 2001 there were no published studies using fMRI’s and
FASD subjects but a preliminary study suggested that the dorsolateral prefrontal
cortex was active in FASD subjects but not controls during a working memory
task (Mattson, Schoenfeld, & Riley, 2001). This was interpreted by the authors
as impaired processing. Due to their noninvasiveness and ability to deal with a
moving infant/child, electroencephalography (EEG) is the functional
neuroimaging technique most commonly used (with infants) in this area of study.
So far, EEG studies have been able to confirm (by reduced resting alpha wave
strength) the structural evidence that the left hemisphere is particularly
susceptible to the effects of alcohol an effect distinguishing FASD from Down’s
syndrome (No authors listed, 2000). An evoked response potential (ERP) called
P300 is delayed in the parietal cortex of children exposed to ethanol prenatally
(Mattson, Schoenfeld, & Riley, 2001). The authors of this study suggest that this
implies information processing deficits. Furthermore, alcohol early and late in
pregnancy has been associated with increased P1 and N1 latencies and
increased N2 latencies respectively, in response to visual stimuli (cited in
Slawecki, Thomas, Riley, and Ehlers, 2004).
Much of the work done with humans and animals includes massive doses
of ethanol prenatally. Indeed, much more is known about the effects of binge
drinking than moderate prenatal ethanol intake. As it is stated in one paper,
“although research has well established that heavy prenatal alcohol exposure
leads to neurobehavioral impairment, the effects of lower levels of alcohol
exposure are not as clear” (No authors listed, 2000, p 35). The goal of the
current proposal is to study how moderate prenatal alcohol affects
responsiveness to chemosensory aspects of alcohol, learning in general and
learning about alcohol (as a conditioned stimulus) at various postnatal ages.
Behavioral effects of early moderate alcohol exposure will be examined along
with possible neuroanatomical and neurochemical mechanisms in a rat model.
This animal model will generate specific hypotheses, which will be further
scrutinized using functional neuroimaging techniques (EEG) first on animals
followed by the use of human subjects.
There is a limited literature on functional neuroimaging on animals. One
article showed that ethanol exposure (6 g/kg/day for 5 days) prenatally increased
peak frequency of EEG readings in the frontal, and parietal lobes. Furthermore,
parietal N1 latencies to a tone were increased subsequent to this alcohol
exposure. The authors suggest that this may be an indicator of attention deficits
(Slawecki, Thomas, Riley, and Ehlers, 2004). This particular study did not
attempt to relate the EEG/ERP readings to a behavioral measure directly.
Hyperactivity was also measured but outside of the EEG techniques. Animal
models of prenatal alcohol exposure have largely converged on the same
findings as humans in terms of the structural brain abnormalities, behavioral
deficits and even facial dysmorphis. Using neurophysiological measures (such
as ERP) in conjunction with an alleged behavioral deficiency would, however, be
a way to solidify the suggestion that certain brain deficits are in fact related to
behavioral abnormalities. The use of exposure to ethanol’s chemosensory cues
(subsequent to prenatal exposure) concurrently with ERP techniques would be a
tremendous tool. Additionally, the present proposal plans to look at the effect of
prenatal alcohol exposure on postnatal learning in general and learning about
A quick look at the search “prenatal alcohol AND learning” reveals that
much work has been done on the effect of prenatal ethanol exposure on
postnatal learning. Spatial memory deficits are a very common area of research.
This is likely due to the consistency of these effects in both humans and rats.
Long-term potentiation is also affected by prenatal alcohol exposure. Motor
learning, such as eye-blink conditioning, also has deficits subsequent to alcohol
exposure in both humans and rats. Another consistent deficit specific to humans
was in verbal learning. It seems for verbal learning as well as in several other
paradigms that deficits tend to be in acquisition rather than memory (Riley &
McGee, 2005). Consistent with this was the finding that free recall is impaired
but not recognition; implicit memory was also spared (Mattson, Schoenfeld, and
Riley, 2001). Furthermore, reversal learning but not discrimination training is
affected by prenatal ethanol. Finally, and possibly relating to attentional deficits
in FASD individuals, habituation (as evidenced by cardiac orienting) is delayed
and slower after alcohol exposure in utero (Hunt & Phillips, 2004).
The effect of prenatal ethanol on later learning about ethanol is not as well
documented however, a related topic: the effect of prenatal ethanol on later
intake patterns has recently been reviewed (Spear & Molina, 20005). Some
studies do indeed look at the interaction of prenatal and neonatal learning about
alcohol but most use ethanol prenatally as a chemosensory cue rather than a
toxic substance (very low doses are used). This is because the focus of these
lines of work is to elucidate how prenatal learning about alcohol affects later
intake of the substance (a very important topic indeed). The present focus,
however, is to study the effect of prenatal alcohol exposure on postnatal learning
abilities. One study using pharmacologically relevant doses of ethanol prenatally
with this same aim found impairment on a passive avoidance task in the
adolescent offspring of exposed dams (Barron & Riley, 1990).
Researching possible mechanisms for alcohol-mediated structural and
functional impairments can help the future of prevention and treatment of FASD.
Cell death (necrotic or apoptotic) plays a major role in the underlying
mechanisms of damage to the alcohol-exposed fetus. Specifically, the
premature neural crest cell death may be linked to facial abnormalities of FAS.
One possible cause of the cell death could be free radicals formed by alcohol
metabolism. Free radicals break down the mitochondria of cells. In addition to
neuronal death, some cells do not properly develop or migrate (ectopic cells).
Growth factors are largely responsible for cell proliferation and are a possible
mechanism of alcohol’s effects. Astrocytes play a large role in cell migration.
Thus, if astrocyte formation is affected by alcohol, as research suggests, this
could be causing faulty cell migration. In addition to glia, alcohol also disrupts
(more specifically delays) the growth of serotonergic neurons. Serotonin and
glutamate are both involved in normal brain development and both systems are
negatively affected by alcohol. Additionally affected by alcohol are glucose
utilization, a cell adhesion molecule called L1, and gene expression.
Prevention is always a better option than treatment. Fortunately, this
disease is 100% preventable. Unfortunately, this fact has not changed incidence
of FASD drastically as the prevalence of drinking during pregnancy and even
FAS cases has increased in the last decade (No authors listed, 2000). This is
not however due to a lack of research in the area. The current proposal however
covers research areas that would have implications for treatment more so than
prevention. Much research in the area of treatment has already been done.
Experiments have found complex motor training initiated in adulthood can
alleviate some of the alcohol induced cerebellar damage (Klintsova, Goodlett, &
Greenough, 2000). Additionally, neonatal handling and enriched environments
also have beneficial effects. Pharmacologically, antioxidants, growth factor
proteins, and glutamate receptor antagonists (to name only a few) have been
shown to protect against or attenuate some of alcohols effects on the fetus
(Chen, Maier, Parnell, and West, 2003). Critical to any treatment or prevention
plan and central to the current proposal is the identification of brain structure
abnormalities with a corresponding mechanism of action.
Barron, S. and Riley, E. P. (1990). Passive avoidance performance following
neonatal alcohol exposure. Neurotoxicology and Teratology, 12, 135-138.
Chen, W. A., Maier, S. E., Parnell, S. E., and West, J. R. (2003). Alcohol and
the developing brain: neuroanatomical studies. Alcohol Research &
Health, 27, 174-180.
Hunt, P. S., and Phillips, J. S. (2004). Postnatal binge ethanol exposure affects
habituation of the cardiac orienting response to an olfactory stimulus in
preweanling rats. Alcoholism: Clinical & Experimental Research, 28,
Kelly, S. J., Day, N., and Streissguth, A. P. (2000). Effects of prenatal alcohol
exposure on social behavior in humans and other species.
Neurotoxicology and Teratology, 22, 143-149.
Klintsova, A. Y., Goodlett, C. R., and Greenough, W. T. (2000). Therapeutic
motor training ameliorates cerebellar effects of postnatal binge alcohol.
Neurotoxicology and Teratology, 22, 125-132.
Mattson, S. N., Schoenfeld, A. M. and Riley, E. P. (2001). Teratogenic effects of
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No authors listed. (2000). Prenatal exposure to alcohol. Alcohol Research
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Riley, E. P., and McGee, C. L. (2005). Fetal alcohol spectrum disorders: an
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Slawecki, C. J., Thomas, J. D., Riley, E. P., and Ehlers, C. L. Neurophysiologic
consequences of neonatal ethanol exposure in the rat. Alcohol, 34, 187
Spear, N. E., and Molina, J. C. (2005). Fetal or infantile exposure to ethanol
promotes ethanol ingestion in adolescence and adulthood: a theoretical
review. Alcoholism: Clinical and Experimental Research, 29, 909-929.
Stanton, M. E., and Goodlett, C. R. (1998). Neonatal ethanol exposure impairs
eyeblink conditioning in weanling rats. Alcoholism: Clinical &
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