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© 1999 Macmillan Magazines Ltd W hen a female mammal makes the transition from virginity to mother- hood, she is forced to refocus her activities dramatically. She must adapt to a multitude of new demands by her offspring or risk losing a significant metabolic and genetic investment. She needs to find and remember the location of food stores, water sources and nest sites, and be able to exploit them to her offspring’s advantage. The per- formance of these tasks may depend on a sharpening of her cognitive abilities. We have tested this idea by using two different strains of rat and two separate spatial tasks, and find that pregnancy and a litter of pups may combine to stimulate spatial learning and memory in females. In the oestrous cycle, which occurs pre- paratory to mating, steroid hormones such as oestradiol and progesterone markedly increase the concentration of hippocampal apical dendritic spines in adult females, par- ticularly when oestradiol levels are raised 1 . Because dendritic spines increase the total surface area of the synapse, such neural changes may improve learning and mem- ory 2 . Pregnancy, which raises the amount of oestradiol and progesterone for significantly longer than the oestrous cycle, may produce long-lived improvements in behaviours that are not maternal as such, but which con- tribute to the survival and rearing of pups. The hormones of pregnancy may prepare sites that regulate learning and memory 3 , in much the same way that they stimulate the medial preoptic area to respond with maternal behaviour when pups are born 4 . We investigated this idea by testing multi- parous (animals that had given birth and lac- tated twice) and age-matched nulliparous (virgin) Sprague–Dawley females on a 16- day regimen of radial-arm maze tests. On the first six days, multiparous females made sig- nificantly more correct choices than nulli- parous females (P*0.0002–0.01; Fig. 1a). In a second experiment, virgin female Long Evans rats were assigned to one of three groups: fosters, maternals and nulli- parous. In the dry-land version of the Mor- ris watermaze 5 , maternal females took significantly less time (43.2 s) than nulli- parous rats (128 s) to recall and locate the food reward (P*0.05; Fig. 1b). No signifi- cant differences were observed between fos- ter (55 s) and maternal rats. Taken together, these results show that a combination of reproductive and pup experience and stimulation is beneficial to learning and memory in female rats. Hormone-induced modifications to the hippocampus (increases in both long-term potentiation 6 and dendritic spine density 1,7 ) may improve the navigation skills involved in parental resource-gathering behaviours. For example, hippocampal volume in parental rodents during the breeding season is correlated with the size of the home range 8 . Concurrent alterations in both the hippocampus and the medial preoptic area may therefore contribute to the pro- nounced behavioural transition that is characteristic of the maternal female, with the medial preoptic area directly regulating maternal behaviour, and the hippocampus controlling supporting behaviours such as foraging. As well as receiving the hormones of pregnancy, the female’s brain is exposed to rich sensory events when pups are born. With each litter of pups comes a plethora of new sights, sounds, tastes, and tactile and suckling stimulation, the last of which can reorganize hypothalamic connections 9 . This sensory stimulation may have effects on brain structure and function that are similar to those caused by other types of enriched physical environment, an idea supported by the observation that the brains of parous, environmentally impoverished rats resemble those of rats in an enriched environment 10 . Dendritic processes called filopodia are triggered by the excitatory activation of N-methyl-D-aspartate (NMDA) 11 , leading to the formation of synapses. Furthermore, new postsynaptic CA1 dendritic spines are rapidly formed after long-term potentiation (LTP, a learning analogue) 12 . Neural activity brought about by pregnancy and the pres- ence of pups may literally reshape the brain, fashioning a more complex organ that can accommodate an increasingly demanding environment. Little attention has been paid to the remarkable neural plasticity that is inherent in reproduction itself and that underlies the subsequent behavioural changes, particu- larly those unique to late pregnancy and the postpartum period. To consider the rela- tionship of a mother caring for her young as unidirectional disregards the potentially rich set of sensory cues in the opposite direction that can enrich the mother’s envi- ronment. By providing such stimuli, the pups may ensure both their own and their mother’s development and survival. Craig H. Kinsley*, Lisa Madonia*, Gordon W. Gifford*, Kara Tureski*, Garrett R. Griffin*, Catherine Lowry†, Jamison Williams†, Jennifer Collins†, Heather McLearie†, Kelly G. Lambert† *Department of Psychology, University of Richmond, Richmond, Virginia 23173, USA e-mail: [email protected] brief communications NATURE | VOL 402 | 11 NOVEMBER 1999 | www.nature.com 137 Motherhood improves learning and memory Neural activity in rats is enhanced by pregnancy and the demands of rearing offspring. Trial day Foster Maternal Nulliparous 123456789 10 11 12 13 14 15 16 Correct responses for first eight arms Time to approach baited food well (s) Multiparous Nulliparous 8 7 6 5 4 3 2 1 0 180 150 120 90 60 30 0 a b Figure 1 Improvement in spatial learning following reproductive and pup experience. a, Effects of distal reproductive experience on spatial learning. Two separate groups of 6–8 adult (~100 days old) randomly chosen female Sprague–Dawley rats were mated and allowed to deliver naturally. Litters were culled to 6 pups and allowed to remain with their mothers until weaning (21 days). Two weeks after weaning, these newly primiparous females were re- mated and the sequence repeated. These newly multiparous females were allowed to remain with their pups until weaning. Two weeks later, the multiparous females and their age-matched nulliparous cohorts were tested on the radial-arm maze. b, Effects of proximal reproductive experience on spatial learning in three groups of 5 virgin Long Evans rats: nulliparous rats, which had no mating or pup contact; fosters, for which females were exposed to three foster pups (exchanged every 24 h for milk-replete pups) for 16 days, when the females met all the criteria for displaying maternal behaviour (retrieving, grouping and crouching over pups within 60 min of exposure); and maternals, which are primiparous females that stayed with pups for 16 days. When pups were removed, females were weighed, food was restricted and behav- ioural testing began in a dry land maze 5 . Further experimental details are available from the authors.
Transcript of document
© 1999 Macmillan Magazines Ltd
When a female mammal makes thetransition from virginity to mother-hood, she is forced to refocus her
activities dramatically. She must adapt to amultitude of new demands by her offspringor risk losing a significant metabolic andgenetic investment. She needs to find andremember the location of food stores, watersources and nest sites, and be able to exploitthem to her offspring’s advantage. The per-formance of these tasks may depend on asharpening of her cognitive abilities. Wehave tested this idea by using two differentstrains of rat and two separate spatial tasks,and find that pregnancy and a litter of pupsmay combine to stimulate spatial learningand memory in females.
In the oestrous cycle, which occurs pre-paratory to mating, steroid hormones suchas oestradiol and progesterone markedlyincrease the concentration of hippocampalapical dendritic spines in adult females, par-ticularly when oestradiol levels are raised1.Because dendritic spines increase the totalsurface area of the synapse, such neuralchanges may improve learning and mem-ory2. Pregnancy, which raises the amount ofoestradiol and progesterone for significantlylonger than the oestrous cycle, may producelong-lived improvements in behaviours thatare not maternal as such, but which con-tribute to the survival and rearing of pups.The hormones of pregnancy may preparesites that regulate learning and memory3, inmuch the same way that they stimulate themedial preoptic area to respond withmaternal behaviour when pups are born4.
We investigated this idea by testing multi-parous (animals that had given birth and lac-tated twice) and age-matched nulliparous(virgin) Sprague–Dawley females on a 16-day regimen of radial-arm maze tests. On thefirst six days, multiparous females made sig-nificantly more correct choices than nulli-parous females (P*0.0002–0.01; Fig. 1a).
In a second experiment, virgin femaleLong Evans rats were assigned to one ofthree groups: fosters, maternals and nulli-parous. In the dry-land version of the Mor-ris watermaze5, maternal females tooksignificantly less time (43.2 s) than nulli-parous rats (128 s) to recall and locate thefood reward (P*0.05; Fig. 1b). No signifi-cant differences were observed between fos-ter (55 s) and maternal rats.
Taken together, these results show that a combination of reproductive and pupexperience and stimulation is beneficial tolearning and memory in female rats. Hormone-induced modifications to thehippocampus (increases in both long-term
potentiation6 and dendritic spine density1,7)may improve the navigation skills involvedin parental resource-gathering behaviours.For example, hippocampal volume inparental rodents during the breeding seasonis correlated with the size of the homerange8. Concurrent alterations in both thehippocampus and the medial preoptic areamay therefore contribute to the pro-nounced behavioural transition that ischaracteristic of the maternal female, withthe medial preoptic area directly regulatingmaternal behaviour, and the hippocampuscontrolling supporting behaviours such as foraging.
As well as receiving the hormones ofpregnancy, the female’s brain is exposed torich sensory events when pups are born.With each litter of pups comes a plethora ofnew sights, sounds, tastes, and tactile andsuckling stimulation, the last of which canreorganize hypothalamic connections9. Thissensory stimulation may have effects onbrain structure and function that are similarto those caused by other types of enrichedphysical environment, an idea supported bythe observation that the brains of parous,environmentally impoverished rats resemblethose of rats in an enriched environment10.
Dendritic processes called filopodia aretriggered by the excitatory activation of
N-methyl-D-aspartate (NMDA)11, leadingto the formation of synapses. Furthermore,new postsynaptic CA1 dendritic spines arerapidly formed after long-term potentiation(LTP, a learning analogue)12. Neural activitybrought about by pregnancy and the pres-ence of pups may literally reshape the brain,fashioning a more complex organ that canaccommodate an increasingly demandingenvironment.
Little attention has been paid to theremarkable neural plasticity that is inherentin reproduction itself and that underlies thesubsequent behavioural changes, particu-larly those unique to late pregnancy and thepostpartum period. To consider the rela-tionship of a mother caring for her young asunidirectional disregards the potentiallyrich set of sensory cues in the oppositedirection that can enrich the mother’s envi-ronment. By providing such stimuli, thepups may ensure both their own and theirmother’s development and survival.Craig H. Kinsley*, Lisa Madonia*, Gordon W. Gifford*, Kara Tureski*, Garrett R. Griffin*, Catherine Lowry†,Jamison Williams†, Jennifer Collins†,Heather McLearie†, Kelly G. Lambert†*Department of Psychology, University of Richmond,Richmond, Virginia 23173, USAe-mail: [email protected]
brief communications
NATURE | VOL 402 | 11 NOVEMBER 1999 | www.nature.com 137
Motherhood improves learning and memoryNeural activity in rats is enhanced by pregnancy and the demands of rearing offspring.
Trial day
Foster MaternalNulliparous
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16
Cor
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s fo
r fir
st e
ight
arm
sTi
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od w
ell (
s)
MultiparousNulliparous
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4
3
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0
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60
30
0
a
b
Figure 1 Improvement in spatial learning following reproductive
and pup experience. a, Effects of distal reproductive experience
on spatial learning. Two separate groups of 6–8 adult (~100 days
old) randomly chosen female Sprague–Dawley rats were mated
and allowed to deliver naturally. Litters were culled to 6 pups and
allowed to remain with their mothers until weaning (21 days). Two
weeks after weaning, these newly primiparous females were re-
mated and the sequence repeated. These newly multiparous
females were allowed to remain with their pups until weaning.
Two weeks later, the multiparous females and their age-matched
nulliparous cohorts were tested on the radial-arm maze. b, Effects
of proximal reproductive experience on spatial learning in three
groups of 5 virgin Long Evans rats: nulliparous rats, which had no
mating or pup contact; fosters, for which females were exposed to
three foster pups (exchanged every 24 h for milk-replete pups) for
16 days, when the females met all the criteria for displaying
maternal behaviour (retrieving, grouping and crouching over pups
within 60 min of exposure); and maternals, which are primiparous
females that stayed with pups for 16 days. When pups were
removed, females were weighed, food was restricted and behav-
ioural testing began in a dry land maze5. Further experimental
details are available from the authors.
© 1999 Macmillan Magazines Ltd
6. Warren, S. G. et al. Brain Res. 703, 26–30 (1995).
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of the mode, which continues down to thediffraction limit of approximately a/4N. Oneven smaller scales, further magnificationdoes not reveal further substructure (Fig. 1,bottom). In the asymptotic limit l↓0(N→÷), the diffractive spreading becomesnegligible, leading to an ideal fractal. Large-N resonators are feasible (for example,N414 has already been realized in ref. 3).
The origin of the self-similarity can beunderstood intuitively: the existence of the round-trip magnification M means thatthe eigenmode must consist of (de)mag-nified copies of itself. Note that the hermite–gaussian modes of stable reson-ators are not self-similar because the round-trip magnification is absent.
A characteristic property of fractals istheir non-integer fractal dimension Df (refs4,5). We found that an accurate determina-tion of Df for two-dimensional modes (asin Fig. 1) would require calculations atmuch higher N than considered here, andhence computational resources beyondthose available; in earlier calculations6 forone-dimensional resonators with N4103,we found that Df was 1.650.1.
It has been shown that a tetraedicalstacking of four reflecting spheres representsa chaotically scattering system with inherentfractal properties7. Our results help toexplain this: light rays repeatedly scatteringin the inner chamber of the tetraedicalstacking undergo geometrical magnificationowing to the reflecting spheres, just as inour unstable resonator. Coincidentally, the‘aperture’ in ref. 7 is approximately triangu-lar, like our aperture in Fig. 1, leading tosimilar patterns. The self-similar aspectseems to be a useful unifying feature of opti-cal-pattern generation, providing insightinto chaotic scattering7 and transverse non-linear optics8. In the latter case, there iscompetition between bulk nonlinear behav-iour and boundary-imposed linear diffrac-tion as pattern-generating mechanisms.G. P. Karman*‡, G. S. McDonald*†, G. H. C. New†, J. P. Woerdman**Leiden University, PO Box 9504, 2300 RA Leiden, The Netherlandse-mail: [email protected]†Blackett Laboratory, Imperial College, London SW7 2BZ, UK‡Present address: Philips Research Laboratories,Prof. Holstlaan 4, 5656 AA Eindhoven, The Netherlands
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†Department of Psychology, Randolph-Macon College, Ashland, Virginia 23005, USA
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brief communications
Laser optics
Fractal modes inunstable resonatorsOne of the simplest optical systems, consist-ing of two mirrors facing each other to forma resonator, turns out to have a surprisingproperty. Stable resonators, in which thepaths of the rays are confined between thetwo mirrors, have a well known mode struc-ture (hermite–gaussian), but the nature ofthe modes that can occur in unstable reson-ant cavities (from which the rays ultimatelyescape) are harder to calculate, particularlyfor real three-dimensional situations1. Herewe show that these peculiar eigenmodes ofunstable resonators are fractals, a findingthat may lead to a better understanding ofphenomena such as chaotic scattering andpattern formation. Our discovery may havepractical application to lasers based onunstable resonators1.
For a confocal unstable resonator con-sisting of a large concave mirror and a smallconvex feedback mirror sharing a commonfocus, the rays spill over the small mirror sothat its size and shape define the outputaperture (Fig. 1, insets); the other mirror isso large that its size and shape are irrele-vant. The properties of an unstable res-onator are determined by the round-trip magnification M and the (equivalent)Fresnel number N of the resonator:N¬(M11)a2/2lL, where l is the opticalwavelength, 2a is the size of the small mir-ror, and L is the cavity length1. The exist-ence of a round-trip magnification M ischaracteristic for unstable resonators, asstable resonators lack such magnification.The value of M determines the round-tripspill-over loss around the small mirror, andN controls the size of the smallest details inthe transverse mode profile as determinedby diffraction. In an unstable-cavity laser,the losses are compensated by the gain pro-vided by the laser medium.
Free-space propagation of the opticalfield is described by the Huygens–Fresnelintegral1, and cavity eigenmodes u(x,y) aredefined by the requirement that, after oneround trip, the field profile remainsunchanged apart from a complex multiplier
(the eigenvalue of the mode). We have cal-culated the mode patterns for a range ofdifferent aperture shapes, concentratingparticularly on regular polygons and rhom-boids. This calculation is numerically verydemanding, so we used a non-orthogonalgrid2 to increase efficiency. For square orcircular apertures, the diffraction problemcan be reduced to a one-dimensional calcu-lation, with the mode patterns for thesquare factorizing into x and y components.
A typical modal intensity distributionfor triangular aperturing is shown in Fig. 1.Smaller and smaller triangles keep appear-ing as the mode pattern is repeatedly mag-nified. This shows the self-similar nature
L2a
Figure 1 Calculated lowest-loss eigenmode of an unstable res-
onator with a triangular mirror. Left inset, a three-dimensional
view; right inset, a side view. M41.5, N410.5. The intensity
distribution äuä2 on the small mirror is shown: white, high; red,
medium; blue, low. Bottom, magnification of the centre part, with
colour coding adjusted to exploit the full dynamical range.
