( 26 , 30 ), despite substantial effects with SuM
activation ( 31 ). Recent work demonstrated
that a subpopulation of SuM units increase
their activity under novel conditions ( 27 ). SuM
cells from this previous study have several
characteristics that overlap with SuMTac1−cells,
including the nonuniform axonal innervation
pattern and mixed neurotransmitter phenotype
in the DG ( 34 ). In another study, optogenetic
inhibition of the SuM affected theta-range spike
timing in SuM-connected structures, but only
atthedecisionpointofthemaze( 26 ). Thus,
SuMTac1−cells may be most influential to hip-
pocampal network patterns in particularly
salient situations, with little contribution to
the mechanisms that underlie spontaneous
theta waves. Consistent with this model,
SuMTac1−activation caused a considerable shift
in firing phase preferences from spontaneously
generated theta rhythm, highlighting poten-
tially different underlying mechanisms for
SuMTac1−-evoked versus spontaneous theta
oscillations.
These data also advance our understanding
of the SuM’s in role in locomotion and identify
a cell type with functional properties relevant
to spatial navigation. In addition to the tight
coupling of SuMTac1+activity to speed, we
found that SuMTac1+activation robustly drove
locomotion while selectively regulating the acti-
vity of speed-sensitive hippocampal neurons.
These data raise the possibility that SuMTac1+
neurons have a role in distributing a speed signal
throughout the SuM’s many axon-termination
sites and complement recent work outlining
a speed-relaying pathway from the MLR to
the entorhinal cortex via the septum ( 38 ).
Given that SuMTac1+cells encode future speed,
the SuM may provide its synaptic partners
with intended speed, whereas executed speed
is propagated from the MLR. Thus, the SuM
may support a role in planning and error cor-
rection during locomotion by broadcasting a
future speed signal.
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ACKNOWLEDGMENTS
The authors thank H. Ito, E. Moser, and M.-B. Moser for generously
sharing a previously published dataset for reanalysis for the
specific purposes of this study and for providing comments on the
manuscript. The authors also thank R. Kumar and S. Felong for
technical assistance.Funding:This work was supported by a
Canadian Institutes of Health Research postdoctoral fellowship(J.S.F.); National Institute of Mental Health (NIMH)
K99MH11284002 (M.L.-B.); an American Epilepsy Society (AES)
Junior Investigator Award (F.T.S.); Stanford Epilepsy Training Grant
5T32NS007280 funded by the National Institute of Neurological
Disorders and Stroke (NINDS) (P.M.K. and E.H.); a Swiss National
Science Foundation Postdoctoral Fellowship (T.G.); National
Science Foundation Fellowship DGE-114747 (B.A.); a HHMI Gilliam
Fellowship for Advanced Study (B.A.); Gates Millennium
Scholarship (B.A.); a Japan Society for the Promotion of
Science (JSPS) Overseas Fellowship (S.T.); an AES Postdoctoral
Fellowship (B.D.); NINDS K99NS117795 (B.D.); National Institute of
Health (NIH) 1U19NS104590 (I.S., A.L., and M.J.S.); NIMH
1R01MH124047 and 1R01MH124867 (A.L.); the Kavli Foundation
(A.L.); and the NIH, NSF, Gatsby, Fresenius, and NOMIS
Foundations (K.D.).Author contributions:Conceptualization:
J.S.F. and I.S. Methodology: B.A., M.O., and G.S. Investigation: J.S.F.,
M.L.-B., P.M.K., F.T.S., T.G., A.L.O., and S.B. Formal analysis: J.S.F.
M.L.-B., P.M.K., F.T.S., T.G., S.T., M.O., E.H., and B.D. Funding
acquisition: M.J.S., K.D., A.L., and I.S. Supervision: M.J.S., K.D., A.L.,
I.S. Writing–original draft: J.S.F. and I.S. Writing–review &
editing: all authors.Competing interests:M.J.S. is a scientific
cofounder of Inscopix. GRIN lenses used in this studied were
purchased from Inscopix.Data and materials availability:Code
used in this study came from publicly available resources
referenced in the materials and methods. Parts of the raw datasets
and additional custom code are openly available at solteszlab.com/
datasets and will continue to be formatted and uploaded.SUPPLEMENTARY MATERIALS
science.org/doi/10.1126/science.abh4272
Materials and Methods
Figs. S1 to S17
References ( 39 Ð 65 )
Movie S111 March 2021; accepted 29 October 2021
10.1126/science.abh4272PHYSIOLOGICAL STRESSPhysiological costs of undocumented human
migration across the southern United States border
Shane C. Campbell-Staton1,2,3*†, Reena H. Walker^4 †, Savannah A. Rogers^5 †‡, Jason De León^6 *,
Hannah Landecker2,7, Warren Porter^8 , Paul D. Mathewson^8 , Ryan A. Long^4 *Political, economic, and climatic upheaval can result in mass human migration across extreme terrain in search
of more humane living conditions, exposing migrants to environments that challenge human tolerance. An
empirical understanding of the biological stresses associated with these migrations will play a key role in the
development of social, political, and medical strategies for alleviating adverse effects and risk of death. We
model physiological stress associated with undocumented migration across a commonly traversed section of
the southern border of the United States and find that locations of migrant death are disproportionately
clustered within regions of greatest predicted physiological stress (evaporative water loss). Minimum values of
estimated evaporative water loss were sufficient to cause severe dehydration and associated proximate causes
of mortality. Integration of future climate predictions into models increased predicted physiological costs
of migration by up to 34.1% over the next 30 years.“You need to put yourself into the most
difficult places that you can where
[Border Patrol] can’t get to. You
understand? Where there are lots of
trees, mountains, rocks...off the trail.
That’s where you need to go. If you
walk in the easiest places, they will
catch you quick.”
—Lucho, 47-year-old migrant from Jalisco,
Mexico, interviewed June 2009 [( 1 ), p. 189]I
n 1994, the US Border Patrol adopted the
border enforcement strategy referred to
as Prevention Through Deterrence. This
policy sought to dissuade undocumented
entry across the southwest border of the
United States by fortifying official ports of en-
try and their surrounding areas for the purpose
of redirecting migrants toward more remote
regions, including areas of the Chihuahuan and
Sonoran Deserts ( 1 , 2 ). Desert environments are1496 17 DECEMBER 2021•VOL 374 ISSUE 6574 science.orgSCIENCE
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