Adult hippocampal neurogenesis and its role in cognition

Adult	hippocampal	neurogenesis	(AHN)	has	intrigued	neuroscientists	for	decades.	Several	lines	of	
evidence	show	that	adult-born	neurons	in	the	hippocampus	are	functionally	integrated	and	
contribute	to	cognitive	function,	in	particular	learning	and	memory	processes.	Biological	properties	
of	immature	hippocampal	neurons	indicate	that	these	cells	are	more	easily	excitable	compared	to	
mature	neurons,	and	demonstrate	enhanced	structural	plasticity.	The	structure	in	which	adult-born	
hippocampal	neurons	are	situated	-the	dentate	gyrus-	is	thought	to	contribute	to	hippocampus	
function	by	disambiguating	similar	input	patterns,	a	process	referred	to	as	pattern	separation.	
Several	ideas	about	AHN	function	have	been	put	forward;	currently	there	is	good	evidence	in	favour	
of	a	role	for	AHN	in	pattern	separation.	This	function	of	AHN	may	be	understood	within	a	
‘representational-hierarchical’	view	of	brain	organisation. The	discovery	of	neurogenesis	in	the	brain	of	adult	mammals1-3
,	including	humans4
,	received	
considerable	attention	as	it	challenged	the	prevailing	dogma	that	the	brain	is	‘post-mitotic’	and	as	
such	is	endowed	with	limited	regenerative	capacity.	In	the	mammalian	brain,	adult	neurogenesis	is	
restricted	to	two	regions:	1.	the	DG,	at	the	border	of	the	granule	cell	layer	and	hilus	(the	subgranular	
zone)	where	adult	neurogenesis	gives	rise	to	the	primary	granule	cells	(GCs),	and	2.	the	
subventricular	zone	of	the	lateral	ventricles;	cells	born	here	subsequently	migrate	to	the	olfactory	
bulb5-7
.	Given	the	well-established	role	of	the	hippocampus	in	learning	and	memory8
,	it	was	soon	
suggested	that	AHN	may	contribute	to	these	functions	in	some	way.	This	idea	was	supported	by	the	
finding	that	memory	demand	correlated	with	AHN	in	birds9
	and	that	in	rats	AHN	could	be	stimulated	
by	learning	a	spatial	task10.	In	this	manuscript,	we	will	review	some	of	the	biological	properties	of	
adult-born	hippocampal	neurons	and	provide	an	overview	of	the	structure	in	which	adult-born	
hippocampal	neurons	are	situated,	the	dentate	gyrus.	This	is	followed	by	an	overview	of	studies	that	
have	addressed	a	putative	role	of	AHN	in	learning	and	memory	function	and	a	discussion	of	the	ideas	
on	how	adult-born	hippocampal	neurons	may	contribute	to	hippocampus	function.The	discovery	of	neurogenesis	in	the	brain	of	adult	mammals1-3
,	including	humans4
,	received	
considerable	attention	as	it	challenged	the	prevailing	dogma	that	the	brain	is	‘post-mitotic’	and	as	
such	is	endowed	with	limited	regenerative	capacity.	In	the	mammalian	brain,	adult	neurogenesis	is	
restricted	to	two	regions:	1.	the	DG,	at	the	border	of	the	granule	cell	layer	and	hilus	(the	subgranular	
zone)	where	adult	neurogenesis	gives	rise	to	the	primary	granule	cells	(GCs),	and	2.	the	
subventricular	zone	of	the	lateral	ventricles;	cells	born	here	subsequently	migrate	to	the	olfactory	
bulb5-7
.	Given	the	well-established	role	of	the	hippocampus	in	learning	and	memory8
,	it	was	soon	
suggested	that	AHN	may	contribute	to	these	functions	in	some	way.	This	idea	was	supported	by	the	
finding	that	memory	demand	correlated	with	AHN	in	birds9
	and	that	in	rats	AHN	could	be	stimulated	
by	learning	a	spatial	task10.	In	this	manuscript,	we	will	review	some	of	the	biological	properties	of	
adult-born	hippocampal	neurons	and	provide	an	overview	of	the	structure	in	which	adult-born	
hippocampal	neurons	are	situated,	the	dentate	gyrus.	This	is	followed	by	an	overview	of	studies	that	
have	addressed	a	putative	role	of	AHN	in	learning	and	memory	function	and	a	discussion	of	the	ideas	
on	how	adult-born	hippocampal	neurons	may	contribute	to	hippocampus	function.This is the author accepted manuscript. The final version is available from Wiley at http://onlinelibrary.wiley.com/doi/10.1002/wcs.1304/abstract