Cool words
Parcellated into functionally-distinct subdivisions
sharply demarcated borders
Part I: Dorsal Ventral Dichotomy – a Brief Introduction
- The hippocampus is traditionally established as playing a key role in learning memory
- More recent evidence proposes a dominant view of a dissociation of function along the
septotemporal poles of the hippocampus
- The dorsal hippocampus (DH) is thought to be primarily responsible for spatial navigation and
memory
- The ventral hippocampus (VH) has a more pronounced role in emotional regulation, such as fear
and anxiety
- This essay will discuss the three lines of evidence that support this “dichotomy” hypothesis
o DH lesions, and not VH lesions, disrupt spatial learning in assays such as the Morris water
maze and elevated T/plus maze
o VH lesions, and not DH lesions, impair fear and anxiety-related behaviours
o The anatomical circuitry of the VH and DH are distinct, projecting to regions associated with
emotion regulation and spatial memory, respectively.
- That being said, this behavioural studies and electrophysiological studies suggest the “dichotomy”
approach should be revisited. Instead, researchers have proposed that the septotemporal axis is
organised along a gradient and that the functional roles of each poles are not as distinct as
originally thought.
- This essay will examine the anatomical, functional, and behavioural data relevant to the “dichotomy”
and “gradient” views.
- I will argue that the differences between the DH and VH may not be as distinct as we think, and the
functions may lie along a gradient rather than a dichotomy
Part I: DH in Spatial Memory
Argument: the DH is solely involved in spatial memory but the VH has NO role in spatial memory
Evidence: Moser et al., (1995) DH lesions impaired performance on Morris water maze
Background Behavioural tests of spatial navigation and memory have been instrumental in our
understanding of hippocampal function
Aim Understand the hippocampal tissue required for normal spatial memory
Method Lesions: injected ibotenic acid into hooded Lister rats within the DH, VH, and complete
hippocampus lesions varied from 20-100% of total hippocampus volume
Behavioural testing
- Used a Morris water maze where rats were allowed 120 seconds to find a
submerged platform in the opaque water
- The platform position was constant for each rat – never moved spot
- Then they made the rats swim for 60s in the maze when the platform was
removed
- Recorded the time spent swimming
Results - Spatial learning in the water maze was impaired with lesions of as little as 25% of
the DH
- A small traverse block of the hippocampus could support spatial learning as long
as it was the dorsal pole the DH supports a large proportion, if not all of spatial
memory in the hippocampus
- Lesions to the VH did not exacerbate this spatial memory impairment and did not
have an effect on behaviour
- VH lesions had to be very large in order to produce such a deficit
Conclusion Does this imply different functioning between the septotemporal poles, or is it just based
on relevant spatial scale?
Critical
thinking
, Evidence: Pothuizen et al., (2004): DH lesions impairs spatial memory in a radial arm maze
Background Better than the Morris water maze is not considered that sensitive in detecting spatial
learning defects
Aim Comparative study: whether the DH and VH are dissociable in their roles in spatial
working memory
Method Lesions: Excitotoxic lesions were achieved by using multiple infusions of N-methyl-D-
aspartate (NMDA) dissolved in phosphate buffered saline within the dorsal, ventral, and
entire hippocampus
- Rats were allowed to recover before behavioural testing
4-baited and 4-unbaited version of an 8-arm radial arm maze allows for concurrent
evaluation of reference and working memory using the same set of spatial cues
- Reference: recognise which of the arms are baited across trials
- Working memory: recognise the rewarded arms that were already visited
compared to those that werent
Results In general, reduction time to complete the trial decreased as training progressed this is
expected and was no different between the type of lesions
DH lesions:
- Produced a significantly larger amount of reference memory errors compared to
VH lesions rats had impaired learning in avoiding the never-baited arms
- Rats with DH and complete lesions were both considered equally impaired in
spatial memory
- Rats had impaired learning in avoiding the once-baited arms
- Rats with DH lesions produced higher working memory “incorrect” errors than the
VH lesion
Conclusion DH and VH lesions of similar size resulted in different effects on spatial learning and
memory
- DH, and not VH lesions led to significant impairments in spatial reference and
working memory
- DH lesions produced the same impairments as complete hippocampal lesions in
all aspects of the study
Critical Advantages
thinking - Better than the Morris water maze
o More sensitive in detecting spatial learning defects
o Water maze may not have been sensitive to detect contributions of VH to
spatial learning
Argument: anatomical evidence is consistent with the role of the DH in spatial reference and working
memory functional dissociation is likely related to the differences in connectivity
Evidence:
Parcellated into functionally-distinct subdivisions
sharply demarcated borders
Part I: Dorsal Ventral Dichotomy – a Brief Introduction
- The hippocampus is traditionally established as playing a key role in learning memory
- More recent evidence proposes a dominant view of a dissociation of function along the
septotemporal poles of the hippocampus
- The dorsal hippocampus (DH) is thought to be primarily responsible for spatial navigation and
memory
- The ventral hippocampus (VH) has a more pronounced role in emotional regulation, such as fear
and anxiety
- This essay will discuss the three lines of evidence that support this “dichotomy” hypothesis
o DH lesions, and not VH lesions, disrupt spatial learning in assays such as the Morris water
maze and elevated T/plus maze
o VH lesions, and not DH lesions, impair fear and anxiety-related behaviours
o The anatomical circuitry of the VH and DH are distinct, projecting to regions associated with
emotion regulation and spatial memory, respectively.
- That being said, this behavioural studies and electrophysiological studies suggest the “dichotomy”
approach should be revisited. Instead, researchers have proposed that the septotemporal axis is
organised along a gradient and that the functional roles of each poles are not as distinct as
originally thought.
- This essay will examine the anatomical, functional, and behavioural data relevant to the “dichotomy”
and “gradient” views.
- I will argue that the differences between the DH and VH may not be as distinct as we think, and the
functions may lie along a gradient rather than a dichotomy
Part I: DH in Spatial Memory
Argument: the DH is solely involved in spatial memory but the VH has NO role in spatial memory
Evidence: Moser et al., (1995) DH lesions impaired performance on Morris water maze
Background Behavioural tests of spatial navigation and memory have been instrumental in our
understanding of hippocampal function
Aim Understand the hippocampal tissue required for normal spatial memory
Method Lesions: injected ibotenic acid into hooded Lister rats within the DH, VH, and complete
hippocampus lesions varied from 20-100% of total hippocampus volume
Behavioural testing
- Used a Morris water maze where rats were allowed 120 seconds to find a
submerged platform in the opaque water
- The platform position was constant for each rat – never moved spot
- Then they made the rats swim for 60s in the maze when the platform was
removed
- Recorded the time spent swimming
Results - Spatial learning in the water maze was impaired with lesions of as little as 25% of
the DH
- A small traverse block of the hippocampus could support spatial learning as long
as it was the dorsal pole the DH supports a large proportion, if not all of spatial
memory in the hippocampus
- Lesions to the VH did not exacerbate this spatial memory impairment and did not
have an effect on behaviour
- VH lesions had to be very large in order to produce such a deficit
Conclusion Does this imply different functioning between the septotemporal poles, or is it just based
on relevant spatial scale?
Critical
thinking
, Evidence: Pothuizen et al., (2004): DH lesions impairs spatial memory in a radial arm maze
Background Better than the Morris water maze is not considered that sensitive in detecting spatial
learning defects
Aim Comparative study: whether the DH and VH are dissociable in their roles in spatial
working memory
Method Lesions: Excitotoxic lesions were achieved by using multiple infusions of N-methyl-D-
aspartate (NMDA) dissolved in phosphate buffered saline within the dorsal, ventral, and
entire hippocampus
- Rats were allowed to recover before behavioural testing
4-baited and 4-unbaited version of an 8-arm radial arm maze allows for concurrent
evaluation of reference and working memory using the same set of spatial cues
- Reference: recognise which of the arms are baited across trials
- Working memory: recognise the rewarded arms that were already visited
compared to those that werent
Results In general, reduction time to complete the trial decreased as training progressed this is
expected and was no different between the type of lesions
DH lesions:
- Produced a significantly larger amount of reference memory errors compared to
VH lesions rats had impaired learning in avoiding the never-baited arms
- Rats with DH and complete lesions were both considered equally impaired in
spatial memory
- Rats had impaired learning in avoiding the once-baited arms
- Rats with DH lesions produced higher working memory “incorrect” errors than the
VH lesion
Conclusion DH and VH lesions of similar size resulted in different effects on spatial learning and
memory
- DH, and not VH lesions led to significant impairments in spatial reference and
working memory
- DH lesions produced the same impairments as complete hippocampal lesions in
all aspects of the study
Critical Advantages
thinking - Better than the Morris water maze
o More sensitive in detecting spatial learning defects
o Water maze may not have been sensitive to detect contributions of VH to
spatial learning
Argument: anatomical evidence is consistent with the role of the DH in spatial reference and working
memory functional dissociation is likely related to the differences in connectivity
Evidence:
