hippocampal formation cell types

Question 1

O’Keefe and Nadel (1976)

Answer 1

• place cells in HC fire when animal moves over place field
• positional and contextual information
• form basis of cognitive map
• allocentric (whereas egocentric coded for by cells outside HC e.g. grid)
  population activity of HC place cells encode whole environments
• formed basis of argument that HC was selectively specialised for processing of spatial info, as HC was required for spatial tasks but not non-spatial
• although, some of these studies also differed in not just spatial/nonspatial but also relational/flexible memory vs rigid/response-only memory
• research now shows that HC is also required for nonspatial aspects of episodic memory (e.g. Eichenbaum 2000)

Question 2

Wilson 1993

Answer 2

• recordings from 80 CA1 neurons

- well-defined place fields

Question 3

Moser 2008

Answer 3

• layer II of mEC contains grid cells
• context independent, egocentric map
• form basis for path integration

Question 4

Gil 2018

Answer 4

• disruption of grid cell firing (but not place cell firing) impairs path integration

Question 5

Kropff 2015

Answer 5

• speed cells in all layers of mEC

- path integration

Question 6

Witter and Moser 2006

Answer 6

• HC not just for spatial navigation!
• damage to HC impairs both spatial and nonspatial info
• T maze spatial alternation task: most HC cells that fired to location on arm of T (common area) only fired if subsequent turn was in specific direction i.e. either left or right
• so also encodes non-spatial aspects of events e.g. intended direction of movement
  separate HC networks encode sequences of behaviours and places separately for left and right turns
• episode-specific encoding of aspects including spatial location = “memory space”
• consistent with neuropsychological findings that show HC is required for factual info in memory episodes

Question 7

Ranck 1984 / Robertson 1999

Answer 7

• HD cells in freely moving rats / primates

- orient spatial maps in HC

Question 8

Eichenbaum 2007

Answer 8

• DNMTS w/rats and 9 locations/scents
• out of firing HC cells, 1/3 spatial and 1/3 non-spatial correlates. some fired for location, some for odour, some for match/mismatch condition etc. most fired to combination

Question 9

Packard & McGaugh (1996)

Answer 9

• T maze: use place strategy in week 1

- lidocaine injection into HC abolished all place memory (performance at chance)

Question 10

heteroassociative networks in HC

Answer 10

• sequential pattern of activity in HC representing each element in memory = episodic memory
• represented by heteroassociative network. cell A synapses onto cell B, then cell C etc, so recalls elements of memories in right temporal order
• recurrent axonal connectivity is established by synapse plasticity and LTP
• local field potentials likely bind together activity of neurons

Question 11

theta and gamma

Answer 11

• single items in an episodic memory e.g. components A-G activated sequentially as a “fast” list on different gamma cycles (30-80Hz)
• 7 +/- 2 gamma cycles on each theta cycle (Ebbinghaus STM capacity)
• theta cycles (4-10 Hz, slow list) with repeated activation of A-G on each one = represent complete episodic memory
• represents STM encoding at cellular level
• as rat moves through place field, place cell fires earlier and earlier in theta phase i.e. phase-specific firing
• centre of place field = fires at trough (time of firing dictates location)
• theta phase precession and population code = predict exact location from temporal code (past, current and future position)

Question 12

Sternberg (1966)

Answer 12

• set of remembered digits
• mean RT increases by 38 ms per item added (length of gamma cycle so time-matched)
• serial scanning mechanism (must scan whole list)
• MEG recordings: theta power increases incrementally with task load, but doesn’t increase once limit of 7 has been reached
• supports role of theta in maintaining WM

Question 13

Hales (2014)

Answer 13

• how do firing patterns of cells in MEC affect physiology of HC cells?
• MEC lesions only partially disrupt HC place cells and specific types of HC-dependent memory
• bilateral lesion of whole MEC in rats caused lower proportion of active HC cells
• remaining cells had place fields but had decreased spatial precision and decreased long-term stability
• impaired on morris water maze task (like HC lesioned rats)
• combined MEC and HC lesion = even more impaired
• MEC lesioned rats not impaired on other HC-dependent tasks e.g. those where object location or context was remembered

= MEC not required for all types of spatial coding / HC-dependent memory, but is necessary for normal acquisition of place memory