- Lyall Watson

1. The brain is impossibly complicated - if it were simple enough to understand, we'd be too simple to understand it. - Lyall Watson

3. Spatial representation • “We live in it, move through it, explore it, defend it… yet we find it difficult to come to grips with space” (O’Keefe and Nadel, 1978). • Space is not a sensory modality (we do not have sensory organs for it – it is a construct of mental processing). • Memory system – evolved in response to environmental demands.

4. Bast, 2007

5. Intra-hippocampal anatomy/circuitry

6. Place cells • Single neurons that fire when the rat moves through a specific location (O’Keefe and Dostrovsky, 1971). • Pyramidal & Granule • Area where a cell fires is its “place field”. • Still fire when landmarks removed, light turned off (Hafting et al., 2005). • Most studies in environments about 1m across (small area for a rat) – this study uses an 18m track –larger spatial scales.

7. Place cells • Replay - Cells coactivated during awake behavior have correlated activity during sleep (consolidation?, Wilson and McNaughton, 1994). • Preplay- firing in a T-maze went ahead in the direction of reward location (Johnson and Redish, 2007). • Retrieval of stored spatial representations. • Hippocampus – active problem solving.

8. Remapping • Memory interference – similar memories. • Place fields can appear, disappear, or move randomly (seemingly). • Rate remapping • Global remapping • Depends upon differences in environment – memory separation.

9. Place cell plasticity Neves et al., 2008

10. Head-direction cells • Neurons that fire when the animals head faces a specific direction in the horizontal plane. • Independent of movement / behavior. • Vestibular input critical to direction signal.

11. Taube, 2008

12. Goal cells?

13. Grid cells • Medial entorhinal cortex – multiple firing fields, forming a grid. • Firing fields dispersed over the entire environment. • Firing fields are generally equally spaced apart, distance from one firing field to all adjacent firing fields is approximately the same.

15. Spatial representation system • Place Cells – hippocampus. • Head direction cells – presubiculum. • Grid – entorhinal cortex. • Thalamus • Parietal cortex • Cell types together provide info for a mapping system. • Location, distance/direction between locations.

16. Aims • Ventral place cells? • How is space represented along the dorsoventral axis of the hippocampus?

17. Materials and methods • Twenty-one male Long-Evans rats ~350-400 g. • Testing occurred in the dark phase. • Rats trained to run back and forth on an 18 m long and 12 cm wide linear track. • Running maintained by chocolate crumbs at each of the turning points. On recording trials, the rats ran approximately 10 consecutive laps in each direction. • On the 18 m track, the rat’s position was tracked by an eye-safe invisible laser beam placed at one end of the track.

18. Materials and methods

19. Materials and methods • The head stage was connected to a 10m long cable that moved freely. • An experimenter consistently followed 1 m behind the rat to ensure that there was no strain from the cable on the rat’s head. • The tetrodes were lowered in steps of 50 μm or less until single neurons could be isolated at appropriate depths. • After each recording, the tetrodeswere moved further until new well-separated cells were encountered. The most ventral recording locations were encountered 30-90 days after the start of tetrode turning.

20. Materials and methods Histology. • The rats were perfused(with electrodes in?). • Sectioned (30 mm) using a cryostat. • The positions of the tips of the recording electrodes were determined.

21. Blue=CA3 Red=CA1 Purple=Subiculum

22. CA3 Place fields Left runs = “Red” Right runs = “green” Top = firing rate as function of position Bottom = spike density on individual laps

23. Most ventral = 5-10m place field

24. Theta phase precession • Theta 4-12Hz oscillation, EEG. • Neuron fires in relation to theta cycle (0-360 degrees). • As a rat moves through a place field the neuron fires earlier and earlier in the cycle. • Each place cell will fire at a different phase of theta - determining location with good precision and possibly providing temporal code.

25. Theta phase precession dorsal Intermediate Ventral

26. Composite rate maps dorsal Intermediate Ventral

27. Spatial scale as function of position along longitudinal axis of hippocampus

28. Conclusions • Ventral place cells? – yes • Dorsal hippocampal neurons – mean of 98cm. • Ventral hippocampal neurons (>10m).

30. Conclusions • Behavioral differences – dorsal / ventral hippocampus. • Smaller scales (watermaze) – Dorsal place fields. • Larger scale (contextual conditioning) – ventral place fields. • Fire everywhere in one room and nowhere in another.