Navigation: Inside the Hippocampus

Transcription

1 9.912 Computational Visual Cognition Navigation: Inside the Hippocampus Jakob Voigts 3 Nov 2008

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4 Functions of the Hippocampus: Memory and Space Short term memory Orientation Memory consolidation(h.m) Spatial memory

5 Entorhinal Cortex (EC) Anatomy Dentate Gyrus(DG) Cornus Ammonis 3 (CA3) Cornus Ammonis 1 (CA1)

6 Place cells (CA1): - Represent mainly location in local environment - Stable over long periods -Can remap (more on that later) -Represent more than just the position! (more on that later) O'Keefe J, DostrovskyJ. : The hippocampus as a spatial map. Preliminary evidence from unit activity in the freely-moving rat Brain Res.

7 Two main pathways: Monosynaptic and Trisynaptic Entorhinal Cortex (EC) Provides main Input from the cortex CA1 Main output to the cortex Evolutionary old, capable of learning, but slow and less stable when cues are changed.

8 Two main pathways: Monosynaptic and Trisynaptic Entorhinal Cortex (EC) Provides main Input from the cortex Dentate Gyrus(DG) First processing step in Trisynaptic pathway CA3 Has recurrent connections CA1 Main output to the cortex Evolutionary newer, can do one-trial learning, way more computational capacities

9 Removing input to CA1 from CA3 - Selective remove NMDA receptors only in CA3 pyramidal cells. -Test animals in water maze with either 4 or 2 distal cues. Full cues:no significant difference between wildtypeand mutant animals, no significant reduction in activity or place field shapes Reduced cues:the mutant animals dropped to pre-training level, considerable loss of Place field specificity. Nakazawa, Wilson, Tonegawa, et al.: Requirement for hippocampal CA3 NMDA receptors in associative memory recall Science

10 Rough Ideas on what the areas do: DG: Pattern separation, dimensionality reduction (has 10x more cells than CA3) CA3: Pattern completion Nakazawastudy, recurrent connections (Hopfield-Attractor) CA1: Novelty detection Place cells, remapping

11 Head direction cells (postsubiculum and anterior thalamic nuclei): - Global orientation in one environment - Can depend on motor&vestibular input (ATN) -Rotate as ensemble if (all) visual cues are rotated Taube et al.: Head-direction cells recorded from the postsubiculumin freely moving rats. II. Effects of environmental manipulations 1990 J. Neurosci.

12 Head direction cells (postsubiculum and anterior thalamic nuclei): Head direction cells are obviously not 100% stable across a given environment. JS Taube.: Head direction cells recorded in the anterior thalamic nuclei of freely moving rats 1990 J. Neurosci.

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14 Grid cells (EC): -Dorsocaudal medial entorhinal cortex (dmec) -Deeper layers have some dependence on head direction. -Relative to environment but present without environmental cues path integration might happen here? -Spacing and size of individual fields increase from dorsal to ventral Hafting et al.: Microstructure of a spatial map in the entorhinal cortex 2005 Nature

15 Nonspatial properties of place cells: -Put mice in M-shaped maze and train alternating left/right visits -Place cell responses in the middle arm show strong dependency on past and upcoming decision. Diverse studies have shown: -Behavioral significance of places shapes place fields in many tasks Traces from 2 cells per area in in/outbound conditions. -Best CA1 correlation with places ~120ms in front of the animal (Muller et al. 1989) Frank LM, Brown EN, Wilson M.: Trajectory encoding in the hippocampus and entorhinal cortex Neuron

16 What happens if we change the environment gradually? - Put mice in square environment -Let them explore - Record single place cells - Change dimensions of environment O Keefe, Burgess.: geometric determinants of the place fields of hippocampal neurons 1996 Neuron

17 What happens if we change the environment gradually? Change in place fields as function of different deformations of the holding box O Keefe, Burgess.: geometric determinants of the place fields of hippocampal neurons 1996 Neuron

18 What happens if we change the environment gradually? Single place fields can be separated into two. O Keefe, Burgess.: geometric determinants of the place fields of hippocampal neurons 1996 Neuron

19 What happens if we change the environment gradually? Behavior can be modeled pretty well as thresholded sum of gaussiansplaced at fixed distances to walls O Keefe, Burgess.: geometric determinants of the place fields of hippocampal neurons 1996 Neuron

20 How does visual input contribute to the hippocampus function in spatial orientation?

21 Seen so far: - Changing box dimensions stretches place fields out, can separate them -Rotating box in room does not rotate place fields with the box -Partial cue removal can disrupt place recognition, this is especially easy when blocking CA3 - Head direction and place cell alignment slowly drifts in darkness

22 Conflicting visual and idiothetic cues Rotate visual cues at different speeds while animal is exploring. Small mismatch (45⁰) : Visual cues dominate and representation rotates Large mismatch (180⁰) : Complete remapping Knierim et al.: Interactions Between Idiothetic Cues and External Landmarks in the Control of Place Cells and Head Direction Cells 2008 J. Neurophysiology

23 Overshadowing of geometrical by featural cues: Train animals with and without featural cues (dark walls) Test without (all plain walls) In rectangle, overshadowing group did worse In the shown kite shaped env. they performed Better than the control group. Pearce et al.: Potentiation, Overshadowing, and Blocking of Spatial Learning Based on the Shape of the Environment 2006 Journal of Experimental Psychology

24 Hippocampalpathology affects viewpoint dependencein spatial memory Test object position memory in VR in Same and shifted viewpoint images in Patients with withfocal bilateral hippocampal damage. Same viewpoint: little impairment Novel viewpoint: strong impairment Pattern matching seems to work w/o Hippocampal involvement Mental rotation, or viewpoint independent representation not King, Burgess, O Keefe et al. : Human Hippocampus and Viewpoint Dependence in Spatial Memory 2002 Hippocampus