Neural mechanisms of Spatial Learning

Spatial LearningMaterials covered in previous lectures

• Historical development– Tolman and cognitive maps

• the classic paper 1948 “cognitive maps in rats and men”

• Multiple memory system, Animal models • Eichenbaum et al (1990), Morris water maze task

– Constant-start-location &

– Variable-start-location

• Kesner et al (1993) (Module 2)

– Spatial location

– Motor response

– Object recognition

Tolman and cognitive maps

Eichenbaum et al (1990) Morris water maze task

Kesner et al (1993) (Module 2)

Sensory cues during Navigation

1. external landmarks1. animals use visual landmarks to guide them to

goal locations2. idiothetic (self-motion) cues: vestibular cues or

proprioceptive cues1. The self-motion information can derive both from

internal sources and from external sources (e.g., optic flow cues).

Spatial frames of reference

• Allocentric processing– Relation between objects– Hippocampaus

• Egocentric processing– Relation between objects and self– Caudate etc

Lesion work, brain structure involved …

Ongoing operation of the human hippocampus and related brain structures during memory performance

•functional neuroimaging methods in normal humans •Module 5

•recording of the activity patterns of single neurons in animals•Module 6

Positron emission tomography (PET) Functional magnetic resonance imaging (fMRI)

1. blood flow and brain oxygen consumption2. level of activation of a brain area 3. temporal resolution: second4. spatial resolution: several thousand neurons

1. at only a gross level that tells us which structures are activated to major shifts in task demands.

5. the simultaneous examination of the entire system

Single cell recording studies in animals

1. monitoring the action potentials of individual neurons 2. the inner workings at the cellular level3. increase in resolution

1. Spatial resolution: particular types of neurons within a specific brain structure

2. temporal resolution: millisecond4. the fundamental coding properties of the units of neural

computation. 5. Drawback: only one part of the system at any time

Landmark studies:1.Tolman, Cognitive map - 2.O’Keefe, place cells

Hippocampus place cells in rats, early findings Initial report: O'Keefe and Dostrovsky's (1971)

These neurons fire only when the rat is in a particular location in its environment and firing is independent of the rat's orientation or ongoing behaviour.

Place field

Evidence for encoding of spatial relationships in the environment 1. place fields move in concert with rotation of salient visual cues 2. they scale with enlargement of all features of the environment, 3. Place cell activity can be related to the rat's memory about its

spatial location

Place fields move in concert with rotation of salient visual cues

They scale with enlargement of all features of the environment

TheoriesPlace cells – cognitive map

O’Keefe and Dostrovksy made the discovery of the place cell that “constructs the notion of a place in the environment by connecting together several multisensory inputs, each of which can be perceived when the animal is in a particular part of the environment”

TheoriesAlterative theories: Eichenbaum “memory space” model

1. Place cells fire in response to the complex interaction of multiple cues and not in response to an internal Cartesian coordinate system.

2. patterns of activation are directly related to particular perceptual, behavioural, and cognitive cues that are being experienced together, in a particular location, and at a particular time.

3. Other factors have been demonstrated to influence the firing of place cells such as

orientation within an environment, direction of movement, speed of movement, angle of movement, presence/absence of landmarks, the intentions related to the movement (e.g., searching for a food

reward).

Neural mechanisms of spatial processing in rats, recent analysis

Cell types

1. Place cells: hippocampus2. Head direction cells

1. parts of the thalamus and limbic system2. fire when the rat’s head is pointed in a

particular direction, regardless of the animal’s location in the environment or the position of the body relative to the head.

Response properties for both types of cells1. Controlled by an interaction between landmarks and idiothetic cues 2. Role of visual landmark (important but not required)

1. rotation of the landmarks -- > an equal rotation of the firing location/ direction of the place cells or head direction cells

2. maintain their location/ direction tuning for many minutes in total darkness, provided that the rat had established its bearings before the light was turned off

3. Role of non-visual input1. vestibular input does appear to be necessary for these cells to

maintain their selective firing properties2. influenced by the ‘‘motor set’’ of the animal3. will shut off or reduce firing dramatically if the rat is restrained

tightly, even if the rat is placed in the proper firing location/ direction of the cell under study