Hippocampus, Spatial, page 1 of 5, 4/16/2020
Ongoing operation of the human hippocampus and related brain structures during memory performance
 functional neuroimaging methods in normal humans
 recording of the activity patterns of single neurons in animals. positron emission tomography (PET) functional magnetic resonance imaging (fMRI)
 involve measurements of blood flow and brain oxygen consumption
 provide good reflections of the level of activation of a brain area
 spatial resolution: several thousand neurons
 temporal resolution: second
 strength: allows the simultaneous examination of the entire system, but at only a gross level that tells us which structures are activated to major shifts in task demands.
Single cell recording studies in animals
 monitoring the action potentials of individual neurons
 closer look at the inner workings of the hippocampus
 allows a major increase in resolution of cellular activity
 allows us to distinguish particular types of neurons within a specific brain structure
 temporal resolution: millisecond
 allow us to monitor only one part of the system at any time, but offer insights into the fundamental coding properties of the units of neural computation.
Imaging work on Human:
Left H: word
Right H: faces objects, spatial memory.
Landmark studies:
Tolman, Cognitive map ( Fig 0-03 Fig 0-04 )
O’Keefe, place cells (described later)
Nature of the task:
Allocentric (world centered) processing
Egocentric (body centered) processing
Navigation and virtual reality, 3 requirements:
1.
Stimulus a.
Morris water maze (VR) for Human i.
Possibility of visual pattern matching b.
Simple corridor mazes i.
Possibility of stimulus response learning c.
Visual texture
2.
Navigation Behavirour

Hippocampus, Spatial, page 2 of 5, 4/16/2020 a.
Map learning vs navigation b.
Active control (with and without body movement)
3.
Scan (head fixed) neuroimaging
Critical structures in the memory system of the medial temporal lobe (Fig 1-11)
1.
the hippocampus a.
the hippocampus proper (Ammon’s horn) b.
the dentate gyrus c.
the subiculum d.
fornix
2.
the parahippocampal region a.
entorhinal cortex b.
perirhinal cortex c.
parahippocampal cortex (= postrhinal cortex in rat)
Imaging studies on Human spatial navigation (debate of the region of activation):

Hippocampus (more posterior region of H) o navigation in context-rich (with landmark), complex environment (large scale space), activate H (Figure 4-0) o simple maze, no activation of H o correlation with behaviour, H

Parahippocampal gyrus o Maguire: Recall landmarks, object location, not recalling complex routes o Only when context rich environment, simple maze, no activation o Spatial information present in visual scenes
Hippocampus place cells in rats, early findings
1.
Initial report: O'Keefe and Dostrovsky's (1971) Figure 4-001
2.
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.
3.
Several lines of evidence indicate that place cell activity reflects an encoding of the spatial relationships between physical stimuli in the environment
1.
place fields move in concert with rotation of salient visual cues, see
Fig. 4-002
2.
they scale with enlargement of all features of the environment, see Fig.
4-003 they are altered when the spatial arrangement of cues is disrupted
3.
Place cell activity can be related to the rat's memory about its spatial location

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Hippocampus place cells in rats, recent analysis
Navigation cues:
 external landmarks o animals use visual landmarks to guide them to goal locations
 idiothetic (self-motion) cues: vestibular cues or proprioceptive cues o animals can find their way back to a starting location in the complete absence of landmarks by using self-motion cues to update a representation of the distance and bearing to that starting location—an ability known as path integration or dead reckoning o The self-motion information can derive both from internal sources and from external sources (e.g., optic flow cues).
Neural mechanisms:

Cell types o Place cells: hippocampus o head direction cells
 parts of the thalamus and limbic system
 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 cells o Controlled by an interaction between landmarks and idiothetic cues o Role of visual landmark (important but not required):
 rotation of the landmarks while the animal is absent from the environment can, under most circumstances, cause an equal rotation of the firing location/ direction of the place cells or head direction cells
 can 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 o Role of non-visual input
 vestibular input does appear to be necessary for these cells to maintain their selective firing properties.
 influenced by the ‘‘motor set’’ of the animal
 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
Theories

Place cells – cognitive map ( Fig. 4-004) o 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”

Alterative theories: Eichenbaum “memory space” model o a vast literature describing the precise characteristics of the places cells seems to have concluded that place cells do not exist and function together as a simple topography of external space as was originally suspected.

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 these cells fire in response to the complex interaction of multiple cues and not in response to an internal Cartesian coordinate system.
 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.

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, ( Fig.
4-005) and the intentions related to the movement (e.g., searching for a food reward).
 important role of the hippocampus in non-spatial learning.