Neural Correlates for Spatial Cognition in Virtual Environments

Aims

Single cell recording in the hippocampus of the rat

Single cells in the awake rat are recorded via chronically implanted electrodes.
Since the seventies it has been shown that neurons in the hippocampus encode spatial information
and fire preferentially when the animal is located in a particular part of the environment (so- called place cells).

What information do place cells encode?

It has been shown that place cells are guided by visual stimuli and also encode vestibular information.

Another source of information is the motor system, eg. The basal ganglia and motor cortex.
However, little research has been dedicated to investigate the importance of contributions from these inputs in detail.

To study the contributions of visual, vestibular and motor information to place cell development in detail,
we are in the process of developing a virtual reality set-up that will enable us to study the orientation of rats
in large environments. The visual stimulus is adapted to the large visual field of rats,
can easily be manipulated and is well controlled (as is the motor activity).

Tetrode recording in the rat

Tetrodes of 4 electrodes each are implanted in the hippocampus. By making use of the tetrode spike analysis
technique, single neurons are identified according to spike profile components.

Spike analysis and sorting (shown are spikes of one tetrode). Spikes are sorted according to principal components (left).
The result of the spike sorting is shown in the right half (summary of all traces and trace average for each channel).

Example of a spatially active neuron (place cell)

These figures show the firing frequency of a single neuron during exploration of a radial arm maze.

Left: Each spike is shown as a blue square, the actual track of the rat is shown in black within the
delineations of the radial arm maze.
Right: Highest firing rate is shown in red. The activity is confined to a smallarea of the maze.
Firing rate is normalised for time spent in each location.

A novel virtual reality set-up for rats

The apparatus consists of an air supported hollow styrofoam sphere of 50 cm diameter on top of which the rat is held in position.
Any locomotion of the animal results in a rotation of the sphere. The rotation is fed back into the projection system which
alters the environmental projection onto the cirular screen accordingly.

Drawing of the set-up

This virtual reality apparatus is under construction by Dr. H. Dahmen.

Photo of the set-up

Left: Visible is the Styrofoam sphere with the motion detectors (red) and the compressed air supply hose to keep the ball afloat.
At the top the projector with the mirror system is seen, along with the projection screen (white torus). The top part will be
lowered during the experiments.
Right: The actual position of the projection screen during an experiment

Scientific questions to be addressed

- What visual information contributes to the orientation of rats in space and to place cell formation? (eg. optic flow, salience of landmarks)

- What is the role of motor activity in spatial orientation and place cell development?

- Does vestibular information play any role for orientation of rats and place cell formation?

- How stable are place fields in rapidly changing environments?

- Are place cells unique for individual environments or are they re-used if the rat is presented with a large number of different environments?

References

Thiele J (2006): Do Rats Use Optical Flow for Motion Control ? In: Bülthoff et al. (eds.), Proceedings of the 9^th Tübingen Perception Conference. p 165.

Hölscher C, Schnee A, Dahmen H, Setia L, Mallot HA(2005):Rats are able to navigate in virtual environments. Journal of Experimental Biology 208, 561-569

Hölscher C. (2001): Long-term potentiation induced by stimulation on the positive phase of theta rhythm: a better model for learning and memory?
In: Neuronal mechanisms of memory formation. Cambridge University Press, pp. 146-167.

Hölscher C, Bliss, TVP, RichterLevin, G (2001): Conclusions and future targets in memory research. In: Neuronal mechanisms of memory formation.
Cambridge University Press, pp. 476-491.

Dahmen, H. (1980) A simple apparatus to investigate the orientation of walking insects. Experimentia 36, 685-686.