I whipped this up this afternoon in case any of your are interested. I tried to gear it towards functionally relevant features. Enjoy
Reference document: The Hippocampal navigational system by Brad Wyble A primer of neurophysiological correlates of spatial navigation in the rodent hippocampus. Why AI enthusiasts should care: The place system is a unique way to study what the rat is "thinking" and how it uses information to compute. Place cells represent a particular way that the rodent brain analyzes spatial location in a way that is cognitively accessible to us. The behavior of place cells are relatively homogenous across the population. Contrast this to recordings from frontal cortex in which cellular activity is extremely varied. Frontal cortical cells are doing very interesting things with respect to behavior, but they are very different from one another which makes it practically impossible to draw conclusions. The structural and functional simplicity of the hippocampus makes it a gateway to understanding the brain, a strong foothold for our first significant steps. It is largely a happy accident that the place system is so easy to study. The hippocampus is arranged in horizontal sheet of very dense cell near the surface of the skull (in rodents at least) which means that it is possible to get yields of 200+ cells *simulteanously* within one rat using current technology, a feat achievable in no other brain area by at least an order of magnitude. This high cell yield allows us to study the behavior of the entire system in the same way the Nielson system studies the television viewing habits of the entire country using data from a tiny fraction of homes. Place Cell The "place cell" was described in the 70's by a group led by John O'Keefe (O'Keefe, 1976) and is the foundation for our understanding of the rodent hippocampal navigational system. A place is a hippocampal neuron(pyramidal, complex spike cell of CA1/CA3/Dentate) that will fire reliably and selectively within a small region of space. The firing pattern for that region of space (usually about 10cm in diameter but varies with the size/shape of the environment) is roughly a 2-dimensional bell-curve if the cell spikes are compiled into a histogram with respect to 2-d location. This region of space is called a "place field" and is defined with respect to a specific neuron in a specific environment. The particular configuration of place fields across all place cells for a given environment is called a "place map". One neuron can have multiple place fields within a single environment, but this is rare. Environment: The concept of what constitutes an enviornment is vital to this discussion. For experimental purposes, a rat is introduced into a chamber it has never seen before. A seemingly arbitrary place map develops over 10-15 minutes. This same rat placed in a different environment will immediately develop an entirely different and *uncorrelated* place map for the new environment. There is alot of complexity in figuring out what constitutes a "different" environment. Alterations in the geometric shape (square-> circle) almost always generate a new map. Variations in visual cues will sometimes cause a remapping, sometimes not. Rats remap in all or none fashion. That is to say, as visual cues are altered, the map will stay constant until some arbitrary threshold in passed, at which point the entire population will remap. Map alterations do not cause gradual shifts in the field. Extreme alterations in behavior can cause a remapping. If a rat is trained to do two different tasks(targetted vs random foraging) in the same environment, it will usually develop two different place maps and switches between them based on the task. Sensory Cue control. In environments for which multiple orientations are available (square, cylinder), the place map will align itself with the most obvious visual cues. If a cylinder has a cue card on the wall, and the card is shifted, the place map will follow the card. The place map is largely immune to the removal of cues. If the lights are turned off and no visual cues are available, the rat will continue to use the same place map. It uses a combination of vestibular and kinesthetic cues to integrate its motion, and keep mental track of its position (as evidenced by the preserved functionality of the place map and behavior). It can use olfactory(excrement) and tactile cues to correct for drift error in the path integration process. This is demonstrated by using environments that allow for no olfactory cues by wiping the environment with alcohol and turning off the lights. The place map is stable with respect to the cylinder walls, but drifts in orientation over time because there ! are no olfactory cues to control for rotational drift, while contact with the walls controls for radial drift. Generally visual cues override olfactory and tactile cues when they are in conflict, but in the absence of vision, rats will use other senses without interruption. A system that uses hippocampal output to control behavior should be able to function despite such cue deletions, as the place map isn't altered when the lights are turned off suddenly. It is interesting to note that the traditional rodent behavior of defecating and urinating at random spots in a novel environment is essentially laying down a gridwork of cues to use for navigational aids. Behavioral relevance. Rat behavior follows the place map, which indicates that the place map is a cognitively relevant part of the rat's behavior. If a rat is trained to prefer a given location in a cylindrical environment and multiple cues are shifted so as to create a contradiction, the place map will pick one interpretation and rotate with it. Behavior will mirror the rotational orientations of the place map. Directionality: In a task in which rats forage randomly for food, place cells are not directional, meaning they fire similarly for all directions of traversal through the field. For tasks which involve running along narrow tracks or paths, place cells are usually uni-directional, meaning they only fire when the rat is traveling in one direction. Redundancy: Approximately 1/3 of the cells within the dorsal area of the hippocampus have place fields within a particular environment. This number is in the 10's of thousands of cells. With place fields of 10 cm diameter in small cylindrical chambers(4-6 feet diameter), obviously some of the place fields overlap heavily. The place code is heavily redundant. A given hippocampal neuron has a 30% chance of having a field in any given environment. A neuron will, usually, have fields in multiple environments. It is extremely important to note that these fields are *not related to one another*. A neuron with a place field in a corner of a rectangular box might have a place field in the center of a different rectangular box. The place fields for a given neuron have no discernable geometric or cue-based preference. According to all possible measurements, the distribution of place fields for a single neuron across a set of environments follows no measureably consistent pattern or tendency. Stability Over Time: Place cells are stable over a long period of time. Within a given environment that has strong visual cues, place cells have been found to be stable for months. That is to say, a given cell fires at a specific spot in the environment and will continue to do so for months afterwards, even if the rat has gone days or weeks without having seen that environment. (note: this is a hard experiment to do, because it requires recording from the same cell for a long period of time. If the electrode or brain drifts on the order of 50 microns, the electrode will lose the cells it was recording and pick up new ones. It's been demonstrated only once or twice) How does a place map form? Noone knows. The hippocampus gets place-like input from the entorhinal cortex, but difficulties from recording from non-hippocampal structures limit the rate at which we can study them. This place-like input seems to heavily reliant on geometry and cues. That is, place cells in the entorhinal cortex are *not* randomly distributed with respect to geometry. A corner cell in one environment will be a corner cell in another environment. The unique aspect of the hippocampus seems to be creating randomly distributed and unique place maps for each environment, essentially a new context for each particular environment. This context doesn't change much over time and is heavily resistant to alteration of the environment, preferring to remap entirely rather than shifting. Where does the place map project? The hippocampus projects to most of the cortical mantle through a series of projections, so basically anywhere. Creating uncorrelated maps from very similar environments seems difficult, how is it done: Probably some sort of system by which the first neuron to fire for any particular combination of cues gets synaptic strengthening, meaning that it will continue to fire for that particular field in the future. How that neuron is prevented from firing at the same spot in a very similar environment with just a few alterations that cause it to remap is a bit puzzling. The map is controlled by visual cues, but yet, the concept of spatial context can always override visual similarity. Why is it done? I view the hippocampus as a paint mixer. You pour in data, shake it up and see what correlations fall out. The place map system could be a substrate on which these correlations are found and stored, within the rat. The rat's entire world is laid out with respect to location and they are possibly better at learning and navigating mazes from a first person perspective than people are. It should not be surprising that they devote a significant amount of brain power to knowing where they are. A similar system for creating contexts may exist in our hippocampus, although it is probably not as tightly tied to spatial location, but rather, the semantic and knowledge based context of a particular area of thought. So when thinking about neuroscience, I might adopt one "concept map", and when switching over to sports, an entirely different one. If the place map analogy holds, the same neuron might fire strongly to the unrelated concepts of dendrites and Tiger Woods. We probably also have place cells. Does the hippocampus do anything else? Yes. These recordings are from the dorsal half of the hippocampus. The ventral half is less well understand because it is harder to get electrodes into it. What records exist show that this part of the hippocampus may have larger, possibly goal-oriented, place fields. Also, there are many details and exceptions glossed over by the above descriptions. Most place cell experiments happen in environments with very simple behavioral contingencies. In environments in which rats are required to perform specific tasks, the place fields adopt secondary firing correlates. ie: fire in this spot only when the cue light is on, or when strawberry is coming from the odor port. I've gone over the basic details, and avoided the controversial topics. The above points are concrete descriptions of underlying tendencies, to which there are the inevitable exceptions when studying natural systems. In any lab, you are sure to find people that disagree with some of them. There are many other types of correlates within the hippocampus. The golden rule of the hippocampus is "if you are looking for a particular behavioral correlate, you will find it", attributed, I believe, to Jim Ranck, a well known researcher. (1)Place units in the hippocampus of the freely moving rat. O'Keefe J. Exp Neurol 1976 Apr;51(1):78-109 ------- To unsubscribe, change your address, or temporarily deactivate your subscription, please go to http://v2.listbox.com/member/[EMAIL PROTECTED]
