Are Place-to-Memory Associations a 2-way Street?

two way narrowPeople who study memory are familiar with the concept of a Memory Palace, a memory scheme for storing information in remembered places. The Memory Palace story is beautifully described in Joshua Foer’s book Moonwalking With Einstein (although it wasn’t clear to me if Einstein was walking on the moon or walking with Michael Jackson). For millennia, mnenonic super stars have memorized gargantuan lists by associating each list item with a location in a virtual home or palace. During learning, the mnemonicist imagines walking through the palace and placing each item in a particular location. At the time of recall, the memorizer imagines walking through the memory palace and, as if by magic, seeing each item where it had been placed. According to this notion, the ability to associate places to content is natural and fairly easy to master. So easy that Joshua Foer, who had no obvious memory potential, became a national memory champion by mastering the technique. I find the accounts impressive and extremely convincing, although I’ve failed miserably when trying to do this. Overall, the Memory Palace phenomenon suggests a strong natural link between location and memory. Some have taken this linkage to suggest that place serves as in indexing system for memory — a kind of lookup table. A point I’ll return to.

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Optical Recording of Maps in the Hippocampus

(Updated with comparison to Mankin et al Feb 20; in red text)

“Long-term dynamics of CA1 Hippocampal Place Codes” is an important study published in the current issue of Nature Neuroscience. The paper is noteworthy in terms of technical achievement, theoretical implications and the potential for future work

headstage

Headstage on mouse brain. Ziv et al fig 1a.

First, the technical achievement. Ziv et al, working in Mark Schnitzer’s group at Stanford have managed to record the firing activity of hundreds of hippocampal neurons over a period of approximately one month. They do this by permanently mounting a miniature camera on the mouse’s head and using Ca+ imaging to identify the firing pattern of each neuron. Continue reading

Are Brains Egocentric?

Hippocampal Representations

Hippocampal Representations
egocentric? allocentric?

The term “egocentric” has nothing to do with Freud or selfishness. It’s a geometric term meaning that part of the self is the center of the spatial coordinate frame (“ego” = self). The contrasting term, “allocentric” means something other than the self is the center of the coordinate frame (“allo”= other). A comparison has helped me: think of geocentric and heliocentric models of the solar system.

The question I’m going to address is whether our brains, our perception of the world, our behavior, and our consciousness operate in egocentric or allocentric coordinate frames.

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David Marr was brilliant, but wrong

David Marr was a brilliant Neuroscientist who died too young, in 1980, at the age of 35. Marr’s work was theoretical — he was at the leading edge of a computational wave.* Marr’s contributions spanned many areas of Neuroscience: cerebellum, hippocampus, and especially vision. Marr is also well known for proposing that brain/behavior function should be approached in three phases that are largely sequential:

  1. The computational level: what is the problem that confronts the animal?
  2. The algorithmic level: How is it logically solved? (including shortcuts)
  3. The implementation level: How does the brain do it?

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Modular Organization of Grid Cells, Part 2

navigation with grid cells

A fanciful depiction of how a grid cell based navigation system could compute a direct path across unvisited regions of space.

A week ago Stensola et al published evidence that Entorhinal Grid Cells are modular, and on the same day I wrote a glowing commentary, The Significance of the Modular Organization of Grid Cells. In addition to praising the paper, I tried to explain why evidence for modular organization was welcome news, in that it supported computational mechanisms that grid cells could perform. In the rest of this post I outline a model Andre Fenton and I have been working on that relies on discrete modules of grid cells. Our model extracts a function we call linear look-ahead and uses this function for efficient navigation. We feel it represents the begining of a process of explaining high order cognitive functions at the neuron network level Continue reading

The Significance of the Modular Organization of Entorhinal Grid Cells

Enthusiastic and Excited response to the publication of “The entorhinal grid map is discretized” (Stensola, et al, Nature, 492, 72-78; 2012)

Background

The hippocampal formation is an amazing place, populated by strange characters called place cells, head-direction cells and grid cells. Hippocampal place cells exhibit “location-specific firing” (see figure). A single place cell willtwo grid cell2 “fire” only when the rat crosses a restricted region of space. The figure below is an overhead-view map of the firing of a single place cell averaged over a 16-minute recording session (the animal was in a cylindrical enclosure; that’s why the map is round). John O;Keefe, followed by many others, has suggested that the collective firing of hippocampal place cells forms the rat’s “cognitive map”, and permits efficient navigation.

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