Cognitive Aspects: How People Perceive, Memorize, Think and Talk About Landmarks - Landmarks: GIScience for Intelligent Services - page 60

the vestibular system, but then, similar to place cells, with drift over time. Head
direction cells are found in many regions of the brain, including the postsubicular
cortex.
Grid cells [ 59 ] seem to code location in a regular tessellation, similar to an
internalized coordinate system. Since the tessellation covers space, firing patterns
are sufficient to decode location in space [ 74 ] . Grid cells are found in the medial
entorhinal cortex, an informational input into the hippocampus. There must be a
mapping of locations identified by firing of the grid cells and locations identified
primarily from path integration in the place cells, and further associations between
place and events to form memories (ibid.).
Another pathway into the hippocampus deals with object recognition establishing
some contextual information about location. With contextual, especially visual
information provided, the brain is capable of allocentric spatial reasoning, probably
in the posterior parahippocampal cortex. Ekstrom et al. [ 49 ] , for example, found
cells in the hippocampus, parahippocampal cortex and other areas that were
responding to location, but in addition also cells that fired dependent on the visual
external cues the person viewed:
We present evidence for a neural code of human spatial navigation based on cells that
respond at specific spatial locations and cells that respond to views of landmarks. The for-
mer are present primarily in the hippocampus, and the latter in the parahippocampal region.
Cells throughout the frontal and temporal lobes responded to the subjects' navigational
goals and to conjunctions of place, goal and view (p. 184).
If grid cells form an internalized coordinate system their resolution and number
becomes interesting. The resolution determines the smallest variation in location
that can be distinguished in firing patterns, and their number determines the size of
the environment that can be represented. Just as a technical equivalent: If the surface
of the Earth should be represented in a tessellation of square elements of 1 m edge
length it needs 40;000;000 2 or 1:6 10 15 elements. It is estimated that the human
brain overall has about 100 billion neurons, thus, human spatial memory must be
differently organized to remember things such as where the keys have been left or
how to travel to Sydney. It needs hierarchic representations. In fact grid cells show
properties that establish multi-resolution memory. First, different areas of similar-
sized grid cells represent the same environment, but with a random offset of their
grids. By nesting, even a small number of neurons can represent a fine level of
granularity [ 139 ] . Secondly, the scale of grid cells varies along the entorhinal cortex.
For rats, grid cells have been found representing distances of about 25 cm in their
dorsal-most sites to about 3 m at the ventral-most sites [ 16 ] . And finally, it has been
shown that when certain channels are knocked off the brain produces a coarser scale
spatial memory [ 65 ] .
In addition to a representation of location the brain shows two additional types of
spatial memory. One is episodic. Sequences of place cells form a route memory that
can be imagined and mentally travelled at ground perspective. Burgess et al. [ 18 ]
put it more precisely: