Memory - Computational Explorations in Cognitive Neuroscience - page 282

Dentate
Gyrus

Entorhinal
Cortex

CA3 CA1

Subiculum

Figure 9.8: Schematic diagram of the principal areas and
patterns of connectivity of the hippocampal formation.

Rat Model

Area Neurons Pct Act Units Pct Act

EC 200,000 7.0 144 25.0

DG 1,000,000 0.5 625 1.0

CA3 160,000 2.5 240 5.0

CA1 250,000 2.5 384 9.4

Tab le 9 . 1 : Rough estimates of the size of various hippocam-
pal areas and their expected activity levels in the rat (data from
Squire et al., 1989; Boss et al., 1987; Boss et al., 1985; Barnes
et al., 1990). Also shown are corresponding values for the

model.

columnar structure, and there are topographic projec-
tions to and from the different cortical areas (Ikeda,
Mori, Oka, & Watanabe, 1989; Suzuki, 1996). The per-
forant path projections from EC to DG and CA3 are
broad and diffuse, but the projection between the DG
and CA3, known as the mossy fiber pathway, is sparse,
focused, and topographic. Each CA3 neuron receives
only around 52-87 synapses from the mossy fiber pro-
jection in the rat, but each synapse is widely believed to
be significantly stronger than the perforant path inputs
to CA3.
The lateral (recurrent) projections within the CA3
project widely throughout the CA3, and a given CA3
neuron will receive from a large number of inputs sam-
pled from the entire CA3 population. Similarly, the
Schaffer collaterals, which go from the CA3 to the CA1,
are diffuse and widespread, connecting a wide range of
CA3 to CA1. Finally, the interconnectivity between the
EC and CA1 is relatively point-to-point, not diffuse like
the projections from EC to DG and CA3

Figure 9.7: Anatomical structure of the hippocampal forma-
tion. Reproduced from Shepherd (1990).

inhibitory neurons to regulate activity levels in the hip-
pocampal system plays an essential role in our model,
as we will see. Also, the same kinds of learning mecha-
nisms are probably at work in the cortex and hippocam-
pus (indeed, the hippocampus is where most of the re-
search on synaptic modification has been done).
The rough sizes and activity levels of the hippocam-
pal layers in the rat are shown in table 9.1. Note that
the DG seems to have an unusually sparse level of ac-
tivity (and is also roughly 4-6 times larger than other
layers), but CA3 and CA1 are also less active than the
EC input/output layer.
A fair amount is known about the detailed patterns of
connectivity within the hippocampal areas (e.g., Squire
et al., 1989).

(Tamamaki,

1991).
Physiological recordings of neural activity in the hip-
pocampus reveal some very important properties of the
representations in different hippocampal areas. For ex-

Starting with the input, the EC has a