learning process, by administering mild electric shocks to the siphon skin of
the sea-hare, led to the identification of so-calledmodulatory interneuronsthat
participate in sensitization. When sensory neurons were activated by the
shock, they excited modulatory interneurons, which in turn increased the
release of neurotransmitters in the synapses of other sensory neurons. As a
result, the animal started to withdraw its gill (breathing organ) intensely even
after touching the siphon only mildly with a paintbrush. Depending on the
number of shocks, sensitization can last for a couple of minutes (short-term
memory) or a few days (long-term memory). Whereas the neuronal mechan-
ism described above is responsible for short-term memory, the formation of
long-term memories involves complex chemical changes in the synapses
(long-term potentiation), the growth of existing synapses and the addition of
new synapses by the activation of genes, processes that have been described
down to the basic molecular level (Squire & Kandel, 1999, pp. 129–55;
Heidelberger et al., 2014; Kalat, 2016, pp. 412–19). Unfortunately, we cannot
study declarative memory in the sea-hare, because it involves processes that
are unique to the mammalian brain.
As we have seen in the case of H.M., the hippocampus plays a crucial role in
the formation of long-term, declarative memories. The hippocampus has
connections to various parts of the brain. Different aspects of new information
(such as visual, spatial, or emotional stimuli) are processed by separate brain
networks, are partly combined in the so-called association areas, and after
further synthesis in the parahippocampal region theyfinally reach the hippo-
campus (Eichenbaum, 2012, pp. 235–42). The chemical process of long-term
potentiation in the synapses between the neurons of the hippocampus creates
a time window of several hours, during which different aspects of memory can
be linked to each other (Squire & Kandel, 1999, pp. 109–38; Eichenbaum,
2012, pp. 51–78). Cells in the hippocampus probably also connect episodes
into sequences as well as creating connections between similar episodes of
separate chains of events (Eichenbaum, 2012, pp. 162–4). There are additional
connections that influence the formation of memories in the hippocampus.
Through connections from the prefrontal cortex, willful attention modulates
what we remember (an everyday example being concentration on something
we want to learn) (Eichenbaum, 2012, pp. 239–40). The amygdala, which plays
a major role in emotions, sends signals to the hippocampus that increase
the strength of some memories (Eichenbaum, 2012, pp. 319–25). With the
help of connections from the hippocampus back to the cortex, memory is
ultimately stored in the very brain parts that process respective information
(Eichenbaum, 2012, pp. 197–217). However, the hippocampus remains crucial
for the consolidation of memories for several years—as demonstrated by
H.M.’s memory loss for events in the years preceding his surgery. Memories
remain essentially malleable during the period of consolidation (Eichenbaum,
2012, pp. 317–50).
64 Cognitive Science and the New Testament