memory. For example, H.M. was not able to remember what happened to
him justfifteen minutes ago due to a deficiency in encoding information.
The tip-of-the-tongue phenomenon is connected to a momentary difficulty
with retrieval. Due to neural damagesthroughout their cortex, Alzheimer
patients have deficiencies in all three stages (Squire & Kandel, 1999, pp. 206–11;
Kalat, 2016, pp. 404–6). H.M.’s example also teaches us about another trad-
itional distinction in memory studies: some memories last only for a short
time, up to a few minutes (such as H.M.’s memories of newly learned
numbers), whereas others last for a long time (for hours, days, or years). The
former is calledshort-term memory, the latterlong-term memory(Squire &
Kandel, 1999, pp. 84–91; Baddeley et al., 2015, pp. 12–13).^2 The hippocam-
pus plays a crucial role in the consolidation of long-term memories, as
research inspired by H.M.’s case demonstrated. Finally, two types of memories
can be distinguished with regard to what is remembered (Squire & Kandel,
1999, pp. 15–16; Eichenbaum, 2012, pp. 98–106). So-calleddeclarativemem-
ories include memories of facts, objects, faces, or events and are available to
unmediated, conscious reflection. Another type of memory includes attitudes,
skills, and emotions of which we are mostly unaware—callednon-declarative
memory. H.M. was able to make new non-declarative memories (e.g., he could
learn new drawing skills), but failed to make any long-term, declarative
memories.^3
The nervous system is an assembly of different types of nerve cells, also
called neurons, most of which follow the same schematic design. In section 3.2
above we have discussed the structure and functioning of the neuron. In a
biological sense, memories are changes in the neurons, mainly occurring in the
synapses. All organisms with a nervous system store memories in rather
similar ways. This is a fortunate situation, as observations about simple
organisms also reveal the mechanism of memorization in animals with vast
nervous systems, like humans. Eric Kandel studied learning in the sea-hare,
which has only twenty thousand nerve cells in total—in contrast to the human
brain that contains a hundred billion nerve cells (Squire & Kandel, 1999,
pp. 36–64; Byrne et al., 2014).Sensitizationis a simple form of learning that
influences subsequent behavior: for example, after hearing a gunshot, we
will be more likely to jump at innocent noises. Study of this elementary
(^2) The use of the terms“short-”and“long-term memory”varies somewhat according to
different underlying theories. For example, Squire & Kandel (1999, p. 131) use the term
“immediate memory”for the momentary stream of memories (see the discussion ofworking
memorybelow). They use“short-term memory”for the period ofconsolidationof memories—
which in other authors’terminology is already part of long-term memory processes.
(^3) There is a related distinction betweenexplicitandimplicitmemories, the former being
conscious and the latter unconscious (Baddeley et al., 2015, pp. 13–15). However, there is some
ambiguity whether this concerns encoding (Kalat, 2016, pp. 400–1) or retrieval (Eysenck &
Keane, 2005, pp. 230–3).
Memory and Transmission 63