194 • CHAPTER 7 Long-Term Memory: Encoding and Retrieval
● FIGURE 7.21 Sequence of events that occur in consolidation.
Connections between the cortex and the hippocampus are initially strong
but weaken as connections within the cortex are established. (Adapted
from Frankland & Bontempi, 2005.)Areas in cortexHippocampusTime
(a) (b) (c)HighLowActivationHippocampus
Cortex(a) Recent
memory(b) Remote
memory● FIGURE 7.22 (a) According to
the standard model of consolidation,
retrieval of recent memories depends
on the hippocampus; cortical
connections have not yet formed.
Thus, for retrieval of recent memories,
hippocampal activation is high and
cortical activation is low. (b) Once
consolidation has occurred, cortical
connections have formed, and the
hippocampus is no longer needed.
Thus, for retrieval of remote memories,
cortical activation is high, and there is
no hippocampal activation.2005). This process involves a reorganization in the nervous system, which occurs at
two levels. Synaptic consolidation occurs at synapses and happens rapidly, over a period
of minutes. The structural changes shown in Figure 7.16 are an example of synaptic
consolidation. Systems consolidation involves the gradual reorganization of circuits
within brain regions and takes place on a longer time scale, lasting weeks, months, or
even years.
Early research, inspired by Hebb’s pioneering
work on the role of the synapse in memory, focused
on synaptic consolidation. More recent research
has focused on systems consolidation, investigating
the role of different brain areas in consolidation.
The case of H.M., who lost his ability to form new
memories after his hippocampus was removed,
indicates the importance of the hippocampus in
consolidation. The hippocampus plays a central
role in the standard model of consolidation.The Standard Model of Consolidation The
graded property of retrograde amnesia, in which
amnesia is worse for experiences that occurred
just before the brain injury, plus other evidence,
led to the proposal of the standard model of con-
solidation. The standard model proposes that
memory retrieval depends on the hippocampus
during consolidation, but that once consolidation
is complete, retrieval no longer depends on the
hippocampus.
● Figure 7.21 shows the steps in the process of
consolidation, as described by the standard model
(Frankland & Bontempi, 2005; Nadel & Moscovitch, 1997). Incoming information
activates a number of areas in the cortex (Figure 7.21a). Activation is distributed
across the cortex because memories typically involve many sensory and cognitive
areas. For example, your memory for last New Year’s Eve could include sights,
sounds, and possibly smells, as well as emotions you were feeling and thoughts
you were thinking at the stroke of midnight. To deal with the fact that the activity
resulting from this experience is distributed across many cortical areas, the cor-
tex communicates with the hippocampus, as indicated by the blue lines in Figure
7.21a. The hippocampus coordinates the activity of the different cortical areas,
which, at this point, are not yet connected in the cortex.
The major mechanism of consolidation is reactivation, a process during
which the hippocampus replays the neural activity associated with a memory.
During reactivation, activity occurs in the network connecting the hippocampus
and the cortex. This activity results in the formation of connections between the
cortical areas (Figure 7.21b). This reactivation process occurs during sleep (see
Chapter 1, page 16, and this chapter, page 188) or during periods of relaxed
wakefulness, and can also be enhanced by conscious rehearsing of a memory
(Frankland & Bontempi, 2005; Huber et al., 2004; Nadel & Moscovitch, 1997;
Peigneux et al., 2004).
Eventually, the cortical connections become strong enough so that the different
sites in the cortex become directly linked, and the hippocampus is no longer neces-
sary (Figure 7.21c). Thus, according to the standard model of consolidation, the
hippocampus is strongly active when memories are fi rst formed (● Figure 7.22a),
but become less active as memories are consolidated, until eventually only cortical
activity is necessary to retrieve remote memories (Figure 7.22b).The Multiple Trace Hypothesis Most researchers accept that both the hippo-
campus and the cortex are involved in consolidation. There is, however, someCopyright 2011 Cengage Learning. All Rights Reserved. May not be copied, scanned, or duplicated, in whole or in part.