Something to Consider • 195disagreement regarding whether the hippocampus is important only at the beginning of
consolidation, as depicted in Figure 7.22, or the hippocampus continues to be impor-
tant, even for remote memories. According to the multiple trace hypothesis, the hip-
pocampus is involved in retrieval of remote memories, especially episodic memories
(Nadel & Moskovitch, 1997). Evidence for this idea comes from experiments like one
by Asaf Gilboa and coworkers (2004), who elicited recent and remote episodic memo-
ries by showing participants photographs of themselves engaging in various activities
that were taken at times ranging from very recently to when they were 5 years old. The
results of this experiment showed that the hippocampus was activated during retrieval
of both recent and remote memories.
The fact that there is evidence supporting both the standard model of consolidation
and the multiple trace hypothesis has led to a great deal of discussion among memory
researchers regarding whether or not the hippocampus is involved in remote memories
(Jadhav & Frank, 2009; Moscovitch et al., 2005). One thing that can be stated, how-
ever, is that memories are not simply “stamped in.” They involve changes at the synapse
and a consolidation process involving both the hippocampus and the cortex. Although
there is no question that consolidation makes memories more stable, recent research,
described next, opens the possibility that even memories that are consolidated can be
modifi ed or eliminated.Something to Consider
Are Memories Ever “Permanent”?
“These are exciting times in memory research. What once seemed simple and settled
now seems complex and open to new ideas” (Nadel & Land, 2000). The “simple and
settled” part of memory research that Lynn Nadel and Cantey Land are referring to are
the following two ideas, which we have just discussed:- Memory is initially fragile, so a disrupting event that occurs shortly after a memory
is formed can disrupt formation of the memory. - Once consolidation has occurred, then the same disrupting event cannot affect the
memory.
New experiments have caused many memory researchers to question the idea that
once memory is consolidated, it cannot be disrupted (Lewis & Maher, 1965; Sara &
Hars, 2006). These experiments have led to the proposal of a process called recon-
solidation that can occur after a memory is initially consolidated. This process, which
occurs after a memory has been retrieved, was demonstrated in the rat by Karim Nader
and coworkers (2000a). Nader used classical conditioning (see Chapter 6, page 165) on
a rat to create a fear response of “freezing”(not moving) to presentation of a tone. This
was achieved by pairing the tone with a shock. Although the tone initially caused no
response, pairing it with the shock caused the tone to take on properties of the shock,
so the rat froze in place when the tone was presented alone.
The design of the experiment is shown in ● Figure 7.23. In each condition, the rat
receives a tone-shock pairing and is injected with anisomycin, an antibiotic that inhib-
its protein synthesis and so prevents changes at the synapse that are responsible for
the formation of new memories. The key to this experiment is when the anisomycin is
injected. The fi rst two conditions indicate the effect of normal memory consolidation
that we have described.
In Condition 1, the rat receives the pairing of the tone and shock on Day 1. It
receives anisomycin on Day 2, and then freezes to the tone when tested on Day 3
(Figure 7.23a). This is exactly what we would expect, because conditioning occurs on
Day 1 and the drug isn’t injected until Day 2, after consolidation has occurred, so the
rat still fears the tone on Day 3.Copyright 2011 Cengage Learning. All Rights Reserved. May not be copied, scanned, or duplicated, in whole or in part.