Science - 06.03.2020

scale(Fig.1C),yetonlysomeofthesecortical
events were also coupled to ripples in the MTL.
Previous evidence has suggested that successful
memory retrieval involves a ripple-mediated
interaction between the cortex and the MTL
( 18 , 25 ). Although each cortical ripple may re-
flect an underlying burst of spiking activity,
only cortical burst events that are coupled to
MTL ripples may be preferentially relevant for
memory retrieval (Fig. 4A). We therefore hy-
pothesized that sequences of spiking activity
that occurred in association with MTL ripples
during retrieval were more similar to encod-
ingsequencesthanburstsofspikingthatwere
not coupled with the MTL. In correct retrieval
trials, we found several examples in which burst
events associated with MTL ripples exhibited
higher average sequence similarity to the en-
coding period than those burst events that oc-
curred in the absence of a MTL ripple (Fig. 4B).
We explicitly quantified replay in the cortex
triggered to the onset of MTL ripples and found
that maximal replay occurred ~100 ms after
MTL ripple onset (significant replay compared
with jittered ~100 to 175 ms) (Fig. 4C). We there-
fore used this temporal relationship to designate
every cortical burst event as coupled or uncou-
pled to a MTL ripple (supplementary materials)
andexaminedthereplaycontentofeachtype
of cortical burst event. In correct retrieval trials
across all participants, burst events coupled to
MTL ripples demonstrated significantly greater
replay of the sequences present during encoding
compared with uncoupled events [n= 6 par-
ticipants,t(5) = 2.85,P= 0.036] (Fig. 4D).
Together, our data demonstrate that ripple
oscillations reflect bursts of spiking activity
in the human cortex and that these bursts
contain item-specific sequences of single-unit

spiking that are established during memory

encoding and replayed during memory retrieval.

Cortical spike replay is enhanced during retrieval

when bursts of spiking in the cortex are coupled

with ripples in the MTL. Our data therefore

suggest that successful memory encoding and

retrievalofindividualitemsinvolvesaspecific

temporal ordering of cortical spiking activity

and provide a link between previous evidence

regarding the role of ripple oscillations in hu-

man memory ( 21 – 26 ) and evidence regarding

the replay of neuronal sequences observed in

rodents ( 7 – 14 ). Studies of spike replay in ro-

dents have largely focused on sequences of

spiking activity that emerge during spatial

navigation and that are replayed during sleep

or during awake periods of rest. These data

have inspired models of memory that posit that

memory formation involves an initial encoding

state, in which the temporal order of spiking

activity is established through sequential expe-

rience, and a subsequent consolidation state,

during which these sequences of spiking ac-

tivity are replayed in a temporally compressed

mannerintheMTL( 17 , 30 ). Our task requires

participants to memorize abstract associations

betweenwordpairsandtoretrievethoseas-

sociations following only a brief distractor pe-

riod. Hence, our data provide direct evidence

that awake human memory retrieval involves

replay of sequences of spiking activity in the

cortex. Whether such cortical spike replay

plays a similar role in longer-term memory

consolidationinthehumanbrainremains

an open question. Moreover, whereas tem-

poral compression and reverse replay may be

common features of spike replay observed in

rodents, their absence here may be related to

our task involving neither sequential experi-

encenor reward. Instead, our data suggest

that spiking sequences may represent indi-

vidual concepts in the human brain through

a temporally stereotyped activation of a neural

ensemble that emerges during the initial ex-

perience of an event and that is replayed when

its memory is retrieved.

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ACKNOWLEDGMENTS

We thank N. Brunel, A. Sanzeni, A. Zadeh, L. Bachschmid-Romano,

V. Sreekumar, J. Chapeton, and Z. Xie for helpful and insightful

comments on the manuscript. We are indebted to all patients who have

selflessly volunteered their time to participate in this study.Funding:

This work was supported by the Intramural Research Program of the

National Institute for Neurological Disorders and Stroke (NINDS).

This work was also supported by National Institute of General Medical

Sciences grant T32 GM007171 and NINDS grant F31 NS113400 to

A.P.V.Author contributions:A.P.V. and K.A.Z. conceptualized the

study; A.P.V. performed all data analysis, software development, and

visualization; A.P.V., J.H.W., S.K.I., and K.A.Z. curated the data; A.P.V.

and K.A.Z. wrote the original draft; K.A.Z. supervised the study; and

A.P.V., J.H.W., S.K.I., and K.A.Z. reviewed and edited the final

manuscript.Competing interests:The authors declare no competing

interests.Data and materials availability:The data that support

the findings of this study are available for public download at

https://neuroscience.nih.gov/ninds/zaghloul/downloads.html.

SUPPLEMENTARY MATERIALS

science.sciencemag.org/content/367/6482/1131/suppl/DC1

Materials and Methods

Supplementary Text

Figs. S1 to S27

Table S1

References ( 31 – 37 )

View/request a protocol for this paper fromBio-protocol.

1 November 2019; accepted 5 February 2020

10.1126/science.aba0672

Vazet al.,Science 367 , 1131–1134 (2020) 6 March 2020 4of4

A B ***

0

.1

MI

MTL

MTG

-2 -1 0

Time to Retrieval (s)

Units

20

40

60

80

100

120

-250 250

MTL Ripple Onset (ms)

MTL C

D

Uncoupled Coupled

Uncoupled Coupled

0

Cortical Replay

MI (Z)

0

1

Cortical Replay

MI (Z)

0

.5

-.5

TrueJittered

Temp Cortex

*

.5

Fig. 4. MTL ripples precede cortical sequence replay.(A) Schematic of proposed mechanism for se-
quence replay in the cortex. Units are not sequentially organized when cortical bursts occur in isolation
(left), whereas sequence replay occurs when MTL ripplesoccur in coordination with cortical bursts (right).
(B) Example of dynamic coupling between MTL and cortical ripples (top), bursting events (middle), and sequence
replay during successful memory retrieval (bottom). (C) Average cortical replay values relative to onset of MTL
ripples in correct retrieval trials (blue), and the same data when MTL ripple temporal indices were jittered
randomly (gray). Error bars represent SEM across all participants (**P< 0.001, permutation test). (D)Sequence
replay during retrieval after dividing all sequences into those that were uncoupled and those that were coupled
to MTL ripples. Each line indicates a single participant, and bars indicate SEM across all participants. Coupled
sequences exhibited higher replay values than those of uncoupled sequences (P<0.05).

RESEARCH | REPORT