Zicong Zhang

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• Zicong Zhang
• 2011.08.15

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Authors

• Wendy A. Suzuki
• Professor of Neural Science and Psychology,
• New York University
• Research interest Organization of memory in the
  medial temporal lobe and the effects of exercise
  on learning, memory and cognition.
• Yuji Naya
• Post-Doc fellow, associate faculty member in
  Suzukis Lab. He is best known for the
  groundbreaking work he did in collaboration with
  Yasushi Miyashita on the representation of
  well-learned associations within the perirhinal
  cortex.

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Backgrounds

• The human medial temporal lobe (MTL) is critical
  for episodic memory presumably because of its
  role in binding individual stimuli or events to
  their temporal and spatial contexts.
• Computation models
• Cortical association areas signal
  information about items
• Parahippocampal regions signal
  information about items along with their temporal
  context
• Hippocampus (HPC) supervises these
  item-context associations.

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Backgrounds

• fMRI studies in humans report both HPC and
  parahippocampal activation during tasks of
  temporal-order memory.
• Recent neurophysiological studies in the rodent
  have highlighted the role of HPC in signaling
  either a particular time within a trial or
  incremental timing across the entire recording
  session.

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Questions and Ideas

• Little is known about the neurophysiological
  basis of how item and timing information is
  integrated within MTL.
• In this study, neural activities from MTL areas
  and a control visual area were recorded as
  nonhuman primates performed a temporal-order
  memory task that required encoding of two visual
  items and their temporal order.

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Training Design
Cue 1 delay period
Time cells neurons whose responses
differentiated between the cue 1 and cue 2
periods, signaling relative timing between cue
presentations or temporal order of cue
presentations. Item cells neurons that showed
significant stimulus-selective activity during
either cue 1 or cue 2.
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Recordings
Time cells neurons whose responses
differentiated between the cue 1 and cue 2
periods, signaling relative timing between cue
presentations or temporal order of cue
presentations. Item cells neurons that showed
significant stimulus-selective activity during
either cue 1 or cue 2.
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Recordings
Two-dimensional plots of the population average
LFP spectrogram in all areas. The gamma band
activity (gt30 Hz) in HPC increased during the cue
1 delay period and was significantly stronger
during cue 2 than cue 1.
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Question 2

• How did the population of cells signal time
  information?

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Population Vector Analysis backgrounds

• Original Idea to explain how populations of
  motor cortex neurons encode movement direction.
• Basis individual neurons tended to discharge
  more for movements in particular directions, the
  so-called preferred directions for individual
  neurons.
• In the population vector model, individual
  neurons 'vote' for their preferred directions
  using their firing rate. The final vote is
  calculated by vectorial summation of individual
  preferred directions weighted by neuronal rates.
• Population vectors were used to examine
  information represented by a population of
  prefrontal activity and its temporal change
  during spatial working memory processes (Takeda
  and Funahashi, 2004)
• In the present study to characterize the
  temporal dynamics of time cells observed
  throughout the MTL.

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Population Vector Analysis for time cells
Population vectors were constructed from the
responses of all individual time cells for 40-ms
time bins throughout the cue 1 delay period.
Their temporal dynamics were characterized
relative to two template time periods defined
as the cue 1 and the cue 2 states. HPC the
distance from the two templates changed
constantly. ERC only the one from cue 1 state
gradually increase. PRC sudden shifted at the
early period and remained relatively constant.
Normalized distances (NDs) from the cue 1 states
(solid circles) and the cue 2 states (open
squares) in each area.
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Population Vector Analysis for time cells
To determine which area provided the most
accurate representation of the cue 2 state before
the presentation of cue 2, we asked which of the
three areas exhibited the shortest distance to
the cue 2 state during the last quarter (240 ms)
of the cue 1 delay period.
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Summary 1

• As a population, HPC time cells provide an
  incremental timing signal that gives an estimate
  of the relative time from the last cue
  presentation as well as an estimate of the
  relative time to the next cue presentation.

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Question 3

• How did the MTL represent item information?

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Item information represented in MTL
PRC and TE represented the same items across the
two cue periods. A time effect was revealed in
PRC such that the response to the neurons
preferred stimulus differed across cue 1 and cue
2.
Mean discharge rates and SEM during cue 1 and cue
2.
The small correlation coefficients in ERC can be
explained by the lack of stimulus selectivity in
one of the two cue periods. This differential
level of stimulus selectivity between cue periods
can serve to integrate item information with a
relative timing signal.
Distributions of correlation coefficients between
cue 1 and cue 2.
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Question 4

• Whether the PRC item cells integrate item and
  temporal order information by modulating their
  stimulus-selective response properties across the
  cue periods?

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Differential time effect in PRC
C Normalized vector distances between cue 1 and
cue 2
D Normalized mean discharge rates
E Kurtosis measure
Suggesting a more prominent time effect in PRC
The differential time effect was not due to
general changes in firing rates
Suggesting that PRC differentiates between the
cue 1 and cue 2 periods by changing the sharpness
of its stimulus-selective response
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HPC provides a robust incremental timing signal
that may serve to anchor the timing to events
within an episode.
ERC neurons can signal incremental timing
information as well as integrate item and time
information at a lower amplitude.
PRC neurons integrate time and item information
by modulating their stimulus-selective response
properties across temporally distinct stimulus
presentations.
Hypothesis the incremental timing signal in HPC
is conveyed to PRC via ERC, where it is
integrated with item information from TE and
converted into a discrete item-based temporal
order signal.
Hypothesized information flow across MTL regions
and TE.
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References

• Takeda, K. and S. Funahashi, Population vector
  analysis of primate prefrontal activity during
  spatial working memory. Cereb Cortex, 2004.
  14(12) p. 1328-39
• Neural ensemble. From Wikipedia.