Qu PASA The PosteriorAnterior Shift in Aging

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Qué PASA? The Posterior-Anterior Shift in Aging

• Simon W. Davis, Nancy A. Dennis, Sander M.
  Daselaar, Mathias S. Fleck, Roberto Cabeza
• Cerebral Cortex, doi10.1093/cercor/bhm155

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Background

• PASA (posterior-anterior shift in aging)
• an age-related reduction in occipital activity
  coupled with increased frontal activity (Grady,
  1994)
• PASA pattern has found in attention, visual
  perception, visuospatial processing, working
  memory, episodic memory encoding retrieval

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Unfinished issues of PASA

• Validity of PASA
• It may simply reflect difference in task
  difficulty (ic. objective subject measures of
  difficulty), especially in its prefrontal cortex
  (PFC) component.
• Function of PASA
• No study has found PFC activations in older
  adults positively associated with performance and
  negatively associated with occipital activation

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Unfinished issues of PASA-Cont.

• Generalizability of PASA
• PASA effect has been observed for activation,
  but not for deactivation
• ?Past findings showed attenuated deactivations in
  posterior midline cortex in healthy older adults.
• possibility compensating this deficit by
  deactivating anterior midline regions to a
  greater extent than younger adults?

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Back to the current study

• 3 predictions
• 1. PASA will exist even eliminating differences
  in difficulty (the validity of PASA)
• 2. PFC activity will be positively correlated
  with cognitive performance and negatively
  correlated with occipital activity, supporting
  the compensation account (the function of PASA)
• 3. Older adults would show weaker deactivations
  in posterior midline regions but stronger
  deactivations in anterior medial regions (the
  generalizability of PASA)

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Methods

• Participants 12 younger 12 older adults were
  matched by a rank order based on corrected scores
  of ER and VP performance

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Methods-cont.

• Stimuli
• For the episodic retrieval (ER) task
• 240 five-letter words 80 five-letter
  pseudowords
• For the visual perception (VP) task
• 120 rectangles were divided by a jagged line to
  2 areas, making different versions of a
  perceptual stimuli with different difficulty of
  subject performance

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Methods-cont.

• Behavioral paradigm
• Prior to scanning
• Showing intermixed real words and pseudowords
• During scanning
• 4 runs of the ER 2 runs of the VP task were
  performed
• accuracy confidence were rated

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Methods-cont.

• MRI scanning
• 4-T GE scanner was used.
• The anatomical MRI was acquired using a 3D T1-
  weighted echo-planar sequence.
• fMRI analyses
• Event-related blood oxygen level-dependent
  responses for each participant were analyzed
  using a modified general linear model

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Results

• Behavioral data

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Results-cont.

• Neuroimaging results

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Results-cont.

• Neuroimaging results show PASA was found when
  differences in task difficulty were eliminated

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Results-cont.

• Neuroimaging results show in older adults PFC
  activity was negatively correlated with activity
  in the occipital cortex, but positively
  correlated with ER/VP performance in younger
  adults, no significant correlations between
  regions or between activity and performance

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Results-cont.

• Neuroimaging results indicated PASA was also
  found for deactivation

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Discussion

• Validity of PASA against a difficulty account
• After removing difficulty difference, a clear
  PASA pattern was found-
• occipital activity in younger gt in older
  adults
• PFC activity in older gt in younger adults

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Discussion-cont.

• Function of PASA for the compensation account
• PFC activity compensates for the decrease in
  occipital activity that is necessary for
  successful task performance

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Discussion-cont.

• Generalizability of PASA posterior-anterior
  shift in deactivations
• Default network- previous studies indicated
• 1. posterior and anterior midline cortices are
  often deactivated during task compared with a
  resting baseline.
• 2. reallocating processing of resources to
  regions involving in task performance
• ? This present study showed only deactivation
  in posterior midline regions were reduced, but
  enhanced in anterior midline regions by aging.

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Conclusion

• PASA pattern acts in a compensatory manner of
  offset posterior-related neuroanatomical declines
  due to aging and is generalizable to
  deactivations across both task and difficulty.

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Q How could a region (eg. PFC) activate and
deactivate simultaneously (see figure BD)? If
the regions of activation and deactivation are
sort of different, is it proper to conclude more
PFC activation and less occipital deactivation in
older group?