Transcript 3FA3M5-C-B1_- FINALRevised_Slides

Mohamed
Knowing Where and Getting
There: A Human Navigation
Network
- Elanor A. Mcguire et al (1998)
By: Group B1
Mohamed Sharif, Meesha Sidhu, Marc Delzotto, and
Brandon Thomas
Mohamed
Overview
• Results
• Further Correlation analysis
• PET limitations
• Further Research
Mohamed
Results
• Results “agree with, and further illuminate, previous
findings”
• Hippocampus provides environment based
representation of space
• Right inferior parietal cortex computes correct
egocentric orientation
• Greater accuracy is correlated with higher right
hippocampal rCBF values
• Higher speed correlated with higher right caudate
nucleus rCBF values
Mohamed
Mohamed
Why do we have this Correlation?
Scenario A ‘Storage’
related discrepancy
Scenario B ‘Retrieval’
related discrepancy
Individuals with higher
accuracy stored spatial
information in a more
effective manner
Individuals with higher
accuracy were more
successful at retrieving
stored spatial
information
Mohamed
How can we test this?
PET Scans during Primary Exploration Period
– Observe hippocampal rCBFs of subjects
– Determine whether a relationship exists between
primary rCBFs and later accuracy
– Use the relationship to infer whether the later
hippocampal rCBF/accuracy correlation is due to
discrepancies in ‘Storage’ or ‘Retrieval’
Mohamed
Primary rCBFs of each
individual
+
Scenario A
‘Storage’
Primary rCBFs
Scenario B
‘Retrieval’
Accuracy in degrees
Mohamed
Scenario A
‘Storage’
Primary rCBFs
Scenario B
‘Retrieval’
Accuracy in degrees
Scenario A
Positive slope implies that
increases in activation
during primary
exploration affects later
accuracy
Scenario B
Lack of correlation
implies that the reason
for later discrepancy is
more dependent on
attempts of retrieving
information
Mohamed
Brandon
Limitations of PET
•What is it actually measuring?
•Blood Flow
•But isn’t there blood everywhere?
• … Yes
20 Year Old
80 Year Old
Brandon
Limitations of PET… con’t
•SO… instead we measure changes in blood flow
•By subtracting moment to moment images, we can
measure which parts of the brain are consistently
active.
•I.e.: If area x was active a moment ago, but not now,
and area y was active a moment ago, and still is…
then area y must be in use, while area x is just noise.
Brandon
PET timescale and resolution
• Delay from neural activity to rCBF change ~2s
• In-plane spatial resolution of the scanner
used: 6.5 mm (and 128x128 px per slice)
• This means that neural populations need to be
big enough to cause a detectable change in
blood flow in at least a 6.5mm x 6.5mm area
of tissue.
• This will be measured ~2 seconds afterward.
Brandon
Neural Circuit Size and Responses
• The basic unit of processing in the cortex is
estimated to be about 30-50 μm in diameter,
containing 80-100 neurons across all layers.
• This means that 6500 μm /50 μm = 130
columns of 80 (*130=10 400) cells are needed
to cause 1 px to light up in a PET scanner.
• These units are hypothesized to operate in
some cases on the timescale of <100-250 ms
Brandon
PET and Cortical Organization
• PET images are averaged across subjects.
• Not all brains are created equal
• Even if populations of 10 000+ neurons across
the cortex are active long enough to be
detectable over several image acquisitions,
they still may average out in the end.
Brandon
What’s the point?
• Basic processing units of cortex are well below
the spatial and temporal resolution of this
technique.
• Thus: important information may be
undetectable, or lost during data processing.
• This logic, of course, supports a conclusion
that the area detected is involved in
navigation at some level… but the technique is
simply too coarse to tell the whole story.
Brandon
On the other hand…
• No one had to die.
• No one was hurt (aside from a bit of radiation
and an IV)
• And it was done in vivo with awake (but
stationary) humans
Mohamed
Further Research
Straight after the turn: The role of the parietal
lobes in egocentric space processing -Janina
Seubert et al. (2008)
- Virtual egocentric navigation schemes were conducted
- Parietal patients were compared with healthy individuals and
other neural patients
Results:
- Parietal patients were unable to correctly orient themselves
to point of origin
Brandon
Further Research
Large-scale recording of neuronal ensembles.–
György Buzsáki (2004) Nat. Neurosci. 7(5): 446-452.
- Multi-tetrode recordings of large areas of cortex
- Theoretically allows real-time data collection from thousands
of neurons
- The technique allows recording of temporally subtle changes
in patterned cortical activity
Brandon
Summary
• Conclusions of the study leave much room for
interpretation
• Further steps can be taken to transform
speculation into empirical results
• PET scans may be misleading
• Recent studies build upon methods of virtual
reality to further assess and localize
egocentric and allocentric navigation