Knowing Where and Getting There: A Human Navigation Network - Elanor A. Mcguire et al (1998)

1. Mohamed Knowing Where and Getting There: A Human Navigation Network - Elanor A. Mcguire et al (1998) By: Group B1 Mohamed Sharif, MeeshaSidhu, Marc Delzotto, and Brandon Thomas

2. Mohamed Overview • Results • Further Correlation analysis • PET limitations • Further Research

3. 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

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5. Mohamed Why do we have this Correlation? Scenario A ‘Storage’ related discrepancy Individuals with higher accuracy stored spatial information in a more effective manner Scenario B ‘Retrieval’ related discrepancy Individuals with higher accuracy were more successful at retrieving stored spatial information

6. 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’

7. Mohamed + Primary rCBFs of each individual Scenario A ‘Storage’ Primary rCBFs Scenario B ‘Retrieval’ Accuracy in degrees

8. Mohamed Scenario A ‘Storage’ Primary rCBFs Scenario B ‘Retrieval’ 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 Accuracy in degrees

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10. Limitations of PET Brandon • What is it actually measuring? • Blood Flow • But isn’t there blood everywhere? • … Yes 20 Year Old 80 Year Old

11. 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.

12. 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.

13. 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

14. 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.

15. 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.

16. 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

17. 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

19. Further Research Brandon 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

20. 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