Transcript Powerpoint

Interaction between different
types of visual information
• Big question: the ‘binding problem’
(how are separate neural
representations re-combined?)
• Smaller question of this paper: where
(not how or why) do motion and
position information interact?
Motion aftereffect (‘static’
MAE)
• Stare at moving stripes/dots for a while
(adapt)
• Inspect a stationary version of the
adapting pattern
• Old story: a sensation of motion without
position shift
• New story: a sensation of motion +
apparent shift of position (in the direction
of the MAE)
Site of interaction?
• Previous suggestion: In V1 or V2, via
connections from V5/MT (Nishida and
Johnston, 1999), because small RFs in V1
would give good position sensitivity.
• Authors suggest it all happens in V5 (more
parsimonious than backwards projections
from V5 to V1)
Overview of experiment
• Perceived misalignment of static test
stimuli produced by prior adaptation to
motion (via MAE)
• Attempt to disrupt this misalignment by
TMS during perception of the test field
• TMS applied to V1 or V5
• Also compared with effects of TMS to V5
after adapting to counter-phased gratings
(don’t produce misalignment in test field)
• If TMS disrupts perceived misalignment,
that area is involved in integration of
motion and position
Experimental design
Author’s conclusion
• TMS affects perceived misalignment when
applied to V5, but not to V1
• Integration of motion and position
information occurs in V5, not V1
Problem (1)
• What is TMS doing?
• Authors suppose it in affecting integration
in V5
• But it may be affecting motion signal in V5
(which affects what goes back down to V1)
and adding a little noise to the integrated
information in V1
• We are not told how the measurements
were made (‘via standard psychophysical
procedures’)
Problem (2)
• How can we be sure that TMS affected
relevant processing in V1?
• Authors say:
In two Ss, coil position was localised by
structural MRI. So TMS was applied to
V1, but its effects uncertain
In other 2 Ss, ‘use of stationary and
moving phosphenes established locus of
V1 and V5’
Web address for more details no longer
exists
Problem (2 cont.)
• Missing condition?:
• Apply TMS to V1 and V5 during adaptation
• (would show whether relevant motionsensitive neurons in V1 as well as V5 are
affected by TMS)
• Does TMS have any effects in V1? We
need to know this in order to interpret the
lack of effect in the alignment task
Problem (3)
• How does TMS affect vision?
• If you hit V5, motion-sensitive neurons are
presumably affected, but how? Temporary
reduction in output?
• How do the Gabor test patches appear
with V1 and V5 TMS? Distorted? Normal?
• Why is only motion apparently affected by
TMS?
My conclusion
• TMS affects V5, but it’s not clear how
• Nishida and Johnston’s idea still alive – if
the TMS to V5 disrupts motion signals
down to V1
• TMS to V1 need not affect integrated
signals
• RAE panel should look hard at papers in
Current Biology