Spatial%20coding%20of%20the%20Predicted%20Impact%20Location%20of%20a%20Looming*%20Object

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Spatial coding of the Predicted Impact Location
of a Looming Object

• M. Neppi-Mòdona
• D. Auclair
• Sirigu
• J.-R. Duhamel

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Approaching Objects

• When will it arrive or pass time to contact
  (TTC)
• Will it hit me?
• How far to one side will it pass?
• Where will it strike me?
• In interception of an object where and when are
  not independent problems
• In this study where is constrained to the plane
  of the eyes

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Main Questions

• How accurate is performance when predicting
  impact location on the face
• For targets originating straight ahead
• For targets with eccentric origin
• What reference frame are approaching objects
  represented in?
• Retinal, visuotopic, intermediate
• Manipulated alignment of the observers retinal
  and visuotopic frames

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LED stimuli (5mm diameter) originated from 40 cm
in a dark and frameless environment Travelled at
20 cm/s for 1 s, then occluded for final 1 s of
approach trajectory 3 start points, 7 end points,
one of which is the cyclopean eye (or midline of
the head)
Fixating central with central LED origin and
central impact location produces same retinal
image as fixation to left with left LED origin
and central impact
Simplest case is fixation directly ahead, and
midline aligned with central Fixation LED,
causing retinotopic and visuotopic frames to lie
on top of one another Task is to judge left /
right of midline (forced choice)
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The effect of eccentric origin on prediction
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Why ipsilateral bias?

• The bias occurs because the judgment is made
  under conditions of uncertainty, forcing the use
  of a heuristic strategy
• The level of uncertainty is greater than in
  normal interception conditions because the
  where judgment must be made at TTC 1000 msec,
  whereas TTC 500 msec would be a more natural
  point to initiate an action. (See handout)
• At TTC 1000 all stimuli starting on the left are
  still on the left (with 2 degrees separating each
  of them), all from the right are still on the
  right, and those from the centre are split 3/1/3

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• At TTC 500 the separation is more like 5 degrees
  between each stimulus, and one of them has
  crossed the midline
• Between TTC 1000 and 500 whether angle alpha is
  growing, shrinking, or constant is above
  threshold, and this gives unambiguous information
  about the destination of the stimulus. (See
  Table)
• Authors do not discuss this optical variable,
  which is also available, but may be below
  threshold in the portion of the trajectory they
  do show.
• Allowing spatial vision of background would help
  bring it above threshold
• Given the impoverished information at TTC 1000,
  a simple heuristic is to respond left if it is on
  the left, and right if it is on the right. For
  the 18 stimuli that dont hit the nose this
  strategy performs at 66 (100 for straight
  origin), and produces the observed biases.

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Why ipsilateral bias?

• A more general point is that natural selection
  would favour a mechanism to intercept impacts,
  not predict where they will occur, which is has
  less adaptive value.
• Those objects observers judged incorrectly as
  left would not have crossed the midline until
  very late in their trajectory (see diagram), so
  actual interception would occur on the left.

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Visuotopic or retinotopic space? (Misaligning the
angle of gaze and the midline)

• Is the approach angle of an object relative to
  the observer correctly perceived, independently
  of retinal position?
• Or, is there an influence of retinal position,
  producing biases in perception of approach angle?
• By pointing the midline towards the central
  target location as before, but fixating one of
  the eccentric positions the foveal origin of the
  retinal coordinate frame is no longer aligned
  with the midline origin of the visuotopic frame
• Observed biases might indicate an influence of
  the retinotopic frame, but a purely retinotopic
  observer could in principle perform accurately
  using the direction of change of alpha

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(No Transcript)
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Group data (individuals differ qualitatively from
each other)
Impact prediction takes place in an intermediate
reference frame?
Right bias
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Individual Differences
Partial-correlation between prediction error and
position in each of the reference frames Presume
based on trials where frames misaligned
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Red line is bias for central origin trajectories
during right fixation
Retinal frame dominant
Intermediate frame dominant
Visuotopic frame dominant
Subject 4 is not consistent with use of direction
of change of alpha strategy at TTC 1000 because
basic ipsilateral bias for central fixation is
still present
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Conclusions

• I predict that all the observed biases, whether
  they are ipsilateral with straight ahead fixation
  or related to the misalignment of retinal frame
  from visuotopic frame would disappear if the
  stimulus was allowed to develop to a more
  realistic value of TTC 500 msec
• This is because direction of change of alpha
  would be well above threshold. Note that this
  would allow observers to behave independently of
  eye position as if they had a visuotopic
  representation, without actually having one at
  all.
• I predict that allowing spatial vision of the
  background would bring change of alpha above
  threshold for earlier TTC values
• Deletion and accretion of texture as alpha
  changes would be a powerful cue, and would
  certainly make it obvious when alpha was
  unchanging, specifying a central impact ( in the
  presented data central impacts are sometimes
  mistaken for lateral ones)

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Conclusions

• Authors agree with me that one explanation for
  the ipsilateral bias is covert interception
• I think the question of reference frames for this
  task is interesting, but I dont believe the
  conditions tested establish that a visuotopic
  frame is needed for this task under more natural
  viewing conditions
• To convince me, authors need to show effects
  under more natural viewing, and rule out the
  alpha explanation