Attention and neglect
Neurophysiology of Attention
• The Superior Colliculus
• MT, MST and PPC
• V2, V4 and IT
Neurophysiology of Attention
• The Superior Colliculus
• MT, MST and PPC
• V2, V4 and IT
The Superior Colliculus
Receptive Field
The Superior Colliculus
Receptive Field
Saccadic Enhancement
The Superior Colliculus
Receptive Field
Is the saccadic enhancement in the superior
colliculus due to attention or to a motor
intent?
The Superior Colliculus
Receptive Field
Attention alone can increase the firing rate
Receptive Field
•Response enhancement could correspond to
the selection of the attended target
•This enhancement seems to be independent
of the behavior
Physiology of Attention
• The Superior Colliculus
• MT, MST and PPC (the posterior
parietal cortex)
• V4 and IT (the infero temporal cortex)
Treue and Maunsell, 1999. MST
• Response to the attended stimulus is
enhanced
• Enhancement is stronger when the distractor
and the target are in the same receptive field
• The response to the distractor is decreased
*
Cue
Firing Rate
PPC: Gottlieb and Goldberg, 1998
1st
Cue: Triangle
Saccade
2nd Saccade
+
Time
1st Saccade
Firing Rate
*
+
Cue: Circle
2nd Saccade
Time
Cells in MT and PPC:
Attention boosts the gain of the response to
the attended stimulus while reducing the
response to the unattended stimulus.
Physiology of Attention
• The Superior Colliculus
• MT and PPC (the posterior parietal cortex)
• V2, V4 and IT
Reynolds et al, 1999
Monkey attends
preferred stimulus of
the cell
Reynolds et al, 1999
Monkey attends null
stimulus for the cell
McAdams and Maunsell, 1999
Colored Gaussian
McAdams and Maunsell, 1999
McAdams and Maunsell, 1999
What’s going on?
• Maunsell:
– Gain modulation
– No baseline shift
– Seen everywhere although strongest when the distractor
and the target are in the same receptive field
• Desimone:
– Not gain: normalization
– Baseline shift
– Seen only when the distractor and the target are in the
same receptive field
Neglect
Line Cancellation
Line Bisection
Patient: Frederico Fellini
Karnath et al., 1993
Patients: Right Parietal
Task: Shape Identification
Condition 1
Condition 2
% Correct
Right Target
Left Target
Prediction for
Retinocentric Neglect
Condition 1
Condition2
% Correct
Right Target
Left Target
Prediction for
Trunk-centered Neglect
Condition 1
Condition2
Patient 1
C1
C2
% Correct
% Correct
Patients
Patient 2
C1
C2
Frames of reference for neglect:
Trunk-centered?
Left hand is
neglected
Left hand is still
neglected even though
it is now on the right
Can neglect be object-centered?
Object-centered or relative neglect?
Object-centered or relative neglect?
Draw the black half
Draw the white half
Axis-centered neglect
Main axis
Hard
Easy
Driver et al, Neuropsychologia, 1994
Frames of Reference of Neglect
• Neglect always affects a mixture of
egocentric frames of reference for all
behaviors
• Neglect is always relative
• In some patients, it can also be axiscentered (object-centered).
• Can we relate the behavior of hemineglect
patients to the response of neurons involved
in spatial representations?
• What do we know of the response
properties of neurons involved in spatial
representations?
Eye Position
Activity
ex = -20
ex = 0
ex = 20
-40
–20
0
20
Retinal Location (deg)
(Andersen et al., 1985)
40
Motor commands, M, are non linear functions
of sensory inputs, S, and posture signals, P:
M = f(S,P)
Such functions can be approximated by linear
combinations of basis functions, Bi(S,P):
M = S ci Bi(S,P)
Claim: parietal neurons compute basis functions
Activity
Eye Position
Activity
ex = -20
ex = 0
ex = 20
-40
–20
0
20
Retinal Location (deg)
40
Superior Colliculus
Retinotopic Map
Premotor Cortex
Head-Centered Map
Eye Movements
Reaching
Eye
Position
Basis Function
Layer
(Parietal Cortex)
Retinal Location
Eye Position
Units
Retinotopic Map
Visual Cortex
Can we generalize the basis function theory
of spatial representations to object-centered
representations?
Neuronal Activity
Trials
Left of the object
Right of the object
Neuronal Activity
Left of the object
Trials
Right of the object
Activity
Left of the object
Saccades Direction (deg)
Right of the object
Explicit object-centered neurons or gain
modulated neurons?
Neuronal Activity
Left of the object
Trials
Right of the object
Activity
Left of the object
Saccades Direction (deg)
Right of the object
It appears that the parietal cortex uses basis
functions for spatial representations in:
• Egocentric coordinates (eye-, head-, bodycentered)
• Object-centered coordinates
Superior Colliculus
Retinotopic Map
Premotor Cortex
Head-Centered Map
Eye Movements
Reaching
Eye
Position
Basis Function
Layer
(Parietal Cortex)
Retinal Location
Eye Position
Units
Retinotopic Map
Visual Cortex
Right
Right
Eye Position
Premotor Cortex
Head-Centered Map
Eye Position
Superior Colliculus
Retinotopic Map
Left
Retinal Position
Right
Left Map
Left
Basis Function
Layer
Retinal Position
Right
Right Map
Eye Position
Units
Retinotopic Map
Visual Cortex
Main assumptions of the
hemineglect model:
• Parietal neurons compute basis functions
• Basis function units are distributed across
hemispheres according to contralateral
gradients
Right
Right
Eye Position
Premotor Cortex
Head-Centered Map
Eye Position
Superior Colliculus
Retinotopic Map
Left
Retinal Position
Right
Left Map
Left
Basis Function
Layer
Retinal Position
Right
Right Map
Eye Position
Units
Retinotopic Map
Visual Cortex
Saliency: summed activity in the basis function layer
Saliency
Model
Error
Error
Patients
Line Length
Error
Error
Line Length
Model
Line Orientation
Patients
Line Orientation
Karnath et al., 1993
Patients: Right Parietal
Task: Shape Identification
Condition 1
Condition 2
Right Target
Left Target
Model
Condition 1
Condition 2
Left Stimulus
Right Stimulus
C2
C1
Left hand is
neglected
Left hand is still
neglected even though
it is now on the right
General principle: any change of posture toward
the ipsilesional side results in an improvement in
performance (extreme case: vestibular recovery)
Relative neglect
Draw the black half
Draw the white half
Object-centered neglect
Activity in the Basis Function Layer
Main axis
Hard
Easy
Conclusions
• Parietal neurons compute basis functions of their
inputs. As a result, they encode the location of
objects in multiple frames of reference
simultaneously
• A lesion of a basis function representation leads to
a syndrome similar to hemineglect if the basis
functions are distributed across hemispheres
according to contralateral gradients
• This approach can be generalized to objectcentered representations