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

PPC: Gottlieb and Goldberg, 1998 Cue: Triangle * 1st Saccade 2nd Saccade Firing Rate Cue + Time Cue: Circle * + 2nd Saccade Firing Rate 1st 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: Desimone: 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

Patients: Right Parietal Task: Shape Identification Karnath et al., 1993 Patients: Right Parietal Task: Shape Identification Condition 1 Condition 2

Retinocentric Neglect Right Target Left Target % Correct Prediction for Retinocentric Neglect Condition 1 Condition2

Trunk-centered Neglect Right Target Left Target % Correct Prediction for Trunk-centered Neglect Condition 1 Condition2

Patients % Correct % Correct Patient 1 Patient 2 C1 C2 C1 C2

Frames of reference for neglect: Trunk-centered?

Left hand is still neglected even though it is now on the right Left hand is neglected

Can neglect be object-centered?

Object-centered or relative neglect?

Object-centered or relative neglect? Draw the white half Draw the black half

Driver et al, Neuropsychologia, 1994 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 axis-centered (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?

Retinal Location (deg) Eye Position ex = -20 ex = 0 ex = 20 Activity -40 –20 0 20 40 Retinal Location (deg) (Andersen et al., 1985)

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

Retinal Location (deg) Activity Retinal Location (deg) Eye Position (deg) Eye Position ex = -20 ex = 0 ex = 20 Activity -40 –20 0 20 40 Retinal Location (deg)

Eye Movements Reaching Premotor Cortex Head-Centered Map Superior Colliculus Retinotopic 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

Saccades Direction (deg) Left of the object Right of the object Saccades Direction (deg) Neuronal Activity Activity Trials

Explicit object-centered neurons or gain modulated neurons?

Saccades Direction (deg) Left of the object Right of the object Saccades Direction (deg) Neuronal Activity Activity Trials

It appears that the parietal cortex uses basis functions for spatial representations in: Egocentric coordinates (eye-, head-, body-centered) Object-centered coordinates

Eye Movements Reaching Premotor Cortex Head-Centered Map Superior Colliculus Retinotopic Map Eye Movements Reaching Eye Position Basis Function Layer (Parietal Cortex) Retinal Location Eye Position Units Retinotopic Map Visual Cortex

Left Retinal Position Right Left Retinal Position Right Premotor Cortex Head-Centered Map Superior Colliculus Retinotopic Map Right Right Basis Function Layer Eye Position Eye Position Left Retinal Position Right Left Retinal Position Right Left Map 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

Left Retinal Position Right Left Retinal Position Right Premotor Cortex Head-Centered Map Superior Colliculus Retinotopic Map Right Right Basis Function Layer Eye Position Eye Position Left Retinal Position Right Left Retinal Position Right Left Map Right Map Eye Position Units Retinotopic Map Visual Cortex

Saliency: summed activity in the basis function layer Right Eye Position Left Left Right Retinal Location

Model Patients Error Error Error Error Patients Line Orientation Line Length Line Length Error Error Patients Model Line Orientation Line Orientation

Patients: Right Parietal Task: Shape Identification 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 Head Position Retinal Location

Left hand is still neglected even though it is now on the right Left hand is neglected

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 object-centered representations