25 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 fieldDesimone:Not gain: normalizationBaseline shiftSeen only when the distractor and the target are in the same receptive field
39 Object-centered or relative neglect? Draw the white halfDraw the black half
40 Driver et al, Neuropsychologia, 1994 Axis-centered neglectMain axisHard EasyDriver et al, Neuropsychologia, 1994
41 Frames of Reference of Neglect Neglect always affects a mixture of egocentric frames of reference for all behaviorsNeglect is always relativeIn some patients, it can also be axis-centered (object-centered).
42 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?
45 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
55 Left Retinal Position Right Left Retinal Position Right Premotor CortexHead-Centered MapSuperior ColliculusRetinotopic MapRightRightBasis FunctionLayerEye PositionEye PositionLeft Retinal Position RightLeft Retinal Position RightLeft MapRight MapEye PositionUnitsRetinotopic MapVisual Cortex
56 Main assumptions of the hemineglect model: Parietal neurons compute basis functionsBasis function units are distributed across hemispheres according to contralateral gradients
57 Left Retinal Position Right Left Retinal Position Right Premotor CortexHead-Centered MapSuperior ColliculusRetinotopic MapRightRightBasis FunctionLayerEye PositionEye PositionLeft Retinal Position RightLeft Retinal Position RightLeft MapRight MapEye PositionUnitsRetinotopic MapVisual Cortex
58 Saliency: summed activity in the basis function layer RightEye PositionLeftLeftRightRetinal Location
59 Model Patients Error Error Error Error Patients Line Orientation Line LengthLine LengthErrorErrorPatientsModelLine OrientationLine Orientation
60 Patients: Right Parietal Task: Shape Identification Karnath et al., 1993Patients: Right ParietalTask: Shape IdentificationCondition Condition 2
61 Right TargetLeft TargetModelCondition Condition 2
62 Left Stimulus Right Stimulus C2 C1 Head Position Retinal Location
63 Left hand is still neglected even though it is now on the right Left hand is neglected
64 General principle: any change of posture toward the ipsilesional side results in an improvement in performance (extreme case: vestibular recovery)
72 Activity in the Basis Function Layer Main axisHard Easy
73 ConclusionsParietal neurons compute basis functions of their inputs. As a result, they encode the location of objects in multiple frames of reference simultaneouslyA lesion of a basis function representation leads to a syndrome similar to hemineglect if the basis functions are distributed across hemispheres according to contralateral gradientsThis approach can be generalized to object-centered representations
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