reading assignmentilab.usc.edu/classes/2002cs564/lecture_notes/24-reaching-grasping … · •sma...

1Itti: CS564 - Brain Theory and Artificial Intelligence. FARS Model

CS564 - Brain Theory and Artificial IntelligenceUniversity of Southern California

Lecture 24.The FARS model of Control of Reaching and Grasping

Reading Assignment:Fagg, A. H., and Arbib, M. A., 1998, Modeling Parietal-Premotor Interactions in Primate Control of Grasping, Neural Networks, 11:1277-1303.

TMB 2.2, 5.3


Final Exam

Tuesday, December 17, 11:00am – 1:00pm, VKC-100

No books, no questions, work alone, everything seen in class.


"What" versus "Where →→→→ How"

VisualCortex

ParietalCortex

InferotemporalCortex

How (dorsal)

What (ventral)

reach programming

grasp programminggraspexecution

reachexecution

“What” versus “Where”: Mishkin and Ungerleider“What” versus “How”: Goodale and Milner


Introducing AIP and F5 (Grasping) in Monkey

F5 - grasp commands inpremotor cortexGiacomo Rizzolatti

AIP - grasp affordancesin parietal cortexHideo Sakata


The Sakata Protocol


Grip Selectivity in a Single AIP Cell

A cell that is selective for side opposition (Sakata)


Differential Timing of Activity Peaks in Different AIP Neurons

Note the need for a broad database of many cells within each region to see that cells are not just “pattern recognizers” but also have a relationship to the time course of the ongoing behavior.


Size Specificity in a Single AIP Cell

This cell is selective toward small objects, somewhat independent of object type ( Hideo Sakata)

Note: Some cells show size specificity; others do not.


Grasp Specificity in an F5 Neuron

Precision pinch (top)

Power grasp (bottom)

(Data from Rizzolatti et al.)


FARS (Fagg-Arbib-Rizzolatti-Sakata) Model Overview

AIP

F5

dorsal/ventral streams

Task Constraints (F6)

Working Memory (46)

Instruction Stimuli (F2)

Task Constraints (F6)Working Memory (46?)Instruction Stimuli (F2)

AIPDorsalStream:Affordances

IT

VentralStream:Recognition

Ways to grab this “thing”

“It’s a mug”PFC

AIP extracts the set of affordances for an attended object.These affordances highlight the features of the object relevant to physical interaction with it.


Secondary Somatosensory Cortex (SII)

In the grasp versus point comparisonin a PET study of humans, we found a marked increase of activity in the secondary somatosensory cortex (SII).

Ablation of SII in non-human primates results in decrements in tactile discrimination and impaired tactile learning.

Focal lesions of the parietal operculum in humans characteristically produce tactile agnosia without loss of simple tactile sensation, or motor control. This deficit can include the inability to sort objects based on size or shape, although sorting on texture is preserved.

The model relates the augmented response to higher order tactile feedback or tactile expectation.


Motor Commands, Expectations, and Feeedback

F5(grasp type)

MI

hand

(muscle assemblies)

(elementary sensory features)

SI

SII

expectation

motor commands

sensory info

(sensory hyperfeatures)

A7(internal model)


Interaction of AIP and F5 During the Sakata Task

Activation Connection

Inhibitory Connection

Priming Connection

AIP precision-related cell

AIP power-related cell

set extension flexion hold release

Precision Pinch

F5

AIP

Go Signal 2nd Go Signalcontact with object

maximum aperture reached

visual

motor

Visual Inputs


The Problem of Serial Order in Behavior (Karl Lashley)

If we tried to learn a sequence likeA → B → A → Cby reflex chaining, what is to stop A triggering B every time, to yield the performance A → B → A → B → A → ….. (or we might get A → B+C → A → B+C → A → …..)A solution:Store the “action codes” (motor schemas) A, B, C, … in one part of the brain (F5 in FARS) and have another area (pre-SMA in FARS) hold “abstract sequences” and learn to pair the right action with each element:

(pre-SMA): x1 → x2 → x3 → x4 abstract sequence

(F5): A B C action codes/motor schemas

Hypothesis: The “Sakata-Protocol Sequencing” is not mediated within F5 --Sequences are stored in pre-SMA and administered by the Basal Ganglia (BG)


The “Visual Front End” of the FARS Model

Visual Cortex

Parietal Cortex

VIP

PIPAIP

F4

How (dorsal)

IT

What (ventral)

(position)

(shape, size, orientation)

(object/grasp transform)

F5

(grasp type)

(arm goal position)


PIP →→→→ IT connections are hard-wired for a simple set of objects

Cylinder Box Sphere

PIP

general

diameter

length

general

width

length

general

diameter

height

IT

Cylinder Box Sphere

PIP

general

diameter

length

general

width

length

general

diameter

height

IT

Cylinder Box Sphere

PIP

general

diameter

length

general

width

length

general

diameter

height

IT

Note the use of coarse coding


Coarse Coding/Population Coding

To code some variable x lying in an interval [a,b) we could take n cells, with cell i (i = 0, …, n-1) firing if and only if the current value of x lies in the ith subinterval

In coarse coding, we achieve much greater discrimination by taking into account the continuously varying firing level fi of each cell, and then we can decode values of x actually varying across each interval, using some such formula as

��

��

� +−+−+n

iaban

iaba )1)((,)(

�

�−

=

−

=��

��

�

−−+

1

0i

1

0i

f

1f

n

i

n

ii

naba


PIP →→→→ AIP connections are hard-wired for a simple set of affordances

Cylinder

general

diameter

length

narrow wide

short long

precisionprecision aperture = 20mm

precision precision aperture = 60mm

PIP

AIP

A B C D


IT →→→→ AIP

The mapping from object identity in IT to maps directly to both the grasp type and the aperture of grasp in AIP when the nature of the object implies such data:

E.g., in the case of AT, the projection from IT can provide the necessary grasp type and parameters for a lipstick but not for a cylinder.

precisionprecision aperture = 20mm

precision precision aperture = 60mm

IT

AIP

A B C D

narrow cylinder (bottle cap) wide cylinder (jar top)

A bottle cap activates a precision grasp with a narrow aperture.A jar top maps to a precision grasp with a wide aperture.


PIP →→→→ IT connections are hard-wired for a simple set of objects

Cylinder Box Sphere

PIP

general

diameter

length

general

width

length

general

diameter

height

IT

Cylinder Box Sphere

PIP

general

diameter

length

general

width

length

general

diameter

height

IT

Cylinder Box Sphere

PIP

general

diameter

length

general

width

length

general

diameter

height

IT

Note the use of coarse coding


F5 activity during execution of a precision graspThe top two traces show the position of the thumb and index finger.Left: The next five traces represent the average firing rate of five F5 neurons (set-, extension-, flexion-, hold-, and release-related). The remaining five traces represent the various external (Ready, Go, Go2) and internal (SII) triggering signals.

Right: Illustrating the temporally distributed coding of F5 cells.


Positioning F2, F6 and Areas 46 and SII in Monkey


Prefrontal Influences on F5

F4

Inferior Premotor

Cortex

F5(grasp type)

F6

46

(arm goal position)

F2 (abstract stimuli)

pre-SMA

Dorsal premotor cortex

Frontal Cortex


Grasp Selection in F5

Within F5, the active grasps compete through a winner-take-all

Area 46, working in conjunction with F6, supplies task-dependent biases for grasp selection in F5, based upon

• task requirements (such as what is going to be done after the grasp), or

• a working memory of a recently executed grasp.

The biasing can • be on the class of grasp (e.g. power versus precision), or • include the parameters of the grasp (e.g. width of aperture) mechanism which incorporates any biases that might be received from area 46.


Supplementary Motor Area (SMA) in Monkey

• a unilateral lesion of the SMA disrupts the monkey's ability to allocate his hands to different subtasks of a bimanual task (Brinkman, 1984)

• SMA is involved in the temporal organization of complex movements (Tanji, 1994).

Luppino, Matelli, Camarda, & Rizzolatti subdivide SMA:

SMA-proper (F3; the caudal region) has heavy projections to the limb regions of F1 and related portions of the spinal cord.

F6 (pre-SMA) does not project to the spinal cord, and has only moderate projections to areas F3 and F2 (the dorsal premotor cortex), but has a very heavy projection to area F5.


Pre-SMA

F6 (pre-SMA) has a very heavy projection to area F5. Inputs into area F6 include VIP, and area 46.F6 contains neurons that become active when an object that the monkey is about to grasp moves from being out of reach into the peripersonal space of the monkey

Interpretation: this class of pre-SMA (F6) neuron is responsible for generating a go signal when it is appropriate for the monkey to begin a reaching movement.

F4

Inferior Premotor Cortex

F5(grasp type)

F6

46

(arm goal position)


FrontalCortex

pre-SMA



Area 46

Area 46 has been implicated as a working memoryin tasks requiring information to be held during a delay period (Quintana & Fuster, 1993).

This memory has been posited to participate in learning tasks involving complex sequences of movements (Dominey, 1995).

Area 46 projects to F6, and also exchanges connections with area F5 (Luppino, et al., 1990; Matelli, 1994).

When a human is asked to imagine herself grasping an object, activated areas (PET or fMRI) involved include:

• Area 46 (Decety, Perani, Jeannerod, Bettinardi, Tadary, Woods, 1994) • Area 44 (a possible F5 homologue)• A site along the intra-parietal sulcus (Grafton, et al., 1996).


Role of Area 46 for Grasping in the Dark

During the performance of the task in the light, area 46 maintains a memory of those F5 cells that participate in the grasp.

To simulate performance in the dark, PIP and IT are then cleared.If a new trial is initiated soon enough, area 46 provides positive support to those F5 cells that were active during the first trial.

The area 46 working memory provides a static description of the grasp that was recently executed, in the sense that the temporal aspects of the grasp are not stored - only a memory of those units that were active at some time during the execution.

[Area 46 is involved in human imagination of grasp execution.]

F4

Inferior Premotor Cortex

F5(grasp type)

F6

46

(arm goal position)


FrontalCortex

pre-SMA



Conditional Tasks and Area F2

Dorsal premotor cortex (F2) is thought to be responsible for the association of arbitrary stimuli (an IS) with the preparation of motor programs (Evarts, et al., 1984; Kurata & Wise, 1988; Mitz, Godshalk, & Wise, 1991; Wise & Mauritz, 1985).

In a task in which a monkey must respond to the display of a pattern with a particular movement of a joystick:

• some F2 neurons respond to the sensory-specific qualities of the input. However, • many F2 units respond in a way that is more related to the motor set that must be

prepared in response to the stimulus. When a muscimol lesion in this region is induced, the monkey loses the ability to correctly make the arbitrary association.

F4

F5(grasp type)

F6

46

(arm goal position)


pre-SMA


In addition to simulating the Sakata task, we simulate conditional tasks.


The Complete FARS Model

Visual Cortex

Parietal Cortex

VIP

PIPAIP

F4

How (dorsal)

ITWhat (ventral)

Inferior Premotor

Cortex(position)



F5(grasp type)

MI

hand

(muscle assemblies)


F6

46

SI

SII

expectation

motor commands

sensory info

(arm goal position)



A7(internal model)


To complete your study of this material

Study the paper:

Fagg, A. H., and Arbib, M. A., 1998, Modeling Parietal-Premotor Interactions in Primate Control of Grasping, Neural Networks, 11:1277-1303.

to ensure that you understand the supplementary slide-set:

18+. FARS Model 1 [Spares] (2000)


F5 activity during execution of a precision graspThe top two traces show the position of the thumb and index finger.Left: The next five traces represent the average firing rate of five F5 neurons (set-, extension-, flexion-, hold-, and release-related). The remaining five traces represent the various external (Ready, Go, Go2) and internal (SII) triggering signals.

Right: Illustrating the temporally distributed coding of F5 cells.


The Complete FARS Model

Visual Cortex

Parietal Cortex

VIP

PIPAIP

F4

How (dorsal)

ITWhat (ventral)

Inferior Premotor

Cortex(position)



F5(grasp type)

MI

hand

(muscle assemblies)


F6

46

SI

SII

expectation

motor commands

sensory info

(arm goal position)



A7(internal model)


Thumb and index finger temporal behavior as a function of cylinder size


F5 cell responses during precision grasps of seven different apertures

This particular cell is active only for narrow precision pinches


F5 movement-related cell (A) and a hold-related (B) cell during the perturbation experiment

20mm/30mm traces correspond to presentation and grasping of a 20mm and a 30mm cylinder, respectively; traces labeled 20→30 and 30 → 20 indicate perturbation trials, in which a 20mm cylinder is switched for a 30mm cylinder, and a 30mm cylinder for a 20mm one, respectively.


Two objects that map to the identical grasp


Comparison of population responses towards two different objects (but identical grasps).

Lighted movement task, AIP (A) and F5 (B) cells; and

AIP populations during fixation (C) and dark movement (D) tasks.


F5 Feedback to AIP

Visual-related AIP receive object-specific inputs; motor-related cells receive recurrent inputs from F5, which do not demonstrate object-specific activity.


A single object mapping to two possible grasps

Before execution, one grasp must be selected based upon the current context (e.g., based upon an Instruction Stimulus).


Two F5 units (A, B) in response to the four conditions: (c,pr), (c,pw), (nc,pr), and (nc,pw). c = conditional; nc = non-conditional; pr = precision pinch; pw = power grasp.


Four boxes of different dimensions

Grasping is performed along the horizontal axis. The two blocks in the left column are grasped using a precision pinch of a 10mm aperture; the blocks on the right require a 20mm aperture.


Comparison of AIP visual responses for objects of the same (A) and different (B) widths; and AIP motor-related responses (dark movement condition) for objects of same (C) and different (D) widths.

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