Space, time and the nervous system

Romi NijhawanUniversity of Sussex

Summary of the talk

• Philosophy; Neutral monism; the flip-flop analogy

• Basics of visual processing. Time delays in the transmission of neural signals

• Neutral monism and the Flash-lag effect

• Neutral monism and the life-span development of the scientist (from zygote embryo fetus newborn adult)

Neutral monism

• Holds that ultimate reality is of one kind

• Intrinsic nature of ultimate reality is neither mental nor physical; it is neutral between the two

– Baruch Spinoza (1632-1677)– Ernst Mach (1838-1916)– William James (1842-1910)– Bertrand Russell (1872-1970)

Bodies do not produce sensations, but complexes of sensations (complexes of elements) make up bodies. If, to the physicist, bodies appear the real, abiding existences, whilst sensations are regarded merely as their evanescent, transitory show, the physicist forgets, in the assumption of such a view, that all bodies are but thought-symbols for complexes of sensations (complexes of elements).

- Mach (1897)

Thus the great gulf between physical and psychological research persists only when we acquiesce in our habitual stereotyped conceptions. A color is a physical object as soon as we consider its dependence, for instance, upon its luminous source, upon other colors, upon temperatures, upon spaces, and so forth. When we consider, however, its dependence upon the retina…it is a psychological object, a sensation.

Not the subject matter, but the direction of investigation, is different in the two domains.

- Mach (1886)

The data of psychology do not differ in their intrinsic character from the data of physics. I have maintained that sensations are data for psychology and physics equally…

- Russell (1921)

Common sense imagines that when it sees a table it sees a table. This is a gross delusion. When common sense sees a table, certain light waves reach its eyes, and these are of a sort which, in its previous experience, has been associated with certain sensations of touch, as well as other people’s testimony that they also saw the table…The light waves caused occurrences in our eyes, and these caused occurrences in the optic nerve, and these in turn caused occurrences in the brain…(Of course, if matter in general is to be interpreted as a group of occurrences, this must also apply to the eye, the optic nerve, and the brain). - Russell (1959)

The modern would-be materialist thus finds himself in a curious position, for, while he may with a certain degree of success reduce the activities of the mind to those of the body, he cannot explain away the fact that the body itself is merely a convenient concept invented by the mind. We find ourselves thus going round and round in a circle: mind is an emanation of body, and body is an invention of mind. Evidently this cannot be quite right, and we have to look for something that is neither mind nor body, out which both can spring.

-Russell (1929)

Neutral monism: an analogy

(In the analogy Flip and Flop represent opposite extremes of a dualistic philosophy)

• Real

• Studied by physicists

• Relevant to psychologists and neuroscientists as a stimulus only

• Well known proponents: Newton, Einstein

• Perceived

• Studied by psychologists and neuroscientists

• Relevant to physicists only as a source of intuitions about ‘forces’ and other quantities; or in interpretation of data (including design of measuring devices)

• Well known proponent: Berkeley

Flip

Flop

In the analogy a move toward neutral monism

Flipflop

Neural delays in the retinaLIGHT

Receptor hyperpolarization

Delays due to signal transmission from retina to primary visual cortex and beyond

Neural delays in response to a discrete visual stimulus

Neural delays in processing of a continuous (moving) visual stimulus

Perceived

Real

Flop?

Flip?

• Assume:• Object velocity = v• Neural processing Delay= t

“vt-lead premise”:

• Due to neural delays a moving object’s real position should lead its perceived position by vt.

• Dilemmas and paradoxes

• Phenomenon: Flash-lag effect

On the ‘real position = perceived position + vt’ premise

• Real and perceived are spatially separated for motion but not for stationary objects (i.e. when v = 0)

• For stationary objects, are the real and the perceived one and the same, or are the real and the perceived co-localized (congruent)?

stationaryobject

Flash-lag effectFlash-lag effect

Light source 1 Light source 2

Dark lab, two light sourcesand a bar painted white.

What do observers see at the instant of the flash?

Percept

“Flash-lag effect”

Movement direction

F

F

M

demo

“I see, therefore, no opposition of the physical and the psychical, no duality, but simply identity.”

-Ernst Mach, 1890

What does the flash-lag effect have todo with neutral monism?

Mach denied quantities suchas absolute space and absolute timedue to the fact that these quantitieswere not observable

“This book exercised a profoundinfluence upon me… while I was a student.”

--Albert Einstein

•Can the spatial lag between the perceivedand the real position of the moving objectbe observed (measured)?

•Is this observation possible even in principle?

NO!

A B

midpoint

01:004

Following Mach: If it is unobservable it does not exist

A B

01:004

midpoint

On the ‘real position = perceived position + vt’ premise the following is a correct picture

But this assumption leads to contradiction

v

Consider an object moving atVelocity = v

Perceptiontrails behind

REAL

sensory delay = t

vt

0 t-t

FLE vtclock

Flash source (off)

20 - 160 ms

0 t-t

FLE vt

0 t-t

FLE vt

t0-t

FLE vt

Either the above picture is completely wrong.

OR

It is partly wrong and there are two different t’s: tm and tf, with tm < tf

RT MOTION

RT FLASH

230

240

250

260

270

280

Me

an

resp

onse

tim

e ( m

s )

0 1.17 2.34 3.51 4.68

Velocity (deg/sec)

Nijhawan et al. (Visual Cognition, in press)

No evidence for faster processing of motion

1.17 2.34 4.68

-25

-15

-5

5

15

25

Velocity (deg/sec)

Lead

of f

lash

for

perc

eive

d si

mul

tane

ity (

ms)

No evidence for faster processing of motion

The above picture is wrongand neutral monism is correct

Conclusion:

Intuition and analysis suggest very different answersconcerning the functioning of the central nervoussystem.

•Intuition suggests that there is a sharp boundary, and a wide gap, between the sensory and motor functions of the CNS.

•Intuition also suggests that there are “real” objectsout there in the world which are “perceived” by theobserver.

•The nervous system development starts out with both sensory (touch-proprioception)and motor functions emerging from the same cells.

•A close look at the brain reveals that more than 95% of neurons in our CNS can neither be categorized as “sensory” nor as “motor”.

•Many neurons in the somatosensory cortexproject to the motor neurons in the spinalcord.

•Many neurons in the premotor cortex respond to sensory stimulation.

The main divide between the sensory and motor functions of the CNS seems to be centered on the following intuition:

Sensory processes are geared to processing objects in the “real” world, while motor processes seem geared to moving limbs etc. of the body to which these processes are attached.

The INPUT vs OUTPUT divide

The INPUT vs OUTPUT divide has, I believe,kept scientists from accepting that sensory andmotor processes are two facets of oneunderlying process.

•OUTPUT: The brain “generates” commandsthat move limbs.

•INPUT: Science is dominated by sightedindividuals, so the concept of “real objects”is synonymous with “objects out there”. Ifobjects are “out there” then information musttravel from objects to our eyes. Hence theconcept of INPUT.

Factors maintaining the INPUT vs OUTPUT divide:

Here is the dilemma: Talking aboutinput in the absence of what it is aninput of is meaningless. If we talkof an object as the source of the inputto the visual system (say), then we areusing the OUTPUT of the brain—“the object” —to call it the sourceof the input. Thus, when we use the terms‘sensory INPUT’ in the traditional sense,we are confusing OUTPUT with INPUT.

I argue that “real” objects are actually a result of one branch (branch B) of the output of the CNS. The other branch of the output (branch A) is dominated by touch, proprioception and the motor functions. Branch A is earlier both in phylogeny and ontogeny than branch B. Thus both sensory and motor functions are outputs.

Speculations based on evolution, development and plasticity

The human nervous system development

41 weeks

Human development: 10 - 41 weeks

… the possibility of learning the significanceof the local signs which belong to oursensations of sight, so as to be able torecognize the actual relations which theydenote, depends, first, on our havingmoveable parts of our own body withinsight.

-- Helmholtz

A. van der Meer, F. van der Weel & D.N. Lee, Science, 1995

Functional significance of arm movements in neonates

10 - 24 days old

Motor flash-lagMotor flash-lag

Voluntary movement of a rod in theabsence of visual feedback

(Nijhawan and Kirschfeld, 2003)

QuickTime™ and aPhoto - JPEG decompressor

are needed to see this picture.

The outcome

sensed positionof rod

Flash-lag Effect for Voluntary Limb Movement

0

2

4

6

8

10

12

PM RJ ME MS

Observer

Fla

sh-l

ag E

ffec

t (cm

)

Motor Flash-lag anisotropyMotor Flash-lag anisotropy

0

90

Motor task

fixation point

0

90

Motor task

fixation point

0

90

Motor task

fixation point

0

90

Motor task

fixation point

0

90

fixation point

Flash

Flash

Vision task

0

90

Vision task

fixation point

0

90

Vision task

fixation point

Fla

sh-l

ag e

ffec

t (cm

)

Movementtowardfixation

Movementaway from

fixation

Movementtowardfixation

Movementaway from

fixation

Motor task Vision task

2

4

6

8

10

12

Fla

sh-l

ag e

ffec

t (cm

)

2

4

6

8

10

12

--Graziano, Hu and Gross, 1997

vision + touch neurons in area F4 of premotor cortex

The visual receptive fields are anchored to a given body part,and move with the body part irrespective of eye position

Many neurons in area F5 of premotor cortex respond when theanimal performs a particular action, and to visual stimulationin the absence of overt action.

-- Rizzolati, Fadiga, Gallese and Fogassi, 1995

Intermediate net

The intermediate system “controls” the kinematicsof limb movements and of “external” visual objects.

Limb movement Object motion

Peripheralinput

Peripheralinput