1-B-19 歩く人・走る人の背景に知覚される仮現運動(2002年度 日本基礎心理学会第21回大会優秀発表賞)

The Japanese Psychonomic Society

NII-Electronic Library Service

The JapanesePsychonomic Society

TVteYitPaneselburnal uf Rs.ychonomicScience

2003,Vol.22,No,1,27-28

Prizewinner's

Summary1-B-19

Backdrop

motion

illusion

from

Kiyoshi

images

FuJiMoTo*･

** _and

University

of

Toleyo

of

a

walking

human

figure

Takao

SATo***

*･***

We have found a novel vi$ual

illusjon

where an ambiguous motion became unidirectional with

the

superimposition of an image of a hurnan

figure

walking on a

trcadmilL

When we

presented

a counterpha$e grating as an ambiguous backdrop, the

grating

appeared to driftin theopposite

directiontothe

bipedal

locomotion. This _phenomenon indicatesthat the visual system would

evaluate

low-level

motion signals relative toa

high-lcvel

representation of an object's movernent

defined

by

its

biological

motion. In thisstudy we presented images of human figurescither walking

forward

or walking backward inorder toexamine the effects of a

directional

correspon-dence between human rnovement and

form

that was important

for

thc recognition of

biological

motion. The results revealed that a

forward

walker _produccd the illusionmore reliably than a

backward

walker, and confirmed thatrecognition of

locomQtion

isa _prirnarydeterminant forthe

illusion,

Key words: visual Musion, motion

perception,

biologicalmotion

Current

models of motion _perccption attempt to explain how the visual system estimates the move-ment of an object from retinal fiow

(Lu

&

Sperling,

2002). However, thevisual system isa]so capable of

representing the movernent of an object even ifan object retains a

fixed

position on thc retinas when

theeyes are stationary. For example, the translation

of a wa]ker isrecognized solely from the articulated

rnovernents of the

limbs.

Our investigationsdemonstrate that such a

high-level

representation of human movements

is

so pow-erful that

it

can modulate low-levelmotion proccss-ing so that an apparent motion in a background

pattern isinduced, Inorder toexplere this

phenomc-non we

have

designed

movie clips inwhich the im-age of a walklng

human

figure

is

superimposed on a counterphase grating

(Figure

1).

The

grating isthe

sum of two _gratings moving at an equal vclocity in

opposite

directions.

This grating isusually per-*

_Intelligent

_Modeling

_{Laboratory, University of}

Tokyo,

1--1-1

Yayoi,

Bunkyo-ku, Tokyo

8657

** _{Present address:}

_Life

_ElectronIcs

_Laboratory,

tiona] Instituteof Advanced

Industrial

Science

and

Technology,

1-8-3

Midori-ga-oka,

Ikeda,

Osaka

563-8577

***

_Department

_{of Psychology, Faculty of Letters,}

University

of

Tokyo,

7-3-1

Hongo, Bunkye-ku,

Tokyo

113-O033

ceived as a

directionless

flicker,which indicatesa non-biased

integration

of the two

dircctional

signals extracted jn the earliest motion

processing.

How-ever, when an

image

of a walking human

figure

is

superimposed on thecounterphase _grating,the

grat-ing

appears tobe unidirectional, as though a striped wall was moving relative to the stationary walker

(for

demonstrations of this illusion,visit http:f!www

1.u-netsurLne,jp/

fff/gait/),

Inthepresent study wc have examined theeffects

of a directional correspondence

between

human

movement and

form

by

using an image of a human

walking ineither oftwo ways: forward or backward.

The backward walker faced

in

theopposite direction

to

locomotion,

Ifthe

form

were crittcal, thc percep-tionof the

direction

ef thegrating movcment would be oppo$ite

'to

the

direction

the

figurc

was facing,

nttrnely, in the same direction as the

backward

Figure1.

Movie

clips of the backdrop rnotion

illusion.

If

the person iswalking toward the Ieft,

thegrating appears to

drift

toward therjght,

The Japanese Psychonomic Society

NII-Electronic Library Service

The JapanesePsychonomic Society

28

The

Japanese

Journal

of Psychonomic

Science

Vol.22, No. 1 walking.

Method

Apparatus. Visual stimuli were

displayed

on a

color

CRT

monitor by using an

Apple

PowerMacin-tosh, A _gamma correction was applied to

_produce

luminance

]inearity.

Each

observer viewed the

dis-play

binocularly

from a distanceof

90

cm while their

head

was supported

by

a

headrest,

The

experiment was conducted

in

a

dark

room.

StimulL

Theimagesofhumanfigureswalkingon

a treadmillwere designed with

Curious

Labs

Poser

4

software. The figures

faced

either leftor right, and they subtended 2.9×

1.2

degrees

of visual angle in

height

and width, respectively. The walkers

com-pleted

one step-cycle ina _periodof 1,2seconds,

The image of each

human

figure

was

superim-posed on asinusoidal counterphase grating that

sub-tended

5

degrees

of visual angle

in

beth

height

and

wiqth.

The

spatial and temporal

frequencies

were 4 cycles _per degree and

8

Hz,

respectively. The

lumi-nance contrast was 60% and the mean

luminance

was 30cdlm2. The luminance contrast was

multi-pliedby a 2-D Gaussian envelepe with a standard

deviation

of

1

degree

of visual angle. The stirnulus

was presented

in

a uniform _gray fieldwith

the

mean

luminance

of the_grating,The stimulus

duration

was O.48seconds.

Procedure.

Eleven

observers were asked to

re-port theirperceptual impre$sion$ ofthe gratingwith

aforcedchoice of three alternatives:

flickering,

drift-ing

left,

or

drifting

right,

The

two

directional

re-sponses were classified as an '`opposite

response" or a

"same

response" according

to

theirrelationship to the walking

direction,

Each

_participantobserved, and reported on,

16

trials

in

which the figurewalked

forward and 16 trialsin which

the

figure

walked

backward.

Results

For the forward walker, _the opposite response was

obtained for

67,1%

of the trials,the sarne response

for

5,1%

of the trials,and the flickerresponse for

27.8%

ofthe trials.For thebackward walker,

38.6%

of the responses were

the

opposite response,

but

36.4% were

the

same response and

25.0%

were the

flickerresponse.

Analysis

of thedata with two-taired

t-tests

indicated

that the opposite response

for

the

forward

walker occurred significantly more often

than the oppesite response or thesame response for

the

backward

walker, ts(1O)>3.25,

Ps<.O1,

Discussion

The

forward

walker

induced

an apparent motien of the _grating

in

a

direction

opposite tothe walking

with the

high

_probability.On

the

other

hand,

images

of the

backward

walker induced apparent motion

in

the opposite and same

directions

with the equal

probabilities,and the percentages were significantly

lower

than that for the opposite response

for

_the

forward

walker,

These

results

indicate

thata

corre-spondence of

direction

between

human motion and

form iscritical ior thc illusion.This isconsistent with arecent model of biologicalmotion recognition, assuming that _the recognition

is

achieved by a

co-operation between motion and form _processing

{Giese

&

Poggio,

2003). Learning also contributes to therecognition, and thismay account

for

the

arnbigu-ous results from the

backward

walker, Backward

walking

is

rarely encountered ineveryday life.

Our

findings

could impact on

the

current models of

motion _perceptienas

follows.

First,

low-level

motion signals could be modulated relative toa high-level representation of object motion which is

defined

by

biologicalmotion. Second. such modu]ation effects

extend toa relatively largevisual

field

over the area inwhich

the

object motion

is

represented. The per-cept reflects a scene thatisfrequentlyexperienced

in

everyday life.Thus, our

findings

suggest the exis-tcnce of a scene-based modulation of motion signals

by a high-levelperceptua] inference.

Referenees

Giese,M,A.

&

Poggio,

T. 2003 Neural mechanisms

for

the

recognition of

biological

movements.

ture

Reviews

Aiburoscience,

4,179-192.

Lu,

Z L.

&

Sperling,G.2001 Three-systems theory of

human

visual motion perception: review and

update.

fournat

of

OPtical

Sociely

ofAmerica

_{A, 18,}