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Levelt, W.J.M.

Friederici, A.

Levelt, W.

MPG.PuRe

Cognitive processes of spatial coordinate assignment: On weighting perceptual cues

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wissenschaften
Cognitive Processes 
of Spatial Coordinate 
Assignment
On Weighting Perceptual Cues
A.D. Friederici and W .J.M . Levelt
M ax-Planck-Insti tu t  für Psycholinguistik, 
NL-6525 X D  Nijmegen
perception o f  space and com m unica tion  abou t 
space requires the co m p u ta t io n  o f  different 
I pes o f  in form ation , such as visual, vestibular and 
S' matosensory inform ation . A lthough  initially 
processed at physiological levels, in tegration and 
interpretation o f  these different perceptual cues
i ke place at a cognitive level. At this level different 
types o f  perceptual in form ation  are weighted and 
mapped on to  some stored, cognitive representation  
o space. In order to achieve unam biguous repre­
ss ntalions o f  what has been perceived or w hat has 
been talked abou t,  the cognitive system has to es- 
t blish a reference frame with respect to which con- 
c pts like “ u p "  and “ d o w n " ,  “ le f t"  and “ r ig h t"  
arc used. It has been argued tha t  the e a r th ’s gravi- 
ional field is one o f  the most fundam enta l  con ­
straints for such a reference choice [5, 6].
! here are, however, at least three types o f  percep­
tual cues which are to be considered when choosing 
a referential frame: the retinal coordinates , the in­
trinsic coord inates  o f  the visually perceived object 
o background and  the gravita tional coord inates  
defined via vestibular an d /o r  som atosensory  input 
information. It is likely tha t  possible conflicts be­
tween coord ina te  systems suggested by different 
types o f  in form ation  are resolved by weighting the
cues differentially [3]. G rav ita t ional inform ation , 
for example, is given a different weight depending 
upon which inform ation  types are b rough t into 
conflict [1, 2, 4]. The prim ary goal o f  this study 
was to determine the role o f  gravity for a spatial 
coord inate  assignment and, moreover, for the m en­
tal representation  o f  space. The general experim en­
tal app roach  was to vary four different factors sys­
tematically: (a) the retinal inform ation , (b) the vi­
sual background  inform ation , (c) the som atosen ­
sory inform ation  and (d) the gravity inform ation.
Experiment I
Method
The subjects were two male payload specialists 
(R .F. and  W.O.), who were part o f  the crew o f  
the D1 mission executed in 1985.
The stimuli were visual arrays varying factor (a), 
represented by two intrinsically non-oriented  o b ­
jects (a white ball and black ball) whose (visual) 
connecting axis could be ro ta ted  to different angles 
with respect to the retinas' vertical meridians, and 
factor (b), represented by two intrinsically oriented 
objects, i.e. two line drawings o f  trees which could 
be displayed to the left and to the right o f  the 
balls or ro ta ted  to some angle. The orien ta tion  o f  
the axis o f  the two aligned balls varied in 
22.5° steps, clockwise off-vertical, from 22.5° to 
360°, the tree orien ta tion  varied in 45° steps off- 
vertical, from 45° to 360°, resulting in 128 different 
stimulus items. These stimuli were presented in 
random  order in a specially designed ap p a ra tu s  
(VISOS). This hardw are  consisted o f  a viewing aid 
m oun ted  on a commercial cam era, using an O lym ­
pus C am era  O M -2 plus O lym pus W inder 2 with 
a remote control,  a Pentax Stereo Viewer II and 
a pair o f  goggles. Developed films, conta in ing  the 
stimulus material, were presented in the goggles. 
A microcassette recorder (Pearlcoder S801), which 
served to record the responses, was a ttached  to 
VISOS. The winder as well as the cassette recorder 
operated  on a battery  basis.
Subjects were required to describe the position o f  
the “ white b a l l"  with respect to the “ black ba l l"  
in each o f  the visual arrays, using words like 
“ a b o v e " ,  “ be low ", “ left" ,  “ r ig h t"  and  co m b in a ­
tions of  these. They were asked to respond as ac­
curately and  as fast as possible. The exposure d u ra ­
tion o f  each trial as well as the presenta tion  o f  
the next trial was controlled by the subject, who 
pressed a rem ote contro l bu tton . The subject’s ver­
bal responses were recorded for later analysis.
Naturwissenschaften 73, 4 5 5 -4 5 8  (1986) ©  Springer-Verlag 1986 455
Tests were per fo rmed  preflight,  inflight and  p o s t ­
flight. D u r in g  preflight and  postf l ight  sessions, 
subjects were s tand ing  upr ight  with their  heads 
upright .  D u r in g  the two inflight sessions, subjects 
were free f loat ing and  were required  to keep their 
heads  s t raight ,  tha t  is, al igned with their  body  axis,
i.e. fac tor  (c) was kept  cons tan t  in this exper iment .
Results
Verbal  responses were ana lyzed  with respect to 
w ha t  type o f  reference was chosen under  the differ­
ent  condit ions .  F r o m  the analysis it is clear  tha t  
subjects did no t  use the visual b a c k g ro u n d  in fo r ­
m a t ion ,  i.e. the trees, as a reference frame,  ei ther  
in prefl ight and  postf l ight  tests o r  in weightless­
ness. Pre/light:  Both subjects used a coo rd ina te  
system indicated by three in fo rm a t io n  types:  the 
gravi ta t ional ,  the body-def ined  vertical and  the ret­
inal vertical,  all o f  these being aligned. Thei r  d e ­
scr ipt ions with respect to this reference were bas i ­
cally w i thou t  e r ro r  (Table  1). Infl ight : Both s u b ­
jects used the retinal,  and  at  the same time body-  
defined vertical coo rd ina te  system as a reference. 
In con t ras t  to W.O. ,  R .F .  displayed a significant 
increase o f  inadequa te  descript ions when c o m p a r ­
ing inflight tests with the prefl ight pe r fo rm ance  
(L +  2 h:  A' 2 =  6.64; p <0.01  an d  L + l  d:  .y2 =  7.43; 
/ ; < 0  .01). M o s t  o f  the descript ions which were in­
correct  with respect to the p r im a ry  chosen refer­
ence, were, however ,  a d e q u a te  when intrinsically 
or iented objects were in terpre ted  as hor izon ta l  (90° 
and  270°) or  as vertical (180° and  360°) c o o r d i ­
nates,  i.e. there was an effect o f  visual b a c k g ro u n d  
in fo rm at ion .  Immedia te ly  postf l ight  bo th  subjects 
chose a vertical system, indicated by the g rav i ta ­
t ional,  the body-def ined  and  retinal coo rd ina tes  as 
the p r im ary  reference. A t  R  + 1 8  h R .F .  d em o n -
Table  1. Exper im ent  I: Visual axis ro ta ted  in steps o f  22.5°. 
H ead  straight.  Percentage  o f  responses which were incorrect  
with respect to the p red o m in an t ly  chosen  reference (L =  
Launch ,  R =  R e tu rn  o f  Space Shutt le ,  h =  hours ,  d =  days)
Subject T ime o f  pe r fo rm ance
Preflight Inflight Postflight
L —85 d L +  2 h a L + l  d R +  18 h a 
R +  19 h
R +  108 d 
R +  104 d
R.F . 0.8 14.3 18.1 11.4 3.2
W.O. 0.8 2.6 2.9 5.7 2.3
a D ur ing  these sessions, shor tened  versions o f  the original s t im ­
ulus set were used
s t ra ted  a higher  rate o f  inadequa te  descript ions 
with respect to the p re d om inan t ly  chosen g rav i ta ­
t ional reference than  preflight (a*2 =  8.05; / ? < 0 .01). 
O n  R - f l d  R . F . ’s pe r fo rm ance  in this task was 
back to the prefl ight  level.
Discussion
The  results f rom the two subjects show tha t  a c o r ­
rect spat ial  coo rd ina te  ass ignment  is possible under  
the absence o f  gravi ty in fo rm a t ion  when retinal 
and  body-def ined vertical coo rd ina te s  were kept 
aligned. The  da ta ,  fu r thermore ,  indicate tha t  c o n ­
flicts between com pe t ing  perceptual  cues can be 
m ore  easily resolved as m ore  in fo rm a t ion  types 
“ c o n s p i r e ” to suggest  the same co o rd in a te  system 
as in 1 g,  where  g rav i ta t ional ,  body-def ined  and 
retinal vertical systems coincide when  s tanding 
upr ight  with the head s traight .  N o te ,  however ,  that  
the qual i ta t ive  analysis  su p p o r t s  this only for one 
o f  the two subjects.
Experiment II
A second exper iment  was  designed in o rde r  to de­
termine  the p r e d o m in a n t  reference choice under  
different  gravi ty cond i t ions  when  retinal and  body- 
defined reference systems conflict.
Method
The  subjects par t ic ipa t ing  in this exper iment  were 
the same as for Exper iment  I.
The  a p p a r a tu s  was the same as in Exper im en t  1. 
The  s t imulus  mater ia ls  differed f rom those used 
in the fo rm er  exper iment  in tha t  the axis o f  the 
two intrinsically non-o r ien ted  objects (balls) as 
well as the o r ien ta t ion  o f  the two intrinsically or i­
ented objects  (trees) were al tered in steps o f  7 
clockwise an d  counterc lockwise  f rom the vertical 
(360° an d  180°), and  the ho r izon ta l  (90° an d  270°), 
result ing in 240 visual arrays .  In add i t ion ,  the n o n ­
or iented  objects  were displayed in all posi t ions 
w i thou t  any  con tex tua l  f rame in fo rm at ion .
As in Exper imen t  I, verbal  descr ip t ions  o f  visually 
presented  a r rays  were required  with the subject 's 
head s t ra igh t  unde r  prefl ight,  inflight an d  p o s t ­
flight condi t ions .  Infl ight  an d  postf l ight  tests were 
also pe r fo rm ed  with the sub jec t ’s head  tilt to the 
left and  r ight  side. Subjects  re sponded  to h a l f  ol 
the s t imulus  i tems when  the head  was tilted by 
app rox im a te ly  30° to 35° to the left, an d  to hall
456
of the s t imulus mater ia l  when the head was tilted 
to the right. D ur ing  inflight sessions, subjects were 
free f loat ing;  du r ing  postf l ight  sessions, subjects 
were s tand ing  upr ight  with their  heads tilted to 
the left or  to the right.
Results
he subjects '  response pa t t e rn  under  the head  
straight condi t ion  basically replicated the findings 
of Exper iment  I (Table  2). Pre/light  and  post/ l ight:  
Both subjects used the grav i ta t iona l  vertical c o o r ­
dinate system, which at  the same time was the reti­
nal an d  the body-def ined  vertical as reference. In 
weightlessness  bo th  subjects chose a vertical sys­
tem, which was indicated by the retinal  and  the 
the body-def ined  axes. The  visual b a c k g ro u n d  in-
Tablc  2. Exper iment  II: Visual axis ro ta ted  in steps o f  7°. H ead  
straight .  Percentage o f  responses which were incorrect  with re­
spect to the p redom inan t ly  chosen reference
Subject T ime o f  pe r fo rm ance
Preflight Inflight Postflight
L —85 d L +  2 h L +  1 d R +  18 h R +  6 d R +  108 d 
R +  10 4  d
R.F. 2.8 16.7 18.8 19.4 20.8 13.9
W.O. 8.3 2.7 6.9 9.7 9.7
fo rm at ion  was not  chosen as a p r imary  reference. 
Subject  R .F .  showed a significant increase o f  i n a d ­
equa te  descript ions when c o m p a r in g  prefl ight and  
inflight tests .y 2 =  4.73;  / ; < 0 . 0 5  and  .y 2 =  7.43; p <
0.01), and  when c o m p a r in g  preflight with each o f  
the postfl ight  tests (all com p ar i so ns  were signifi-
a
101) - ,
90 -
30 -
71) -
M l  -
5( -
JO -
20
10 -
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I I
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hi*.id bodv head 
♦ bod v
100 -
90 -
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70 -
fiO -
SO -
40 -
*0 -
JO -
10 -
REFERENCE
head body«3 head 
gravity ♦body
REFERENCE
100 -
90 -
80 -
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50 -
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gravity ♦body
REFERENCE
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50 -
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io -
< *  . . . . . .
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head body* head 
gravity ♦body
REFERENCE
INFLIGHT 
1. ♦ I dav
POSTFLIGHT 1 
R + I dav
P0STFLIGHT II 
R + ft days
OO-i
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♦ bodv
REFERENCE
INFLIGHT 
L ♦ I day
100 -
90 -
80 -
70 -
bO  -
50-
40-
10 -
20-
10 -
Ill
x v X 'X * :  
■ • v . v . v . v  
. v . v . v . %  
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. v . v . v  
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REFERENCE
POSTFLIGHT I 
K ♦ I day
100-
90 -
80 -
70 -
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50 -
4 0  -
lo­
di­
lo -
head body* head 
gravity ♦body
REFERENCE
POSTFLIGHT III 
R ♦ 104 days
POSTFLIGHT III 
R ♦ 108 days
Fig. 1. Exper iment  II: Visual 
axis ro ta ted  in steps o f  7°, 
head tilt, subjects R.F.  (a) and  
W.O. (b). Percentage o f  
chosen reference. Head  verbal 
responses which are correct  
with respect to the retinal 
(head)  coord ina tes .  Body 
verbal responses which are 
correct  with respect to the 
body-def ined (in 1 g  also the 
gravi ta t ional)  coord ina tes .  
Head  4- body verbal responses 
which are correct  with respect 
to bo th  retinal and  body-  
defined coord ina tes ,  e.g. when 
visual axis o f  the balls is 7° 
and  the head is tilt to the right 
by app rox im a te ly  30° to 35°, 
the correct  descr ipt ion with 
respect to each o f  the 
coord ina tes  would  be “ right 
a b o v e ”
457
Table  3. Exper im ent  I I : Visual axis ro ta ted  in steps o f  7°. Head 
tilt. Percentage o f  responses which were incorrect  with respect 
to the p redom inan t ly  chosen reference
Subject T ime o f  per fo rm ance
Inflight Post High t
L +  I d R + 1  d R +  6 d R +  108 d 
R +  104 d
R.F. 28.5 0 23.6 23.6
w.o. 23.6 14.1 ----- 9.7
can t  a t  /?<0 .01) .  In con t ras t  to preflight pe r fo r ­
mance,  R . F . ’s postf l ight  pe r fo rm ance  showed a 
slight influence o f  the visual b a c k g ro u n d  in fo rm a ­
tion in choos ing  the reference frame.
The  results o f  the head tilt cond i t ion  u n a m b ig u o u s ­
ly de te rmined  w ha t  type o f  reference was used 
under  the different gravity cond i t ions  for  this type 
o f  task. The  subjects verbal  responses could ei ther 
be correct  with respect to the visual b a c k g ro u n d  
in fo rm at ion ,  the retinal vertical or  the body-de-  
llned vertical,  which under  1 g  coincided with the 
gravi ta t ional  vertical,  or  co m b in a t io n s  o f  these. Vi­
sual b a c k g ro u n d  in fo rm a t ion  was  not  used as a 
p r imary  reference. Reference choice for  the two 
subjects are displayed in Fig. 1, rates o f  incorrect  
responses within the chosen  reference f rame are 
listed in Table  3.
F ro m  these d a ta  it is evident  that  for references 
in microgravi ty  both  subjects p re d om inan t ly  used 
the retinal (head) vertical as a reference, which in 
this condi t ion  did not  coincide with any  o the r  c o ­
o rd ina te  system suggested by o the r  perceptual  in­
format ion .  In 1 g  g rav i ta t iona l  an d  body-defined 
coord ina tes  were preferred as a reference. The  dif­
ference o f  the d is t r ibu t ion  o f  chosen references in 
weightlessness and  immedia te ly  postfl ight  (R + 1  d) 
was statistically significant for bo th  R.F .  (a 2 =  
28.2 ; p  <  0.001 ) and  W.O.  (a 2 =  5.04; p  <  0.05). Per­
fo rmance  o f  later postfl ight  tests (R - f -104d  and  
R +  108d)  indicated tha t  the subjects chose gravi­
ta t ional  coord ina tes  as a reference in 1 g. R.F.  
d em o n s t ra ted  a cer tain  aftereffect o f  weightlessness 
in the s t rength  o f  this behavior :  immedia te ly  post-  
flight (R  +  l d), a gravi ta t ional  reference was c h o ­
sen m ore  often than  on bo th  o the r  postf l ight  occa­
sions (R +  6 d : .v2 =  24.54; /; <0 .001  and  R +  108 d : 
a 2 =  24.54; p <  0.001).
Discussion
The  findings from the two subjects tested here, in­
dicate tha t  different  reference systems are used for 
spat ial  ass ignment  in 1 g  when s tand ing  upright  
and  in microgravi ty .  In  weightlessness subjects p re ­
d o m in an t ly  use the retinal vertical coord ina tes  as 
reference, whereas  on ea r th  they prefer  the gravita-  
t ionally defined vertical which,  when s tanding  
upright ,  coincides with the body-def ined  vert i­
cal.
The  com bined  da ta  show tha t  a l though  gravity 
normal ly  plays an  im p o r t a n t  role in the reference 
choice, consis tent  spat ial  ass ignment  can be made  
when gravity is absent .  This f inding suggests that  
the menta l  represen ta t ion  o f  space is an abs t rac t  
one  and  can be used ra ther  independent ly  o f  differ­
ent  perceptual  input  parameters .  F u r th e rm o re ,  the 
a s su m p t io n  is sup p o r ted  tha t  an  u n a m b ig u o u s  sp a ­
tial ass ignment  is achieved by weight ing different 
perceptual  in fo rm a t ion  differential ly;  a par t icu la r  
cue loses weight when two or  m ore  o the r  cues c o n ­
spire agains t  it. In weightlessness spatial  ass ign­
ment  is mos t  successful when d o m in a n t  weight  is 
given to retinal  in fo rmat ion .  W h e th e r  such weight­
ing and  con diet resolving procedures  are u n d e r  at- 
tent ional  contro l  is open  for fu r ther  research.
The  research repor ted  here was su p po r ted  by the M ax-Planck-  
Gesel lschaft  and  the Bundesminis ter ium für F o rsc h u n g  und 
Technologie .
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J.J. K oender ink ,  eds.). U t rech t :  V N U  Science Press 1984
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Das Heft enthält eine Beilage des Gustav Fischer Verlages. Stuttgart
458


Cognitive processes of spatial coordinate assignment : on weighting perceptual cues

NARCIS 

-N tur wlssenschaflen Cognitive Processes of Spatial Coordinate Assignment On Weighting Perceptual Cues A.D. Friederici and W.J.M. Levelt Max-Planck-Institut fiir Psycholinguistik, NL-6525 XD Nijmegen p erception of space and communication about space requires the computation of different types of information, such as visual, vestibular and somatosensory information. Although initially processed at physiological levels, integration and interpretation of these different perceptual cues take place at a cognitive level. At this level different types of perceptual information are weighted and mapped onto some stored, cognitive representation of space. In order to achieve unambiguous repre- sentations of what has been perceived or what has been talked about, he cognitive system has to es- tablish a reference frame with respect to which con- cepts like "up"  and "down",  "left" and "right" are used. It has been argued that the earth's gravi- tational field is one of the most fundamental con- straints for such a reference choice [5, 6]. There are, however, at least three types of percep- tual cues which are to be considered when choosing a referential frame: the retinal coordinates, the in- trinsic coordinates of the visually perceived object or background and the gravitational coordinates defined via vestibular and/or somatosensory input information. It is likely that possible conflicts be- tween coordinate systems suggested by different types of information are resolved by weighting the cues differentially [3]. Gravitational information, for example, is given a different weight depending upon which information types are brought into conflict [1, 2, 4]. The primary goal of this study was to determine the role of gravity for a spatial coordinate assignment and, moreover, for the men- tal representation f space. The general experimen- tal approach was to vary four different factors sys- tematically: (a) the retinal information, (b) the vi- sual background information, (c) the somatosen- sory information and (d) the gravity information. Experiment I Method The subjects were two male payload specialists (R.F. and W.O.), who were part of the crew of the DI mission executed in 1985. The stimuli were visual arrays varying factor (a), represented by two intrinsically non-oriented ob- jects (a white ball and black ball) whose (visual) connecting axis could be rotated to different angles with respect o the retinas' vertical meridians, and factor (b), represented by two intrinsically oriented objects, i.e. two line drawings of trees which could be displayed to the left and to the right of the balls or rotated to some angle. The orientation of the axis of the two aligned balls varied in 22.5 ~ steps, clockwise off-vertical, from 22.5 ~ to 360 ~ , the tree orientation varied in 45 ~ steps off- vertical, from 45 ~ to 360 ~ , resulting in 128 different stimulus items. These stimuli were presented in random order in a specially designed apparatus (VISOS). This hardware consisted of a viewing aid mounted on a commercial camera, using an Olym- pus Camera OM-2 plus Olympus Winder 2 with a remote control, a Pentax Stereo Viewer II and a pair of goggles. Developed films, containing the stimulus material, were presented in the goggles. A microcassette r corder (Pearlcoder $801), which served to record the responses, was attached to VISOS. The winder as well as the cassette recorder operated on a battery basis. Subjects were required to describe the position of the "white ball" with respect o the "black ball" in each of the visual arrays, using words like "above ", "below ", "left", "r ight" and combina- tions of these. They were asked to respond as ac- curately and as fast as possible. The exposure dura- tion of each trial as well as the presentation of the next trial was controlled by the subject, who pressed a remote control button. The subject's ver- bal responses were recorded for later analysis. Naturwissenschaften 73, 455-458 (1986) 9 Springer-Verlag 1986 455 Tests were performed preflight, inflight and post- flight. During preflight and postflight sessions, subjects were standing upright with their heads upright. During the two inflight sessions, subjects were free floating and were required to keep their heads straight, that is, aligned with their body axis, i.e. factor (c) was kept constant in this experiment. strated a higher rate of inadequate descriptions with respect to the predominantly chosen gravita- tional reference than preflight (x2= 8.05; p < 0.01). On R+I  d R.F.'s performance in this task was back to the preflight level. D&cussion Results Verbal responses were analyzed with respect to what type of reference was chosen under the differ- ent conditions. From the analysis it is clear that subjects did not use the visual background infor- mation, i.e. the trees, as a reference frame, either in preflight and postflight ests or in weightless- ness. Preflight: Both subjects used a coordinate system indicated by three information types: the gravitational, the body-defined vertical and the ret- inal vertical, all of these being aligned. Their de- scriptions with respect to this reference were basi- cally without error (Table 1). Inflight: Both sub- jects used the retinal, and at the same time body- defined vertical coordinate system as a reference. In contrast o W.O., R.F. displayed a significant increase of inadequate descriptions when compar- ing inflight tests with the preflight performance (L+2h:  xZ=6.64;p<0.01 and L+I  d: x 2=7.43; p <0.01). Most of the descriptions which were in- correct with respect o the primary chosen refer- ence, were, however, adequate when i trinsically oriented objects were interpreted as horizontal (90 ~ and 270 ~ ) or as vertical (180 ~ and 360 ~ ) coordi- nates, i.e. there was an effect of visual background information. Immediately postflight both subjects chose a vertical system, indicated by the gravita- tional, the body-defined and retinal coordinates as the primary reference. At R+18 h R.F. demon- Table 1. Experiment I: Visual axis rotated in steps of 22.5 ~ Head straight. Percentage of responses which were incorrect with respect to the predominantly chosen reference (L= Launch, R = Return of Space Shuttle, h = hours, d = days) Subject Time of performance Preflight Inflight Postflight L -85d  L+2h"  L+ld  R+18h ~ R+108d R+19h R+104d R.F. 0.8 14.3 18.1 11.4 3.2 W.O. 0.8 2.6 2.9 5.7 2.3 a During these sessions, shortened versions of the original stim- ulus set were used The results from the two subjects how that a cor- rect spatial coordinate assignment is possible under the absence of gravity information when retinal and body-defined vertical coordinates were kept aligned. The data, furthermore, indicate that con- flicts between competing perceptual cues can be more easily resolved as more information types "conspire" to suggest he same coordinate system as in 1 g, where gravitational, body-defined and retinal vertical systems coincide when standing upright with the head straight. Note, however, that the qualitative analysis supports this only for one of the two subjects. Experiment IIA second experiment was designed in order to de- termine the predominant reference choice under different gravity conditions when retinal and body- defined reference systems conflict. Method The subjects participating in this experiment were the same as for Experiment I.The apparatus was the same as in Experiment I.The stimulus materials differed from those used in the former experiment in that the axis of the two intrinsically non-oriented objects (balls) as well as the orientation of the two intrinsically ori- ented objects (trees) were altered in steps of 7 ~ clockwise and counterclockwise from the vertical (360 ~ and 180~ and the horizontal (90 ~ and 270~ resulting in 240 visual arrays. In addition, the non- oriented objects were displayed in all positions without any contextual frame information. As in Experiment I, verbal descriptions of visually presented arrays were required with the subject's head straight under preflight, inflight and post -  flight conditions. Inflight and postflight ests were also performed with the subject's head tilt to the left and right side. Subjects responded to half of the stimulus items when the head was tilted by approximately 30 ~ to 35 ~ to the left, and to half 456 of the stimulus material when the head was tilted to the right. During inflight sessions, subjects were free floating; during postflight sessions, subjects were standing upright with their heads tilted to the left or to the right. Results The subjects' response pattern under the head straight condition basically replicated the findings of Experiment I (Table 2). Preflight and postflight: Both subjects used the gravitational vertical coor- dinate system, which at the same time was the reti- nal and the body-defined vertical as reference. In weightlessness both subjects chose a vertical sys- tem, which was indicated by the retinal and the the body-defined axes. The visual background in- Table 2. Experiment II : Visual axis rotated in steps of 7 ~ Head straight. Percentage of responses which were incorrect with re- spect to the predominantly chosen reference Subject Time of performance Preflight Inflight Postflight L -85d  L+2h L+ld  R+18h R+6d R+108d R+104d R.F. 2.8 16.7 18.8 19.4 20.8 13.9 W.O. 8.3 2.7 6.9 9.7 - 9.7 formation was not chosen as a primary reference. Subject R.F. showed a significant increase of inad- equate descriptions when comparing preflight and inflight tests x ~ = 4.73 ; p < 0.05 and x 2 = 7.43 ; p < 0.01), and when comparing preflight with each of the postflight tests (all comparisons were signifi- I00  - 90.  80. 70"  60-  50"  .x+x-x. 4o. ~~ .............. 2o - i!i@!ii~! '7:117:11:71111: ~0 - 8 100-  90-  80-  70"  z 60-  ~a 50- 40.  30" b 20- 10- ::::~:::~ ;::5::::::::". 6.'::;:;::::::: iIN head body  head §  REFERENCE INFL IGHT L + I day -:.-:.'.-. i:!:.::~$~:i:! :::::s ::::::: ZZII .:-:.::;:.,.:;:.: !Nil i!i@ill :!:f:!:!:!:!:? head body  head +body INFL ICHT L +]  day 100 90 80 60 5O o il 2O 10 head body= bead grav i ty  +body  REFERENCE POSTFL IGHT I R + ] day :/i'{i!i)!/:: 8o - il:ilil;i: :8:Z:Z: :.::::x: 7o - ::ii1!:!!:!: :;i:i:Z: 60 - ZIII:II::I: :::.:..: ":!8!i!:::i: 50 - "Z$!:?;! 30-  20-  10-  head  body-  head grav i ty  +body REFERENCE POSTFL IGNT I R + ! day ] O0 90 80 70 60 50 40 30 20 i0 : 00 -  90 -  80-  70-  bO-  50-  40-  30" 20-  I0- Nii . . . .  N% !i@ii ,....,.,, 9 :7:-:,: 84 9 w::,::i "i:i!131:i: ]:il:i: ~ ,,,..,> :i:.i ,-,,.., ":,::::: :!i:i:i -:...:?,: ,::.,x-" 7.':':":7 ~:,.:::~,: body= head grav i ty  +body REFERENCE POSTFL IGNT II R + 6 days i~:i~il! ;;:~;i;iiiiii :::.i:i:i:i ::::.:.:. ::::::::i!: :::7::i.1 :}?:::::~ .::::.:.:: i:i:i:!.:$ .:::::::: :)i!i!i:iiii! . : : : .  ;~i.iii:ii: head bc~dF= bead grav i ty  +body RE~ERENCE POSTFLs  III R + ]04 days i00 - 90 80 70 60 50 30 ---  20 i0 :::.::.T iii!i$ili!i~ ::::::::::::: ){:!:!:i8!: iiii!;iiiiii  iiiiiiii:,iiii  !!ii!i!i!{!i!]! i[ii{i!ili!iill :::::::::::::: ::::::::::::: !!!!i[i!!{iiii! "~iii~il - r  :i:i:!:!:!8! iiii !ilili ili!!:i!!!!!{!i:!! i!i!iiiiii!i!i head body-  head grav i ty  +body REFERENCE POSTFL IGHT I I I  R + 108 days Fig. 1. Experiment II: Visual axis rotated in steps of 7 ~ head tilt, subjects R.F. (a) and W.O. (b). Percentage of chosen reference. Head verbal responses which are correct with respect to the retinal (head) coordinates. Body verbal responses which are correct with respect to the body-defined (in 1 g also the gravitational) coordinates. Head + body verbal responses which are correct with respect to both retinal and body- defined coordinates, e.g. when visual axis of the balls is 7 ~ and the head is tilt to the right by approximately 30 ~ to 35 ~ the correct description with respect to each of the coordinates would be "right above" 457 Table 3. Experiment II: Visual axis rotated in steps of 7 ~ Head tilt. Percentage of responses which were incorrect with respect to the predominantly chosen reference Subject Time of performance Inflight Postflight L+ld  R+ld  R+6d R+108d R+104d R.F. 28.5 0 23.6 23.6 W.O. 23.6 14.1 - 9.7 cant at p<0.01). In contrast o preflight perfor- mance, R.F.'s postflight performance showed a slight influence of the visual background informa- tion in choosing the reference frame. The results of the head tilt condition unambiguous- ly determined what type of reference was used under the different gravity conditions for this type of task. The subjects verbal responses could either be correct with respect o the visual background information, the retinal vertical or the body-de- fined vertical, which under 1 g coincided with the gravitational vertical, or combinations of these. Vi- sual background information was not used as a primary reference. Reference choice for the two subjects are displayed in Fig. I, rates of incorrect responses within the chosen reference frame are listed in Table 3. From these data it is evident hat for references in microgravity both subjects predominantly used the retinal (head) vertical as a reference, which in this condition did not coincide with any other co- ordinate system suggested by other perceptual in- formation. In I g gravitational and body-defined coordinates were preferred as a reference. The dif- ference of the distribution of chosen references in weightlessness and immediately postflight (R + 1 d) was statistically significant for both R.F. (x 2= 28.2; p < 0.001 ) and W.O. (x 2 = 5.04; p < 0.05). Per- formance of later postflight ests (R+ 104 d and R + 108 d) indicated that the subjects chose gravi- tational coordinates as a reference in 1g. R.F. demonstrated a certain aftereffect of weightlessness in the strength of this behavior: immediately post- flight (R + 1 d), a gravitational reference was cho- sen more often than on both other postflight occa- si ons (R + 6 d: x z = 24.54; p < 0.001 and R + 108 d: x a = 24.54; p < 0.001). Discussion The findings from the two subjects tested here, in- dicate that different reference systems are used for spatial assignment in 1 g when standing upright and in microgravity. In weightlessness subjects pre- dominantly use the retinal vertical coordinates as reference, whereas on earth they prefer the gravita- tionally defined vertical which, when standing upright, coincides with the body-defined verti- cal. The combined data show that although gravity normally plays an important role in the reference choice, consistent spatial assignment can be made when gravity is absent. This finding suggests that the mental representation f space is an abstract one and can be used rather independently of differ- ent perceptual input parameters. Furthermore, the assumption is supported that an unambiguous spa- tial assignment is achieved by weighting different perceptual information differentially; a particular cue loses weight when two or more other cues con- spire against it. In weightlessness patial assign- ment is most successful when dominant weight is given to retinal information. Whether such weight- ing and conflict resolving procedures are under at- tentional control is open for further esearch. The research reported here was supported by the Max-Planck- Gesellschaft and the Bundesministerium ffir Forschung und Technologic. 1. Bischof, N., Scheerer, E. : Psychol. Forsch. 34, 99 (1970) 2. Corballis, M.C., et al. : Percept. Psychophys. 24, 263 (1978) 3. Levelt, W.J.M., in : Limits in Perception (A.J. Van der Grind, J.J. Koenderink, eds.). Utrecht: VNU Science Press 1984 4. Parker, D.E., Poston, R.L., Gulledge, W.L.: Percept. Psy- chophys. 33, 139 (1983) 5. Rock, I. : Orientation and Form. New York : Academic Press 1973 6. Shepard, R.N., Hurwitz, Sh. : Cognition 18, 161 (1984) Verantwortlich fiirden Anzeigenteil: E. Ltickermann, Heidelberger Platz 3, D-1000 Berlin 33. - 9 Springer-Verlag, Berlin-Heidelberg 1986. Springer-Verlag GmbH & Co. KG, D-1000 Berlin 33. - Gesamtherstellung: U iversit~tsdruekerei H. St irtz AG, Wfirzburg. Printed in Germany. Das Heft enth~dt eine Beilage des Gustav Fischer Verlages, Stuttgart 458 

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Cognitive processes of spatial coordinate assignment : on weighting perceptual cues

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