Two studies were performed to determine whether a subject's response to naturalistic optical flow specifying egomotion would be affected by a concurrent attention task. In the first study subjects stood in a moving room in which various areas of the optical flow generated by room movement were visible. Subjects responded to room motion with strong compensatory sway when the entire room was visible. When the side walls of the room were completely obscured by stationary screens, leaving only the front wall visible, sway was significantly reduced, though it remained greater than in an eyes-closed control. In Exp. 2 subjects were presented with either the full room (large sway response) or the room with only the front wall visible (moderate response), each in combination with either a hard or easy verbal addition task. Preliminary results show that swaying in the fully visible room and in the room with only the front wall visible increased when combined with either the hard or easy tasks. These preliminary results suggest that at the least the pick-up of optical flow specifying egomotion is not affected by concurrent attentional activity

Stoffregen, T.

English

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THE INTERACTION OF FOCUSED ATTENTION WITH FLOW-FIELD SENSITIVITY 
Tom Stoffregen 
Department of Psychology 
Cornell University 
Ithaca, New York 14853 
ABSTRACT 
Two studies were performed to d.termine whether a subject's 
response to naturalistic optical flow specifying egomotion would 
be affected by a concurrent attention task. In the first study 
subjects stood in a 'moving room' in which various areas of the 
optical flow generated by room movSmentwere visible. Subjects 
responded to room motion with strong compensatory sway when the 
entire room was visible. When the side walls of the room were 
completely obscured by stationary screens, leaving only the 
front wall visible, sway was significantly reduced, though it 
remained greater than in an eyes-closed control. In Exp. 2 
subjects were presented with either the full room (large sway 
response) or the room with only the front wall visible (moderate 
response), each in combination with either a hard or easy verbal 
addition task. Preliminary results show that swaying in the 
fully visible room and in the room with only the front wall 
visible increased when combined with either the hard or easy 
tasks. These preliminary results suggest that at the least the 
pick-up of optical flow specifying egomotion is not affected by 
concurrent attentional activity, supporting the notion of dual 
visual systems, and of the direct, non-attentional nature of the 
pick-up of optical flow. 
INTRODUCTION 
In recent years a growing amount of work has investigated the 
role of optical information in the control of both postural 
stability and guidancE! of actions, s ... ch as standing, running, 
and flying. Generally these st~dies have dealt with automatic 
pick-up of flow information, and have tacitly assumed that 
active attentional processes are ... nimportant in these areas. In 
fact the role of active, exploratory attentional pick-up during 
egomotion has hardly been addressed at all. One of the few 
studies related to this issue was carried out by Fischer, Haines 
and Price (1981) who investigated pilots performance in simula-
tors with Head-Up Display (HUD) instrumentation. The tasks 
involved this study were quite complex, and while subjects 
typically showed no decrement in simulator performance while 
reading HUD, there were some cases in which flight-critical 
information went completely undetected. The present paper re-
ports the results of the first two experiments in a series 
551 
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devoted to the basic question "What is the role of attention 
during locomotion?" To what extent, if any, must we actively 
attend to the optical information accompanying motion in order 
to successfully get around? Can we pick up information for our 
own motion through the environment while at the same time at-
tending to some task? 
For an initial look at this issue it was decided to use a situa-
tion for which something is already known about the usefulness 
of optical flow. In a series of experiments with the well-known 
'swinging room' Lee (Lee & Lishman 1975, Lee & Aronson, 1974) 
has shown that large scale optical flow is naturally used in the 
control of standing posture. Subjects in the SWinging room sway 
in response to exclusively optical room motion; the effect is 
robust, and the sensation of egomotion very compelling. Such a 
paradigm could easily be augmented by a variety of attention 
tasks. 
If attention does have an effect on concurrent pick-up of flow 
field information, we would expect that such effects would be to 
some extent a function of the difficulty of the attention task. 
Similarly, a given level of attention task difficulty,could have 
differential effects on the use of flow information depending on 
the ease with which the latter could be picked up; pick-up of 
restricted or otherwise impoverished flow could be less effi-
cient while active attention was being used than otherwise. 
Experiment 1 sought to determine conditions under which optical 
flow specifying egomatian might be rendered less' effective in 
controling posture. 
EXPERIMENT 1 
~gthQg A moving room was constructed for us. in these exper-
iments. The 'room' is a large cu~ical box, 2.5m on a side, 
mounted on. four wheels such that it can be rolled along the 
floor. The walls have reinforced wooden frames, faced on the 
inside with rigid cardboard, which is itself covered with a 
semi-random visual texture. The room has no floor, such that a 
subject inside it stands on the floor of the laboratory as the 
room moves around them. One wall of the room is left open; sub-
jects stand with their backs to this open wall, facing into the 
room. 
Postural adjustments in response to room motion were registered 
by a potentiometer. A grooved wheel was fixed to the axle of 
the potentiometer, and a string passed over the wheel and around 
the subject's neck, such that anterior/posterior movements 
caused the wheel to turn, generating a position-specific voltage 
which could be recorded. A.second potentiometer registered 
motion of the room; data from the two could be correlated as 
552 
time series to determine the effect of room motion on stance. 
Stationary cardboard screens could be placed in the room in 
order to restrict the optical motion available to subjects. The 
screens could be placed so as to occlude the three vertical 
walls of the room. Subjects wore a hat with a wide bill which 
prevented their seeing the ceiling. 
The room was moved sinusoidally along an axis parallel with the 
subject's line of sight. The total magnitude of the movements 
was 2.5cm, with a period of 12 seconds per cycle. Each one 
minute trial consisted of a contin~ous series of five of these 
cycles. Subjects were instructed to look straight ahead, 
keeping their gaze within a small square outlined on the front 
wall. They were not given any task to do, and were not imformed 
about room movements in advance~ but were simply told to stand 
still and look at the wall. 
Left Front Right 
Wall 
Full Room Eyes Closed 
S1 
F2 
Figure 1. Experimental conditions. Dotted areas were blocked 
by stationary screens. The ceiling was alway~ 
blocked. The floor was visible but did not move. 
553 
~QnQi1iQn§ Conditions are illustrated schematically in figure 1. 
The Full Room condition served as a baseline to establish the 
magnitude of the basic sway response. In the control condition 
subjects stood as normal in the room, but kept their eyes closed 
throughout the trial. In the four experimental conditions the 
side and front walls of the room were blocked off by the sta-
tionary cardboard screens, leaving flow available to either the 
retinal center or periphery. Each of 27 subjects participated 
in the five experimental conditions, and 12 of these also were 
i n the eyes closed control. 
8g§~!.1§ 
(across 
The data shown in figure 2 are the mean 
subjects) between room motion and subject 
correlations 
motion for 
each condition. 
1.0 
r 
.5 
39 
Eyes 
Closed 
IN=12J 
Figure 2. 
3 
Full F1 
Room 
:123 
F2 51 
Experiment 1 
N=27 
52 
Mean correlations between room movement and sub-
ject sway. 
With their eyes closed subjects movements were unrelated to 
those of the room. With eyes open and the entire room visible 
there was a strong and consistent sway response. In the four 
experimental conditions the greatest overall sway came in re-
sponse to the larger peripheral exposure, such that for this 
condition subjects swayed nearly as much as when the whole room 
was visible. By contrast, the larger front wall exposure pro-
duced only half as much sway as its peripheral counterpart, and 
554 
significantly less than the full room. Neither of the 
exposures produced any more sway than in the eyes 
control. 
smaller 
closed 
~i§£~§§ieQ Experiment 1 showed that by limiting optical flow to 
the front wall of the experimental room <and thereby blocking 
off flow from the far retinal periphery), compensatory sway 
could be significantly reduced, though the larger front wall 
exposure still produced significantly more sway than the eyes 
closed control. These results suggest a peripheral dominance 
for the pick-up of flow information for postural stability, and 
are consistent with the findings of Brandt, Dichgans and Koening 
(1973), who found a similar peripheral dominance for sensations 
of egomotion induced by rotatory optical stimulation. With 
respect to the goal s of thi s project, ,the major resul ti s the 
finding that exposing the entire front wall of the room results 
in a significant but reduced sway' response; the dlfference 
between Full Room and Front Wall candi tion,s caul d be used in the 
next experiment. 
EXPERIMENT 2 
The second experiment combined the two levels of sway response 
established in experiment 1 with two levels of difficulty in a 
verbal addition task as a preliminary test of the interaction 
between attention and flow pick-up. Subjects were presented 
with both the full room and with the room with the side walls 
completely occluded by the stationary screens. Each of these 
exposures was paired with both the hard and easy verbal tasks. 
In the easy task the subject was presented with a' three digit 
number at the beginning of a trial, which they would increment 
by 2 continuously over the course of the trial, announcing the 
sums in time with the beating of a metronome (50 beats/minute). 
The hard task was identical, except that subjects added 3 in-
stead of two. Nine subjects were run. 
Bg§~lt§ Since this was a preliminary study only sway data were 
analyzed; task performance as a function of flow exposures will 
be evaluated in future studies. Results are presented in fi-
gure 3. As can be seen from the figure, the addition of either 
the hard or the easy task produced no decrement in sway in 
either full room or front wall conditions. On the contrary, the 
presence of either of the verbal tasks produced an iQ&C~~§~ in 
induced sway. Multiple comparisons done with the Tukey test 
sho~ed no significant differences between the full room by 
its~lf and in combination with either of the tasks, or between 
the partially blocked room by itself and in combination with 
either of the tasks. 
555 
T 
F i ur.e 3. 
P 
~9 
n - a i gnS bet. a 
j s:c:t s: 
Q~§£~§a~Qn These results, though preliminary, indlcate tha i n 
the present experiment the presence of an ongoing attention task 
did not diminish adaptive responding to large scale optical flow 
specifying egomotion. This is consistent with the notion that 
optical information used for maintaining postural stability is 
picked up 'automatically'. The suggestion in these data that a 
concurrent verbal task may increase sway would be more difficult 
to interpret. 
Future studies in this series will again investigate the effects 
of attention tasks on response to visual motion, but also the 
reverse; the effects, if any, of flow pick-up on performance of 
an attention task. They will also examine the interaction of 
flow pick-up with visual attention task~ (such as might be found 
in flight situations), and will extend the requirements on flow 
pick-up by having subjects execute an active movement task which 
is dependent on the pick-up of flow information. 
556 
REFERENCES 
Br~ndt, T., Dichgans, J. and Koenig, E., "Differential Effects 
of Central Versus Peripheral Vision on Egocentric and Exocentric 
Motion Perception", 1973, Experimental Brain Research, 16, 476-
491. 
Fischer, E., Haines, R. and Price, T., "Cognitive hssues in 
Head-Up Displays", 1980, NASA Technical Paper 1711. 
Lee, D. and Aronson, E., 
Standing in Human Infants", 
15(3), 529-532. 
"Visual Proprioceptive Control of 
1974, Perception and Psychophysics, 
Lee, D. and Lishman, R., "Visual Proprioc!!ptive Control of 
Stance", 1975, Journal of Human Movement Studies, 1, 87-95. 
557 


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