Readiness to attend to visual foreground cues

Duane M. Rumbaugh Timothy V. Gill Sue C. Wright Georgia State University and Yerkes Primate Research Center, Emory University, Atlanta, Ga., U.S.A. Received 19 June 1972

Readiness to Attend to Visual Foreground Cues* Nonhuman primate genera are differentially inclined to attend to stimuli of the immediate foreground. When great apes were trained to a criterion on each of a series of two-choice visual discrimination problems and then given critical test trials with irrelevant visual cues (4 in wire mesh) positioned immediately in front of each problem’s objects, accuracy of performance was significantly more disrupted in orangutans (Pvngo) than in chimpanzees (Pan) and gorillas (Gorilla). Two groups of chimpanzees known to differ profoundly in complexlearning shills did not differ in their readiness to attend to irrelevant foreground cues; hence, it is concluded that the observed differences among the three genera of great apes must be species related and associated with how arboreal/terrestial they are in their natural habitats.

1.lntrodlaction All non-human primates rely heavily upon vision for adaptation to the ecological niches within which they live. Whereas the monkey/ape mother might compensate for the incompetence of a binocularly blind infant for a few weeks or months, blindness is such a profound deterrent to adaptation and survival that to date no blind adult non-human primates have been reported in field studies. Young & Farrer (1971) have discussed the basic, static anatomical-physiological characteristics of non-human primate and human eyes, concluding that when the refractive characteristics are equated, the visual acuities of non-human primates are comparable to that of humans. Further, their review of research on color vision supports the conclusion that the photopic spectrosensitivity functions of macaques (Macacu) and chimpanzees (Pun) are parallel to those for humans, with only minor differences being discerned. Differences in anatomical-physiological characteristics are, then, mitigated so as to produce near equivalence so far as visual perception is concerned. Among the apes, the gorilla eye is much more human-like than, in turn, is that of the chimpanzee and orangutan. The anterior portion of the gorilla eye is virtually identical to that of Man. It is generally believed that the visual world of the apes and monkeys is essentially the same as Man’s. This belief notwithstanding, there are differences in the readiness of non-human primate forms to attend to visual stimuli that are relatively proximal to the eye. The experimental evidence in support of this conclusion is the main subject of the present paper. That non-human primates are differentially disposed to attending to relatively proximal visual stimuli was first inferred by Rumbaugh & McCormack (1967). In their report of a comprehensive, comparative study of the learning-set (LS; Harlow, 1949) skills of monkeys and apes, they tentatively concluded that their test procedures, which entailed + Paper presented at the NATO Advanced Study Institute on Comparative Biology of Primates, held in Montaldo (Turin) Italy, 7-19 June 1972. Journal

of Human Evolution (1973) 2, 181-188

182

D.

the encasement differentially reliably

of stereometric

compromised

attending

fragile

than

frontal

for example,

when objects however, squirrel

handicapped

remained training

monkeys developed

manifested intrinsic

encasement

procedure

was reviewed

Plexiglas

condition

moderately

of encasing

of

was implemented

to

in support

high-level

LS

of the con-

forms were, there

of object

bins

than through the (Hylobates) were

encasement.

Squirrel

over the course of 500, 6-trial

problems

bins (Rumbaugh,

where

Ternes

the Plexiglas

high LSs.

training

the test objects. performances,

& Abordo,

bins were

One gibbon,

the visual irrelevancies

handicapped than

the

of limited LS proficiency,

to ignore

better

1965) ;

not used,

On the other hand,

many of the apes tested could not have been appreciably for they achieved

bins,

behavior

the primate

by the method

high LS after it was given special

to the method

transparent-fronted to the critical

to the fronts of the transparent-fronted

learning-setless

were encased within

in another

S. C. WRIGHT

within

with regard

Evidence

to attend

AND

objects encased within, to look at rather Squirrel monkeys (Saimiri) and gibbons

of the bins.

to be particularly

monkeys,

objects

studied (The

GILL

with how arboreal/terrestrial

predisposition

to the problem

planes

thought

T. V.

problem

test objects.)

that in accordance

was a differential rather

LS

the species

to the test objects.

allow use of small, clusion

M. RUMBAUGH,

90%

macaques

and

by that procedure responses

correct

on

trial 2 on a final series of 100 problems. In 1969 Rumbaugh

& McCormack

great apes were differentially cues (but not background orang-utan. series

2-trial

$-in wire-mesh

animals,

particularly

number

quite

by those statistical Center

chimpanzee, When

cues,

given a

consisting

the same irrelevant training

the wire-mesh.

Indeed, better

several

than

all

of the

ever before.

was not so great as to allow for analysis

for concluding

2. Experiment

cues

procedures,

with confidence

that there was

Access to the great ape colony of the Yerkes Regional

has allowed for an additional

of

of the bins and the test objects,

did significantly

orang-utans,

that the

foreground

gorilla,

foreground

By criterional

to ignore

tests that are a requisite

order:

By contrast,

levels.

in this study, however,

truly a species difference. Research

to chance

suggesting

of LS training.

the irrelevant

the glass fronts

rapidly

evidence

effects of irrelevant

ascending histories

were not disruptive.

the older

of subjects

with

between

were reduced

learned

had extensive

problems

behind the test objects

of the animals The

LS

interposed

their performances placed

additional

to the disruptive

cues) in the following

All of their animals

of 100,

reported

disposed

and definitive

Primate

study on this question.

1

Procedure The

subjects

extensive

were 24 great apes, each

test histories

in LS-type

genera

tasks,

represented

and were

by N = 8.

thoroughly

All subjects

familiar

with

had

the test

apparatus, essentially a modified Wisconsin General Test Apparatus (WGTA; Harlow, 1949). Their recent test histories were identical, and their performances as genera on a LS-relevant

task, the Transfer

Index

complex-learning

skills

chimpanzee

= O-86, orang-utan

g TI

(Rumbaugh,

(TI),

designed for equitable

1970))

were

comparable

8 TI

= O-95).

cross-species (gorilla

8

studies of

TI

= 0.95,

Whereas the subjects were not food-deprived, they, nonetheless, worked readily for the M&M candy rewards which they received for correct choices. Phase I consisted of training each subject on each of a series of two-choice visual discrimination

problems,

characteristically

used in LS experiments,

to the criterion

of nine

READINESS

correct (l-4

responses in)

The

and

10 consecutive

were encased

minimum

assignments

within

number

within

trials.

trials.

Subsequent

removed

VISUAL

for the next five training

The

of the criterion

a trial

on which

Plate

right-left

position

discrimination

training

the

of Q-in wire-mesh

bins, hence interposed

test trial was presented.

1 (a).

for each problem

sections

1 (b) portrays

to this first test trial, five additional critical

10.

sizes

in Plate

so that only by object

Upon achievement

183

CUES

were of varying

bins as portrayed

behind the fronts of the Plexiglas

then a second

FOREGROUND

test objects

was, of course,

the fronts of the bins and the test objects. wire removed,

The

unpredictably

test trials was given,

were placed immediately

TO

Plexiglas-fronted

changed

be achieved.

first of three critical

ATTEND

of trials to criterion

of the objects

could the criterion

TO

the nature

between

of these test

trials were given with the The wire was once again

trials, and then a third and final critical

test trial was

presented. In addition

to the intra-problem

was required.

The latter criterion

not be in error (10 problems

on more

x

than

criterion

Phase II consisted

of 50,

trial of each problem

P-trial

was a critical

as portrayed

duction

of the wire-mesh

changed

trial 2.

This phase provided

for a terminal

genera,

as a result

I training,

by the introduction

criterion the animal

10 problems

perform

prevailed;

in Plate

1 (b).

test to determine

The right-left

were terminally

better

whether

equivalent foreground

of

the second

Only by ignoring

significantly

of the irrelevant

On the first trial

l(a)

unpredictably.

could the subjects

of Phase

problems. in Plate

test trial as portrayed

assignments

induced

of the objects

problems

test trials from those

new discrimination

the condition

an interproblem

= 30).

position

influence

above,

that on 10 consecutive

two of the 30 critical

3 test trials per problem

each of these problems,

described

required

the intro-

than chance

on

or not the three

with regard

to the

cues (IFC).

Results

Figure

1 portrays

performances critical

the results of both Phase I and II.

were

more

test trials from

disrupted

It is apparent

by the introduction

the first 10 problems

than

were the chimpanzees

Assuming

that at the time of the first test trial all subjects’

accuracy,

the gorillas’

orang-utans’ significant

about

disruption

30 ‘A. Analysis

(F = 9.42, df = 2,21,

significantly

from one another

orang-utans

(Newman-Keuls,

that the orang-utans’

of the wire-mesh

on the 30 and gorillas.

performances

were at 100 %

was about

10 %, the

chimpanzees’

20 %, and

of variance

identified

the species variable

P < O-01), with gorillas and chimpanzees

but both being less disrupted

the

as highly

not differing

in choice behavior

than the

P < 0.05). The main effect for test trials was also significant (F = 3.71, df = 2, 42, P < O-05). Performance on trial 2 was significantly better than on trial 1 (P < O-05). That the gorillas required only 10.9 problems to reach

the inter-problem

orang-utans,

further

by the introduction significant indicated required

source

criterion,

indicates

of the wire-mesh of variance

no reliable

(F =

difference

fewer total problems

utans (P < 0.01). On the average,

in contrast screen.

to achieve

gorilla,

for chimpanzees

and

they were only minimally

Analysis

of variance

df = 2, 14, P < O-01).

17.27,

between

the gorillas

were learned

1 l-3 ; chimpanzee,

defined

criterion

for

species as a tests

but they both

than did the orang-

at equivalent

10.8;

21.8

distracted

Newman-Keuls

and chimpanzees,

the inter-problem

even the first 10 problems

in mean trials to criterion:

to 13.9

that by comparison

rates as reflected

and orang-utan,

10.6.

184

D.

M. RUMBAUGH,

Figure 1. Phase I performance on the critical test trials (three per problem) from the first 10 problems of the study, and Phase II trial-two performances on a series of two-trial problems on which the second trials were as portrayed in Plate l(b). A-A Gorilla, N = 8; +-0 Pan, N=8; n -m Pongo, N = 8.

T.

V.

GILL AND

S. C. WRIGHT

100 -

50 -

4 2

I

3

T-t

IFC Viols

Discussion The

results of this study indicate

readiness eye.

to attend

quite

clearly

If this were not so, their performances

Great

that great

to stimuli of the visual foreground of Phase

genera

in Phase

however,

II of the experiment.

of irrelevant

by the equivalent The

of their

proximal

to the

accounting

stimuli to the visual

performances

for the observed

of the three difference

is,

problematic.

That ground

this point being substantiated

in terms

I would have been comparable.

apes can and do learn to ignore the introduction

foreground,

apes differ

that are relatively

the orang-utans

were more sensitive

cues than were the chimpanzees

some aspect of the arboreal/terrestial utans are much

more arboreal

ward on the ground trees. Chimpanzees uniquely

adapted

nonetheless, the apes;

to the introduction

and gorillas is in support

dimension

than are either

of the irrelevant

fore-

of the hypothesis

that

might be the underlying chimpanzees

or gorillas.

factor.

Orang-

They

are awk-

and spend the majority of their time, both day and night, in the are less arboreal than are orang-utans and, as with gorillas, are

for rather

well adapted

efficient

quadrupedal

for life in the trees.

the older and the heavier

stay in the lower branches

the animals

terrestial

Gorillas

locomotion.

They

are the most terrestial

are,

of all of

get with age, the more likely they are to

of the trees, if not on the ground.

It might be that readiness

to attend to foreground cues is an adaptation to protect the eyes from obstructions that would otherwise threaten harm as locomotion in trees occurs either through semibrachiation thought

or climbing,

as with

orang-utans

and

chimpanzees.

suggests that as the gorilla has come to be adapted

way of life, the selective are so terrestrial

force for such a perceptual

This

same

for an increasingly

propensity

has weakened.

that they are not known to engage in even semi-branchiatio

line

of

terrestrial (Gorillas

nin the field.)

There are other possible explanations for the observed differences. It might be, for example, that the orang-utans were more distracted because of greater curiosity and a

READINESS

TO

ATTEND

TO

VISUAL

FOREGROUND

CUES

185

greater investigative propensity than characterizes chimpanzees and gorillas. Whereas this is an alternative explanation that should be pursued by empirical tests, it does not seem too plausible that their attraction to the wire-mesh as introduced on the average of 66 times per animal (22 problems on the average to reach the inter-problem criterion with three test trials per problem) should support a response of curiosity; as the wire was inaccessible to touch, it would seem that its visual attractiveness as something “interesting to look at” would quickly wane. As stated earlier, these animals were comparable in their complex-learning capabilities. Also, their average trials to criterion were about the same. Accordingly, it seems unlikely that the observed differences on the critical test trials can be attributed to differences in basic cognitive capabilities. That assumption not withstanding, this possibility was put to critical test in the following experiment. 3. Experiment

2

At the Yerkes field station there is a group of 16 adult chimpanzees, eight of which were feral-born and eight captive-born, now about 14 years of age. The captive-born animals were raised in very restricted environments for upwards of the first two years of life. A detailed report of the rearing conditions and the long-term behavioral damage associated therewith is provided by Davenport & Rogers (1970). The restricted-reared chimpanzees have been noted for their poor formal test performance relative to that of the wild-born chimpanzees. This being the case, if the wildborn and restricted-reared groups of chimpanzees were not to differ in terms of their readiness to attend to the introduction of irrelevant foreground cues, the conclusion that the observed differences reported in Experiment 1 are reliable species’ differences of other than a cognitive nature would be supported. Procedure and results

They were first tested to obtain Transfer Index values in order to determine with this refined cross-species assessment of complex-learning skills whether or not the previously reported differences by Davenport and Rogers would be corroborated (Rogers, Davenport & Rumbaugh, 1972). Only six of the eight restricted-reared chimpanzees proved testable. Their performances along with those from the group of eight wild-born chimpanzees are presented in Figure 2. It is apparent that the two groups differed profoundly in terms of their learning-set capabilities as assessed by Transfer Index procedures. The main effect for groups was highly significant (F = 29.81, df = 1, 12, P < O-01). Across the course of the four successive Transfer Index measurements there was reliable improvement, but it did not interact significantly with groups. Transfer Index testing procedure entails criterional acquisition, in the case of the present study the 84% correct level (see Rumbaugh, 1970, for details), prior to the critical test trials on which the cue values are reversed for 10 test trials. The Transfer Index measurement is based on reversal performance for ten consecutive problems. It is important to note that these two groups did not differ significantly in terms of the average number of trials required to reach criterion (TTC, Figure 2). Their simple discrimination learning skills were equivalent; however, their discrimination-reversal performances subsequent to achievement of criterion were profoundly different.

186

D.

M.

RUMBAUGH,

T.

V.

GILL

AND

S.

C.

WRIGHT

Figure 2. Performance according to early rearing conditions for two groups of chimpanzees on four successive TI measurements, 10 (N = 8); 04 Reproblems per measurement. A-A Wild-born stricted (N = 6).

‘.OO OBO -

f

.

0.60

-

0.40

-

o,20rwll A* 3

2

20

” I%?

A* 4

Blocks of IO triols

While

not a prime

the long-term

groups of chimpanzees These

results

through

trials to criterion

of the chimpanzee Figure

3 clearly

as readiness

rearing

to underscore

which

these two

were then tested in accordance

foreground

cue experiment

reached

reliably

both the intra-

from one another

nor their average

group in Experiment reveals

with the procedures

animals was dropped because of intolerably

the course of TI testing.

of the irrelevant

rates, not differing

their average

it is important

had during the first two years of life.

two groups of chimpanzees

equivalent

paper,

with the differential

1. One of the restricted-reared

slow progression These

of the present

associated

two groups of chimpanzees

of Experiment The

consideration

cognitive deficit

are portrayed

on each of the three critical significant differences.

3.

criteria

at

and inter-problem

in either measurement.

problems

to criterion

Neither

are unlike those

1.

that these two groups of chimpanzees

to attend to the irrelevant

in Figure

foreground

were equivalent

cues of the wire-mesh

test trials per problem.

Analysis

so far

when introduced

of variance

revealed

no

of the data of Experiment

1,

Discussion The results of Experiment namely

that the observed

1 are not to be attributed

2 corroborate differences

the interpretation

among the great apes’ performances

to differences

in their complex-learning

of Experiment

capabilities.

These

READINESS TO ATTEND

Figure 3. Performances from Phases I and II of the experiment to determine sensitivity to the introduction of irrelevant foreground cues for two groups of chimpanzees with different earlyrearing experiences. A-A Wildborn (N = 8); 0-O Restricted (N = 5).

TO VISUAL

FOREGROUND

CUES

187

r 100 -

5 goL $ 5 80c” z E 70E 8 $

1

60

two groups of chimpanzees, though profoundly different in their complex-learning capabilities, were very comparable in terms of their distractability and/or their readiness to attend to irrelevant foreground cues when introduced. These data lend strong support to the conclusion that the differences obtained in Experiment 1 are species-linked characteristics, not attributable to either differential early-rearing conditions or to complexlearning skills. 4. Experiment

3

As an additional test of the hypothesis that the differences obtained among the great ape genera of Experiment 1 is in some manner related to their respective arboreal/terrestrial characteristics, four adult talapoin monkeys (Cercopithecus talapoin) were tested in basic accordance with the procedures of Experiment 1. The only point of departure was that by necessity a diminutive test apparatus had to be employed to accommodate their small size, their average weight being l-7 kg. As talapoins are highly arboreal, it was predicted that their performances should approximate that of the orang-utans on the critical test trials of Phase I. On the first 10 problems the talapoins required more trials on the average to reach the intra-problem criterion (X = 20*7), but only slightly fewer problems to reach the inter-problem criterion than did the orang-utans. On the critical test trials of Phase I the talapoins’ performances were disrupted in a manner commensurate with that of the orang-utans: trial 1 = 75 %, trial 2 = 83 %, and trial 3 = 65 % for an average of 74 ‘A responses correct, the orang-utans being disrupted on the average to 70 % responses correct. Their Phase II trial 2 performance was 57 % correct, but this value should not be directly compared with the trial 2 performances of any of the apes for the apes are far superior to talapoins on trial 2 learning-set performance under standard test conditions. The talapoins were, then, markedly disrupted on the critical test trials of Phase I and, also, required about as many problems to reach the inter-problem criterion of Phase I as did the orang-utans. It is concluded, therefore, that their data supports the hypothesis that some aspect of arboreality predisposes attention to cues of the immediate foreground. 2

D. M.

188

RUMBAUGH,

T.

V.

GILL

AND

S.

C.

WRIGHT

5. Conclusions

Whereas

the three great ape genera

have visual systems that most likely provide for them

the same visual world as that experienced to different

aspects

of that world.

great apes remarkable

abilities

are able to peer through

in the distant

field.

common

that in the ontogeny

of human

ness to be distracted

by, or to attend

In conclusion,

attend

variable,

additional

it affords

of a variety

notwithstanding,

to, irrelevant appears

evidence

to underscore Whatever

of behaviors.

be the case

degrees of readi-

cues of the visual field.

to attend

to be reliably

visual targets

it might different

foreground

inclined

to the

aspects of his visual field.

we might determine

great apes are differentially As this difference

has by way of comparison to various

a morass of visual noise to minute

observation

perception

visual foreground. determinant

of course,

to selectively

Adult humans

This

by Man, it is clear that they are prone to attend

Man,

to cues of the immediate

associated

the power

its underlying

with the species

of that variable mechanisms

as a

might be,

it would seem that just as some people “fail to see the wood for the trees”,

the orang-utan,

more

to see the trees

so than

either

the chimpanzee

or the gorilla,

might

be “unable

for the Ieaves”.

This research Research

was supported

Center

of Emory

by NIH

grant

University

RR-00165

to the Yerkes

and by NSF grants GB-7161

Regional

Primate

and 11850.

References Davenport, R. K. & Rogers, C. M. (1970). Differential rearing of the chimpanzee. A project survey. In (G. H. Bourne, Ed.), The Chimpanzee. Vol. III, pp. 337-360. Base]: S. Karger. Harlow, H. F. (1949). The formation of learning sets. PsychologicalReview 56, 5 l-65. Rogers, C. M., Davenport, R. K. & Rumbaugh, D. M. (1972). The effects of early restricted-rearing upon the complex-learning skills of adult chimpanzees. In preparation. Rumbaugh, D. M. (1970). Learning skills of anthropoids. In (L. A. Rosenblum, Ed.) Primate Behmior: Deoelopmentsin Field and Laboratory Research. Vol. I, pp. I-70. New York: Academic Press. Rumbaugh, D. M. & McCormack, C. (1967). The learning skills of primates: a comparative study of apes and monkeys. In (D. Starck, R. Schneider & H. J. Kuhn, Eds), Progress in Primatology, pp. 289-306. Stuttgart: Gustav Fischer. Rumbaugh, D. M. & McCormack, C. (1969). Learning and attentional skills of great apes compared with those of gibbons and squirrel monkeys. Proceedings of the Second International Primate Congress pp. 167172. Rumbaugh, D. M., Terncs, J. W. & Abordo, E. (1965). Learning set in squirrel monkeys as affected by encasement of problem objects in Plexiglas bins. Perceptual Motor Skills 21, 531-534. Young, F. A. & Farrcr, D. N. (1971). Visual similarities of non-human and human primates. In (E. I. Goldsmith & J. Moor-Jankowski, Eds), Medical Primatology 19i0, pp. 316328. Basel: S. Karger.