IMPLICIT VERSUS EXPLICIT COGNITION AND MEMORY A hallmark of evolutionary psychological explication is that adaptive

behavior is predicated on adaptive thought (Tooby & Cosmides,

1992).

Human behavior, as well as the behavior of other species, is said to be governed

by

information-processing systems and “strategies” designed

by

evolution to solve recurrent problems faced by our ancestors. Such language connotes a self-aware being who is conscious, at some level, of the strategies he or she is executing and of the processes underlying such decisions.

This

bias may be especially potent for cognitive developmentalists, whose science has been strongly influenced

by

Piaget’s theory to focus on high-level cogni- tive tasks, quite different from the basic-level and often unconscious pro- cesses that have characterized the study of adult cognition (Siegler & Ellis,

1996).

But this is obviously not the definition of cognition and strategies that evolutionary psychologists intend. Rather, brains and minds have been shaped

by

natural selection to

be

sensitive to certain classes of information and to respond in certain ways outside of the immediate awareness of the individual. Such cognition is implicit.

This

is in contrast to the explicit

cognition that most people think about when they think about “thought”

(Karmiloff-Smith, 1992; Nelson & Bloom,

1997;

Parkin, 1997; Schacter, 1992).

But explicit cognition-occurring with conscious awareness-is obvi- ously of substantial significance to Homo supiens.

It

is because we are con- scious that we are writing this book and you are reading it. It is only self- conscious individuals who are capable of pursuits such as science, literature, art, and city governments, and consciousness may be responsible for the complex social patterns characteristic of human culture, as well as for culture itself. Although some researchers have speculated that human consciousness is a by-product of a large brain

(S. J.

Gould & Lewontin,

1979),

we propose, as have others (e.g., Rebers, 1992), that self-awareness had strong adaptive value for a long-lived, large-brained, socially complex species with an ex- tended juvenile period and has been selected over the course of hominid phylogeny. In this section, we examine briefly the nature of implicit and explicit cognition and memory, their development, and their possible evolution.

Implicit versus Explicit Memory

Cognitive psychologists and neuropsychologists have made two broad distinctions for the contents of memory. Explicit, or declarative memory, refers to memory with awareness and can be assessed using conventional memory tests (i.e., free recall, recognition). Implicit, or nondechrutive or procedural memory, in comparison, refers to memory without awareness and is reflected in memory for routinized skills (i.e., procedural memory), priming, and operant and classical conditioning. Explicit memory is usually conceived as consisting of two interacting systems: semantic memory, which is world knowledge, particularly of language, rules, and concepts, and episodic memory, which refers to a knowledge of one’s past experiences or personal history, as in autobiographical memory (Tulving, 1985).

This distinction has more than heuristic value, for implicit and explicit memory appear to be governed

by

different brain systems, as revealed

by

research with people with brain damage (Schacter,

1992;

Schacter, Norman,

& Koutstall, 2000). For instance, the hippocampus is involved in transferring

new explicit information from the short-term store (the “location” of imme- diate awareness) to the long-term store. People with damage to the hippo- campus can acquire a new skill as a result of repeated practice but will have no awareness of ever learning such a skill. For example, neuropsychologist Brenda Milner

(1964)

reported the case of

H. M.,

who had hippocampal damage.

H. M.

was given a mirror-drawing task over several days, in which he had to trace figures while watching his hand in a mirror.

H.

M.’s per- formance was quite poor initially but improved after several days of practice,

CLASSIFYING COGNITION

115

despite the fact that he had no recollection of ever performing the task previously. The enhancement of performance as a result of practice is a reflection of implicit (procedural) memory, whereas

H.

M.’s failure to recall previously performing the task is a reflection of a lack of explicit memory.

It seems obvious that the memory of animals and of human infants is of the implicit type, making implicit memory both ontogenetically and phylogenetically ancient (Rebers,

1992).

One interesting question concerns human infants’ first display of explicit memory and whether there is any evidence of explicit memory in nonhuman animals, such as the great apes.

One area of research that has shed some light on this issue is deferred imitation, which refers to an individual observing a model and, after some significant amount of time

(5

minutes or longer), reproducing that behavior.

Such imitation requires that the observed event be stored in memory and retrieved at a later time. Piaget

(1962)

proposed that deferred imitation is first seen in children at about 18 months of age and is a reflection of symbolic (mental representational) functioning. Recent research has shown that chil- dren as young as

9

months of age can display deferred imitation for simple behaviors (Carver & Bauer, 1999) and that preverbal toddlers can retain such memories for as long as

1

year (Bauer, Wenner, Dropik, & Wewerka,

2000;

Bauer & Wewerka, 1995; Mandler & McDonough,

1995),

suggesting to some that symbolic functioning is present in human infants late in the first or early in the second year of life (Mandler,

1998;

Meltzoff, 1990) and that deferred imitation reflects a preverbal form of explicit memory (Bauer, 1997; Meltzoff & Moore,

1997).

Evidence for the explicit nature of deferred imitation was provided in a study of patients with anderograde amnesia who, similar to

H. M.,

were unable to acquire new explicit information due, presumably, to hippocampal damage (McDonough, Mandler, McKee, & Squire,

1995).

Patients were ad- ministered a series of explicit memory tasks, which they obviously failed. They were also given a series of deferred-imitation tasks, similar to those passed

by

preverbal toddlers (Bauer,

1997). If

deferred imitation is a form of implicit memory, amnesic patients should perform the tasks well, just as

H. M.

eventu- ally learned to copy figures while watching

his

hand in a mirror.

If,

however, deferred imitation is a form of explicit memory, the patients should perform poorly on these tasks as well. McDonough and colleagues reported that the amnesic patients did indeed fail the deferred-imitation tasks, suggesting that it reflects an explicit ability. The fact that 1-year-old infants can pass these tasks suggests “that the neurological systems underlying long-term recall are present, in at least rudimentary form,

by

the beginning of the second year of life” (Schneider & Bjorklund, 1998, p.

474).

If

deferred imitation is a reflection of nonverbal explicit memory, it should be possible to assess such cognition in animals. This has been done

with nonhuman primates. Although it is almost axiomatic that monkeys and apes imitate (“monkey see, monkey do”; the verb “to ape”), relatively little empirical evidence exists of deferred imitation in nonhuman primates (see Tomasello & Call,

1997).

What evidence there is of deferred imitation in great apes comes from animals who have had significant human contact early in life. For example, observational research with rehabilitant (i.e., formerly captive) orangutans (Russon, 1996) and an enculturated (human- reared) orangutan (Miles, Mitchell, & Harper,

1996)

reported evidence of deferred imitation in animals as young as 2 years,

11

months of age.

Several experimental studies, using procedures with appropriate con- trols similar to those used with human infants (Bauer,

1997;

Meltzoff,

1995),

also have been conducted (Bering et al.,

2000;

Bjorklund, Bering, & Ragan,

2000;

Bjorklund et al., in press; Tomasello, Savage-Rumbaugh, & Kruger, 1993). In these experiments, enculturated great apes (chimpanzees, bo- nobos, or orangutans) interacted with target objects during a baseline phase.

A

human model then displayed some target behavior with the objects (e.g., putting pegs in a form board and hitting the pegs with a hammer) and, after a delay ranging from 10 minutes (Bering et al.,

2000;

Bjorklund et al.,

2000,

in press) to

24

or

48

hours (Tomasello, Savage- Rumbaugh, & Kruger,

1993),

the apes were re-presented the objects (deferred phase). The incidence of imitating the modeled behavior during the deferred phase was contrasted with the apes’ behavior at baseline, before they had seen the modeled actions. Tomasello, Savage-Rumbaugh, and Kruger

(1993)

also tested a group of mother-reared (i.e., nonencul- turated) chimpanzees and bonobos and

18-

and 30-month-old human children.

The deferred imitation results of the Tomasello, Savage-Rumbaugh, and Kruger study are shown in Figure

5.1. As

can be seen, the enculturated apes displayed the highest level of deferred imitation, significantly greater even than that of the children. Following the interpretation of the results of McDonough and her colleagues

(1995)

of an inability of amnesic patients to perform explicit memory tasks, including deferred-imitation tasks, the finding of deferred imitation in great apes is consistent with the position that these animals possess symbolic representation and explicit (“conscious”)

,

as opposed to only implicit, memory. The fact that these abilities have been demonstrated convincingly only in enculturated animals suggests that great apes have a flexible cognition that is amenable to modification toward a more “human-like” form when they experience a human-like rearing environment. As we commented in chapter

4,

we believe that evidence of species-atypical cognition in a large-brained, slow-growing, genetic cousin to Homo supiens, under species-atypical rearing conditions, has important implications for theories of human cognitive evolution.

CLASSIFYING COGNITION

117

100

80

3

rn 60

c

.c

0 a, 0 m

C a, a,

c

p 40 a

20

0

1 8-month-old 30-month-old Enculturated Nonenculturated

Children Children Chimps Chimps

figure 5.1. Percentage successful deferred imitation for enculturated apes, mother- reared apes, and 18- and 30-month-old children.

Note. From “Imitative Learning of Actions on Objects by Children, Chimpanzees, and Enculturated Chimpanzees,” by M. Tomasello, S. Savage-Rumbaugh, and A. C.

Kruger, 1993, Child Development, 64, pp. 1688-1 705. Copyright 1993 by Society for Research in Child Development. Adapted with permission.

Implicit Cognition and Its Development

Most research in cognitive development beyond infancy has dealt with explicit cognition, particularly in the field of memory development (see Blasi & Bjorklund, 2001; Schneider & Bjorklund, in press; Schneider &

Pressley,

1997).

However, over the past decade or so, research into children’s implicit learning and memory has increased, led, in large part, by an innova- tive theory of developmental psychologist Annette Karmiloff-Smith ( 199 1, 1992). Central to Karmiloff-Smith’s theory is the concept of representational redescription, which involves the mind re-representing knowledge it already possesses. According to Karmiloff-Smith ( 199 1 ), “human development

crucially involves the passage from representations that constitute knowledge in the mind to representations that acquire the status of knowledge to other parts of the mind” (p. 175). From this perspective, representational redescription is similar to Piaget’s reflective abstraction, whereby children discover new information by examining the contents of their own mind (i.e., introspecting). Karmiloff-Smith proposed four levels of representation, the first being implicit, followed in development by three levels of explicit representation

(El, E2,

and

E3).

Implicit Representations

Implicit representations reflect the earliest stage and the most primitive type of representation. They correspond to special-purpose knowledge (e.g., knowledge related to physical objects, language, and cause and effect) that is modular in nature, is activated by specific environmental stimuli, and is inaccessible to other parts of the cognitive system (i.e., outside of conscious awareness). This is complex, procedural knowledge that is similar to that possessed

by

other “unconscious” species (e.g., a bird’s knowledge about building a nest or a spider’s about building a web). Implicit knowledge may still require some experience for it to be expressed properly, but it is limited in its application to a narrow range of situations and objects, and it is not easily modified. There is no redescription of implicit representations.

Consistent with Karmiloff-Smith’s idea, implicit learning and memory are early developing abilities that show relatively little improvement over the course of ontogeny in comparison to explicit learning or memory (Hayes

& Hennessy, 1996;

RUSSO,

Nichelli, Gibertoni, & Cornia,

1995;

Vinter &

Perruchet,

2000).

For instance, research has demonstrated that

6-

and 10- year-old children learn serial sequences of responses as well as adults, despite having no explicit (verbalizable) knowledge of what they have learned (Meulemans, Van der Linden, & Perruchet, 1998).

In

other research,

9-

and 10-year-old children were shown pictures of

4-

and 5-year-olds and asked to determine whether each picture was a former preschool classmate (Newcombe & Fox,

1994).

In addition to their verbal (explicit) responses, changes in the electrical conductance of children’s skin were also recorded and used as an indication of implicit recognition memory (higher skin conductance being predicted when the children saw pictures of former classmates than when they saw pictures of unknown children).

As

might be expected, performance was relatively poor on both the explicit and implicit tasks but greater than expected by chance, indicating that the children had some memory of their preschool classmates. Importantly, there was no difference in skin conductance between children who performed well on the explicit task and those who performed poorly, suggesting that even children whose performance on the explicit memory task was no greater

CLASSIFYING COGNITION

119

than chance still “recognized,” implicitly, as many of their former classmates as those children who performed well on the explicit task. This pattern of data indicates that some children “remembered” (implicitly) more than they

“knew” (explicitly; see also Lie & Newcombe,

1999).

One example of young children’s possession of implicit knowledge on a typically “explicit” task was provided

by

developmental psychologists Wendy Clements and Josef Pemer (1994). False-belief tasks (Wimmer &

Pemer,

1983)

assess theory of mind, specifically children’s understanding that other people have knowledge that may be different from their own.

(Theory of mind is discussed in chapter

7.)

In the standard false-belief task, children watch as a treat is hidden in one location in the presence of two other people. One of those people, Maxi, then leaves the room, and the treat is moved to a new location. The child is then asked where Maxi will think the treat is hidden when he returns. Most children

by

age

4

pass this test, saying that Maxi will mistakenly think the treat is hidden in its original location. Children much younger than

4,

however, typically say that Maxi will look in the new location, not understanding, apparently, that Maxi has different knowledge than they have.

Clements and Pemer conducted a variant of this task, assessing chil- dren’s implicit as well as explicit understanding of false belief. Preschool- age children were told a story about a mouse named Sam who had placed a piece of cheese in a specific location (Location

A)

so that he could get it later when he was hungry. While Sam was sleeping, Katie mouse moved the cheese to a new place (Location

B).

When Sam returned, children were asked where he would look to find his cheese. Most of the younger preschoolers erroneously said that he would look in Location

B,

reflecting their lack of understanding of false belief. This is the standard, or explicit, false-belief task, and it replicates previous research findings. However, Clem- ents and Pemer also recorded where children first looked, at Location

A

or Location

B.

This is an implicit task, requiring (seemingly) no conscious awareness or verbal response. Figure

5.2

shows the level of performance for children of different ages on the implicit and explicit tasks.

As

can be seen, only the youngest children (ages 2 years, 10 months or younger) “failed”

the implicit false-belief task.

By

age 2 years,

11

months, children typically looked at the proper location (Location

A),

despite providing the incorrect verbal response (Location B). What these findings imply is that by about age

3

years, children have a well-developed implicit understanding of false belief that exceeds their explicit (verbalizable) knowledge. (This finding has been replicated by Clements, Rustin, & McCallum, 2000, and by Gamham & Ruffman, 2001; a similar finding has been reported for infants’

performance on A-not-B object permanence tasks, with infants looking at the right location before searching at the right location; Ahmed &

Ruffman,

1998.)

2.5

1.5

0.5

-0.5

Implicit

- -

⁰

- -

^Explicit

2 yrs. 5 mos.

-2 yrs. 10 mos. ^{2 yrs. 11} mos.

-3 yrs. 2 ^{mos. -3 yrs. 3 yrs. 3} 6 ^{mos. mos. 3 yrs.} 8 ^rnos.

-4 yrs. 6 mos.

Age

Figure 5.2. Average implicit and explicit understanding scores on false-belief task by age.

Note. From “Implicit Understanding of Belief,” by W. A. Ciements and J. Perner, 1994, Cognitive Development, 9, pp. 377-395. Copyright 1994 by Cognitive Science Society, Inc. Adapted with permission.

Explicit Representations

Redescription begins with the first level of explication

(El).

Knowledge is now available to other cognitive systems but, according to Karmiloff- Smith, it is not yet conscious. For example, as young children acquire language, they are able to ignore aspects of the external stimuli and to focus on the internal meaning of language, which they use to formulate linguistic theories. Young children are very good, for example, at determining what is and is not a grammatical sentence-but they can rarely tell you why this is so. Because such knowledge is unavailable to conscious awareness, we feel more comfortable describing such cognitions as implicit. But the existence of such knowledge, which has been substantially modified by experience but

CLASSIFYING COGNITION

121

is still unavailable to consciousness, suggests that much sophisticated knowledge may exist in an implicit (or

E l )

form before it can be realized con- sciously.

According to Karmiloff-Smith, knowledge requires greater explication to become conscious

(E2),

and greater explication yet before it can be verbalized and shared with others

(E3).

For example, Karmiloff-Smith

(1979)

described English-speaking children’s correct use of the articles a and the during the preschool years. Yet, it is not until

9

or

10

years of age that they become consciously aware of them. According to Karmiloff-Smith, it is not until this time that children are able to redescribe their representations so that these representations are available not only to other cognitive systems but also to consciousness.

It is not surprising that children are born with well-developed implicit learning and memory abilities that develop rapidly. What is somewhat surprising, perhaps, is that human cognition develops beyond sophisticated implicit knowledge. Explicit memory is a phylogenetically recent innovation found only among humans and perhaps, to a lesser extent, among some great apes, and it develops slowly. Unlike age differences in implicit memory, age differences in explicit memory are substantial and do not asymptote until young adulthood (see Schneider & Bjorklund,

1998;

Schneider &

Pressley,

1997).

Although the origins and functions of explicit memory/

representation systems may forever remain a mystery, researchers have pro- vided interesting speculations, some of which we present below.

Evolutionary Considerations of Different Memory and Representational Systems

Humans and great apes are not the only species that need memory to survive and thrive. Learning and memory abilities, for any species, are adaptive specializations that have been shaped

by

the processes of natural selection to solve particular problems confronted in ancestral environments (Rozin,

1976;

Sherry & Schacter,

1987).

Memory is critical for finding food; avoiding predators; and identifying kin, friends, and foes and may be especially important for a species with a complex social network involving social exchanges (Cosmides & Tooby,

1992).

Implicit memory theoretically exists to varying degrees, in all species with a nervous system (Rebers, 1992), but explicit memory seems to be unique to humans and perhaps to great apes raised in species-atypical conditions. Explicit memory’s particular adaptive function and how it evolved are important questions for evolutionists.

One possibility was proposed by Sherry and Schacter

(1987),

who suggested that functional incompatibility, which exists when an adaptation that is dedicated to serving one function cannot serve other functions, led to the evolution of at least two memory systems. The implicit memory