Pii: s0959-4388(00)00132-x

Cognitive neuroscience
Thomas D Albright*, Eric R Kandel† and Michael I Posner‡
The last decade of the 20th century has seen the development disparate subfields of biology into one coherent disci- of cognitive neuroscience as an effort to understand how the pline. Almost all of the other unifications have been brain represents mental events. We review the areas of spearheaded by the synthetic power of molecular biology.
emotional and motor memory, vision, and higher mental Cognitive neuroscience is distinctive in that the impor- processes as examples of this new understanding. Progress tant impetus has come from other sources; in particular, a in all of these areas has been swift and impressive, but much large part of the impetus has come from psychology and needs to be done to reveal the mechanisms of cognition at the local circuit and molecular levels. This work will require newmethods for controlling gene expression in higher animals and That psychology should be a driving force for such a syn- in studying the interactions between neurons at multiple levels.
thesis is perhaps not surprising. Psychology provides theagenda for the brain sciences; it poses the questions about Addresses
mental activity that we ultimately want to address. In *Howard Hughes Medical Institute and Salk Institute, 10010 North addition, within the first half of the 20th century, psychol- Torrey Pines Road, La Jolla, California 92037, USA; ogy underwent a remarkable increase in explanatory power, evolving from a philosophical tradition based on † Howard Hughes Medical Institute and Center for Neurobiology andBehavior, College of Physicians and Surgeons of Columbia University, introspection to an independent scientific discipline cen- 722 West 168th Street, New York, New York 10032, USA; tered on psychophysics and behaviorism. As psychology matured in the first half of the 20th century, it increasing- ‡ Sackler Institute, Department of Psychiatry, Box 140, Weill Medical ly advocated an empirical behaviorist approach that ended College of Cornell University, 1300 York Avenue, New York,New York 10021, USA; e-mail: mip2003@mail.med.cornell.edu up reducing the focus of interest in psychology to observ-able aspects of behavior. Psychologists argued that Current Opinion in Neurobiology 2000, 10:612–624
attempts to quantify non-observable mental events, such as perception, imagery, thinking, retention, problem solv- 2000 Elsevier Science Ltd. All rights reserved.
ing, or consciousness were at best speculative andunstable. This emphasis on observable indices of behav- Abbreviations
5-HT
ior resulted in making psychology a rigorously Ca2+/calmodulin-dependent protein kinase II experimental discipline but one that was so narrowly focused that it excluded from the study of behavior most of the really fascinating features of mental life. The re- awakening of interest in internal events led to the emergence of modern cognitive psychology by making us realize that our knowledge of the world is based on per- ception, and that perception is an act of construction that depends not only on the information inherent in the stim- ulus but also on the mental structure of the perceiver.
Ulric Neisser laid out the task of cognitive psychology. In his classic monograph of 1967 [1], he wrote: “…the world of experience is produced by the man who experiences it…There certainly is a real world of trees and people and cars and even books, and it has a great deal to do with our experience of these objects. However, we have no direct immediate tetracycline-responsive transcription factor access to the world, nor to any of its properties… Whatever we know about reality has been mediatednot only by the organs of sense but by complex sys- Introduction
tems which interpret and reinterpret sensory The last decade of the 20th century, the Decade of the Brain, has also been the Decade of Cognitive “. . . the term ‘cognition’ refers to all the processes Neuroscience. It has been the decade in which the merg- by which the sensory input is transformed, reduced, er of cognitive psychology and neural science has begun elaborated, stored, recovered and used . . .” to realize its promise. The joining of neural science andcognitive psychology is the most recent in a series of sci- Neisser, and the other earlier pioneers in cognitive psy- entific unifications that have brought together the chology, pointed out that to study mental operations, Cognitive neuroscience Albright, Kandel and Posner 613
cognitive psychologists needed to focus on the flow of sen- different sites within the brain. We briefly consider the sory information, from its transduction by appropriate systems problem before focusing on the molecular prob- sensory receptors to its eventual use in memory and action.
lems of storage (for a current review, see [5]).
This implied that each perceptual or motor act has aninternal representation in the brain, evidenced by a pattern The systems problem of memory
of activity in a specific set of interconnected cells. The pat- Procedural (implicit) memory: there are multiple forms tern of connections also stores information, in memory, It is now generally accepted that declarative (explicit) about the perception and the motor act.
memory requires for storage the medial temporal lobe andthe hippocampus, whereas procedural (implicit) memory In the 1970s and 1980s, the early years of cognitive psychol- does not. One of the advances of the past decade has been ogy, most types of internal representations were not really the further documentation that procedural memory has accessible for a meaningful experimental analysis (for inter- many subcomponents, and that essentially all cognitive esting exceptions, see [2–4]). Fortunately, the emergence of systems — be they perceptual or motor — involving a cognitive psychology was soon followed by, and indeed large variety of different neural systems may be capable of helped stimulate, significant progress in systems neural sci- storing some types of procedural memory. For example, ence and in brain imaging. By the 1990s, these approaches various types of motor learning involve the cerebellum; had come together, making it possible, for the first time, to learned fear the amygdala; operant conditioning and habit study directly internal sensory and motor representations in learning the basal ganglia; priming the neocortex; and per- both nonhuman primates and human beings. A major goal of ceptual learning, including habituation and sensitization, this new direction was to map elementary cognitive func- the primary sensory pathways. This distribution of proce- tions onto specific neuronal systems.
dural memory across neural systems was already wellappreciated in the 1980s, and, in some cases, such as motor We cannot, in this brief review, document the substantial learning involving the cerebellum, the circuitry involved in progress that has been made in mapping internal represen- learning was already well studied (see reviews in [6–10]).
tations of cognitive functions. We therefore will restrict our Here, we illustrate how our knowledge of implicit memory review to three areas in which we have direct experience: mechanisms has expanded in the 1990s by focusing briefly memory storage, perception, and higher mental functions.
on just two examples, fear conditioning in the amygdala Our purpose in considering memory storage is to illustrate and motor learning in the cerebellum.
that some elementary aspects of cognitive processes, inparticular the switch from short- to long-term memory, can now be studied at the molecular level. In discussing per- Considerable evidence from both humans and experimen- ception, we illustrate the influence that psychophysics, tal animals now indicates that the amygdala is critical for especially psychophysical studies in intact, awake behav- the expression of emotion and intervenes between the ing primates, has exerted on the study of the neuronal hypothalamus and brainstem nuclei — the regions con- organization and function of the visual system. Finally, we cerned with the somatic expression of emotion — and the illustrate the remarkable power that we have gained in the cingulate, parahippocampal, and prefrontal cortices — the study of complex cognitive processes in humans from the neocortical areas concerned with conscious feeling. For ability to image the living, behaving human brain.
example, electrical stimulation of the amygdala in humansproduces feelings of fear and apprehension, whereas dam- Memory storage
age to the amygdala in experimental animals produces During the past decade, the study of memory storage has tameness. Consistent with this idea, studies using positron been characterized by the attempt to map the molecular emission tomography (PET) and functional magnetic res- biology of synaptic plasticity onto ideas about memory onance imaging (fMRI) clearly show that recognition of systems that emerged from cognitive psychology in the emotional expression in faces involves the amygdala [11].
1980s. The earlier, cognitive psychological studies ofpatients with brain lesions made it apparent that memory As early as 1956, Lawrence Weiskrantz [12] had demon- was not a unitary faculty of mind but had at least two strated that lesions of the amygdala in experimental major forms: a declarative (explicit) form concerned with animals also impair learned fear. In subsequent work, the knowledge of what something was about — a knowl- Michael Davies [13], Michael Fanselow [14], Joseph edge about facts and events — and a procedural (implicit) LeDoux [15] and their colleagues have delineated the out- form concerned with the knowledge of how to do some- lines of a neural circuit for learned fear. They found that thing — a knowledge about perceptual and motor one subnucleus of the amygdala, the lateral nucleus, is procedures. In studying these two independent memory required for procedural memory of fear conditioning to a processes, it has proven convenient to divide the study of neutral tone [15]. Information about the CS, the tone, is memory storage into two parts: the systems problem of carried to the lateral nucleus via two pathways: the thalamo- memory, which is concerned with where in the brain amygdala projection from the auditory thalamus and the memories are stored, and the molecular problem of mem- cortico-amygdala projection from the auditory cortex.
ory, which is concerned with how memories are stored in Long-term potentiation (LTP) has been observed in vivo A decade of Current neurobiology
in the projection from the thalamus to the amygdala after the cerebellar cortex and the deep nuclei, with each playing tetanization of the thalamic input [16]. Importantly, synap- a different role. The cerebellar cortex has the special func- tic change resembling LTP has been observed in this tion of learning the timing of movement. In so doing, the pathway after naturally occurring fear conditioning [17].
cerebellar cortex guides the learning in the deep nuclei,which may allow learning to be transferred from the cortex Because of the anatomical complexity of the amygdala to the deep nuclei. Since one of the issues in declarative (especially when compared with more ordered structures learning is the transfer of information from the hippocampus such as the hippocampus and the cerebellum), precise and the medial temporal lobe to other areas of neocortex, molecular characterization of the plasticity at these defined this insight may prove to be of general significance (see dis- synapses has not yet been achieved. However, recent find- cussion of Declarative memory storage below).
ings have begun to make some progress. Thus, blockade ofnoradrenaline receptors has been found to interfere with Recent studies indicate that the cerebellum probably does formation of emotional memory in humans, suggesting even more than participate in motor learning; it also par- that the cAMP pathway in the amygdala may be required ticipates in associative word learning [29], learning of [18]. Disruption of cAMP-dependent kinase (PKA) in fact tactile mazes [30], and perception of time intervals disrupts fear conditioning [19,20]. More recently, LTP has been described in slices of the amygdala at both thesynapse from the thalamus and the cortex to the lateral Declarative (explicit) memory storage: the medial temporal amygdala, and, like LTP in the hippocampus, it has been system appears to have only a temporary role found to have an early phase and a protein-synthesis- A key feature of declarative memory is that the medial tem- dependent late phase [21]. The induction of the early poral lobe is involved in memory for a limited period of phase is postsynaptic, but the expression is presynaptic time. The initial evidence came from studies of the patient H.M., as well as the two other patients described byPenfield and Milner [32] who had good memory for remote Memory for motor acts and the cerebellum events that occurred years before their surgery. Recently, Well into the 1970s and 1980s, many neural scientists still studies in experimental animals have tested this idea rigor- thought that the function of the cerebellum was restricted ously and obtained clear evidence for temporally graded to coordinating voluntary movements: gait, posture, retrograde amnesia, which covered a period ranging from a speech, and other skilled movements. This view began to few days to about a month before surgery (reviewed in [5]).
change in the early 1980s, when a series of studies byRichard Thompson, Masao Ito, Mitchell Glickstein, Tom These data have given rise to the idea that the medial tem- Thach and their colleagues provided a variety of indepen- poral lobe structures direct a gradual process of dent evidence that the cerebellum is also critical for the reorganization and stabilization by changing the organiza- formation of learned motor responses ([6–9]; for early tion of cortical representations, perhaps by binding together theoretical discussions, see also [24,25]). For example, the separate cortical regions that store memory for a whole Thompson and co-workers [6,26,27] found that lesions of event [33,34]. After sufficient time has passed, the hip- the cerebellar cortex produced deficits in conditioned eye- pocampal formation may not be needed to support storage blink responses while sparing the unconditioned or retrieval of declarative memory, and long-term memory responses. The site for this plasticity appears to reside in is fully dependent on the neocortex (reviewed in [35]).
the mossy-fiber–parallel-fiber Purkinje cell synapse [27].
Indeed, conditioned responses can be obtained by simply The molecular mechanisms of memory storage
substituting for the CS direct electrical stimulation of A role for CREB in procedural (implicit) and declarative brainstem (pontine) nuclei and pairing that with electrical stimulation of fibers from the inferior olive as the US [27].
Although declarative and nondeclarative memory use dif-ferent neural systems and different logic, the two The work on eyeblink conditioning is part of a larger tra- memory systems have in common distinct temporal phas- dition of work on the role of the cerebellum in learning, a es. There is a short-term memory lasting minutes to tradition that includes the studies of Masao Ito [9] on the hours, and a long-term memory lasting days or even modification of the vestibular–ocular reflex and the studies longer. In both memory systems, long-term memory dif- of Thomas Thach [28] on the role of mossy and climbing fers from short-term memory not only in time course but fibers in monkeys learning to adjust their wrist movements also in molecular mechanisms. Long-term memory, but not short-term memory, requires the synthesis of newprotein (reviewed in [36,37]). Studies during the past A comparison of learning in the vestibular–ocular reflex to decade in Drosophila, Aplysia, and mice suggest that learning of classical eyeblink conditioning by Raymond et al. cAMP, PKA, and the cAMP-responsive transcription fac- [10] suggests that these two quite different behaviors show tor CREB are critically involved in the conversion of a surprisingly conserved role for the cerebellum in motor short-term to long-term memory for both declarative and learning. In each case, the plasticity is distributed between Cognitive neuroscience Albright, Kandel and Posner 615
Indeed, in both Drosophila and Aplysia, CREB learning- protein synthesis and the kinase PKA [46]. Moreover, related transcriptional activation appears to be the there is increasing evidence for rapid receptor insertion rate-limiting step in the conversion of short-term to long- and remodeling during E-LTP and for actual structural term synaptic plasticity and behavioral memory. This was changes during L-LTP (reviewed in [47]).
first shown for synaptic plasticity by injecting into thenucleus of the sensory neuron of Aplysia an oligonucleotide Development of gene targeting by homologous recombina- with a cAMP-responsive element (CRE) [38]. This tion in embryonic stem cells has made it possible to test oligonucleotide selectively blocked long-term facilitation some of these ideas genetically. The initial studies exam- without affecting short-term facilitation. A similar result ined mice with targeted knockout of CaMKII and the was later obtained by injecting antibodies specific for tyrosine kinase fyn, kinases that had previously been impli- CREB1 [39]. Conversely, injecting a phosphorylated form cated in LTP in pharmacological studies [44,48]. The of the recombinant CREB1a protein, a key regulator of genetic studies revealed that mice lacking CaMKIIα dis- transcription, was sufficient to induce long-term facilita- played a partial loss of E-LTP in the CA1 neurons of the tion [40]. Injection of antibodies against Aplysia CREB2 hippocampus and severe impairment on spatial memory (ApCREB2), a negative regulator that inhibits CREB1- tasks [49–51]. Similarly, analysis of mice with targeted dele- mediated transcription, led to long-term facilitation tions of the tyrosine kinase fyn also showed deficits in accompanied by a growth of new synaptic connections [40] E-LTP as well as in spatial memory. Mice with deletions of when paired with a single pulse of 5-HT (which normally the non-receptor tyrosine kinases src and yes were normal produces short-term facilitation). Thus, ApCREB2 acts as [52]. By contrast, expression of a dominant-negative a repressor of long-term facilitation by functionally com- inhibitor of PKA in neurons of the forebrain using the peting with the CREB1a activator. In parallel experiments CaMKIIα promoter [53] resulted in mice that had a normal in Drosophila, the behavioral switch from short- to long- E−LTP but a dramatically attenuated L-LTP. These genet- term memory was found to be regulated by changing the ically modified mice learned a contextual task as well as activity ratio between CRE binding activator dCREB2a wild-type animals, had a perfectly good short-term memory and repressor dCREB2b proteins, the fly homologues of when tested 1 hour after training, but were impaired in mammalian CREB and Aplysia CREB1 [36,41]. For exam- selective long-term memory when tested 24 hours later.
ple, when the expression of the CREB activator is induced Thus, as in Aplysia and Drosophila, the PKA signaling path- before training, a single odor–shock pairing produces a way in mice seems to be important for maintaining both LTP and memory for prolonged periods of time.
These results in Drosophila suggest that this CREB-medi- Previously, Roussoudan Bourtchouladze et al. [54] had ated induction of transcription is necessary to produce the examined the role of CREB in the mouse. They found that long-lasting changes in synaptic strength required for the a deletion that eliminated only the alpha and delta isoforms long-term storage of memories [36,37]. The results in led to impairment in LTP and in long-term memory storage Aplysia suggest that the CREB switch functions at the level [54]. The memory deficit was similar to that seen in mice of the individual synapse to convert a short-lasting increase expressing the PKA inhibitor; initial learning and short- in synaptic strength produced by covalent modifications of term memory were intact, whereas long-term memory was existing proteins to one that is long-lasting and produced by impaired. Moreover, work by Daniel Storm and colleagues the synthesis of new proteins (see also [42,43]).
[55,56] has provided further strong evidence that CREB isindeed involved in mouse synaptic plasticity. They pro- duced a transgenic mouse in which a lacZ reporter gene is What about the switch to long-term declarative memory? activated by a CREB-responsive promoter, and they found In the past decade, methods for modifying individual that this reporter is activated both by L-LTP in vitro [55] genes in mice have become available and have helped and by certain forms of hippocampus-dependent learning define the mouse as the mammalian model system par in vivo [56]. This demonstrates that CREB or CREB-like excellence for the genetic study of declarative memory stor- transcription factors are in fact activated under circum- age. Mice exhibit a memory for space and objects that stances that lead to plasticity and suggests a causal role corresponds to human declarative memory, requiring the similar to that seen in Aplysia and Drosophila.
hippocampus and the medial temporal lobe. Moreover,the hippocampus has a form of synaptic plasticity, called Regionally and temporally restricted gene expression LTP, thought to be a candidate mechanism for this sort of The traditional genetically modified mouse lines men- memory storage. Pharmacological experiments in the tioned above have identified some genes that are necessary 1980s and early 1990s first indicated that LTP itself has to develop a normal learning and memory phenotype in the stages, much like long-term facilitation in Aplysia. There adult, and suggest genes that may vary in their allelic form is an early-stage LTP (E-LTP) that requires covalent in a normal population to give rise to the normal variation in modification mediated by Ca2+/calmodulin-dependent cognitive ability. These lines of mice, however, suffer from protein kinase II α (CaMKIIα) and the tyrosine kinase the limitation that many molecules likely to be important fyn [44,45], and a late-phase LTP (L-LTP) that requires for adult learning are also likely to be important for normal A decade of Current neurobiology
development. In addition, these gene manipulations often Overexpression of calcineurin leads to an impairment in an affect a variety of different brain regions. One example of intermediate form of LTP (I−LTP) in hippocampus and to how to circumvent this problem is provided by work on a defect in spatial memory in the Morris water maze regional and regulated gene expression in the brain [57–59].
[63,64,66]. Since the transgene can readily be switched onand off by giving or removing doxycycline, it has the great Mice with a targeted deletion in the NR1 subunit of the advantage that it can be used to study not only memory NMDA receptor die shortly after birth [60]. Previously, Joe storage but also memory retrieval. Mutant mice that Tsien and colleagues [57,58] succeeded in obtaining condi- express the calcineurin transgene transiently, after learning tional deletion restricted to forebrain neurons by using the has been acquired and after spatial memory has already CRE-loxP systems. By combining the forebrain-specific been stored, have an apparent defect in the retrieval of the CaMKIIα promoter with the bacterial CRE-loxP recombi- spatial information. This retrieval defect is not attributable nase system, they were able to knock out the NR1 gene to a disruption in memory storage because it could be specifically in the CA1 region of the postnatal hippocampus, reversed when the transgene expression was turned off by without affecting other structures. This restricted knockout stopping doxycycline administration. Thus, with the use of avoided the perinatal lethality of a complete NR1 knockout.
regulated genetic modification one can not only control for As predicted, the mice showed a deficit in CA1 LTP and a potential developmental abnormalities associated with a severe deficit in spatial learning, strongly supporting a role genetic change but also begin to explore the various for NMDA-dependent LTP in hippocampus-dependent phases of memory acquisition, storage and retrieval [64].
learning. More recently, Tsien and colleagues [61] have car-ried out the reverse experiment. They expressed a mutant Future directions
form of NR1 that allows greater Ca2+ influx, and found that The use of mouse genetics to investigate complex behav- ioral traits such as learning and memory is at an early stage,but it promises to extend cognitive neuroscience into a A further advance has been the ability to regulate gene new, molecular genetic direction. Moreover, the work in expression not just regionally but also temporally, using both invertebrates and mice suggests that many of the the tetracycline system (see e.g. [62]). In a group of exper- basic molecular mechanisms for memory may be con- iments, a cell-type-specific promoter, such as the served across species, allowing insights from invertebrates CaMKIIα promoter, is used to drive expression of the to be applied to the mammalian brain. In the fly, for exam- tetracycline-responsive transcription factor tTA in fore- ple, the characterization of a new learning and memory brain neurons of one line of mice. In a second line, the mutants will hopefully provide a fuller understanding of transgene to be regulated is linked to a promoter consist- the critical genes involved in memory storage. However, ing of multiple repeats of the tet operator (tetO) linked to since the developmental integrity of many brain regions is a minimal eukaryotic promoter element. When these two necessary for the proper performance of even the simplest lines are mated so that both transgenes are introduced into memory task, and since memory storage requires some of a single mouse, the tetO-linked gene is activated specifi- the most basic cellular signaling mechanisms, there will be cally in those cells that express tTA. The expression of the many developmentally important genes that affect learn- tetO-linked transgene can then be suppressed by oral ing and memory. The difficult task facing the field, administration of the tetracycline analogue doxycycline therefore, will be to distinguish those mutations that affect [59]. In another class of experiments, a mutant form of the the core cellular signaling mechanisms that are used to tetracycline repressor is used that induces transcription encode memories from those mechanisms that modulate only in the presence of doxycycline [62]. This reverse tTA, these mechanisms or affect the development of the basic or rtTA, has been used to obtain inducible and reversible circuits important for performing the learning task or for expression of a Ca2+-activated protein phosphatase cal- storing the learned information. The use of progressively cineurin in forebrain neurons [63,64]. Calcineurin opposes better anatomically restricted and temporally regulated the action of PKA and other protein kinases. Calcineurin genetic modification in the mouse will be critical for dis- has a high affinity for Ca2+, even higher than that of tinguishing those genes that directly affect memory CaMKIIα. At low-frequency stimulation, the amount of encoding from those that affect memory indirectly through Ca2+ coming into the cell through the NMDA receptor is developmental, motivational, or perceptual mechanisms.
small and activates calcineurin but not CaMKIIα.
Calcineurin, in turn, can dephosphorylate protein phos- phatase Inhibitor-1, which activates protein phosphatase 1 In his pioneering text, which first appeared 50 years ago, (PP1) and leads to long-term depression (LTD) of synaptic Donald Hebb [67] observed that “we know virtually noth- transmission, which is the mirror opposite of LTP [65]. In ing about what goes on between the arrival of an excitation contrast, higher frequencies of stimulation lead to greater at a sensory projection area and its later departure from the Ca2+ influx. This, in turn, activates kinases, including motor area of the cortex…” “Something like thinking inter- PKA, that phosphorylate and block Inhibitor-1, thereby venes,” and although it would be hard to disagree with that shutting off the phosphatase cascade. In fact, PKA and cal- proposition, the goal of cognitive neuroscience has been to cineurin target the same residue on Inhibitor-1.
flesh out that ‘something’ in a form that is more satisfying Cognitive neuroscience Albright, Kandel and Posner 617
to both psychologists and neurobiologists alike. In part In a set of experiments conducted by Thomas Albright, because its operations span the chasm that Hebb lament- Gene Stoner, and colleagues [71–73], contextual cues ed, the visual system has served as a proving ground for unrelated to visual motion (e.g. luminance or stereoscopic this goal. By tracing the flow of visual information from cues that elicit a percept of surface depth ordering) were retina to motor control circuits we can, in principle, deter- found to markedly alter perceived motion, even though mine how its representation by the brain contributes to the retinal stimulus motion remained unchanged. In addition, various cognitive processes that constitute thinking, such the responses of many motion-sensitive neurons in cortical as perception, recognition, imagery, decision making, and visual area MT were found to co-vary with perceived motion rather than with retinal motion, demonstrating thatthe formation of perceptual or ‘scene-based’ neuronal rep- The 1990s will long be remembered as a turning point in resentations is accomplished at early stages in the visual this effort. As often recorded in these pages, recent advances in cognitive neuroscience are many. This state ofaffairs owes much to the fact that neurobiologists have Related evidence for scene-based representations has with increasing frequency turned to experimental psychol- come from experiments in which perceived features are ogy (and vice versa) for guidance, inspiration, and tools.
not physically present in the retinal stimulus, but their Products of this new-found interdisciplinary success presence in the visual scene is implied by contextual cues.
include discoveries regarding the correspondence between In one such study, Rüdiger von der Heydt and colleagues neuronal and perceptual events, the role of context in per- [76] studied the neuronal basis of a perceptual phenome- ceptual processing, the neuronal substrates of attention non known as ‘illusory contours’, in which contextual cues and decision making, the plasticity of adult sensory repre- imply the presence of an occluding surface and the edges sentations, and the role of such in perceptual learning and of the surface are seen, even though they are not actually sensory–motor spatial coordinate transformations.
present in the stimulus. These investigators discoveredthat many neurons fire in a way that matches the percept, Linking neuronal and perceptual events
as though a real contour had been placed in the receptive We entered the past decade secure in the knowledge that field. In another study, John Assad and John Maunsell [77] the stimulus selectivities of visual neurons resemble the exploited the fact that observers generally infer the con- basic elements of perceptual experience, such as simple tinuous motion of an object when it moves behind an forms, motions, and colors. Lacking, however, was evidence occluder. By using remote contextual cues to place the for a specific causal relationship between neuronal and per- path of occluded motion within the receptive fields of ceptual events. The problem was remedied by William motion-sensitive MT neurons, these investigators found Newsome and colleagues, who united classic methods from that many neurons respond in a way that matches the per- experimental psychology with modern neurobiological ceptual inference of motion, in the absence of any real techniques (for a review, see [68]). These investigators dis- motion in the receptive field. The significance of these covered a close relationship between perceptual motion findings lies, of course, in the fact that the neuronal rep- sensitivity and the sensitivity of neurons in cortical visual resentations reflect — as does perception — the enduring area MT [69], which strongly suggested that neuronal activ- structural and relational qualities of the observer’s ity within area MT is a constituent of the perceptual experience of motion. Icing came in the form of anotherstudy in which Newsome and colleagues modified the per- Visual attention
ceptual experience of motion by artificially activating small The primate visual system has a limited information pro- collections of MT neurons [70]. The results from these cessing capacity. An exciting area of research in the 1990s experiments have offered sound reassurance that the stim- has been that addressing the means and conditions under ulus selectivities of visual neurons account for perceptual which this limited capacity — visual attention — is experience, and they have paved the way for an under- dynamically allocated. Work in this area has revealed two standing of the underlying mechanisms.
basic types of attentional phenomena, which may havedistinct neuronal substrates. One effect, known as ‘atten- Sensation versus perception
tional facilitation’, is the improved processing of a The Newsome experiments solved one puzzle, but as stimulus when it appears at an attended location. Early many neurobiologists of the 1990s gained sophistication in investigations of the effects of focal brain lesions in perceptual psychology, they were forced to confront another: humans implicated the parietal lobe in attentional facili- as understood up to that point, the stimulus selectivities of tation. In subsequent physiological studies of parietal visual neurons encoded properties of the retinal stimulus.
cortex in non-human primates, Michael Goldberg and But things perceived reflect the ‘meaning’ of the stimulus, colleagues [79] found that for many neurons an attended as defined by the content of the visual scene that leads to visual stimulus elicited a much larger sensory response its appearance. Several studies carried out during the past than did an identical unattended stimulus. Similar facili- decade have sought to distinguish neuronal representa- tatory effects have since been reported for other cortical tions of sensory and perceptual events.
A decade of Current neurobiology
The other basic attentional effect that has been studied search for neuronal responses that ‘predict’ an impending extensively is known as ‘attentional selection’. This effect motor response to a visual cue (see e.g. [89]). In a recent refers to the phenomenon in which a target stimulus (i.e.
study, Michael Shadlen and colleagues [90] exploited the the thing you’re looking for) is selected from among other fact that difficult decisions generally require time to accu- stimuli that are competing for attention. In the mid-1980s, mulate relevant information, hence predictive neurons Robert Desimone and colleagues [82] found that receptive should exhibit responses that increase in magnitude in par- field profiles of individual neurons in cortical areas V4 and allel with the observer’s decision confidence. These IT contract around the attended stimulus, excluding unat- investigators found that neurons in prefrontal cortex do tended stimuli. These findings of selection at the neuronal exactly that. Collectively, these novel studies of the deci- level imply that information about an attended stimulus is sion process have identified relevant neuronal substrates carried to higher processing stages, at the expense of infor- and have led to promising theories regarding ways in which mation about unattended stimuli. Selective effects have visual information is flexibly mapped to action.
now been reported for many visual areas, including areasV1, V2, V4, MT, MST, and IT (see e.g. [83–86]), indicat- Perceptual learning
ing that selective mechanisms operate simultaneously on It is a central tenet of neurobiology that the sensory neo- cortex reaches its mature state of organization following abrief period of postnatal plasticity known as the critical Physiological studies are beginning to target the underly- period. One of the most important discoveries of the past ing mechanisms of selective attention. Much of the work decade, however, is the large extent to which this plastici- in this area has been inspired by the ‘biased competition’ ty continues throughout life. Adult plasticity enables forms model advocated by Desimone and Duncan [87].
of cognitive flexibility such as perceptual learning, which According to this view, pieces of incoming sensory infor- is the improvement with practice in the ability to discrim- mation compete for neuronal representation (and, inate sensory attributes. Early hints of this plastic potential ultimately, control of the observer’s actions), and the com- came from studies demonstrating that the adult cortex petition is biased such that behaviorally relevant inputs are undergoes a local functional reorganization to compensate facilitated. Support for this model comes from a recent for damage to the sensory periphery [91–93]. This com- study by Desimone and colleagues [86], who found that pensation may be mediated by intrinsic cortical the neuronal response to two unattended stimuli placed connections, which appear to undergo rapid changes in together in the receptive field was approximately the aver- synaptic efficacy, as well as a slower process of sprouting age of the responses elicited by the two stimuli presented and synaptogenesis [94,95]. The belief that this form of independently. By contrast, when either stimulus was plasticity also underlies adult perceptual learning is sup- attended — thus giving it a competitive advantage — the ported by evidence that training on perceptual tasks leads neuronal response approximated that elicited by the to reorganization of cortical sensory maps [96] and attended stimulus alone. Although these findings beg improvements in the sensitivities of cortical neurons [97]. important and difficult questions regarding the source ofthe biasing signal, as well as the local circuit and synaptic Transforming signals from visual space to motor space
interactions that lead to changes in the receptive field pro- Another success story from cognitive neuroscience in the file, they provide a striking example of the gains afforded 1990s centers on the problem of converting visual inputs to by the convergence of psychology and neurobiology.
signals that can guide actions. One long-standing viewholds that retinal signals, which represent visual space in a Perceptual decisions
coordinate frame that shifts with every movement of the While intelligent behavior depends upon knowledge of eyes, are re-mapped into a more generic and stable coordi- one’s external environment (i.e. perception), it also nate frame based, for example, on the positions of objects requires deciding which actions are appropriate given that relative to the observer’s head or body. Consistent with this knowledge. This ‘decision process’ has been the focus of view, Richard Andersen and colleagues [98] discovered that several revealing experiments over the past decade. One the magnitude of response to a visual stimulus varies with goal has been to identify neuronal activity that is correlat- the angle of gaze. Because they take eye position into ed with the decision to execute a particular action in account, these ‘gain field’ responses yield a head-centered response to a particular sensory stimulus, rather than sim- map of visual space that is distributed across a population of ply correlated with either stimulus or action alone. In a parietal neurons [99]. More recent studies have obtained series of studies, Earl Miller and colleagues [88] found that intriguing evidence for explicit cellular representations of the responses of neurons in prefrontal cortex — an area space in pre-motor cortex. Carl Olson and Sonya Gettner long believed to play a role in the organization of complex [100], for example, found that some premotor neurons rep- behavior — change as new sensory–motor relationships are resent spatial location relative to the parts of a visible object learned, such that individual neurons come to represent (i.e. ‘object-based’ coordinates), independent of the posi- new behaviorally relevant conjunctions of stimulus and tion of the object’s image on the retina. Perhaps even more action. Other groups have adopted a different approach to remarkable are the findings of Michael Graziano, Charles the decision process, the principal feature of which is a Gross and colleagues [101], who reported the existence of Cognitive neuroscience Albright, Kandel and Posner 619
premotor neurons that represent the position of a visual their organization into circuits, their pathologies and stimulus relative to the position of the observer’s forearm.
Body-part-centered representations of this sort appear wellsuited for orchestrating specific limb movements to stimuli Imaging has strengthened the correspondences
between the brain anatomy of humans and that of
experimental animals
The binding problem
A major advance in making cross-species comparisons has In reviewing major themes of cognitive neuroscience been the development of flat maps that provide a two- research in the 1990s, we would be remiss to exclude a dimensional surface for mapping the complex folds of the topic that has captured enormous attention in disciplines human brain [106]. There has been progress in efforts to ranging from visual physiology to philosophy, while relate retinotopic human visual areas to the maps nonetheless remaining one of the most unsettled. The obtained from cellular recording in primates [107].
topic in question is the use of temporal binding codes to Evidence that attention enhances activity in V1 (for a represent complex conjunctions of information carried by review, see [108]), has made it possible to explore atten- individual neurons. The potential utility of such codes has tional influences in a brain area where the detailed long been recognized. Empirical support came in the form cellular structure is better understood. Cellular studies in of physiological data from Charles Gray, Wolf Singer and monkeys indicate the importance of attention in integrat- colleagues [102], which suggested that visual features (e.g.
ing visual effects that occur outside the classic receptive edges) perceived as parts of the same object are represent- field of V1 neurons, for example, in perceiving contours ed by neurons that fire synchronously. Others, however, [109]. The interaction of attention with V1 circuitry may have failed to find such support (or have raised objections also be important in understanding the early visual sys- on theoretical grounds), and the unprecedented polariza- tem plasticity described in the previous section.
tion of opinions on the subject remains palpable as thedecade draws to a close (e.g. see reviews in the October Integration of human and animal studies is not limited to 1999 issue of Neuron). In light of the importance of the the early visual system. Studies of parietal neurons, locat- binding problem for both a functional and mechanistic ed within brain areas shown to be active in imaging studies understanding of cognition, and the attention and of spatial attention [110,111], have provided evidence of a resources that have been directed at the problem in recent map of locations organized by their current importance years, this persistent lack of consensus is both surprising [112] that could serve as the basis for human working and greatly disappointing. One can only hope that resolu- memory for location [113]. Using new fMRI methods, it tion will come from the application of new concepts and has been possible to separate operations performed by the superior and inferior areas of the parietal lobe. Theseresults [111] suggest that the temporal–parietal junction is Imaging higher cognitive functions
critical for shifts of attention toward unexpected visual Imaging has been critical for the localization of mental
stimuli, a finding that supports the importance of this area processes
in the neglect of space opposite the lesioned hemisphere As one reads journals devoted to cognitive neuroscience or found in patients suffering from strokes that affect the human brain mapping, it is hard to imagine that there could ever have been doubts that there was a specificanatomy related to higher mental processes. Only in the Timing and the functional connectivity of neural circuits
past decade have neuroimaging studies using PET and Of equal importance to precision in space is the ability to fMRI demonstrated a pervasive form of localization in a say exactly when and for how long an anatomical area is wide variety of cognitive and emotional tasks [103,104].
active and when information is being exchanged between The localization of mental operations, as described in the areas. Because mental operations occur in the range of tens first section of this review, has made psychology a full to hundreds of milliseconds, it has been useful to relate partner in efforts to understand human brain mechanisms.
areas of activity found in imaging studies to the distribu-tion of electrical activity recorded from the scalp [114] or The distributed nature of the activations in any real cog- by depth electrodes [115]. For example, using combined nitive task helps explain why Karl Lashley [105] and ERP and fMRI methods, it has been shown that although others could have thought that the brain operated as a attention influences activity in primary visual cortex, this whole. However, in tasks involving language, mental takes place only after the information has been processed imagery, spatial navigation and working memory, where we have been able to dissect them into plausible compu-tations, it is these components not the task themselves A different approach to the circuitry of high-level cognition that are localized. Of course localization is only a start is to examine functional connectivity by studying the cor- toward the achievement of cognitive neuroscience. As relation between brain areas on the basis of hemodynamic described below, imaging has provided an important [117] or electro-magnetic [118] measurements. These impetus for exploring the evolution of mental operations, efforts provide an approach to the transfer of information A decade of Current neurobiology
between brain areas by specifying within a task exactly the Neuroimaging studies have provided us with some mech- time when their activity is correlated.
anisms by which experience at different time scales mightchange anatomy or circuitry on a temporary or permanent Recently, an adaptation of magnetic imaging called diffu- basis. One way in which brain circuitry can be altered is sion tensor imaging has been used to produce images of called priming, which refers to changes in the efficiency of the white matter connecting brain areas [119,120]. The processing a target when part or all of the pathway involved time course of myelination of neuronal pathways can be has been previously activated. Combined neuroimaging used to test theories of when in development particular and cellular studies show that priming works by reducing behaviors emerge. Perhaps this method, when combined or tuning the number of neurons required to process the with measuring correlations in electrical activity, will allow target [131]. Studies in human subjects have shown that us to predict when a given brain circuit is sufficiently priming can take place within a second and that it may developed to support the learning of complex skills. In a take place even when the person is unaware of the identi- recent study [121], for example, differences in white mat- ty of the prime [132]. Priming may help account for ter tracts in the temporal parietal area of the left moment-to-moment differences in the thoughts generated hemisphere were related to reading skill within both a nor- in response to a given environmental challenge or strate- mal and a reading-impaired population.
Pathology: the search for subtle functional changes in
A few minutes of practice has been shown to be sufficient to change the circuit that processes information from one that Brain damage due to strokes and tumors can be observed involves a high level of complex computation to one that has readily in structural images, but other abnormalities may an already compiled answer waiting for output [123,133].
involve more subtle functional changes. The use of struc- Changes of pathways can help explain the shifts that take tural images to map brain lesions has allowed investigators place when extensive practice renders a skill automatic.
to combine the data from sets of patients with large cere-bral lesions showing by the overlap in lesion location the Somewhat slower are the task-related increases in cerebral brain areas that appear to cause the deficit. For example, a tissue found in sensory [134] and motor [135] systems with set of patients with large lesions of the left hemisphere all extended practice. These changes in visual areas were dis- showed expressive aphasia provided that they had a loss of cussed in the previous section. The effects of extensive neurons in the anterior insula [122]. This finding support- practice have also been documented in studies of adults ed results obtained with PET showing that the insula is an who learned languages either as children or later in life important pathway for highly automated verbal output, [136] and musicians who had extensive practice on musical instruments [137]. Although there are strong hints that therelative plasticity of these skills may differ between chil- Tasks involving monitoring of emotion and cognition acti- dren and adults, full documentation of this form of critical vate separate areas of the frontal midline [124–126]. In some studies, the cognitive and emotional areas appear tobe mutually inhibitory, with cognitive tasks reducing blood The next decade?
flow in areas related to emotion, as well as the reverse Donald Hebb and his fellow pioneers in cognitive neural [126]. These areas appear to be important in some forms of science would surely be pleased at the promiscuous bed- psychopathology. For example, PET scans of patients with fellows that psychologists and neurobiologists have now schizophrenia who have never been on medication indicat- become, and with their offspring, which is a fuller demon- ed an abnormality in the left globus pallidus [127], which stration that specific cognitive information is represented is the outflow of a major dopamine circuit that modulates in the activities of specific neuronal populations. These cellular activity in midline frontal areas such as the anteri- recent successes notwithstanding, it seems likely that or cingulate gyrus [128]. This abnormal function might Hebb would also recognize the weakness in our current help to explain both the curious neglect of the right side of view: lack of information about how these neuronal repre- space shown in early schizophrenia and the changes in cir- sentations are achieved mechanistically. How, for example, cuitry within the anterior cingulate observed in the brains are contextual cues assessed by cortical neurons to form of schizophrenic patients postmortem [129].
visual representations that coincide with perceptual expe-rience? And how are such representations altered as a Plasticity and shaping of neural circuits by experience
function of experience? Answers to these and other mech- A major achievement of infancy research in the past anistic questions require, at the very least, detailed decade has been to show that human infants enter the information about the patterns of anatomical connections world with some mechanisms related to the processing of in the cerebral cortex, and the functional properties con- language, objects, faces and numbers, as well as with the ferred by specific circuit components.
ability to imitate motor routines (for a review, see [130]).
How does experience shape these initial mechanisms into Determining the local circuit organization of the cerebral cortex and how that organization relates to the processing Cognitive neuroscience Albright, Kandel and Posner 621
of region-specific information is, however, dauntingly powerful molecular and genetic tools find their calling in complex, and its elucidation — important though it may the service of cognitive neuroscience, and that the field will be — is among the most formidable challenges facing cog- continue to advance through a global circuit-based nitive neuroscience in the next decade. Hope lies in some approach to cognitive representation by the brain. These remarkable new experimental approaches, which promise are indeed heady times for the young field of cognitive both fine-scale assessment of functional circuitry in the neuroscience. Although, as noted by Hebb 50 years ago, cortex and pictures of the global patterns of neuronal activ- there still is “a long way to go before we can speak of under- ity associated with specific cognitive states.
standing the principles of behavior to the degree that weunderstand the principles of chemical reaction”, the time One of the most exciting prospects for fine-scale analysis for that understanding is now — at least — in full view.
of functional circuitry can be found in methods for region-ally restricted and temporally regulated control of gene Acknowledgements
expression. These methods took the stage in the 1990s in We thank Marc Tessier-Lavigne for his helpful editing of this paper.
TD Albright and ER Kandel are Investigators of the Howard Hughes the form of mouse germ line transgenic manipulations, and Medical Institute. The work of MI Posner was supported by a grant from they have become extremely powerful tools for analysis of the James S McDonnell Foundation to the Sackler Institute.
the cellular and molecular bases of learning and memory.
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