A signal for action

Originally, 'to stimulate' meant literally to prick or stub (stimulus, Latin for a goad or spur). Over the years, various concepts and usages of 'stimulus' have played a central part in the behavioural and neural sciences. Unless otherwise indicated, definition 1 is the one discussed here. Stimuli could be or could include "cues; further, definition 2 overlaps with definitions 3 and 4 of'cue'.

The "classic usage of'stimulus' in memory research refers to an event that triggers a response in the behaving organism. The concept, however, spans "levels, from the behavioural to the molecular. At all levels, stimulus is a packet of information that triggers specific, or less specific, processes in the receiving system.

1. On stimuli and information. There are two basic approaches to information. One, 'syntactic', considers the quantity without considering the content. The other, 'semantic', considers the meaning and significance. The 'syntactic' approach is used in information theory, where 'information' is a mathematical abstraction that refers to the uncertainty in coding and transmitting data, irrespective of the semantics ("system). This efficacy of transfer is measured in 'bits'. One bit is the choice between two equally likely possibilities; the number of bits required to select among n alternatives is log2n. It is currently difficult, if not impossible, to translate the data in sensory stimuli into bits

("capacity). Even when the number of bits will be known, still, itself it will tells us nothing about what these bits signify to the receiver. To understand this, we must either observe the behavioural or physiological effect of the stimulus under "controlled conditions, or decipher the neuronal codes used by the sender and the receiver ("internal representation). The semantics of the stimulus depends on at least some of the stimulus "dimensions.

2. On the dimensions of stimuli. Autonomous dimensions, which are independent of the "context and of the state of the receiver, involve modality (e.g. wavelength, chemical composition), intensity, duration, rate, frequency, and location in time and space. Contextual dimensions, e.g. saliency, refer to the relation of the stimulus to other stimuli. Subjective dimensions depend on the interaction of the stimulus with the receiver and include functional threshold ("sensitization), familiarity, hedonic valence, saliency, and efficacy.

3. On the source of stimuli. From the point of view of source-receiver interaction, stimuli differ in their pur-posiveness. Purposive stimuli1 differ in the specificity of their target. Non-purposive stimuli are released inadvertently by animate or inanimate systems. Consider a rabbit hopping in an open field. The moving object is a stimulus that triggers attack by the fox. This stimulus is not released purposively by the source. When the rabbit discovers the attack, it may freeze ("fear conditioning), which is already a purposive stimulus to abort the attack; this purpose was selected by evolution and shaped the innate, "a priori behavioural repertoire of the species. As the meaning of purposive stimuli is context and receiver dependent, it is not necessarily what the sender meant to deliver. Nowhere is it better epitomized than in 'pragmatics', the discipline that studies the meaning of sentences in language (i.e. verbal stimuli, definitions 1 and 2) in the particular contexts in which they are uttered. The same is true for body language. Travel guides to certain countries carry special warnings about this problem.

4. On the effect of stimuli. Stimuli could do one of the following: (a) induce new pattern(s) of activity in the receiver, in which case the stimulus is said to be instructive; (b) augment or inhibit non-discriminatorily endogenous patterns of activity in the receiver, in which case the stimulus is adjustive. (c) augment or inhibit differentially endogenous patterns of activity in the receiver, in which case the stimulus is selective; or (d) leave the system as it is, in which case the stimulus is ineffective.ln each of a-c above, the effect of the stimulus could be either immediate or latent, and may or may not "persist after the stimulus is gone ("memory, "plasticity).

A *reductive *taxonomy of stimuli encountered in learning and memory research:

a. The types of stimuli that affect the behaviour oforgan-isms. The behaviour of organisms is affected by sensory stimuli from the outside world, and by endogenous stimuli, which are either sensory stimuli from within the organism, or global mental states (e.g. hunger, anxiety), or specific mental operations (e.g. one thought as a stimulus for another, "associative memory). In most learning "paradigms, emphasis is placed on exogenous sensory stimuli. It is the experimenter who selects these stimuli, but the experimental "subject may not honour the selection (e.g. "classical conditioning, "context).

b. Sensory stimuli are delivered from the external world to the front end of a specialized sensory channel in the organism, causing sensation, i.e. a functional change in a sensory "receptor. Sensory physiology textbooks distinguish four types of dimensions in sensory stimuli, which can be quantitatively correlated with sensation. These are sensory modality, intensity, duration, and location in sensory space (Martin 1991). 'Proximal stimuli' affect the sensory receptor directly, for example, chemicals that interact with taste receptors, or pressure applied to somatosensory receptors on the skin. 'Distal stimuli' interact with the receptor via proximal stimuli; hence a visual scene is a distal stimulus that interacts with the visual receptor via photons, which are proximal stimuli.

This is a proper point to digress briefly into a conceptual issue. Dominant schools in philosophy trust that sensory stimuli are not autonomous entities but rather 'sense data', i.e. entities that exist as such only because they are sensed (Price 1950; "percept). This view is in accord with the stand taken in this book, that the world drives behaviour only via internal representations, which are inherently "biased "models of the world. In important chapters in the history of psychology, however, 'stimuli' did not connote mental processing. For example, Skinner (1938; "behaviourism) regarded 'stimulus'as part of the environment that affects behaviour, and 'response' as part of behaviour that is affected by the stimulus. The basic idea was that behaviour and environment can be broken into parts, which interact but retain their identity throughout an experiment. That brain processes are involved was not deemed relevant. This type of attitude was accompanied by highly sophisticated stimulus-response

('operant') "methodology and terminology (Guthrie 1935; Skinner 1938; Hull 1943; Bower and Hilgard 1981; "instrumental conditioning).2 Another discipline in which the focus is on the phe-notype of stimuli is ethology. A central notion is that of a 'sign stimulus', or 'releaser', which elicits a particular pattern of innately predisposed behaviour. A popular example is the spring fighting of the male stickleback, which is released by the sight of a conspecific male, or by elongated fish-like dummies with a red belly (Tinbergen 1969; for releasers consider also "birdsong, "imprinting).

c. Perceptual stimuli are extracted by the brain from sensory stimuli, creating a "percept, which could become a memory. At this stage, the stimulus is already converted into an internal representation, encoded in the spatiotemporal activity of neuronal circuits, or populations. Further processing could be subserved by stimuli arriving from other neuronal populations ("cell assembly), i.e. 'interpopulation stimuli'.

d. Interpopulation stimuli are delivered from one neuronal population to another. They are either the outcome of a percept, or of a global brain state (e.g. "attention), or of endogenous activity of the source circuit. The notion that there are 'stimulus-independent' states in the brain (e.g. McGuire et al. 1996) should therefore be regarded only as a pragmatic heuristic of a test protocol, not a "reallife state.

e. *Synaptic stimuli are either chemical ligands ("neu-rotransmitter) or ion currents. They engage receptors, "ion channels, and other membrane proteins, and subserve interpopulation stimuli (d above). Included here are also diffused ('volume') transmission and circulating hormones. So are glia-neuron messages (Araque et al. 1999).

f. Intracellular stimuli are delivered from a molecular sender to a molecular receiver within a neuron. For example, a G-protein shuttling between a surface receptor and an enzyme, or a transcription factor shuttling between the "protein kinase and a regulatory site of a gene (e.g. "CREB, "immediate early gene, "intracellular signal transduction cascade). Intracellular stimuli hence operate downstream of intercellular stimuli (e above) and ultimately actualize the information in the latter. Ion currents could also serve as intracellular stimuli.

The breakdown of stimulus types by level reminds one of the idea that the world rests on turtles, that rest on turtles, that rest on turtles, 'all the way down' (Geertz

1973; "reduction). For if we proceed in it we could end up with elementary particles as stimuli. This is a dilemma of reductionism, which must be solved by reconciling the multilevel nature of memory with the need to focus on those levels of organization and function in which the most important attributes of memory emerge (Dudai 1992). In the future science of memory, 'correspondence rules' of reductive theories will permit investigators to translate events from the language of one level to that of another. In the meantime, suffice it to remember that e and f above relate to cellular plasticity and cellular storage, whereas points c and d relate to representational change, hence bona fide memory; and that from a phylogenetic point of view, the survival value of all the stimuli is judged at the behavioural level, a.

Finally, in considering memory, it is important to reiterate that not all effective stimuli are successful teachers, and that in many cases the system quickly relaxes into the pre-stimulus state ("homeostasis). Therefore, not every poststimulus change in the brain is a manifestation of learning and memory, even if the report ends up as a catchy title in a respected journal. It is also noteworthy that our interaction with stimuli is use dependent. Sensory systems change their sensitivity to change (e.g. Torre et al. 1995; "metaplasticity); the brain learns to extract stimulus attributes by perceptual learning (Goldstone 1998); and we construe stimuli in the context of "culture (e.g. Clark and Clark 1980). We also learn to anticipate some stimuli but not others. If there is a discrepancy between the expectation and the actual stimulus, we experience "surprise, which itself is an effective incentive to form a memory of that stimulus, or, more accurately, of its percept.

Selected associations: Attention, Cue, Dimension, Generalization, Percept

1Purposiveness does not imply *declarativeness. The emission of the stimulus by the source may subserve a discrete purpose, e.g. warning, without the source being consciously aware of it. Purposiveness should also not be confounded with intentionality, which is the dimension of 'aboutness', considered to distinguish the mental from the physical (see *system).

2To do justice to subschools of behaviourism: not all those who considered overt behaviour as the only legitimate type of datum in psychology, discarded the role of internal processes; all, however, regarded stimuli as critical circumscribed variables of the behavioural experimentation and theory (Tolman 1952).

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