'Percepts' are glimpses of the sensory world captured by the brain in real-time (definition 1; from per- + capere, 'to seize' in Latin). These short-lived representations serve as raw material for more advanced analysis of the sensory world. The brain then decides, within a few milliseconds, whether to pay "attention and use the information for further processing, or simply forget about it. Research on perception is an extremely rich and dynamic discipline at the core of the neural and cognitive sciences (Hochberg 1998). It addresses the whole gamut of processes and mechanisms by which the nervous system extracts information from sensory input. In the context of this type of research, 'percept' is also referred to not as the initial internal representation of the stimulus, but as the intermediate or end-product of central analysis (definitions 2 and 3). Construed this way, 'percept' denotes a "memory, as it outlives the stimulus. As noted below, the distinction between 'percept' and 'memory', specifically 'immediate' memory ("phase), is not trivial. In discussions of learning and memory, however, there are merits to such a distinction. Furthermore, for the purpose of the present discussion, considering percept as dependent on awareness (definition 3), which is central to cognitive treatments of percept, is not obligatory. Hence the more restricted, "reductive definition 1 is here preferred.
Information about the world arrives at our brain via sensory channels. The interaction of sensory energy with peripheral receptors at the front end of these specialized channels is termed 'sensation'. 'Perception' involves processes downstream of the peripheral receptors. Both the peripheral receptors and the central sensory units could be characterized using multiple "criteria; two common ones are modality, i.e. the type of energy detected, and the 'receptive field', i.e. that portion of the world from which that energy can affect the detector. Over the years, philosophers and psychologists have been debating and re-debating the relative role of the external and internal (brain) world in perception. What the frog's eye tells the frog's brain (Lettvin et al. 1959) may depend not only on what is in front of the eye, but also on what is at its back. The debate can be illustrated by contrasting theories of 'direct' vs. 'indirect' perception. Both types of theories agree on the basic good old philosophical tenet that 'perceiving is knowing' (Price 1950; Hall 1964; Burnyeat 1990). They disagree, however, on critical elements of this tenet. Adherents of 'direct' theories (alias 'ecological', 'stimulus-centred') suggest that the focus of perceiving is in the animal-environment interaction rather than in the animal, and perception is merely the selection and capture of information that is awaiting us outside there (Gibson 1979;
Michaels and Carello 1981). In contrast, proponents of the 'indirect' theories (alias 'information-processing', 'constructivist') propose that in perception, sensory data interact with endogenous information and are subjected to neuronal computations before becoming meaningful and useful (Ullman 1980; Rollins 1998). The view taken in this book is the latter. Ample data indicate that sensory areas in the brain function as filters tuned to specific sets of features (Dosher and Sperling 1998; Nakayama 1998). The information flows in both bottom-up and top-down streams (Knierim and Van Essen 1992; Ullman 1996; Jones et al. 1997; Nakayama 1998), with the latter possibly involving interactions with endogenously generated pre-representations ("a priori) as well as with previously acquired representations. The process probably becomes accessible to consciousness only at brain stations far from the sensory periphery (e.g. Crick and Koch 1995). Ultimately it "binds into a coherent percept. Thus, sense data do not represent the whole physical object but only selected aspects of it, biased by the species' and the individual's experience.
The fine tuning of central sensory systems and, as a consequence, the perception of attributes of sensory stimuli, is altered during brain "development in the young and by experience in the adult. This experience-dependent modification in the ability to extract information from sensory stimuli is termed 'perceptual learning' (Hebb 1949; Sagi and Tanne 1994; Goldstone 1998). Perceptual learning is manifested in multiple alterations in perceptual competence, including discrimination, categorization, and the attentional valence attributed to sensory features. It could involve the formation of perceptual 'sets',1 and is implicated in a wide range of behaviours. In addition to the overall maturation of sensory abilities, these behaviours include, among others, filial and sexual "imprinting during sensitive periods early in life; re-adjustment of sensory capabilities in response to insult ("map, "plasticity); and the acquisition of sensory "skill. Fine discrimination of odours in wine tasters is but one refreshing example for the latter (Bende and Nordin 1997). Whereas the normal brain is destined to benefit from improved perception over time, certain pathologies do the opposite. 'Agnosias' lead to failure in perceptual processing and recognition in the absence of disorders of sensation or language (Peach 1986). Their investigation assists in identifying brain regions that are involved in specific aspects of normal perception.
'Percept' by definition 1 above is a transient internal representation. It refers to the cognitive present. What is 'transient' and 'present' in this context? And can one at all distinguish a percept from very short-term memory?
A reasonable "criterion is to rely on the estimate of the minimal time that is required by the brain to extract information from sensory input. The exact value is expected to vary according to the complexity of input, but still, converging evidence from both cellular and psychophysical investigations points to 20-30 ms as the elementary cognitive stroke (e.g. Rolls and Tovee 1994; Horowitz and Wolfe 1998; "binding, "cell assembly). Even the complete processing of a complex visual scene, including the immediate response to it, requires < 150ms (Thorpe et al. 1996). This is shorter than the commonly accepted life span of 'sensory memory' (0.25-2 s; Dosher and Sperling 1998). So do 'percepts', as defined above, live 20-200 ms before they either become memories or die out? Possibly, but not necessarily. Circuits encoding percepts may sustain the primordial representation longer than that. First, because of their inherent biophysical properties. Second, because the brain may had evolved to always allow some more time for decision to be taken before sensory data are discarded. For example, is the endurance of evoked activity detected in cortical 'face' cells (200-300 ms after the offset of a stimulus; Rolls and Tovee 1994) evidence for the representation of a percept, or of an immediate memory trace? In real-life, the transition from percept to very short-term memory is probably gradual. Attempts to dissociate the two by behavioural "assays were unsuccessful (Haber 1966; but see Neisser 1967). Advanced psychophysical and cellular methodologies (e.g. Horowitz and Wolfe 1998; Parker and Newsome 1998) may clarify the issue, set new criteria, or declare the problem solely semantic and therefore practically irrelevant.
Yet even if at the end of the day 'percept' as a phase that precedes memory remains hypothetical and elusive, it is still a notion worthwhile to retain. This is because it sharpens the conceptual differentiation among memory phases and subphases. 'Percept' as considered here refers to the 'encoding' but not the 'registration' function in "acquisition (see there). It is also the concrete agent of 'stimulus-driven' attention, capable of pushing the input into the road to the "engram; for the brain, a stimulus has no meaning unless it results in a percept. A percept could also serve as a 'sign stimulus', which activates an innate response programme (Lorenz 1981), in which case the registration of the input is not a must. And, finally, percepts could provide "cues for "retrieval. Percepts thus induce major 'rites of passage' in the life of a memory.
Selected associations: Binding, Cue, Internal representation, Phase, Stimulus
1On what a 'set' is, see "learning set.
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