Habituation

Habituation is usually considered the simplest form of learning, although its mechanisms and correlation with memory processes are still not completely clarified.

Habituation may be defined as the relatively persistent waning of a response as a result of repeated or continuous simulation that is not followed by any kind of reinforcement (Thorpe, 1963; Hinde, 1970), or a decline in the responsiveness to a stimulus as a result of its repeated presentation (Pearce, 2000). In its widest sense, habituation is simply learning not to respond to specific stimuli that tend to be without significance. Habituation is probably one of the more widely occurring types of learning responses and this may be found and experimentally demonstrated in great variety of organisms. Indeed, one finds habituation everywhere. One of the first experimental examples came from Paramecia: Jennings (1906) found that an infusorium reacts to being touched by contracting. With continued touching the number of stipulations, needed to produce this response increases to 20 or 30. Similar patterns may be observed in different species, from Hydra that drops its responses for tapping a glass where it lives if nothing happens, to human being who ceases to respond to noise and other stimuli that are given repeatedly when nothing either pleasant or unpleasant happens.

Timing is critical in habituation. The marine ragworm Nereis will withdraw into its burrow in response to a wide range of stimuli, from touches and shadows to bright light and electrical shocks. Habituation to a bright flash or light occurred in less than forty trials when the stimuli were given at thirty-second intervals; but eighty trials were necessary for habituation to occur if the stimuli were given five minutes apart (Clark, 1960).

The paired with habituation process is sensitisation. This is an increase in the response to an innocuous stimulus when that stimulus occurs after a significant stimulus. For example, when the siphon of the sea slug Aplysia is gently touched, the animal withdraws its gill for a brief period. However, if preceded by an electrical shock to its tail, the same gentle touch to the siphon will elicit a longer period of withdrawal. The sensitisation response to a single shock dies out after about an hour, and returns to baseline after a day. In this case sensitisation underlies short-term memory. However, if the animal is sensitised with multiple shocks given over several days, its subsequent response to a gentle touch on the siphon is much larger and retained longer. This is an example of long-term memory and requires protein synthesis (Kandel, 2001).

Habituation also can be long - lasting. Thus, if you make an unusual sound in the presence of a dog, it will respond - usually by turning its head toward the sound. If the stimulus is given repeatedly and nothing either pleasant or unpleasant happens to the dog, it will soon cease to respond. When fully habituated, the dog will not respond to the stimulus even though weeks or months have elapsed since it was last presented.

It has been demonstrated in some experiments that habituation and sensitisation are moving in opposite directions over time, i.e. habituation increases whereas sensitisation declines.

There are a number of factors that can increase or decrease the response strength. Also, it was found for different species, that each type of stimulus (light, electricity, shadows, or touch) has its own characteristic rate of habituation at any trial frequency.

Habituation as the basis of test procedures is widely used by modern experimenters. These tests often allow researchers to obtain principally new results in different fields, from molecular mechanisms of memory to remarkable linguistic abilities of human neonates. For illustration, let us consider an example from a set of experiments supporting a hypothesis that human newborn infants during the first few months of life can discriminate phonetic constrains. Human newborns provide scientists with the same type of challenge that is offered by non human species: we cannot simply ask them what they perceive, we must use some tricks to obtain the answer. The trick used by developmental psychologists is just a procedure of habituation/dishabituation (Kellman and Spelke, 1983). Infants are habituated to a particular stimulus, and after habituation had occurred, they are shown a somehow changed stimulus. Dishabituation indicates that the changed stimulus is perceived by babies as a novel one. The ability to discriminate phonemes in babies was firstly studied in 70-th by behavioural methods, namely, by repeating an acoustic stimulus for several minutes until some behavioural response of the infant habituates, and then examining whether the response recovers when the stimulus is changed. For example, babies always stop to suck a pacifier when they react to a novel stimulus and return to this pastime when habituate. Later high-density recordings of event-related potentials were applied in order to reveal how fast three- months infants can detect phonetic changes and what brain mechanisms are involved (Dehaene-Lambertz and Dehaene, 1994). The behavioural basis was the same. Infants were seated in a carrier affixed to their parents, and their heads were covered with a geodesic sensor net in a very soft way. They faced a loudspeaker placed on top of TV monitor where a silent video showed attention-grabbing coloured objects. The video was not synchronised with the auditory stimuli, thus preventing any visually evoked potentials. On each trial, a sequence of five syllabes was presented. In half the trials, one syllable, designated as the standard, was repeated five times. In the other half (deviant trials), the standard was repeated only four times, followed by one instance of the other syllable, designated as the deviant. Because repeated and deviant trials were randomly mixed, infants could not predict the nature of the fifth stimuli. Thus any significant difference in event-related potentials indicated that the two syllables had been discriminated. The infants thus reacted to a new stimulus against a background of habitual ones. The infant brain recognises a phonetic change in less than 400 ms. Thus, two to three months old children may already possess a supramodal anterior network for novelty detection which can be activated in less than one second. So the method which basically relies on habituation/dishabitation turned out to be useful for studying cognitive development processes in humans. As we will see further in this book, this method has been successfully applied in many cognitive experiments with animals.

The adaptiveness of habituation in animals may readily be seen in social or colonial animals that habituate to each other and therefore save time and energy that would have been spent in needless squabbling. It also plays an important role in habitat selection: once habituated to an area an animal may feel at ease only in that familiar surrounding where novel stimuli are rare. In any case, the context is important for animals in the wild: a stimulus that has been habituated to must appear in its usual context if it is to be ignored. Out of context, it may arouse the original reactions (Wallace, 1979).