Latent inhibition


Latent inhibition is a technical term used in classical conditioning to refer to the observation that a familiar stimulus takes longer to acquire meaning than a new stimulus. The term "latent inhibition" dates back to Lubow and Moore. The LI effect is "latent" in that it is not exhibited in the stimulus pre-exposure phase, but rather in the subsequent test phase. "Inhibition", here, simply connotes that the effect is expressed in terms of relatively poor learning. The LI effect is extremely robust, appearing in both invertebrate and mammalian species that have been tested and across many different learning paradigms, thereby suggesting some adaptive advantages, such as protecting the organism from associating irrelevant stimuli with other, more important, events.

Theories

The LI effect has received a number of theoretical interpretations. One class of theory holds that inconsequential stimulus pre-exposure results in reduced associability for that stimulus. The loss of associability has been attributed to a variety of mechanisms that reduce attention, which then must be reacquired in order for learning to proceed normally. Alternatively, it has been proposed that LI is a result of retrieval failure rather than acquisition failure. Such a position advocates that, following stimulus pre-exposure, the acquisition of the new association to the old stimulus proceeds normally. However, in the test stage, two associations are retrieved and compete for expression. The group not pre-exposed to the stimulus performs better than the pre-exposed group because for the first group there is only the second association to be retrieved.

Variation

LI is affected by many factors, one of the most important of which is context. In virtually all LI studies, the context remains the same in the stimulus pre-exposure and test phases. However, if context is changed from the pre-exposure to the test phase, then LI is severely attenuated. The context-dependency of LI plays major roles in all current theories of LI, and in particular to their applications to schizophrenia, where it has been proposed that relationship between the pre-exposed stimulus and the context breaks down; context no longer sets the occasion for the expression of the stimulus-no consequence association. Consequently, working-memory is inundated with experimentally familiar but phenomenally novel stimuli, each competing for the limited resources required for efficient information processing. This description fits well with the positive symptoms of schizophrenia, particularly high distractibility, as well as with research findings.

Physiology

The assumption that the attentional process that produces LI in normal subjects is dysfunctional in schizophrenia patients has stimulated considerable research, with humans, as well as with rats and mice. There is much data that indicate that dopamine agonists and antagonists modulate LI in rats and in normal humans. Dopamine agonists, such as amphetamine, abolish LI while dopamine antagonists, such as haloperidol and other anti-psychotic drugs, produce a super-LI effect. In addition, manipulations of putative dopamine pathways in the brain also have the expected effects on LI. Thus, hippocampal and septal lesions interfere with the development of LI, as do lesions in selective portions of the nucleus accumbens. With human subjects, there is evidence that acute, non-medicated schizophrenics show reduced LI compared to chronic, medicated schizophrenics and to healthy subjects, while there is no difference in the amount of LI in the latter two groups. Finally, symptomatically normal subjects who score high on self-report questionnaires that measure psychotic-proneness or schizotypality also exhibit reduced LI compared to those who score low on the scales.
In addition to LI illustrating a fundamental strategy for information processing and providing a useful tool for examining attentional dysfunctions in pathological groups, the LI procedure has been used to screen for drugs that can ameliorate schizophrenia symptoms LI. LI has also been used to explain why certain therapies, such as alcohol aversion treatments, are not as effective as might be expected. On the other hand, LI procedures may be useful in counteracting some of the undesirable side-effects that frequently accompany radiation and chemo-therapies for cancer, as for example food aversion. LI research also has suggested techniques that may be efficacious in the prophylactic treatment of certain fears and phobias. Of popular interest, several studies have attempted to relate LI to creativity.
In summary, the basic LI phenomenon represents some output of a selective attention process that results in learning to ignore irrelevant stimuli. It has become an important tool for understanding information processing in general, as well as attentional dysfunctions in schizophrenia, and it has implications for a variety of practical problems.

Pathology

Low latent inhibition

Most people are able to ignore the constant stream of incoming stimuli, but this capability is reduced in those with low latent inhibition. Low latent inhibition seems to often correlate with distracted behaviors. This distractedness can manifest itself as general inattentiveness, a tendency to switch subjects without warning in conversation, and other absentminded habits. This is not to say that all distractedness can be explained by low latent inhibition, nor does it necessarily follow that people with low LI will have a hard time paying attention. It does mean, however, that the higher quantity of incoming information requires a mind capable of handling it.
Those of above average intelligence are thought to be capable of processing this stream effectively, enabling their creativity and increasing their awareness of their surroundings. Those with average and, less than average intelligence, on the other hand, are less able to cope and as a result are more likely to suffer from mental illness and sensory overload. It is hypothesized that a low level of latent inhibition can cause either psychosis or a high level of creative achievement or both, which is usually dependent on the individual's intelligence. When they cannot develop the creative ideas, they become frustrated and/or depressive.
High levels of the neurotransmitter dopamine in the ventral tegmental area of the brain have been shown to decrease latent inhibition. Certain dysfunctions of the neurotransmitters glutamate, serotonin and acetylcholine have also been implicated.