People with schizophrenia fail to identify odors on a popular smell identification test that those without schizophrenia easily name, according to new research published in APA's Journal of Abnormal Psychology (Vol. 113, No. 2).
The findings suggest that those with schizophrenia may experience an insensitivity to particular smells, rather than a deficit in their general olfactory abilities--a phenomenon that's also observed in people with Parkinson's disease, says one of the study authors, Kelly L. Minor, PhD, a National Institutes of Health postdoctoral research fellow in psychology at Stanford University.
To test the ability to label common smells, the researchers asked 136 healthy participants and 54 with schizophrenia to scratch and sniff the 40 items of the University of Pennsylvania Smell Identification Test (UPSIT). Participants chose a particular odor's name from a list of four possibilities--such as chocolate, rose, gasoline and orange.
Then, using a statistical technique known as Rasch analysis, the researchers scrutinized group differences at the item level. They created a difficulty rating for every smell, making it possible to determine whether participants were making errors in keeping with their overall smell identification ability.
The reasoning was, "if the test is a pure measure of odor identification, then we should be able to predict which items any given respondent will miss based upon her estimated ability level," says Minor.
The researchers found that adults with schizophrenia were often unable to identify the smells of cheese and bananas--mistakes rarely made by healthy people, even those with olfactory deficits.
Additionally, people with the disorder easily identified some items that were often missed by people with Parkinson's disease--a group previous research has shown to demonstrate similar gross smell identification deficits. While both groups miss five items on average, people with schizophrenia almost never fail to name the smell of pizza, clove and wintergreen--items Parkinson's patients frequently miss.
According to some researchers, this could mean that Parkinson's disease and schizophrenia result in specific cognitive deficits, which lead to particular sensory deficits. But Minor believes that the UPSIT's methodology, such as the strength it accords individual scents, may be biased against people with these disorders.
"We don't know why, but the preliminary findings suggest this test may measure something different in these two patient populations," says Minor.
And because of that uncertainty, Minor cautions investigators against inferring olfactory ability and brain functioning from the UPSIT--a new practice, but one that is gaining in popularity in the schizophrenia research literature, says Minor.
"While some studies have relied upon this test as a measure of orbitofrontal brain functioning, that may be a premature leap," she notes.