For our sense of the world to, well, make much sense, we rely on the assumption of a reasonably constant sensory experience.
The same stop sign is going to look the same tomorrow as it does today, which looks the same as it did yesterday. If this were not the case, we might very quickly start having some anxiety about which real is real anyway.
Most of us can take this for granted, but there's a growing body of research suggesting that for schizophrenics, the ability to reliably filter and interpret sensory information is compromised. The world as the brain perceives it can't quite be counted on and this ties into the general schizophrenic syndrome of "disordered thinking." Hallucinations are a natural consequence.
By now a large number of studies have linked deficits in auditory and visual sensory processing with schizophrenia, but there has still existed a significant overlap in these observed deficits between schizophrenic and healthy populations. In other words, sensory deficits don't make for great predictors of the disorder or its future emergence in pre-schizophrenic patients.
That may be about to change, however. A study published this week in the open-access journal Translational Psychiatry offers a new way into the question of sensory processing in schizophrenic via a process known as sensory (or neural) adaptation. Simply, this is how the brain changes its response to a continuous, unchanging sensory input. That is, if I were to blink a light at you from a fixed distance for some period of time, your brain would progressively "get used to it"—repetitive inputs will be filtered more and more rather than be treated as novel sensory experiences. In other words, the brain will turn the series of flashes into one flash.
"This hypothesized reduction in representational redundancy likely serves to enhance the brain's ability to detect more relevant novel environmental changes or novel stimuli," the study notes. Tuning out the repetitious stuff means being better able to process new inputs.
What the current study wants to know is how and if this process of sensory adaptation varies between schizophrenic and non-schizophrenic individuals. To test this, the researchers, an NIH-funded group from the University of Rochester, took 15 schizophrenic subjects and 15 control subjects and recorded their neural responses via scalp electrodes to repetitive visual and touch stimuli.
The visual component of the Rochester experiments consisted of showing subjects kind of trippy checkerboard patterns that repeated at regular intervals. The somatosensory (touch) component swapped in a "custom-built vibrotactile stimulator," which basically consisted of a vibrating thing strapped to subjects' wrists right above their median nerves. As with the visual component, the wrist things were vibed at regular intervals as EEG data was collected.
What should happen is that initial, intense neural activity tapers off over time so long as the sensory input remains constant. The brain adapts and moves on. However, the schizophrenic patients were less able to adjust to the repeated stimulation—enough that such a test could help diagnose the disease.
"In the visual system, robust adaptation deficits, detectable at the individual-subject level, were observed over lateral occipital sites," the study concludes. These deficits were sufficient to classify schizophrenic subjects with an 80 percent success rate. The touch-based experiments were far less conclusive, with some differences between schizophrenic/non-schizophrenic subjects observed, but not enough to say anything conclusive. There's more work to be done.
The Rochester team hopes that their work will lead to better predictors of schizophrenia in those likely to develop it in the future, something that doesn't really exist now.
"A key point with this study is that we find these dramatic differences in patients who are already suffering from full-blown schizophrenia," offered John Foxe, the lead author of the study, in a statement. "Schizophrenia is a disease that typically strikes during late adolescence or early adulthood, but what we also know is that long before a person has their first major psychotic episode, there are subtle changes occurring that precede the full manifestation of the disease. Our hope is that these new measures can allow us to pick up on these people before they ever become seriously ill."