As anyone who has ever been startled—and who hasn’t?—knows, the startle response is involuntary. Indeed, it appears to be hard-wired early in development, and to reflect fundamental factors in brain architecture and function. And as many people on the autistic spectrum also know to their cost, sudden loud noises in particular can be especially difficult to deal with, and can leave them feeling angry and upset. A likely explanation is that autistics have heightened auditory sensitivity, perhaps along with deficits in their ability to filter out some sounds—something that might certainly explain their often-reported problems with listening to particular voices against a background of general loud conversation.

Startle response can be objectively measured by the pre-pulse inhibition (or PPI) test in which a low-stimulus warning signal precedes one loud enough to induce the startle reflex, for example as registered in involuntary blinking (above left). Evidence that PPI is indeed hard-wired and fundamental comes from the finding that PPI lessens startle response by notably different amounts in heterosexual women (13%), heterosexual men (40%), lesbians (33%) and gay men (32%).

Reduced PPI has been widely reported in patients suffering from schizophrenia (where the same sex difference in PPI as in the general population is also found). And of course, if the problem really is deficits in sensory filtering (or gating, as it sometimes called), sufferers from autism spectrum disorders (ASD) should be the same, and some studies have indeed reported this.

But according to the diametric model of mental illness proposed by the imprinted brain theory, schizophrenia is a paradigmatic psychotic spectrum disorder (PSD) and as such its primary symptoms should be the opposite of those of ASD. Indeed, in a previous post I pointed out that such should indeed be the case where laboratory experiments with hearing voices is concerned. But given how basic startle response is to brain function, it would be hard to dismiss PPI findings as reflecting secondary, or derived symptoms.

Now a new study suggests that the prediction implicit in the diametric model is correct and that PPI responses in ASD are in fact the reverse of what is found in PSD.

In the study by Gitte Falcher Madsen, Niels Bilenberg, Cathriona Cantio, and Bob Oranje, a total of 35 ASD patients aged between 8 and 12 years, and 40 normal controls matched on age, sex, IQ, and parental socioeconomic status, were recruited for participation in a PPI experiment. The authors report that

This is the first study to show that patients with ASD do not display the drop in percentage PPI … that healthy controls exhibit. In addition, this is the first study showing an increased sensitization in ASD children… If true, then our findings indicate different filter irregularities in ASD children than in adult schizophrenic patients. … To conclude, the present study is the first to show evidence of increased PPI and sensitization in children with ASD, suggesting that ASD children are hypersensitive to auditory stimulation compared with healthy controls. These results may point toward a hypersensitive sensory filter in ASD children … The results do not support that the aberrant sensorimotor gating currently found in ASD subjects is related to the gating deficits usually reported in schizophrenic patients. 

In other words, where PPI is concerned, autistics are the reverse of schizophrenics, and show enhanced sensitivity to auditory stimuli, explaining both the PPI result, and autistics' greater startle response to sudden loud noises. 

Leading authorities in psychiatry are already on record as saying regarding the diametric model that “If mental illness ends up being that conceptually straightforward, that would be shocking,” so I don't feel too unjustified in headlining this finding as "startling." And as an outstanding new study of the current state of our knowledge of schizophrenia co-edited by Steven Silverstein (who kindly brought these findings to my attention) makes patently clear, it's high time that psychiatrists woke up and started to listen! (And perhaps I should warn them that this finding is just the pre-pulse: a much bigger and even more startling one is coming!)

 (With thanks to Bernard Crespi and Gitte Falcher Madsen.)

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