Schizophrenia (SCZ) is among the most debilitating syndromes within the neuropsychiatric
domain, with patients suffering from this condition displaying a rich nosographic
characterization. Besides, anhedonia, social withdrawal, and a progressive cognitive
decline, positive manifestations such as delusions and hallucinations are perhaps the
hallmark symptomatological features underlying this disorder. Notably, prodromal stages
that entail subclinical cognitive and/or psychotic symptoms have often been shown to
anticipate SCZ onset, corroborating a dimensional approach toward the understanding of
such psychiatric disorder. Indeed, it has been proposed that schizotypal traits exist
within the general population and that schizotypy and SCZ may correspond to quantitative
poles within the same continuum, with the former (i.e., individuals characterized with
high schizotypal traits, HST) including subjects displaying many SCZ-related subthreshold
clinical signs, and thus, more likely to develop the latter. However, no reliable
biomarker to early detect such psychosis-prone individuals has been so far, leaving
clinicians without pre-emptive policies toward risk mitigation.
It should be nonetheless highlighted that both demeaned microstructural and white matter
integrity have been reported in schizotypy and SCZ alike, the magnitude of which
predicting the severity of positive symptoms at the individual level. Given the pivotal
role these neuroanatomical features play in relaying electrical signals across the brain,
it is not surprising that SCZ patients, as well as HST individuals, oftentimes exhibit
aberrant oscillatory dynamics within the low-frequency ranges, such as in the alpha band
(7-13 Hz). These brain oscillations are the dominant rhythm in both the resting and
active brain. As such, they have been found to subserve many functions closely tied to
sensory processing. Among multifold operational features these rhythms appear to be
endowed with, their occipital cycling speed (i.e., individual alpha frequency peak, also
known as IAF) has been recently associated with the efficiency by which sensory
information is integrated or segregated, according to the task at hand . In agreement
with the discrete sampling hypothesis of perception, the faster (or slower) these rhythms
oscillate, the higher (or lower) the observers' visual sampling rate and their perceptual
or cognitive accuracy.
Of note, this electrocortical index bears trait-like features that have been shown to
parametrize perceptual performance, especially in multisensory contexts.
For instance, the propensity to report an illusory percept in audiovisual tasks such as
the Sound-Induced Flash Illusion (SIFI) was found to negatively correlate with the IAF as
recorded from occipital loci: the slower the IAF, the greater the number of illusory
percepts and vice-versa. These results align with pieces of research showing a critical
contribution of alpha oscillatory speed when individuals are asked to perform cross-modal
simultaneity judgments and visuo-tactile tasks as well. Intriguingly, evidence for a tie
between the IAF and multisensory temporal binding windows (TBW) has been further
reinvigorated by data showing slower IAFs in patients suffering from schizophrenia or
displaying schizotypal traits. Namely, those clinical and subclinical populations
performing poorly at the SIFI, and, more broadly, multisensory paradigms, due to enlarged
TBWs.
As for the neurocomputational mechanics subsiding this intricate pattern of alterations,
it remains a matter of debate whether they might be driven either by an over-exploitation
of auditory-relayed priors, or poor bottom-up sensory processing. Indeed, although
alterations in sensory integration/segregation are a core hallmark of SCZ, it is not
entirely clear whether such behavioral and cognitive impairments might be due to
defective sensory processing (i.e. poor sensory input due to reduced feedforward
connectivity), or aberrant perceptual bias mechanisms (i.e. disproportionately high
top-down signaling), and to which extent they may be present at prodromal stages of the
disease. To this aim, we will therefore investigate the performance of SCZ patients, and
individuals displaying either high (HST) or low (LST) schizotypal traits, in a series of
tasks relying on multisensory interaction. We will do so by implementing signal detection
theory (SDT) matched with logistic fitting, to clarify at a finer-grained level whether
alterations in multisensory processes can be accounted for by deficits at the level of
sensory input (d'), top-down control (c), or both.