Top-Down Expectancy Modulates Sensory Distortion and Restores Cognitive Function in the Degenerating Brain: A Randomized Trial of Placebo-Induced Mitigation of Hallucinations

Abstract

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Background: Neuropsychiatric symptoms, including visual and auditory hallucinations, are primary drivers of morbidity in dementia. While pharmacological interventions are standard, the capacity for top-down cognitive modulation specifically through expectation-based (placebo) mechanisms to mitigate sensory distortions and cognitive deficits remains poorly understood, particularly in non-Western clinical populations. This study aimed to evaluate the impact of experimentally induced positive and negative hallucinations on semantic memory retrieval, reaction time, and affective states (depression/distress) in patients with mild dementia. Furthermore, it sought to determine the efficacy of a placebo intervention in reversing hallucination-induced cognitive and emotional impairment.

Method: This study used a mixed-factorial, placebo-controlled, double-blind, randomized block design, sixty participants (n=30 mild dementia; n=30 healthy individuals) were recruited from different hospitals in Rawalpindi   and Islamabad, Pakistan between June 1, 2024, and August 2024. Sensory hallucinations were induced using standardized paradigms via PsychoPy software. Cognitive performance was measured through semantic memory retrieval tasks and reaction time (RT) measurements, while emotional symptoms were assessed using validated psychometric instruments for dementia and depression. A placebo intervention was introduced in the final experimental block to induce positive expectancy.

Results: Induction of both auditory and visual hallucinations significantly impaired semantic memory retrieval accuracy and exacerbated depressive symptoms across both cohorts (p < .05). Negative hallucinations produced the most profound disturbances, particularly in the dementia group, characterized by increased RT and clinical disorientation. Notably, the introduction of the placebo intervention significantly attenuated these deficits, yielding a marked recovery in semantic retrieval performance and a reduction in affective distress in dementia patients.

Conclusions: These findings demonstrate that even in the presence of neurodegeneration, the brain retains a robust capacity for expectancy-mediated modulation of sensory processing. The placebo effect can successfully override the cognitive and emotional burden of hallucinations, suggesting that top-down pathways remain viable therapeutic targets. These findings demonstrate that the neurodegenerative brain maintains a functional capacity for expectancy-mediated modulation, suggesting that top-down non-pharmacological interventions can effectively attenuate the cognitive and emotional burden of hallucinations and reduce clinical reliance on antipsychotic medications.