Department of Psychiatry and Behavioural and Clinical Neuroscience Institute, University of Cambridge, Cambridge, United Kingdom; Brain and Mind Centre, University of Sydney, Sydney, Australia.
Brain and Mind Centre, University of Sydney, Sydney, Australia; School of Social Sciences and Psychology, Western Sydney University, Sydney, Australia.
Biol Psychiatry Cogn Neurosci Neuroimaging. 2017 Nov;2(8):680-688. doi: 10.1016/j.bpsc.2017.04.007. Epub 2017 May 3.
Models of hallucinations emphasize imbalance between sensory input and top-down influences over perception, as false perceptual inference can arise when top-down predictions are afforded too much precision (certainty) relative to sensory evidence. Visual hallucinations in Parkinson's disease (PD) are associated with lower-level visual and attentional impairments, accompanied by overactivity in higher-order association brain networks. PD therefore provides an attractive framework to explore contributions of bottom-up versus top-down disturbances in hallucinations.
We characterized sensory processing during perceptual decision making in patients with PD with (n = 20) and without (n = 25) visual hallucinations and control subjects (n = 12), by fitting a hierarchical drift diffusion model to an attentional task. The hierarchical drift diffusion model uses Bayesian estimates to decompose task performance into parameters reflecting drift rates of evidence accumulation, decision thresholds, and nondecision time.
We observed slower drift rates in patients with hallucinations, which were less sensitive to changes in task demand. In contrast, wider decision boundaries and shorter nondecision times relative to control subjects were found in patients with PD regardless of hallucinator status. Inefficient and less flexible sensory evidence accumulation emerges as a unique feature of PD hallucinators.
We integrate these results with evidence accumulation and predictive coding models of hallucinations, suggesting that in PD sensory evidence is less informative and may therefore be down-weighted, resulting in overreliance on top-down influences. Considering impaired drift rates as an approximation of reduced sensory precision, our findings provide a novel computational framework to specify impairments in sensory processing that contribute to development of visual hallucinations.
幻觉模型强调感官输入与感知的自上而下影响之间的不平衡，因为当自上而下的预测相对于感官证据具有过高的精度（确定性）时，可能会产生虚假的感知推断。帕金森病（PD）中的视觉幻觉与较低层次的视觉和注意力损伤有关，同时伴随着高级联想大脑网络的过度活动。因此，PD 为探索幻觉中自下而上与自上而下干扰的贡献提供了一个有吸引力的框架。
我们通过将分层漂移扩散模型拟合到注意力任务中，来描述 PD 患者（有视觉幻觉者 n=20，无视觉幻觉者 n=25）和对照组受试者（n=12）在进行感知决策时的感觉处理。分层漂移扩散模型使用贝叶斯估计来将任务表现分解为反映证据积累漂移率、决策阈值和非决策时间的参数。
我们观察到有幻觉的患者漂移率较慢，对任务需求的变化不敏感。相比之下，PD 患者的决策边界更宽，非决策时间更短，无论是否有幻觉者。与对照组相比，PD 患者的感觉证据积累效率较低且灵活性较差，这是 PD 幻觉者的一个独特特征。
我们将这些结果与幻觉的证据积累和预测编码模型相结合，表明在 PD 中，感觉证据的信息量较少，因此可能会被低估，导致过度依赖自上而下的影响。考虑到漂移率受损可以近似于感觉精度降低，我们的研究结果提供了一个新的计算框架，可以说明导致视觉幻觉发展的感觉处理损伤。
