
Hallucinations, vivid sensory experiences without external stimuli, have long intrigued scientists, clinicians, and the general public. The open access article "Hallucinations as Top-Down Effects on Perception" by Powers and colleagues (2016) explores the psychological and neurological mechanisms underlying these phenomena, proposing that hallucinations result from top-down effects—where the brain’s expectations and prior experiences shape perception in the absence of external sensory input. This perspective provides a nuanced understanding of hallucinations, emphasizing their role in both mental health and normal cognition.

What Are Top-Down Effects on Perception?
Perception is traditionally understood as a combination of sensory input (bottom-up processing) and the brain’s interpretation of that input (top-down processing). Bottom-up processing involves raw data from the senses, whereas top-down processing incorporates prior knowledge, context, and expectations to construct our experience of the world (Gregory, 1980).
Top-down processing plays a critical role in filling gaps in sensory input and resolving ambiguities in perception. For example, when you recognize a familiar face in a crowd or interpret a blurry image, your brain uses prior knowledge and context to make sense of incomplete information. This predictive function of the brain is highly adaptive, allowing us to navigate complex environments efficiently. However, the same predictive mechanisms can go awry, leading to hallucinations.
When top-down processes dominate or override bottom-up sensory input, the brain generates sensory experiences based on internal expectations rather than external reality (Powers et al., 2016). This mismatch can produce vivid and often convincing hallucinations, highlighting the active and interpretive nature of perception.
The article further explains that this theory challenges earlier models of hallucinations that framed them as random sensory misfires or purely bottom-up disturbances. Instead, it emphasizes the brain’s active role in constructing reality, even in the absence of external stimuli. This understanding provides a framework for examining how hallucinations emerge from the interplay of cognition and sensory processing.
Key Neurological Findings
Advances in neuroscience have provided substantial evidence supporting the role of top-down processes in hallucinations. Functional imaging studies reveal increased activity in the prefrontal cortex and sensory association areas during hallucinatory episodes, suggesting that these regions are central to generating internally-driven perceptions (Allen et al., 2008). This hyperactivity aligns with predictive coding models, which propose that hallucinations arise when the brain prioritizes internal expectations over sensory inputs, leading to vivid but false sensory experiences.
These findings underscore how disruptions in predictive mechanisms can make internally generated perceptions indistinguishable from external stimuli (Powers et al., 2016).
Further research highlights altered connectivity between brain regions involved in expectation and sensory integration, such as the prefrontal cortex and primary sensory areas (Ford & Mathalon, 2004). This disrupted communication impairs the brain’s ability to reconcile bottom-up sensory data with top-down predictions, resulting in a sensory disconnection that fosters hallucinatory experiences.
Neurological evidence from conditions like schizophrenia and Parkinson’s disease demonstrates how specific impairments in these networks exacerbate the mismatch between perception and reality. Together, these insights highlight the critical role of predictive processing in both typical and atypical sensory experiences.
Hallucinations Fall Along a Spectrum: They Are Not Always Pathological
Hallucinations are relatively common in non-clinical populations, occurring under various circumstances across a wide range of individuals. Imaginative play and developmental reliance on top-down processes can lead to vivid sensory experiences in around 12% of children, such as hearing voices or seeing imaginary companions, which are typically benign and transient (Maijer et al., 2018). In religious and spiritual contexts, hallucinations are often interpreted as sacred or meaningful. In healthy adults, approximately 5–15% experience hallucinations, including hearing one’s name in an empty room or seeing fleeting shapes during stress, sleep deprivation, or sensory isolation (Maijer et al., 2018). In one study, 25% of surveyed Evangelical Christians reported hearing the voice of God or seeing visions, with similar patterns observed in other religions (Luhrmann, 2012). [We are not categorizing these profound spiritual experiences as imaginary.] Hypnagogic and hypnopompic hallucinations, which occur at the boundaries of sleep, are examples of normative sensory experiences nearly everyone encounters in their lifetime.
Distinguishing pathological hallucinations from normative ones depends on several factors. Pathological hallucinations typically cause significant distress, impair daily functioning, or occur with high frequency and intensity. For example, hallucinations associated with schizophrenia may be persistent, intrusive, and emotionally disturbing, often accompanied by delusional beliefs. By contrast, normative hallucinations are brief, context-dependent, and not distressing.
This distinction is crucial for clinicians, as it helps to differentiate adaptive or benign sensory experiences from those requiring therapeutic intervention. Hallucinations, alone, are not pathological.
Hallucinations Across Disorders
Hallucinations due to psychiatric illnesses often involve disruptions in the brain’s top-down processing systems, leading to persistent and intrusive sensory experiences. In schizophrenia, auditory hallucinations such as hearing voices are among the most common symptoms. These experiences are thought to arise from heightened activity in brain regions responsible for expectation and prediction, such as the prefrontal cortex (Powers et al., 2016). This overreliance on top-down processing can create a disconnection from sensory reality, where internally generated expectations are misinterpreted as external stimuli. Substance use and withdrawal can also provoke hallucinations by amplifying or distorting top-down effects. Hallucinogens like LSD and psilocybin, for instance, enhance predictive processing, resulting in vivid and often surreal sensory experiences (Carhart-Harris et al., 2012).
Neurological conditions, such as Parkinson’s disease and Charles Bonnet Syndrome, also produce hallucinations through disruptions in top-down and bottom-up interactions. In Parkinson’s, visual hallucinations often occur due to impaired dopamine pathways, which affect the brain’s ability to reconcile sensory input with internal expectations (Onofrj et al., 2006). Similarly, Charles Bonnet Syndrome, a condition affecting individuals with vision loss, demonstrates how the brain compensates for missing sensory input by generating visual hallucinations. These hallucinations reflect the brain’s attempt to fill sensory gaps using predictive mechanisms, highlighting the interplay of cognition and sensory processing in neurological disorders. Together, these cases illustrate how mismatched top-down and bottom-up processing contributes to hallucinations across a spectrum of conditions.
Implications for Treatment
Understanding hallucinations as top-down effects opens new avenues for treatment. Current therapies, such as antipsychotic medications, primarily target the bottom-up pathways by dampening sensory input. However, the top-down perspective suggests neurocognitive and behavioral methods may serve as complementary approaches, including:
Cognitive-Behavioral Therapy (CBT): Helping patients recognize and challenge distorted expectations and beliefs that fuel hallucinations. It is common among patients with schizoaffective disorder, for example, to have hallucinations that are congruent with their mood, such as hearing others’ thoughts confirming self-doubts or paranoid ideation. As such, CBT techniques help people recognize these hallucinations as external manifestations of their mood, which they can influence through cognitive (e.g., cognitive reappraisal) and behavioral methods (e.g., finding a calming space).
Mindfulness-Based Interventions: Teaching individuals to observe sensory experiences without immediate interpretation can help reduce the influence of top-down processes. For example, patients experiencing auditory hallucinations might be taught to recognize these experiences as transient and unrelated to external threats, reducing their emotional salience. Through mindfulness practices such as focused breathing or body scanning, individuals can cultivate a nonjudgmental awareness of their sensations, which helps them detach from the hallucination’s perceived meaning.
Neuromodulation: Techniques like transcranial magnetic stimulation (TMS) target overactive brain regions involved in prediction and expectation, offering a direct approach to modifying the neural activity underlying hallucinations. For instance, TMS applied to the temporoparietal junction has been shown to reduce auditory hallucinations in patients with schizophrenia by modulating the overactive networks responsible for generating these experiences. Similarly, repetitive TMS (rTMS) can disrupt maladaptive predictive signals, restoring a balance between top-down and bottom-up processes.
Key Takeaways
Top-Down Processing: Hallucinations occur when the brain’s internal expectations and prior experiences override sensory input, actively shaping perception without external stimuli.
Predictive Coding:Â Elevated activity in brain regions such as the prefrontal cortex supports the idea that hallucinations stem from disrupted predictive mechanisms, where internal predictions dominate over actual sensory data.
Spectrum of Experience: Hallucinatory phenomena span a continuum—from benign, normative experiences observed in non-clinical populations to distressing, pathological symptoms seen in various mental health disorders.
Disrupted Sensory Integration: Altered connectivity between regions responsible for top-down and bottom-up processing impairs the brain’s ability to reconcile incoming sensory information with expectations, leading to false perceptions.
Therapeutic Implications:Â Recognizing hallucinations as effects of maladaptive top-down processing opens new treatment avenues, including cognitive-behavioral therapy, mindfulness practices, and neuromodulation techniques.
Conclusion
Hallucinations, while often associated with pathology, are better understood as part of a broader spectrum of human cognition. By distinguishing between normative and pathological hallucinations, we can challenge stigmas and provide more effective care for those experiencing them. Normative hallucinations, such as those occurring during sleep transitions or in spiritual contexts, demonstrate the brain’s creative and predictive powers. Pathological hallucinations, on the other hand, cause significant distress or impair functioning, often arising from disruptions in the brain’s balance between top-down and bottom-up processes.
This distinction not only highlights the brain’s interpretive nature but also reframes hallucinations as an opportunity to study human perception and creativity. By understanding their underlying mechanisms, we can better support individuals across the spectrum of experience, fostering both clinical innovation and empathy.
Glossary
auditory-verbal hallucinations (AVH): auditory hallucinations of spoken words, often without a corresponding external stimulus.
Bayesian brain: a framework suggesting that the brain infers environmental states by combining sensory inputs with prior knowledge to minimize prediction errors.
bottom-up processing: sensory information flow from the external environment to the brain, starting with basic perception and progressing to higher-level cognitive interpretation.
cognitive-behavioral therapy (CBT): a structured, goal-oriented psychotherapy focusing on changing negative thought patterns and behaviors.
cognitive penetration: the influence of higher-level cognitive states, like beliefs and emotions, on lower-level perceptual processes.
Charles Bonnet Syndrome: a condition in which visually impaired individuals experience complex visual hallucinations without underlying psychosis.
default mode network (DMN): a network of interacting brain regions active during rest and involved in self-referential thought and prediction.
functional connectivity: statistical dependencies or correlations between brain regions measured via imaging techniques, indicating communication.
hallucination: perception without an external stimulus, often consistent with a person’s mood or expectations.
hypnagogic hallucinations: vivid sensory phenomena occurring at the onset of sleep.
hypnopompic hallucinations: vivid sensory phenomena occurring upon waking from sleep.
illusory perception: incorrect perception of real sensory input, often involving misinterpretation or distortion.
limbic system: a set of brain structures, including the amygdala and hippocampus, involved in emotion, behavior, and memory.
mindfulness: a mental practice emphasizing present-moment awareness and acceptance without judgment.
modularity: a theoretical concept that the mind is composed of independent modules responsible for specific functions.
neuromodulation: the regulation of neural activity through targeted electrical, magnetic, or chemical stimulation.
normative hallucinations: hallucinations experienced by the general population under certain non-clinical conditions, such as grief or sensory deprivation.
Parkinson's disease: a neurodegenerative disorder characterized by motor symptoms (e.g., tremors, rigidity) and often non-motor symptoms such as hallucinations or cognitive impairment.
pathological hallucinations: hallucinations arising from underlying medical or psychiatric conditions, impairing functioning.
perception: the process by which sensory information is interpreted and organized by the brain to form a coherent representation of the external environment. This involves integrating sensory input from various modalities (e.g., visual, auditory, tactile) with prior knowledge, memories, and expectations to construct a meaningful understanding of the world.
predictive coding: a theory that the brain continuously generates predictions to explain incoming sensory inputs and updates them to minimize errors.
repetitive transcranial magnetic stimulation (rTMS): a non-invasive neuromodulation technique using repeated magnetic pulses to stimulate specific brain regions.
rich-club hubs: highly interconnected nodes in the brain that facilitate global information integration across networks.
salience network: a brain network that detects and filters relevant stimuli and contributes to attention and decision-making.
sensory conditioning: the process by which repeated associations between stimuli modify sensory perception.
top-down processing: information flow from higher cognitive processes, such as expectations and prior knowledge, to influence lower sensory processing.
transcranial magnetic stimulation (TMS): a non-invasive technique using magnetic fields to stimulate specific brain areas.
visual cortex: the part of the brain responsible for processing visual information, including recognition and interpretation.
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