Lewy body disease encompasses Parkinson’s disease and dementia with Lewy bodies, together representing the second most common form of degenerative dementia after Alzheimer’s disease. More than 80 percent of people with Parkinson’s eventually develop dementia, though it may take more than a decade to emerge. When dementia does arrive, brain imaging shows a telltale signature: reduced metabolism and depleted cholinergic signaling in the posterior cortical regions that handle visual information. Yet despite this clear anatomical link, visuoperceptual impairment has remained under-researched compared to attention and executive function, largely because the available tests were poorly suited to detect it.

The problem with existing tools is one of dynamic range. Bedside screening tasks such as clock drawing or copying intersecting pentagons suffer from both ceiling and floor effects, and they demand a motor response that can be muddied by the movement disorder itself. Purified perceptual batteries like the Benton Judgment of Line Orientation and the Visual Object and Space Perception battery remove the motor confound but are so easy for many patients that early deficits slip through undetected. The research team, led by Emily McCann and Peter J. Nestor of the University of Queensland, set out to build tests with none of these weaknesses.

The two new instruments target the brain’s dual visual streams, a division first described by Ungerleider and Mishkin in 1982. The ventral stream, running from the occipital cortex down into the temporal lobe, identifies what an object is — its form, color, and identity. The dorsal stream, projecting up into the parietal lobe, processes where things are and how multiple visual elements fit together spatially. To probe the ventral stream, the researchers created the Graded Blurry Image task, a 20-item object identification test in which pictures are blurred by progressively larger Gaussian filters, from coarse to fine, until the patient can name them. Items that proved too easy, too hard, or too variable during piloting were discarded, and item-specific difficulty scaling ensured that no patient could reach ceiling and no one was stuck at floor.

For the dorsal stream, the team designed a Simultanagnosia Test built around the phenomenon in which a person cannot perceive multiple visual elements at once. In each of 40 items, a digit from zero to nine is hidden inside random black noise — the digit is defined not by a positive image but by the absence of background speckle, so the brain must integrate the missing information to complete the shape. Healthy pilot performance on the final items ranged from perfect down to near chance, and participants had eight seconds per item. The engineers of the test took care to keep patients engaged by inserting an easy item every eight trials.

Crucially, the researchers validated the tasks against brain anatomy. Because dementia in Lewy body disease typically causes little cortical shrinkage, they regressed test scores against MRI-derived cortical thickness in cerebrospinal fluid biomarker-confirmed Alzheimer’s patients instead. The results were striking: performance on the Graded Blurry Image task peaked in correlation with thinning of the right ventral occipitotemporal cortex, exactly where the ventral stream resides, with a Spearman correlation of 0.83. The Simultanagnosia Test correlated most strongly with the right posterior parieto-occipital junction, the dorsal hub, at 0.82. The tasks were measuring precisely the circuits they were designed to probe.

Armed with validated instruments, the team tested 53 people with Lewy body disease — from cognitively asymptomatic Parkinson’s patients through to dementia — alongside 29 patients with biomarker-confirmed Alzheimer’s disease. The findings were unambiguous. Visual impairments were more prominent in Lewy body disease than in Alzheimer’s, and strikingly, they struck both visual streams equally. In Alzheimer’s, by contrast, ventral stream function was relatively preserved in early disease, with dorsal deficits appearing first. The Graded Blurry Image task flagged impairment in Lewy body patients once their global cognitive scores dipped below 86.5 on the Addenbrooke’s Cognitive Examination, whereas Alzheimer’s patients did not cross the impairment threshold until a far more advanced score of 63.4. Most remarkably, 44 percent of cognitively asymptomatic Parkinson’s patients scored in the impaired range on the Simultanagnosia Test, and 30 percent on the blurry image task — with the impaired individuals tending to be older.

The connection to visual hallucinations, a core clinical feature of Lewy body disease, proved especially powerful. Patients with a history of well-formed, complex hallucinations performed dramatically worse on both tasks: 88 percent fell in the impaired range on the ventral stream test and 95 percent on the dorsal test. Normal performance on either task carried a negative predictive value for hallucination history of 91 to 95 percent. The authors argue this reframes the relationship: hallucinations are not merely a predictor of coming dementia but a manifestation of it, driven at least in part by failing visual cortex. The dissociation between the diseases adds a tantalizing clue — Lewy body hallucinations typically feature people, animals, and objects, the specialty of the ventral stream, while hallucinations are rare or late in Alzheimer’s, where ventral function holds up longer.

Perhaps the study’s most provocative implication concerns how clinicians interpret routine cognitive testing. Standard measures of attention and executive function, such as the Trail Making Test and the Digit Symbol Substitution Test, are presented visually — and in the Lewy body patients, performance on them was overwhelmingly explained by visuoperceptual impairment rather than by non-visual executive measures like Digit Span and phonemic fluency. The Simultanagnosia Test alone accounted for 63 to 64 percent of the variance on these classic tasks. Effect sizes for the novel visual tests in demented patients dwarfed those of the non-visual measures, reaching a Cohen’s d of 1.88. The authors warn that deficits long attributed to frontostriatal dopamine circuits may in part be misread symptoms of a failing visual system — a misattribution that could be particularly misleading in Parkinson’s patients who appear cognitively intact on standard measures.

The study has limitations: it was cross-sectional, so the precise trajectory of visual decline over time remains to be charted, and the Lewy body diagnoses rested on clinical criteria rather than pathological confirmation, albeit with careful longitudinal follow-up. Retinal disease, which is common in these patients, could theoretically contribute, though the tasks were designed with large, high-contrast stimuli to minimize that confound. Still, the message is compelling and potentially transformative: dual-stream visual impairment is an early hallmark of Lewy body disease, detectable before dementia announces itself, and measuring it precisely may offer clinicians both an earlier diagnosis and a cleaner yardstick for future therapies. The mind’s eye, it turns out, may be the first window into the disease.

Subject of Research: Visuoperceptual processing deficits in dorsal and ventral visual streams in Lewy body disease

Article Title: Dorsal and ventral stream visuoperceptual processing deficits in Lewy body disease

Article References: McCann, E., Coleman, F., Lee, S., Fazlollahi, A., O’Sullivan, J. D., & Nestor, P. J. (2026). Dorsal and ventral stream visuoperceptual processing deficits in Lewy body disease. Journal of Neurology, 273(10), Article 620. https://doi.org/10.1007/s00415-026-14166-5

Image Credits: AI Generated

DOI: 10.1007/s00415-026-14166-5

Keywords: Lewy body disease, Parkinson's disease, dementia with Lewy bodies, visual perception, dorsal stream, ventral stream, neuropsychology, visual hallucinations, Alzheimer's disease, cognitive testing, posterior cortex, MRI