Mid-level visual processing represents a crucial stage between basic sensory input and higher-level object recognition. The conventional model posits that fundamental visual qualities like color and motion are processed in specialized, retinotopic brain regions (e.g., V4 for color, MT/V5 for motion). Using atlas-based lesion-symptom mapping and disconnectome maps in a cohort of 307 ischemic stroke patients, we examined the neuroanatomical correlates underlying the processing of eight mid-level visual qualities. Contrary to the standard model's predictions, our results did not reveal consistent relationships between processing impairments and damage to traditionally associated brain regions. While we validated our methodology by confirming the established relationship between visual field defects and damage to primary visual areas (V1, V2, and V3), we found no reliable evidence linking processing deficits to specific regions in the posterior brain. These findings challenge the traditional modular view of visual processing and suggest that mid-level visual processing may be more distributed across neural networks than previously thought. This supports alternative models where visual maps represent constellations of co-occurring information rather than specific qualities.
Acoustic-phonetic perception refers to the ability to perceive and discriminate between speech sounds. Acquired impairment of acoustic-phonetic perception is known historically as "pure word deafness" and typically follows bilateral lesions of the cortical auditory system. The extent to which this deficit occurs after unilateral left hemisphere damage and the critical left hemisphere areas involved are not well defined. We tested acoustic-phonetic perception in 73 individuals with chronic left hemisphere stroke and performed multivariate lesion-symptom mapping incorporating controls for non-specific task confounds, pure tone hearing loss, response bias, and lesion size. Separate analyses examined place of articulation, manner of articulation, voicing, and vowel discriminations. Overlap of the lesion map with transcallosal pathways linking left and right temporal lobes was examined using a probabilistic diffusion tensor tractography map of these pathways obtained from a healthy control cohort. Compared to an age- and education-matched control sample, 18% of the patients had impaired acoustic-phonetic perception overall, with 44% impaired on voicing, 26% on manner, 15% on place, and 14% on vowel discrimination. Lesion-symptom mapping revealed the most critical areas to be the transverse temporal gyrus (TTG) and adjacent medial belt cortex, the acoustic radiation, and the posterior superior temporal sulcus (pSTS). There were notable differences between lesion correlates for the different types of discrimination, with place discrimination linked to medial TTG, vowel discrimination to lateral TTG and planum temporale, manner discrimination to posterior planum temporale, and voicing discrimination to pSTS. Overlap of the main lesion map with transcallosal temporal lobe pathways was minor but included a deep white matter component at the base of the middle and inferior temporal gyri. The extent of overlap between individual lesions and the transcallosal pathway map was not correlated with acoustic-phonetic perception. The results add further evidence that acoustic-phonetic impairments, particularly impairments of voicing perception, are relatively common after unilateral left temporal lobe damage, and they clarify the lesion correlates of these deficits. Differences between the lesion maps for the discrimination types likely reflect differential reliance on spectral versus temporal analysis for these discriminations.
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