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Martin Kanowski

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9 papers
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9

YNICL Journal 2022 Journal Article

Structural changes to primary visual cortex in the congenital absence of cone input in achromatopsia

  • Barbara Molz
  • Anne Herbik
  • Heidi A. Baseler
  • Pieter B. de Best
  • Richard W. Vernon
  • Noa Raz
  • Andre D. Gouws
  • Khazar Ahmadi

Autosomal recessive Achromatopsia (ACHM) is a rare inherited disorder associated with dysfunctional cone photoreceptors resulting in a congenital absence of cone input to visual cortex. This might lead to distinct changes in cortical architecture with a negative impact on the success of gene augmentation therapies. To investigate the status of the visual cortex in these patients, we performed a multi-centre study focusing on the cortical structure of regions that normally receive predominantly cone input. Using high-resolution T1-weighted MRI scans and surface-based morphometry, we compared cortical thickness, surface area and grey matter volume in foveal, parafoveal and paracentral representations of primary visual cortex in 15 individuals with ACHM and 42 normally sighted, healthy controls (HC). In ACHM, surface area was reduced in all tested representations, while thickening of the cortex was found highly localized to the most central representation. These results were comparable to more widespread changes in brain structure reported in congenitally blind individuals, suggesting similar developmental processes, i.e., irrespective of the underlying cause and extent of vision loss. The cortical differences we report here could limit the success of treatment of ACHM in adulthood. Interventions earlier in life when cortical structure is not different from normal would likely offer better visual outcomes for those with ACHM.

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YNIMG Journal 2020 Journal Article

Foveal pRF properties in the visual cortex depend on the extent of stimulated visual field

  • Gokulraj T. Prabhakaran
  • Joana Carvalho
  • Azzurra Invernizzi
  • Martin Kanowski
  • Remco J. Renken
  • Frans W. Cornelissen
  • Michael B. Hoffmann

Previous studies demonstrated that alterations in functional MRI derived receptive field (pRF) properties in cortical projection zones of retinal lesions can erroneously be mistaken for cortical large-scale reorganization in response to visual system pathologies. We tested, whether such confounds are also evident in the normal cortical projection zone of the fovea for simulated peripheral visual field defects. We applied fMRI-based visual field mapping of the central visual field at 3 T in eight controls to compare the pRF properties of the central visual field of a reference condition (stimulus radius: 14°) and two conditions with simulated peripheral visual field defect, i. e. , with a peripheral gray mask, stimulating only the central 7° or 4° radius. We quantified, for the cortical representation of the actually stimulated visual field, the changes in the position and size of the pRFs associated with reduced peripheral stimulation using conventional and advanced pRF modeling. We found foveal pRF-positions (≤3°) to be significantly shifted towards the periphery (p<0. 05, corrected). These pRF-shifts were largest for the 4° condition [visual area (mean eccentricity shift): V1 (0. 9°), V2 (0. 9°), V3 (1. 0°)], but also evident for the 7° condition [V1 (0. 5°), V2 (0. 5°), V3 (0. 9°)]. Further, an overall enlargement of pRF-sizes was observed. These findings indicate the dependence of foveal pRF parameters on the spatial extent of the stimulated visual field and are likely associated with methodological biases and/or physiological mechanisms. Consequently, our results imply that, previously reported similar findings in patients with actual peripheral scotomas need to be interpreted with caution and indicate the need for adequate control conditions in investigations of visual cortex reorganization.

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YNIMG Journal 2019 Journal Article

Population receptive field and connectivity properties of the early visual cortex in human albinism

  • Khazar Ahmadi
  • Anne Herbik
  • Markus Wagner
  • Martin Kanowski
  • Hagen Thieme
  • Michael B. Hoffmann

In albinism, the pathological decussation of the temporal retinal afferents at the optic chiasm leads to superimposed representations of opposing hemifields in the visual cortex. Here, we assessed the equivalence of the two representations and the cortico-cortical connectivity of the early visual areas. Applying fMRI-based population receptive field (pRF)-mapping (both hemifield and bilateral mapping) and connective field (CF)-modeling, we investigated the early visual cortex in 6 albinotic participants and 4 controls. In albinism, superimposed retinotopic representations of the contra- and ipsilateral visual hemifield were observed on the hemisphere contralateral to the stimulated eye. This was confirmed by the observation of bilateral pRFs during bilateral mapping. Hemifield mapping revealed similar pRF-sizes for both hemifield representations throughout V1 to V3. The typical increase of V1-sampling extent for V3 compared to V2 was not found for the albinotic participants. The similarity of the pRF-sizes for opposing visual hemifield representations highlights the equivalence of the two maps in the early visual cortex. The altered V1-sampling extent in V3 might indicate the adaptation of cortico-cortical connections to visual pathway abnormalities in albinism. These findings thus suggest that conservative developmental mechanisms are complemented by alterations of the extrastriate cortico-cortical connectivity.

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YNIMG Journal 2017 Journal Article

In vivo MRI assessment of the human locus coeruleus along its rostrocaudal extent in young and older adults

  • Matthew J. Betts
  • Arturo Cardenas-Blanco
  • Martin Kanowski
  • Frank Jessen
  • Emrah Düzel

The locus coeruleus (LC), a major origin of noradrenergic projections in the central nervous system (CNS), may serve a critical role in the pathogenesis of neurodegenerative disorders such as Alzheimer's disease (AD) and Parkinson's disease (PD). As such, there is considerable interest to develop magnetic resonance imaging (MRI) techniques to assess the integrity of the LC in vivo. The high neuromelanin content of the LC serves as an endogenous contrast for MRI but existing protocols suffer from low spatial resolution along the rostrocaudal axis of the LC rendering it difficult to differentiate its integrity in caudal and rostral portions. This study presents a novel approach to investigate the human LC in vivo using T1-weighted Fast Low Angle Shot (FLASH) MRI at 3 T (T). Using high-resolution isotropic imaging to minimise the effect of low spatial resolution in the slice direction, this study aimed to characterise the rostrocaudal distribution of LC signal intensity attributed to neuromelanin from 25 young (22–30) and 57 older (61–80) adults. We found a significant age-related increase in maximum but not median signal intensity, indicating age-related differences were not homogenous. Instead, they were confined to the rostral third of the LC with relative sparing of the caudal portion. The findings presented demonstrate in vivo T1-weighted FLASH imaging may be used to characterise signal intensity changes across the entire rostrocaudal length of the LC (a corresponding standardised LC map is available for download), which may help to identify how the human LC is differentially affected in aging and neurodegenerative disease.

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YNIMG Journal 2008 Journal Article

Contemporary ultrasound systems allow high-resolution transcranial imaging of small echogenic deep intracranial structures similarly as MRI: A phantom study

  • Uwe Walter
  • Martin Kanowski
  • Jörn Kaufmann
  • Annette Grossmann
  • Reiner Benecke
  • Ludwig Niehaus

Transcranial sonography (TCS) of small deep brain structures, such as substantia nigra and brainstem raphe, is increasingly used for assessment of neurodegenerative disorders. Still, there are reservations against TCS because of the smallness of evaluated structures and constraints on image resolution that is discussed to be lower compared to magnetic resonance imaging (MRI). To evaluate two different-generation TCS systems in visualizing fine intracranial structures, we studied image resolution on a phantom consisting of 0. 80 mm×1. 05 mm regular meshwork of nylon threads embedded in a wet, gel-filled ex vivo human skull. Imaging was performed with a former-generation and a present-day clinical ultrasound system and for comparison with MRI. In axial direction of insonation both TCS systems resolved 0. 80-mm and 1. 05-mm thread-to-thread distance at depths between 55 and 120 mm using transmission frequencies ≥2. 5 MHz. The meshwork, however, was recognizable as such only with the contemporary TCS system at depths between 60 and 85 mm due to its higher lateral resolution. MRI resolved the meshwork if image resolution was chosen sufficiently high but not if realistic clinical conditions were applied with its trade-offs between image SNR, resolution, total scan time, and unavoidable head motion during the latter. Hence, if the requirements for optimal TCS image resolution are fulfilled, i. e. sufficient acoustic bone window, increased echogenicity of target structure and its localization in a distance of maximum ±15 mm from midsagittal plane, findings suggest that contemporary TCS systems achieve higher image resolution of intracranial structures in comparison not only to former-generation systems, but also to MRI under clinical conditions.

Details DOI
v2026.09.13