fMRI of the rod scotoma elucidates cortical rod pathways and implications for lesion measurements

Abstract

Are silencing, ectopic shifts, and receptive field (RF) scaling in cortical scotoma projection zones (SPZs) the result of long-term reorganization (plasticity) or short-term adaptation? Electrophysiological studies of SPZs after retinal lesions in animal models remain controversial, because they are unable to conclusively answer this question because of limitations of the methodology. Here, we used functional MRI (fMRI) visual field mapping through population RF (pRF) modeling with moving bar stimuli under photopic and scotopic conditions to measure the effects of the rod scotoma in human early visual cortex. As a naturally occurring central scotoma, it has a large cortical representation, is free of traumatic lesion complications, is completely reversible, and has not reorganized under normal conditions (but can as seen in rod monochromats). We found that the pRFs overlapping the SPZ in V1, V2, V3, hV4, and VO-1 generally (i) reduced their blood oxygen level-dependent signal coherence and (ii) shifted their pRFs more eccentric but (iii) scaled their pRF sizes in variable ways. Thus, silencing, ectopic shifts, and pRF scaling in SPZs are not unique identifiers of cortical reorganization; rather, they can be the expected result of short-term adaptation. However, are there differences between rod and cone signals in V1, V2, V3, hV4, and VO-1? We did not find differences for all five maps in more peripheral eccentricities outside of rod scotoma influence in coherence, eccentricity representation, or pRF size. Thus, rod and cone signals seem to be processed similarly in cortex. can adult human visual cortex reorganize after the removal of visual input?" This question can be studied through the effects of retinal lesions (causing scotomas), in which input from the retina has been removed, but cortical representations of the scotoma projection zone (SPZ) remain intact. Accordingly, emphasis must be placed on teasing apart effects of scotomas that relate to short-term cortical adaptation from those of long-term cortical plasticity (1) (terminology review is in ref.2). Here, we investigate this question in human cortex by using functional MRI (fMRI) to measure the immediate cortical SPZ responses in the unique paradigm of the naturally occurring rod scotoma.The photoreceptors in humans can be divided into two classes: cones, which are primarily responsible for vision under high-luminance (photopic) conditions, and rods, which are primarily responsible for vision under low-luminance (scotopic) conditions when the cones are inactive. The cones are an order of magnitude more highly concentrated in the fovea relative to the periphery, where they inform our most detailed visual experience (3). In contrast, the greatest concentrations of rods are more than ∼10°e ccentric from fixation and become increasingly sparse toward fixation until they are completely absent. This roughly circular rodfree zone covers a radius of ∼0.6-0.8°of visual angle about the fixation point (diameter = ∼1.25-1.7°) (4, 5). Under scotopic conditions, a...

Primary visual cortical remapping in patients with inherited peripheral retinal degeneration

“…Some authors have suggested the use of an artificial scotoma to mimic the patients' visual field extent, to help elucidate the origin of remapping (Goesaert et al, 2014, Masuda et al, 2010). Previous works applying central artificial scotomata in healthy participants (Barton and Brewer, 2015, Baseler et al, 2011b, Haak et al, 2012) reported a visual cortical remapping similar to the effects usually seen in patients. Given that we did not use this approach, two possible explanations remain possible to explain functional remapping in our group of patients: fast adaptation or long term reorganization mechanisms.…”

Predictive masking is associated with a system-wide reconfiguration of neural populations in the human visual cortex

“…In response to the AS, pRFs shift their preferred spatial position towards the AS border. While such shifts have been reported previously for pRFs initially located inside the natural SPZ (31) and ASPZ (10–12, 32), our study is the first to show that this reconfiguration is not restricted to the ASPZ, but is a system-wide phenomenon. Within the ASPZ, the pRFs shifted their preferred position towards the AS border, which could be consistent with an extrapolation process.…”

Potenzial von fMRT für die Funktionsüberprüfung des pathologischen Sehsystems

“…Der Vergleich der Patientendaten mit denen von Kontrollen mit simuliertem Skotom zeigt, dass die beobachtete Aktivität nicht durch Plastizität, sondern mit dem Auftreten von kortikalen Neuronen mit ektopen rezeptiven Feldern auch im Normalfall mit simuliertem Zentralskotom assoziiert ist. Dies zeigt, wie wichtig es ist, Patientendaten mit adäquaten Kontrollen zu vergleichen, wie auch eine aktuelle Studie zum skotopischen Zentralskotom unterstreicht [26].…”