Low frequency transcranial magnetic stimulation of right posterior parietal cortex reduces reaction time to perithreshold low spatial frequency visual stimuli

Elkin-Frankston, Seth; Rushmore, Richard J.; Valero-Cabré, Antoni · 2020 · Crossref

DOI: 10.1038/s41598-020-59662-4

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Summary

This study investigates the role of the right posterior parietal cortex (PPC) in modulating early visual processing through top-down interactions. While the PPC is known to integrate perceptual information and guide attention, its specific influence on early visual cortical areas remains unclear. The researchers aimed to determine whether inhibiting the right PPC affects contrast sensitivity and detection response times for visual stimuli. They hypothesized that suppressing the PPC would degrade contrast sensitivity, particularly for low spatial frequency stimuli, which are processed by the magnocellular/dorsal stream that projects to the PPC. The experiment involved 36 healthy participants divided into independent groups receiving either low-frequency (1 Hz) repetitive transcranial magnetic stimulation (rTMS) to the right PPC or a control stimulation to the scalp vertex. Participants performed a two-alternative forced-choice visual detection task using centrally presented Gabor patches at low (3 cycles per degree) and high (12 cycles per degree) spatial frequencies. Performance was measured at three time points: baseline (pre-rTMS), immediately after 15 minutes of rTMS (post-rTMS), and one hour post-rTMS. Stimulation sites were verified using individual structural MRI scans with vitamin E markers. Contrast sensitivity thresholds were determined using an adaptive staircase method, and reaction times were recorded for correct responses. The results showed that low-frequency rTMS to the right PPC did not significantly alter contrast sensitivity for either low or high spatial frequency stimuli compared to baseline or vertex stimulation. However, the stimulation significantly reduced reaction times for low spatial frequency (3 cpd) stimuli immediately after rTMS, an effect that returned to baseline one hour later. No significant changes in reaction time were observed for high spatial frequency (12 cpd) stimuli or following vertex stimulation. Statistical analysis confirmed a significant interaction between stimulation site and time point for the low spatial frequency condition, indicating that the speed-up in response was specific to right PPC inhibition. The authors conclude that suppressing the right PPC facilitates processing speed for low spatial frequency visual stimuli without affecting contrast sensitivity. They propose that this paradoxical facilitation results from disinhibition of the homotopic left PPC via transcallosal projections. Since the right PPC is involved in allocating attention to high-salience peripheral stimuli, its suppression may shift processing resources toward the left PPC, which is more responsive to low-salience central stimuli. These findings suggest that rTMS of the right PPC could potentially bias visual processing toward central fields, offering implications for treating visual dysfunctions in conditions like schizophrenia or autism, where magnocellular stream deficits or peripheral processing excesses are observed.

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• contrast sensitivity