Abstract
Recent evidence indicates that inhibition within the visual cortex is greater in older than young adults. Increased inhibition has been associated with reduced visual cortex plasticity in animal models. We investigated whether age-related increases in human visual cortex inhibition occur in conjunction with reduced visual cortex plasticity. Visual cortex inhibition was measured psychophysically using binocular rivalry alternation rates (AR) for dichoptic gratings. Slower ARs are associated with a greater concentration of the inhibitory neurotransmitter GABA within the human visual cortex. Visual cortex plasticity was measured using an established paradigm for induction of long-term potentiation (LTP) -like increases in visually evoked potential (VEP) amplitude. Following rapid visual stimulation, greater increases in VEP amplitude indicate greater visual cortex plasticity. The study involved two groups; young (18–40 years, n = 29) and older adults (60–80 years, n = 18). VEPs were recorded for a 1 Hz onset/offset checkerboard stimulus before and after 9 Hz visual stimulation with the same stimulus. ARs were slower in older than young adults. In contrast to most previous studies, VEP amplitudes were significantly reduced following the rapid visual stimulation for young adults; older adult VEP amplitudes were unaffected. Our AR results replicate previous observations of increased visual cortex inhibition in the older adults. Rapid visual stimulation significantly altered VEP amplitude in young adults, albeit in the opposite direction than predicted. VEP amplitudes did not change in older adults suggesting an association between increased inhibition and reduced plasticity within the human visual cortex.
Highlights
Visual cortex plasticity is high during an early “critical period” of development and gradually declines as the brain matures
Magnetic resonance spectroscopy (MRS) studies have shown that the neuromodulation technique anodal transcranial direct current stimulation (a-tDCS) reduces gamma aminobutyric acid (GABA) concentration in the adult motor cortex and that larger GABA reductions are associated with increased motor learning (Stagg et al, 2011; Patel et al, 2017)
Post hoc t-tests conducted on the late period data for each group separately indicated a significant reduction in visually evoked potential (VEP) amplitude from pre to post tetanization for the young adult group (t28 = 5.3, p < 0.001) but not for the older adult group (t17 = 0.5, p = 0.6)
Summary
Visual cortex plasticity is high during an early “critical period” of development and gradually declines as the brain matures. Critical periods for visual cortex characteristics such as ocular dominance can be reopened in mature, postcritical period animals through pharmacological, or environmental manipulations that reduce GABA concentration (Sale et al, 2007; Vetencourt et al, 2008; Harauzov et al, 2010). The current evidence from animal models strongly suggests that GABA concentration plays a key role in modulating visual cortex plasticity. Less is known about the time course of visual cortex plasticity in humans; there is evidence that plasticity is high in early childhood and declines with increasing age (Sale et al, 2010; Siu et al, 2017) as is the case in animal models. A-tDCS can temporarily improve vision in adults with amblyopia, perhaps by modulating GABA concentration (Spiegel et al, 2013a,b; Ding et al, 2016; Castaño-Castaño et al, 2017)
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