Abstract
Transcranial Direct Current Stimulation (tDCS) has recently been employed in traditional psychophysical paradigms in an effort to measure direct manipulations on spatial frequency channel operations in the early visual system. However, the effects of tDCS on contrast sensitivity have only been measured at a single spatial frequency and orientation. Since contrast sensitivity is known to depend on spatial frequency and orientation, we ask how the effects of anodal and cathodal tDCS may vary according to these dimensions. We measured contrast sensitivity with sinusoidal gratings at four different spatial frequencies (0.5, 4, 8, and 12 cycles/°), two orientations (45° Oblique and Horizontal), and for two stimulus size conditions [fixed size (3°) and fixed period (1.5 cycles)]. Only contrast sensitivity measured with a 45° oblique grating with a spatial frequency of 8 cycles/° (period = 1.5 cycles) demonstrated clear polarity specific effects of tDCS, whereby cathodal tDCS increased and anodal tDCS decreased contrast sensitivity. Overall, effects of tDCS were largest for oblique stimuli presented at high spatial frequencies (i.e., 8 and 12 cycles/°), and were small or absent at lower spatial frequencies, other orientations and stimulus size. Thus, the impact of tDCS on contrast sensitivity, and therefore on spatial frequency channel operations, is opposite in direction to other behavioral effects of tDCS, and only measurable in stimuli that generally elicit lower contrast sensitivity (e.g., oblique gratings with period of 1.5 cycles at spatial frequencies above the peak of the contrast sensitivity function).
Highlights
Neuro-stimulation techniques have recently been combined with traditional psychophysical paradigms in an effort to obtain a measure of direct manipulation on spatial frequency channel operations in the early visual system
The 20 observers (10 per orientation group) that continued onto the Transcranial Direct Current Stimulation (tDCS) portion of this study showed no statistically significant increment or decrement in contrast sensitivity across the final eight stimulus blocks completed during baseline
A-tDCS induced decreases in contrast sensitivity remained stable across spatial frequency, but at the case-level, we found that observers were progressively more likely to have contrast sensitivity values 1 SD below the grand mean than pre-stimulation contrast sensitivity values as spatial frequency increased
Summary
Neuro-stimulation techniques have recently been combined with traditional psychophysical paradigms in an effort to obtain a measure of direct manipulation on spatial frequency channel operations in the early visual system (review: Antal et al, 2006). The polarity specific facilitation and inhibitory effects of tDCS may be opposite to those reported in motor cortex (Antal et al, 2001; Accornero et al, 2007; Lang et al, 2007; Chaieb et al, 2008; Spiegel et al, 2012; Peters et al, 2013; Pirulli et al, 2014). Part of the variability in tDCS effects for different cortical loci can be attributed to structural (e.g., cell type and morphology and the direction of current flow in relation to the somatodendritic axis), or functional differences between stimulated areas (Rushton, 1927; Ward and Weiskrantz, 1969; Shipp, 2005; Radman et al, 2009; Reato et al, 2010; Bikson et al, 2013). Given that the visual cortex is both structurally and functionally different from motor cortex, it should come as no surprise that the effects of tDCS over the visual cortex are less clear
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