Modulation of Visual Cortical Excitability by Working Memory: Effect of Luminance Contrast of Mental Imagery

Alberto Pisoni,Costanza Papagno,Juha Silvanto,Zaira Cattaneo

doi:10.3389/fpsyg.2011.00029

Alberto Pisoni, Costanza Papagno + Show 2 more

Open Access

https://doi.org/10.3389/fpsyg.2011.00029

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Abstract

Although much is known about the impact of stimulus properties such as luminance contrast, spatial frequency, and orientation on visually evoked neural activity, much less is known about how they modulate neural activity when they are properties of a mental image held in working memory (WM). Here we addressed this question by investigating how a parametric manipulation of an imagined stimulus attribute affects neuronal excitability in the early visual cortex. We manipulated luminance contrast, a stimulus property known to strongly affect the magnitude of neuronal responses in early visual areas. Luminance contrast modulated neuronal excitability, as assessed by the frequency of phosphenes induced by transcranial magnetic stimulation (TMS) with the exact nature of this modulation depending on TMS intensity. These results point to a strong overlap in the neuronal processes underlying visual perception and mental imagery: not only does WM maintenance selectively engage neurons which are tuned to the maintained attribute (as has previously been shown), but the extent to which those neurons are activated depends on the image contrast (as is the case with visually evoked responses). From a methodological viewpoint, these results suggest that assessment of visual cortical excitability using TMS is affected by the TMS intensity used to probe the neuronal population.

Highlights

The formation of representations in visuo-spatial working memory (VSWM) in the absence of bottom-up sensory input is referred to as visual mental imagery
These results point to a strong overlap in the neuronal processes underlying visual perception and mental imagery: does WM maintenance selectively engage neurons which are tuned to the maintained attribute, but the extent to which those neurons are activated depends on the image contrast
These results suggest that assessment of visual cortical excitability using transcranial magnetic stimulation (TMS) is affected by the TMS intensity used to probe the neuronal population

Summary

Introduction

The formation of representations in visuo-spatial working memory (VSWM) in the absence of bottom-up sensory input is referred to as visual mental imagery. Much is known about the impact of various stimulus properties (such as luminance contrast and spatial frequency) on visually evoked neural activity, much less is known about how they modulate neural activity when they are properties of a mental image This is an important question because previously reported facilitatory effects of mental imagery on phosphene perception could be due to factors such as attention (which has been shown to facilitate phosphene perception; Bestmann et al, 2007). In this view, the increased visual cortical excitability might reflect imageryinduced attentional facilitation of the maintained attributes rather than the actual content of the mental image. In human early visual areas the BOLD signal increases with stimulus contrast, and contrast discrimination thresholds correlate with the strength of V1 activation (e.g., Boynton et al, 1999; Buracas and Boynton, 2007)

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