The effect of TMS on visual motion sensitivity: an increase in neural noise or a decrease in signal strength?

Abstract

Ruzzoli M, Abrahamyan A, Clifford CW, Marzi CA, Miniussi C, Harris JA. The effect of TMS on visual motion sensitivity: an increase in neural noise or a decrease in signal strength? J Neurophysiol 106: 138-143, 2011. First published May 4, 2011 doi:10.1152/jn.00746.2010The underlying mechanisms of action of transcranial magnetic stimulation (TMS) are still a matter of debate. TMS may impair a subject's performance by increasing neural noise, suppressing the neural signal, or both. Here, we delivered a single pulse of TMS (spTMS) to V5/MT during a motion direction discrimination task while concurrently manipulating the level of noise in the motion stimulus. Our results indicate that spTMS essentially acts by suppressing the strength of the relevant visual signal. We suggest that TMS may induce a pattern of neural activity that complements the ongoing activation elicited by the sensory signal in a manner that partially impoverishes that signal. V5/MT; single pulse of transcranial magnetic stimulation IN BEHAVIORAL STUDIES THAT employ transcranial magnetic stimulation (TMS), the most often analyzed dependent variables are accuracy and reaction time (for a review, see Sandrini et al. 2011). Both of these variables are affected by the signal-tonoise ratio: higher ratios are associated with greater accuracy and shorter reaction times (cf. the linking hypothesis: Teller 1984). TMS has an effect on this ratio (Harris et al. 2008;Rauschecker et al. 2004;Sack et al. 2006;Silvanto et al. 2008) by reducing the strength of the signal (Harris et al. 2008), increasing the neural noise (Ruzzoli et al. 2010), or both.The "virtual lesion" hypothesis, by analogy with animal and human neuropsychological studies, states that TMS-induced effects are interpretable as a temporary and reversible halt of the functionality of the stimulated area as a consequence of the discharge of magnetic pulses (Pascual-Leone et al. 1999;Walsh and Cowey 1998). Although the virtual lesion is the most common theoretical framework used to interpret the behavioral effects of TMS, it does not provide a clear explanation of the underlying neural mechanisms . Recent human (Harris et al. 2008;Ruzzoli et al. 2010;Silvanto et al. 2008) Harris and colleagues (2008) used an added noise paradigm (Pelli and Farell 1999) to compare the functional effects of TMS on signal strength vs. noise. They applied a single pulse of TMS (spTMS) to the occipital pole while subjects discriminated the orientation of sinusoidal luminance gratings to which were added varying levels of spatial white noise. The orientation detection thresholds were established using an adaptive staircase procedure. The primary aim of that experiment was to ascertain the impact of TMS on the linear function between the square of the detection thresholds (contrast energy) and the square of the amount of noise added to the stimulus [noise variance (2 )]. Harris et al. (2008) found that the slope of the psychometric function increased more than double when spTMS was applied to visual cortex, compared with ...

Transcranial magnetic stimulation modulates the brain's intrinsic activity in a frequency-dependent manner

“…Studies have emphasized interindividual variability in the neuromodulatory effects of rTMS as a function of the rTMS frequency applied (23). Furthermore, as the effects of TMS have been shown to be state-dependent (37), lpIPL may have responded differently to the two frequencies because of disparities in ongoing levels of tonic activation (38). In this light, the prerTMS activity could serve to partially determine the degree and direction of cortical excitability changes at pIPL as a way of maintaining a homeostatic level of cortical activation within a physiologically useful range (39).…”

Effects of TMS over Premotor and Superior Temporal Cortices on Biological Motion Perception

“…No increase in false alarms was found for the nonbiological motion task, indicating that local biological motion information may trigger specific neural computations and/or populations and may even selectively engage the match-to-body process. Studies in which signal and noise are manipulated independently could help test these possibilities (Ruzzoli et al, 2011). Eye tracking can be used to test whether TMS affects how observers scan the noise-masked displays.…”

Direct injection of noise to the visual cortex decreases accuracy but increases decision confidence

“…This conclusion is in line with several studies that have argued that TMS acts by adding neural noise to the perceptual process rather than by affecting signal strength (Ruzzoli et al 2010;Schwarzkopf et al 2011). Nevertheless, two other studies argued for the opposite conclusion (Harris et al 2008;Ruzzoli et al 2011). This suggests that the precise influence of TMS may depend on the specific stimulation and task parameters.…”

“…To ensure that subjects do not consciously adjust their criteria to compensate for the change in signal variability, ideally, the difference between the conditions of interest should be subtle and mainly focused on increasing variability in one of the conditions. Although transcranial magnetic stimulation (TMS) to the visual cortex is often used to completely "knock out" conscious perception (e.g., Boyer et al 2005;Breitmeyer et al 2004;Kastner et al 1998;Koivisto et al 2010Koivisto et al , 2011Ro et al 2004), some recent studies have shown that low-intensity stimulation can effectively inject noise to the visual system (Ruzzoli et al 2010;Schwarzkopf et al 2011; although, see Harris et al 2008;Ruzzoli et al 2011). In the present study, we applied single-pulse TMS at an intensity below the threshold for the conscious perception of phosphenes.…”