Visual motion adaptation increases the susceptibility of area V5/MT to phosphene induction by transcranial magnetic stimulation

State-dependent TMS effects in the visual cortex after visual adaptation: A combined TMS-EEG study

OBJECTIVE

The impact of transcranial magnetic stimulation (TMS) has been shown to depend on the initial brain state of the stimulated cortical region. This observation has led to the development of paradigms that aim to enhance the specificity of TMS effects by using visual/luminance adaptation to modulate brain state prior to the application of TMS. However, the neural basis of interactions between TMS and adaptation is unknown. Here, we examined these interactions by using electroencephalography (EEG) to measure the impact of TMS over the visual cortex after luminance adaptation.

METHODS

Single-pulses of neuronavigated TMS (nTMS) were applied at two different intensities over the left visual cortex after adaptation to either high or low luminance. We then analyzed the effects of adaptation on the global and local cortical excitability.

RESULTS

The analysis revealed a significant interaction between the TMS-evoked responses and the adaptation condition. In particular, when nTMS was applied with high intensity, the evoked responses were larger after adaptation to high than low luminance.

CONCLUSION

This result provides the first neural evidence on the interaction between TMS with visual adaptation.

SIGNIFICANCE

TMS can activate neurons differentially as a function of their adaptation state.

Common framework for "virtual lesion" and state-dependent TMS: The facilitatory/suppressive range model of online TMS effects on behavior

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Transcranial magnetic stimulation can either facilitate or impair behavior.

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Nature of behavioral effects depends on factors such as brain state and intensity.

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We present a common framework to account for these effects.

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There are distinct intensity ranges for facilitatory and suppressive effects of TMS.

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Changes in excitability shift these ranges and account for behavioral effects.

The behavioral effects of Transcranial Magnetic Stimulation (TMS) are often nonlinear; factors such as stimulation intensity and brain state can modulate the impact of TMS on observable behavior in qualitatively different manner. Here we propose a theoretical framework to account for these effects. In this model, there are distinct intensity ranges for facilitatory and suppressive effects of TMS – low intensities facilitate neural activity and behavior whereas high intensities induce suppression. The key feature of the model is that these ranges are shifted by changes in neural excitability: consequently, a TMS intensity, which normally induces suppression, can have a facilitatory effect if the stimulated neurons are being inhibited by ongoing task-related processes or preconditioning. For example, adaptation reduces excitability of adapted neurons; the outcome is that TMS intensities which inhibit non-adapted neurons induce a facilitation on adapted neural representations, leading to reversal of adaptation effects. In conventional “virtual lesion” paradigms, similar effects occur because neurons not involved in task-related processes are inhibited by the ongoing task. The resulting reduction in excitability can turn high intensity “inhibitory” TMS to low intensity “facilitatory” TMS for these neurons, and as task-related neuronal representations are in the inhibitory range, the outcome is a reduction in signal-to-noise ratio and behavioral impairment.

Initial activation state, stimulation intensity and timing of stimulation interact in producing behavioral effects of TMS

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TMS effects depend on various factors such as intensity, brain state and timing.

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We examined how these factors interact to give rise to behavioral effects.

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TMS was applied while participants performed a behavioral priming task.

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State dependency of TMS effect was found to interact with intensity and timing.

Behavioral effects of transcranial magnetic stimulation (TMS) have been shown to depend on various factors, such as neural activation state, stimulation intensity, and timing of stimulation. Here we examined whether these factors interact, by applying TMS at either sub- or suprathreshold intensity (relative to phosphene threshold, PT) and at different time points during a state-dependent TMS paradigm. The state manipulation involved a behavioral task in which a visual prime (color grating) was followed by a target stimulus which could be either congruent, incongruent or partially congruent with the color and orientation of the prime. In Experiment 1, single-pulse TMS was applied over the early visual cortex (V1/V2) or Vertex (baseline) at the onset of the target stimulus – timing often used in state-dependent TMS studies. With both subthreshold and suprathreshold stimulation, TMS facilitated the detection of incongruent stimuli while not significantly affecting other stimulus types. In Experiment 2, TMS was applied at 100 ms after target onset –a time window in which V1/V2 is responding to visual input. Only TMS applied at suprathreshold intensity facilitated the detection of incongruent stimuli, with no effect with subthreshold stimulation. The need for higher stimulation intensity is likely to reflect reduced susceptibility to TMS of neurons responding to visual stimulation. Furthermore, the finding that in Experiment 2 only suprathreshold TMS induced a behavioral facilitation on incongruent targets (whereas facilitations in the absence of priming have been reported with subthreshold TMS) indicates that priming, by reducing neural excitability to incongruent targets, shifts the facilitatory/inhibitory range of TMS effects.

Differential effect of visual motion adaption upon visual cortical excitability

The objectives of this study were 1) to probe the effects of visual motion adaptation on early visual and V5/MT cortical excitability and 2) to investigate whether changes in cortical excitability following visual motion adaptation are related to the degree of visual dependency, i.e., an overreliance on visual cues compared with vestibular or proprioceptive cues. Participants were exposed to a roll motion visual stimulus before, during, and after visual motion adaptation. At these stages, 20 transcranial magnetic stimulation (TMS) pulses at phosphene threshold values were applied over early visual and V5/MT cortical areas from which the probability of eliciting a phosphene was calculated. Before and after adaptation, participants aligned the subjective visual vertical in front of the roll motion stimulus as a marker of visual dependency. During adaptation, early visual cortex excitability decreased whereas V5/MT excitability increased. After adaptation, both early visual and V5/MT excitability were increased. The roll motion-induced tilt of the subjective visual vertical (visual dependence) was not influenced by visual motion adaptation and did not correlate with phosphene threshold or visual cortex excitability. We conclude that early visual and V5/MT cortical excitability is differentially affected by visual motion adaptation. Furthermore, excitability in the early or late visual cortex is not associated with an increase in visual reliance during spatial orientation. Our findings complement earlier studies that have probed visual cortical excitability following motion adaptation and highlight the differential role of the early visual cortex and V5/MT in visual motion processing.NEW & NOTEWORTHY We examined the influence of visual motion adaptation on visual cortex excitability and found a differential effect in V1/V2 compared with V5/MT. Changes in visual excitability following motion adaptation were not related to the degree of an individual's visual dependency.

Information-Based Approaches of Noninvasive Transcranial Brain Stimulation

Progress in cognitive neuroscience relies on methodological developments to increase the specificity of knowledge obtained regarding brain function. For example, in functional neuroimaging the current trend is to study the type of information carried by brain regions rather than simply compare activation levels induced by task manipulations. In this context noninvasive transcranial brain stimulation (NTBS) in the study of cognitive functions may appear coarse and old fashioned in its conventional uses. However, in their multitude of parameters, and by coupling them with behavioral manipulations, NTBS protocols can reach the specificity of imaging techniques. Here we review the different paradigms that have aimed to accomplish this in both basic science and clinical settings and follow the general philosophy of information-based approaches.

Occipital TMS at phosphene detection threshold captures attention automatically

Strong stimuli may capture attention automatically, suggesting that attentional selection is determined primarily by physical stimulus properties. The mechanisms underlying capture remain controversial, in particular, whether feedforward subcortical processes are its main source. Also, it remains unclear whether only physical stimulus properties determine capture strength. Here, we demonstrate strong capture in the absence of feedforward input to subcortical structures such as the superior colliculus, by using transcranial magnetic stimulation (TMS) over occipital visual cortex as an attention cue. This implies that the feedforward sweep through subcortex is not necessary for capture to occur but rather provides an additional source of capture. Furthermore, seen cues captured attention more strongly than (physically identical) unseen cues, suggesting that the momentary state of the nervous system modulates attentional selection. In summary, we demonstrate the existence of several sources of attentional capture, and that both physical stimulus properties and the state of the nervous system influence capture.

Why is "blindsight" blind? A new perspective on primary visual cortex, recurrent activity and visual awareness

The neuropsychological phenomenon of blindsight has been taken to suggest that the primary visual cortex (V1) plays a unique role in visual awareness, and that extrastriate activation needs to be fed back to V1 in order for the content of that activation to be consciously perceived. The aim of this review is to evaluate this theoretical framework and to revisit its key tenets. Firstly, is blindsight truly a dissociation of awareness and visual detection? Secondly, is there sufficient evidence to rule out the possibility that the loss of awareness resulting from a V1 lesion simply reflects reduced extrastriate responsiveness, rather than a unique role of V1 in conscious experience? Evaluation of these arguments and the empirical evidence leads to the conclusion that the loss of phenomenal awareness in blindsight may not be due to feedback activity in V1 being the hallmark awareness. On the basis of existing literature, an alternative explanation of blindsight is proposed. In this view, visual awareness is a "global" cognitive function as its hallmark is the availability of information to a large number of perceptual and cognitive systems; this requires inter-areal long-range synchronous oscillatory activity. For these oscillations to arise, a specific temporal profile of neuronal activity is required, which is established through recurrent feedback activity involving V1 and the extrastriate cortex. When V1 is lesioned, the loss of recurrent activity prevents inter-areal networks on the basis of oscillatory activity. However, as limited amount of input can reach extrastriate cortex and some extrastriate neuronal selectivity is preserved, computations involving comparison of neural firing rates within a cortical area remain possible. This enables "local" read-out from specific brain regions, allowing for the detection and discrimination of basic visual attributes. Thus blindsight is blind due to lack of "global" long-range synchrony, and it functions via "local" neural readout from extrastriate areas.

Causal evidence for a privileged working memory state in early visual cortex

Emerging evidence suggests that items held in working memory (WM) might not all be in the same representational state. One item might be privileged over others, making it more accessible and thereby recalled with greater precision. Here, using transcranial magnetic stimulation (TMS), we provide causal evidence in human participants that items in WM are differentially susceptible to disruptive TMS, depending on their state, determined either by task relevance or serial position. Across two experiments, we applied TMS to area MT+ during the WM retention of two motion directions. In Experiment 1, we used an "incidental cue" to bring one of the two targets into a privileged state. In Experiment 2, we presented the targets sequentially so that the last item was in a privileged state by virtue of recency. In both experiments, recall precision of motion direction was differentially affected by TMS, depending on the state of the memory target at the time of disruption. Privileged items were recalled with less precision, whereas nonprivileged items were recalled with higher precision. Thus, only the privileged item was susceptible to disruptive TMS over MT+. By contrast, precision of the nonprivileged item improved either directly because of facilitation by TMS or indirectly through reduced interference from the privileged item. Our results provide a unique line of evidence, as revealed by TMS over a posterior sensory brain region, for at least two different states of item representation in WM.

Unidirectional visual motion adaptation induces reciprocal inhibition of human early visual cortex excitability

OBJECTIVES

Behavioural observations provided by the waterfall illusion suggest that motion perception is mediated by a comparison of responsiveness of directional selective neurones. These are proposed to be optimally tuned for motion detection in different directions. Critically however, despite the behavioural observations, direct evidence of this relationship at a cortical level in humans is lacking. By utilising the state dependant properties of transcranial magnetic stimulation (TMS), one can probe the excitability of specific neuronal populations using the perceptual phenomenon of phosphenes.

METHOD

We exposed subjects to unidirectional visual motion adaptation and subsequently simultaneously measured early visual cortex (V1) excitability whilst viewing motion in the adapted and non-adapted direction.

RESULT

Following adaptation, the probability of perceiving a phosphene whilst viewing motion in the adapted direction was diminished reflecting a reduction in V1 excitability. Conversely, V1 excitability was enhanced whilst viewing motion in the opposite direction to that used for adaptation.

CONCLUSION

Our results provide support that in humans a process of reciprocal inhibition between oppositely tuned directionally selective neurones in V1 facilitates motion perception.

SIGNIFICANCE

This paradigm affords a unique opportunity to investigate changes in cortical excitability following peripheral vestibular disorders.

How visual short-term memory maintenance modulates the encoding of external input: evidence from concurrent visual adaptation and TMS

The impact of memory representations on the encoding of visual input has been the subject of much debate. Here we investigated this issue by examining how visual short-term memory (VSTM) maintenance of orientation information modulates the strength of the tilt aftereffect (TAE) induced by a concurrent visual adapter. We reasoned that if VSTM maintenance facilitates visual processing of stimuli that match the VSTM content, then the magnitude of the TAE should be enhanced when the orientations of the memory item and the adapter are identical. In contrast, if VSTM content inhibits visual processing, then the TAE induced by the adapter should be reduced. Our results are consistent with the latter hypothesis, and a TMS study demonstrated that the reduction of the TAE by VSTM maintenance of orientation information occurs in the early visual cortex. VSTM maintenance of shape information also reduced the TAE magnitude, but to a smaller extent than maintenance of orientation information. A TMS experiment did not implicate the early visual cortex in this phenomenon. In summary, our results indicate that VSTM maintenance under these circumstances inhibits the encoding of concurrent visual input, and that this inhibition occurs at various levels of the visual cortex.

Vestibular activation differentially modulates human early visual cortex and V5/MT excitability and response entropy

Head movement imposes the additional burdens on the visual system of maintaining visual acuity and determining the origin of retinal image motion (i.e., self-motion vs. object-motion). Although maintaining visual acuity during self-motion is effected by minimizing retinal slip via the brainstem vestibular-ocular reflex, higher order visuovestibular mechanisms also contribute. Disambiguating self-motion versus object-motion also invokes higher order mechanisms, and a cortical visuovestibular reciprocal antagonism is propounded. Hence, one prediction is of a vestibular modulation of visual cortical excitability and indirect measures have variously suggested none, focal or global effects of activation or suppression in human visual cortex. Using transcranial magnetic stimulation-induced phosphenes to probe cortical excitability, we observed decreased V5/MT excitability versus increased early visual cortex (EVC) excitability, during vestibular activation. In order to exclude nonspecific effects (e.g., arousal) on cortical excitability, response specificity was assessed using information theory, specifically response entropy. Vestibular activation significantly modulated phosphene response entropy for V5/MT but not EVC, implying a specific vestibular effect on V5/MT responses. This is the first demonstration that vestibular activation modulates human visual cortex excitability. Furthermore, using information theory, not previously used in phosphene response analysis, we could distinguish between a specific vestibular modulation of V5/MT excitability from a nonspecific effect at EVC.

Randomized trial of supervised versus unsupervised optokinetic exercise in persons with peripheral vestibular disorders

BACKGROUND

Visual vertigo (VV) symptoms improve only when customized vestibular rehabilitation (VR) integrates exposure to optokinetic stimuli (OK). However, equipment is expensive, biweekly sessions are not standard practice, and therapy is often unsupervised.

METHODS

A controlled, parallel-group comparison was made of patients' responses to an 8-week customized program incorporating OK training via a full-field visual environment rotator (group OKF) or DVD (an optokinetic disc or drum rotating at 40° or 60° s(-1)), supervised (group OKS) or unsupervised (group OKU). A total of 60 participants with chronic peripheral vestibular symptoms were randomly allocated to 1 of 3 treatment groups: group OKF (n = 20) or OKS (n = 20), in which participants attended weekly sessions and were prescribed customized home exercises incorporating the DVD, or group OKU (n = 20) who practiced customized exercises and the DVD unsupervised. Treatment response was assessed at baseline and at 8 weeks with dynamic posturography, Functional Gait Assessment (FGA), and questionnaires for symptoms, symptom triggers, and psychological state.

RESULTS

No significant between-group differences were present at baseline or at post interventions. All groups showed significant within-group improvements for vestibular (ie, lightheadedness), VV, and autonomic symptoms (P < .05). Posturography and FGA improved significantly for groups OKF and OKS (P ≤ .01) as well as anxiety scores for group OKS (P < .05) and depression for group OKF (P < .05). Migraine significantly affected VV improvement (migraineurs improved more; P = .01). The drop-out rate was 55% for group OKU and 10% for each supervised group (P < .01).

CONCLUSIONS

The DVD may be an effective and economical method of integrating OK into VR. However, rehabilitation should be supervised for greater compliance and improvements, particularly for postural stability and psychological state.