Role of primary visual cortex in the binocular integration of plaid motion perception

Long-Lasting Enhancement of Visual Perception with Repetitive Noninvasive Transcranial Direct Current Stimulation

Understanding processes performed by an intact visual cortex as the basis for developing methods that enhance or restore visual perception is of great interest to both researchers and medical practitioners. Here, we explore whether contrast sensitivity, a main function of the primary visual cortex (V1), can be improved in healthy subjects by repetitive, noninvasive anodal transcranial direct current stimulation (tDCS). Contrast perception was measured via threshold perimetry directly before and after intervention (tDCS or sham stimulation) on each day over 5 consecutive days (24 subjects, double-blind study). tDCS improved contrast sensitivity from the second day onwards, with significant effects lasting 24 h. After the last stimulation on day 5, the anodal group showed a significantly greater improvement in contrast perception than the sham group (23 vs. 5%). We found significant long-term effects in only the central 2–4° of the visual field 4 weeks after the last stimulation. We suspect a combination of two factors contributes to these lasting effects. First, the V1 area that represents the central retina was located closer to the polarization electrode, resulting in higher current density. Second, the central visual field is represented by a larger cortical area relative to the peripheral visual field (cortical magnification). This is the first study showing that tDCS over V1 enhances contrast perception in healthy subjects for several weeks. This study contributes to the investigation of the causal relationship between the external modulation of neuronal membrane potential and behavior (in our case, visual perception). Because the vast majority of human studies only show temporary effects after single tDCS sessions targeting the visual system, our study underpins the potential for lasting effects of repetitive tDCS-induced modulation of neuronal excitability.

Is theta burst stimulation applied to visual cortex able to modulate peripheral visual acuity?

Repetitive transcranial magnetic stimulation is usually applied to visual cortex to explore the effects on cortical excitability. Most researchers therefore concentrate on changes of phosphene threshold, rarely on consequences for visual performance. Thus, we investigated peripheral visual acuity in the four quadrants of the visual field using Landolt C optotypes before and after repetitive stimulation of the visual cortex. We applied continuous and intermittend theta burst stimulation with various stimulation intensities (60%, 80%, 100%, 120% of individual phosphene threshold) as well as monophasic and biphasic 1 Hz stimulation, respectively. As an important result, no serious adverse effects were observed. In particular, no seizure was induced, even with theta burst stimulation applied with 120% of individual phosphene threshold. In only one case stimulation was ceased because the subject reported intolerable pain. Baseline visual acuity decreased over sessions, indicating a continuous training effect. Unexpectedly, none of the applied transcranial magnetic stimulation protocols had an effect on performance: no change in visual acuity was found in any of the four quadrants of the visual field. Binocular viewing as well as the use of peripheral instead of foveal presentation of the stimuli might have contributed to this result. Furthermore, intraindividual variability could have masked the TMS- induced effects on visual acuity.

Anodal transcranial direct current stimulation reduces psychophysically measured surround suppression in the human visual cortex

Transcranial direct current stimulation (tDCS) is a safe, non-invasive technique for transiently modulating the balance of excitation and inhibition within the human brain. It has been reported that anodal tDCS can reduce both GABA mediated inhibition and GABA concentration within the human motor cortex. As GABA mediated inhibition is thought to be a key modulator of plasticity within the adult brain, these findings have broad implications for the future use of tDCS. It is important, therefore, to establish whether tDCS can exert similar effects within non-motor brain areas. The aim of this study was to assess whether anodal tDCS could reduce inhibitory interactions within the human visual cortex. Psychophysical measures of surround suppression were used as an index of inhibition within V1. Overlay suppression, which is thought to originate within the lateral geniculate nucleus (LGN), was also measured as a control. Anodal stimulation of the occipital poles significantly reduced psychophysical surround suppression, but had no effect on overlay suppression. This effect was specific to anodal stimulation as cathodal stimulation had no effect on either measure. These psychophysical results provide the first evidence for tDCS-induced reductions of intracortical inhibition within the human visual cortex.

A double dissociation between striate and extrastriate visual cortex for pattern motion perception revealed using rTMS

The neural mechanisms underlying the integration and segregation of motion signals are often studied using plaid stimuli. These stimuli consist of two spatially coincident dynamic gratings of differing orientations, which are either perceived to move in two unique directions or are integrated by the visual system to elicit the percept of a checkerboard moving in a single direction. Computations pertaining to the motion of the individual component gratings are thought to take place in striate cortex (V1) whereas motion integration is thought to involve neurons in dorsal stream extrastriate visual areas, particularly V5/MT. By combining a psychophysical task that employed plaid stimuli with 1 Hz offline repetitive transcranial magnetic stimulation (rTMS), we demonstrated a double dissociation between striate and extrastriate visual cortex in terms of their contributions to motion integration. rTMS over striate cortex increased coherent motion percepts whereas rTMS over extrastriate cortex had the opposite effect. These effects were robust directly after the stimulation administration and gradually returned to baseline within 15 minutes. This double dissociation is consistent with previous patient data and the recent hypothesis that both coherent and transparent motion percepts are supported by the visual system simultaneously and compete for perceptual dominance.

Non-invasive brain stimulation in the detection of deception: scientific challenges and ethical consequences

Tools for noninvasive stimulation of the brain, such as transcranial magnetic stimulation (TMS) and transcranial direct current stimulation (tDCS), have provided new insights in the study of brain-behavior relationships due to their ability to directly alter cortical activity. In particular, TMS and tDCS have proven to be useful tools for establishing causal relationships between behavioral and brain imaging measures. As such, there has been interest in whether these tools may represent novel technologies for deception detection by altering a person's ability to engage brain networks involved in conscious deceit. Investigation of deceptive behavior using noninvasive brain stimulation is at an early stage. Here we review the existing literature on the application of noninvasive brain stimulation in the study of deception. Whether such approaches could be usefully applied to the detection of deception by altering a person's ability to engage brain networks involved in conscious deceit remains to be validated. Ethical and legal consequences of the development of such a technology are discussed.

Neural activation state determines behavioral susceptibility to modified theta burst transcranial magnetic stimulation

Transcranial magnetic stimulation (TMS) allows one to investigate the effects of temporary interference of neural processing in neurologically intact subjects. In a previous study [J. Silvanto et al. (2007) Eur. J. Neurosci., 25, 1874-1881] we found that online TMS perceptually facilitates the attributes encoded by the least active neural populations. The objective of the present experiment was to extend this work to determine whether such state-dependent effects can be observed when offline high-frequency TMS is applied to suppress neural activity. The activity levels of direction-selective neural populations in the V1/V2 region were modulated by asking subjects to passively view either leftward or rightward motion during offline TMS. In a subsequent motion direction-discrimination task, their ability to discriminate motion direction was dependent on the type of motion they had passively viewed during offline TMS: detection of the congruent direction (i.e. direction viewed during offline TMS) was unaffected, whereas detection of the incongruent direction (i.e. opposite direction to the one viewed during offline TMS) was impaired. As the activity level of neurons tuned to the incongruent direction was presumably lower during the TMS than of those tuned to the congruent direction, this behavioral result demonstrates that the offline TMS preferentially suppressed attributes encoded by the least active neural populations. In contrast to direction discrimination, motion detection was not impaired in a direction-specific manner. This shows that the requirements of the psychophysical task, in conjunction with the relative activity states of neuronal populations when TMS is applied, can be used to selectively interfere with overlapping neuronal populations.

Neural adaptation reveals state-dependent effects of transcranial magnetic stimulation

Transcranial magnetic stimulation (TMS) is now widely used as a 'virtual' lesion paradigm to investigate behavioural functions, but the mechanisms through which it influences neural processing are unclear. To understand the differential effects of TMS on spatially overlapping populations of neurons we manipulated the relative activity levels of visual neurons by adapting subjects to a range of visual stimuli. By applying TMS to the visual cortex representing the central visual field we have shown in two experiments that the behavioural and perceptual effects of TMS depend on the state of adaptation of the neural population stimulated by TMS. Specifically, we have demonstrated that within the stimulated area TMS perceptually facilitates the attributes encoded by the less active neural population. We have demonstrated the generality of this principle for both suprathreshold and subthreshold TMS as well as for colour and orientation-contingent colour using both subjective reports and psychophsyical measures. These findings can explain how TMS disrupts cognitive functions and therefore have implications for all studies which use TMS to disrupt behaviour.