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We used concurrent transcranial magnetic stimulation and functional MRI (TMS-fMRI) during a visuospatial cueing paradigm in humans, to study the causal role of the right angular gyrus (AG) as a source of attentional control. Our findings show that TMS over the right AG (high vs. low intensity) modulates neural responses interhemispherically, in a manner that varies dynamically with the current attentional condition. The behavioural impact of such TMS depended not only on the target hemifield but also on exogenous cue validity, facilitating spatial reorienting to invalidly cued right visual targets. On a neural level, right AG TMS had corresponding interhemispheric effects in the left AG and left retinotopic cortex, including area V1. We conclude that the direction of covert visuospatial attention can involve dynamic interplay between the right AG and remote interconnected regions of the opposite left hemisphere, whereas our findings also suggest that the right AG can influence responses in the retinotopic visual cortex.

Combining transcranial magnetic stimulation (TMS) with concurrent functional magnetic resonance imaging (fMRI) allows study of how local brain stimulation may causally affect activity in remote brain regions. Here, we applied bursts of high- or low-intensity TMS over right posterior parietal cortex, during a task requiring sustained covert visuospatial attention to either the left or right hemifield, or in a neutral control condition, while recording blood oxygenation-level–dependent signal with a posterior MR surface coil. As expected, the active attention conditions activated components of the well-described “attention network,” as compared with the neutral baseline. Also as expected, when comparing left minus right attention, or vice versa, contralateral occipital visual cortex was activated. The critical new finding was that the impact of high- minus low-intensity parietal TMS upon these visual regions depended on the currently attended side. High- minus low-intensity parietal TMS increased the difference between contralateral versus ipsilateral attention in right extrastriate visual cortex. A related albeit less pronounced pattern was found for left extrastriate visual cortex. Our results confirm that right human parietal cortex can exert attention-dependent influences on occipital visual cortex and provide a proof of concept for the use of concurrent TMS–fMRI in studying how remote influences can vary in a purely top–down manner with attentional demands.

The posterior parietal cortex (PPC) has been associated with the encoding of events in peripersonal space, but little is known about the precise segregation of parietal areas involved specifically in 'near-space' visuospatial processing. This study applied transcranial magnetic stimulation (TMS) to two parietal areas: the right angular gyrus (ANG) and the right supramarginal gyrus (SMG) in addition to a control site, cortical visual area V5, while subjects performed symmetry judgements on lines presented simultaneously in the left and right visual fields. Eight subjects performed the task with the stimuli presented either in peripersonal or extra personal space. As expected, TMS over the right ANG caused subjects to report lines as being longer ipsilateral to the stimulation site, but only in near space. TMS of the right SMG, however, induced a bias shift in both near and far space, but the change only reached significance in near space. Our findings suggest that, consistent with its role in somatosensory functions, the SMG, like the ANG, is specialised for near space.

Summary Background Regions in human frontal cortex may have modulatory top-down influences on retinotopic visual cortex, but to date neuroimaging methods have only been able to provide indirect evidence for such functional interactions between remote but interconnected brain regions. Here we combined transcranial magnetic stimulation (TMS) with concurrent functional magnetic resonance imaging (fMRI), plus psychophysics, to show that stimulation of the right human frontal eye-field (FEF) produced a characteristic topographic pattern of activity changes in retinotopic visual areas V1-V4, with functional consequences for visual perception. Results FEF TMS led to activity increases for retinotopic representations of the peripheral visual field, but to activity decreases for the central field, in areas V1-V4. These frontal influences on visual cortex occurred in a top-down manner, independently of visual input. TMS of a control site (vertex) did not elicit such visual modulations, and saccades, blinks, or pupil dilation could not account for our results. Finally, the effects of FEF TMS on activity in retinotopic visual cortex led to a behavioral prediction that we confirmed psychophysically by showing that TMS of the frontal site (again compared with vertex) enhanced perceived contrast for peripheral relative to central visual stimuli. Conclusions Our results provide causal evidence that circuits originating in the human FEF can modulate activity in retinotopic visual cortex, in a manner that differentiates the central and peripheral visual field, with functional consequences for perception. More generally, our study illustrates how the new approach of concurrent TMS-fMRI can now reveal causal interactions between remote but interconnected areas of the human brain.

We used concurrent transcranial magnetic stimulation and functional MRI (TMS-fMRI) during a visuospatial cueing paradigm in humans, to study the causal role of the right angular gyrus (AG) as a source of attentional control. Our findings show that TMS over the right AG (high vs. low intensity) modulates neural responses interhemispherically, in a manner that varies dynamically with the current attentional condition. The behavioural impact of such TMS depended not only on the target hemifield but also on exogenous cue validity, facilitating spatial reorienting to invalidly cued right visual targets. On a neural level, right AG TMS had corresponding interhemispheric effects in the left AG and left retinotopic cortex, including area V1. We conclude that the direction of covert visuospatial attention can involve dynamic interplay between the right AG and remote interconnected regions of the opposite left hemisphere, whereas our findings also suggest that the right AG can influence responses in the retinotopic visual cortex.

We used concurrent TMS–fMRI to test directly for hemispheric differences in causal influences of the right or left fronto-parietal cortex on activity (BOLD signal) in the human occipital cortex. Clinical data and some behavioral TMS studies have been taken to suggest right-hemisphere specialization for top–down modulation of vision in humans, based on deficits such as spatial neglect or extinction in lesioned patients, or findings that TMS to right (vs. left) fronto-parietal structures can elicit stronger effects on visual performance. But prior to the recent advent of concurrent TMS and neuroimaging, it was not possible to directly examine the causal impact of one (stimulated) brain region upon others in humans. Here we stimulated the frontal or intraparietal cortex in the left or right hemisphere with TMS, inside an MR scanner, while measuring with fMRI any resulting BOLD signal changes in visual areas V1–V4 and V5/MT+. For both frontal and parietal stimulation, we found clear differences between effects of right- versus left-hemisphere TMS on activity in the visual cortex, with all differences significant in direct statistical comparisons. Frontal TMS over either hemisphere elicited similar BOLD decreases for central visual field representations in V1–V4, but only right frontal TMS led to BOLD increases for peripheral field representations in these regions. Hemispheric differences for effects of parietal TMS were even more marked: Right parietal TMS led to strong BOLD changes in V1–V4 and V5/MT+, but left parietal TMS did not. These data directly confirm that the human frontal and parietal cortex show right-hemisphere specialization for causal influences on the visual cortex.

Transcranial magnetic stimulation (TMS) has been used to document some apparent interhemispheric influences behaviorally, with TMS over the right parietal cortex reported to enhance processing of touch for the ipsilateral right hand (Seyal et al., 1995). However, the neural bases of such apparent interhemispheric influences from TMS remain unknown. Here, we studied this directly by combining TMS with concurrent functional magnetic resonance imaging (fMRI). We applied bursts of 10 Hz TMS over right parietal cortex, at a high or low intensity, during two sensory contexts: either without any other stimulation, or while participants received median nerve stimulation to the right wrist, which projects to left primary somatosensory cortex (SI). TMS to right parietal cortex affected the blood oxygenation level-dependent signal in left SI, with high- versus low-intensity TMS increasing the left SI signal during right-wrist somatosensory input, but decreasing this in the absence of somatosensory input. This state-dependent modulation of SI by parietal TMS over the other hemisphere was accompanied by a related pattern of TMS-induced influences in the thalamus, as revealed by region-of-interest analyses. A behavioral experiment confirmed that the same right parietal TMS protocol of 10 Hz bursts led to enhanced detection of perithreshold electrical stimulation of the right median nerve, which is initially processed in left SI. Our results confirm directly that TMS over right parietal cortex can affect processing in left SI of the other hemisphere, with rivalrous effects (possibly transcallosal) arising in the absence of somatosensory input, but facilitatory effects (possibly involving thalamic circuitry) in the presence of driving somatosensory input.

It has often been proposed that regions of the human parietal and/or frontal lobe may modulate activity in visual cortex, for example, during selective attention or saccade preparation. However, direct evidence for such causal claims is largely missing in human studies, and it remains unclear to what degree the putative roles of parietal and frontal regions in modulating visual cortex may differ. Here we used transcranial magnetic stimulation (TMS) and functional magnetic resonance imaging (fMRI) concurrently, to show that stimulating right human intraparietal sulcus (IPS, at a site previously implicated in attention) elicits a pattern of activity changes in visual cortex that strongly depends on current visual context. Increased intensity of IPS TMS affected the blood oxygen level–dependent (BOLD) signal in V5/MT+ only when moving stimuli were present to drive this visual region, whereas TMS-elicited BOLD signal changes were observed in areas V1–V4 only during the absence of visual input. These influences of IPS TMS upon remote visual cortex differed significantly from corresponding effects of frontal (eye field) TMS, in terms of how they related to current visual input and their spatial topography for retinotopic areas V1–V4. Our results show directly that parietal and frontal regions can indeed have distinct patterns of causal influence upon functional activity in human visual cortex. Key words: attention, frontal cortex, functional magnetic resonance imaging, parietal cortex, top--down, transcranial magnetic stimulation

Signal detection theory tests an observer's ability to discriminate between signal and noise. Deciding whether or not a patient's symptoms warrant further investigation or treatment is an example of this task in the clinical setting. Noise can exist within the observer--for example, in the brain of a tired or inexperienced doctor--or can arise from an external source such as the patient. Patients can produce external noise by giving numerous unrelated presenting complaints, providing overly detailed accounts of their symptoms, or simply talking too quickly. The more noise that is present, the harder the signal (such as a new disease or a notable change in an old condition) is to detect. Patients in the neurology clinic seem to be 'noisier' than average, perhaps owing to the long duration of their condition in many cases and the relatively high proportion of patients with medically unexplained symptoms. The ability to interpret such 'noisy' histories often underpins the neurological diagnosis. This Review aims to promote the relevance of signal detection theory to the overworked neurologist on the ward or in the clinic and explores strategies to reduce the noise generated both within the brain of the doctor and by patients.

Localized repetitive transcranial magnetic stimulation was used to disrupt visuospatial perception in the near and far space of six healthy volunteer subjects. In addition to the baseline condition, they were stimulated over the right posterior parietal cortex, the right or left dorsal occipital cortex or the right ventral occipital cortex, during the brief presentation of a transected horizontal line. Subjects had to indicate whether the part of the line to the left or right of the transection appeared longer. The stimulus display was back-projected on a screen at a viewing distance of either 50 or 150 cm ("near" and "far" space, respectively). Reaction times and choices were measured. In a forced-choice paradigm, subjects showed "pseudoneglect", the natural tendency of neurologically intact subjects to perceive the left side of a centrally transected line as slightly longer than the right. These errors occurred more for lines in near space than for lines in far space. Magnetic stimulation of the right posterior parietal cortex or the right ventral occipital lobe selectively induced a significant shift to the right in the perceived midpoint for near- and far-space lines, respectively. The results reproduced in normal subjects the dissociation between neglect in near and far space that has been described in patients with different right-hemisphere lesions. This dissociation supports the contention that there is a dorsal/near space-ventral/far space segregation of processing in the visual system which reflects the behavioural goals of the two putative visual streams.