Twenty-ninth Bartlett Lecture

Publication Summary

Introduction: Magnetic Stimulation in studies of cortical function. Vincent Walsh University of Oxford. Magnetic stimulation of the human brain is one of many techniques available for studies of perceptual, motor and cognitive functions. The main advantages claimed for TMS are the reversible nature of the interference, its brevity and the ability to selectively interfere with different cortical functions. I will discuss the unique problem space occupied by studies using TMS, outline the main methodological modes in using it and discuss the spatial, temporal and functional resolution. I will give examples of the use of TMS in studies of learning, cortical inteactions and speech which serve as examples of the kinds of contribution use of the technique can make to studies of cortical function. Transcranial magnetic stimulation as a tool for probing and manipulating motor cortical organisation J C Rothwell Institute of Neurology, London Transcranial magnetic stimulation is now commonly used in experimental psychology as a method of inducing a temporary functional "lesion" in parts of the cerebral cortex. Here, I will summarise three new pieces of work that use TMS in a different way, to study and manipulate the excitability of connections between two areas of cortex. All three studies examined the connection between the premotor areas and the motor cortex in healthy human subjects. In the first study, we used a paired pulse design to test whether single stimuli over sites anterior to motor cortex could affect its excitability as measured by the EMG response to a standard test stimulus. Two small figure of eight coils were used so that the stimuli could be given as close a 3cm apart, and the intensity of the anterior, conditioning stimulus was set to be lower than the active motor threshold for the motor cortex. Two points (A and B) were found to produce suppression of the response to the test shock, with a time course that reached a peak at an interstimulus interval of 6ms. Point A was 4-6 cm anterior to the hand area of motor cortex; point B was in the midline and 6cm anterior to the vertex. Since stimuli at these sites had no effect on the size of H-reflexes in arm muscles, nor on the EMG responses evoked by transcranial electrical stimulation of the motor cortex, we presume that the interaction we observed was due to activity in cortico-cortical connections between A or B and the motor cortex. In the second and third studies we used repetitive TMS given at 1Hz for up to 25min to manipulate the excitability of the connection for a short period outlasting the end of the rTMS. Stimuli that were subthreshold for stimulating motor cortex in active subjects were used, and motor cortex excitability was again tested before and after the train by measuring the EMG response to a standard test shock. Stimuli given over a point 3cm anterior to the hand area could reduce motor cortical excitability in the resting state for up to 30min after the end of the train. There was no effect from stimulation over motor cortex itself. We conclude that it is possible to produce effects at distant sites by repetitive stimulation of one area of cortex. Finally, we examined the nature of this effect in more detail by testing how a subthreshold rTMS of premotor cortex affected paired pulse testing of motor cortex excitability. The train reduced the amount of cortico-cortical inhibition in motor cortex for up to 30min after the train, but only at a small range of interstimulus intervals (6, 7ms). This implies that the train over premotor areas affected the excitability of intrinsic circuits in the motor cortex. Thus conditioning one part of the cortex with TMS can affect the way another cortical area processes incoming data. Tracking developmental change in the human motor system with TMS. Janet Eyre University of Newcastle Medical School Abstract not yet availablenot yet available Visual awareness studied by transcranial magnetic stimulation in blindsight and in retinal blindness Alan Cowey University of Oxford Subject G.Y. is a much studied hemianope whose left striate cortex is almost totally destroyed. He has blindsight (good visual detection and discrimination without visual awareness) for many visual stimuli presented within his blind-hemifield but sometimes reports faint conscious visual percepts especially for swiftly moving stimuli of high luminance contrast. To determine whether the latter reflect residual visual processing in extra-striate visual area MT/V5 we used repetitive transcranial magnetic stimulation (rTMS) over MT/V5 in his normal and his damaged hemisphere. On the normal side, "silvery swirling" visual phosphenes could be elicited by rTMS applied at an area over the position of MT/V5 as previously shown in this subject by fMRI.; but not by stimulating MT/V5 in the damaged hemisphere even following dense sampling of the adjacent regions. However, phosphenes were occasionally produced in the hemianopic field by TMS close to the midline which probably stimulated V3 of the damaged hemisphere. The size and position of the phosphenes was determined by asking the subject to fixate the centre of radial graph paper while rTMS was delivered and to mark the centre and edges of any phosphene on the paper. In contrast, phosphenes could be elicited in a peripherally blinded subject by stimulating MT/V5 in either hemisphere by rTMS. The phosphenes were achromatic and within 20 degree of the visual axis, the latter estimated by asking the blind subject to "fixate" the forefinger of one hand held at the centre of the graph paper while he used the other hand to mark any phosphene. We also studied the effects of rTMS in the blind subject when delivered over the sagittal midline. "Bright", often coloured, but stationary phosphenes were readily elicited with increasing eccentricity in the lower visual field the more rostral the rTMS. We conclude that even years after the eyes have been severed from the brain, striate cortex and visual area MT/V5 remain excitable and can generate visual percepts. But in the absence of V1, magnetic stimulation of area V5 on that side no longer yields conscious visual percepts, at least with the stimulation conditions we used. Unravelling movement and action with TMS Patrick Haggard Institute of Cognitive Neuroscience, University College London The information-processing underlying human action is characterised by a hierarchy, extending from very high-level intentions (e.g., "I want a drink") to the lowest level of motorneuronal firing and muscle contraction. The psychology of action has greatly suffered from an inability to control the input to, and to selectively interfere with, the higher and lower levels of control. TMS over the motor cortex can produce involuntary movements, which correspond in some ways to normal operation of the lower (movement) part of the hierarchy, without operation of the higher (action) part. As such, they provide an important method for psychologists wishing to individuate the two kinds of processing. I shall report a number of behavioural and psychophysical studies which have taken this approach. First, I shall report data showing that the conscious awareness of (TMSinduced) movement and the conscious awareness of intentional action differ fundamentally. Second, I shall report data on attempts to use TMS in a completely differen way, in which, rather than evoking movements at the lower level of the hieararchy, it is used to try to intervene on the higher processes in the hierarchy directly. Using TMS to study attention Matthew Rushworth University of Oxford Transcranial magnetic stimulation (TMS) can be used to transiently disrupt the normal patterns of neuronal activity in association cortex. While TMS is being applied , a brain area will be unable to function normally. TMS, like the investigation of permanent brain lesions, can provide evidence about whether a brain area is critical for a cognitive process. In addition TMS, because it only induces a very brief disruption, can be used to investigate when a brain area is playing its most critical role. In a recent experiment we used fMRI to identify areas of blood oxygen level dependent (BOLD) signal increase in the dorsomedial frontal cortex during two set switching tasks. One task required subjects to switch between two different attentional sets involving the allocation of attention to different stimulus features. In the second task subjects switched between two different intentional or response sets. Although there were BOLD signals increases in the dorsomedial frontal cortex in both experiments, TMS only disrupted performance in the second case of intentional set switching. Further experiments demonstrated that medial frontal TMS only disrupted task performance when it was delivered at the time of actual set-switching, not when it was delivered at the time that subjects selected individual task responses. Comparing the effects of medial frontal and dorsal premotor TMS revealed a temporal double dissociation of function. The time course of false recognition memory Evan Heit, Noellie Brockdorff and Koen Lamberts University of Warwick In the Deese/Roediger-McDermott false memory paradigm (e.g., Roediger & McDermott, 1995), after subjects study a list of inter-related words, they falsely recognise semantically similar lures nearly to the extent that they correctly recognise studied items. This result seems to persist even when subjects are forewarned about the nature of the illusion. In two experiments we examined the time course of false recognition judgments. Using an old-new recognition task, we elicited responses at different time lags from presentation of stimulus through a response-signal procedure. We investigated the effects of study list length and forewarning on the time course of false recognition judgments and examined the results in terms of changes in accuracy and changes in response bias over time. We report cha