Elena Rusconi

The use of transcranial magnetic stimulation in cognitive neuroscience: A new synthesis of methodological issues

Transcranial magnetic stimulation (TMS) has become a mainstay of cognitive neuroscience, thus facing new challenges due to its widespread application on behaviorally silent areas. In this review we will summarize the main technical and methodological considerations that are necessary when using TMS in cognitive neuroscience, based on a corpus of studies and technical improvements that has become available in most recent years. Although TMS has been applied only relatively recently on a large scale to the study of higher functions, a range of protocols that elucidate how this technique can be used to investigate a variety of issues is already available, such as single pulse, paired pulse, dual-site, repetitive and theta burst TMS. Finally, we will touch on recent promising approaches that provide powerful new insights about causal interactions among brain regions (i.e., TMS with other neuroimaging techniques) and will enable researchers to enhance the functional resolution of TMS (i.e., state-dependent TMS). We will end by briefly summarizing and discussing the implications of the newest safety guidelines.

Dexterity with numbers: rTMS over left angular gyrus disrupts finger gnosis and number processing.

Since the original description of Gerstmanns syndrome with its four cardinal symptoms, among which are finger agnosia and acalculia, the neuro-cognitive relationship between fingers and calculation has been debated. We asked our participants to perform four different tasks, two of which involved fingers and the other two involving numbers, during repetitive transcranial magnetic stimulation (rTMS) over the posterior parietal lobe of either hemisphere. In the finger tasks, they were required to transform a tactile stimulus randomly delivered on one of their fingers into a speeded key-press response either with the same or with the homologous finger on the opposite hand. In the numerical tasks, they were asked to perform a magnitude or a parity matching on pairs of single digits, in the context of arithmetically related or unrelated numerical primes. In accordance with the original anatomical hypothesis put forward by Gerstmann [Gerstmann, J. (1924). Fingeragnosie: eine umschriebene Stoerung der Orienterung am eigenen Koerper. Wiener clinische Wochenschrift, 37, 1010-12], we found that rTMS over the left angular gyrus disrupted tasks requiring access to the finger schema and number magnitude processing in the same group of participants. In addition to the numerous studies which have employed special populations such as neurological patients and children, our data confirm the presence of a relationship between numbers and body knowledge in skilled adults who no longer use their fingers for solving simple arithmetical tasks.

Number magnitude orients attention, but not against one's will

Recent evidence has shown that uninformative numbers can trigger attention shifts congruent with the spatial representation of number magnitude (Fischer, Castel, Dodd, & Pratt, 2003). In the present study, three spatial-cuing experiments whose aim was to qualify the automaticity of this numbermediated orienting are described. Experiment 1 replicated the phenomenon, showing that uninformative numbers can evoke orienting in a simple detection task. In Experiment 2, target location was random, but the participants were encouraged to shift attention to the left in response to large numbers and to the right in response to small numbers. No evidence for strong automaticity was observed, since the participants’ performance was better when left-side targets were preceded by large numbers than when they were preceded by small numbers and vice versa. Experiment 3 corroborated this pattern by comparing gaze- and number-mediated cuing under conditions of real counterpredictiveness. The results indicate that unlike gaze-driven orienting, number-mediated orienting is not obligatory.

A disconnection account of Gerstmann syndrome: Functional neuroanatomy evidence†

To examine the functional neuroanatomy that could account for pure Gerstmann syndrome, which is the selective association of acalculia, finger agnosia, left‐right disorientation, and agraphia.

Two orienting mechanisms in posterior parietal lobule: An rTMS study of the Simon and SNARC effects

The aim of the present study was to investigate, with repetitive transcranial magnetic stimulation (rTMS), the involvement of posterior parietal lobule (PPL) circuits in producing the Simon effect and the spatial-numerical association of response codes (SNARC) effect. The Simon effect is indexed by faster responses to left-side stimuli with left- than with right-side key-presses and faster responses to right-side stimuli with right- than with left-side key-presses. The SNARC effect is indexed by faster responses to smaller numbers with left- than with right-side key-presses and faster responses to larger numbers with right- than with left-side key-presses. Participants were required to perform a parity judgement on numbers ranging from 1 to 9 (without 5), by pressing a left or a right response key. The numbers were presented to either the left or the right side of fixation. Two bilateral PPL areas lying along the intraparietal sulcus (IPS) were stimulated with rTMS. Results suggested a causal role of the anterior portion of PPL of both hemispheres in the Simon effect and of the posterior portion of PPL of both hemispheres in the Simon effect and the SNARC effect.

A brain for numbers

Healthy human brains come equipped with several circuits that contribute to number processing. Nature and nurture interact to produce a unique combination of core skills and more sophisticated abilities, by building on a handful of auxiliary routes (e.g., verbal language, body knowledge and visuospatial attention). Transcranial magnetic stimulation (TMS) studies on number processing will be here succinctly reviewed, in light of their most stimulating and challenging contributions. New research directions will be pointed out, that might enhance their theoretical impact.

Unimanual SNARC effect: hand matters

A structural representation of the hand embedding information about the identity and relative position of fingers is necessary to counting routines. It may also support associations between numbers and allocentric spatial codes that predictably interact with other known numerical spatial representations, such as the mental number line (MNL). In this study, 48 Western participants whose typical counting routine proceeded from thumb-to-little on both hands performed magnitude and parity binary judgments. Response keys were pressed either with the right index and middle fingers or with the left index and middle fingers in separate blocks. 24 participants responded with either hands in prone posture (i.e., palm down) and 24 participants responded with either hands in supine (i.e., palm up) posture. When hands were in prone posture, the counting direction of the left hand conflicted with the direction of the left–right MNL, whereas the counting direction of the right hand was consistent with it. When hands were in supine posture, the opposite was true. If systematic associations existed between relative number magnitude and an allocentric spatial representation of the finger series within each hand, as predicted on the basis of counting habits, interactions would be expected between hand posture and a unimanual version of the spatial–numerical association of response codes (SNARC) effect. Data revealed that with hands in prone posture a unimanual SNARC effect was present for the right hand, and with hands in supine posture a unimanual SNARC effect was present for the left hand. We propose that a posture-invariant body structural representation of the finger series provides a relevant frame of reference, a within-hand directional vector, that is associated to simple number processing. Such frame of reference can significantly interact with stimulus–response correspondence effects, like the SNARC, that have been typically attributed to the mapping of numbers on a left-to-right mental line.

Arithmetic priming from neglected numbers

Patient AM, with left visuospatial neglect, and 31 healthy participants performed a parity judgment task on numbers presented to their right parafovea. Target numbers were preceded by a pair of digits (prime) appearing peripherally for 100 ms either in their left (LVF) or in their right visual field (RVF), which participants had to ignore. In healthy participants, when primes were arithmetically related to the following target, performance was significantly slower than when primes and targets were not related. In contrast, AMs performance was slower in the related than in the control condition when prime digits appeared in his RVF, whereas it was faster in the related than in the control condition when prime digits appeared in his LVF. This suggests that neglected numbers were nevertheless processed at least until the level of stored arithmetic knowledge. Thus, visuospatial neglect does not prevent neglected numbers from accessing their representations in arithmetic networks, which seems to be a highly automatised skill. Moreover, AMs pattern of data (i.e., interference from RVF primes vs. facilitation from LVF primes) supports the hypothesis of a link between conscious attention and inhibitory processes, as proposed by Fuentes and Humphreys (1996).

The mental space of pitch height.

Abstract: Through stimulus‐response compatibility we tested whether sound frequency (pitch height) elicits a mental spatial representation. Musically untrained and, mostly, trained participants were shown a stimulus‐response compatibility effect (Spatial‐Musical Association of Response Codes or SMARC effect). When response alternatives were either vertically or horizontally aligned, performance was better when the lower (or leftward) button had to be pressed in response to a low sound and the upper (or rightward) button had to be pressed in response to a high sound, even when pitch height was irrelevant to the task.

Singing Numbers… in Cognitive Space — A Dual‐Task Study of the Link Between Pitch, Space, and Numbers

We assessed the automaticity of spatial-numerical and spatial-musical associations by testing their intentionality and load sensitivity in a dual-task paradigm. In separate sessions, 16 healthy adults performed magnitude and pitch comparisons on sung numbers with variable pitch. Stimuli and response alternatives were identical, but the relevant stimulus attribute (pitch or number) differed between tasks. Concomitant tasks required retention of either color or location information. Results show that spatial associations of both magnitude and pitch are load sensitive and that the spatial association for pitch is more powerful than that for magnitude. These findings argue against the automaticity of spatial mappings in either stimulus dimension.