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Frontal eye field modulates of visual cortical oscillations: an online TMS EEG study
Abstracts responses to small numbers are faster when the effector is in left space; responses to large numbers are faster with the effector in right space (SNARC effect). It is generally claimed that the spatial representation of number is function of the posterior parietal cortex (Hubbard et al 2005), however the representation of space and covert orienting to it can involve frontal brain areas (Corbetta & Shulman 2002). Moreover, different spatial frames of reference can be elicited depending on response arrangements (Wood, Nuerk & Willmes 2006). 5-Hz TMS was delivered in each trial at stimulus onset for 400 ms (110%MT) with 50-mm coils, while participants (Expt1:N 5 10, Expt2:N 5 8) performed magnitude comparison or parity judgment on central digits. Responses were given through left and right key-presses with the index and middle fingers ipsilateral to stimulation. SNARC effect was used to probe the spatial representation of numbers during rFEF and rIFg stimulation in Expt1, and lFEF and lIFg in Expt2. The baseline was collected with rTMS over vertex. A behavioural experiment [Expt3:N 5 24] was also conducted to control the fate of the SNARC effect when unimanual left responses are requested (as in Expt2). In magnitude comparison, the SNARC effect was completely eliminated by stimulating rIFg, whereas stimulating rFEF eliminated the SNARC effect for small numbers only, i.e. those in left representational space. In parity judgment, the SNARC effect survived stimulation at both sites. rTMS did not suppress core magnitude or parity representations, since overall performance was not affected by it in either task. In Expt2, only stimulation over lFEF interfered with the processing of contralateral numbers as indicated by the direction of the SNARC effect, regardless of the task. No effect was found during lIFg stimulation. Expt3 confirmed that when unimanual left responses are requested, the SNARC effect is not present in its standard form (as in Expt2). These results add two important qualifications to models of interactions between numbers and space. First, the spatial coding for numbers is not a purely parietal matter. Second, in contrast with the claim that a number stimulus always elicits the same representations, we support the claim that spatial-related effects can be strongly dependent on response arrangement.
tDCS Poster Only 242
Long-term potentiation and depotentiation by direct current in the hippocampal slice.
Rotenberg A1, Muller P1, Pascual-Leone A2, Jensen F1, 1Children’s Hospital, Harvard Medical School (Boston, US); 2Berenson-Allen Center for Noninvasive Brain Stimulation, Harvard Medical School (Boston, US) Objectives: The lasting changes in cortical excitability that are induced in vivo by anodal or cathodal transcranial direct current stimulation (tDCS) appear similar in character to long-term potentiation (LTP) and long-term depression (LTD) that are best seen with repetitive high frequency or low frequency electrical stimulation of the hippocampal slice. However, whether LTP- or LTD-like changes in synaptic strength can be induced by direct current stimulation (DCS) in vitro has not been extensively studied. Accordingly, to examine mechanisms underlying the in vivo effects of tDCS, we tested (1) whether LTP or LTD can be induced in the rat hippocampal slice by DCS, and (2) whether these in vitro changes are dependent on magnitude, duration and directionality of the stimulating direct electrical current. Methods: Extracellular CA1 field recordings were obtained by repetitive Schaffer collateral stimulation in hippocampal slices prepared from young (postnatal day 18-25) Long-Evans rats. Following 50-minute baseline recordings, DCS was applied with two metal electrodes placed outside of the slice adjacent to CA1 and to CA3. Stimulating conditions were designated as ‘‘anodal’’ when the anode DCS electrode was adjacent to CA1, and ‘‘cathodal’’ when the cathode was adjacent to CA1. Extracellular field recordings were continued for an additional
317 60 minutes after DCS. To test the effect of current amplitude and duration, cathodal DCS was delivered to three groups (n 5 3 slices per group) with the following settings: 75 uA for 30 minutes, 25 uA for 30 minutes, and 25 uA for 5 minutes. Additionally to test for effects of DCS current directionality, anodal DCS was delivered to one group (n 5 3 slices) at 25 uA for 30 minutes. Results: We found robust (P , 0.01) potentiation of the excitatory postsynaptic potential (EPSP) by anodal DCS in each group, and the magnitude of change in EPSP slope directly correlated with current amplitude and duration. In contrast, modest but significant (P , 0.01) depotentiation was produced by 30 minutes of anodal DCS. Conclusion: We demonstrate the capacity of in vitro DCS to produce lasting effects closely resembling those of classic LTP and LTD. Concordant with observations in vivo, the effects in the hippocampal slice are dependent on direction, magnitude and duration of DCS. Our findings support the notion that LTP- and LTD-like mechanisms are involved in the physiologic changes induced by tDCS.
TMS Poster Only 243
Frontal eye field modulates of visual cortical oscillations: an online TMS EEG study
Hoogenboom N, Lauder J, Grosbras MH, Centre for Cognitive Neuroimaging (Glasgow, UK) Objective: A single pulse of transcranial magnetic stimulation (TMS) applied over the frontal eye field (FEF) can facilitate the subsequent detection of near-threshold stimuli (Grosbras and Paus, 2003) or can increase the excitability of other visual regions (Silvanto et al., 2005). This is consistent with a role of FEF in attention control. How FEF TMS influences cortical processes remains elusive, however. Attention enhances P1 and N1 components of visual evoked potentials and diminishes oscillatory synchronization in the alpha frequency range (8-12 Hz). Taylor et al. (2006) have shown that repetitive TMS applied over the FEF can perturb the attentional modulation of the visual evoked potentials. In the present experiment we aim to combine electroencephalography (EEG) and single pulse TMS to explore the timing of the modulation of EEG as well as changes in the oscillatory activity. Method: Three subjects viewed small gratings briefly (40ms) flashed in their lower left or right visual field while we recorded EEG. In some trials we applied a single pulse of TMS over the right FEF or a control site (vertex) at one of 3 latencies relative to stimulus onset: -100, 0 and 50ms. In some trials we applied TMS without any visual stimulation. We excluded from the analyzes the 30ms during which TMS artifact occurred. We estimated the power in all frequencies between 2 and 40Hz during the 500ms after visual stimulus onset. Result: In these three subjects TMS reduced the power of the parietooccipital alpha synchronization. When there was no visual stimulation the decrease in alpha power was larger for the FEF TMS than for the vertex TMS, and for the right hemisphere (ipsilateral to TMS). This suggests that, beyond the non-specific, alerting, effect of TMS, FEF stimulation can change the background activity of visual areas. When a visual stimulus was present, the only condition showing a greater decrease in alpha synchronization for FEF TMS compared to vertex TMS, was when TMS was applied 100ms before the onset of a left (contralateral) visual stimulus. Conclusion: The TMS reduction in alpha synchronization in visual cortex during background activity is reminiscent of similar desynchronization during attention orienting and in line with a role of FEF in top-down control of visual perception. The reduction in alpha when a pulse is applied 100ms before visual onset could provide a mechanism for the visual detection facilitation observed in earlier studies.
tDCS Poster Only 242
Long-term potentiation and depotentiation by direct current in the hippocampal slice.
Rotenberg A1, Muller P1, Pascual-Leone A2, Jensen F1, 1Children’s Hospital, Harvard Medical School (Boston, US); 2Berenson-Allen Center for Noninvasive Brain Stimulation, Harvard Medical School (Boston, US) Objectives: The lasting changes in cortical excitability that are induced in vivo by anodal or cathodal transcranial direct current stimulation (tDCS) appear similar in character to long-term potentiation (LTP) and long-term depression (LTD) that are best seen with repetitive high frequency or low frequency electrical stimulation of the hippocampal slice. However, whether LTP- or LTD-like changes in synaptic strength can be induced by direct current stimulation (DCS) in vitro has not been extensively studied. Accordingly, to examine mechanisms underlying the in vivo effects of tDCS, we tested (1) whether LTP or LTD can be induced in the rat hippocampal slice by DCS, and (2) whether these in vitro changes are dependent on magnitude, duration and directionality of the stimulating direct electrical current. Methods: Extracellular CA1 field recordings were obtained by repetitive Schaffer collateral stimulation in hippocampal slices prepared from young (postnatal day 18-25) Long-Evans rats. Following 50-minute baseline recordings, DCS was applied with two metal electrodes placed outside of the slice adjacent to CA1 and to CA3. Stimulating conditions were designated as ‘‘anodal’’ when the anode DCS electrode was adjacent to CA1, and ‘‘cathodal’’ when the cathode was adjacent to CA1. Extracellular field recordings were continued for an additional
317 60 minutes after DCS. To test the effect of current amplitude and duration, cathodal DCS was delivered to three groups (n 5 3 slices per group) with the following settings: 75 uA for 30 minutes, 25 uA for 30 minutes, and 25 uA for 5 minutes. Additionally to test for effects of DCS current directionality, anodal DCS was delivered to one group (n 5 3 slices) at 25 uA for 30 minutes. Results: We found robust (P , 0.01) potentiation of the excitatory postsynaptic potential (EPSP) by anodal DCS in each group, and the magnitude of change in EPSP slope directly correlated with current amplitude and duration. In contrast, modest but significant (P , 0.01) depotentiation was produced by 30 minutes of anodal DCS. Conclusion: We demonstrate the capacity of in vitro DCS to produce lasting effects closely resembling those of classic LTP and LTD. Concordant with observations in vivo, the effects in the hippocampal slice are dependent on direction, magnitude and duration of DCS. Our findings support the notion that LTP- and LTD-like mechanisms are involved in the physiologic changes induced by tDCS.
TMS Poster Only 243
Frontal eye field modulates of visual cortical oscillations: an online TMS EEG study
Hoogenboom N, Lauder J, Grosbras MH, Centre for Cognitive Neuroimaging (Glasgow, UK) Objective: A single pulse of transcranial magnetic stimulation (TMS) applied over the frontal eye field (FEF) can facilitate the subsequent detection of near-threshold stimuli (Grosbras and Paus, 2003) or can increase the excitability of other visual regions (Silvanto et al., 2005). This is consistent with a role of FEF in attention control. How FEF TMS influences cortical processes remains elusive, however. Attention enhances P1 and N1 components of visual evoked potentials and diminishes oscillatory synchronization in the alpha frequency range (8-12 Hz). Taylor et al. (2006) have shown that repetitive TMS applied over the FEF can perturb the attentional modulation of the visual evoked potentials. In the present experiment we aim to combine electroencephalography (EEG) and single pulse TMS to explore the timing of the modulation of EEG as well as changes in the oscillatory activity. Method: Three subjects viewed small gratings briefly (40ms) flashed in their lower left or right visual field while we recorded EEG. In some trials we applied a single pulse of TMS over the right FEF or a control site (vertex) at one of 3 latencies relative to stimulus onset: -100, 0 and 50ms. In some trials we applied TMS without any visual stimulation. We excluded from the analyzes the 30ms during which TMS artifact occurred. We estimated the power in all frequencies between 2 and 40Hz during the 500ms after visual stimulus onset. Result: In these three subjects TMS reduced the power of the parietooccipital alpha synchronization. When there was no visual stimulation the decrease in alpha power was larger for the FEF TMS than for the vertex TMS, and for the right hemisphere (ipsilateral to TMS). This suggests that, beyond the non-specific, alerting, effect of TMS, FEF stimulation can change the background activity of visual areas. When a visual stimulus was present, the only condition showing a greater decrease in alpha synchronization for FEF TMS compared to vertex TMS, was when TMS was applied 100ms before the onset of a left (contralateral) visual stimulus. Conclusion: The TMS reduction in alpha synchronization in visual cortex during background activity is reminiscent of similar desynchronization during attention orienting and in line with a role of FEF in top-down control of visual perception. The reduction in alpha when a pulse is applied 100ms before visual onset could provide a mechanism for the visual detection facilitation observed in earlier studies.