State-dependent variability of neuronal responses to transcranial magnetic stimulation of the visual cortex

Authors:

• Brian Pasley

• Elena A. Allen

• Ralph D. Freeman

Date: 2009

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Abstract:

Electrical brain stimulation is a promising tool for both experimental and clinical applications. However, the effects of stimulation on neuronal activity are highly variable and poorly understood. To investigate the basis of this variability, we performed extracellular recordings in the visualcortex following application of transcranialmagneticstimulation (TMS). Our measurements of spiking and local field potential activity exhibit two types of response patterns which are characterized by the presence or absence of spontaneous discharge following stimulation. This variability can be partially explained by state-dependent effects, in which higher pre-TMS activity predicts larger post-TMS responses. These results reveal the possibility that variability in the neural response to TMS can be exploited to optimize the effects of stimulation. It is conceivable that this feature could be utilized in real time during the treatment of clinical disorders.

Transcranial magnetic stimulation elicits coupled neural and hemodynamic consequences

Authors:

• Brian Pasley

• Elena A. Allen

• T. Duong

• Ralph D. Freeman

Date: 2007

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Abstract:

Transcranial magnetic stimulation (TMS) is an increasingly common technique used to selectively modify neural processing. However, application of TMS is limited by uncertainty concerning its physiological effects. We applied TMS to the cat visual cortex and evaluated the neural and hemodynamic consequences. Short TMS pulse trains elicited initial activation (~1 minute) and prolonged suppression (5 to 10 minutes) of neural responses. Furthermore, TMS disrupted the temporal structure of activity by altering phase relationships between neural signals. Despite the complexity of this response, neural changes were faithfully reflected in hemodynamic signals; quantitative coupling was present over a range of stimulation parameters. These results demonstrate long-lasting neural responses to TMS and support the use of hemodynamic-based neuroimaging to effectively monitor these changes over time.

Analysis of oxygen metabolism implies a neural origin for the negative BOLD response in human visual cortex

Authors:

• Brian Pasley

• B. A. Inglis

• Ralph D. Freeman

Date: 2007

PubMed: 17113313

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Abstract:

The sustained negative blood oxygenation level-dependent (BOLD) response in functional MRI is observed universally, but its interpretation is controversial. The origin of the negative response is of fundamental importance because it could provide a measurement of neural deactivation. However, a substantial component of the negative response may be due to a non-neural hemodynamic artifact. To distinguish these possibilities, we have measured evoked BOLD, cerebral blood flow (CBF), and oxygen metabolism responses to a fixed visual stimulus from two different baseline conditions. One is a normal resting baseline, and the other is a lower baseline induced by a sustained negative response. For both baseline conditions, CBF and oxygen metabolism responses reach the same peak amplitude. Consequently, evoked responses from the negative baseline are larger than those from the resting baseline. The larger metabolic response from negative baseline presumably reflects a greater neural response that is required to reach the same peak amplitude as that from resting baseline. Furthermore, the ratio of CBF to oxygen metabolism remains approximately the same from both baseline states (approximately 2:1). This tight coupling between hemodynamic and metabolic components implies that the magnitude of any hemodynamic artifact is inconsequential. We conclude that the negative response is a functionally significant index of neural deactivation in early visual cortex.