Complementary Techniques Help Probe Brain Networks

by Jim Schnabel

March 23, 2009

Researchers in Japan have reported combining transcranial magnetic stimulation (TMS) with electroencephalography (EEG) to verify an important visual-attention pathway in the brain. The study, published in December in Nature Neuroscience, is noteworthy for its illumination of how the prefrontal cortex influences activity elsewhere in the brain, but it also suggests how useful the combination technique can be.

“The combined TMS/EEG method used here is just starting to take off,” says Paul Taylor, a cognitive neuroscientist at the University of London who has published work in this area but was not involved with this study. “Combining stimulating with recording means you can start asking questions such as, ‘How are these brain areas communicating with each other?’ ”

The Japanese study was meant to confirm something that researchers have long assumed:  When visual attention is directed to an image, signals from attentional areas at the front of the brain run to the back of the brain, to the visual cortex, where they influence activity in the visual processing areas relevant for that image.  Previous studies have come up with results consistent with this idea, but largely due to limitations in brain-imaging technology, none has really shown that prefrontal attention areas control visual cortex activity in a task-specific manner.

 In the study, Yosuke Morishima and his colleagues in the University of Tokyo laboratory of Katsuyuki Sakai asked 15 student subjects to watch a visual image with two components, each requiring processing in a distinct area of the visual cortex.  One component was a human face; the other, overlaying it, was a moving pattern of darker and brighter lines known as a luminance grating.  At a cue the subjects were meant to focus either on the grating, to determine whether it was moving right or left, or on the face, to determine whether it was male or female.

 For half of these task intervals, a TMS device over the subject’s forehead created a brief, mild pulse of activity in the “frontal eye field” of the prefrontal cortex, where signals that guide visual attention are believed to originate.  The researchers’ hypothesis was that a subject’s attention to the face, for example, would effectively open the attentional pathway to the face-recognition area, so that the TMS signal, riding that pathway, would be transmitted more strongly to that area (as measured with scalp-mounted EEG electrodes).  Similarly, the subject’s shift of attention to the moving grating would, in turn, shift the attentional pathway to the motion-processing areas of the visual cortex, and the TMS signal would become more measureable there.

Essentially that is what Morishima and his colleagues found:  When subjects attended to the grating, the EEG-measured “hotspot” of the TMS signal moved over the motion-processing area. When subjects attended to the face, the hotspot shifted to the face-processing area.  The timing of the EEG-measured signals suggested that the TMS signal had come directly from the frontal eye field.  And when the researchers increased the strength of the TMS signals to moderately disrupt frontal eye field neurons, the subjects’ performance on the visual discrimination task worsened.

 “Previous studies have shown that activation originating from prefrontal regions modulates neural activity in the human visual cortex, consistent with proposals that such top-down modulation is important for cognitive control,” says Juha Silvanto, a cognitive neuroscientist at the University of Essex who also has done work in this area. “The work by Morishima and colleagues provides direct evidence for this view by demonstrating the behavioral significance of the prefrontal top-down modulation.”

The study also highlights the promise of the TMS/EEG technique, and others like it, as a basic set of tools for probing neural pathways. “You can also combine TMS with brain scanning methods such as positron emission tomography [PET] or functional magnetic resonance imaging [fMRI], and all of these enable you to study cortical interactions noninvasively in people,” Taylor says.

Taylor notes that EEG, despite being a relatively old technology, is much better than PET or fMRI at measuring the precise timing of signals and thus has more power to illuminate causal relationships among events in the brain: “This is an important issue not only in the attentional selection of visual stimuli but also in other types of attention and decision making.”