While it is clear that the brain creates our mind, the vast mechanism linking the two remains mysterious in many respects. The connection is attested by countless conditions and experiments, more or less tragic, but while the science has been progressing well, the brain is an inscrutible organ, and much remains to be done.
A recent paper describes coordinated fMRI and EEG experiments on humans to tell whether top-down attention uses the mechanism of brain waves to filter visual processing coming up from the sensory system, so that we can look at what we want. The attention system is very interesting and perhaps not commonly understood. The executive areas of the brain reach back into all the sensory areas and elsewhere, such that information processing is a two-way street. Signals from the senses do not just all flood into consciousness at the same time, but we attend to one or another, and that requires not only filtering against all the other streams of incoming data, but also management of the active stream, to focus on what we are "interested" in. Indeed, with no sensation at all, we can, though the attention process, imagine a sensory stream, which actually animates the sensory regions involved to create an attenuated, but physically accurate, stream of internally-generated "sensation" information.
The researchers here were interested in the correlation between mental activity, particularly in the visual areas, and brain wave activity. They were looking at alpha waves, (7-12 kHz), not the gamma wave activity that has been associated (loosely) with consciousness. Alpha waves are known to be associated with relaxed, non-processing, but ready, states. The primary example is the visual (occipital, rear) area of the brain, with eyes closed and the subject relaxed but awake. The waves then decline in power / coherence when eyes are opened and a visual scene is being actively processed. Other forms of the alpha wave are strong in specific areas and kinds of sleep.
"... several studies using transcranial magnetic stimulation and visual entrainment of 10-Hz activity have demonstrated that the alpha rhythm plays a causal role in the allocation of attention, perception, and working memory maintenance."
Not surprisingly, the researchers found an inverse correlation between alpha waves and the attention someone is paying in a part of a visual scene. This suggests that alpha waves are used to quiet the waters in unattending areas, rather than to synchronize or convey data, as has been thought for other wave patterns. The unattending areas are still working and processing their visual fields, but they are closed off from attention and consciousness, more or less.
|Visual processing pathway, showing how visual hemifields map to the respective occipital cortex hemispheres.|
The setup was to place human subjects in an MRI machine, with EEG electrodes attached as well. They were shown visual images that had two parts, a left side and a right side. The left side (coming from each eye) maps in the brain to a left "hemifield", which is processed in the right side of the brain. The researchers instructed the subjects to pay attention to one or the other side, though images were presented to both sides. The attention was enforced by asking/testing whether the image was the same or different than a previous one.
|Example of images presented to the subjects, where attention was directed to one or the other side (hemifield; arrows). This implies that the subject's eyes were kept straight ahead, focused on the center "X".|
So the subjects were staring at a full visual field, but spent a couple of seconds attending (mentally) to one or the other side, either a face or a landscape. The MRI showed increased activity (i.e. blood flow) on the corresponding side of the brain's visual processing area, the occipital lobe at the back. It also showed increased activity in the facial recognition area (for faces; fusiform face area) or in a place recognition area (for landscapes; parahippocampal place area).
But the alpha wave pattern in the EEG consistently showed lower power in the areas active above, and higher power in the corresponding areas in the other hemisphere- the hemisphere where attention was not being focused.
|Alpha wave power during directed visual attention, reflected by higher MRI signal on the R side, corresponding to attention to the Left visual hemifield. Alpha power is dramatically reduced as attention is being paid.|
|MRI signal simultaneous with the image above, with higher activity on the attending side (R).|
What do they make of all this? They note that "Our findings add support to the notion that alpha band activity serves an active role in the allocation of resources." But not exactly in the way I had expected. As a last project, they look at something called the dorsal attention network, which is thought to be a mechanism where the frontal executive areas reach back to focus attention. This set of regions show similarly inverse MRI correlation with the alpha wave power in the visual area.
"We have demonstrated that the suppression of unattended information is reflected by active inhibition via increased alpha band activity in the unattended visual stream. Our findings add support to the notion that alpha band activity serves an active role in the allocation of resources."
"We suggest that the mechanism of this gating by alpha activity is via phasic manipulation of higher frequency oscillations in the gamma band. In particular, the increase in alpha activity is important for the routing of information by depressing irrelevant processing of sustained stimulation, which is in stark contrast to the view that posterior alpha activity reflects idling or drowsiness."
Sadly, the work seems quite weak. They basically find that if one directs a person's attention to some item in a visual scene, the corresponding visual region in the brain shows dampened alpha wave activity, while the contralateral area in the other hemisphere shows heightened activity. That part was quite clear. The attention network result was not very strong. And the interpretation of all this is very unclear. Do some waves have something to do with attention and consciousness in some areas of the brain, while other waves at other frequencies and places relate to the opposite? That seems to be the lesson, which thus leaves us waiting for more work and insight into the matter.
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