Where does visual input become conscious? (Baars, hat tip to Prof. Alumit Ishai)
There are at least forty visual “maps” in the visual cortex — each sensitive to a different aspect of vision. The first map in visual cortex, conveniently dubbed V1, responds to a visual scene pixel-by-pixel. At higher levels, visual feature maps respond to color, motion, objects, faces, and entire scenes. These higher-level visual maps “live” outside of the occipital lobe, in the parietal and temporal lobes. The medial temporal lobe (MTL) is close to the “evolutionary root” of the cortex, from which that giant structure grew hundreds of millions of years ago. There we find object-sensitive, face-sensitive, and scene-sensitive visual maps, where single neurons can evoke whole scenes.
Conscious vision combines multiple maps into a single, integrated experience with color, motion, location, contrast, faces, emotional expressions, and more. To trigger all those maps to combine their information into a single, integrated experience it helps to stimulate cortex high up in the visual hierarchy. V1 only triggers a few layers. The Face Area or the Place Area (both in lower temporal cortex) trigger many other maps more easily.
Pierre Megevand and colleagues now report in the Journal of Neurosciences that direct electrical stimulation of the human PPA area (parahippocampal place area) can result in conscious glimpses of entire places. “Para” means “near,” so that “para-hippocampal” means “near the hippocampus.” But the hippocampus is wrapped like a sausage inside of the temporal lobe, which seems to emerge from the huge cortex like an arm emerging from the body. Because so many important structures reside inside of the “arm” of the temporal lobe, or even in the “armpit,” this is a confusing part of the brain to visualize. The metaphor of a sausage in a bun is not a bad first approximation. To understand the brain slices shown in this article, imagine that the “sausage-in-a-bun” is sliced to reveal the inside (the hippocampus). In this region the two temporal lobes also come together in one place, the medial temporal lobe (MTL). So the MTL is neocortex that wraps around hippocampus. (Which is actually the ancient ancestral cortex with only three layers).
Got that? Never mind. We’ll look at it later on. It’ll all one giant knotty structure, and you have to mentally fly around it many times before it becomes clear. (See the figures here and here).
OK. Now here’s the cool part. Direct low-voltage electrical stimulation in this “place area” of the visual cortex produces ***visual hallucinations of whole scenes and places.*** For example, a conscious surgical patient with implanted electrodes in this area experiences “specific topographic hallucinations: the patient reported seeing … the train station in his neighborhood, the stairs, a blue cabinet and the office of his optician.” In contrast to the “place” area, Electrical stimulation of the neighboring face zone, triggered “visual hallucinatioms with other content, such as his team of doctors and scientists who seemed to be present in the operating room, and Italians wearing aprons working at a pizza parlor. ”
Other sites of stimulation triggered a “sense of déjà vu” (“I’ve seen this before!”) but without the visual content. Thus an abstract “feeling of knowing” emerged from some very small and specific part of this region. Other nearby regions triggered NO particular conscious experience at all. This is not a simple part of the brain.
Experiments in human patients always have to worry about the fantastic adaptive capacities of the brain, especially after injury. But we can only study human patients if it is medically justified to do so — which means they have always experienced some brain injury, either local or very widespread. The brain is extremely dependent on a constant flow of oxygen and glucose, as well as physical protection against impact. Thus we never study a human brain directly with invasive methods that is exactly the way it was before injury. Fortunately we now have a dozen non-invasive methods like EEG and fMRI, which have created a whole new world of insight.
Notice that visual cortex is ALSO the place where we can sometimes elicit *CONSCIOUS* visual hallucinations and *CONSCIOUS* feelings of deja vu.
Also notice that people can stay conscious and able to communicate even during brain surgery, as long as they are given local anesthetic around the surgical incisions in the scalp. In this way brain surgery is much like a dental operation. Local anesthetic works well, because the brain itself does not have pain-sensitive receptors.
The pioneering work on conscious brain surgery was done by Wilder Penfield and colleagues in Montreal circa 1950. It has taken six decades for the operation to become a gold standard for both treatment and scientific research.