Warning: neuroscience weeds.
Every so often we get into discussions about where in the brain consciousness lies. Sometimes it’s asserted to be in the brainstem, other times in the thalamus, sometimes in the parietal lobe, and yet other times in the prefrontal cortex. Myself, I’ve concluded that conscious perception requires activation of a network including sensory cortical areas, the posterior association cortex, and regions in the frontal lobes. In other words, a number of areas in the thalamo-cortical system are necessary.
One interesting data point for this discussion has been the phenomenon of blindsight. Sometimes people sustain brain injuries in the visual cortex in the Occipital Lobe. Their eyes are physically fine, but the cortical region in their brain that processes vision is damaged, so their ability to consciously perceive what the eyes are seeing is reduced or eliminated.
However, often people with this condition, if forced to make a decision about whether something is in front of them, are able to to do so with a success rate significantly higher than chance. Such people are also often able to perceive emotions on people’s faces, even if they have no conscious perception of what they’re seeing. In essence, they seem to have only a vague feeling about what their eyes are seeing.
There have been a number of theories about how this happens, but the one most cited proposes that there are alternate pathways for visual information to reach the patient’s executive centers in their frontal lobes. The majority of the axons in the optic nerve go to the thalamus and then to the visual cortex, but about 10% go to a region in the upper brainstem or midbrain region called the superior colliculus.
The visual processing that happens in this region is interesting because we have no conscious access to it. It’s like a subterranean perception area. None of the axons from the color sensitive cones cells project there, so the sensory images that form there are low resolution (blurry) and colorless. These images typically drive reflexive eye movements including saccades, low level attention impulses, and other functions.
The alternate pathway theory assumes that signals from this region somehow reach the executive centers of the brain, allowing the accurate but “blind” guesses.
A new study by Australian researchers using fMRI has confirmed the existence of this alternate pathway. It appears to go from the superior colliculus, to the pulvinar region in the thalamus, and from there to the amygdala, which communicates emotions to the executive center. This is how the feeling of something in front of the person reaches their conscious perception without the visual part of the perception.
This seems like additional evidence that conscious perception is a cortical phenomenon. If it were in the upper brainstem, we would likely have at least some conscious access to the visual images there. And since the alternate pathway goes through the thalamus, that also seems to rule out that region as a seat for consciousness. (At least by itself, since the thalamus is a signaling hub that the cortical regions use to communicate with each other.)
Of course, alternate explanations are always possible, but we have to keep Occam’s razor in mind.
Unless of course I’m missing something?