Ok, let's see if I can post this before I fall asleep (which I hope to do soon, since I've got wicked insomnia lately)....
Been reading "Visual Awareness" by Geraint Rees in
The Cognitive Neurosciences.
He starts out essentially paralleling Koch, but then takes a wicked little left turn that's really fascinating, and I hope perhaps others will have relevant cites (if I haven't ticked off all other potential posters by now).
Maybe I'll get to that plot twist tonight, maybe I won't....
Rees agrees with Koch, Gazzaniga, and others that "a significant body of brain-imaging studies now shows that many regions of human visual cortex can be activated by visual stimuli that do not reach [conscious] awareness".
In other words, we're beginning to distinguish between the brain real-estate that's responsible for perception versus the real-estate responsible for conscious awareness.
For example, when a simple achromatic disc is flashed briefly, activity is elicited in the corresponding retinotopic location of primary visual cortex (V1) even when the target is rendered completely invisible by a surrounding mask. This activation of visual cortex by stimuli that cannot be accurately reported continues to higher stages of processing. For example, activation of functionally specialized areas is consistently observed for masked words, faces, and objects that observers do not report seeing....
Such observations are not restricted to different types of visual masking, as unconscious activation of the ventral visual pathway during the "attentional blink" can reflect object category and semantic analysis of visually presented words.
The "attentional blink" is a kind of blind spot produced by exposure to rapid sequences of stimuli. Essentially, if you get a quick exposure to a sight or a sound while your brain is busy processing one that came just before it, you won't be able to consciously perceive it. It's kind of the flip side of the masking effect in which the imposition of a consciously perceptible sight or sound very rapidly after an earlier one prevents you from being aware of the first.
In the former case (the "attentional blink") the attention you're giving to the first image or sound occupies your brain activity, preventing you from being consciously aware of something that you would have been consciously aware of if you'd been exposed to it in isolation. It's a kind of mental distraction.
In the latter case (the backward masking) the brief stimulus comes first, but it's not given enough time to reverberate into conscious awareness because another sound or image of longer duration follows it and swamps it out.
But in both cases, some parts of the brain are indeed processing the masked or blinked input and can recall and use that information. These may be earlier or later in the impulse chain.
What Rees is saying here is that even though we're not consciously aware of images we see during an attentional blink, parts of our brains process it in the same way they process the stuff we are aware of seeing.
Not only that, but words that we're exposed to during the blink are handled by higher-level language processing areas of the brain even though we're not aware of seeing the words.
So it would seem that we're getting closer to distinguishing the brain real-estate responsible for conscious awareness from the other areas, as we saw in Koch's discussion of binocular optical illusions. (But like I said, there's a twist on the way.)
Similarly, visual motion rendered invisible through "crowding" can still activate V5/MT. Unconscious activation also extends to cortial areas considered part of the dorsal stream of visual processing; images of tools rendered invisible by continuous flash suppression nevertheless activate the human dorsal stream.
Such neuroimaging evidence converges with electrophysiological studies showing that electrical potential associated with higher levels of visual procesing can be elicited by invisible stimuli.
Things here are already more blurry than the binocular illusion study, since we've got higher-level processing involved without any reportable conscious percept.
Taken together, they suggest that under conditions that render stimuli subjectively or objectively invisble, a substantial degree of processing continues in visual cortex, including higher stages of visual processing.
Rees also mentions studies that parallel the studies Gazzaniga mentions regarding interpretation of emotions from facial expressions by people with certain types of blindness.
Subcortical pathways can also show activation in respoinse to subjectively invisible and unreported emotional visual stimuli. For example, the amygdala responds selectively to fearful faces under conditions of masking and binocular suppression, even when observers are unable to report their presence.
In fact, researchers can determine which scene a subject is viewing by observing the subject's brain activity, even when the subject has no idea what s/he's looking at.
Multivox pattern analysis can successfully predict which one of two oriented stimuli a participant is viewing, even when masking renders that stimulus invisble.
And it's not just a matter of immediate perception. People will show changes in behavior depending on "priming" by sights and sounds they're never aware of perceiving. Or as Rees puts it, these stimuli can "elicit considerable processing outside [conscious] awareness, and this can influence subsequent behavior".
I'm reminded of a study in which subliminal images of animals made test subjects measurably more likely to think they'd seen those images before, even though they never reported seeing them in the first place.
Ok, but here's the weird part.... Rees describes studies in which the
percept appears to influence the activity of brain real estate earlier in the chain! Now, this is bizarre, and if anybody has any related research to report, I'd really be interested in it.
In V1, retinotopic activity can reflect conscious perception of illusory features. When a moving grating is divided by a large gap, observers report seeing a moving "phrantom" in the gap, and there is enhanced activity in the locations in early retinotopic visual cortex corresponding to the illusory percept. Moreover, when phantom-inducing gratings are paired with competing stimuli that induce binocular rivaly, spontaneous fluctuations in conscious perception of the phantom occur together with changes in early visual activity. Similarly, V1 activation can be found on the path of apparent motion and is associated with strengthened feedback connections to that retinotopic location from cortical area V5/MT. Finally, when two objects subtending identical angles in the visual field are made to appear of different sizes using three-dimentional context, the spatial extent of activation in the V1 retinotopic map reflets the perceived rather than actual angular size of the objects.
Rees even cites studies showing that "responses in human V1 can... be altered by sound, and can reflect subjective perception rather than the physically present visual stimulus".
So is this perhaps further support for global workspace theory? Does it demonstrate that human consciousness does indeed involve reverberations in extended neuronal loops across levels of neocortex?
