Lecture 5: access consciousness
— "seeing" vs. "seeing as", revisited
— checks and balances
— access is key
— the neural correlates of consciousness
— the last word on qualia
— "seeing" vs. "seeing as", revisited
— checks and balances
— access is key
— the neural correlates of consciousness
— the last word on qualia
A close-up photograph of a common bay
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Unless you see some of the boulders here
To be seen
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A concept-rich system that is capable of "seeing as" is still prone
to a certain kind of occasional (at best) or endemic (at worst)
stupidity — the kind that stems from the lack of
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Popperian creatures make a big advance by asking themselves, "What should I think about next?" before they ask themselves, "What should I do next?"
Gregorian creatures take a further big step by learning how to think better about what they should think about next — and so forth, a tower of further internal reflections with no fixed or discernible limit.
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When the to be changed item is cued in advance (b), subjects perform almost 100% correct.
However, when subjects are cued after the disappearance of Stimulus 1 but before the onset of Stimulus 2 (c), they perform almost as well.
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Four models of visual awareness and its relation to attention.
According to (d), CB and IB are not necessarily failures of consciousness, but of conscious memory. In other words, we are 'conscious' of many inputs but, without attention, this conscious experience cannot be reported and is quickly erased and forgotten.
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Attentional selection is a convolution of memory and processing. Selection is necessary when two stimuli (labelled A, B) reach the brain but only one response is possible. Competition, typically at the level of the extrastriate areas, prevents all inputs from reaching output areas of the brain.
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Left: a visual stimulus activates the primary cortical area for vision in the occipital cortex.
Middle: activity spreads to higher-order visual areas in the parietal and temporal lobes.
Right: recurrent interaction between these areas results in a widespread pattern of activity that corresponds to the animal becoming able to report perceiving the stimulus.
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Phenomenal versus access awareness: interaction between recurrent processing and mechanisms of attentional selection.
Competition between the neural representations of multiple stimuli (A, B) can prevent the feedforward transfer from V1 to the executive areas of all but a few stimuli (in this case A). At lower levels, however, simultaneous representations (of both A and B) might exist.
Feedforward activation (green dots), both of selected (i.e. attended) and non-selected inputs, is unconscious, even though it might trigger or modify behavior.
Meanwhile, neurons in activated regions start to engage in recurrent interactions, which are accompanied by increased activity or synchronous firing (yellow dots). This produces phenomenal awareness of the visual inputs (and iconic memory after removal of the stimulus).
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Visual discrimination task with metacontrast masking (Lau and Passingham, 2006).
The stimuli were presented on a black background. The mask overlaps with part of the contour of the target without leaving gaps or overlapping with the target spatially.
After the presentation of the target and the mask, the participants were first asked to decide whether a diamond or a square was presented. Then they had to indicate whether they actually saw the target or whether they simply guessed the answer.
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Behavioral (upper) and fMRI (lower) results showing a dissociation between awareness and performance in the metacontrast discrimination task.
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Activity in the mid-DLPFC reflects visual consciousness (long SOA > short SOA). The activity in this area is higher in the long SOA condition than in the short SOA condition, despite the fact that the two conditions did not yield different discrimination accuracy. There were, however, more trials during which the stimuli were classified by the participants as consciously seen in the long SOA condition than in the short SOA condition.
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In this condition, one of the two stimuli is usually suppressed; sometimes the two are combined in a patchwork fashion, with different pieces coming from the different images.
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Right amygdala neuron that follows the subjective percept.
(a) Stimulus presentation: an image is presented monocularly for 1,000 ms. The same image then is flashed onto the same ipsilateral eye while a different image is flashed to the contralateral eye for 500 ms. (Left) A picture of Clinton is presented monocularly, and a black and white pattern is later flashed onto the other eye. (Right) The pattern is presented monocularly, and Clinton is flashed to the contralateral eye.
(b) Subjective percept. (Left) The picture of Clinton is suppressed during the binocular period by the pattern. (Right) The picture of Clinton perceptually suppresses the pattern.
(c) This neuron responded selectively to Clinton among 49 different stimuli. (Left) Clinton shown first; (Right) ineffective stimulus shown first.
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Right amygdala neuron that follows the subjective percept.
(d-f) Responses of the same neuron to a different image of Clinton.
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Responses of a neuron in the right amygdala that showed a selective response to four different faces from a set of 42 different stimuli. For two of those stimuli, we did not have a sufficient number of presentations during the binocular period. We therefore show the average response only to the other two stimuli (a photograph of Paul McCartney and one of the Ekman faces).
(a) Responses in those cases where the effective stimuli were shown monocularly and perceptually suppressed during the flash by an ineffective stimulus.
(b) Responses during those trials in which an ineffective stimulus was shown monocularly and the effective stimuli were flashed. The cell responded if and only if the effective stimulus was perceived subjectively.
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n is the total number of units recorded in each location.
Amy, amygdala;
Hipp, hippocampus;
EC, entorhinal cortex;
PHG, parahippocampal gyrus.
"Category selective" indicates the units that were category-selective during the monocular presentation and were tested during the flash presentation with a sufficient number of repetitions and stimuli.
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In one study, Kupers et al. (2006) used transcranial magnetic stimulation (TMS) of the occipital lobe to induce tactile sensations in blind subjects. The subjects had been trained to use a sensory substitution device — an electrode array placed on the tongue — to determine the orientation of visual stimuli, which were captured by a camera and fed to the electrode array.
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On the objective level, this is really a non-issue:
On the subjective level, the problem boils down to the difference between first-person and third-person access to phenomenal states — a difference that is illusory, insofar as phenomenal states are virtual constructs.
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Lecture 5 Last modified on Wed Feb 21 09:21:56 2007 |
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