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Lesions of Retinostriate Pathway
• Lesions (usually due to
stroke) cause a region of
blindness called a scotoma
• Identified using perimetry
• note macular sparing
X
Retinocollicular Pathway
independently mediates orienting
• Blindsight patients have been shown to posses a
surprising range of “residual” visual abilities
– better than chance at detection and discrimination of some
visual features such as direction of motion
• These go beyond simple orienting - how can this be?
Retinocollicular Pathway
independently mediates orienting
• Recall that the feed-forward sweep is not a single
wave of information and that it doesn’t only go
through V1
• In particular, MT seems to get very
early and direct input – probably
from tectopulvinar pathway, but
also directly from LGN
Retinocollicular Pathway
independently mediates orienting
• Recall that the feed-forward sweep is not a single
wave of information and that it doesn’t only go
through V1
• In particular, MT seems to get very
early and direct input – probably
from tectopulvinar pathway, but
also directly from LGN
• This input guides behaviour but
doesn’t support awareness
Searching for the NCC
• Dorsal/Ventral dichotomy suggests that not all neural
processes “cause” consciousness
• Can we find similar dissociations in healthy brains?
• Can we attribute awareness to certain classes of
neural events?
Searching for the NCC
• What is needed is a situation in which a perceiver’s
state can alternate between “aware” and “unaware”
of some information in ways that we can correlate
with neural events
• One such situation is called Binocular Rivalry
Rivalrous Images
• A rivalrous image is
one that switches
between two
mutually exclusive
percepts
Binocular Rivalry
• What would happen if each eye receives
incompatible input?
Left Eye
Right Eye
Binocular Rivalry
• What would happen if each eye receives
incompatible input?
• The percept is not usually the amalgamation of the
two images. Instead the images are often rivalrous.
– Percept switches between the two possible images
Binocular Rivalry
• Rivalry does not entail suppression of one eye and dominance
of another – it is based on parts of objects:
Stimuli:
Left Eye
Percept:
Right Eye
Or
Binocular Rivalry
• Percept alternates randomly (not regularly) between
dominance and suppression - on the order of
seconds
– What factors affect dominance and suppression?
Time ->
Binocular Rivalry
• Percept alternates randomly (not regularly) between
dominance and suppression - on the order of
seconds
– What factors affect dominance and suppression?
– Several features tend to increase the time one image is
dominant (visible)
• Higher contrast
• Brighter
• Motion
Binocular Rivalry
• Percept alternates randomly (not regularly) between
dominance and suppression - on the order of
seconds
– What factors affect dominance and suppression?
– Several features tend to increase the time one image is
dominant (visible)
• Higher contrast
• Brighter
• Motion
• What are the neural correlates of rivalry?
Neural Correlates of Rivalry
• What Brain areas “experience” rivalry?
• Clever fMRI experiment by Tong et al. (1998)
– Exploit preferential responses by different regions
– Present faces and buildings in alternation
Neural Correlates of Rivalry
• What Brain areas “experience” rivalry?
• Clever fMRI experiment by Tong et al. (1998)
– Exploit preferential responses by different regions
– Present faces to one eye and buildings to the other
Neural Correlates of Rivalry
• What Brain areas “experience” rivalry?
• Apparently activity in areas in ventral pathway
correlates with awareness
• But at what stage is rivalry first manifested?
• For the answer we need to look to single-cell
recording
Neural Correlates of Rivalry
• Neurophysiology of Rivalry
– Monkey is trained to indicate
which of two images it is
perceiving (by pressing a
lever)
– One stimulus contains
features to which a given
recorded neuron is “tuned”,
the other does not
– What happens to neurons
when their preferred stimulus
is present but suppressed?
Neural Correlates of Rivalry
• The theory is that Neurons in the LGN mediate rivalry
Neural Correlates of Rivalry
• The theory is that Neurons in the LGN mediate rivalry
• NO – cells in LGN respond similarly regardless of
whether their input is suppressed or dominant
Neural Correlates of Rivalry
• V1? V4? V5?
• YES – cells in primary and early extra-striate cortex
respond with more action potentials when their
preferred stimulus is dominant relative to when it is
suppressed
• However,
– Changes are small
– Cells never stop firing altogether
Neural Correlates of Rivalry
• Inferior Temporal Cortex (Ventral Pathway)?
• YES – cells in IT are strongly correlated with percept
Neural Mechanisms of Consciousness?
• So how far does that get us?
• Not all that far – we still don’t know what is the mechanism that
causes consciousness
• But
– we do know that it is probably distributed rather than at one locus
– We do know that ventral structures seem to play a critical role in visual
awareness
• Thus the question remains: what is special about the activity of
networks of neurons that gives rise to consciousness?