PSYC10003 Lecture Notes - Lecture 11: Visual Cortex, Homonymous Hemianopsia, Cortical Blindness
11. The Neural Correlates of Attention and Consciousness
The Concept of Attention
• Difficult to define
• Most people have general intuition of what attention is
• But these ideas do not always conform to our understanding based on empirical evidence derived
from experimental psychology and neuroscience
• Everyone knows what attention is. It is the taking possession by the mind, in clear and vivid form, of
one out of what seem several simultaneously possible objects or trains of thought. Focalisation,
concentration, of consciousness are of its essence. It implies withdrawal from some things in order
to deal effetiel ith othes… (James, 1890)
• according to James, attention can be considered as that set of processes used by brain to select
stimuli and actions that are currently relevant for controlling ongoing behaviour, and to inhibit stimuli
and actions that are currently irrelevant
A link between Attention and Consciousness?
• Number of noise-masks appears, masking pattern does not directly cover any changes but captures
attention, making it less likely that your attention will be grabbed by the transient visual signals
associated with the changed objects themselves
• Now attention has been drawn to location of changed items, becomes much easier to spot
• Brain not was representation of location and identity of these changes in visual short-term memory
(a set of processes that enables storage of a limited amount of visual information over a relatively
short duration)
Visual Pathways & Field Defects
• Damage to primary visual cortex (area V1) causes blindness to region of visual field presented by
affect area of cortex
• ∴ a small unilateral lesion of V1 will lead to a scotoma (a small patch of blindness) in one hemi field
• unilateral destruction of V1 entirely will cause blindness in the whole of the contralateral visual field
(an homonymous hemianopia)
• total destruction of V1 bilaterally will result in complete cortical blindness (absence of conscious
vision in both visual fields)
• experiments in monkeys with extensive or complete removal of V1 bilaterally showed that animals
could respond in simple ways to visual stimuli, by making hand or eye movements to visual targets,
or by detecting and discriminating between different visual stimuli at above-chance levels
• ∴ even if monkeys were blind by cortical ablation of primary visual cortex, could still use vision to
perform certain tasks
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2
Blindsight – a Disorder of Conscious Vision
• Weiskrantz et al. Were first to demonstrate residual visual functioning in a patient with surgical
ablation of primary visual cortex
• Coined blindsight to denote such preserved visual capacities in the absence of conscious visual
experience
• These preserved capacities include reflexive responses, accurate localization of visual targets using
hand and eye movements, and shape and wavelength discrimination
• Weiskrantz (1986) found that hemianopic patient DB could discriminate an X from an O in his blind
field
• However, DB was unable to perform more difficult discriminations between shapes in which neither
the orientation of edges nor the overall area could be used as cues, implying some limit in the extent
unconscious visual processing
• Recent studies revealed that reaching, pointing and grasping to unseen targets may also be preserved
to varying degrees in blindsight patients (e.g., Perenin & Rossetti, 1996).
Perimetric Testing in Blindsight
• Experiment by Weiskrantz, patient DB
• required to detect small spots of light flashed on various regions of his visual field
• first part of study:
o DB asked to indicated when he saw spots
o Unaware of visual stimuli that fell within a large region to the left of fixation
o Reports the same visual stimuli was normal in other parts of visual field
• Second experiment:
o “ho sae spots of light at diffeet positios ithi lid egio of is isual field
o Required to guess whether a spot was present or absent on given trial (on 50% of occasions
a spot was presented, and on remaining 50% spot was not present
• Task seemed ridiculous to DB because he never consciously perceived the spots
• DB was significantly above chance in detecting spots in blind field despite having no conscious
percept of them
Manual Pointing in Blindsight
• Another blindsight patients, GY (damage in LH)
• Weiskrantz examined whether spots of light presented in blind field could be correctly localized by a
simple pointing response
• Was required to point to location
• Task seemed ridiculous to GY again
• Had no conscious perception of visual stimuli in that region
• Weiskrantz foud that GYs ailit to loalise spots alost as auate i his lid field as i his itat
field
• Could not consciously see
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3
How does Visual Information get to the Brain in Blindsight?
• Beyond optic chiasm, axons in the optic tracts continue posteriorly until they form synapses with
neurons in a part of the thalamus called the lateral geniculate nucleus (LGN, there is one in each
hemisphere)
• Neurons in LGN send their axons posteriorly where they form synapses with neurons in the Primary
Visual Cortex
• 90% of retinal axons course through the LGN on their way to the primary visual cortex forming the
geniculostriate pathway
• after damage to primary visual cortex, these inputs are rendered ineffective, resulting in blindness
• 10% retinal axons bypass LGN and project instead to the superior colliculus (SC; part of midbrain)
and pulvinar nucleus of the thalamus
• in turn send axons on to other visual areas of the cortex
• SC is involved in controlling and orienting response to sudden visual and auditory stimuli
• It seems likely that in the SC, pulvinar, and the cortical regions to which they project, underlies the
remarkable preservation of some visual capacities in blindsight patients
• Without direct input from LGN to primary visual cortex, there is no longer and conscious visual
experience
• ∴ blindsight provides an example of neuropsychological dissociation between conscious and
unconscious perception
What is Attention?
• Attention may be selective in the sense that it restricts processing to just those aspects of the sensory
input that occupy a particular region of space, or that occur at a particular time
o Eg. May keep attention focused on a particular face in a crowd, or on a perceptual input that
corresponds to a specific point in time
o We seem unable to pay attention to everything at once; attention has limited capacity
• ∴, we cannot simultaneously attend to every face in a crowd to check for a friend or acquaintance,
despite our subjective impression of seeing everyone at once
• can only attend to around three or four separate objects at a time
• attention must be redeployed elsewhere if we are to take in more information from a complex scene
• limit in vigilance: the period of time for which we can effectively sustain attention
• difficult to sustain attention for more than a few minutes at a time without becoming fatigued and
missing targets
• attention also involved in establishment and maintenance of expectations or perceptual set
• tend to focus or processing resources on stimulus inputs and behaviours that are relevant to the
current environment and task demands
• attention is essential for flexible switching from one task to another, or form one environment to
another
• without ability to switch we would be forced to respond in a stereotypical fashion regardless of task
demands or stimuli, an impairment that is sometimes apparent in neurological patients with frontal
lobe damage
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Document Summary
The neural correlates of attention and consciousness. It is the taking possession by the mind, in clear and vivid form, of one out of what seem several simultaneously possible objects or trains of thought. Focalisation, concentration, of consciousness are of its essence. Blindsight a disorder of conscious vision: weiskrantz et al. What is attention: attention may be selective in the sense that it restricts processing to just those aspects of the sensory input that occupy a particular region of space, or that occur at a particular time, eg. Spatial neglect is most frequently encountered in stroke-patients with rh lesions, particularly to inferior parietal lobe (ipl). Other areas include: superior temporal gyrus (stg), inferior frontal gyrus (ifg), middle frontal gyrus (mfg). Ipl i(cid:374)fe(cid:396)io(cid:396) pa(cid:396)ietal lo(cid:271)ule a(cid:374)gula(cid:396) g(cid:455)(cid:396)us sup(cid:396)a(cid:373)a(cid:396)gi(cid:374)al g(cid:455)(cid:396)us. Extracellular microelectrode recordings in macaque monkeys have shown that single neurons in monkey parietal cortex modulate their rate of firing in accordance with the attentional demands of a visual task.
