PSYC10003 Lecture Notes - Lecture 11: Homonymous Hemianopsia, Cortical Blindness, Visual Cortex
12 Jun 2018
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Lecture 11, Thursday 24 March 2016
PSYC10003 - MIND, BRAIN & BEHAVIOUR 1
LECTURE 11
THE NEURAL CORRELATES OF ATTENTION &
CONSCIOUSNESS
THE CONCEPT OF ATTENTION
•The concept of attention has proven remarkably difficult to define. Most people have a 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. The view
proposed by William James more than 100 years ago was as follows:
•‘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 effectively with others...’ (James, 1890)
•According to William James, then, attention can be considered as that set of processes used by the
brain to select stimuli and actions that are currently relevant for controlling ongoing behaviour,
and to inhibit stimuli and actions that are currently irrelevant.
THE LINK BETWEEN ATTENTION & CONSCIOUSNESS
•What role does attention play in determining conscious perception? In this demonstration, you
are required to find the small differences between two nearly identical images. During the switch
from one image to another, a number of small noise-masks appears briefly. This masking pattern
does not directly cover any of the changes between the two scenes, but it does have the effect of
capturing attention, thus making it less likely that your attention will be grabbed by the transient
visual signals associated with the changed objects themselves.
•In this slide some of the items that changed have been circled. How many changes did you
notice?
•Note that once your attention has been drawn to the location of the changed items, it now
becomes much easier to spot the changes in the original dynamic display. Your brain now has a
representation of the 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
•Before considering the role of attention in conscious perception, it is instructive to look at a
neurological condition in which there is a profound
loss of conscious vision, but a number of preserved
visual abilities of which the patient remains
unaware.
•Damage to the primary visual cortex (area V1)
causes blindness in the region of the visual field
represented by the affected area of cortex. Thus, a
small unilateral lesion of V1 will lead to a scotoma
(i.e., a small patch of blindness) in one hemifield;
unilateral destruction of V1 in its entirety will cause
blindness in the whole of the contralateral visual
field (an homonymous hemianopia); and total
destruction of V1 bilaterally will result in complete
cortical blindness (i.e., an absence of conscious
vision in both visual fields). Early experiments in
monkeys with extensive or complete removal of
area V1 bilaterally showed that the animals could !
Lecture 11, Thursday 24 March 2016
PSYC10003 - MIND, BRAIN & BEHAVIOUR 1
!
nevertheless 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.
In other words, even though the monkeys were rendered ‘blind’ by cortical ablation of the
primary visual cortex, they could still use vision to perform certain tasks.
BLINDSIGHT - A DISORDER OF CONSCIOUS VISION
•Similar preservation of visual abilities has been reported in studies of humans with unilateral or
bilateral lesions of area V1. Weiskrantz and his colleagues (Sanders, Warrington, Marshall, &
Weiskrantz, 1974; Weiskrantz, Warrington, Sanders, & Marshall, 1974) were the first to
demonstrate residual visual functioning in a patient with surgical ablation of primary visual
cortex. They coined the term blindsight to denote such preserved visual capacities in the absence
of conscious visual experience. These preserved capacities include reflexive responses (e.g.,
pupillary reflexes), accurate localisation of visual targets using hand and eye movements, and
shape and wavelength discrimination. Thus, for example, Weiskrantz (1986) found that the
hemianopic patient DB could discriminate an X from an O in his blind field. Interestingly,
however, patient 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 of unconscious visual processing.
•More recent studies have 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
•In one experiment by Weiskrantz, patient DB
was required to detect small spots of light
flashed into various regions of his visual field.
In the first part of the study, DB was asked to
indicate when he saw the spots. He was
unaware of visual stimuli that fell within a
large region to the left of fixation (indicated in
black in the figure; note that his damage
involved the right visual cortex), but his
reports of the same visual stimuli was normal
in other parts of the visual field. In a second
experiment, DB was shown the same spots of
light at different positions within the ‘blind’
region of his visual field, but now he was
required to guess whether a spot was present or absent on a given trial (on 50% of occasions a
spot was presented, and on the remaining 50% there was no spot). Of course, the task seemed
ridiculous to DB because he never consciously perceived the spots. Remarkably, though, DB was
significantly above chance in detecting spots in his blind field, even though he reported having no
conscious percept of them! When told of his performance DB was left speechless – how had he
managed to detect visual stimuli he couldn’t see?
MANUAL POINTING IN
BLINDSIGHT
•In further experiments with another
blindsight patient, GY (whose damage
was in the left hemisphere), Weiskrantz
examined whether spots of light
presented in the blind field could be
correctly localised by a simple pointing
response. After a spot of light was
flashed briefly either in GY’s intact or !
