} Spatial attention
Lecture 2
Chapter 7
Lecture 2
Chapter 7
} Introduction
Where is your left hand now?
Where is your cell phone?
How would you get home this afternoon?
How do you get to the nearest bank from here?
Point 1:
The brain represents space at different levels/reference points
NOT a single continuous entity
- Egocentric space: relative to position of body
(eye gaze: retinocentric space)
- Allocentric space: finding location of one object relative to another object
Point 2:
Space = common dimension of most perceptual systems & action/motor system
The brain makes use of this property to integrate information across sensory modalities (cross-modal perception)
Point 3:
Space also important for prioritizing incoming information
Spatial attention: Selectively processing information in particular regions of space & filtering out unwanted info
Therefore: spatial representations usually involve discussion of attentional mechanisms in brain
} Attention
Definition: The process by which certain information is selected for further processing & other information is discarded
} Necessary to avoid sensory overload
} Attention likened to filter/bottleneck
Ø Visual attention
– Spotlight: highlights particular location in space
– Locus of spotlight need not be the same as eye fixation
– Can be split across 2 locations
} But how does the spotlight know where to go?
} Mechanisms of visual attention
1) Orientation of attention
• Overt & covert (with or without eyes)
• External - Exogenous orienting:
• Passive
• Attracted by sudden change in environment
• Internal - Endogenous orienting:
• Active
• Observer guide attention according to goals
Ø Visual Search:
◦ Makes use of endogenous orienting
◦ Detect presence/absence of specified target object amongst distracters
◦ Influential theory - FIT (later)
} Posner Study (1980, 84)
• Famous study of spatial cueing
• Example of exogenous orienting (externally driven by the stimulus)
• Illustrated that attention operates on spatial basis
Paradigm:
} Asymmetrical orientation to space in humans
Pseudoneglect: In a non-lesioned brain, there is over-attention to the left side of space (RH spatial dominance)
Theories:
◦ RH more specialized for spatial processing than the LH Mesulam (1981). Consequences for RH damage
◦ RH concerned with global aspects of attention, whereas LH primarily allocates attention to detail (Robertson & Lamb, 1991)
Attention can thus under different conditions select spatial locations, features, whole objects or combinations thereof, a view that most current attention theories can accommodate
We do not, however, have perfect control over the deployment of this attentional selection
} Feature – integration theory
Lecture 3
Chapter 7
Lecture 3
Chapter 7
} Feature Integration Theory (FIT)
Assumes:
1) All rudimentary perceptual features of objects (i.e. colour, shape) are coded in parallel & prior to attention
2) The attentional “spotlight’ is the ‘glue’ that binds these features together
Which attention theory is this an example of???
Early selection model
Array 1:
If an object doesn’t share features with other objects, – it appears to “pop-out”
Array 2:
Ø Distracters now have same features that define the target
Ø To detect the target one has to bring together information from different features
Ø This occurs by allocating spatial attention to the object location
Ø If not target – “spotlight” moves on & inspects next candidate in serial fasion
} Evidence for Theory
Visual search experiment:
Dependent measure = time
Ø Target determined by single feature:
↑distracters, = time
(“pops out”)
Ø Target defined by conjunction of features:
↑distracters, ↑time
(thus each object must be serially inspected)
NB - Pop-out is not affected by number of items to be searched
} Evidence against FIT Theory
How do you define a “feature”?
T & L’s: Both consist of vertical and horizontal lines
Duncan & Humphreys (1989):
- Data can also be explained in terms of how easy it is to perceptually group objects together
- Similarity between target & diff distractor NB
Other evidence: Negative priming
- Ignored object was, in fact, processed meaningfully, rather than being excluded purely on the basis of colour
} Evidence for Semantic Processing of Unattended Stimuli – Negative priming
} So which is it: Early or Late?(How can the evidence for and against FIT be reconciled?)
The selection of objects for further processing may sometimes be early and sometimes late (depending on demands of task)
Nillie Lavie’s Perceptual Load Theory (1995):
- NB – brain has limited capacity!!
- Early selection occurs under high perceptual load, whereas
- Late selection occurs under low load conditions
Therefore:
Ø Large arrays (i.e. visual search) ® early selection
Ø Conditions of low load (i.e. negative priming) ® enough capacity to process all objects meaningfully ® late selection
“Lavie's perceptual load theory (1995; Lavie & DeFockert, 2001; Lavie & Tsal, 1994) holds that observers can efficiently filter out task-irrelevant distractors when performing under high levels of perceptual load (e.g., large display size), but fail to do so under low levels of perceptual load (e.g., small display size).
On this account, perceptual load modulates selectivity by way of limiting or freeing processing capacity. When load is high, capacity is exhausted and there are no available resources left to process irrelevant distractors; while the reverse holds true for low load.”
Neural basis of spatial attention:
- Number of regions implicated, but parietal lobes particularly NB
From early visual processing: 2 pathways:
Ventral (temporal
lobe) = what
Dorsal (parietal
lobe) = where
} Evidence for parietal lobe involvement in spatial cognition:
1) Single cell recordings
Parietal neurons that combine visual spatial info with postural info
- Essential for locating objects in external space
2) Left/right differentiation
Parietal lobes contain more neurons that selectively respond to left and right space respectively
Neglect syndrome (usually far more severe following RIGHT parietal injury - failure to attend to left)
3) Posner task: parietal lobes necessary to disengage attention
Not so important for initial orienting to cue, rather disengage
Evidence from neglect patients
4) Visual search tasks
TMS applied over parietal lobe slows conjunction searches, but not single-feature searches
Patients with parietal lesions often show high level of illusory conjunction errors
} Cross-modal Integration
Spatial frames of ref may be used to integrate information from diff sensory modalities
} Memory for Space: Does the Hippocampus contain a Spatial Map?
Ø Different mechanisms for egocentric & allocentric space (memory)
Ø NB - Hippocampus
Rat Hippocampus:
Ø O’Keefe & Nadel (1978) argued that rat hippocampus stores spatial maps of the environment
Ø Single-cell recordings
Ø Measured firing rate of neuron at particular locations
Ø Place cells respond maximally when animal in certain locations
Ø Place cells often context sensitive (“remapping”)
} Memory for Space: Does the Human Hippocampus have a Spatial Map?
Ø Recent studies found place cells in human hippocampus, but thinking about location similar to being in that location
Ø Imaging and lesion studies suggest:
Ø Right HC more related to spatial memory in humans
Ø Left HC more specialised for remembering other contextual detail
Ø Virtual environment: fMRI shows activation in right HC for finding one’s own way relative to being shown route
Ø Patients with right HC damage poor at remembering layout of virtual town, but patients with left HC damage impaired at remembering events
“the right hippocampus is recruited specifically for navigation in large-scale spatial environments”
} Does Driving a Taxi increase Hippocampal Volume?
Ø London taxi drivers required to learn nearest route between any 2 points in the city ("The Knowledge")
Ø VBM shows greater right hippocampal grey matter volume in taxi drivers than IQ matched controls
Ø Did they choose the job because of their spatial memory? Probably not, the longer they have been in the job the greater the volume in this area
Ø Thus region might expand with usage
} Summary
Parietal Lobes: Transforms sensory-based maps of space into various egocentric maps of space that facilitate interaction with the environment Integrate info from diff senses Prioritize incoming information through spatial attention} Hippocampus:
◦ Long-term storage of spatial maps of environment
}
Can one perceive an object but not its location?
DEF of Balint’s syndrome:
A severe difficulty in spatial
processing normally following
bilateral lesions of the parietal lobe
(A double form of neglect)
Three clinical characteristics of Balint’s syndrome
(1) Inability to perceive more than one object at a time (simultanagnosia)
(2) Inability to reach in the proper direction for an object under visual guidance (optic ataxia)
(3) Fixation of gaze without a primary deficit of eye movement (optic apraxia)
} Patient RM has Balint’s syndrome…
◦ Basic visual abilities normal (e.g. visual acuity, colour perception, contrast sensitivity)
◦ Unable to locate objects by reaching and pointing
◦ Also impaired at locating sounds
◦ Can state which side of body and which body part touched (spatial frames for body preserved)
◦ Thus different mechanisms to compute body space and external space
} Simultanagnosic patients can recognize single objects but cannot compute spatial relationships between objects
} But when is an object an object?
E.g. is a face an object or several objects?
} Top-down factors important too
} Patient GK could compute face orientation (91%) but not location of ovals (55%)
} Performance improves if other face-like features added (bottom)
} Balint’s patients show interesting parallel to inattention errors of normals:
Show illusory conjunctions even with plenty of time
} Neglect Definitions:
A failure to report, respond to or orient towards stimuli in contralesional space Variety in syndrome:- Predominantly personal neglect
- Extrapersonal neglect
- Patients may neglect left half of objects
Sub-syndromes:
Neglect dyslexia: - Patient omits letters on left side of word/page when reading
Extinction} Extinction:
· Phenomenon of visual extinction suggests different perceptual representations are competing for attention (and visual awareness)
} Testing for Neglect
} Critical Lesions?
} Problem in Perception
or Attention?
or Attention?
Lines of Evidence:
◦ fMRI – activity in neglected field
◦ They are often able to detect objects on the left if cued there (Posner task)
◦ Often fare worse in tasks where they have to voluntary orient attention and where several stimuli compete for attention (search times increase with more ipsilesional objects)
◦ Affects other senses too: auditory and tactile judgements
◦ Extinction further proof
} Perceptual vs Representational neglect
} Neglect can also occur for spatial mental images
} Piazza del Duomo experiment (Bisiach & Luzzatti, 1978)
} Object-based vs space-based Neglect
} Some neglect patients attend to objects on the left side of space but omit to attend to one half of the object itself (object-based neglect)
} Forms a double dissociation with space-based neglect
} How does this impact on reading?
(‘home’ as ‘come’)
} Driver & Halligan (1991) – patient with object-based neglect cannot detect differences on left side of an object even when falling into right side of space
} This is called “object-centered” neglect!
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