seminar presentation
DESCRIPTION
Gore, et al,; and Shihui, et al.TRANSCRIPT
Presentation of Shafritz, Gore, Morois (2002); and Shihui, et al.
(2005)
By Kyle Stevens and Julia Leone
Shafritz, Gore, Morois (2002)
The Role of the Parietal Cortex in Visual Feature Binding
Introduction
“Binding Problem” -- When we see multiple objects, what neural mechanisms attribute features (shape, color, motion) to their appropriate objects?
Previous Work Treisman and Gelade (1980) propose that
spatial attention to objects' locations integrates features
Little evidence to support this. Parietal cortex is likely candidate, already associated with representation of spatial information.
Goals and Predictions Use fMRI to demonstrate parietal cortex's role
in feature binding. Localize involved regions of parietal cortex. Regions should show conjunction-related
activation (more activation for more features). Activation should occur when multiple objects
are in visual field simultaneously.
Experiment 1 (spatial attention localizing)
Subjects are shown two snowflakes, then a mask, then a test snowflake, and asked whether the test matched either of the two in either location, or shape, but not both.
Cue (2 s) Two flakes (1080 ms) Mask (40 ms) Test flake (1080 ms)
Previous studies define the location task as involving greater spatial attention than the shape task.
Cont
Fig. 1. Definition of spatial attention ROIs. (A) Trial design. Each block oftrials began with a screen instructing the subjects which feature to attend. Ineach trial, two snowflake objects were presented, followed by a mask and atest object. Subjects determined whether the test matched one of the twosample objects in the attended feature (shape or location). Object colors wereuninformative for the shape and location judgments and were used for pilotexperiments of feature conjunctions (data not shown).
Results Areas engaged in the spatial
attention/localization task: Intra-parietal cortex Superior-parietal cortex The right parietal cortex was more activated than the left. Consistent with previous studies.
Cont.
(B) Brain activation for location (red) compared to shape judgment (blue). Location judgment activated two regions of the parietal cortex: anterior intra-parietal (green arrow) and superior parietal (BA7) cortex (purple arrow).
Experiment 2 (feature conjunction)
Subjects asked to match a test object to two previous objects based on either shape, color, or both.
One type of presentation was simultaneous (two objects presented at the same time), and one was sequential.
ResultsAccuracy In simul. presentations, conjunction was more
difficult than color, but not shape.
Reaction Times In sequen., conjunction took longer than both
shape and color; but in simul., conjunction took longer than color but not shape.
(B) Behavioral performance during the fMRI sessions.
Results Cont.Brain Activation Simul. presentation activated parietal cortex
more Simul. presen., conjunction tasks activated
brain more than either single feature Sequen. presen., conjunction tasks activated
brain more than color, but not shape In right hemisphere, activation was more
mathematically significant than in left
Fig. 3. (A) Direct comparison of conjunction-related activation in the simultaneous (yellow) and sequential (blue) presentations. Green and purple boxesindicate the position of the intra- and superior parietal ROIs, respectively. The simultaneous-presentation activation largely overlaps with the parietal ROIs. (B)Activation of parietal ROIs with conjunction and single feature judgments. Both the right intra- and superior parietal ROIs were more activated during theconjunction judgments than during either of the single feature judgments in the simultaneous presentation mode only (asterisks). Yellow voxels in the statisticalparametric map
represent brain areas significantly more activated for the conjunction judgment than for both of the single feature judgments during the simultaneous presentations condition (voxels met criteria of P%0.05 for each of the two conjunction vs. single-feature comparisons). The activated voxels overlapwith the ROIs.
What does this mean? Experiment 1
establishes specific regions of parietal cortex activated by spatial attention task
Experiment 2 parietal cortex is more activated when the objects were
presented simultaneously than sequentually
Reinforces parietal cortex's role in spatial attention
Cont. same parietal regions engaged in both tasks (spatial
attention, feature conjunction), “are suggestive of a role for spatial attention in feature integration”
parietal regions more activated for conjunction tasks, “may reflect the representation or manipulation of spatial information or attention during the conjunction task”
supported by the right hemisphere lateralization supports theory that spatial attention/localizaiton
promotes binding, as both occur in same regions of parietal cortex
Shihui, et al., (2005)
Attentional Modulation of Perceptual Grouping in Human Visual Cortex: Functional MRI Studies
Introduction When we see many objects, we must organize
pieces into useful parts Gestalt laws of visual organization--we
organize information based on proximity, color, etc
Ex. Gestalt Law of Closure
What are neural bases? Is this organization early or late?
Previous Work
Han, et al. (2001, 2002) show in event-related potential (ERP) studies that stimuli grouped by proximity generated activation within 100-120 ms in visual cortex
Implicates relationship between proximity-grouping and visual cortex
ERP study does not specify whether the grouping activity arises in visual cortex or some other part of brain
Mack, et al. (1992) show that identification of perceptual groups is affected by attention, when attention is concurrently engaged
Goals and Predictions
Use fMRI to see whether grouping affects processing in visual cortex, whether grouping is affected by attention
Experiment 1 Subjects asked to identify “row- or column-
grouped stimuli (regardless of whether proximity or similarity cues produced grouping)”
Displays was shown for 200 ms, with blank screen in between for either 3400 ms or 4200 ms
Figure 1.Illustration of the stimulus arrays used in Experiment 1. Each stimulus array consists of a square lattice of gray circles or squares on a black background. Alternate circles and squares distributedevenly across the lattice in the uniform stimulus, but are grouped into rows or columns by either proximity or similarity of shape in the grouped stimuli. The target stimuli are the same as the nontargets except that they are smaller.
Results Response time was faster for proximity-
grouped targets than similarity-grouped targets
Activated brain areas in proximity-grouping right calcarine area inferior bilateral parietal cortices right superior temporal cortex
Activated areas in similarity-grouping right middle occipital cortex left middle temporal cortex
Figure 3.Brain activations associated with proximity and similarity groupingin Experiment 1. a: Brain areas showing stronger activations forthe proximity-grouped nontargets than the uniform nontarget inExperiment 1. These include the right calcarine cortex, bilateralinferior parietal cortices, and right superior temporal cortex. b:The time course of BOLD responses after proximity-grouped anduniform stimulus onset (time zero on abscissa) in calcarine area. c:Brain areas showing stronger activations for similarly-groupedthan for uniform stimuli. These include the right middle occipitalcortex and left middle temporal cortex.
Experiment 2
Display consisted of small white line segments on a black screen
Local shapes were uniformly rotated line segments
Peripheral dots on corners of display and fixation cross would light up in two of the tasks
Tasks Task 1
“subjects discriminated the orientation of the uniform or the grouped stimuli”
Task 2 subjects discriminated the color of the peripheral dots
(green or red)
Task 3 subjects discriminated the color of the fixation cross
Figure 2.Illustration of the stimuli used in Experiment 2. a: A nontarget grouped stimulus. b: A nontarget uniform stimulus. c: A proximitytarget stimulus used in Task 1. d: An uniform target stimulus used in Task 1. e: A nontarget stimulus used in Task 2, in which theperipheral dots are white. The dots were either red or green intarget stimuli. f: A nontarget stimulus used in Task 3, in which thefixation is white. The fixation was either red or green in nontargetstimuli.
Results No significant different in response time
between tasks Task 1
calcarine area and left sup. parietal sulcus showed increased activation in proximity-grouped stimuli
Task 2 only calcarine area showed increased activation in
proximity-grouped stimuli compared to uniform
Task 3 “No activation was observed in the same contrast in Task
3”
Figure 4.Brain areas showing stronger activations for proximity-grouped than for uniform stimuli in Task 1 of Experiment 2. a: Grouping related activation in the calcarine cortex. b: Grouping-related activation in the superior parietal sulcus. c: The time courses of the signal change in the calcarine cortex.
Figure 5.Brain areas showing stronger activations for proximity-grouped than for uniform nontargets in Task 2 of Experiment 2. a: Grouping- related activation in the calcarine cortex. b: The time courses of the signal change in the calcarine cortex.
What does this mean? Experiment 1
early proximity-grouping is localized to calcarine cortex, especially right side
Experiment 2 design filtered out low spatial frequencies calcarine activation suggests it is involved in proximity-
grouping regardless of spatial frequency
Cont. because calcarine cortex was not activated for shape
grouping, it is assumed that the calcarine cortex is specific to proximity-grouping in visual processing
Cont. Other parts of brain
Proximity grouping shows activation in inferior parietal cortices
suggests a two-stage system for proximity-grouping early grouping by the calcarine cortex late grouping by the inferior cortices (supported by previous
research) Shape grouping shows activation in right middle occipital
cortex and left temporal cortex previous research associated these areas with shape processing
Attentional Window
No calcarine activation in Experiment 2, Task 3 (color of fixation cross), suggests size of attentional window can impact perceptual grouping
small attended window=lack of outside perceptual grouping, even at early stages
Supported by previous research