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