Proceedings of 2005 4th IEEE International Conference on Development and Learning
Impaired visual working memory capacity in case of motion direction andcolor-shape feature binding Masahiro Kawasaki Department of Quantum Engineering and System Science, Graduate School of Engineering, University of Tokyo, Japan 2-11-16, Yayoi, Bunkyo-ku, Tokyo, 113-0032, Japan
[email protected]
Masataka Watanabe Department of Quantum Engineering and System Science, Graduate School of Engineering, University of Tokyo, Japan 2-11-16, Yayoi, Bunkyo-ku, Tokyo, 113-0032, Japan
[email protected]
Department of Complexity Science and Engineering, Graduate School of Frontier Science, University of Tokyo, Japan 4-6-1, Komaba, Meguro-ku, Tokyo, 153-8505, Japan
[email protected]
the other hand, Wheeler & Treisman (2002) failed to replicate Luck & Vogel's finding for bicolored squares while their results for color-location or color-shape binding were similar. Accordingly, they claim that features from the same dimension compete for capacity, whereas features from different dimensions can be stored in parallel and working memory capacity is limited both by the independent capacity of simple feature stores as an explanation of memory for multi-dimensional objects. The hypothesis that the number of remembered objects is the same when the features come from the different dimensions is now widely accepted, but these previous studies using change detection tasks were related to objects defined by features processed by the ventral pathway (e.g. color, shape, orientation of lines, size, etc). That is to say, these earlier change detection tasks ignored motion features because they used only static objects, although the actual visual representations are composed of not only color and shape but also motion features. In this article, we will use the term "ventral feature" to refer to features processed by ventral pathway, and "dorsal feature" to refer to motion features processed by dorsal pathway. Moreover, according to the results of the multiple object tracking task (MOPT task), the color-shape conjunction was not observed when the objects were moving (Saiki, 2003). However, the MOPT task measured feature-location binding of multiple objects and dorsal features was not dealt as one single dimension like ventral features since these dynamic objects moved with a regular rotation pattern. No studies have so far been made at binding motion feature with color and shape in a visual working memory task. This study carried out the change detection tasks using dynamic color random dots in a several frame as sample stimulus and tested the feature parallel storage when the dorsal features added. In terms of three features; color, shape, and motion direction, we compared with three single-feature conditions for each feature, three two-features-binding conditions (color-shape binding, color-motion binding, and shape-motion binding) and one three-features-binding (colorshape-motion binding) condition.
Abstract – It has been proposed that visual working memory can hold a set of approximately four objects and the number of remembered objects is the same whether the objects have one relevant single feature, or two, or even four (Luck & Vogel; Wheeler & Treisman). Although the actual visual images have color-shape and motion features, traditional change detection tasks were mainly related to only color and shape. This study using dynamic color random dots dealt with motion direction and color-shape equally and then tested previous proposals. The capacity significantly decreased when motion features were added, that is to say, the performance in motion and both color and shape binding conditions was worse than in a single feature condition, unlike in color and shape binding condition. Our results suggest that motion feature competes for color-shape feature, therefore visual working memory capacity for multidimensional objects is impaired contrary to a widely held view. Index Terms – visual working memory, memory capacity, feature binding
I. INTRODUCTION Visual information can be held for short amounts of time in a store termed visual working memory. The capacity of visual working memory is proposed to be approximately four objects (Pashler, 1988) limited by the number of objects, independently of the number of features making up objects (Luck & Vogel, 1997; Vogel, Woodman, & Luck, 2001). For example, observers were able to memorize about four colors or four oriented lines in visual working memory, but not eight of each feature. However, all eight features could be remembered when four colors and four orientations were conjoined to form four colored lines (Olson & Jiang, 2002). Luck & Vogel support an object-based storage hypothesis that what is stored in visual working memory is a small set of bound objects, to which any number of features can be added for free if they characterize one of the original objects, even though they are same dimension such as bicolored squares. On
58
0-7803-9225-6/05/$20.00 ©2005 IEEE.
Kazuyuki Aihara
II. MATERIAL AND METHODS
TABLE I TYPES OF FEATURES IN EACH CONDITION OF EXPERIMENTS.
We performed seven experiments; Experiment1 Color only, Experiment2 Shape only, Experiment3 Motion direction only, Experiment4 Color- shape binding, Experiment5 Colormotion binding, Experiment6 Shape-motion binding, Experiment7 Color-shape-motion binding.
condition
A. Participants Ten graduated students participated in each experiment. Their ages ranged from 22 to 30 years (mean (±s.d.) 25.10±2.17 years). All had normal color vision and normal or corrected-to-normal visual acuity.
$. Material
200msec
Test display
250msec
1000msec
250msec
motion
Ex.1
color only
8
fix(䂔)
fix(㸣)
Ex.2
shape only
fix(gray)
8
fix(㸣)
Ex.3
motion only
fix(gray)
fix(䂔)
8
Ex.4
color-shape binding
8
8
fix(㸣)
Ex.5
color-motion binding
8
fix(䂔)
8
Ex.6
shape-motion binding
fix(gray)
8
8
Ex.7
color-shape-motion binding
8
8
8
%. Procedure The same time sequence and basic single-probe test of change-detection paradigm were used as in previous experiments. At the beginning of each trial, a central arrow cue instructed the participants to remember the items in either the left or the right hemi-field. Each trial consisted of a 250ms sample display followed by a 1000-ms inter-stimulus interval and then a 250-ms test display. The sample display contained two, three, four or five items presented at random locationsin each hemi-field. The test display contained only one item. The participants' task was to detect whether the features of item were the same as the features located at that position on the sample display by a keypress (right key: not change, left key: change). Each participant completed seven separate conditions presented in a counterbalanced order in seven blocks of trials, one condition per block. Participants knew which they would be tested on. Each block contained 288 experimental trials (i.e., 4 [set size] 2 [probe change absent vs. present] 2 [stimulus onset instruction right vs. left] 18 cases), for a total of 2016 trials. Each block of trials had a different set of instructions. 1. Single-feature condition (Color only, Shape only, and Motion only): Participants were told that only one feature (color or shape or motion direction) would be tested. Each item was defined as eight different tested feature and the other two features were neutral. For the test,one single item was presented at the location in each hemi-field of the initial display.The item of the test display was identical with one of the sample display on 50% of trials or could contain either a
(a) ISI
shape
In these experiments, visual stimuli were displayed on a 21inch computer screen of Macintosh and these programs were written using a Matlab 5.2.0 with the extensions provided by the Psychophysics Toolbox (Brainard, 1997) and the Video Toolbox (Pelli, 1997). On shape-only, motion direction only and shape-motion binding conditions, one color, gray, was used as the neutral color. On color-only, motion-only and color-motion binding conditions, one shape, a square, was used as the neutral shape. On color-only, shape-only and color-shape binding conditions, one motion-direction, down, was used as the neutral shape (See Table I).
All stimulus arrays were presented within two invisible 3×5 cell matrix that subtended 4.0°×6.8° rectangular regions that were centered 2.0° to the left and right of a central fixation cross on a black background (0.41 cd/m2). Each memory array consisted of 2-5 sets of dynamic color random dots (1.0°×1.0°) in each hemi-field. One of these random dots was 0.033°×0.033°. Each item was randomly placed within the region. Each item had three features; color, shape and motion direction. Eight colors produced by RGB colors were used for the items. The x, y, and luminance values for the color were measured with a Color CAL using the 1931 CIE color coordinate system. The luminance of each item was identical (12.50 cd/m2). The colors were as follows; gray (x=.279, y=.303), red (x=.615, y=.344), green (x=.286, y=.591), blue (x=.153, y=.076), yellow (x=.394, y=.510), magenta (x=.275, y=.147), cyan (x=.209, y=.294), orange (x=.570, y=.378). All colors were highly distinctive. The sets of dynamic color random dots moved within frames that defined eight shapes (Fig.1 (b)). The size of each shape was identical. The directions of dynamic color random dots moving were eight patterns (up, down, right, left, upper right, upper left, lower right, and lower left). The velocity of these dynamic random dots was 0.067°per second. The set sizes were randomly intermixed within blocks with the constraint of balanced presentation across all levels. Colors, shapes and motion directions were never repeated within any sample display.
Sample display
color
(b)
Fig. 1 (a) Example of a visual memory trial for the right hemi-field in Experiment1. ISI, inter stimulus interval. (b) Shapes presented in Experiment.2, 4, 6, and 7.
59
feature of another item located at the different position in same hemi-field. Participants determined whether this item had been present in this particular location in the sample display. 2. Two-features-binding conditions (Color- shapebinding, Color-motion binding, and Shape-motion binding): Take color-shape binding for example. Participants were told that the color or the shape of items would be tested. On different trial that there is change, the relationship between color and shape change for two items. The item of the test display was identical with one of the sample display on 50% of trials or could contain either a color of another item located at the different position in same hemi-field on 25% of trials or a shape on 25% of trials. The other conditions were the same. 3. Three-features-binding conditions (Color- shape-motion binding): Participants were told that the color or shape or motion direction of items could change. On different trial, at test an item which one feature switched for the other item presented. The item of the test display was identical with one of the sample display on 50% of trials or could contain either a color of another item located at the different position in same hemi-field on 16.6% of trials or a shape on 16.6% of trials or a motion direction on 16.6% of trials. As measurements of capacity of visual working memory, effects of experiment factors were analyzed using accuracy rates for each condition.
Accuracy (% Correct)
㪈㪇㪇
㪐㪇 㪺 㫊 㫄 㪺㫊 㪺㫄 㫊㫄 㪺㫊㫄
㪏㪇
㪎㪇
㪍㪇
㪌㪇 㪉
㪊
㪋
㪌
Set size Fig. Mean accuracy (including change and no-change trials) for conditions of experiments. c: color only, s: shape only, m: motion only, cs: color-shape binding, cm: color-motion binding, sm: shape-motion binding, csm: colorshape-motion binding.
TABLE II RESULTS OF POST-HOC ANALYSIS FOR CONDITION. The upper right of this table indicated p-values and the lower left indicated disparity between raw and column conditions.
III. RESULTS Figure 2 plots mean accuracy as a function of the number of to-be-remembered objects and conditions. We tested performance between conditions. Running a two-way ANOVA with factors of condition and set size, the main effect of condition was significant (F(6,252)=16.94, p
