Storage and binding of object features in visual working memory

Abstract

An influential conception of visual working memory is of a small number of discrete memory "slots", each storing an integrated representation of a single visual object, including all its component features. When a scene contains more objects than there are slots, visual attention controls which objects gain access to memory. A key prediction of such a model is that the absolute error in recalling multiple features of the same object will be correlated, because features belonging to an attended object are all stored, bound together. Here, we tested participants' ability to reproduce from memory both the color and orientation of an object indicated by a location cue. We observed strong independence of errors between feature dimensions even for large memory arrays (6 items), inconsistent with an upper limit on the number of objects held in memory. Examining the pattern of responses in each dimension revealed a gaussian distribution of error centered on the target value that increased in width under higher memory loads. For large arrays, a subset of responses were not centered on the target but instead predominantly corresponded to mistakenly reproducing one of the other features held in memory. These misreporting responses again occurred independently in each feature dimension, consistent with 'misbinding' due to errors in maintaining the binding information that assigns features to objects. The results support a shared-resource model of working memory, in which increasing memory load incrementally degrades storage of visual information, reducing the fidelity with which both object features and feature bindings are maintained.

Figure 1. Precision of working memory in a dual-feature reproduction task

(a) Subjects were presented with an array of colored, oriented bars, followed by a pattern mask. After a blank retention interval, a probe appeared and subjects used two response dials to adjust its color and orientation to match the item at the corresponding location in the memory array (the target). (b & c) Turning each dial cycled the probe through a circular parameter space of possible colors or orientations. Some examples of orientations (b) and colors (c) are shown corresponding to different points in each response space. (d) Recall precision as a function of memory load: 1 object (low) v 6 objects (high). Precision is defined as the reciprocal of the standard deviation of error in subjects’ responses: zero indicates chance performance.

Figure 2. Distribution of errors relative to target feature values

(a) Frequency of response as a function of the deviation between reported and target feature values in the low-load (1 object) condition: for color (top), orientation (right), and conjunction of both features (heat map). Colored lines indicate the response probabilities predicted by a mixture model combining a gaussian distribution centered on the target value and a uniform distribution spread equally across the response space. (b) Error distributions plotted as in (a) for the high-load (6 object) condition. (c) Predicted distributions of color and orientation responses under conditions of full correlation (top) and full independence (bottom) between feature dimensions. Compare with the observed distribution of errors shown in (b). (d) Proportion of trials on which both responses are centered on target values (TT); both are uniformly distributed (UU); the orientation response is centered on the target and the color response is uniform (TU); and vice versa (UT). Estimates obtained by fitting a mixture model to observed responses are shown along with predictions under full correlation and full independence models.

Figure 3. Distribution of errors relative to non-target feature values

(a) Frequency of response as a function of the deviation between reported feature values and those of each non-target (unprobed) item, in the high-load (6 object) condition, for color (top), orientation (right), and conjunction of both features (heat map). Note the strong central tendency in each distribution, indicating that subjects frequently mistakenly report the features of non-target items. (b) Proportion of trials on which color and orientation responses are both centered on the same non-target (N=N); each is centered on a different non-target (N≠N); the orientation response is centered on the target and the color response on a non-target (TN); and vice versa (NT). Estimates obtained by fitting a mixture model to observed responses are shown along with predictions of correlated-misreporting and independent-misreporting models.