Abstract
Intuition suggests that increased viewing time should allow for the accumulation of more visual information, but scant support for this idea has been found in studies of voluntary averaging, where observers are asked to make decisions based on perceived average size. In this paper we examine the dynamics of information accrual in an orientation-averaging task. With orientation (unlike intensive dimensions such as size), it is relatively safe to use an item's physical value as an approximation for its average perceived value. We displayed arrays containing eight iso-eccentric Gabor patterns, and asked six trained psychophysical observers to compare their average orientation with that of probe stimuli that were visible before, during, or only after the presentation of the Gabor array. From the relationship between orientation variance and human performance, we obtained estimates of effective set size, i.e., the number of items that an ideal observer would need to assess in order to estimate average orientation as well as our human observers did. We found that display duration had only a modest influence on effective set size. It rose from an average of ∼2 for 0.1-s displays to an average of ∼3 for 3.3-s displays. These results suggest that the visual computation is neither purely serial nor purely parallel. Computations of this nature can be made with a hybrid process that takes a series of subsamples of a few elements at a time.
Highlights
Attneave (1954) noted, ‘‘When some portion of the visual field contains a quantity of information grossly in excess of the observer’s perceptual capacity, he treats those components of information . . . as a statistician treats ‘error variance,’ averaging out particulars and abstracting certain statistical homogeneities’’ (p. 188)
If voluntary averaging were mediated by a purely serial process, i.e., one that estimates the orientations of individual items, one at a time, (a) the effective set size should grow with the time available for processing the stimulus, and (b) it should be possible to prevent the serial process from having sufficient time to estimate the properties of more than one item in the array
The results did not support the hypothesis of reduced efficiency with increased memory load
Summary
Attneave (1954) noted, ‘‘When some portion of the visual field contains a quantity of information grossly in excess of the observer’s perceptual capacity, he treats those components of information . . . as a statistician treats ‘error variance,’ averaging out particulars and abstracting certain statistical homogeneities’’ (p. 188). To quantify how well summary statistics like average orientation are calculated, we have adopted an Equivalent Noise (Nagaraja, 1964; Pelli, 1990; Dakin, 2001) framework for collecting and analyzing psychophysical data Within this framework, there are two distinct limits on visual performance. If voluntary averaging were mediated by a purely serial process, i.e., one that estimates the orientations of individual items, one at a time, (a) the effective set size should grow with the time available for processing the stimulus, and (b) it should be possible to prevent the serial process from having sufficient time to estimate the properties of more than one item in the array. As for the other independent variable, we expected a general facilitation of performance when probes were exposed before the Gabor array (no memory load) and a general reduction in performance when probes were exposed after it (high memory load)
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