Academic journal article Attention, Perception and Psychophysics

Coherent Plaids Are Preattentively More Than the Sum of Their Parts

Academic journal article Attention, Perception and Psychophysics

Coherent Plaids Are Preattentively More Than the Sum of Their Parts

Article excerpt

We investigated whether plaids activate preattentive mechanisms that are distinct from those activated by their component gratings. Observers searched for a target plaid, the sum of two perpendicular components in a circular window (radius = 0.65°). The target was present on half the trials. On all trials, half of the distractors had the same frequency and orientation as one component of the plaid, and the rest were the same as the other component. The target and the distractors were arrayed evenly on a circle (radius = 2.36°) around fixation. Target and distractor contrasts were randomly perturbed up to ±30%. The following results held for each of the 6 participants tested. (1) When F1 = 2 c/deg and F2 = 5.25 c/deg, response times (RTs) increased significantly when set size (number of distractors plus target, if present) was increased from four to eight. (2) When the spatial frequencies of both plaid components were the same (i.e., both 2 c/deg or both 5.25 c/deg), RTs increased very slightly, if at all, when set size was increased from four to eight. These results suggest the existence of a preattentive, plaid-sensitive mechanism with band-limited input that does not respond to individual grating components.

The research reported here was based on several assumptions about low-level visual processing, all of which are common to the standard model of visual texture perception (e.g., Bergen & Landy, 1991). First, we follow Adelson and Bergen (1991) in assuming that, before extracting such things as object boundaries, identities, and locations from the retinal input, the visual system first applies a battery of fast, spatially parallel image transformations whose response images reflect "the amounts of various kinds of visual 'substances' present in the image" (p. 3). It is useful to imagine these substance-sensing transformations as being implemented in retinotopically organized neural arrays. In a given array, all neurons are assumed to perform the same computation on the visual input impinging on a small region of the retina but at different locations in the visual field. Thus, any such substance-sensing array operates like a movie camera to make a neural image available to higher level vision (Robson, 1980), reflecting the rapidly changing distribution of a particular substance across the visual field. Because these hypothetical arrays are assumed to operate automatically and prior to any conscious effort on the part of the participant, we call them preattentive mechanisms. Each preattentive mechanism requires a dedicated array of neurons, all of which are continuously active in normal vision. The high expense in neural computational resources of a single such mechanism makes it likely that human vision has only a modest number of them, raising the prospect that we may be able to catalog them. Thus, two of the most compelling open questions in the field of low-level vision are as follows: How many preattentive mechanisms exist in human vision, and what properties of the visual input do these mechanisms sense? Or, more grandly, what are the elementary substances of human vision?

The picture sketched above echoes the original theory offered by Treisman and Gelade (1980) to account for instances of parallel versus serial search. They proposed that, when search is parallel (i.e., when a target pops out from a field of distractors irrespective of how many there are), it is because there exists in human vision one or more preattentive mechanisms (of the sort hypothesized above) activated by the target but not by the distractors. Wolfe and Horowitz (2004) have argued that this picture is too simple. They pointed out that there exist certain cases of parallel search that seem to require more complicated preprocessing than we imagine as being achieved by a preattentive mechanism. In particular, search is parallel for an oriented bar among bars perpendicular to the target bar, even when all bars in the scene are largely occluded by amorphous blobs (one for each bar) so that the small bits of bar that remain revealed offer no cues of the sort that might drive a standard, orientation-selective preattentive mechanism. …

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