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W B Cowan

Publications and source records attributed to W B Cowan.

15 recordsLinked to original sources
PubMed2001

Interactions between search mechanisms in conjunction search.

We report the results of a technique designed to measure interactions between different visual search processes. We interrupted pop-out search before it produced a detection response, by adding extra distractors to the display so that a target initially defined by a single feature difference (e.g., a yellow horizontal line among yellow vertical lines) could then only be found on the basis of the conjunction of two features (a yellow horizontal line among yellow vertical lines and pink horizontal lines; difficult search). This technique has been used to measure the duration of the perceptual components of pop-out search, independent of over-all response time, for targets presented among different sets of distractors. In addition, when pop-out failed because it was interrupted, past work has shown that it nevertheless provided useful information to the processes responsible for difficult search. That is, partial pop-out assisted difficult search, when extra distractors made search difficult because the target was between the two types of distractors in the relevant feature space (Olds, Cowan, & Jolicoeur, 2000a,b,c). The present results demonstrate that partial pop-out also assists difficult search when difficult search is a conjunction search, and therefore these interactions may occur at a stage where information from different feature dimensions is combined.

Adult↗
PubMed2000

The time-course of pop-out search.

Olds, Cowan and Jolicoeur [2000. Tracking visual search over space and time. Psychonomic Bulletin & Review (in press)] interrupted pop-out search by adding distractors to a display after a delay. They analyzed the response time distributions from conditions with different delays for interruption and showed that when pop-out search fails, its partially completed computations can be used to assist other, slower search processes. This paper demonstrates that expectancies, numbers of items and colors in the display, and color onsets do not explain those results. Finally, an experiment in which the target was moved mid-trial demonstrates that partial pop-out assists difficult search by indicating something about where the target is, or where the target is not.

Adult↗
PubMed2000

Partial orientation pop-out helps difficult search for orientation.

We interrupted pop-out search before it produced a detection response by adding extra distractors to the search display. We show that when pop-out for an orientation target fails because of this interruption, it nevertheless provides useful information to the processes responsible for difficult search. That is, partial pop-out assists difficult search. This interaction has also been found for color stimuli (Olds, Cowan, & Jolicoeur, 2000a, 2000b). These results indicate that interactions and/or overlap between the mechanisms responsible for pop-out and the mechanisms responsible for difficult search may be quite general in early visual selection.

Adult↗
PubMed2000

Tracking visual search over space and time.

Visual perception consists of early preattentive processing and subsequent attention-demanding processing. Most researchers implicitly treat preattentive processing as a domain-dependent, indivisible stage. We show, however, by interrupting preattentive visual processing of color before its completion, that it can be dissected both temporally and spatially. The experiment depends on changing easy (preattentive) selection into difficult (attention-demanding) selection. We show that although the mechanism subserving preattentive selection completes processing as early as 200 msec after stimulus onset, partial selection information is available well before completion. Furthermore, partial selection occurs first at locations near fixation, spreading radially outward as processing proceeds.

Adult↗
PubMed1999

Convex hull test of the linear separability hypothesis in visual search.

Visual search for a colour target in distractors of two other colours is dramatically affected by the configuration of the colours in CIE (x, y) space. To a first approximation, search is difficult when a target's chromaticity falls directly between (i.e. is not linearly separable from) two distractor chromaticities, otherwise search is easy (D'Zmura [1991, Vision Research, 31, 951-966]; Bauer, Jolicoeur, & Cowan [1996a, Vision Research, 36, 1439-1466]; Bauer, Jolicoeur, & Cowan [1996b, Perception, 25, 1282-1294]). In this paper, we demonstrate that the linear separability effect transcends the two distractor case. Placing a target colour inside the convex hull defined by a set of distractors hindered search performance compared with a target placed outside the convex hull. This is true whether the target was linearly separable in chromaticity only (Experiments 1 and 2), or in a combination of luminance and chromaticity (Experiments 3 and 4).

Adult↗
PubMed1999

Stimulus-determined discrimination mechanisms for color search.

Visual attention can be goal driven, stimulus driven, or a combination of the two. Here we report evidence for an unexpectedly stimulus-driven component of visual search for a target defined by color. Observers demonstrated a surprisingly cost-free ability to incorporate multiple classifiers in search for a target of one color from among distractors of other colors. A target color was presented among distractors that could change from trial to trial (intermixed presentation) or that remained constant across all trials in a block (blocked presentation). For blocked presentation, a single search classifier (a mechanism that segregates the target from distractors in color space) could be adopted, whereas for intermixed presentation different classifiers had to be used when the distractor colors changed. The benefit of blocked presentation was very small, suggesting that the appropriate classifier was determined very quickly in trials for which the classifier changed. The results suggest that the stimulus-driven activation of an appropriate stimulus classifier can be very efficient.

Adult↗
PubMed1998

The linear separability effect in color visual search: ruling out the additive color hypothesis.

Bauer, Jolicoeur, and Cowan (1996b, 1996c) demonstrated difficult visual search for color targets that were not linearly separable (in color space) from two distractor colors and easier search for linearly separable targets. This suggested that search is mediated by a chromatically linear discrimination mechanism (see D'Zmura, 1991). However, in those experiments, the targets that were not linearly separable fell midway between the distractor colors and thus corresponded to the admix of the distractor colors. An alternate interpretation of the results of Bauer et al. is that search was more difficult when the target corresponded to the distractor admix than when it did not. We tested this hypothesis in three experiments by contrasting conditions in which a target that was not linearly separable did or did not correspond to the admix of the distractor colors. In all cases, a target that was not linearly separable produced difficult search, demonstrating that linear separability determines search performance.

Color Perception↗
PubMed1996

Visual search for colour targets that are or are not linearly separable from distractors.

D'Zmura [(1991) Vision Research, 31, 951-966] reported qualitative differences in visual search rates for a target colour in a background of differently coloured distractors depending on their colour configuration in CIE(x,y) space. A target colour that was chromatically mid-way between the distractor colours resulted in steep search slopes. A target off the distractor-distractor line, "popped out". We replicated his finding in several loci, investigated several potential confounds, and discovered boundary conditions for the phenomenon: for a given target, the effect of collinearity dissipates with increasing distractor-distractor colour difference. Furthermore, within limits, performance was dependent on the target to distractor-line distance.

Adult↗
PubMed1989

Black light: how sensors filter spectral variation of the illuminant.

Visual sensor responses may be used to classify objects on the basis of their surface reflectance functions. In a color image, the image data are represented as a vector of sensor responses at each point in the image. This vector depends both on the surface reflectance function and on the spectral power distribution of the ambient illumination. Algorithms designed to classify objects on the basis of their surface reflectance functions typically attempt to overcome the dependence of the sensor responses on the illuminant by integrating sensor data collected from multiple surfaces. In machine vision applications, we show that it is often possible to design the sensor spectral responsivities so that the vector direction of the sensor responses does not depend upon the illuminant. We state the conditions under which this is possible and perform an illustrative calculation. In biological systems, where the sensor responsivities are fixed, we show that some changes in the illumination cause no change in the sensor responses. We call such changes in illuminant black illuminants. It is possible to express any illuminant as the sum of two unique components. One component is a black illuminant. We call the second component the visible component. The visible component of an illuminant completely characterizes the effect of the illuminant on the vector of sensor responses.

Artificial Intelligence↗
PubMed1984

On the ability to inhibit simple and choice reaction time responses: a model and a method.

This article reports four experiments on the ability to inhibit responses in simple and choice reaction time (RT) tasks. Subjects responding to visually presented letters were occasionally presented with a stop signal (a tone) that told them not to respond on that trial. The major dependent variables were (a) the probability of inhibiting a response when the signal occurred, (b) mean and standard deviation (SD) of RT on no-signal trials, (c) mean RT on trials on which the signal occurred but subjects failed to inhibit, and (d) estimated RT to the stop signal. A model was proposed to estimated RT to the stop signal and to account for the relations among the variables. Its main assumption is that the RT process and the stopping process race, and response inhibition depends on which process finishes first. The model allows us to account for differences in response inhibition between tasks in terms of transformations of stop-signal delay that represent the relative finishing times of the RT process and the stopping process. The transformations specified by the model were successful in group data and in data from individual subjects, regardless of how delays were selected. The experiments also compared different methods of selecting stop-signal delays to equate the probability of inhibition in the two tasks.

Choice Behavior↗
PubMed1983

The chromatic Cornsweet effect.

The Cornsweet effect was measured using equiluminous chromatic gradients as well as with an achromatic gradient. The chromatic Cornsweet effect is smaller than the achromatic effect.

Color Perception↗
PubMed1982

Changes in perceived color due to chromatic interactions.

Studies of chromatic induction have generally examined either (a) the effects of a chromatic surround on a neutral test field, or (b) the effects of one spectral hue on another. To investigate how colors interact in other regions of color space an experiment was designed using fifteen test stimuli scattered through C.I.E. color space. The perceived hue of each stimulus was matched on its own and in the presence of five inducing stimuli. Matching was done both with and without a lens to correct axial chromatic aberration, which was found to be a significant prereceptoral factor influencing perceived colour. With chromatic aberration corrected the overall pattern of chromatic changes can be explained neither by receptor processes alone, nor by opponent channel processes alone. But a reasonable fit can be obtained if changes are allowed to take place in both levels of the system.

Color Perception↗
PubMed1981

Deuteranomalous color matching in the deuteranopic eye.

Two observers were classified as deuteranopes by standard tests including two-degree anomaloscope matches. Color matching similar to the Rayleigh type was then carried out for a 10-degree field size at retinal illuminance ranging from 1 to more than 3000 trolands (td). The results show that at the larger field size and higher levels of retinal illuminance, a third independent color-mediating mechanism with the sensitivity of the deuteranomalous cone is participating in the color match. The results also confirm participation of a different third mechanism with rod sensitivity at levels below about 100 td. There is a range of transition between the two as the level increases above 100 td. Therefore large-field color matching in these deuteranopes is trichromatic at the levels tested, not dichromatic, and a third cone system is found to operate at typical photopic light levels under static viewing conditions in a dichromatic eye.

Color Perception Tests↗