Search

A two-stage visual search model is described. Four experiments show that in a VM paradigm sequential presentation of the display leads to increases in accuracy and decreases in latency. Slower presentation rates allow for faster transfer of the target from iconic to short-term memory. When target and background items are dissimilar, transfer to short-term memory need not take place and decisions can be made on the basis of the Stage 1 analysis. These results are taken as confirming evidence for the proposed model.

To what extent does successful search for a target letter in a visual display depend on the allocation of attention to the target’s spatial position? To investigate this question, we required subjects to discriminate the orientation of a briefly flashed U-shaped form while searching for a target letter. Performance operating characteristics (POCs) were derived by varying the relative amounts of attention subjects were to devote to each task. Extensive tradeoffs in performance were observed when the orientation form and target letter occurred in nonadjacent display positions. In contrast, the tradeoff was much more restricted when the two targets occurred in adjacent positions. These results suggest that the interference between simultaneous visual discriminations depends critically on their separation in visual space. Both visual search and form discrimination require a common limited-capacity visual resource.

Previous research had indicated that when subjects are instructed to report one of a number of visually displayed items, both the number and spacing of the presented material affect report accuracy and latency. The present experiment sought to determine the nature and temporal course of the interference provided by nonattended visual material. Subjects reaction times were measured for deciding which of two targets occupied the indicated position in one or eight element displays. Placing replicas of the target in nonindicated display positions was equivalent to presenting the target alone. Members of the opposite response set produced maximum interference, while encodable and unencodable noise elements not belonging to a response set produced an intermediate decrement. For all display types, presenting the indicator prior to display onset decreased reaction time. Dividing each of the display elements into two parts and presenting the parts asynchronously provided evidence that subjects were indifferent to the presence of complete forms for the first 50 msec. These results were interpreted as supporting the existence of a hiararchical sequence of stages consisting of a preattentive stage which segregates the input into objects and an attentive stage which is likened to a spatial scanner responsible for synthesizing the crude preattentive features into recognized forms. The concurrent operation of these stages provides for the redirection of attention when changes in the input are detected.

When a multiletter display is preceded by a bar designating one of the letters for report, reaction time (RT) to voice the indicated letter decreases. Previous research had indicated that the efficiency of this selective mechanism decreases as the number of display elements increases. Two experiments were conducted to determine whether the effect of display size could be eliminated when the indicator precedes the display at long intervals. Results indicated that the display size effect was maintained. The results could not be attributed to eye movements, but were interpreted in terms of a central encoding mechanism that is limited in its precision of localization and exclusion. At the phenomenal level, we can look at an object and see it as an entire object without concentrating on specific details. For example, we may look at the clock on the wall, even noticing what time it is, but not paying "attention" to specific characteristics of the numerals. Or we can phenomenally direct our attention to one of the numerals on the clock such as the 3 and in the process seem to exclude the details of the other numerals. This experience can occur even though our distance from the clock is such that the whole clock face is falling within the fovea and assuring adequate resolution of detail for all parts of the clock. It is as though we had a limited capacity for processing information per unit of time, and we can select which information and in what order it is going to be processed and encoded. We have been employing an experimental analog of this phenomenal experience. Circular displays of a small subset of letters are presented to Ss with a black bar indicator designating one of the letters in the display. The display as a whole is foveal. sub tending approximately 2 deg of visual angle on the diameter. The bar indicator is quite conspicuous, both in terms of its size and shape, but primarily in terms of its location as an extension of an imaginary radius from the center of the display extending beyond the circumference of the imaginary circle along which the letters are arrayed. Ss are instructed to name the designated or target letter as quickly as possible. By manipulating the display variables, we have hoped to delineate some of the characteristics by which selective attention or encoding occurs.

The time required for Ss to voice a target letter in a visual display was studied as a function of the spatial proximity of two kinds of noise elements (letters or disks) to the target and as a function of when the noise elements were presented following the onset of the target letter. The results were not consistent with a focusing model of attention or selective encoding. Instead, there appears to be a small area in the visual field (about 1 deg of angle) in which all stimuli are processed in detail.

Circular visual displays of 12 elements, consisting of A, H, M, and U, were presented to S. She responded with a lever movement in one direction if a given letter designated by a bar indicator was a member of the set A-U and in the opposite direction if it was from the other set. The principal experimental variables were the SOA by which the indicator preceded the display and whether the target letter was flanked by letters of the same or opposite set. The results indicated that the interference produced by noise letters is primarily on the response as opposed to the processing side. Also, there is a limit to the precision of selective attention in the visual field. The interference of opposite-set letters was inversely proportional to their distance from the target.

Previous research had found that the accuracy with which an S could report an indicated letter or target in a briefly exposed multiletter display decreased as the number of irrelevant letters increased, and accuracy increased if the position of the target letter was indicated 150 msec before the display was presented. Iii the present experiment, these variables were reinvestigated using vocal reaction time as the dependent variable. An interpretation of the accuracy measures in terms of differential processing times was supported. The results of the two experiments were discussed in terms of a model of attentional selectivity.