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W A Johnston

Publications and source records attributed to W A Johnston.

At least 19 recordsLinked to original sources
PubMed2001

Driven to distraction: dual-Task studies of simulated driving and conversing on a cellular telephone.

Dual-task studies assessed the effects of cellular-phone conversations on performance of a simulated driving task. Performance was not disrupted by listening to radio broadcasts or listening to a book on tape. Nor was it disrupted by a continuous shadowing task using a handheld phone, ruling out, in this case, dual-task interpretations associated with holding the phone, listening, or speaking, However significant interference was observed in a word-generation variant of the shadowing task, and this deficit increased with the difficulty of driving. Moreover unconstrained conversations using either a handheld or a hands-free cell phone resulted in a twofold increase in the failure to detect simulated traffic signals and slower reactions to those signals that were detected. We suggest that cellular-phone use disrupts performance by diverting attention to an engaging cognitive context other than the one immediately associated with driving.

Adolescent↗
PubMed2000

Novel popout is an attention-based phenomenon: an ERP analysis.

When observers are given a brief glimpse of a display containing one novel object and three repeated objects, they are often better able to report the location of the novel object than the location of any one of the repeated objects. The present study contrasted two interpretations of this "novel popout" effect. The attention-based interpretation suggests that the novel popout is an attentional phenomenon, occurring during the initial processing of the four-object display. The retrieval-based interpretation suggests that novel popout is due to differential processing occurring when observers are subsequently probed for the location of one of the objects in the display. ERP measures recorded while subjects performed the novel popout task revealed differences during the initial processing of the four-object display but not subsequent to the presentation of a localization probe. The findings are most consistent with the attention-based interpretation of novel popout, which suggests that attention is rapidly drawn to the novel object in an otherwise familiar display.

Adolescent↗
PubMed1996

In vitro stage-specific sensitivity of Plasmodium falciparum to quinine and artemisinin drugs.

The inhibitory effects of quinine, chloroquine and 4 qinghaosu drugs, artemisinin, artemether, artesunate and dihydroartemisinin, on 4 culture-adapted isolates and 2 standard clones of Plasmodium falciparum were determined in vitro. All isolates were sensitive to the widely used antimalarial drugs quinine (EC50 range 3 x 10(-8)-1 x 10(-7) mol/L) and chloroquine (EC50 range 1 x 10(-9)-7 x 10(-9) mol/L), irrespective of the geographical origin or treatment history of the patients from which they were taken. In general, the qinghaosu drugs were more potent than the conventional antimalarials, having EC50 values of 3 x 10(-11)-3 x 10(-8) mol/L. Stage-specific data indicated that quinine has a primary mode of action on mature parasite forms, achieving 80-100% growth inhibition within 2-4 h of drug exposure. The stage-specific activity of the 3 qinghaosu drugs artemisinin, artemether and dihydroartemisinin differed from that of quinine, and each derivative displayed a unique stage-specific profile. Artemisinin was rapidly effective against both rings and schizonts, achieving 100% growth inhibition within 6-8 h. The inhibitory effects of artemether were less rapid, requiring 10 h to achieve 70-80% ring stage growth inhibition. Dihydroartemisinin was highly effective against all parasite stages in most cases achieving 100% growth inhibition within 2-4 h of exposure. The results confirm that the qinghaosu drugs are potent antimalarials, and suggest different stage-specific profiles compared to conventional antimalarial drugs.

Adult↗
PubMed1994

Novel popout with nonsense strings: effects of predictability of string length and spatial location.

Recent studies have shown that when one of four expected words is replaced by a single unexpected word, the unexpected word may capture attention. In three experiments, we explored the generality of this effect. In each experiment, observers viewed arrays composed of four computer-generated "nonsense" strings. Accuracy of string localization was assessed after each array. Some strings, called familiar, appeared in many arrays, whereas others, called novel, appeared in only one. In each experiment, novel strings in arrays composed of one novel and three familiar strings were localized more accurately than were novel strings in arrays composed entirely of novel strings, and familiar strings in these arrays were localized less accurately than were familiar strings in arrays composed entirely of familiar strings. These two effects, termed novel popout and familiar sink-in, respectively, were observed even when novel and familiar strings were rendered less discriminable by holding their lengths constant (Experiment 2) and when familiar strings always appeared in the same spatial locations (Experiment 3). The data suggest that novel objects can capture attention even when the objects lack any clear linguistic referent, when they are superficially similar to the familiar objects that surround them, and when the spatial locations of familiar objects are completely predictable.

Acoustic Stimulation↗
PubMed1991

Spontaneous selective attention in schizophrenia.

A visual word identification task was used to measure the type of selective attention that occurs spontaneously when there are multiple stimuli, all potentially relevant, and insufficient time to process each of them fully. This task presents two words simultaneously, one above the other, for 200 ms, and periodically requires the subject to identify either the upper or the lower word. We tested schizophrenic patients, manic-depressive patients, and normal controls under a baseline divided attention condition with no predictability and then introduced a degree of predictability into the upper location that normally results in selective attention to the lower nonpredictable location. In both the divided attention and the selective attention conditions, the schizophrenic group was just as accurate in identifying words as the other two groups, indicating no deficit in the rate of information processing. However, spontaneous selective attention under conditions of predictability was abnormal in the schizophrenic patients: They paid more attention to the predictable than to the nonpredictable source of information, consequently processing different, not less, information than normal subjects. Abnormal values on a ratio measure of selective attention occurred in 85% of the schizophrenic patients compared with only 30% of the manic-depressives and 20% of the normal subjects.

Adult↗
PubMed1991

Long-term perceptual memory for briefly exposed words as a function of awareness and attention.

Two experiments investigated the possibility that perceptual memory for words is dependent on level of awareness of those words. In Experiment 1, subjects attempted to report briefly exposed words in a study phase and then identify words that faded into view in a test phase. Old words appeared in both the study and test phases, whereas new words appeared only in the test phase. Perceptual memory, indexed as the faster identification of old vs. new words, was observed only for words correctly reported in the study phase. In the study phase of Experiment 2, words were flanked by digits, and the distribution of attention between words and digits was varied. Perceptual memory increased from nil to high levels as more attention was allocated to the words. These findings suggest that long-term perceptual memory is dependent on level of awareness of words in the study phase.

Adult↗
PubMed1991

Contribution of perceptual fluency to recognition judgments.

Following a shallow (count vowels) or deep (read) study task, old and new words were tested for both fluency of perception and recognition memory. Subjects first identified a test word as it came gradually into view and then judged it as old or new. Old words were identified faster than new words, indicating implicit, perceptual memory for old words. Independently of this effect, words judged old were identified faster than words judged new, especially after shallow study. Eight experiments examined the possible causal relationship between perceptual fluency and recognition judgements. Experiments 1 to 4 showed that fast identifications per se do not promote old judgments. Accelerating the identification of test items by semantically priming them or making them come more quickly into view did not affect recognition judgments. Experiment 5 showed that the usual association of fast identifications with old judgments is not an artifact of item selection because the association disappeared when the identifications and judgements were segregated into different phases of the test task. Experiments 6 and 7 showed tha the likelihood of old judgments increases directly with the pretested perceptibility of test words, but only after shallow study. Experiment 8 showed that the dependency of recognition judgments on perceptual fluency continues to hold when the requirement to identify the words before judging them is eliminated. We conclude that fluency of perception contributes to recognition judgments, but only when the fluency is produced naturally (e.g., through perceptual memory) and explicit memory is minimal.

Adult↗
PubMed1990

Attention capture by novel stimuli.

In several experiments, observers were given glimpses of 4-word arrays. Accuracy of word localization was tested after each array. Some words, called familiar, appeared many times across the series of arrays; others, called novel, appeared only once. The ratio of novel to familiar words in an array ranged from 0:4 to 4:0. When familiar and novel words were not intermixed (in 0:4 and 4:0 arrays), localization accuracy was higher for familiar words. However, when they were intermixed, especially in 1:3 arrays, accuracy tended to be higher for the novel words. This novel popout effect was the outcome of the suppressed localizability of the familiar words (relative to the 0:4 baseline) and the enhanced localizability of the novel words (relative to the 4:0 baseline). Novel popout may reflect an automatic orientation of attention away from more fluently unfolding regions of the perceptual field (familiar objects) and toward less fluently unfolding regions (novel objects).

Adolescent↗
PubMed1988

The influence of expertise on X-ray image processing.

Observers with four different levels of radiological experience performed a recognition memory task on slides of faces and chest X-ray films. Half of the X-ray films revealed clinically significant abnormalities and half did not. Recognition memory for faces was uniformly high across all levels of radiological experience. Memory for abnormal X-ray films increased with radiological experience and, for the most experienced radiologists, was equivalent to memory for faces. Surprisingly, recognition memory for normal films actually decreased with radiological experience from above chance to a chance level. These results indicate that radiological expertise is associated with selective processing of clinically relevant abnormalities in X-ray images. Expert radiologists appear to process X-ray images the way that we all process faces, by quickly detecting and devoting processing resources to features that distinguish one stimulus from another. However, the selective processing of X-ray films appears to be restricted to clinically relevant abnormalities. As they develop the ability to detect these abnormalities, radiologists appear to lose the ability to detect variations in normal features.

Clinical Competence↗
PubMed1985

Levels of selection and capacity limits.

Subjects performed a visual target-detection task in eight experiments. We examined the effects of word relevancy (word in relevant or irrelevant location) and display load (1-4 words) on physical, semantic, and controlled processing of nontargets. Interwoven with the detection task was a test-word identification task that was used to measure priming potency of nontargets. Physical and semantic levels of processing were measured in terms of identity and semantic priming, respectively. Nontarget primes were repeated as test words in identity priming. Nontarget primes were semantic associates of test words in semantic priming. Controlled processing of nontargets was measured in terms of recognition memory on a subsequent test. All measures increased with word relevancy and decreased with display load. The priming effects remained intact even when word presentation was speeded up and controlled processing was sharply curtailed. The data indicate that all levels of processing are selective and capacity limited.

Adolescent↗
PubMed1982

In defense of intraperceptual theories of attention.

A distinction is drawn between intraperceptual and extraperceptual theories of attention. Only the former class allows for the selective modulation of amount of nonconscious, perceptual processing of concurrent stimuli. The adequacy of intraperceptual theory has been questioned on the basis of diverse empirical findings. This literature is critically examined and is considered inconclusive partly because of the lack of an appropriate measure of amount of perceptual processing. A new experiment that attempts to provide such a measure is reported. Subjects detected names of states of the United States in dichotically presented lists. Subjects either could focus on just one list (relevant list) or had to divide their attention between the two lists. Amount of perceptual processing of a word (e.g., voice or titre) was measured in terms of the extent to which it primed subjects' interpretations of a visually presented homograph (e.g., spoke). Priming potency of words was greatest in the relevant list and was least in the other list. The data support a broad version of intaperceptual theory that allows for birth perceptual facilitation of relevant stimuli and perceptual suppression of irrelevant stimuli.

Attention↗
PubMed1979

Depth of nontarget processing in an attention task.

Two studies examined the effect of the sensory discriminability of targets from nontargets on depth of nontarget processing. Subjects shadowed target words that were binaurally presented with coincident nontarget words. Targets and nontargets were spoken in the same male voice under low sensory discriminability and in male and female voices, respectively under high sensory discriminability. Across the two studies, depth of nontarget processing was assessed in three ways: extent to which shadowing accuracy was disrupted by a semantic overlap between targets and nontargets, expenditure of capacity (reaction time to subsidiary light signals), and nontarget recall. All three possible measures of depth of nontarget processing decreased as sensory discriminability increased. The data support the assumption of multiple-loci theories of attention that nontargets can be perceptually inhibited; they contraindicate the assumption of late-selection theories that perceptual processing is automatic and irrepressible.

Attention↗