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Biomedical subjects

H L Hawkins

Publications and source records attributed to H L Hawkins.

13 recordsLinked to original sources
PubMed1996

Mechanisms of visual-spatial attention: resource allocation or uncertainty reduction?

Many studies have found that stimuli can be discriminated more accurately at attended locations than at unattended locations, and such results have typically been taken as evidence for the hypothesis that attention operates by allocating limited perceptual processing resources to attended locations. An alternative proposal, however, is that attention acts to reduce uncertainty about target location, thereby increasing accuracy by decreasing the number of noise sources. To distinguish between these alternatives, we conducted 6 spatial cuing experiments in which target location uncertainty was eliminated. Despite the absence of uncertainty, target discriminations were more accurate at the attended location, consistent with resource allocation models. These cue validity effects were observed under a broad range of conditions, including central and peripheral cuing, but were absent at very short cue-target delay intervals.

Attention↗
PubMed1994

Effects of spatial cuing on luminance detectability: psychophysical and electrophysiological evidence for early selection.

Three experiments were conducted to determine whether attention-related changes in luminance detectability reflect a modulation of early sensory processing. Experiments 1 and 2 used peripheral cues to direct attention and found substantial effects of cue validity on target detectability; these effects were consistent with a sensory-level locus of selection but not with certain memory- or decision-level mechanisms. In Experiment 3, event-related brain potentials were recorded in a similar paradigm using central cues, and attention was found to produce changes in sensory-evoked brain activity beginning within the 1st 100 ms of stimulus processing. These changes included both an enhancement of sensory responses to attended stimuli and a suppression of sensory responses to unattended stimuli; the enhancement and suppression effects were isolated to different neural responses, indicating that they may arise from independent attentional mechanisms.

Adolescent↗
PubMed1992

Aging, exercise, and attention.

The authors investigated the relationship among aging, attentional processes, and exercise in 2 experiments. First they examined age differences on 2 attentional tasks, a time-sharing task and an attentional flexibility task. Young adults alternated attention between 2 sequenced tasks more rapidly and time-shared the processing of 2 tasks more efficiently than older adults. They then investigated the effects of aerobic exercise on the same 2 attentional tasks in older adults. Following the 10-week exercise program, older exercisers showed substantially more improvement in alternation speed and time-sharing efficiency than older controls. Interestingly, this exercise effect was specific to dual-task processing. Both groups of subjects showed equivalent effects on single-task performance. These results indicate that aerobic exercise can exert a beneficial influence on the efficiency of at least 2 different attentional processes in older adults.

Adult↗
PubMed1990

Visual attention modulates signal detectability.

The mechanism by which visual-spatial attention affects the detection of faint signals has been the subject of considerable debate. It is well known that spatial cuing speeds signal detection. This may imply that attentional cuing modulates the processing of sensory information during detection or, alternatively, that cuing acts to create decision bias favoring input at the cued location. These possibilities were evaluated in 3 spatial cuing experiments. Peripheral cues were used in Experiment 1 and central cues were used in Experiments 2 and 3. Cuing similarly enhanced measured sensitivity, P(A) and d', for simple luminance detection in all 3 experiments. Under some conditions it also induced shifts in decision criteria (beta). These findings indicate that visual-spatial attention facilitates the processing of sensory input during detection either by increasing sensory gain for inputs at cued locations or by prioritizing the processing of cued inputs.

Adult↗
PubMed1978

Perceptual flexibility in word recognition: strategies affect orthographic computation but not lexical access.

Four tachistoscopic forced-choice recognition experiments explored the flexibility of processes underlying word perception. Stimuli were words, orthographically regular but unfamiliar pseudowords, and orthographically irregular nonsense strings. In the first two experiments, subjects knew that several different kinds of stimuli would occur in each block of trials and that one kind would occur much more often than the others. No matter which stimulus subjects expected to see most often, accuracy on words and pseudowords differed little, and both were identified considerably better than nonsense. In the third and fourth experiments, subjects were led to believe that only on stimulus type would occur but were surreptitiously shown another type on a small number of trials. Words were again identified more accurately than nonsense, and the size of the effect was independent of expectations. However, when either words or nonsense strings were expected exclusively, pseudoword accuracy did not differ from nonsense accuracy. Only when subjects knew that pseudowords would occur did they identify pseudowords more accurately than nonsense. This dissociation between word and pseudoword identification indicates the operation of two independent encoding mechanisms during tachistoscopic recognition, a stimulus-specific or logogenlike system sensitive to particular familiar strings and an orthographic mechanism sensitive to generally applicable constraints on letter sequencing. The stimulus-specific mechanism appears to be utilized automatically, but use of the orthographic mechanism is under strategic control. As shown in the first two experiments, however, rather extraordinary measures were required to demonstrate the flexibility of the orthographic processes used in this task.

Cognition↗
PubMed1977

Abnormal plasma lipoproteins and lecithin-cholesterol acyltransferase deficiency in alcoholic liver disease.

Serial studies of plasma lipids and lipoproteins were performed in 4 patients with acute alcoholic liver disease characterized by a massive fatty liver and laboratory evidence of intrahepatic cholestasis. There were striking alterations in the plasma lipoprotein electrophoretic patterns characterized by the absence of alpha- and pre-beta-lipoprotein bands and the presence of a single band of abnormal mobility. These changes were associated with an extreme decrease in plasma lecithin-cholesterol acyltransferase activity, resulting in greatly reduced levels of plasma cholesteryl esters and increased levels of unesterified cholesterol. In 2 patients hypertriglyceridemia and hypercholesterolemia were present, the latter because of an increase in unesterified cholesterol. Lipoproteins were isolated from the plasma by sequential ultracentrifugation at the densities used for separation of normal very low density, low density, and high density lipoproteins; however, the patients' lipoproteins were different from normal in lipid composition and ultrastructure. All of the lipoprotein fractions were decreased in cholesteryl esters and the major lipoprotein was a triglyceride-rich low density lipoprotein. Electron microscopic studies of the low and high density lipoprotein fractions revealed the presence of bilamellar vesicles and stacked discs. All of the changes in lipoprotein composition and ultrastructure gradually returned to normal with clinical improvement. These observations indicate that alcoholic liver injury is associated with profound alterations in lipoprotein composition and metabolism which may be related in part to lecithin-cholesterol acyltransferase deficiency.

Acyltransferases↗
PubMed1976

Flexible coding in word recognition.

An experiment was designed to examine the contribution of phonetic information in the processing of words in tachistoscopic recognition masking. Following stimulus presentation, subjects were required to indicate which of two alternatives had appeared. On trials containing word stimuli, the alternatives were either phonetically identical (SENT, CENT) or not (SOLD, COLD). Recognition performance was inferior in the former case, provided conditions were not structured to discourage reliance on phonetic information. The findings were interpreted as showing that more than one type of coding process can underly the word superiority effect. Phonetic information is ordinarily used to code words in this type of task, but an alternative processing tactic (e.g., one relying on visual or perhaps semantic codes) can also be effectively used in word recognition when phonetic information does not discriminate well among response alternatives.

Discrimination, Psychological↗