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Biomedical subjects

G R Lockhead

Publications and source records attributed to G R Lockhead.

6 recordsLinked to original sources
PubMed1989

McCollough aftereffects in strabismus and amblyopia.

In strabismic people, the functioning of the deviating eye is impaired. This impairment is binocular in origin since it depends on stimulation of the straight eye; sensitivity and acuity of the deviating eye are usually better when the straight eye is patched than when both eyes are viewing. We investigated interactions between the eyes in strabismic people by using the McCollough color aftereffect as an exploratory procedure. Both normal and strabismic people adapted to black and colored bars and then examined black and white test bars. They reported colors in the achromatic test bars that were complementary to the adapting colors; this is the McCollough aftereffect. Both a monocular McCollough effect and a binocular McCollough effect were induced in people having normal vision. Strabismic people did not show a binocular McCollough effect. Aftereffect strengths were the same in each eye of normal people but were stronger in the impaired eye of strabismic people. A speculation is offered as to why the "non-seeing" eye of strabismic people "sees" the aftereffect better.

Adaptation, Ocular↗
PubMed1987

Brightness of luminance distributions with gradual changes.

The perception of gradually changing luminance distributions was investigated. Luminance changed across the radius of a disk by a linear, quadratic, or cubic function with varying magnitudes. Subjects selected matching luminances for the inner and outer edges of each stimulus. The threshold for reporting that the inner and outer matches were different occurred at approximately 20% contrast between those regions. This threshold did not vary with the particular function which described the luminance distribution. Further, as the magnitude of luminance change across the stimulus increased, subjects judged the inner and outer edges to differ more in brightness. Matching luminances also depended upon the background configuration with greater differences perceived across the disk radius when the surround and center dot of the stimulus were of opposite, rather than the same, luminance. These results indicate that models of brightness must consider all luminance changes in the stimulus, not just changes of a particular type such as second differences or changes at a luminance step.

Humans↗
PubMed1979

Holistic versus analytic process models: a reply.

Two classes of stimulus process models are considered in this reply to Dykes and Cooper. It is shown that analytic models which assume that stimuli are initially processed in terms of constituent dimensions do not account for large amounts of published data. It is also shown that the holistic-discriminability model that Dykes and Cooper reject is nonetheless consistent with their results and predicts all of the data for which their analytic model was constructed to account.

Discrimination Learning↗
PubMed1977

Classifying intergral stimuli.

Two reported experiments support holistic, as opposed to analytic, processing models for integral stimuli. Speeded classification data from different information-processing tasks (univariate and correlated) were predicted by distance between stimuli in similarity space but not by redundancy. The results of the filtering and condensation tasks and the notion of configural stimuli are also explicable in these terms. It is shown that some operational definitions commonly used to define integral stimuli are usually confounded with stimulus similarity. The assumption of independence between the attributes that combine to form multidimensional stimuli is not always met and is always an empirical question. When these attributes are not independent, physical and psychological spaces are not necessarily the same. Similarity structure is a crucial concern if inferences of cognitive processing are to be based on information-processing task results.

Association↗
PubMed1977

Parameters of stimulation and perception in an artificial sensory feedback system.

The relationships between stimulus parameters and perceptions in a prosthetic feedback system were measured using psychophysical methods. Electrical stimulation of the median nerve produced a monotonic relation between frequency of stimulation and the perceived magnitude of the stimulus. There were two qualitatively different perceptions of the stimulation; one for low frequencies and one for high. These two qualities fit different psychophysical continuua, kind of stimulation, and amount of stimulation.

Adult↗