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Biomedical subjects

H W Leibowitz

Publications and source records attributed to H W Leibowitz.

At least 37 records · Page 2Linked to original sources
PubMed1988

Age differences in Vistech near contrast sensitivity.

Contrast sensitivity was measured at near using the Vistech 6000 Contrast Test System (VCTS) in 287 persons aged 4 to 87 years. Obtained data compared well with those previously reported for the Vistech charts. Results indicate an age-related loss in sensitivity to intermediate and higher spatial frequencies. This is consistent with the age trends observed using other methods for measuring contrast sensitivity functions (CSFs). However, the Vistech charts yield contrast sensitivity values which are lower than those obtained previously with the Nicolet system. These between-method differences are greatest at lower spatial frequencies, and are attributed to the small number of grating cycles in the Vistech charts. The data are used to develop provisional age norms for the Vistech charts which allow the assessment of an individual's performance relative to their age cohort. Recommendations for use and future development of the Vistech charts are also given.

Adolescent↗
PubMed1986

Reaction times to different spatial frequencies as a function of detectability.

Simple reaction time to sine-wave gratings of 1, 4 and 10 c/deg was measured as a function of the detectability of the gratings. Reaction time increased with spatial frequency over a range of detectabilities. Assuming that equally detectable gratings produce equivalent levels of response in the visual system, this increase in reaction time reflects an increase in perceptual latency as spatial frequency is increased.

Form Perception↗
PubMed1986

Two modes of processing visual information: implications for assessing visual impairment.

The recognition of visual detail and the ability to orient spatially to visual stimuli are apparently subserved by distinct modes of processing, termed "focal" and "ambient," respectively. Current clinical techniques are well suited to evaluate aspects of focal vision, but do little to evaluate ambient visual functions. As a result, the impairment of ambient functions may be difficult to specify or may be overestimated. Implications for the development of assessment techniques for ambient functions are discussed.

Humans↗
PubMed1985

Viewing distance and the sustained detection of high spatial frequency gratings.

Although conventional visual acuity tests measure the ability to resolve fine detail, many visual tasks demand prolonged detection. By evaluating the ability to maintain visual resolution, we observed that detection of stimuli subtending a small but constant visual angle depends on viewing distance and is probably determined by accommodative control. Detection was longest when viewing distance corresponded to the individual observer's accommodative resting, or tonus, position and fell off markedly for further distances. Since most visual assessment procedures involve a momentary detection criterion, they will not predict the ability to sustain accommodation and may thus overestimate visual resolution for prolonged viewing tasks.

Accommodation, Ocular↗
PubMed1985

A revised analysis of the role of efference in motion perception.

The analysis of motion perception historically has included efferent as well as afferent mechanisms to account for the perception of motion during eye movement. The application of efferent mechanisms to motion perception has been limited, however, by several illusions which are apparently inconsistent with the notion that oculomotor mechanisms contribute to motion perception. An alternative account is presented of the manner in which efference may contribute to the perception of motion. It is proposed that distinct smooth eye-movement systems contribute differentially to object motion perception. Specifically, activity in the smooth pursuit system gives rise to the perception of object motion, whereas activity in the smooth component of reflexive eye movements does not. Tracking of a moving object results in object motion perception as a result of efference in the pursuit system. However, the pursuit system may be activated to oppose the smooth component of reflexive eye movements in order to preserve fixation on a stationary object. In such cases neither the fixated object nor the eye is moving but illusory movement results from the efference in the pursuit system. A number of illusory movement phenomena are interpreted in terms of this model.

Eye Movements↗
PubMed1985

Spatial orientation mechanisms and their implications for falls.

Postural stability is only one function on a multisensory system that also subserves spatial orientation and gaze stability. The problem of falling is discussed in the context of understanding the nature and integration of these sensory loops, and the possibility of their modification is discussed. The characteristics of the visual system contributing to spatial orientation are analyzed. The role of gaze stability in postural balance and its modification as a function of age are also discussed. It is suggested that our understanding of the problem of falling will be enhanced by a research strategy that views the problem as a special case of spatial orientation.

Accidents↗
PubMed1985

Reduction of body sway by stimuli imaged within a cortical scotoma: a case study.

The reduction of body sway by visual stimulation was equally effective for stimuli imaged within a cortical scotoma or in the mirror image position in the normal visual field. The results are consistent with the concept of distinct visual orientation and discrimination modes of processing visual information, which suggests that spatial orientation functions do not necessarily involve awareness.

Adult↗
PubMed1985

Tachygastria and motion sickness.

Cutaneously-recorded electrogastrograms (EGGs) were obtained from 21 healthy volunteers who were seated within a drum, the rotation of which produced vection or illusory self-motion. Fourteen subjects developed symptoms of motion sickness during vection and in each the EGG frequency shifted from the normal 3 cpm to 5-8 cpm, tachygastria, an abnormal pattern. In 6 of 7 asymptomatic subjects, the 3 cpm EGG pattern was unchanged during vection. It was concluded that illusory self-motion produces tachygastria and motion sickness in susceptible subjects.

Adolescent↗
PubMed1984

The effect of contrast on sustained detection.

During a 1 min observation, the percentage of time a high spatial frequency grating can be detected is influenced by the contrast and spatial frequency of a second, superimposed and orthogonally-oriented sine wave grating. Increasing contrast of the second pattern aids detection of the first by providing a more effective accommodative stimulus. The function relating spatial frequency and the minimum contrast needed to activate accommodation is similar in shape to the classical contrast sensitivity function. However, an order of magnitude more contrast is required to stabilize accommodation than is required to simply detect a pattern. These results suggest that performance on visual tasks requiring sustained rather than brief detection may be markedly impaired under low contrast conditions.

Accommodation, Ocular↗
PubMed1982

The effect of convergence on the vestibulo-ocular reflex and implications for perceived movement.

The apparent motion of a fixated stimulus during head translation in the dark was found to depend on the magnitude of the vestibulo-ocular reflex (VOR). Absolute convergence level determines VOR magnitude and thereby influences apparent motion during head movement by determining the magnitude and direction of the pursuit effort required to maintain gaze stability. The results are discussed in terms of the biological utility of the coupling between convergence and the VOR in maintaining image stability, and the role of the pursuit eye movement system in illusory movement.

Fixation, Ocular↗
PubMed1982

Autokinesis and peripheral stimuli: implications for fixational stability.

The reduction in autokinesis of a fixated stimulus was determined as a function of the eccentricity of an identical second stimulus. When the stimuli were separated by 1 deg, autokinesis was reduced by approximately 50% but with increasing separation the attenuation was diminished. The results are interpreted in terms of fixational stability models of autokinesis. It is suggested that autokinesis as well as other examples of apparent movement result from fixational instability.

Adult↗