Biomedical subjects

Balamurali Vasudevan

Publications and source records attributed to Balamurali Vasudevan.

6 recordsLinked to original sources

PubMed2007

Conceptual model of human blur perception.

An empirically based, conceptual model of human blur perception is presented. It incorporates the concepts of blur detection and blur discrimination in depth, and across the central and peripheral retina, in two- and three-dimensional visual space. Key aspects of the model are its dynamic nature, predictability regarding the blur-based depth-ordering of objects, patterns of retinal defocus with far and near viewing, and interactions related to retinal defocus between the central and peripheral retina. Furthermore, a two-dimensional schematic representation of the blur-free region during near viewing is depicted in dioptric space. This model has implications with respect to accommodative control, depth perception, and refractive error development and progression.

Accommodation, Ocular↗

PubMed2006

Depth-of-focus: control system implications.

The depth-of-focus (DOF) plays a major role in accommodative control. It provides a neurological tolerance for retinal defocus and the perception of blur. Hence, the DOF has been an important component in bioengineering models of the accommodative system, but with its model-based control function limited to the fovea. However, new studies have demonstrated the influence of the near retinal periphery on the DOF: with increased target extent, the DOF increases. Thus, the blur control derived from the combined fovea and near retinal periphery should be incorporated into future models of accommodation.

Accommodation, Ocular↗

PubMed2006

Effect of blur adaptation on blur sensitivity in myopes.

Although blur adaptation in myopia has been investigated, knowledge regarding its effect on blur sensitivity remains unknown. In the present study, changes in three blur thresholds (i.e., noticeable, bothersome, and non-resolvable blur) were assessed monocularly after 1h of blur adaptation in myopes. A Badal optical system was used to present either an isolated 20/50 Snellen E or 20/50 lines of text, with the full text field used in the latter condition for all blur judgments. Eight visually normal adult myopes were tested with paralyzed accommodation. All subjects exhibited blur adaptation, with a significant improvement in group mean visual acuity of -0.16 LogMAR. There was a consistent and concurrent significant decrease of 0.15-0.19 D in all blur thresholds for the isolated 20/50 E. However, there was no significant effect of blur adaptation on blur thresholds for the 20/50 text, with large intersubject variability evident. The enhanced blur sensitivity for the isolated E target may in part be attributed to the increased visual resolution following blur adaptation. Differences found in the blur thresholds for the two targets may be related to a variety of neuroperceptual phenomena, in particular lateral masking.

Accommodation, Ocular↗

PubMed2006

Objective blur thresholds in free space for different refractive groups.

The aim of the current study was to measure the objective depth-of-focus (DOF) of myopes (MYO) in free space and compare them with emmetropic (EMM) and hyperopic (HYP) cohorts. The objective DOF was measured in 35 visually normal, young adults including 16 MYO, 13 EMM, and 6 HYP using the Power Refractor (PR II). The DOF was larger in the MYO and HYP than in the EMM. The larger objective DOF found in the MYO subgroup is in agreement with previous studies assessing the DOF subjectively. This finding is consistent with current thinking that increased amounts of retinal-defocus integrated over extended time may be myopiogenic. In agreement with earlier clinical and computer simulation studies, hyperopes demonstrate a relative resistance to increased retinal defocus.

Accommodation, Ocular↗

PubMed2005

An objective technique to measure the depth-of-focus in free space.

BACKGROUND: Determining the depth-of-focus (DOF) objectively in free space is important, as this provides information regarding the range of clear vision under more natural viewing conditions in which target blur, size, and proximal information, as well as other monocular depth cues, are present. METHODS: The DOF was assessed objectively in free space in young adults [n=20, ages 24.9+/-3.1years: myopes (n=14), emmetropes (n=5), hyperope (n=1)] by monitoring accommodation continuously using the commercially available Power Refractor II (PR II) under monocular viewing conditions. A high-contrast target ( approximately 83%) subtending a mean visual angle of 2.3 deg was placed on an optical bench at 25 cm (4 D) along the line-of-sight of the viewing right eye and was displaced slowly ( approximately 0.1-0.15 D/s) proximally and distally. This target provided blur, size, and proximal information, and other depth cues, as normally found in free space. The first consistent change in steady-state accommodative baseline level reflected one edge of the DOF. The mean dioptric difference between these distal and proximal endpoints was averaged, and this represented the total objective DOF. RESULTS: The group mean total objective DOF (n=20) was +/-0.61+/-0.09 D, with a range from +/-0.46 D to +/-0.81 D. Repeatability was excellent. CONCLUSION: A technique was developed and tested to measure the DOF in free space objectively. Further development of this technique will allow the assessment of blur perception under more complex natural viewing conditions simulating the everyday environment.

Accommodation, Ocular↗

PubMed2005

"Bothersome blur": a functional unit of blur perception.

Knowledge regarding the amount of blur perceived to be "bothersome" to an individual, namely that which is assumed to be annoying and to adversely affect task performance, remains limited. A Badal optical system was used to measure the blur detection, bothersome blur, and non-resolvable blur dioptric thresholds monocularly either to an isolated 20/50 or 20/200 Snellen E, or to three 20/50 lines of text. Subjects were comprised of 13 visually normal young adults and 3 absolute presbyopes. Cycloplegia was used to paralyze accommodation in the young adults. Within each target type for the young adults, the mean bothersome blur threshold was always significantly larger than that found for blur detection and significantly smaller than that found for non-resolvable blur. Across target types and blur criteria, the bothersome blur thresholds for the isolated 20/50 E (1.02 D) and the 20/50 text (1.34 D) were not significantly different, although in 12 of the 13 subjects the latter were larger (p<0.002, sign test). However, both were significantly smaller than for the isolated 20/200 E (1.80 D). In a subset of young adult subjects, bothersome blur was found to be repeatable over time. The results were similar in the absolute presbyopes. The bothersome blur threshold was primarily influenced by target detail and secondarily by target extent. These findings have important implications with respect to tolerances for optical lens design and refractive surgery outcomes, as well as provide insight into basic aspects of human blur perception.

Accommodation, Ocular↗