Progressive addition lenses (PALs) are an increasingly preferred mode for the correction of presbyopia, gaining an increased share of the prescription lens market. Sales volumes are likely to increase over the next few years, given the increasing cohort of presbyopic patients in the population. This research investigated adaptation to PAL wear, investigating head movement parameters with and without progressive lenses in everyday visual tasks, and examined symptoms of spatial distortions and illusory movement in a crossover wearing trial of three PAL designs. Minimum displacement thresholds in the presence and absence of head movement were also investigated across the lens designs. Experiment 1 investigated head movements in two common visual tasks, a wordprocessing copy task, and a visual search task designed to replicate a natural environment task such as looking for products on supermarket shelving. Head movement parameters derived from this experiment were used to set head movement amplitude and velocity in the third experiment investigating minimum displacement thresholds across three PAL designs. Head movements were recorded with a Polhemus Inside Track head movement monitoring system which allows real time six degrees of freedom measurement of head position. Head position in azimuth, elevation and roll was extracted from the head movement recorder output, and data for head movement angular extent, average velocity (amplitude/duration) and peak velocity were calculated for horizontal head movements Results of the first experiment indicate a task dependent effect on head movement peak and average velocity, with both median head movement average and peak velocity being faster in the copy task. Visual task and visual processing demands were also shown to affect the slope of the main sequence of head movement velocity on head movement amplitude, with steeper slope in the copy task. A steeper slope, indicating a faster head movement velocity for a given head movement amplitude, was found for head movements during the copy task than in the search task. Processing demands within the copy task were also shown to affect the main sequence slopes of velocity on amplitude, with flatter slopes associated with the need for head movement to bring gaze to a specific point. These findings indicate selective control over head movement velocity in response to differing visual processing demands. In Experiment 2, parameters of head movement amplitude and velocity were assessed in a group of first time PAL wearers. Head movement amplitude, average and peak velocity were calculated from head movement recordings using the search task, as in Experiment 1. Head movements were recorded without PALs, on first wearing a PAL, and after one month of PAL wear to assess adaptation effects. In contrast to existing literature, PAL wear did not alter parameters of head movement amplitude and velocity in a group of first time wearers either on first wearing the lenses or after one month of wear: this is due to task related effects in this experiment compared to previous work. Task demand in this experiment may not have required wearers to use the progressive power corridor to accomplish identification of visual search targets, in contrast to previous studies where experimental conditions were designed to force subjects to use the progressive corridor. In Experiment 3, minimum displacement thresholds for random dot stimuli were measured in a repeated measures experimental design for a single vision lens as control, and three PAL designs. Thresholds were measured in central vision, and for two locations in the temporal peripheral field, 30° temporal fixation and 10° above and below the horizontal midline. Thresholds were determined with and without the subjects' head moving horizontally in an approximate sinusoidal movement at a frequency of about 0.7 Hz. Minimum displacement thresholds were not significantly affected by PAL design, although thresholds with PALs were higher than with a single vision lens control. Head movement significantly increased minimum displacement threshold across lens designs, by a factor of approximately 1.5 times. Results indicate that the local measures of minimum displacement threshold determined in this experiment are not sensitive to lens design differences. Sensitivity to motion with PAL lenses may be more a global than a localized response. For Experiment 4, symptoms of spatial distortion and illusory movement were investigated in a crossover wearing trial of three PAL designs, and related to optical characteristics of the lenses. Peripheral back vertex powers of the PALs were measured at two locations in the right temporal zone of the lenses, 15.6 mm temporal to the fitting cross, and 2.7 m above and below the horizontal to the fitting cross. These locations corresponded to the zones of the lenses through which minimum displacement thresholds were measured in the previous experiment. The effect of subjects' self movement on symptoms is able to discriminate between PAL designs, although subjective symptoms alone were not related to the lens design parameters studied. Subjects' preference for one PAL design over the other designs studied in this experiment is inversely related to the effect on subject movement on their symptoms of distortion. An optical parameter, blur strength, derived from the power vector components of the peripheral powers, may indicate preference for particular PAL designs, as higher blur strength values are associated with lower lens preference scores. Head movement amplitude and velocity are task specific, and are also influenced by visual processing demands within tasks. PALs do not affect head movement amplitude and velocity unless tasks are made demanding or performed in less natural situations designed to influence head movement behaviour. Both head movement and PALs have large effects on minimum displacement thresholds; these effects may be due in part to complexity of the subjects' task within the experiment. Minimum displacement thresholds however were not influenced by PAL design. The most sensitive indicator for subject's preference of PALs was the effect of subjects' self movement on their perception of symptoms, rather than the presence of actual symptoms. Blur strength should be further investigated for its role in PAL acceptance.

Yes / Purpose: A recent randomised controlled trial indicated that providing long-term multifocal wearers with a pair of distance single-vision spectacles for use outside the home reduced falls risk in active older people. However, it also found that participants disliked continually switching between using two pairs of glasses and adherence to the intervention was poor. In this study we determined whether intermediate addition multifocals (which could be worn most of the time inside and outside the home and thus avoid continual switching) could provide similar gait safety on stairs to distance single vision spectacles whilst also providing adequate ‘short-term’ reading and near vision. Methods: Fourteen healthy long-term multifocal wearers completed stair ascent and descent trials over a 3-step staircase wearing intermediate and full addition bifocals and progression-addition lenses (PALs) and single-vision distance spectacles. Gait safety/caution was assessed using foot clearance measurements (toe on ascent, heel on descent) over the step edges and ascent and descent duration. Binocular near visual acuity, critical print size and reading speed were measured using Bailey-Lovie near charts and MNRead charts at 40 cm. Results: Gait safety/caution measures were worse with full addition bifocals and PALs compared to intermediate bifocals and PALs. The intermediate PALs provided similar gait ascent/descent measures to those with distance single- vision spectacles. The intermediate addition PALs also provided good reading ability: Near word acuity and MNRead critical print size were better with the intermediate addition PALs than with the single-vision lenses (p < 0.0001), with a mean near visual acuity of 0.24 0.13 logMAR (~N5.5) which is satisfactory for most near vision tasks when performed for a short period of time. Conclusions: The better ability to ‘spot read’ with the intermediate addition PALs compared to single-vision spectacles suggests that elderly individuals might better comply with the use of intermediate addition PALs outside the home. A lack of difference in gait parameters for the intermediate addition PALs compared to distance single-vision spectacles suggests they could be usefully used to help prevent falls in older well-adapted full addition PAL wearers. A randomised controlled trial to investigate the usefulness of intermediate multifocals in preventing falls seems warranted.

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