The Activity Inventory: an adaptive visual function questionnaire.

PURPOSE: The Activity Inventory (AI) is an adaptive visual function questionnaire that consists of 459 Tasks nested under 50 Goals that in turn are nested under three Objectives. Visually impaired patients are asked to rate the importance of each Goal, the difficulty of Goals that have at least some importance, and the difficulty of Tasks that serve Goals that have both some importance and some difficulty. Consequently, each patient responds to an individually tailored set of questions that provides both a functional history and the data needed to estimate the patient's visual ability. The purpose of the present article is to test the hypothesis that all combinations of items in the AI, and by extension all visual function questionnaires, measure the same visual ability variable. METHODS: The AI was administered to 1880 consecutively-recruited low vision patients before their first visit to the low vision rehabilitation service. Of this group, 407 were also administered two other visual function questionnaires randomly chosen from among the Activities of Daily Living Scale (ADVS), National Eye Institute Visual Functioning Questionnaire (NEI VFQ), 14-item Visual Functioning Index (VF-14), and Visual Activities Questionnaire (VAQ). Rasch analyses were performed on the responses to each VFQ, on all responses to the AI, and on responses to various subsets of items from the AI. RESULTS: The pattern of fit statistics for AI item and person measures suggested that the estimated visual ability variable is not unidimensional. Reading-related and other items requiring high visual resolution had smaller residual errors than expected and mobility-related items had larger residual errors than expected. The pattern of person measure residual errors could not be explained by the disorder diagnosis. When items were grouped into subsets representing four visual function domains (reading, mobility, visual motor, visual information), and separate person measures were estimated for each domain as well as for all items combined, visual ability was observed to be equivalent to the first principal component and accounted for 79% of the variance. However, confirmatory factor analysis showed that visual ability is a composite variable with at least two factors: one upon which mobility loads most heavily and the other upon which reading loads most heavily. These two factors can account for the pattern of residual errors. High product moment and intraclass correlations were observed when comparing different subsets of items within the AI and when comparing different VFQs. CONCLUSIONS: Visual ability is a composite variable with two factors; one most heavily influences reading function and the other most heavily influences mobility function. Subsets of items within the AI and different VFQs all measure the same visual ability variable.

Visual disability variables. I: the importance and difficulty of activity goals for a sample of low-vision patients.

OBJECTIVE: To test the validity and reliability of latent trait measures estimated from ratings by low-vision patients of the importance and difficulty of selected activity goals. DESIGN: Validation of a telephone-administered functional assessment instrument using Rasch analysis of self-assessment ratings. SETTING: Telephone interviews of respondents in their homes. Participants Consecutive series of 600 outpatients with low vision. INTERVENTIONS: Not applicable. MAIN OUTCOME MEASURES: Ratings of the importance and difficulty of achieving 41 activity goals. Person and item traits were measured with the Andrich rating scale model. Measurement validity and reliability were tested statistically by comparing response patterns and distributions with measurement model expectations. RESULTS: Patients could distinguish only 3 categories of importance and 4 categories of difficulty. The distributions of person and item measure fit statistics were consistent with 2 unidimensional constructs: value of independence estimated from importance ratings and visual ability estimated from difficulty ratings. However, 8 of 41 activity goals were poor estimators of value of independence and 7 of 41 activity goals were poor estimators of visual ability. Person measure distributions could be divided into 3 statistically distinct strata for estimates from both importance ratings and difficulty ratings. Item measure distributions could be divided into 21 strata for estimates from importance ratings and 7 strata for estimates from difficulty ratings. CONCLUSIONS: The 2 variables that define visual disability-value of independence and visual ability-are valid constructs that can be estimated accurately and reliably from patient ratings of the importance and difficulty of activity goals.

Visual disability variables. II: The difficulty of tasks for a sample of low-vision patients.

OBJECTIVES: To test the validity and reliability of measures of visual ability and to evaluate the relation between measurements made at the task level and measurements made at the goal level of a hierarchical model for visual disability. DESIGN: Validation of a telephone-administered functional assessment instrument using Rasch analysis on self-assessment ratings. SETTING: Telephone interviews of respondents in their homes. PARTICIPANTS: Consecutive series of 600 outpatients with low vision. INTERVENTIONS: Not applicable. MAIN OUTCOME MEASURES: Ordinal ratings of the difficulty in performing a subset of 337 tasks. Measures of the visual ability of each patient and the required visual ability to perform each task were made using the Andrich rating scale model. Measurement validity and reliability were tested statistically by comparing response patterns and distributions to measurement model expectations. RESULTS: Results were consistent with a single visual ability construct. Patients' visual ability estimated from task difficulty ratings agreed with estimates from goal difficulty ratings ( r =.74); the difficulty of individual goals was equal to the weighted average of the difficulties of subsidiary tasks ( r =.79). However, conclusions from the Rasch analysis were not confirmed by principal components analysis of item residuals, which indicated that visual ability had a 2-dimensional structure, with 1 factor related to mobility and the other related to reading. Factor analysis on person measures estimated from subsets of functionally grouped items confirmed the 2-dimensional structure of visual ability. CONCLUSIONS: Our study results confirm the hierarchical structure of the Activity Breakdown Structure model and show how the individualized Activity Inventory can produce measures of limitations in functional vision.

Facial recognition using simulated prosthetic pixelized vision.

PURPOSE: To evaluate a model of simulated pixelized prosthetic vision using noncontiguous circular phosphenes, to test the effects of phosphene and grid parameters on facial recognition. METHODS: A video headset was used to view a reference set of four faces, followed by a partially averted image of one of those faces viewed through a square pixelizing grid that contained 10x10 to 32x32 dots separated by gaps. The grid size, dot size, gap width, dot dropout rate, and gray-scale resolution were varied separately about a standard test condition, for a total of 16 conditions. All tests were first performed at 99% contrast and then repeated at 12.5% contrast. RESULTS: Discrimination speed and performance were influenced by all stimulus parameters. The subjects achieved highly significant facial recognition accuracy for all high-contrast tests except for grids with 70% random dot dropout and two gray levels. In low-contrast tests, significant facial recognition accuracy was achieved for all but the most adverse grid parameters: total grid area less than 17% of the target image, 70% dropout, four or fewer gray levels, and a gap of 40.5 arcmin. For difficult test conditions, a pronounced learning effect was noticed during high-contrast trials, and a more subtle practice effect on timing was evident during subsequent low-contrast trials. CONCLUSIONS: These findings suggest that reliable face recognition with crude pixelized grids can be learned and may be possible, even with a crude visual prosthesis.