Biomedical subjects

F J Blommaert

Publications and source records attributed to F J Blommaert.

6 recordsLinked to original sources

PubMed1990

An object-oriented model for brightness perception.

An object-oriented model for the brightness perception of static images is presented. The philosophy behind the model is that the visual system aims at a brightness representation that displays object properties, and consequently is insensitive to variations in light source and changes in viewing position. The model assumes an ensemble of neural units that differ in receptive field size, i.e. identical operations are performed at a number of (spatial) scales. The brightness at each retinal position is the weighted sum of the neural activities that exist at this position in the different scales. The weighting function is such that the brightness impression is robust against variation in viewing distance. The operating characteristics of the neural units are derived from ganglion cell behaviour, and are robust against variation in light level. The model is able to unify different aspects of brightness perception, such as brightness induction and brightness assimilation. In order to gain in transparency, the approach adopted in this paper is a general one: we aim at showing how different aspects of brightness perception can be explained and unified within the model, at the expense of a less detailed description of psychophysical data.

Humans↗

PubMed1988

Early-visual factors in letter confusions.

For the purpose of quantifying models of letter recognition, similarities are often specified in terms of stimulus properties. In this paper, an approach is advocated based on similarities between internal letter representations or internal letter images, i.e. it is argued that optical and retinal factors play a more prominent role in letter confusions than is usually assumed. To illustrate this, letter images were calculated on the basis of earlier experimentally determined point spread functions (Blommaert et al., Spatial Vision 2, 99-115, 1987). Next, data on confusion matrices from Bouma (Vision Res. 11, 459-474, 1971) were taken to evaluate different measures which might be useful for quantifying similarities between internal letter representations. In the analysis of experimental data, Luce's (In: Handbook of Mathematical Psychology, 1963) choice model was used. It was found that if similarities were expressed in terms of differences between image contours, a fair first order approximation of Bouma's experimental results could be formulated (overall correlation coefficient of 0.95). Other measures like correlations between spatial frequency spectra of letter images were found to be less successful. The method used provides a means to relate quantitatively stimulus features and optical and early-visual factors to letter confusions.

Form Perception↗

PubMed1987

Prediction of thresholds and latency on the basis of experimentally determined impulse responses.

As was shown before (Roufs and Blommaert 1981), temporal impulse responses and step responses can be obtained psychophysically using a drift-correcting perturbation technique. In this paper, experimentally determined impulse responses are given for eight subjects using different experimental conditions, i.e. a 1 deg stimulus field at background luminances of 1200 Td and 100 Td, and a point source superimposed on an extended background of the same luminances, which is a possibility to separate transient and sustained processing. For a large class of stimuli, predictions of threshold curves and latency of different time functions are calculated on the basis of these measured impulse responses. Predictions are tested against experimental data. It will be shown that a simple model, only consisting of a linear filter followed by a noisy peak detector, suffices for a fair quantitative description of the available data.

Humans↗

PubMed1987

Letter recognition at low contrast levels: effects of letter size.

Contrast variation was used to measure recognition thresholds for lowercase letters, with the aim of obtaining a better understanding of the role that early stages of visual processing play in letter recognition. Frequency-of-recognition curves were measured for alphabets of different letter size. Since variation of the adaptational state of the eye changes the characteristics of primary visual processing in a quantifiable way, recognition thresholds were measured both at a high (150 cd m-2) and at a low (0.9 cd m-2) adaptation level. Thresholds decreased as letter size increased, in a way comparable with data on visual acuity. At the lower adaptation level, recognition thresholds became higher, which is also in accordance with visual acuity data. Furthermore, the slopes of the frequency-of-recognition curves for alphabets as a function of log contrast decreased with decreasing letter size. It is argued that this is mainly caused by an increasing dispersion of internal representations of individual letters on the internal psychological scale as letter size decreases.

Adaptation, Ocular↗

PubMed1987

Point spread functions and detail detection.

Point spread functions, intended to characterize local spatial transfer of the visual system, can be obtained psychophysically using a perturbation technique. Data of such point spread functions are shown for three experimental conditions: foveally at adaptation levels of 1200 and 10 Td, and in the parafovea at an eccentricity of 2 deg using a 1200 Td adaptation level. The results are consistent with earlier findings (Blommaert, F. J.J. and Roufs, J.A.J. 1981, Vision Res. 21, 1223-1233). On the basis of such point spread functions a simplified multiple unit model was constructed, the parameters of which were fitted to thresholds of discs with varying diameter. Threshold predictions from this model for annuli, thin lines and broad lines were found to be in fair quantitative agreement with experimental results. It is argued that for a certain class of slender stimuli, including alphanumeric characters, thresholds can be described with a single channel model containing only the experimentally determined point spread function as a basic component.

Humans↗

PubMed1987

Reading fields of magnifying loupes.

For the purpose of understanding the influence of the use of magnifying loupes on the reading process, the reading field is a relevant concept. Three possible reading fields are distinguished: the monocular reading field, the binocular reading field, and the composite reading field. Theoretical expressions for the widths of reading fields are derived as a function of the physical parameters of the loupe and the geometry of the reading situation. Variation of reading-field width as a function of magnification is illustrated. For verification, experimentally determined reading-field widths were compared with the theoretical ones. There were small systematic deviations, probably caused by achromatic aberrations. In order to learn about the strategies that subjects use when reading with the aid of a loupe, loupe displacement was measured while subjects read text under conditions that provided a variety of reading-field widths. It was found that individual subjects use different strategies (i.e., they use different reading-field widths).

Fovea Centralis↗