Linear analysis of detection and discrimination of complex stimuli in the amblyopic visual system
Amblyopia is generally regarded as an anomaly in the spatial domain. In addition to the reduced visual acuity, amblyopes show a loss of contrast sensitivity over a broad range of spatial frequencies. In normal spatial vision, there is strong evidence which suggests that 1) there are multiple spatial frequency "channels", and 2) to a first approximation, the visual system is, within certain limits, linear. In the present study, the contrast relationships of the first two harmonics of a ramp wave-form at detection and discrimination thresholds as well as their independent thresholds, were examined. Two psychophysical methods, method of limits and temporal two- alternative forced-choice, were used to investigate linearity of detection and discrimination in the amblyopic visual system. The results of this study show that discrimination of phase sensitivity is decreased across a broad range of spatial frequencies, with the greatest loss occurring at high spatial frequencies relative to the sine-wave modulation sensitivity curve. The response by the visual normal and non- amblyopic eye displays linearity, whereas at low spatial frequencies, there is summation of the first two harmonics. The amblyopic eye does show summation at low spatial frequencies, but a geater decrease in the sensitivity of phase at higher spatial frequencies and evidence of non-linear function. Results of central mask experiments indicate an increased extent of non-linearity across the visual field in the amblyopic eye. This nonlinearity within the fovea and parafovial area in the amblyopic eye may be more of an applicable explanation to acuity loss. A scotoma model may at least in part explain decreased central sensitivity for the detection of a stimulus, but it does not explain the decreased loss of sensitivity for phase discrimination as does a non-linearity model. It may be in fact, then, that the scotoma represent an extension of the non-linearity.