Abstract
There is evidence that letter identification is mediated by only a narrow band of spatial frequencies and that the center frequency of the neural channel thought to underlie this selectivity is related to the size of the letters. When letters are spatially filtered (at a fixed size) the channel tuning characteristics change according to the properties of the spatial filter (Majaj et al., 2002). Optical aberrations in the eye act to spatially filter the image formed on the retina—their effect is generally to attenuate high frequencies more than low frequencies but often in a non-monotonic way. We might expect the change in the spatial frequency spectrum caused by the aberration to predict the shift in channel tuning observed for aberrated letters. We show that this is not the case. We used critical-band masking to estimate channel-tuning in the presence of three types of aberration—defocus, coma and secondary astigmatism. We found that the maximum masking was shifted to lower frequencies in the presence of an aberration and that this result was not simply predicted by the spatial-frequency-dependent degradation in image quality, assessed via metrics that have previously been shown to correlate well with performance loss in the presence of an aberration. We show that if image quality effects are taken into account (using visual Strehl metrics), the neural channel required to model the data is shifted to lower frequencies compared to the control (no-aberration) condition. Additionally, we show that when spurious resolution (caused by π phase shifts in the optical transfer function) in the image is masked, the channel tuning properties for aberrated letters are affected, suggesting that there may be interference between visual channels. Even in the presence of simulated aberrations, whose properties change from trial-to-trial, observers exhibit flexibility in selecting the spatial frequencies that support letter identification.
Highlights
In 1994 Solomon and Pelli used critical band masking to show that, despite letters being broadband stimuli, their identification is mediated by a single narrow band of spatial frequencies
Since their ideal observer model exhibited low-pass filtering characteristics, rather than bandpass characteristics as derived from the performance of human observers, it was suggested that the low-frequency fall-off of the human-derived filter represents a visual constraint upon letter identification
In addition to shifting the visual channel in response to filtering of the stimulus, Oruç and Landy (2009) suggested that, when the masking noise that is added to the stimulus dominates the equivalent noise that is associated with the contrast sensitivity function of human observers, it is possible for an observer to switch visual channels, not necessarily optimally
Summary
In 1994 Solomon and Pelli used critical band masking to show that, despite letters being broadband stimuli, their identification is mediated by a single narrow band of spatial frequencies Since their ideal observer model (based on the requirement to discriminate differences between letters) exhibited low-pass filtering characteristics, rather than bandpass characteristics as derived from the performance of human observers, it was suggested that the low-frequency fall-off of the human-derived filter represents a visual constraint upon letter identification. This account is in accordance with human observers’ inability to identify severely low-pass filtered letters, such as those with optical blur. In addition to shifting the visual channel in response to filtering of the stimulus, Oruç and Landy (2009) suggested that, when the masking noise that is added to the stimulus dominates the equivalent noise that is associated with the contrast sensitivity function of human observers (which describes how spatial frequencies are transmitted by the visual system), it is possible for an observer to switch visual channels, not necessarily optimally
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