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Critical band masking reveals the effects of optical distortions on the channel mediating letter identification

Lookup NU author(s): Dr Laura YoungORCiD



This work is licensed under a Creative Commons Attribution 4.0 International License (CC BY 4.0).


© 2014 Young and Smithson.There is evidence that letter identification is mediated by only a narrow band of spatial frequencies and that the centre 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.

Publication metadata

Author(s): Young LK, Smithson HE

Publication type: Article

Publication status: Published

Journal: Frontiers in Psychology

Year: 2014

Volume: 5

Online publication date: 30/09/2014

Acceptance date: 04/09/2014

Date deposited: 20/05/2020

ISSN (electronic): 1664-1078

Publisher: Frontiers Research Foundation


DOI: 10.3389/fpsyg.2014.01060


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We would like to acknowledge the John Fell Fund for supporting this work.