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

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.

Acquired Visual Deficits Independent of Lesion Site in Acute Stroke

“…Recognition of an acuity target involves the distinction of a static optotype from its background (52). The addition of luminance noise elements raises the threshold of retinal sensitivity as well as the subsequent neural processing needed for stimulus identification (56). This visual processing originates in the primary visual cortex, and involves the dorsal stream via the parietal cortex, for visually guided spatial location and orientation of objects (57).…”

Comparison between optical and digital blur using near visual acuity

“…Background. Wavefront aberrations, W(r, θ) , such as spherical refractive error can be quantified in terms of their Zernike coefficients 22 according to the Zernike polynomial expansion 23,24 ,…”

“…is the wavelength of the light used, D is the diameter of the aperture and α is the oversampling factor 24 . We use α = 2 in our study.…”

Editorial: Using Noise to Characterize Vision

“…Letter identification is mediated by an octave-wide spatial frequency channel (Solomon and Pelli, 1994 ). Young and Smithson ( 2014 ) use spatially bandpass noise to reveal the letter identification channel in the presence of optical distortions, and find changes in the central spatial frequency of the letter-identification channel. Hall et al ( 2014 ) find that adding white noise increased the center spatial frequency of the letter-identification channel for large but not small letters.…”