Neural sources of letter and Vernier acuity

Abstract

Visual acuity can be measured in many different ways, including with letters and Vernier offsets. Prior psychophysical work has suggested that the two acuities are strongly linked given that they both depend strongly on retinal eccentricity and both are similarly affected in amblyopia. Here we used high-density EEG recordings to ask whether the underlying neural sources are common as suggested by the psychophysics or distinct. To measure visual acuity for letters, we recorded evoked potentials to 3 Hz alternations between intact and scrambled text comprised of letters of varying size. To measure visual acuity for Vernier offsets, we recorded evoked potentials to 3 Hz alternations between bar gratings with and without a set of Vernier offsets. Both alternation types elicited robust activity at the 3 Hz stimulus frequency that scaled in amplitude with both letter and offset size, starting near threshold. Letter and Vernier offset responses differed in both their scalp topography and temporal dynamics. The earliest evoked responses to letters occurred on lateral occipital visual areas, predominantly over the left hemisphere. Later responses were measured at electrodes over early visual cortex, suggesting that letter structure is first extracted in second-tier extra-striate areas and that responses over early visual areas are due to feedback. Responses to Vernier offsets, by contrast, occurred first at medial occipital electrodes, with responses at later time-points being more broadly distributed—consistent with feedforward pathway mediation. The previously observed commonalities between letter and Vernier acuity may be due to common bottlenecks in early visual cortex but not because the two tasks are subserved by a common network of visual areas.

Neural Sources Underlying Visual Word Form Processing as Revealed by Steady State Visual Evoked Potentials (SSVEP)

“…However, the SSVEP paradigm has long been limited to the field of low-level visual perception and attention (for a review, see Norcia et al 2015). Only recently, this paradigm has been extended to more complex visual stimuli processing, such as objects (Stothart et al, 2017), faces (Alonso-Prieto et al, 2013;Farzin et al, 2012;Liu-Shuang et al, 2014), numerical quantities (Guillaume et al, 2018;Van Rinsveld et al, 2020), text (Yeatman & Norcia, 2016), letters (Barzegaran & Norcia, 2020), and words (Lochy et al, 2015(Lochy et al, , 2016(Lochy et al, , 2018(Lochy et al, , 2020. These SSVEP studies of higher-level processes have used different presentation 2 .…”

“…Of present interest, two SSVEP studies on text and letter recognition have revealed multiple underlying sources with different temporal dynamics and scalp topography (Barzegaran & Norcia, 2020;Yeatman & Norcia, 2016) either by defining different regions of interest (Yeatman & Norcia, 2016) or employing a spatial filtering approach (Barzegaran & Norcia, 2020). Other SSVEP work has focused on only a single source of word-related processing in the left hemisphere (Lochy et al, 2015(Lochy et al, , 2016(Lochy et al, , 2018 by analyzing periodic responses from several pre-selected (literature-based) sensors.…”

“…A number of linear spatial filters, such as Principal Components Analysis (PCA) and Common Spatial Patterns (CSP) (Blankertz et al, 2008), have been applied to SSVEP data, mainly in the brain-computer interface (BCI) field (e.g., Mohanchandra et al 2014). In cognitive neuroscience, a spatial filtering technique referred to as Reliable Components Analysis (RCA) has been increasingly used (Barzegaran & Norcia, 2020;Dmochowski et al, 2012Dmochowski et al, , 2015. RCA derives a set of spatial components (i.e., spatial filters operationalized as topographic weights) that maximize across-trial or across-subject correlations ("reliability") while minimizing noise ("variance").…”

“…Employing the spatial filtering component analysis (RCA) approach used in Barzegaran & Norcia (2020) and Norcia et al (2020), here we reproduced and extended a previous SSVEP study of French word processing (Lochy et al, 2015), with 4-letter English word versus pseudofont comparisons. Our goal was to determine whether multiple sources could be revealed in conditions where only a single source of activity was described.…”

“…Based on the evidence from Lerma-Usabiaga et al (2018) and Barzegaran & Norcia (2020), we hypothesized that word deviants among pseudofont base stimuli would elicit more than one neural discrimination source when subjected to RCA, producing at least two reliable signal sources with distinct temporal and topographical information. We sought to further investigate whether RCA component topographies and time-courses were specific to particular experimental parameters or whether similar components could be elicited across a wider range of presentation rates and changes in stimulus locations.…”