Accurate reading with sequential presentation of single letters

Rapid, accurate reading is possible when isolated, single words from a sentence are sequentially presented at a fixed spatial location. We investigated if reading of words and sentences is possible when single letters are rapidly presented at the fovea under user-controlled or automatically-controlled rates. When tested with complete sentences, trained participants achieved reading rates of over 60 words/minute and accuracies of over 90% with the single letter reading (SLR) method and naive participants achieved average reading rates over 30 wpm with >90% accuracy. Accuracy declined as individual letters were presented for shorter periods of time, even when the overall reading rate was maintained by increasing the duration of spaces between words. Words in the lexicon that occur more frequently were identified with higher accuracy and more quickly, demonstrating that trained participants have lexical access. In combination, our data strongly suggest that comprehension is possible and that SLR is a practicable form of reading under conditions in which normal scanning of text is not possible, or for scenarios with limited spatial and temporal resolution such as patients with low vision or prostheses

Testing Neuronal Accounts of Anisotropic Motion Perception with Computational Modelling

<div><p>There is an over-representation of neurons in early visual cortical areas that respond most strongly to cardinal (horizontal and vertical) orientations and directions of visual stimuli, and cardinal- and oblique-preferring neurons are reported to have different tuning curves. Collectively, these neuronal anisotropies can explain two commonly-reported phenomena of motion perception – the oblique effect and reference repulsion – but it remains unclear whether neuronal anisotropies can simultaneously account for both perceptual effects. We show in psychophysical experiments that reference repulsion and the oblique effect do not depend on the duration of a moving stimulus, and that brief adaptation to a single direction simultaneously causes a reference repulsion in the orientation domain, and the inverse of the oblique effect in the direction domain. We attempted to link these results to underlying neuronal anisotropies by implementing a large family of neuronal decoding models with parametrically varied levels of anisotropy in neuronal direction-tuning preferences, tuning bandwidths and spiking rates. Surprisingly, no model instantiation was able to satisfactorily explain our perceptual data. We argue that the oblique effect arises from the anisotropic distribution of preferred directions evident in V1 and MT, but that reference repulsion occurs separately, perhaps reflecting a process of categorisation occurring in higher-order cortical areas.</p></div

Adaptation.

<p>The change in performance of direction judgement following adaptation at 0° (red) and 45° (blue X), and their dependence on the stimulus direction relative to adaptation. Mean (± <i>SE</i>) changes in orientation accuracy (A), precision (B) and reversal fraction (C) are shown for 13 naïve participants. All values are expressed as <i>Adapt</i> – <i>Control</i>.</p

Isolated effects of population anisotropy.

<p>Maximum likelihood (A–C) and vector averaging (D–F) decoding of simulated anisotropic neuronal populations at integration times that produce similar perceptual precision to humans. Each coloured point quantifies the mean cardinal repulsion (<a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0113061#pone.0113061.e006" target="_blank">Eq. 3</a>) and oblique effect (<a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0113061#pone.0113061.e007" target="_blank">Eq. 4</a>) observed in 1000 instantiations of a neuronal population with a defined level of anisotropy. The colour of the point defines the level of anisotropy: mid-level (grey) RGB values correspond to neuronal populations with no anisotropies; higher blue values correspond to populations with more cardinal-preferring neurons; higher red values to lower spiking rates for neurons with cardinal preferences; and higher green values to narrower tuning curves for cardinal-preferring neurons. In each panel, the effect of varying a single anisotropy metric in isolation is shown: (A,D) variable anisotropy in direction preference (<i>k</i>) while enforcing no anisotropy in spiking rate or bandwidth (<i>w_R</i> = 0; <i>w_BW</i> = 0); (B,E) variable anisotropy in spiking rate (<i>w_R</i>) while enforcing no anisotropy in direction preference or bandwidth (<i>k</i> = 0; <i>w_BW</i> = 0); (C,F) variable anisotropy in bandwidth (<i>w_BW</i>) while enforcing no anisotropy in direction preference or spiking rate (<i>k</i> = 0; <i>w_R</i> = <i>0</i>).</p

Duration-dependent perceptual performance.

<p>Increasing stimulus duration leads to better orientation accuracy (A, D), better orientation precision (B, E) and fewer reversals (C, F). Individual results are shown for three experienced observers (A–C) and for eleven participants (mean ± <i>SD</i>) tested over a different set of duration conditions (D–F).</p

Human-like perceptual masking is difficult to observe in rats performing an orientation discrimination task.

Visual masking occurs when the perception of a brief target stimulus is affected by a preceding or succeeding mask. The uncoupling of the target and its perception allows an opportunity to investigate the neuronal mechanisms involved in sensory representation and visual perception. To determine whether rats are a suitable model for subsequent studies of the neuronal basis of visual masking, we first demonstrated that decoding of neuronal responses recorded in the primary visual cortex (V1) of anaesthetized rats predicted that orientation discrimination performance should decline when masking stimuli are presented immediately before or after oriented target stimuli. We then trained Long-Evans rats (n = 7) to discriminate between horizontal and vertical target Gabors or gratings. In some trials, a plaid mask was presented at varying stimulus onset asynchronies (SOAs) relative to the target. Spatially, the masks were presented either overlapping or surrounding the target location. In the absence of a mask, all animals could reliably discriminate orientation when stimulus durations were 16 ms or longer. In the presence of a mask, discrimination performance was impaired, but did not systematically vary with SOA as is typical of visual masking. In humans performing a similar task, we found visual masking impaired perception of the target at short SOAs regardless of the spatial or temporal configuration of stimuli. Our findings indicate that visual masking may be difficult to observe in rats as the stimulus parameters necessary to quantify masking will make the task so difficult that it prevents robust measurement of psychophysical performance. Thus, our results suggest that rats may not be an ideal model to investigate the effects of visual masking on perception

Analyses of cardinal-centric symmetry in orientation performance.

<p>(A) Comparison of population-mean precision and accuracy metrics illustrates that cardinal directions are associated with both higher accuracy and precision. Cluster analysis to the level of two groups resulted in one cluster containing the four cardinal directions and 300°. (B) The circular cross-correlation of each participants' direction-dependent orientation bias with its reflection – a measure of symmetry in the anisotropic judgement of directions. Each colour corresponds to a different participant, and curves have been vertically offset for clarity. Some participants exhibited correlations with 90° periodicity whereas others with 45° periodicity. In both cases, the highest correlations tended to occur across the cardinal axes. The histogram counts all local maxima in the cross-correlations of each participant; the most common axes of symmetry was indeed in the cardinal planes (0° and 90° rotation).</p

Population anisotropies.

<p>Three types of anisotropy that can be parametrically varied in the model; each curve represents the direction tuning of a single neuron. The model could independently specify anisotropies in the preferred direction (A), gain or peak firing rate (B), or tuning bandwidth (C). Note that for clarity, only one parameter is varied in each panel, but in the model, all parameters could be independently and simultaneously varied.</p

Relative performance for oblique versus cardinal directions.

<p>As in <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0113061#pone-0113061-g008" target="_blank">Figure 8</a>, results are shown separately for three experienced observers (A–C) and averaged across eleven participants (D–F; mean ± <i>SD</i>). The ratio of oblique to cardinal orientation accuracy (A, D) and precision (B, E) was used to measure the strength of the oblique effect. Values greater than 1 indicate that orientation performance was better for cardinal directions than obliques. Error bars show geometric standard deviation. (C, F) The difference between arcsine-transformed oblique and cardinal reversal fractions; values greater than 0 indicate that direction performance was better for cardinal directions than obliques.</p