Saeideh Ghahghaei

Effects of load on the time course of attentional engagement, disengagement, and orienting in reading.

We examined how the frequency of the fixated word influences the spatiotemporal distribution of covert attention during reading. Participants discriminated gaze-contingent probes that occurred with different spatial and temporal offsets from randomly chosen fixation points during reading. We found that attention was initially focused at fixation and that subsequent defocusing was slower when the fixated word was lower in frequency. Later in a fixation, attention oriented more towards the next saccadic target for high- than for low-frequency words. These results constitute the first report of the time course of the effect of load on attentional engagement and orienting in reading. They are discussed in the context of serial and parallel models of reading.

Depth Perception and Grasp in Central Field Loss

Purpose We set out to determine whether individuals with central field loss benefit from using two eyes to perform a grasping task. Specifically, we tested the hypothesis that this advantage is correlated with coarse stereopsis, in addition to binocular summation indices of visual acuity, contrast sensitivity, and binocular visual field. Methods Sixteen participants with macular degeneration and nine age-matched controls placed pegs on a pegboard, while their eye and hand movements were recorded. Importantly, the pegboard was placed near eye height, to minimize the contribution of monocular cues to peg position. All participants performed this task binocularly and monocularly. Before the experiment, we performed microperimetry to determine the profile of field loss in each eye and the locations of eccentric fixation (if applicable). In addition, we measured both acuity and contrast sensitivity monocularly and binocularly, and stereopsis by using both a RanDot test and a custom stereo test. Results Peg-placement time was significantly shorter and participants made significantly fewer errors with binocular than with monocular viewing in both the patient and control groups. Among participants with measurable stereopsis, binocular advantage in peg-placement time was significantly correlated with stereoacuity (ρ = −0.78; P = 0.003). In patients without measurable stereopsis, the binocular advantage was related significantly to the overlap in the scotoma between the two eyes (ρ = −0.81; P = 0.032). Conclusions The high correlation between grasp performance and stereoacuity indicates that coarse stereopsis may benefit tasks of daily living for individuals with central field loss.

Texture segmentation influences the spatial profile of presaccadic attention

Attention is important for selecting targets for action. Several studies have shown that attentional selection precedes eye movements to a target, and results in an enhanced sensitivity at the saccade goal. Typically these studies have used isolated targets on blank backgrounds, which are rare in real-world situations. Here, we examine the spatial profile of sensitivity around a saccade target on a textured background and how the influence of the surrounding context develops over time. We used two textured backgrounds: a uniform texture, and a concentric arrangement of an inner and an outer texture with orthogonal orientations. For comparison, we also measured sensitivity around the target on a blank background. The spatial profile of sensitivity was measured with a brief, dim, probe flashed around the saccade target. When the target was on a blank or a uniformly textured background, spatial sensitivity peaked near the target location around 350 ms after cue onset and declined with distance from the target. However, when the background was made up of an inner and outer texture, sensitivity to the inner texture was uniformly high, peaking at about 350 ms after cue onset, suggesting that the entire inner texture was selected along with the target. The enhancement of sensitivity on the inner texture was much smaller when observers attended the target covertly and performed the probe-detection task. Thus, our results suggest that the surface representation around the target is taken into account when an observer actively plans to interact with the target.

The crowding factor method applied to parafoveal vision.

Crowding increases with eccentricity and is most readily observed in the periphery. During natural, active vision, however, central vision plays an important role. Measures of critical distance to estimate crowding are difficult in central vision, as these distances are small. Any overlap of flankers with the target may create an overlay masking confound. The crowding factor method avoids this issue by simultaneously modulating target size and flanker distance and using a ratio to compare crowded to uncrowded conditions. This method was developed and applied in the periphery (Petrov & Meleshkevich, 2011b). In this work, we apply the method to characterize crowding in parafoveal vision (<3.5 visual degrees) with spatial uncertainty. We find that eccentricity and hemifield have less impact on crowding than in the periphery, yet radial/tangential asymmetries are clearly preserved. There are considerable idiosyncratic differences observed between participants. The crowding factor method provides a powerful tool for examining crowding in central and peripheral vision, which will be useful in future studies that seek to understand visual processing under natural, active viewing conditions.

The effect of load on spatial attention depends on preview: Evidence from a reading study

ABSTRACT The spatio‐temporal distribution of covert attention has usually been studied under unfamiliar tasks with static viewing. It is important to extend this work to familiar tasks such as reading where sequential eye movements are made. Our previous work with reading showed that covert spatial attention around the gaze location is affected by the fixated word frequency, or the processing load exerted by the word, as early as 40ms into the fixation. Here, we hypothesised that this early effect of frequency is only possible when the word is previewed and thus pre‐processed before being fixated. We tested this hypothesis by preventing preview. We investigated the dynamics of spatial attention around the gaze location while the observer read strings of random words. The words were either always exposed (normal preview) or only exposed while being fixated (masked preview). We probed spatial attention when a target word with either high or low printed frequency – or low or high load, respectively – was fixated. The results confirmed that, early in a fixation, allocation of spatial attention 6 characters from the gaze was affected by the words frequency but only when the word was exposed before being fixated, so that processing of the word could start before it was fixated. Our results indicate that the ongoing processing load of a word is modulated by its pre‐processing and affects the dynamics of covert spatial attention around the word once it is fixated.

Feedback about gaze position improves saccade efficiency

When searching for an unknown number of noisy targets in a limited time, looking at uncertain locations is more efficient than looking at locations with a high probability of the target. Previously, we showed that immediate saccadic feedback that revealed the true identity of a noisy stimulus was effective in improving saccade efficiency (Verghese, Ghahghaei, 2013). However, the stimuli in that task were not limited by visibility, and the feedback artificially removed any ambiguity about the identity of the stimulus as soon as a saccade landed at that location. Here we examine if the increase in visibility upon naturally foveating a target combined with simple knowledge about eye position will encourage a strategy to fixate informative locations. Observers actively searched a brief display (900 ms) with six Gabor patches in noise, located 3 degrees from fixation to locate an unknown number of horizontal targets, among vertical distractors. The contrast of the Gabors was high or low such that the orientation of the high-contrast Gabor was perfectly discriminable at 3 degrees, but that of the low-contrast Gabor only upon foveation. Thus, saccades to low-contrast rather than high-contrast locations were more informative. In separate blocks participants received either (i) no gaze feedback, (ii) delayed gaze feedback at the end of the trial, or (iii) immediate gaze feedback after each saccade in the trial. Feedback was provided by changing the color of the ring that surrounded each location. In the absence of feedback, participants differed in the proportion of saccades to informative locations, with a greater proportion for more experienced participants. Both immediate and delayed feedback increased the proportion of informative saccades for four out of five participants. Furthermore, gaze feedback increased the latency of the first saccade and reduced the number of reflexive saccades to salient locations, making saccades more informative. Meeting abstract presented at VSS 2015.