The human visual system has learned to assume that light originates from above, most likely because of the persistent natural overhead light source – the sun (Ramachandran, 1988). Asymmetries of perception in neurologically normal individuals, like assuming light is coming from above, in part result from efficiency measures of the visual system. Not only is light assumed to come from above, but light from above and to the left has been found to decrease reaction times in target finding as well as increase aesthetic preference (Sun & Perona, 1998; Smith & Elias, 2013). The underlying cause of the bias towards upper-left lighting is debated, and may have a relationship with another peculiar phenomenon in neurologically normal individuals where greater attention is paid to leftward space, called pseudoneglect (Bowers & Heilman, 1980). Alternatively, an explanation suggesting that directional reading influences lighting preferences has been proposed, as Smith and Elias (2013) found native right-to-left readers to be significantly different from leftward biased left-to-right readers. The current set of experiments used eye-tracking and a target finding paradigm to assess differences between left-to-right and right-to-left readers. Manipulating the position of the light illuminating a field of spheres generated targets, creating either 1 convex bubble among 15 concave depressions, or vice-versa. Results from these studies are mixed, and highlight differences between both upper and lower and lateral visual space. Light originating from above facilitated shorter average duration times for both groups, whereas left-to-right readers tended to prefer light from the upper-left, while right-to-left readers preferred light from the upper-right. No one target location in the array facilitated shorter average duration times for right-to-left readers, although left-to-right readers tended to exhibit shorter durations when identifying targets in the upper-left quadrant. Participants spent the greatest amount of time examining the upper quadrants of the array, tending to focus more on the side of the image that their native reading direction begins on. The influence of directional reading on light source perception, and the potential problems of using exclusively Western participant samples are discussed.

In everyday life our eyes are exposed to massive amounts of visual stimuli. However, even though the stimuli’s features grab our attention, we have a natural tendency to focus on the centre of the scenes. This central spatial bias is not steady; in fact, while freely viewing a scene, the eyes shift towards the left for two seconds and then return to the centre (Ossandón, Onat, & König, 2014). The leftward spatial bias has also been reported in other behavioural studies, suggesting the role of the lateralization of the attention network. The attention network is activated dominantly on the right hemisphere when detecting new/novel stimuli, causing stronger effects on the contralateral (left) hemispatial side. Hence, in this thesis a series of behavioral studies were conducted using an eye-tracking technique to modulate the leftward spatial bias using different types of primes. Five different eye-tracking experiments were performed in the Neurobiopsychology (NBP) Department at Osnabrück University to investigate the interplay between the horizontal spatial bias and multiple different primes displayed prior to the presentation of images in free viewing tasks. The goal of Experiment 1 was to investigate if different reading directions can alter the leftward spatial bias. The results showed that native right-to-left (RTL) readers showed RTL spatial bias after reading RTL texts and left-to-right (LTR) spatial bias after reading LTR texts. This result suggests the dynamic role that the reading direction has on modifying the horizontal spatial bias. On the other hand, native LTR readers who learned RTL languages later in life showed a leftward spatial bias after reading both LTR and RTL texts. While these results suggest the crucial role of mastering RTL languages in modulating the spatial bias, a larger sample size is required to confirm these findings. The aim of Experiment 2 was to investigate if the reader’s second language has a different effect than his/her native language on the leftward spatial bias. Compared to native language LTR texts, LTR/LTR bilinguals demonstrated a slight increase in the leftward spatial bias after reading second language LTR texts. This finding demonstrates the effect that the second language has on enhancing and reinforcing the leftward spatial bias. The goal of Experiment 3 was to study the difference between habitual reading and non-habitual reading (mirrored reading) on the leftward spatial bias. LTR bilinguals read LTR and mirrored LTR (mLTR) texts prior to image exploration and showed a strong leftward bias after reading both texts. The outcome of this experiment suggests that there is an influence of habitual (normal LTR) reading and not of non-habitual (mLTR) reading on the horizontal spatial bias, even though the same language was used in the primes. Experiment 4 investigated if the oculomotor control of the eye movement, without reading, can modulate the leftward spatial bias as in habitual reading. Thus, LTR and RTL moving-dot primes without reading were presented prior to image exploration, mimicking the readers’ eye movement. Native LTR readers showed a leftward bias after primed with LTR and RTL moving-dots. However, in a pilot study within this experiment, native RTL bilinguals demonstrated rightward bias after RTL moving-dots and a weak leftward bias after LTR moving-dots. These findings strengthen the effect of the habitual reading direction and exclude the role of language in reshaping the leftward horizontal bias. Following this, Experiment 5 studied the effect of different factors, including age, gender, first language, second language, second language proficiency, and age of second language acquisition, on the magnitude of the horizontal spatial bias. This Experiment is considered an extension of Experiment 1 in order to study the interindividual differences among native RTL readers after reading RTL texts in a free viewing task. Compared to the native LTR/LTR readers of Experiment 2, the rightward spatial bias among individuals of native RTL readers was strong and profound, but with a large variance of the measurements, suggesting inter-individual differences. This study found no correlation between the magnitude of the RTL spatial bias and the age, gender, first language, second language, second language proficiency, and age of second language acquisition of the participants. Thus, these findings strengthen the profound role that the habitual reading direction has on the RTL spatial bias, regardless of the biological and cultural variables mentioned above. Overall, the thesis proves that the RTL habitual reading direction has a flexible role in modulating the leftward spatial bias (Experiment 1). In addition, the LTR habitual scanning direction can reinforce the leftward bias among native LTR readers to a certain degree (Experiment 2). Yet, non-habitual reading process (Experiment 3) and oculomotor control without language involvement (Experiment 4) showed no influence on the horizontal spatial bias. Moreover, there was no evidence to suggest whether or not age, gender, first language, second language, second language proficiency, and age of second language acquisition influence the magnitude of the rightward horizontal spatial bias (Experiment 5). This leads to the conclusion that forming a habit of scanning direction is a strong factor in changing the natural spatial bias. Furthermore, even though no correlation was found between several biological/cultural factors and the magnitude of the RTL spatial bias, certain speculations can be proposed. First, the strength of the LTR and RTL scanning habits among RTL individuals could lead to an antagonizing effect and yield to interindividual differences. Second, the interindividual differences at the structural and functional cortical level among healthy individuals could cause interindividual differences in the horizontal spatial bias. Third, the narrow group sample of the LTR readers could lead to a small variance in comparison to the diversity of the RTL group sample. Overall, these five experiments have shed light on the dynamic effect of reading direction on the natural spatial bias and opened the door for potential cross-cultural studies regarding visuospatial attention.