Does perceiving a letter of the alphabet cause spatial shifts of attention?

The association between numbers and space has been widely investigated and there is mounting evidence that perception of numbers involves a spatial component. It has been demonstrated that there is an association between number magnitude and response preference with faster left-hand responses for small numbers and faster right-hand responses for large numbers (SNARC effect for Spatial Numerical Association of Response Codes), suggesting the existence of a “Mental Number Line”. Involuntary shifts of attention to the left or right visual field have been shown to occur when stimuli with a strong meaning (e.g. numbers, direction arrows, etc) abruptly appear in the visual field, even when the observers know that these stimuli are irrelevant to the task and must be ignored. Since numbers convey not only magnitude but also order, the question of whether non-numerical ordinal information is also spatially coded follows. Although initially the SNARC effect was found exclusively with numbers as stimuli further studies showed that non-numerical information such as letters of the alphabet, months of the year, days of the week, etc. is also spatially coded. Based on these findings in the present study we investigated whether perceiving letters of the alphabet can induce such a shift of attention to the left or right visual field. 23 subjects did a detection task in which they had to fixate a central white cross on a black screen, followed by either one of the two letters A or Z (the first and the last letters of the Italian alphabet) or a neutral cue (a diamond) presented for one of the durations (500 ms, 800 ms or 1000 ms) randomly. After the letter disappeared, the target, a small white square, appeared on the left or the right of the fixation point. In 20% of the trials a white circle appeared instead of the square, as catch trials. Participants were to press the space bar as soon as they detected the target (a white square). The results showed that there were significant shifts of attention after perceiving Z but not A, and these shifts were generated only for the trials where the letter was presented the longest (1000ms). This suggests that the spatial component of letters might not be as strong as those of the numbers, and it probably takes longer for letter perception to induce shifts of attention.