Odor localization in humans has repeatedly been subject of recent olfactory research. However, the question whether we are capable, in principle, of creating a spatial representation of the external world purely based on olfactory information is still controversial. Though experience- or gender-based differences in individual performance might exist 
, in laboratory settings people generally only seem to succeed in odor localization if the odorant additionally stimulates the trigeminal nerve 
. While the olfactory nerve conveys odor-specific chemical information from olfactory sensory neurons, the trigeminal nerve endings are excited by tactile or nociceptive stimuli and relate to the subjective experience of burning or tickling sensations 
. On the other hand, behavioral experiments have shown that directional smelling can be found in higher vertebrates 
. In addition, separated sensory epithelia and lateralized neuronal projections into the primary olfactory cortex would in principle allow for spatial comparisons across hemispheres in humans 
One reason for the negative findings in human studies might be an inappropriate experimental approach. Former experiments exclusively used explicit judgments to investigate the ability for directional smelling. However, olfaction is known to act to a substantial extent on a preconscious level, which is supposed to be the result of its anatomical and functional organization 
. Hence, a paradigm that requires the explicit
assessment of odor location could exceed our capacity for directional smelling. An alternative approach is to assess spatial processing in an implicit way, thereby rendering assessment of conscious access to spatial information unnecessary. A well-known procedure to study implicit spatial processing is the spatial cueing paradigm, which was originally developed by Posner 
to investigate visual spatial attention. In these paradigms the location of a target is preceded by a cue at either the same (valid cue) or a different location (invalid cue). Typically, people respond faster and more accurately to validly compared to invalidly cued targets. Here, the focus is set on exogenous cueing which relies on the automatic, involuntary spatial orienting response caused by the detection of an otherwise irrelevant cue. These cues do not share any features with the target and do not predict the subsequent target location (for review, see 
). Since its influential adoption in visuospatial processing, the spatial cueing paradigm has been extended to study the interaction between stimuli of different modalities, including tactile and auditory stimuli. Notably, it appears that a stimulus in one modality can facilitate the processing of a target in a different modality irrespective of task-relevance 
. For instance, response times and accuracy to tactile stimuli have been shown to be enhanced by visual and auditory cues preceding the target at the same side 
. Similar findings have been reported for different cue-target relations (e.g. tactile or auditory cues preceding visual targets 
To date, no study has used olfactory stimuli for exogenous spatial cueing of targets in a different modality. Nonetheless, priming procedures have also been used in olfactory research, with many studies demonstrating the high potential of a preceding odorant to enhance processing of subsequent (emotional) stimuli 
. However, these priming studies in olfaction have been limited to semantic and emotional contexts, and never included spatial attention.
The present study implemented the spatial cueing paradigm as a novel approach to study lateralization of human olfaction. An olfactory stimulus was used as crossmodal, exogenous cue for a visual target. We hypothesized that a residual ability for directional smelling will become manifest as a spatial cueing effect, with valid cues leading to a significant gain in behavioral performance compared to invalid cues. As women have repeatedly been shown to have better olfactory performance than men 
, we included sex as a variable of interest in order to investigate a corresponding female advantage in implicit spatial processing of olfactory stimuli. In a second experiment, we sought to replicate former findings of an inability to explicitly localize olfactory stimuli across both nostrils.