Spatiotemporal masking in pure olfaction

In olfaction, it is not possible to determine which nostril is being stimulated, i.e., lateralize, when a pure olfactory substance, e.g., phenylethyl alcohol or vanillin, is administered into one nostril, and, simultaneously, an odorless, solvent blank into the contralateral nostril. We subjected volunteers to extensive training, with feedback on each trial, in an attempt to determine whether it was possible, in these well-trained subjects, to overcome this apparent impossibility. We failed to obtain any evidence to support the notion that a pure olfactory stimulus could be lateralized when the odorant and blank were presented simultaneously. The task, however, became simple when the odorant and the blank entered each nostril sequentially. We investigated, using a two-channel olfactometer, temporal parameters that enabled such discrimination. We controlled the duration of odorant and blank air puffs, as well as their mutual timing, to determine the threshold stimulus onset-disparity, i.e., the interval between stimulus onset and blank onset, that resulted in an inability to lateralize. Latencies shorter than the threshold interval would be perceived as simultaneous stimulation. We determined that the onset interval was between 200 and 400 ms, depending on the duration of the stimuli (a shorter interval was noted for stimuli of 150-ms duration relative to 300- and 450-ms stimuli). This was also true when two odorants were applied, rather than an odorant and a blank, and the subject was instructed to focus on the sequence of odorant delivery and side of stimulation. The temporal onset threshold was the same for lateralization and for order of stimulation. Whether the olfactory system per se mediates this discrimination or whether inputs from olfaction and chemesthesis, via trigeminal free nerve endings stimulated by air-stream onset, combine to allow this discrimination has yet to be determined.