assumed, when the olfactory stimulants simultaneously excite the trigeminal somatosensory system. Thus it is possible to distinguish between right and left side when the substances additionally or mainly excited the trigeminal nerve. RHD patients with left tactile and visual neglect were reported to exhibit neglect and extinction of olfactory stimuli to the left nostril, in spite of the anatomically constrained projection of the olfactory input from that nostril to the intact left hemisphere. This finding was taken to suggest an impaired processing of all inputs from the contralesional side of space, regardless of whether such inputs were primarily directed to the damaged right hemisphere or the intact left hemisphere. Yet this interpretation is questionable because normal subjects appear unable to localize to a nostril a lateralized olfactory stimulus without the aid of an associated stimulation of the crossed trigeminal input from that same nostril. Further, and in keeping with the above notion, on a number of unilateral and bilateral olfactory stimulations those patients identified the left nostril input correctly, but misplaced it to the right nostril, possibly because of a rightward response bias related to left-sided neglect. Specifically, when two different stimuli were delivered to each nostril, RHD patients consistently failed to report the stimulus delivered to the left nostril. The olfactory system predominantly projects its fibers ipsilaterally thus these results are evidence supporting the representational theory of neglect. Also patients affected by olfactory extinction showed a large number of displacements in that the correctly identified stimuli presented to the left nostril were described as being in the right nostril. Nevertheless, it is not completely possible to determine the exact influence exerted by the nasal somatosensation in the olfactory extinction reported, since one of the odours considered as being pure odorants was later found to be processed probably also by the trigeminal. It appears that the human olfactory system is able to localize the source of the olfactory stimulation only when the odour elicits also a trigeminal response. This contradicts the idea that trained participants can localize both trigeminal stimuli and pure odorants between the two nostrils. Moreover, recently it was shown that naive participants were able to reliably localize pure odorants between the two nostrils. Clearly, if the ability of the olfactory system to extract spatial information from non-trigeminal stimuli turns out to be true, new light could be shed on the extinction phenomena described for odors. The olfactory