Brain mechanisms for extracting spatial information from smell

On the trigeminal percept of androstenone and its implications on the rate of specific anosmia

Specific anosmia is a term that describes an inability to perceive a particular odorant in the context of an otherwise normal olfactory acuity. The most common example, for the odor of androstenone, has been ascribed a prevalence ranging from 2 to 45%. In two experiments we sought to determine whether this wide range could be explained by the difference in steroid concentrations used, and by the degree to which the trigeminal system contributes to perception of androstenone. Experiment 1 demonstrated that high concentrations of androstenone stimulated the trigeminal system, as indicated by electrophysiological recordings. Experiment 2 demonstrated that conscious detection of androstenone is possible based solely on the trigeminal system. Interestingly, detection seems to interact with olfactory acuity in that subjects with a low olfactory sensitivity to androstenone were better able to detect its trigeminal component. The agreement between conscious experience and behavioral discrimination was not well calibrated, in that subjects demonstrated a clear overconfidence in their abilities. Altogether, the current study suggests that androstenone is an odorant that produces a concentration-dependent degree of trigeminal stimulation. This trigeminal component explains the diversity of the reported prevalence of specific anosmia for androstenone and might have implications on future use of specific anosmia as a tool to understand odor processing.

Temporal integration in nasal lateralization of ethanol

Two experiments examined the trade-off between concentration and stimulus duration in nasal lateralization of n-ethyl alcohol. In nasal lateralization, a common measure of irritation threshold, subjects receive chemical vapor in one nostril and clean air in the other. Subjects try to determine which nostril received the chemical. Within experimental runs, subjects received fixed concentrations (1650-5000 ppm) of ethanol, and duration was varied to find the shortest, lateralizable stimulus. In Experiment 1, a small group of subjects was tested intensively to obtain stable individual data. In Experiment 2, a larger group was studied using more rapid methods. In both cases, subjects could lateralize increasingly weaker concentrations with longer stimulus presentations. Hence integration occurred. However, more than a twofold increase in duration was required to compensate for a twofold decrease in concentration to maintain threshold lateralization. These results suggest that an imperfect, mass-integrator model can describe short-term integration of nasal lateralization of ethanol.

Spatial neglect--a vestibular disorder?

The phenomenon of spatial neglect after right brain damage greatly helps our understanding of the normal mechanisms of directing and maintaining spatial attention, of spatial orientation, and the characteristics of neural representation of space. The intriguing symptom is a spontaneous orientation bias towards the right leading to neglect of objects or persons on the left. Interestingly, we observe similar symptoms namely a spontaneous bias of eyes and head along the horizontal dimension of space in patients with unilateral vestibular dysfunction. Further similarities concern anatomical findings. Both spatial neglect and vestibular processing at cortical level show dominance in the right hemisphere and involve common brain areas. Lesion studies in human and monkey, electrical and transcranial magnetic stimulation, as well as functional imaging results have revealed the superior temporal cortex, insula and the temporo-parietal junction to be substantial parts of the multisensory (vestibular) system as well as to be affected in spatial neglect. We argue that these structures are not strictly 'vestibular' but rather have a multimodal character representing a significant site for the neural transformation of converging vestibular, auditory, neck proprioceptive and visual input into higher order spatial representations. Neurons of these regions provide us with redundant information about the position and motion of our body in space. They seem to play an essential role in adjusting body position relative to external space. This view may initiate further development of those strategies to treat spatial neglect that use routes to rehabilitation based on specific manipulations of sensory input feeding into this system.

A truffle in the mouth is worth two in the bush: odor localization in the human brain

It is widely thought that locating the source of a smell is an ability best left to nonhuman members of the animal kingdom. In this issue of Neuron, two complementary articles highlight the neural mechanisms underlying the localization of an odor, either to the left or right side of the nose (Porter et al.) or to the inside or outside of the mouth (Small et al.). Together, these studies validate the idea that the human brain is equipped with the apparatus necessary to pinpoint the location of an odor source.

Setting occupational exposure limits in humans: contributions from the field of experimental psychology

Psychophysical methods from the field of experimental psychology are evaluated for their utility in the derivation of occupational exposure limits (OELs) for volatile chemicals based on acute sensory irritation in humans. The lateralization threshold method, which involves the localization of trigeminal vapor to the stimulated nostril, is evaluated for its underlying assumptions, reliability and validity. Whole body exposures, on the other hand, which involve the controlled, ambient exposure of human subjects to the irritant at one or a series of concentrations for an extended period are also discussed. It is concluded that the single-organ psychophysical method is largely resistant to response bias is practical and economical. However, its reliability and validity need further assessment. Whole body exposures, while having enhanced ecological validity, are more prone to demand characteristics, response bias, and subject beliefs than the traditional psychophysical procedures. An approach that involves the exposure of only the most sensitive organs such as the eyes and nose, via a mask or facebox, could facilitate the administration and alternation of odorant/irritant stimuli over a wide range of concentrations while enhancing ecological validity.