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Learning
to smell.
When exposed to the sex-steroid androstenone
repetitively, we become sensitized to it. Our threshold drops and simultaneously
there is an increase in the olfactory evoked potential and the olfactory
event-related potential.
The same happens for the putative human signalling compound androstadienone. Additionally, as the exposure-induced sensitization occurs, there is a change in the perceptual quality of the odour; from no smell, to sweet/honey/minty, to urinous/sweaty.
- Wang, L., Chen, L. and Jacob, T.J.C. (2004) Evidence for peripheral plasticity in human odour response. Journal of Physiology 554, 236-244.
- Tim J.C. Jacob, Liwei Wang, Sajjida Jaffer and Sara McPhee (2006) Changes in the odour quality of androstadienone during exposure-induced sensitization implies multiple receptors. Chemical Senses 31, 3-8. [First published on-line 2005, doi:10.1093/chemse/bji073]
- Boulkroune, N., Wang, L., March, A., Walker, N. and Jacob, T.J.C. (2007) Repetitive olfactory exposure to the biologically significant steroid androstadienone causes a hedonic shift and gender dimorphic changes in olfactory evoked potentials. Neuropsychopharmacology 32, 1822-1829.
Good
vs. bad smells.
We have shown that, contrary to expectations,
we adapt to bad smells more quickly than good smells. We
turn off our awareness of a bad smell more quickly than to a good smell.
This may seem bad strategy since bad smells warn us of danger (poisons,
toxins, off food etc) and you would think we would need to remain alert
to possible danger. But, we are more sensitive to changes in concentration
of bad smells than good smells. Nature seems to have arranged that once
we have taken in the smell information conveyed by a bad smell, we either
act (avoidance) or, if we assess that it is safe for the moment, we switch
off. As soon as the level of bad smell changes we are aware of it again.
This doesn't happen with good smells, emphasizing the point that good
smells have little biological significance - at least in in terms of survival.
Tell that to the perfume manufacturers!
Bad smells are warning signals and need
to be responded to quickly. We have measured reaction times to good and
bad smell and found that, indeed, we react quicker to bad smells (at equal
intensities).
- Jacob, T.J.C., Fraser, C.S., Wang, L., Walker, V.E. and O'Connor, S. (2003) Psychophysical evaluation of responses to pleasant and mal-odour stimulation in human subjects; adaptation, dose response and gender differences. Int. J. Psychophysiol. 48, 67-80.
- Tim J. C. Jacob and Liwei Wang (2006) A new method for measuring reaction times for odour detection at iso-intensity: comparison between an unpleasant and pleasant odour. Physiology and Behavior 87(3):500-5.
Correlation
of psychological and physiological responses to olfactory stimulation.
The aim of this study was
to correlate the perception of varying degrees of olfactory stimulation
(a conscious event) with the olfactory event related potential recorded
on the scalp (OERP). The OERP reflects both sensory transmission (physiological)
and cognitive (psychological) events.
Olfactory Event-Related Potentials (OERPs) were recorded by EEG using monopolar electrodes located at Fz , Cz , Pz, T3, C3, C4, T4 electrode sites (International 10/20 System) in 28 young students (range 18-25 years ). Brain potentials were amplified and digitised and an average was created for all subjects for 20 trials, while the subjects concentrated on a visual task. Pulses of amyl acetate (saturated vapour) were mixed 1:3 with medical air for 35, 50, 75, 100 and 200ms duration and delivered at 2.5, 5,10 and 60s inter-stimulus intervals (ISI) respectively. In each condition, the odour was delivered between 5 to 15 times. Each session was separated by a 2 min break to relieve fatigue and allow for neuronal recovery. Airflow, temperature and humidity were kept constant. The amplitude of the OERP was measured as the difference between the negative and positive peaks (sometimes referred to as N1-P2 or N1-P3). For the psychometric test the subjects were given the same stimulus protocol. A number of pulses, between 15-25, were delivered and they were asked to record how many they detected.
There was a decline in the number of odour pulses detected both with decreasing stimulus and ISI. In the matching set of physiological experiments there was also a decline in the OERP when either odour concentration or ISI were decreased. The decline in perception as ISI decreases probably results from the process of adaptation occurring in the olfactory receptor neurones, although there may also be a contribution from central habituation, and this is reflected in the decreased OERP amplitude. The increased amplitude of the OERP with increasing stimulus strength results in increased perception once the stimulus has achieved a threshold value. Perceptual and physiological thresholds were similar.
- Journal of Physiology (2000) 523, 92-93P.
- Wang, L., Walker, V.E., Sardi, H., Fraser, C. and Jacob, T.J.C. (2002) The correlation between physiological and psychological responses to odour stimulation in human subjects. Clinical Neurophysiology 113, 542-551.
Secondary physiological effects of odour
We are investigating the effects of certain odours on the following physiological parameters:
- heart rate
- respiration rate
- EEG activity
- blood oxygen
- skin resistance
- blood pressure
Smell has the ability to affect our physiological and psychological state via two mechanisms; (1) the instrinsic pharmacological properties of the odour molecule itself and (2) contextual association and memory. This latter has been extended to conditioned reflexes in animals.
We respond differently to different smells. While the biological significance of malodours is clear, the reason for the existence of pleasant odours is less obvious. Can we observe differences in our psychophysiological response to malodours and pleasant smells?
Recording of olfactory function & detection of anosmia
To detect and measure anosmia (loss of
smell), to help people who have lost their sense of smell, or to evaluate
treatments to cure anosmia, we need to be able to measure the activity
of the olfactory system. At present this is difficult. To measure it requires
the recording of electrical activity in response to a stimulus (smell)
by placing wire electrodes far up the nose. Current methods are invasive
and uncomfortable at best.
We have developed a method (see Clin. Neurophys 115 (2004) below)
for recording the activity of the olfactory system non-invasively by placing
electrodes on the surface of the nose. We simultaneously record the electro-olfactogram
(EOG) from the olfactory epithelium, and the olfactory event-related potential
from the scalp.
We hope to be able to investigate:
- anosmia
- recovery from anosmia
- therapy for anosmia
- physiological vs psychological components of the olfactory event-related potential (OERP)
- Wang, L., Hari, C., Chen, L. and Jacob, T.J.C. (2004) A new non-invasive method for recording the electro-olfactogram using external electrodes. Clinical Neurophysiology 115, 1631-1640.
- Hari, C.K., Wang, L. and Jacob, T.J.C. (2001) Olfactory receptor potentials in humans. Chemical Senses 26(6), 790
- Jacob, T.J.C., Hari, C.K., and Wang (2001) Simultaneous recording of EOG, OERP and "bulbar" potentials in humans. Chemical Senses 26(8), 1088.
The characteristics of the psychological perception of the malodours butyric acid and n-valeric acid were studied by olfactometry. The odours were delivered to the nostrils via a teflon canula in a continuous air-stream with a total flow rate of 3 L/min. The duration of a random number of odour pulses was set by computer-controlled solenoid valves to last for 35, 50, 75, 100 and 200 ms with inter-stimulus intervals of 2.5, 5, 10 or 60 s. Subjects were required to indicate the number of pulses they could detect. The results showed that the number of odour pulses detected increased with increasing the pulse duration or inter-stimulus interval. 3D curve fitting with an exponential function revealed that the perception of odour (the percentage of odour pulses detected) was positively correlated to the concentration (pulse duration) and the inter-stimulus interval in both odours tested. However, more interesting phenomenon were revealed by analysing the data in terms of gender. The perception of n-valeric acid was different between male and female subjects. The perception was correlated both to the concentration of the odour and the inter-stimulus interval in male subjects, while it was only correlated to the inter-stimulus interval in females. As for butyric acid, there was no significant gender difference in the correlation of perception with the concentration and inter-stimulus interval. Furthermore, the threshold of the perception of both odours was higher in male than in female subjects. The results suggest that there are different perceptual models for different odours and that, for certain malodours, women are more sensitive than men.
published in Chemical Senses 26(6), 789: (2001)
OLFACTORY RECEPTOR POTENTIALS IN HUMANS
The electro-olfactogram (EOG) is considered
to be the summated generator potential of olfactory receptor cells and
therefore represents peripheral olfactory events. Recording of human EOG
is technically difficult due to poor access to the olfactory mucosa and
the nasal irritation is tolerated by few subjects. The evoked potentials
at recorded on either side of the bridge of the nose in response to two
odorants, n-amyl acetate and benzaldehyde, were recorded simultaneously
with the EOG, recorded conventionally with an intranasal electrode, and
the olfactory event-related potential (OERP) recorded using scalp electrodes.
The extranasal potential recorded at the root of the nose, 0.5-1cm below
the nasion, 1cm from the ridge and ipsilateral with the stimulus, had
the highest degree of correlation with the intransasal EOG. We refer to
this site as N1 (left side) and N2 (right side). Further analysis demonstrated
that the latency, the time constant of the rising phase and the amplitude
of the evoked potential recorded at N1 also had a higher correlation coefficient
with the EOG than did those potentials recorded at other sites. Statistical
analysis indicated that the latency and time constant of the response
recorded externally at N1 were the same as those of the EOG recorded intranasally.
We conclude that an olfactory evoked potential, with many of the characteristics
of the EOG recorded from the olfactory mucosa, can be recorded externally
at a site close to the bridge of the nose.
Significance This non-invasive method of recording the EOG will have benefits for the objective assessment of olfactory function.
- Wang, L., Hari, C., Chen, L. and Jacob, T.J.C. (2004) A new non-invasive method for recording the electro-olfactogram using external electrodes. Clinical Neurophysiology 115, 1631-1640.
- Hari, C.K., Wang, L. and Jacob, T.J.C. (2001) Olfactory receptor potentials in humans. Chemical Senses 26(6), 790
- Jacob, T.J.C., Hari, C.K., and Wang (2001) Simultaneous recording of EOG, OERP and "bulbar" potentials in humans. Chemical Senses 26(8), 1088.
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