Chemical Senses Vol. 30 No. suppl 1 © Oxford University
Press 2005; all rights reserved
Temporal Characteristics of Human Taste Judgements as Calibrations for Gustatory Event-related Potentials and Gustatory Magnetoencephalographs
Departments of Psychology and Neurobiology and Behavior, Cornell University, Ithaca NY 14853-7601 USA
Correspondence to be sent to: Bruce P. Halpern, e-mail: bph1{at}cornell.edu
Key words: evoked potential, reaction time, MEG, GEM, ERP, gustatory
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Introduction |
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 Top Introduction CNS measurementsAcknowledgementsReferences |
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Activity in the central nervous system (CNS) constantly fluctuates. CNS changes that are potential responses to sensory stimulation must occur before an observable external outcome of the stimulation. If the external change is an overt, measurable behavior, then the time interval between a controlled stimulus and the behavior is a reaction time (RT) (Halpern, 1986
, 1991, 1994). Human RT
can be used to predict when relevant changes in human CNS activity in response to a
specified controlled stimulus will occur. Therefore, human RT both indicate the time
after stimulus onset (latency) when relevant human CNS changes should be found and
provide a means of excluding CNS changes for which the latency is too long.
Human gustatory RT require controlled stimuli with known arrival times, concentration
profiles and durations. They can be provided by rapidly changing from a carrier liquid
(solvent only) to a stimulus liquid (solvent plus solute), both delivered at a fixed flow
rate and temperature over a consistent and limited area of the human tongue for
predetermined durations and then rapidly changing back to the carrier liquid (e.g.
Kelling and Halpern, 1983, 1987,
1988). Physical measurements at the tongue of concentration changes over time provide
calibration of stimulus duration and concentration profile. Effects of the rapid change
events on RT are identified by ‘changing’ from carrier liquid to carrier
liquid, thus controlling for responses to alterations in liquid flow. If subjects are
asked to respond only to taste changes and are given identified practice trials during
which there is, or is not, a change from carrier liquid to stimulus liquid and back to
carrier liquid, then reports of a change in taste during unidentified simple taste
reaction time (RTs) control trials average <10%; error rates are
higher for brief duration stimulus trials, but fall below 10% for durations
>100 ms (Kelling and Halpern,
1987). RT responses may require movement of a button or lever, or a spoken
word; in some instances, a computer display gives feedback. All timing accuracy can be at
the millisecond level.
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CNS measurements |
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TopIntroductionCNS measurementsAcknowledgementsReferences |
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A number of non-invasive techniques for measurement of CNS activity are available. In many cases there is a reciprocal relationship between precision of time registration and degree of spatial location or representation of the structural aspects of CNS regions. Two approaches with relatively high temporal resolution are evoked potentials, also known as event-related-potential (ERP) recording and magnetoencephalography (MEG; see Plattig, 1991
;
Näätänen et al.,
2002). MEG has a better spatial resolution (Endo et al., 1999). ERP
In a comparison of ERP and MEG, the gustatory evoked potentials (GEP) included a
positive-going change (P1) with a mean latency of 127 ms, a negative going change at 263
ms (N1) and a second (and sustained for 
200 ms) positive-going potential at 432 ms
(P2) (Mizoguchi et al.,
2002). All three GEP latencies were less than the 446 ms mean human simple
taste reaction time (RTs) to 500 mM NaCl presented under similar conditions
(Kelling and Halpern, 1987). However,
because one subject in the
Kelling and Halpern (1987) study had a
RTs of 283 ms, it may be that P2 and perhaps N1 denoted some cortical
processing of the gustatory input.
Mizoguchi et al. (2002)
reached a similar conclusion based upon relations between N1, P2 and responses
simultaneously recorded using MEG. On the other hand, the latencies of P1, N1 and P2 were
all briefer than the mean complex taste reaction times (RTC) of 600 ms or more
associated with taste quality identifications (RTCi) (Yamamoto and Kawamura, 1981, 1984;
Halpern, 1986, 1991). This might imply
that the degree of cortical processing that was indicated by N1 and P2 of the measured
GEP (Mizoguchi et al.,
2002), although perhaps more than sufficient for RTs, may not have
been at the level of RTCi.
MEG
Measurements of changes in cortical magnetic fields (MEG) evoked by sensory
stimulation provide ms timing and high spatial resolution, as studies of relationships
between visual RT and CNS preparatory motor activity have demonstrated (e.g.
Endo et al., 1999). There
appears to be less distortion than with ERP (Murayama et al., 1996). MEG recordings in
gustatory areas of the CNS have been done using either electrical stimulation of the
tongue (see
Frank and Smith, 1991) or with flowing
tastants. For electrical stimulation of the tongue (electrogustometry) with currents that
evoked reports of taste but not irritation (Yamamoto et al., 2003) the latency for MEG
responses was considerably longer than that produced by flowing tastants (Mizoguchi et al., 2002) and longer than many GEM
(see below). This raises questions about the use of electrogustometry.
Both MEG responses to flowing tastants (GEM) and RTs have been examined in
a number of studies (e.g.
Kobayakawa et al., 1996;
Saito et al., 1998). GEM
onset latencies and RTs were correlated. One possible issue is the extent to
which the 1 M NaCl that was used might have been both a trigeminal and a gustatory
stimulus and therefore elicited chemesthetic (Bryant and Silver, 2000) as well as taste responses. This
probably did not affect the GEM data because the onset latency did not change with NaCl
concentration.
A later study (Yamamoto et al.,
2000) with flowing tastants observed GEM to tastants but no responses to flow
of H2O. Furthermore, after subjects chewed a taste-modifier that results in
humans perceiving citric acid as sweet (‘miracle fruit’), the GEM latency for
citric acid approached that for sucrose. These data provided strong support for
interpretation of the MEG data as GEM, apparently with little or no contamination from
chemesthetic input.
A series of studies illuminated GEM latency differences between several cortical
gustatory areas (Kobayakawa et al.,
1999;
Saito et al., 2000;
Mizoguchi et al., 2002). It
is possible that the 1 M NaCl that was used may have been both a gustatory and a
chemesthetic stimulus, but the saccharin and the lower NaCl concentrations that were
employed were likely to be only taste stimuli. Latencies ranged from a few hundred to
>1000 ms. The shorter GEM latencies were similar to those reported in pervious
investigations. The later GEM latencies, which were found in regions other that primary
gustatory areas, may have been associated with CNS processing related to the taste
quality or intensity judgements which subjects were asked to report after each recording.
This series of studies is important because they suggest that sequential cortical
processing of gustatory input can be studied using GEM and related to perceptual and
cognitive judgements more demanding than RTs.
In general, the ERP and GEM studies have focused on RTs. Since this represents only the earliest and perhaps least sophisticated level of gustatory processing, it would be valuable for future studies to be designed such that RTC for taste quality and intensity, as well as gustatory time-intensity and time-quality tracking, can be related to measures of gustatory ERP and to GEM.
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Acknowledgements |
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TopIntroductionCNS measurementsAcknowledgementsReferences |
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I thank P. Halpern, S. Saito and the anonymous referee for helpful recommendations on earlier drafts of this manuscript.
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