Clinical NeuroscienceHigh resolution time–intensity recording with synchronized solution delivery system for the human dynamic taste perception
Introduction
Taste perception occurs in a dynamic way at the taste receptors in the mouth, in the central nervous system, and at the response organs (Gotow and Kobayakawa, 2014, Halpern, 1986, Nakamura et al., 2012). Previous studies have attempted to characterize the temporal aspects of human taste perception in terms of reaction time and time–intensity. The reaction time is the time interval between the arrival of a stimulus at a receptor organ and the response of an organism to the stimulus (Halpern, 1986). The reported taste reaction time for a response (pressing a switch) to a single stimulus on the tongue, known as the simple reaction time, is more than 400 ms (Kelling and Halpern, 1983). For example, in a study on adults, simple reaction times to NaCl and sucrose ranged from 433 to 930 ms and 652 to 1926 ms, respectively (Yamamoto and Kawamura, 1981). Reaction times for a response to any change in taste are typically less than 1 s if suprathreshold concentrations are used (Halpern, 1986). The taste quality reaction time, which is the time taken to specify taste quality (for example, recognizing ‘sweet’), is longer than the simple reaction time. Reported taste quality reaction times range from 538 to 1182 ms for NaCl and from 755 to 2155 ms for sucrose (Yamamoto and Kawamura, 1981).
The measurement of reaction time requires the precise time starting from the initial contact of the solution on the tongue surface. Delivery systems for this purpose have used a plexiglass bar and tube (Lester and Halpern, 1979) or a drinkometer and pipette (Yamamoto and Kawamura, 1981, Yamamoto and Kawamura, 1984). In both of these systems, the solution was delivered downwards over the anterior dorsal surface of the extended tongue through the closed lips of participants in a sitting position.
In contrast to reaction time measurement, time–intensity evaluation requires participant to subjectively report the intensity of the perceived sensation over time. Although both types of data would be useful, time–intensity evaluation studies have usually not reported reaction times. In one such study, participants were asked to taste 15 mL of wine while rating bitterness intensity by mouse-clicking on a scale at 1 s intervals (Sokolowsky and Fischer, 2012). This methodology is good for investigating long-lasting flavors but cannot readily detect physiological sensations occurring within 1 s (Halpern, 1986). Furthermore, the method of delivery and controlled duration of taste stimuli may have affected the results. A single sip during unrestricted drinking has a duration of about 1300 ms, whereas the interval between the first two sips of multiple sips is 1800 ms (Halpern, 1986). Another time–intensity evaluation study used the finger-span technique and a mechanical taste delivery system to deliver microquantities of sour stimuli (Lugaz et al., 2005).
Results of a time–intensity evaluation can also be affected by the recording procedure. Early mechanical methods were limited by diagramming the perceived intensity on paper at fixed times (Neilson, 1957), or by using a moving paper recorder (Birch and Munton, 1981, Larson-Powers and Pangborn, 1978). Early computerized methods used a joystick (Guinard et al., 1985), whereas most current computer systems use an on-screen slider scale that is moved by a mouse (Dijksterhuis and Piggott, 2000, Yoshida, 1986). Still, several practical issues exist such as computer mouse ‘accidents’ in which the mouse becomes jammed, does not work, or is accidentally moved and results in ‘false’ peaks on the time–intensity curve (Lallemand et al., 1999). Besides time–intensity, computer mouse system is also not optimal for the investigation of taste reaction time, because computer mouse reaction times are about 300–340 ms (Peters and Ivanoff, 1999).
To our knowledge, publications reporting reaction time measurement together with time–intensity taste sensory evaluation could not be seen. We assume that the improvement of the time resolution and delivery method of the measurement system may enable this and help to profile time–intensity taste sensory evaluation. Therefore, it was of our interest to develop a system which could satisfy five features simultaneously, namely: (1) intra-oral device delivering taste solution to the anterior, lateral and posterior tongue in standardized condition (delivery of solution with a fixed amount, flow rate and duration), (2) the elimination of the swallow so that taste buds at the pharynx, soft palate and gut would not be stimulated so that repeated measurements are possible, (3) a millisecond-resolution time–intensity sensory evaluation meter linked to synchronized taste solution delivery system, which allows time–intensity profiles to be recorded in high temporal resolution, (4) a system that can provide consistent data and allow us to detect a perception difference among conditions (for example, comparison of solutions of different taste qualities or same taste qualities in different concentrations), and (5) a data analysis method to investigate the corrected taste quality reaction time from raw sensory perception data and following sensory evaluation profile.
Section snippets
Design
Our hand-held time–intensity sensory evaluation meter consisted of a rotary dial that allowed the participant to select the level of their subjective taste intensity over time. The scale on the dial corresponded to a visual analog scale from 0 (no taste) to 10 (strongest taste imaginable) (Fig. 1). The analog signal generated by the rotary dial was transferred to a microcontroller (Arduino Uno R3) and was converted to a digital signal by a computer. A challenge in designing this analog device
The latency for stimulus arrival on the tongue
The latency from the moment of starting recording (computer sending triggering signal to solenoid valve) to the moment when the solution flowed to the tongue of the participant was 196 ± 22 ms (mean ± SD), upon 30 trials.
The duration for stimulus arrival on the suction tube
The mean inner width at the end of the intra-oral device and the mean length from the end of the inlet tube to the suction tube upon 16 devices were 3.91 ± 0.44 cm and 4.30 ± 0.57 cm respectively. The duration from the moment of starting recording to the moment when the solution flowed to
Discussion
Our system allowed us to provide a precise and thorough picture of how the perceived taste intensity on the whole tongue varied with time. Our method was able to determine taste quality reaction timing, maximum intensity, and maximum intensity timing from the raw sensory perception data. It was difficult to define the absolute value of reaction time in this system, while relative comparison of time–intensity sensory profile among conditions will be available in future study. It would provide us
Conflict of interest
We do not have conflict of interest to disclose.
Acknowledgements
We thank Dr. Johnson Chun Ming Lee, Mr. Ray Tik Yuen Tang, Mr. Peter Yuanjun Liu, Mr. Billy Ka Lung Chan, and Mr. Matthew Wing Fai Lee for their assistance; Mr. Alan Wong for his technical support; and Ms. Kar Yan Li for her help with the statistics.
The study was supported by the Seed Funding Programme for Basic Research, The University of Hong Kong (Code: 20121159140) to Goto TK and Ninomiya Y and partially supported by a General Research Fund grant from the Research Grants Council of the Hong
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