Coactivation of gustatory and olfactory signals in flavor perception

Abstract

It is easier to detect mixtures of gustatory and olfactory flavorants than to detect either component alone. But does the detection of mixtures exceed the level predicted by probability summation, assuming independent detection of each component? To answer this question, we measured simple response times (RTs) to detect brief pulses of one of 3 flavorants (sucrose [gustatory], citral [olfactory], sucrose-citral mixture) or water, presented into the mouth by a computer-operated, automated flow system. Subjects were instructed to press a button as soon as they detected any of the 3 nonwater stimuli. Responses to the mixtures were faster (RTs smaller) than predicted by a model of probability summation of independently detected signals, suggesting positive coactivation (integration) of gustation and retronasal olfaction in flavor perception. Evidence for integration appeared mainly in the fastest 60% of the responses, indicating that integration arises relatively early in flavor processing. Results were similar when the 3 possible flavorants, and water, were interleaved within the same session (experimental condition), and when each flavorant was interleaved with water only (control conditions). This outcome suggests that subjects did not attend selectively to one flavor component or the other in the experimental condition and further supports the conclusion that (late) decisional or attentional strategies do not exert a large influence on the gustatory-olfactory flavor integration.

Figure 1

Distribution of RTs (averaged over trials ± SD) to the single-component flavorants per subject in the baseline measurement session for condition A (panel A) and condition B (panel B).

Figure 2

The TASTE apparatus, showing its workings (hidden from the subjects). For a detailed description, see Ashkenazi et al. (2004). The hidden workings include a sliding nozzle array that allows for rapid switching between stimuli. Because each line and each nozzle was dedicated to a specific flavorant, cross-contamination and sniffing of the stimuli before stimulus onset was precluded. The subjects readied themselves for stimulus presentation by extending the tongue to the Teflon guide under the outflow point of the preselected nozzle. After stimulus presentation, the subject indicated, as quickly as possible, if a target stimulus was present (sucrose, citral or, sucrose–citral mixture) by pressing the response button. Between trials, subjects rinsed with deionized water and expectorated into the sink. This figure appears in color in the online version of Chemical Senses.

Figure 3

Cumulative probability density functions for the observed sucrose–citral mixtures (gray diamonds), the sucrose stimulus (black squares), and the citral stimulus (gray triangles) averaged over subjects (in panel A). In panel A, we also plotted the pooled (across subjects) false-positive responses to the water stimulus (black crosses). In panel B, we reproduced the observed sucrose–citral mixtures (black triangles) and added the predicted model of independent activation and probability summation (gray diamonds) averaged over subjects. Panel C gives a sample of data of individual subjects (averaged over trials) CDFs. Significant differences (at α = 0.05) are indicated with an asterisk.

Figure 4

Cumulative probability density functions in the interleaved (black triangles) and control blocks (gray diamonds) for sucrose, citral, and sucrose–citral mixtures (averaged over subjects). Significant differences (at α = 0.05) are indicated with an asterisk.

Figure 5

Cumulative probability density functions in a selected window of time for the observed sucrose–citral mixtures (black triangles), the predicted model of independent activation and probability summation (black diamonds), and the predicted model of independent activation and probability summation including the probability from false positives (gray diamonds), averaged over subjects. The black crosses depict the cumulative probability density function of the pooled observed false positives.