Licking microstructure reveals rapid attenuation of neophobia

Abstract

Many animals hesitate when initially consuming a novel food and increase their consumption of that food between the first and second sessions of access-a process termed attenuation of neophobia (AN). AN has received attention as a model of learning and memory; it has been suggested that plasticity resulting from an association of the novel tastant with "safe outcome" results in a change in the neural response to the tastant during the second session, such that consumption increases. Most studies have reported that AN emerges only an hour or more after the end of the first exposure to the tastant, consistent with what is known of learning-related plasticity. But these studies have typically measured consumption, rather than real-time behavior, and thus the possibility exists that a more rapidly developing AN remains to be discovered. Here, we tested this possibility, examining both consumption and individual lick times in a novel variant of a brief-access task (BAT). When quantified in terms of consumption, data from the BAT accorded well with the results of a classic one-bottle task-both revealed neophobia/AN specific to higher concentrations (for instance, 28mM) of saccharin. An analysis of licking microstructure, however, additionally revealed a real-time correlate of neophobia-an explicit tendency, similarly specific for 28-mM saccharin, to cut short the initial bout of licks in a single trial (compared with water). This relative hesitancy (i.e., the shortness of the first lick bout to 28-mM saccharin compared with water) that constitutes neophobia not only disappeared between sessions but also gradually declined in magnitude across session 1. These data demonstrate that the BAT accurately measures AN, and that aspects of AN-and the processes underlying familiarization-begin within minutes of the very first taste.

Keywords: attenuation of neophobia; brief-access task; licking microstructure.

Figure 1

Analysis of licking. Shown is example data from the BAT—the first part of a single trial of licking (each lick is a vertical hash mark as time progresses toward the right). Licks in bouts tend to be separated by reliable interlick intervals, such that the longer pauses separating bouts are easily visible. Summing the number of licks provides an estimate of consumption; bout length in general has been related to taste palatability, and the length of the initial bout in particular—the length of the licking bout before the first hesitation—is examined here in relation to neophobia and AN.

Figure 2

Concentration-dependent AN in the one-bottle task. (A) Raw consumption of water (dotted lines) and saccharin (solid lines), in milliliters, across the first and second tasting session. More saccharin (8.9mM) was consumed than water in session 2; less saccharin (28 and 89mM) was consumed than water in session 1, and in addition, less saccharin (89mM) was consumed than water in session 2. (B) The preference scores (saccharin consumption divided by the sum of saccharin and water consumption) are shown for sessions 1 (dashed line) and 2 (solid line) across concentrations (x axis); consumption above 0.5 means that the saccharin is preferable to water (horizontal dotted line). AN is observed between sessions 1 and 2 for the 3 highest saccharin concentrations. In both panels, the 2 concentrations identified for further testing (“low sacc” and “high sacc”) are noted. Error bars represent 1 standard error of the mean. *P ≤ 0.05, **P ≤ 0.01, ***P ≤ 0.001.

Figure 3

Concentration-dependent AN in the BAT, conventions same as for Figure 2. (A) The pattern of raw consumption data, here measured in number of licks, is strikingly similar to that of the one-bottle task, in that the only saccharin–water difference is a preference for water observed in session 1 of high sacc availability. (B) Preference scores shown for low (solid line) and high sacc (dashed line). As in the one-bottle task, AN occurs only for high sacc. Error bars represent 1 standard error of the mean. **P ≤ 0.01.

Figure 4

Analysis of initial lick bout length reveals AN within the first session of high sacc. The difference between number of licks to water and to saccharin in the first bout changes across the first session (A) but is stable across the second session (B) in the high-sacc group. (C) First-bout preference for high sacc significantly increases (i.e., neophobia attenuates) across the first session (left panel, P = 0.01); preference is stable across session 2 (right panel, P = 0.54). Error bars represent 1 standard error of the mean.

Figure 5

AN is not observed within the first session of low sacc. The relationship between licks in the first bout for water and saccharin is stable for the first (A) and second (B) day in the low-sacc group. Additionally, there is no significant linear relationship for the preference for either day within the first bout of licking (C, session 1: P = 0.29; session 2: P = 0.56), demonstrating that rapid AN occurs only in the neophobia-inducing concentration of neophobia, and it is not an artifact of analysis. Error bars represent 1 standard error of the mean.

Figure 6

Within-session AN is not a function of declining consumption. (A) The relationship between within-session AN (quantified in terms of the difference between the lick bout preference ratio for the first and last quarter of the session) and within-session reduction of water consumption (similarly calculated), for the entire data set. Neither the overall correlation (thick regression line) nor the correlation for any individual session was significant. (B) The same analysis, but comparing AN to total consumption (sacc + water).

Figure 7

Within-session AN is only observed under short pause criteria. As the pause length used to delineate the length of initial lick bouts is increased, observable within-session AN to high sacc (solid line; quantified in terms of change in ratio from beginning to end of session 1) decreases in magnitude. The horizontal dashed line represents a difference of zero.