General response characteristics
Responses to between three and 21 presentations (median = 11) of each of four taste stimuli (two exemplars of each of two taste qualities) were recorded from 38 cells. Across all cells the average spontaneous rate was 3.2 ± 0.7 sps. Among the 35 cells that had the initial response profiles available, 20 were identified as NaCl best, 5 as HCl best, 4 as sucrose best and 6 as quinine best. Complete response profiles are not available for 3 cells because of experimenter error. One cell responded equally well to NaCl and sucrose in the initial response profile; this cell was classified as NaCl best because it responded better to salts than sugars on all subsequent trials.
Recording sites were determined for 20 cells. Seven of these (35%) were located within the intermediate subdivision of the NTS, 7 (35%) within the lateral subdivision, 2 (10%) within the medial subdivision, 3 (15%) within the dorsomedial subdivision and 1 (5%) in the dorsolateral subdivision.
Response magnitude varied across trials
Several observations attest to the variability of response magnitude with stimulus repetition. For example, although most cells showed repeated significant responses throughout the recording session to the stimuli to which they responded initially, 16 cells (of 38; 42%) failed to respond to a given stimulus on at least one trial (). In addition, across repeated blocks of four trials (consisting of two pairs of tastants of similar qualities) both the order of effectiveness of stimuli of the same quality and that of different qualities changed in the majority of cells. These changes were found more frequently for responses to tastants of the same quality (33 cells with a median of 4 changes in order of effectiveness) compared with responses to tastants of different qualities (15 cells with a median of 2 changes in order of effectiveness). shows examples of both effects in two cells.
Table 1 Response variability and frequency of significant responses in NTS cells. “CV” is the coefficient of variation (standard deviation / mean response across trials) averaged across tastants for each cell. "Blocks of Trials" is the number (more ...)
Figure 2 Response rate (sps) across trials for salty and sour tastants in two cells. Abbreviations are as follows: N, NaCl; L, LiCl; H, HCl; C, citric acid. Top graph shows a cell with responses to different qualities (salty and sour) that reverse their order (more ...)
The coefficient of variation (CV; SD/mean), a measure of variability, was used to evaluate the stability of each cell’s response to each stimulus across trials. The average CV of all 2 sec responses, whether significant or not, to all tastants was 0.43 ± 0.05 SEM. shows the mean CV ± SEM for each stimulus. (Only stimuli that evoked at least 3 significant responses were included.) There were no significant differences in CVs across stimuli (one-way ANOVA, F7,132 = 0.91, p < 0.50), suggesting that responses to all stimuli were equally variable across repetitions. Even when responses to stimuli of the same quality were pooled, there were no significant differences in CV according to taste quality (one-way ANOVA, F3,136 = 1.51, p < 0.22). The fact that the CV did not differ across tastants reflects, in part, the strong correlation of the mean response to a given stimulus within a cell with the standard deviation of responses across trials (r = 0.69, p < 0.01). This correlation was equally strong when responses to individual taste stimuli were examined, with the exception of the sweet tastants where the sample was small (n = 5). These results suggest that stronger responses showed larger standard deviations; however, the significant negative correlation between the mean response and the CV (r = −0.33, p < 0.001) suggests that stronger responses were less variable than weaker ones.
Figure 3 Graph of the mean ± SEM coefficient of variation (CV, standard deviation divided by the mean) across cells for each stimulus. Only those cells that showed at least three significant responses were included. Abbreviations and numbers of cells included (more ...)
Although the mean response magnitudes across cells for NaCl, LiCl, HCl and citric acid were all quite similar (see ), it was still possible that response magnitudes for these tastants might differ considerably within a particular cell. To assess this possibility, the mean of the absolute value of the difference between each pair of tastants across blocks of trials was calculated for each cell. These values were then averaged across cells. Because there were relatively few cells tested with sweet and bitter tastants, only those cells that were tested with both salty and sour tastants were used (n = 24 cells). The results, shown in , show that tastants of similar qualities evoke more similar response magnitudes within cells than tastants of dissimilar qualities. A one-way ANOVA of these differences revealed a main effect of paired tastant (F5,113 = 2.424, p < 0.04), but Newman-Keuls pairwise tests showed no significant differences. However, when comparisons across tastants of similar qualities (NaCl-LiCl and HCl-citric acid) were pooled and compared with all other comparisons as a group, differences between tastants of similar and dissimilar tastants were evident (Student’s t test, p < 0.001).
Differences (sps) between responses to salty and sour tastants within cells.
We next examined correlations between mean firing rates elicited by different tastants. To do this, we calculated Pearson’s correlation coefficients. shows the results of these analyses. shows that responses to both similar and dissimilar stimuli did not covary over time; within-cell correlations therefore provide no indication that as a group they detect similarity between the two salty and the two sour tastants, as might be indicated by larger interstimulus correlations between similar-tasting stimuli. It should be noted, however, that the interval between stimulus presentations was two minutes for dissimilar stimuli, and four minutes for similar stimuli; fluctuations over shorter periods of time could not be detected with our experimental design. Median correlations across tastants were also quite low. Across cells, correlations between average responses (across trials) to similar tastants were greater than correlations between dissimilar tastants, though all interstimulus correlations were high, as seen in .
Information conveyed by spike count and spike timing for comparisons of salty and sour tastants. Numbers are mean ± SEM.
Collectively, results of analyses of interstimulus correlations show that neither individual cells nor the across neuron patterns of response are competent to both distinguish among tastants as well as recognize the similarity of tastants of like taste quality. These results are based on the assumption that the response magnitude, i.e. spike count or firing rate, in the initial 2 sec of response can convey the critical information necessary for this task. It is possible, perhaps probable, that only certain cells use this coding mechanism and that cells may also use other coding mechanisms, e.g. temporal coding, to accomplish this discrimination. We therefore analyzed the contribution of the temporal characteristics of taste responses to the detection of differences among taste stimuli.
Temporal coding of tastants of similar and dissimilar quality
To analyze the contribution of temporal coding to the neural code for tastants of similar and dissimilar qualities, we utilized metric space analyses, as described in the Materials and Methods section above. These procedures allowed the quantification of the amount of information that was contributed by spike count alone, by the rate envelope (time course of the response), or by spike timing. Naturally, all three of these mechanisms might, and usually were, observed in any given comparison of responses to tastants. In addition, it is important to note that the maximum information that can be conveyed in any pairwise discrimination is 1 bit. In the present data only a few cells that used spike count alone achieved that value, though some cells for which spike timing and/or rate envelope contributed information came close. Of 40 stimulus-stimulus comparisons where Hmax = 1.0, 32 (80%) were between stimuli of different taste qualities and 30 (75%) also showed Hcount = 1. – show examples of the results of analyses of temporal coding in two cells.
Figure 4 A. Peristimulus-time histograms (PSTHs) of responses to NaCl, LiCl and HCl. B. Responses (sps) to NaCl (N), LiCl (L), HCl (H) and citric acid (C) across trials. C. Metric space analysis of information contributed by responses (filled squares) at various (more ...)
Figure 5 Example of a cell for which the rate envelope primarily accounts for the distinction between responses to HCl and citric acid. A. PSTHs of responses to HCl and citric acid. B. Responses (sps) to NaCl (N), LiCl (L), HCl (H) and citric acid (C) across trials. (more ...)
Twenty-five cells had sufficient numbers of trials of each of four tastants to permit the analyses of temporal coding. Twenty-four cells were tested with salty (NaCl and LiCl) tastants, 19 cells were tested with sour (HCl and citric acid) tastants, four cells were tested with salty and bitter (quinine and urea) tastants and two cells were tested with sour and sweet (sucrose and fructose) tastants. The proportion of cells tested with each pair of taste qualities reflects the relative incidence of responsiveness to these stimuli among NTS cells. Thus, since almost all cells responded well to NaCl and HCl, salty and sour tastants were most often tested and most of our conclusions are based on data obtained from cells that responded well to these stimuli.
Analyses of information contributed by temporal coding showed that comparisons of tastants of similar qualities were encoded differently than those for tastants of dissimilar qualities. As shown in , the amount of information contributed by spike count alone was significantly less for tastants of similar quality than that for tastants of dissimilar quality (Student’s t test, p < 0.001). This was not surprising given the observation that the average difference between response magnitudes was higher for pairs of tastants of dissimilar qualities than for pairs of tastants of similar qualities (see ). Moreover, the larger the mean absolute difference in response magnitude, the larger the amount of information conveyed by spike count (r = 0.49, p < 0.01).
The amount of information contributed by the temporal characteristics of the response in addition to that contributed by spike count was about the same for all pairwise comparisons. Given the fact that the total amount of information contributed by the temporal characteristics of a response was significantly larger for tastants of dissimilar quality than for those of similar quality (Student’s t test, p < 0.001), the temporal characteristics of responses contributed proportionately more information than spike count for distinguishing among tastants of similar quality than for tastants of dissimilar quality. The amount of information provided by the spike timing was therefore proportionally greater for distinctions between tastants of similar quality.
It is important to distinguish between classifying discriminations as easy vs. difficult, and classifying discriminations on the basis of whether the tastants are of similar vs. dissimilar qualities. We are operationally defining an “easy” discrimination as one that can be made by spike count, and a “difficult” discrimination as one that cannot be made by spike count). As shown in , on average, the response magnitudes evoked by two tastants of similar quality are similar. For cells where this applies, distinction between these two stimuli would qualify as difficult. However, in some cells, two tastants of different qualities evoke similar response magnitudes. This condition would also qualify as a difficult distinction. So, the question that arises is whether temporal coding is used most often for distinguishing between tastants of similar quality, or for difficult discriminations regardless of taste quality. To answer this question, in , we plotted the information conveyed by spike count (Hcount) vs. the additional information contributed by temporal characteristics of the response (Hmax –Hcount) for tastants of similar (filled circles) and dissimilar qualities (open circles). This figure shows that there was a range of coding strategies (the scatter across the entire triangle), with temporal coding contributing more when Hcount was low, i.e., a difficult discrimination. This happens whether the comparison is between tastants of similar or dissimilar quality.
Figure 6 Information conveyed by Hcount relative to the contribution of the temporal characteristics of a response to the total amount of information conveyed by taste responses. A. Graph of Hcount plotted against Hmax – Hcount for all stimulus-stimulus (more ...)
The proportional contribution of temporal coding is shown in . This value was calculated as follows:
Values of this ratio that are less than 1.0 (left side of graph, corresponding to the lower left triangle of ) indicate that spike count conveys proportionately more information than the temporal characteristics of the response. Conversely, values of this ratio that are greater than 1.0 (right side of graph, corresponding to the upper right triangle in ) indicate that the temporal characteristics of the response contribute proportionately more information than the spike count alone. This figure reveals differences between comparisons of tastants of similar and dissimilar quality. For comparisons of tastants of dissimilar qualities (hashed), the percentage of cells that convey proportionately more information by spike count (left side of graph) was quite high compared to the corresponding percentage of cells that convey proportionately more information by spike timing. This was due primarily to the large number (43 of 99 comparisons, 43%) of comparisons for which spike count provided the maximum amount of information conveyed by the cell for that comparison. However, for comparisons of tastants of similar quality (solid), ratios > 1.0 were more common than ratios < 1.0 suggesting that spike timing conveyed proportionately more information in these comparisons than spike count. In fact, 66% of all comparisons of tastants of similar qualities had ratios > 1.0, compared with only 36% of comparisons of tastants of dissimilar qualities. Thus, when equated for “difficulty” of discrimination (i.e., similar values of Hcount
), discriminations between tastants of similar quality rely more heavily on spike timing than discriminations between tastants of dissimilar quality.
Data presented in further support the idea that temporal coding is evident more frequently for comparisons of tastants of similar quality than dissimilar quality. It can be seen, for example, that the comparison between the two salty and the two sour tastants showed the largest proportion of cells with a significant contribution of spike timing (NaCl vs. LiCl, 12 of 24 cells, 50%; HCl vs. citric acid, 11 of 19 cells, 58%) relative to all other pairwise comparisons (≤ 32% of the cells). In contrast, fewer cells used spike count to differentiate NaCl vs. LiCl (1 of 24 cells, 4%) and HCl vs. citric acid (3 of 19 cells, 16%) than were used with the other pairwise comparisons (range of 5 to 10 cells of 19, 26 to 53%). If the number of cells with responses showing a significant contribution of spike timing and the rate envelope were combined, it was evident that the temporal characteristics of response were more widely available to convey information about NaCl vs. LiCl (21 of 24 cells, 88%) and HCl vs. citric acid (13 of 19 cells, 68%) than the other comparisons (range of 8 to 10 cells of 19, 42 to 53%).
Figure 7 Incidence of temporal coding by spike timing (blue squares), rate envelope (yellow squares) and spike count (pink squares). Empty squares indicate that the total information contributed by the response was low; i.e. Hmax < 0.1. Numbers indicate (more ...)
Several additional results are worth noting. First, spike timing contributed information to at least one, but not necessarily to every (see ), pairwise comparison in almost all NTS cells (15 of 19, 79%). Second, the level of precision at which spike timing was significant was generally between ~70–125 msec (median q for all pairwise comparisons ranged between 8 and 13.7). Third, as seen in , four out of five cells tested with salty and bitter tastants used spike timing; however, none of these cells used spike timing to differentiate between quinine and urea.