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Previous Article | Next Article
The Journal of Neuroscience, May 15, 2002, 22(10):4205-4216
Odor Discrimination and Odor Quality Perception in Rats with Disruption of Connections between the Olfactory Epithelium and Olfactory Bulbs
Burton Slotnick1 and Natalya Bodyak2 1 Department of Psychology, American University, Washington, DC 20016, and 2 Department of Medicine, Endocrinology Division, Beth Israel Deaconess Medical Center and Harvard Medical School, Boston, Massachusetts 02215
ABSTRACT
TOP
ABSTRACT
INTRODUCTION
Rats were trained using olfactometry and operant conditioning to discriminate among homologous fatty acids, homologous aldehydes, and a series of unrelated odors. Their memory for the positive and negative assignment of each odor (tested under extinction) was assessed before and after they had received selective lesions of the olfactory bulbs or injection of the olfactory epithelial toxin 3-methyl indole (3-MI). Response accuracy on the memory test provided a measure of the extent to which treatments altered the remembered perceptual quality of the odors. The degree of deafferentation of the bulb by treatment with 3-MI was assessed using anterograde transport of horseradish peroxidase applied to the olfactory epithelium. Rats treated with 3-MI had a detectable reaction product only in varying numbers of glomeruli on the lateral and, in some cases, posterior medial walls of the olfactory bulb. Bulbar lesions destroyed the dorsal and dorsomedial bulbar areas that have been identified in optical and electrophysiological studies as showing responses to fatty acids. Rats with bulbar lesions had good to near perfect retention on their post-treatment memory test on all odor pairs, as did 3-MI-treated rats that still had substantial input to glomeruli on the lateral or posterior medial wall of the bulb. 3-MI-treated rats with substantially fewer afferent connections had severe retention deficits, particularly for the aldehyde and fatty acid odors, but this loss was secondary to deficits in the ability to discriminate among these odors. The results indicate that input to bulbar areas that are activated by a series of homologous odors may not be essential for odor discrimination and that deafferentation of the majority of bulbar glomeruli may be primarily without effect on odor quality perception as assessed by the memory test. These outcomes point to a much higher degree of redundancy within the olfactory bulb than that envisioned by current combinatorial or odotopic hypotheses of odor quality coding or, alternatively, to mechanisms of odor coding used in the awake behaving animal that have not yet been elucidated.
Key words: odor coding; odor quality; olfactory bulb; olfactory memory; olfactory discrimination; olfatoxin
INTRODUCTION
Most current models of odor coding assume that odor quality is represented by patterns of activation across specific sets of olfactory bulb glomeruli (Xu et al., 2000
). Consonant with this view are optical imaging studies (Rubin and Katz, 1999
Of particular interest to the present study are previous findings (Imamura et al., 1992
In general, these results support the notion that sensory input to the bulb is organized "odotopically" and that odor quality coding involves a combinatorial mechanism in which each odor produces a pattern of activity across some subset of bulbar glomeruli. Differences in these patterns together with synaptic interactions among bulbar neurons (Yokoi et al., 1995
Because odor quality perception is clearly a behavioral variable, physiological and anatomical explanations of odor quality coding must, ultimately, be assessed by examining behavior. The present study is based on a prediction from this combinatorial model: that a disruption of patterned input should produce a change in the perceived quality of an odor. Because a homologous series of odors share a common domain in the olfactory bulb, disruption of that domain should have a particularly marked effect on the perception of quality and, possibly, ability to discriminate among those odors. To assess perceptual quality, we used a test of odor memory for homologous odors. The assumption underlying this test was that if an experimental treatment had no significant effect on odor quality perception, then there should be no change in the memory for a closely related series of odors; e.g., the perception of the odor should match a stored image of that odor.
MATERIALS AND METHODS
Subjects
Ten male and nine female adult Long-Evans strain rats were housed individually on sawdust in plastic cages in a temperature- and humidity-controlled vivarium maintained on a 12 hr light/dark cycle (lights on at 7 A.M.). Ralston Purina (St. Louis, MO) pelleted rat chow was continuously available in the home cage. Access to water was restricted, and rats were given a total of 7-10 ml of tap water each day. All procedures were performed under the auspices of a protocol approved by the American University Institutional Animal Care and Use Committee.Apparatus
Rats were trained in four identical Knosys eight-channel olfactometers. Each unit contained eight independent odor lines, an operant chamber, and a digital interface (Fig. 1). Each odor line consisted of a 200 ml glass odor saturator bottle that contained 50 ml of odorant material and whose input and output lines (C-flex tubing) were controlled by normally closed pinch valves. Operating the upstream and downstream valves of an odor line added a 50 ml/min stream of odor-saturated air from the saturator bottle to a 1950 ml/min stream of clean air. The downstream end of the clean air line was connected to a glass odor-sampling tube on the operant chamber via the normally open and common ports of a three-way pinch valve (Fig. 1, Final Valve). The normally closed port of this valve was connected to an exhaust line.
Air for all systems was provided from an 80 gallon tank maintained at 70 psi by oil-free compressors. Tank air was refrigerant-dried, filtered through three columns of activated charcoal and a fritted glass particle filter, and controlled at 6 psi for each system. Airflow in each olfactometer was controlled by Teflon and glass needle valves and monitored by calibrated flow meters. The entire system was washed with 95% ethanol and air-dried before use. The use of pinch valves, the replacement of their tubing, and an alcohol wash of other components when new odors were used completely eliminated any possibility of valve or line contamination.
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Figure 1. Diagrammatic representation of the eight-odor liquid dilution olfactometer. Each of eight independent channels contained a different odor. Air passing through an odor saturator bottle was controlled by upstream and downstream normally closed pinch valves (Control Valves). The carrier flow (A) was set at 1950 cc/min, and the odor flow (B) was set at 50 cc/min.
Test chamber
The Plexiglas test chamber was similar to that used by Lu and Slotnick (1998)Odor concentrations
Odor concentrations given below are those of the liquid odorant in the saturator bottle. Because the odor vapor, generated by passing 50 cc/min over the surface of the odorant material in the saturator bottle, was manifolded with a 1950 cc/min stream of clean air before being introduced to the sampling port, the concentration of odor stimuli experienced by the rat at the sampling port was ~2.5% of the concentration of the head space above the liquid odorant.Pretreatment training procedures
Initial training. Beginning 2 weeks after the initiation of their water deprivation schedule, rats were trained in two stages using standard operant conditioning procedures. First, rats were trained to insert their snout into the odor-sampling tube and then respond by licking on the reinforcement tube. The odor stimulus (2% aqueous solution of ethyl acetate) was present for 1 sec after a snout insertion was detected, and on termination of the stimulus, responses on the water tube were recorded for the next 1.5 sec (response window). Licking at the water tube during this response window produced a 0.04 ml water reward. These trials were separated by a 10 sec intertrial interval (ITI). In the second stage of training, the first snout insertion after the end of the ITI activated the stimulus and final valve assembly, resulting in introduction of the odor into the carrier stream and, while the final valve was energized, directing that stream to an exhaust line. The final valve was de-energized 0.8-1.2 sec later, thus introducing the stimulus into the odor-sampling port. To obtain reinforcement in this training stage, the rat was required to keep its snout in the sampling tube during the final valve period and for at least 120 msec after the stimulus appeared and then to respond on the water delivery tube. Responding during the final valve period (i.e., when no odor was present) or not sampling the stimulus for at least 120 msec immediately terminated the trial and initiated the intertrial interval. Each rat completed this preliminary training in two or three 30 min sessions. Detection and discrimination training. The initial training procedures were continued in a 200-trial odor detection session. The procedures followed in initial training were used, except that 2% ethyl acetate was the odor stimulus for half of the trials (S+ trials), and the remaining trials (Strials) were presentations of the vapor from deionized water, the solvent for the ethyl acetate stimulus. S+ and S
Post-treatment memory and postmemory tests
Procedures in the first post-treatment session were identical to those of the pretreatment memory test, except that after the first 80 trials (extinction trials), a minimum of 120 additional trials were run using 100% reinforcement (retraining trials). The memory test and retraining trials were given 5 or 6 d after treatment for rats injected with 3-methyl indole (3-MI) and 14 d after surgery for rats with olfactory bulb lesions. Rats that performed poorly on the memory test and in the 120-trial postmemory test were given additional training until they reach criterion performance or for a maximum of 360 trials given in one session. Rats with partial olfactory bulb lesions were given additional post-treatment tests on the day after the memory test. First, they were tested on set A odors, except that the liquid concentration of each odor was reduced by 1 log unit. In the next session, given 3 d later, each rat was trained on a set of eight novel odors (vanilla, butanol, pyridine, and isopropyl acetate served as S+ odors, and maple, geraniol, benzaldehyde, and isoamyl acetate served as SControl procedures
Our extensive experience with these olfactometer units demonstrates that discriminative responding is based only on vapor cues. Thus, in previous studies using these units, olfactory bulbectomized rats performed entirely at chance over hundreds of training trials, and in psychophysical or odor mixture discrimination tests, performance accuracy of intact controls decreases as problem difficulty is increased (Bodyak and Slotnick, 2000Treatments
Two complementary methods were used to disrupt patterned input to the olfactory bulbs: discrete aspiration lesions of bulbar tissue and more widespread bulbar deafferentation by intraperitoneal injection of 3-MI. 3-MI is a known olfactory epithelial toxin that produces dose-dependent degenerative changes of the olfactory epithelium (Peele et al., 1990Anatomical control
Deafferentation of sensory input to the olfactory bulbs in 3-MI-treated rats was assessed using anterograde transport of horseradish peroxidase-wheat germ agglutinin (HRP-WGA) from the olfactory epithelium to the olfactory bulbs (Slotnick et al., 2001Data analysis
Percent correct responding and errors to achieve criterion performance of 85% responding in a block of trials were used as dependent measures of behavior. Within- and between-group comparisons were made using t tests with anlevel of 0.05.
RESULTS
Rats with bulbar lesions: anatomical results
Each of the six experimental rats with bulbar lesions had bilateral damage to the dorsal and medial aspects of both olfactory bulbs. The lesions in the right bulb were deliberately made larger than those on the left and removed completely the rostral 2-3 mm of the bulb and extended posteriorly, removing virtually all of the remaining dorsal and dorsal half of the medial wall of the main olfactory bulb rostral to the accessory olfactory bulb. The lesions to the left bulb were less extensive, but in each case, most or all of the rostral 2 mm of the bulb was removed. In most cases (R1, R11, R13, and R14), the lesions extended posteriorly through the dorsal and dorsomedial quadrants of the bulb to the rostral level of the accessory olfactory bulb. Figures 2 and 3 show photomicrographs illustrating the lesions in two of these rats (R11 and R1). In two rats (R12 and R40), the lesions did not extend as far posteriorly, although in both, the first 2.5 mm of the dorsal, dorsolateral, and dorsomedial areas of the bulb were removed.
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Figure 2. Four representative sections from the right olfactory bulb of rat R11. Lesions in the left olfactory bulb were larger. The lesion removed the anterior 1 mm of the bulb and the remaining dorsal and dorsomedial areas of the bulb to the level of the anterior aspect of the accessory olfactory bulb.
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Figure 3. Four representative sections from the right olfactory bulb of rat R1. Lesions in the right olfactory bulb were larger. The lesion removed the anterior 1 mm of the bulb and the remaining anterior 3 mm of the dorsomedial area of the bulb.
Three control rats (R16, R42, and R43) had most of the right bulb removed, but the left bulb was intact. Both olfactory bulbs were removed completely in the bilaterally bulbectomized rat (R10).
Rats with bulbar lesions: pretreatment behavior scores
There were no differences in performance between the three control rats with only unilateral bulbar lesions and the two 3-MI control rats on any pretreatment or post-treatment test, and for descriptive statistics and inferential tests, these rats were combined into a single control group. Controls and rats in the designated bilateral bulbar lesion group performed well on the preoperative memory test on set A odors (controls, 97.6%; lesion group, 94.4%). Similar high levels of performance were achieved for memory of set B odors (controls, 97.7%; lesion group, 95.4%). All rats also performed well on the postmemory test training trials on both sets of odors, and mean performance over all rats in the last two blocks of training (the last training trials before experimental treatments) was 96.7% (Fig. 4).
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Figure 4. Left, Performance in the last 120 trials (Training) before the pretreatment memory test (Preop Mem Test), the last 120 training trials before treatment, the 80 trial post-treatment memory test (Postop Mem Test), and the post-treatment retraining trials (Postop Retraining) for one control rat (R42) and six of the seven rats with bulbar lesions. All data points other than the memory test represent the mean score on 40 trials. Right, Diagrammatic representations of the lesions for each rat.
Rats with bulbar lesions: post-treatment behavior scores
Pretreatment and post-treatment performance of individual rats on each set of odors is shown in Figure 4. Post-treatment differences in memory test scores between controls and rats with bilateral bulbar lesions for set A odors (control rats, 95.2%; experimental rats, 91.9%) and set B odors (control rats, 95.4%; experimental rats, 89.6%) were not significant. Each rat had scores of
85% on the memory test and had near perfect performance on the postmemory retraining trials (Fig. 4).
As shown in Figure 4, there was no obvious relationship between test scores and the size of a lesion within the lesion group. The two rats with relatively large lesions, R11 (Fig. 2) and R1 (Fig. 3), performed either as well as controls or no worse than those with smaller lesions.
The postoperative memory score on set A odors for the bilaterally bulbectomized rat (R10) was 52%, and this rat continued to perform at chance on the retraining trials (Fig. 4). It was subsequently tested on an odor detection task [1% valeric acid (S+) vs air (S
After the post-treatment odor set A and odor set B memory test and retraining, control and experimental rats were trained on 0.1% concentrations of set A odors and then on a set of eight novel odors (see Materials and Methods). Mean errors to criterion on the lower concentration set A odors for experimental rats (26.8) were greater than those for controls (19.1), but the between-group difference was not significant (Fig. 5). Both groups made more errors in discriminating acid odors (controls, 14.3; experimental rats, 21.3) than in discriminating aldehyde odors (controls, 7.7; experimental rats, 12.6). However, the ratio of errors on the acid-aldehyde discrimination problems for the two groups was similar (controls, 1.86; experimental rats, 1.69).
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Figure 5. Top, Mean performance of three control rats and the seven rats with bulbar lesions trained on 0.1% concentrations of odor set A (see Results). Bottom, Acquisition of a novel eight-odor discrimination task by these same rats. Each data point for both graphs represents the mean performance on 40 trials. For each task, all eight odors were presented in random order in the same session.
The two groups made equivalent numbers of errors in the novel eight-odor task (controls, 65.1; experimental rats, 67.7) (Fig. 5). However, two of the rats with bulbar lesions did not reach criterion within the 320-trial session, and the mean error score for experimental rats included all errors made by these two rats.
3-MI-treated rats: anatomical results
In control rats treated with only the vegetable oil solvent, virtually all bulbar glomeruli contained a dense or moderately dense HRP reaction product (Fig. 6). In each of the six experimental rats, most glomeruli contained no detectable reaction product. Glomeruli judged as containing no reaction product appeared completely clear in polarized light (Fig. 7A) and could be discriminated from glomeruli that contained only a light sprinkling of a reaction product that was difficult to detect in the bright field (Fig. 7C) but could be clearly discerned using polarized light (Fig. 7B,D).
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Figure 6. Section through the olfactory bulb in a control rat showing anterograde transport of HRP-WGA to the olfactory nerve layer and glomeruli. Dorsal is to the top, and lateral is to the right. Bottom, Detail of the nerve layer and glomeruli on the medial wall of the bulb.
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Figure 7. A, Polarized light photomicrographs of glomeruli on the dorsomedial wall of the right olfactory bulb for rat R3. No reaction product within these glomeruli can be detected. B, Glomeruli on the posterior lateral wall of the bulb from the same rat showing a light sprinkling of a reaction product (arrows). C, Glomeruli from the posterior lateral wall of rat R7 photographed using bright-field optics. A dense reaction product in the glomerulus on the bottom right is seen clearly, but the lighter reaction product in the more dorsal glomeruli is difficult to discern in the bright field. D, Same area as in C but photographed using partly polarized light. A light reaction product in several glomeruli adjacent to the glomerulus with dense reaction product is clearly visible.
In all but rat R6, there was a complete absence of the reaction product in glomeruli in the dorsal, dorsomedial, and dorsolateral walls, and no glomeruli on the medial wall of the bulb rostral to the level of the accessory olfactory bulb contained a detectable reaction product. However, in each case, at least some glomeruli on the posterior lateral wall contained a light to dense reaction product. In three rats (R4, R7, and R8), glomeruli in the ventral half of the posterior medial wall of the bulb contained a moderate to dense reaction product. Counts of the number of glomeruli with a light, moderate, and dense reaction product for each rat are given in Table 1, and diagrammatic representations of the location of these glomeruli are shown in Figure 12.
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Table 1. Number of glomeruli with a light, moderate, and dense reaction product in each 3-MI-treated rat, post-treatment memory score, and the highest score obtained in a block of 40 trials in the postmemory retraining trials Rat R3 had the lowest glomerular count, and the reaction product was limited primarily to a cluster of glomeruli in the ventral half of the posterior lateral wall of the bulb and to another cluster of glomeruli on the posterior ventral medial wall (Fig. 8). The reaction product in most of these glomeruli was judged as light or moderate in density.
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Figure 8. Representative frontal sections from left olfactory bulb of rat R3. The only glomeruli containing a reaction product were in a restricted region of the posterior ventrolateral bulb (bottom left panel, arrow).
The remaining 3-MI experimental rats had appreciably more glomeruli with a reaction product. Most labeled glomeruli were located on the posterior ventrolateral to midlateral wall, but R7, R8 (Fig. 9), and R4 (Fig. 10) had labeled glomeruli on the ventral half of the posterior medial wall of the bulb. In each of these cases, the glomerular regions containing input consisted of a mixture of glomeruli with little or no reaction product and a dense and moderately dense reaction product, respectively.
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Figure 9. Representative frontal sections from the left olfactory bulb of rat R8. Only glomeruli along the posterior lateral and posterior medial wall of the bulb contained reaction product.
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Figure 10. Representative frontal sections of the right olfactory bulb from rat R4. A relatively narrow band of glomeruli along the midlateral and on the ventral posterior wall of the olfactory bulb contained reaction product. No reaction product was detected in glomeruli on the dorsal third or anterior medial wall of the bulb.
Rat R6 had the greatest number of glomeruli with a reaction product (Table 1). This rat had a few glomeruli with a light or moderate reaction product in the dorsomedial, dorsolateral, and ventrolateral glomeruli in the most rostral sections available (Fig. 11). At more posterior levels, most glomeruli on the lateral wall contained a dense reaction product. No glomeruli on the posterior medial wall of the bulb had a detectable reaction product. As shown in Table 1, the other five experimental rats had far fewer glomeruli with a moderate or dense reaction product.
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Figure 11. Representative frontal sections of the left olfactory bulb from rat R6. A dense reaction product was found in a cluster of glomeruli in the anterior dorsomedial area and in a broad band of glomeruli along the midlateral aspect of the bulb. Glomeruli in the dorsal and more posterior medial surface of the bulb contained no detectable reaction product.
3-MI-treated rats: pretreatment behavior scores
Each rat in the designated 3-MI group had essentially perfect retention on the pretreatment memory test for odor set A (mean, 97.0%) and odor set B (mean, 97.5%). Each performed well on the pretreatment training trials on both sets of odors, and mean performance in the last two blocks of training for both sets of odors (the last training trials before experimental treatments) was 98.2%.
3-MI-treated rats: post-treatment behavior scores and extent of bulbar inputs
As a group, post-treatment memory scores for set A odors (73.5%) and set B odors (84.2%) of 3-MI-treated rats were significantly lower than those of controls (p < 0.004; p < 0.04, respectively). Pretreatment and post-treatment performance of individual rats for each set of odors is shown in Figure 12.
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Figure 12. Left, Performance in the last 120 trials (Training) before the pretreatment memory test (Preop Mem Test), the last 120 training trials before treatment, the 80 trial post-treatment memory test (Postop Mem Test), and the post-treatment retraining trials (Postop Retraining) for one control rat (R4) and each of the six rats treated with 3-MI. All data points other than the memory test represent the mean score on 40 trials. Right, Diagrammatic representations of glomerular areas for each rat that contained a dense reaction product (filled circles), moderately dense reaction product (open circles with dots), or only a light sprinkling of a reaction product (open circles).
In contrast to rats with bulbar lesions, performance among rats in group 3-MI was quite variable. On the basis of memory and relearning scores, particularly those on set A odors, three subgroups of 3-MI-treated rats could be identified. R3 and R7 constituted one subgroup; both had near chance performance on the set A odor memory test and poor performance on the retraining trials (Fig. 12). Rat R3 failed to reacquire the set A discrimination task. R7 also demonstrated little memory for set A odors but slowly reacquired the discrimination task and, within the 360 trials allowed, achieved 72.5% accuracy. Both of these rats had few glomeruli with a dense reaction product, and glomerular scores were lower for R3 than for R7 (Table 1).
The second subgroup, consisting of rats R5 and R8, also had near chance scores on the set A odor memory test, but both quickly reacquired the discrimination task and reached criterion performance of 85% accuracy after the first block of 40 training trials (Fig. 12). Both rats had appreciably better memory scores on the set B odors (Fig. 12). The glomerular scores of R8 were similar to those of R7, a rat that performed more poorly, whereas R5 had approximately three times as many glomeruli with a dense reaction product than did R7 and R8 (Table 1).
The remaining two 3-MI-treated rats (R4 and R6) performed well on the memory and retraining trials on both sets of odors, and their post-treatment scores were virtually identical despite the fact that R4 had far fewer glomeruli with a reaction product. The behavior scores of both (Fig. 12) were similar to those of rats in the control and lesion groups.
The excellent performance of rat R4 was particularly impressive in view of its greatly reduced input. To quantify this input, all glomeruli in the 19 sections available for this rat were counted. A total of 1498 glomeruli were identified, of which 260 or 17% contained a detectable reaction product. Of these 260 glomeruli, only 112 (or ~7.5% of the total glomeruli identified) contained a moderate to dense reaction product.
Figure 13 shows the mean performance of these three subgroups of 3-MI-treated rats together with mean scores of controls and rats with bulbar lesions. As shown, controls, rats with bulbar lesions, and the R4 and R6 3-MI subgroup had good retention and reacquisition scores in the post-treatment tests. As shown in Figures 12 and 13, the rats that had little or no memory for the set A odors (R3, R7, R5, and R8) were those with severe bulbar deafferentation and chance or near chance performance in initial retraining trials.
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Figure 13. Summary of mean performance on set A odors for control rats, rats with bulbar lesions, and subgroups of rats treated with 3-MI on their last 120 training trials before the pretreatment memory test, the pretreatment memory test, the last 120 trials of training before treatment, the post-treatment memory test, and post-treatment retraining. The rats treated with 3-MI were divided into subgroups based both on performance during the post treatment memory test and on anterograde transport scores (see Results).
Distribution of errors on the odor set A post-treatment memory test
In their post-treatment memory test, controls made, on average, 1 error on the aldehyde odors and 2.2 errors on the acid odors. The six rats with bilateral bulb lesions made, on average, 2 errors on the aldehyde odors and 3.6 errors on the acid odors. The six rats treated with 3-MI made, on average, 8.3 errors on the aldehyde odors and 12 errors on the acid odors. Thus, although lesion and 3-MI experimental rats made more errors on the acid than on the aldehyde odors, this was also true for control rats. In fact, the mean of ratios of acid to aldehyde errors for rats in the lesion group (2.05) and the 3-MI group (1.97) was essentially identical to that for controls (1.90), and differences among groups for this ratio score were not significant (F test).
DISCUSSION
The present study addressed the question of whether disruption of afferent connections to the olfactory bulb would significantly alter odor quality perception or discrimination among homologous odors. Our results demonstrate that after deafferentation or lesions of bulbar areas identified as involved in mediating these odors, there remained considerable savings in odor recognition and discrimination. The outcomes are relevant to current concepts of odor coding derived from molecular biological studies of the olfactory system, particularly the combinatorial view of odor coding and the role of odotopic organization within the olfactory bulb in odor perception. The extent to which these results bear on these issues depends importantly on the validity of the behavior tests and adequacy of the experimental interventions.
Methodological considerations
Assessing odor quality perception
Because two odors may be highly discriminable even when their perceived quality has changed, odor discrimination tasks do not adequately assess odor quality perception. Several methods have been proposed to test odor quality perception, but each has significant shortcomings (Slotnick and Schellinck, 2001Odor concentration
At least for some odors, higher concentrations produce more widespread activation across bulbar glomeruli (Johnson and Leon, 2000Relevance of outcomes to odotopic organization within the olfactory bulb
Lesions of the olfactory bulb were essentially without effect on odor memory for homologous fatty acids, homologous aldehydes, and a group of unrelated odors. Because these lesions removed and, in most cases, extended well beyond the boundaries of areas identified in the electrophysiological and optical imaging studies as being responsive to fatty acid odors, they certainly would have disrupted the normal pattern of bulbar activation. Thus, we conclude that the areas so identified are not essential for discriminating among the odors used or for whatever unique perceptual qualities these odor have that allowed rats to recognize them as being essentially identical to those experienced before their bulbar lesions. Obviously, these functions were mediated by other bulbar areas.
In the studies of Johnson et al. (1999)
Another potential role for multiple representations is to ensure one intact pattern in the case of injury or mild nasal infections affecting part of the olfactory epithelium (Johnson et al., 1999
However, it might prove difficult to reconcile a redundancy explanation with the combinatorial view of odor coding. Redundancy implies a certain degree of equipotentiality among the different bulbar areas activated by an odor, whereas, according to a combinatorial mechanism, odor quality is determined by a unique pattern of bulb activation. Of course, a subset of bulbar representations might serve redundancy, whereas others participate in encoding odor quality, or perhaps variations in patterns of activation within a single domain are sufficient for odor recognition, and these patterns are multiply represented. At present, these hypotheses must be viewed as speculative attempts to account for the behavioral savings in rats with bulb lesions that would not have been predicted by proposed mechanisms of odor coding based primarily on patterns of bulbar activation revealed in optical imaging and high-resolution 2-DG studies.
Other considerations
If patterns of activation identified in the mapping studies correspond in a one-to-one manner with how odors are discriminated and recognized, then it might be argued that the training and test procedures did not adequately assess these functions. If so, then what might be the basis for the behavioral savings obtained in the present study?
Assessing quality perception is a relatively new and difficult aspect of animal psychophysics. For some modalities, a stimulus generalization gradient provides insight to how an animal judges the similarity of stimuli along a defined continuum. Application of similar methods in olfaction, however, is problematic because odors differ simultaneously along several or many dimensions. The memory test provides one approach to this problem. However, odor quality could be represented by multiple stimulus features, and patterns of bulb activation may code for some but not all such determinants. The excellent scores of experimental animals in this study may reflect the fact that the memory test is not sensitive to all changes in odor quality perception produced by our experimental interventions.
It is also the case that training may alter patterns of bulbar activation (Coopersmith and Leon, 1986
The failure of this and previous related behavioral studies to confirm hypotheses based on anatomical and physiological evidence is not without precedent in the animal behavior literature. Thus, on the basis of the progressive sharpening of receptive fields of neurons from the cochlear nucleus to the cortex, it was reasonable to expect that the auditory cortex played an important role in frequency discrimination. Lesions of auditory cortex, however, were mostly without effect on the ability of dogs, cats, and rodents to discriminate auditory frequencies (Heffner, 1978
The mapping studies, particularly those of Johnson and colleagues (Johnson et al., 1998
FOOTNOTES Received Nov. 14, 2001; revised Feb. 14, 2002; accepted Feb. 15, 2002.
This work was supported in part by National Institutes of Health Grants DC/0D029870, MH6111801, and DC04671.
Correspondence should be addressed to Burton Slotnick, Department of Psychology, American University, Washington, DC 20016. E-mail: slotnic{at}american.edu.
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