The problem with two-event sequence learning by pigeons.

Bonobos appear to show little evidence of learning to make one response (R1) to an AB sequence and a different response (R2) to sequences BB, AA, and BA (Lind et al. PLoS ONE 18(9):e0290546, 2023), yet under different conditions, pigeons can learn this (Weisman et al. Exp Psychol Anim Behav Process 6(4):312, 1980). Aspects of the bonobo procedure may have contributed to this failure. Most important, no response was required in the presence of the stimuli to encourage attention to them. Furthermore, learning to make one response to the target sequence and another to the other sequences involves a bias that allows for better than chance responding. With the two-alternative forced-choice procedure used with the bonobos, the R1 response is correct for one sequence, whereas the R2 response is correct for three sequences. To correct for this, there are three times as many AB trials as each of the other sequences. However, this correction allows a bias to develop in which reinforcement often can be obtained by using only the last stimulus seen as the basis of choice (e.g., when the last stimulus is B respond R1 when the last stimulus is A respond R2). This solution yields reinforcement on five out of six, or 83%, of the trials. In the present experiment with pigeons, using this two-alternative forced choice procedure, most subjects tended to base their choice on the last-seen stimulus. This design allowed subjects to use a suboptimal but relatively effective choice strategy.

Serial pattern learning: The anticipation of worsening conditions by pigeons.

In general, animals are known to be sensitive to the immediacy of reinforcers. That is, they are generally impulsive and outcomes that occur in the future are generally heavily discounted. Furthermore, they should prefer alternatives that provide reinforcers that require less rather than greater effort to obtain. In the present research, pigeons were given a choice between (1) obtaining reinforcers on a progressively more difficult schedule of reinforcement; starting with four pecks, then eight pecks, then 16 pecks, then 32 pecks, and finally 64 pecks on each trial, and (2) a color signaling a number of pecks for a single reinforcer: red = six, green = 11, blue = 23, or yellow = 45. If pigeons choose optimally, most of the time they should choose the progressive schedule to obtain five reinforcers rather than switch to a color to receive only one. However, if they are sensitive primarily to the number of pecks to the next reinforcer, they should choose the progressive schedule once before switching to red, twice before switching to green, three times before switching to blue, and four times before switching to yellow. Instead, they systematically switched too early. Rather than choose based on the rate of reinforcement or even based on the time or effort to the next reinforcer, they appear to anticipate that the progressive schedule is going to get more difficult, and they base their choice suboptimally on the serial pattern of the worsening progressive schedule.

"Distractor" effects in delay discounting of probability by pigeons.

Impulsive behavior can be measured by performance on a successive delay-discounting task, in which a response to a stimulus provides a small reinforcer sooner (SS), but in the absence of a response, a larger reinforcer later (LL). Previous research suggests that the presence of a concurrent "distractor" stimulus, to which responding has no programed consequence, can result in increased LL reinforcers. In the present experiments, we used differences in the probability of reinforcement between SS and LL (rather than magnitude of reinforcement) and tested the hypothesis that the concurrent stimulus may become a Pavlovian conditioned stimulus. For the Red-Only group, a response to the SS stimulus resulted in a reinforcer with a low probability (SS), whereas the absence of a response resulted in a reinforcer with a high probability (LL). For the Red-Green group, (analogous to the more typical simultaneous choice between an SS and LL stimulus) the absence of a response to the SS stimulus replaced the SS stimulus with the LL stimulus and a response to the LL stimulus resulted in the reinforcer. Thus, for the Red-Green group, the concurrent stimulus should have been less effective because responding to the concurrent stimulus was not immediately followed by the reinforcer. In Experiment 1, the concurrent stimulus was a yellow key-light; in Experiment 2, it was a houselight. In both experiments, the concurrent stimulus was effective in increasing the number of LL reinforcers and the effect was larger for the Red-Only group than for the Red-Green group.

Memory for where and when: pigeons use single-code/default strategy.

Memory for what, where, and when an event took place has been interpreted as playing a critical role in episodic memory. Moreover, such memory is likely to be important to an animal's ability to efficiently forage for food. In Experiment 1 of the present study, pigeons were trained on a task in which on each trial, one lit stimulus color and location was presented and then another. A cue presented after the last stimulus location signaled that the pigeon was to choose either the first location presented, or the last location presented, to receive a reinforcer. After learning this task, in Experiment 2, the color cue was removed, requiring the pigeons to choose based on location and order alone. In Experiment 3, when a delay was inserted between presentation of the two locations, it had little effect on task accuracy. Results suggested that the pigeons had acquired the task using a single-code/default rule. When presented with the cue indicating that the last location was correct, pigeons selected the location just presented. When presented with the cue indicating that the first location was correct, pigeons chose the other location, by default. In support of this hypothesis, in Experiment 4, when a delay was inserted, prior to receiving the instructional cue, it had a disruptive effect on task accuracy proportional to the delay. Although the present results do not provide evidence for episodic memory, they do suggest that the pigeons have developed a single-code/default strategy that appears to be an efficient means of performing this task.

1-Back reinforcement symbolic-matching by humans: How do they learn it?

For humans, a distinction has been made between implicit and explicit learning. Implicit learning is thought to involve automatic processes of the kind involved in much Pavlovian conditioning, while explicit learning is thought to involve conscious hypothesis testing and rule formation, in which the subject's statement of the rule has been taken as evidence of explicit learning. Various methods have been used to determine if nonverbal animals are able to learn a task explicitly - among these is the 1-back reinforcement task in which feedback from performance on the current conditional discrimination trial is provided only after completion of the following trial. We propose that it is not whether an organism can learn the task, but whether they learn it rapidly, all-or-none, that provides a better distinction between the two kinds of learning. We had humans learn a symbolic matching, 1-back reinforcement task. Almost half of the subjects failed to learn the task, and of those who did, none described the 1-back rule. Thus, it is possible to learn this task without learning the 1-back rule. Furthermore, the backward learning functions for humans differ from those of pigeons. Human subjects who learned the task did so all-or-none, suggesting explicit learning. In earlier research with pigeons, they too showed significant learning of this task; however, backward learning functions suggested that they did so gradually over the course of several sessions of training and to a lower level of asymptotic accuracy than the humans, a result suggesting implicit learning was involved.

Pigeons learn two matching tasks, two nonmatching tasks, or one of each.

When pigeons learn matching-to-sample or nonmatching-to-sample there is good evidence that they can transfer that learning to novel stimuli. But early evidence suggests that in the rate of task acquisition, there is no benefit from a matching relation between the sample and the correct or incorrect comparison stimulus. In the present research we trained three groups of pigeons, each on two two-stimulus tasks simultaneously, matching-matching, nonmatching-nonmatching, or matching-nonmatching. If a common matching or nonmatching relationship benefits acquisition, the first two groups should acquire their tasks faster than the third group, for which the two tasks ought to be incompatible. The results indicated that all three groups acquired their tasks at about the same rate. A secondary goal of the experiment was to determine the basis of learning for the each of the three groups. During testing, for each task, there were test trials in which one of the stimuli from the other task replaced either the correct or the incorrect comparison stimulus. Surprisingly, neither comparison stimulus appeared to show complete control over comparison choice. Although replacing either comparison stimulus resulted in a decrement in task accuracy from about 90% to 70% correct, independent of which comparison stimulus was replaced, the pigeons chose correctly at well above chance accuracy. Suggestions to explain this unexpected outcome are discussed.

Pigeons acquire the 1-back task: Implications for implicit versus explicit learning?

In humans, a distinction can be made between implicit or procedural learning (involving stimulus-response associations) and explicit or declarative learning (involving verbalizable rules) that is relatively easy to make in verbal humans. According to several investigators, it is also possible to make such a distinction in nonverbal animals. One way is by training them on a conditional discrimination task (e.g., matching-to-sample) in which reinforcement for correct choice on the current trial is delayed until after a choice is made on the next trial - a method known as the 1-back procedure. According to Smith, Jackson, and Church ( Journal of Comparative Psychology, 134(4), 423-434, 2020), the delay between the sample-correct-comparison response on one trial and reinforcement obtained on the next trial is too long for implicit (associative) learning. Thus, according to this theory, learning must be explicit. In the present experiments we trained pigeons using the 1-back procedure. In Experiment 1, pigeons were trained on red/green 1-back matching using a non-correction procedure. Some of the pigeons showed significant learning. When a correction procedure was introduced, all the pigeons showed evidence of learning. In Experiment 2, new pigeons learned red/green 1-back matching with the correction procedure. In Experiment 3, new pigeons learned symbolic 1-back matching with yellow and blue conditional stimuli and red/green choice stimuli. Thus, pigeons can learn using 1-back reinforcement. Although it would appear that the pigeons acquired this task explicitly, we believe that this procedure does not adequately distinguish between implicit and explicit learning.

Visual alternation by pigeons: Learning to select or learning to avoid.

In the visual alternation task, pigeons learn to alternate between two stimuli (e.g., red and green) that vary randomly in location from trial to trial. The task is inherently difficult because animals tend to return to a stimulus to which they have just received reinforcement for responding. Williams (1971, Journal of the Experimental Analysis of Behavior, 15, 129-140) suggested that pigeons learn this task by learning to avoid the stimulus most recently chosen. The present experiment tested this hypothesis by involving three groups. The Standard Group replicated Williams' design. For the New Correct Group, following a correct (reinforced) response, on the next trial, the color of the new correct stimulus changed. For example, if it had been green, it changed to either blue or yellow, but the color of the new incorrect stimulus (the one that was just correct) remained the same (i.e., red). For the New Incorrect Group, following a correct response, on the next trial, the color of the new incorrect stimulus changed. For example, if it had been red, it changed to blue or yellow, but the color of the new correct stimulus remained (i.e., green). The Standard Group replicated Williams's finding that pigeons can learn the alternation task. Consistent with Williams's hypothesis, pigeons in the New Correct Group showed evidence of learning the alternation task, whereas pigeons in the New Incorrect Group showed little evidence of learning. Acquisition of the visual alternation task suggests that pigeons are cognitively flexible enough to overcome their natural tendency to repeat their most recently reinforced response to a stimulus.

Pigeons can learn a difficult discrimination if reinforcement is delayed following choice.

Delaying reinforcement typically has been thought to retard the rate of acquisition of an association, but there is evidence that it may facilitate acquisition of some difficult simultaneous discriminations. After describing several cases in which delaying reinforcement can facilitate acquisition, we suggest that under conditions in which the magnitude of reinforcement is difficult to discriminate, the introduction of a delay between choice and reinforcement can facilitate the discrimination. In the present experiment, we tested the hypothesis that the discrimination between one pellet of food for choice of one alternative and two pellets of food for choice of another may be a difficult discrimination when choice consists of a single peck. If a 10-s delay occurs between choice and reinforcement, however, the discrimination is significantly easier. It is suggested that when discrimination between the outcomes of a choice is difficult and impulsive choice leads to immediate reinforcement, acquisition may be retarded. Under these conditions, the introduction of a brief delay may facilitate acquisition.

Animal procrastination: Pigeons choose to defer experiencing an aversive gap or a peck requirement.

When humans procrastinate, they delay completing a required relatively aversive task. In the present experiments with pigeons, we considered the possibility that completing the task close to the deadline results in the formation of a stronger conditioned reinforcer. In Experiment 1, pigeons were given a choice between two chains: (a) a signaled long period, followed by a dark gap, followed by a signaled short conditioned reinforcer, and food and (b) a signaled short period, followed by a dark gap, followed by a signaled long conditioned reinforcer, and food. We found a reliable preference for the delayed gap. In Experiment 2, we let pigeons choose between two chains: (a) walking to a near panel to peck a key, followed by a long walk to peck a key for reinforcement and (b) walking to a far panel to peck a key followed by a short walk to peck a key for reinforcement. When a single peck was required to either key, the pigeons were indifferent. When ten pecks were required to the near key but only one peck to the far key, the pigeons preferred the far key. When ten pecks were required to either key, the pigeons preferred the far key. The results of both experiments suggest that pigeons prefer to defer a relatively aversive event but, in keeping with Fantino's Delay Reduction Theory, this effect may result from the development of a strong conditioner reinforcer that occurs when the event (the gap or required pecking) comes close to reinforcement.

Transitive inference in pigeons may result from differential tendencies to reject the test stimuli acquired during training.

In the five-term, transitive inference task used with animals, pigeons are trained on four simultaneous discrimination premise pairs: A + B -, B + C -, C + D -, D + E -. Typically, when tested with the BD pair, most pigeons show a transitive inference effect, choosing B over D. Two non-inferential hypotheses have been proposed to account for this effect but neither has been reliably supported by research. Here we test a third non-inferential hypothesis that the preference for B arises because the animals have not had as much experience with B - in the A + B - discrimination as they have had with the D - in the C + D - discrimination. To test this hypothesis we trained the Experimental Group with the A + B - discrimination in which, over trials, there were four possible A + stimuli that could appear. This was done to encourage the pigeons to learn to reject the B - stimulus. For the Control Group there was only one A + stimulus over trials, as is typically the case. We also varied the nature of the stimuli between groups, such that colors served as the stimuli for half of the pigeons, whereas flags of different counties served as stimuli for the remaining pigeons. In both stimulus conditions, for the Experiment Group, we found little preference for stimulus B over stimulus D, whereas for the Control Group we found the typical preference for stimulus B. Thus, we propose that it is not necessary to attribute the transitive inference effect to an inferential process.

Midsession reversal learning by pigeons: Effect on accuracy of increasing the number of stimuli associated with one of the alternatives.

The midsession reversal task involves a simultaneous discrimination in which choice of one stimulus (S1) is correct for the first 40 trials and choice of the other stimulus (S2) is correct for the last 40 trials of each 80-trial session. When pigeons are trained on the midsession reversal task, they appear to use the passage of time from the start of the session as a cue to reverse. As the reversal approaches, they begin to make anticipatory errors, choosing S2 early, and following the reversal they make perseverative errors, continuing to choose S1. Recent research suggests that anticipatory errors can be reduced (while not increasing perseverative errors) by reducing the probability of reinforcement for correct S2 choices from 100% to 20%. A similar effect can be found by increasing the response requirement for choice of S2 from one peck to ten pecks. In the present experiments, we asked if a similar effect could be attained by increasing the number of stimuli that, over trials, could serve as S2. Instead, in both experiments, we found that increasing the number of S2 stimuli actually increased the number of anticipatory errors. Several interpretations of this result are provided, including the possibility that attention to the variable S2 stimuli may have interfered with attention to the S1 stimulus.

Cognitive and behavioral correlates of caudate subregion shape variation in fragile X syndrome.

Individuals with fragile X syndrome (FXS) exhibit frontal lobe-associated cognitive and behavioral deficits, including impaired general cognitive abilities, perseverative behaviors, and social difficulties. Neural signals related to these functions are communicated through frontostriatal circuits, which connect with distinct regions of the caudate nucleus (CN). Enlargement of the CN is the most robust and reproduced neuroanatomical abnormality in FXS, but very little is known on how this affects behavioral/cognitive outcomes in this condition. Here, we investigated topography within focal regions of the CN associated with prefrontal circuitry and its link with aberrant behavior and intellect in FXS. Imaging data were acquired from 48 individuals with FXS, 28 IQ-matched controls without FXS (IQ-CTL), and 36 typically developing controls (TD-CTL). Of the total participant count, cognitive and behavioral assessment data were obtained from 44 individuals with FXS and 27 participants in the IQ-CTL group. CN volume and topography were compared between groups. Correlations were performed between CN topography and cognitive as well as behavioral measures within FXS and IQ-CTL groups. As expected, the FXS group had larger CN compared with both IQ-CTL and TD-CTL groups. Correlations between focal CN topography and frontal lobe-associated cognitive and behavioral deficits in the FXS group supported the hypothesis that CN enlargement is related to abnormal orbitofrontal-caudate and dorsolateral-caudate circuitry in FXS. These findings deepen our understanding of neuroanatomical mechanisms underlying cognitive-behavioral problems in FXS and hold promise for informing future behavioral and psychopharmacological interventions targeting specific neural pathways.

Conditional discrimination learning by pigeons: Stimulus-response chains or occasion setters?

In conditional discrimination, the conditional stimulus or sample indicates which of two choice or comparison stimuli is associated with a reinforcer. Two hypotheses have been proposed concerning the role of the sample stimulus. According to Hull (1952), the sample and the response to the correct comparison form a stimulus-response chain. According to Skinner (1938), however, the sample serves as an occasion setter, setting the occasion for the choice of the correct comparison stimulus. In a conditional discrimination, if the sample stimulus forms part of a stimulus-response chain, then presenting the sample in the absence of the comparison stimuli should weaken the association. If the sample serves as an occasion setter, however, presenting the sample alone should not weaken its occasion-setting ability. In two experiments we tested these predictions. In Experiment 1, following conditional discrimination training with vertical and horizontal line samples and red and green comparison stimuli, we found that the presentation of the samples without the comparison stimuli (followed sometimes by a reinforcer) had little effect on conditional discrimination accuracy. In Experiment 2, two different houselights served as samples. When we presented the samples without comparison stimuli and without the reinforcers we found similar results. The results support the hypothesis that in conditional discrimination, the samples serve as occasion setters. (PsycInfo Database Record (c) 2024 APA, all rights reserved).

What makes the ephemeral reward task so difficult?

The ephemeral reward task involves providing subjects with a choice between two distinctive stimuli, A and B, each containing an identical reward. If A is chosen, the reward associated with A is obtained and the trial is over. If B is chosen, the reward associated with B is obtained but A remains, and the reward associated with A can be obtained as well. Thus, the reward-maximizing solution is to choose B first. Although cleaner fish (wrasse) and parrots easily acquire the optimal response by choosing B, paradoxically, several nonhuman primate species, as well as rats and pigeons, do not. It appears that some species do not associate their choice and reward with the second reward. Surprisingly, research in an operant context with pigeons and rats suggests that inserting a delay between the choice and reward facilitates optimal choice. It is suggested that impulsivity may be, in part, responsible for the difficulty of the task. In an attempt to better understand this task, we trained human subjects on an operant version of this task, with and without a brief delay between choice and reward and found that many subjects failed to learn to choose optimally, independent of the delay. Furthermore, performance on this task was not correlated with a task thought to measure impulsivity, the Balloon Analog Risk Task or with the Abbreviated Impulsivity Survey. We concluded that, for humans, the task is confusing because there is no incorrect response, only good and better, and better is not easily discriminated. (PsycInfo Database Record (c) 2024 APA, all rights reserved).

Interference of same/different learning by a spatial discrimination.

Same/different learning by pigeons has long been of interest to experimental psychologists. In one of these procedures, matching-to-sample, responses to a sample stimulus result in the presentation of two comparison stimuli, one of which matches the sample, the other of which does not, and choice of the matching stimulus is reinforced. Evidence of a matching concept has been found when transfer has been found to new stimuli. Given the transfer results, it is surprising that acquisition of two matching tasks (or two mismatching tasks), has not been found to be any faster than one matching and one mismatching task (i.e., two compatible tasks do not appear to facilitate each other). In the present experiment, we asked if matching acquisition involving three colors would be retarded if the correct response to a fourth color was not matching but was spatial (e.g., if the sample is red choose the red comparison, if the sample is green choose the green comparison, if the sample is yellow choose the yellow comparison, but if the sample is blue choose the left comparison). We found that acquisition of this task was slower than acquisition of a four color matching task (i.e., when the sample was blue, the blue comparison was correct). The results suggest that there is an interaction among matching associations, such that common rules facilitate learning compared with having to learn an inconsistent (spatial) rule. This result provides further evidence of the development of a matching concept by pigeons.

Implicit learning of the one-back reinforcement matching-mismatching task by pigeons.

Humans can learn tasks explicitly, as they can often describe the rules they have used to learn the task.1,2,3 Animals, however, are thought to learn tasks implicitly (i.e., purely associatively).2,3 That is, they gradually learn the correlation or association between the stimulus (or response) and the outcome. Both humans and pigeons can learn matching, where a sample stimulus indicates which one of two stimuli matches the sample. The 1-back reinforcement task is a difficult version of matching in which a correct response on trial N is rewarded only following a response on trial N + 1 (independent of the response on trial N + 1),4 and the correct response on trial N + 1 indicates whether a reward will occur on trial N + 2, and so forth. Humans do not appear to be able to learn the 1-back rule.5 Pigeons, however, do show 1-back reinforcement learning,6,7 and they appear to do so implicitly by gradually learning the correlation between their response on one trial and the outcome on the next trial (because all other relations are uncorrelated with the outcome). They learn the task slowly and to a level below what would be expected had they learned it explicitly. The present results, together with research with humans,7 suggest that there are times when human explicit learning may interfere with the ability of humans to learn. Pigeons, however, are not "distracted" by attempts at explicit learning, and thus they are able to learn this and other similar tasks.6,7,8.

What enables "distraction" to reduce delay discounting for pigeons (Columba livia).

In a successive delay-discounting task, a small reward can be obtained immediately but a larger reward can be obtained if one waits. There is evidence that the larger reward can be obtained more easily if one is "distracted" from obtaining the small reward. It is proposed here that a distractor stimulus may function as a Pavlovian conditioned stimulus (sign tracking) because orienting to it may be directly associated with the larger reinforcer. In the present study with pigeons, we examined two successive procedures: (a) a peck to a red light resulted in one pellet of food, and waiting for the red light to turn off resulted in five pellets (Red-Only). (b) If the pigeon pecked a red light, it received one pellet of food, and if it waited for the red light to turn to green, a peck to the green light resulted in five pellets of food (Red-Green). For both groups, on some trials, a concurrent (distractor) stimulus appeared with the red light but responses to it had no programed consequence. Results indicated that the pigeons in both groups waited for the larger reward more often when the distractor was present than when it was absent and that pigeons in the Red-Only group waited longer than those in the Red-Green group. The results are consistent with the hypothesis that the concurrent stimulus served as a conditioned stimulus for the Red-Only group and as a higher order conditioned stimulus for the Red-Green group. (PsycInfo Database Record (c) 2023 APA, all rights reserved).

Matching is acquired faster than mismatching by pigeons when salient stimuli are presented manually.

Same/different learning by pigeons has been studied using several different procedures. One of these procedures is matching-to-sample or mismatching-from-sample in which responses to a sample stimulus result in the presentation of two comparison stimuli, one of which matches the sample, the other of which does not. In the matching task, choice of the matching stimulus is reinforced. In the mismatching task, choice of the stimulus that does not match the sample is reinforced. Most research that has compared acquisition of the two tasks has not reported a difference between them. Research with transfer of training, in which either the matching stimulus or the mismatching stimulus is replaced with a new stimulus, suggests that the matching stimulus is selected in the matching task, but the matching stimulus is rejected in the mismatching task. In the present experiment, pigeons were trained on either matching or mismatching with salient stimuli presented manually and the reinforcer was presented under a colored slide that covered it. In Phase 1 with a noncorrection procedure and a reinforcer for pecking the sample, pigeons did not acquire either task, however, in Phase 2 they learned both tasks readily without reinforcement for pecking the sample and with a correction procedure. Furthermore, the pigeons learned matching significantly faster than mismatching, suggesting that sameness may be a more natural stimulus relation than mismatching.

Scrotal Wall Metastasis from Adenocarcinoma of Unknown Origin, with Concurrent Extramammary Paget's Disease - a Case Report.

INTRODUCTION: Scrotal cancer is a very rare disease, with the most common subtype being squamous cell carcinoma. Metastatic carcinoma to the scrotal wall is very rare. A histological finding of adenocarcinoma in a scrotal malignancy invariably suggests a metastasis from another primary cancer. We describe an enigmatic case of metastatic adenocarcinoma to the scrotum managed as metastatic adenocarcinoma of unknown origin. Attempts to identify a primary cancer were complicated by ambiguous diagnostic results. This is the first case in literature of metastatic cancer to the scrotum from an adenocarcinoma of unknown origin, and this was complicated by concurrent extramammary Paget's disease. CASE PRESENTATION: A 70-year-old male presented with painless progressive scrotal skin swelling, which was shown on histology to be adenocarcinoma. Immunohistochemistry showed prostatic lineage markers. However, the argument for a prostatic primary was weakened by negative prostate transrectal ultrasound biopsy findings and negative radiological findings. The scrotal metastatic adenocarcinoma was managed as metastatic adenocarcinoma of unknown origin. A differential of occult poorly differentiated prostatic primary was considered in view of the clinical phenotype of an elderly male patient with extensive sclerotic bony metastases, immunohistochemistry results and relatively low PSA level in relation to systemic burden of disease. The patient was managed with palliative systemic chemotherapy (carboplatin/paclitaxel) with initial disease response, but eventually developed progressive disease. DISCUSSION AND CONCLUSION: Finding of adenocarcinoma in scrotal skin malignancy indicates a metastasis and should prompt further work-up to identify a primary cancer, particularly of other genitourinary or lower gastrointestinal origin, so that treatment can be targeted at the underlying primary malignancy. However, attempts to identify a primary cancer might be complicated by ambiguous diagnostic results.