Stephen B. Fountain

Extrapolation of serial stimulus patterns by rats

This experiment determined if rats could extrapolate a familiar serial sequence of diminishing food quantities by accurately anticipating a novel quantity added to the end of the sequence. In 13 days of training, rats ran in a straight runway to obtain quantities of food pellets presented in sequential order. A strongly monotonic group received repetitions of a formally simple pattern of 14-7-3-1 pellets of food, while weakly monotonic and nonmonotonic groups received formally more complex 14-5-5-1 and 14-3-7-1 patterns, respectively. In subsequent transfer, a 0-pellet quantity was added to each pattern, thus extending pattern length to five elements. Results of the very first pattern repetition containing the added 0-pellet element indicated that rats in the strongly monotonic condition, but not in the others, anticipated the reduced quantity before actually experiencing it. This result supports a cognitive, rule-learning hypothesis for serial learning by rats.

Formal structure and pattern length in serial pattern learning by rats

When rats learn to anticipate a sequence of stimulus events, such as a serial pattern of different food quantities, they are sensitive to the rule-based formal structure relating the magnitude of successive stimuli. Earlier research has shown that if formal structure is simple (e.g., if a single “less than” rule relates the size of each successive quantity), patterns are learned faster than if formal structure is complex (e.g., if two or more rules such as “less than” and “greater than” relate successive pattern quantities). Two experiments tested the hypothesis that pattern length modulates the role of pattern complexity. We predicted that pattern length and pattern complexity interact in determining pattern difficulty. That is to say, long complex patterns should be learned more slowly than short complex patterns. However, long simple patterns should be learned faster than short simple patterns. In Experiment 1, rats ran a straight runway to receive repeated sequences of food quantities. The long-monotonic group received a formally simple 18-10-6-3-1-0 pattern, in which each number represents a quantity of food pellets. The long-nonmonotonic group received a formally complex 10-1-3-6-18-0 pattern. Similarly, the short-monotonic and short-nonmonotonic groups received 18-1-0 and 1-18-0 patterns. Pattern tracking—fast and slow running in anticipation of large and small quantities of food, respectively—was taken as an index of pattern learning. In Experiment 2, comparable patterns were used, but rats leverpressed in a discrete-trial procedure; response latencies measured pattern tracking. In both experiments, rats learned formally simple patterns faster than they did formally complex patterns. In Experiments 1 and 2, but less clearly in Experiment 2, the predicted interaction was obtained. The results support and generalize the idea that rats encode and use some representation of the formal rule structure of serial patterns as they learn them.

Differential impairments of rat serial pattern learning and retention induced by MK-801, an NMDA receptor antagonist

Three experiments investigated the role of NMDA receptor dependent systems in sequential learning and memory. Rats tracked serial patterns after systemic administration of MK-801, an NMDA receptor antagonist that blocks plasticity in the hippocampus and other structures. Experiments 1 and 2 sought to describe the effects of MK-801 on acquisition and retention of serial patterns. Patterns were 24 elements long and highly organized, with some groups receiving a “violation” element that did not fit the organization of the rest of the pattern. Experiment 3 evaluated MK-801’s effects on the integration of new information that was either consistent or inconsistent with a previously learned pattern structure. The results of these experiments indicated that MK-801 interfered with serial pattern learning and, to a lesser degree, retention. Learning about structure was relatively spared, whereas learning about interruptions or violations of pattern structure was impaired. The latter differential effects are consistent with the contention in earlier literature that serial pattern learning is subserved by at least two learning/memory systems.

Chunking, sorting, and rule-learning from serial patterns of brain-stimulation reward by rats

Three studies tested the notion that rats would treat brain-stimulation reward (BSR) as a stimulus alphabet from which rules could be abstracted to learn serial patterns. In Experiment 1, rats learned to track a serial pattern of 18-10-6-3-1-0 pulses of BSR, responding fast in anticipation of large quantities of BSR and slowly or not at all in anticipation of small quantities of BSR. In Experiment 2, rats learned to track a formally simple 18-6-1-0 pattern faster than a formally complex 18-1-6-0 pattern in a within-subjects procedure, indicating that rats can learn to discriminate between simple and complex pattern structures. Finally, in Experiment 3 rats learned either a formally simple 25-18-10-3-1-0 or a formally complex 25-3-10-18-1-0 pattern whose successive elements were separated by an embedded three-element 6-6-0 subpattern. Rats learned to “chunk together” the dispersed pattern elements, and rats receiving the simple pattern learned to track their pattern, whereas rats receiving the complex pattern did not. The latter results suggest that when simple pattern structure is available, rats can simultaneously track rule structures in at least two memory locations. The results of these experiments, using a new testing procedure and, presumably, a new stimulus alphabet, generalize and extend the idea that rats can abstract relational rules to learn serial patterns.

Serial-pattern-learning processes dissociated by trimethyltin exposure in rats

Trimethyltin (TMT) is a neurotoxic organometal which produces a variety of learning and memory impairments in laboratory animals and humans, including impairments of avoidance learning, maze learning, and problem solving. Two studies investigated the effects of TMT exposure on serial-pattern learning in rats. Rats in both experiments were intubated once with either 0 or 7.0 mg/kg TMT 1 week prior to the pattern-learning procedure. Rats learned serial patterns composed of various quantities of brain-stimulation reward (BSR) pulses; they received BSR quantities in a predetermined order for leverpresses in a discrete-trial operant task. In Experiment 1, all rats received two serial patterns (20-10-0 vs. 1-29-0 pulses of BSR) that alternated within each daily session of 100 patterns. In Experiment 2, all rats received two serial patterns (18-10-6-3-1-0 vs. 18-1-3-6-10-0 pulses of BSR) that alternated within each daily session. In Experiment 1, TMT-exposed rats learned both their patterns more slowly than did controls. In Experiment 2, TMT-exposed rats learned the formally simple 18-10-6-3-1-0 pattern of BSR quantities faster than did controls, but were significantly slower than controls in learning the formally more complex 18-1-3-6-10-0 pattern. In both experiments, however, TMT exposure did not affect either the reinforcing properties of BSR or rats’ asymptotic performance. These results support the notion that TMT exposure impaired some aspects of the rote processes involved in serial-pattern learning in rats, yet spared the rats’ ability to encode some representation of the formal rule-based structure of the pattern. The results thus suggest that the processes involved in learning simple versus complex serial patterns may be mediated by different systems in the brain.

Neuronal Plasticity in the Mammalian Brain: Relevance to Behavioral Learning and Memory

Much recent activity in the neurosciences relates to the search for the brain mechanisms underlying learning and memory. In recent years a brain circuit in cerebellum and brainstem has been discovered that is responsible for the learning of a simple motor response (nictitating membrane movement). This has provided a model for neuroscientists to use in understanding the brain circuits involved in this simple form of learning and, by extension, to more complex forms ultimately, and a means of exploring the changes in neural function underlying the learning. An enduring change in neural function is represented by long-term potentiation (LTP), an alteration in synaptic efficacy seen in hippocampus and other areas. LTP can be induced experimentally and occurs as a concomitant of learning. We review data suggesting that different brain circuits may underlie different forms of learning and memory. Several current theories of learning and memory with respect to hippocampal and other brain circuit involvement are considered. We conclude with the behavioral and physiological effects of exposure to teratogens or toxins and the CNS alterations associated with dementia.

Blocking in rat serial pattern learning.

Both associative and rule-learning theories have been proposed to account for rat serial pattern learning, but individually they are unable to account for a variety of recent behavioral and psychobiological phenomena. The present study examined the role of rule learning versus discriminative learning in rat pattern learning using a classic associative phenomenon: blocking. Rats learned to press levers in an 8-lever circular array according to a rule-based serial pattern, 123-234-345-456-567-678-781-812, where digits indicate the correct lever in the array for each trial. Each pattern presentation contained a chunk with a final element violation, such as 454 instead of 456. Rats learned in a first phase that a noise signaled the violation chunk; then, a concurrent spatial cue was added in a second phase. A test with spatial cues alone showed that blocking occurred. The results suggest that associative learning mediated cuing of violation elements. Taken together with other behavioral and psychobiological evidence already reported in the literature implicating rule learning when rats learn this pattern in this paradigm, these results implicate multiple concurrent learning processes in rat serial pattern learning.

Rule learning in rats: serial tracking in interleaved patterns

Abstract Humans have the ability to chunk together information from nonadjacent serial positions in sequential patterns. For example, human subjects can extrapolate the pattern, A-M-B-N-C-O-D-P-E-..., to find the missing element, Q, by sorting pattern elements into two component interleaved subpatterns: A-B-C-D-E and M-N-O-P-... Two experiments investigated the ability of rats to reorganize pattern elements from nonadjacent serial positions into chunks not presented by the experimenter. Rats learned either a structured or unstructured sequence interleaved with elements of a repeating sequence (experiment 1) or an alternation sequence (experiment 2). In both experiments, rats learned the interleaved subpatterns at different rates. Acquisition rate was correlated with the structural properties of component subpatterns and the nature of the rules required to describe the interleaved subpatterns. The results indicate that rats are sensitive to the organization of nonadjacent elements in serial patterns and that they can detect and sort structural relationships in interleaved patterns.

Behavioral consequences of intraperitoneal carbon monoxide administration in rats

Blood carboxyhemoglobin (HbCO) was determined 15, 30, 45, 60, 90, and 120 min following ip injection of 2.5, 5, 10, 20, and 40 ml pure carbon monoxide (CO)/kg body wt in rats. These CO doses produced HbCO concentrations of 12, 24, 35, 45, and 60%, respectively, at 30 min postinjection. Once these normative data were obtained, a group of eight naive rats were trained to produce a rapid sequence of responses in a stimulus-tracking task, then they were exposed to each of the doses of CO. The 10-ml/kg dose produced a slight decrement in performance, the 20-ml/kg dose reduced correct responses by nearly half, and the 40-ml/kg dose resulted in virtually complete cessation of responding. CO exposure resulted in longer pauses in responding with increasing dose, but the distribution of errors produced in the stimulus-tracking task remained relatively parallel across the range of CO exposures. Thus CO exposure impaired tracking performance but had relatively little effect on the pattern of errors rats produced during the stimulus-tracking test. Taken together these results contradict previous reports purporting to show that CO by ip administration has no behavioral effects; instead, the results indicate that CO administration via the ip route has very similar effects to inhaled CO on behavior.

SERIAL-PATTERN LEARNING IN MICE: PATTERN STRUCTURE AND PHRASING

In two experiments, mice learned 24-element serial patterns. In Experiment 1, patterns either were perfectly structured or had a single violation element and were either phrased by temporal pauses or unphrased. In Experiment 2, the same violation pattern of Experiment 1 was phrased by temporal cues, visual cues, or a combination of the two. For mice, as for rats and humans in earlier studies, pattern structure predicted pattern learning difficulty and also the nature and relative frequency of errors. Mice, like rats and humans, also found a violation element especially difficult to learn and at that point in the pattern made errors consistent with the structure of the remainder of the pattern. However, in both experiments, phrasing interfered with responding correctly on the element after the phrasing cue. In a third experiment, mice were able to use temporal intervals and, to a lesser degree, visual stimuli as discriminative cues to control spatial responses in the same apparatus used in earlier studies. The results support the view that mice are sensitive to pattern organization but may have difficulty using phrasing cues in the context of serial patterns.