The arguments of associations

This chapter considers associative solutions to “non‐linear” discrimination problems, such as negative patterning (A+ and B+ vs AB‐) and the biconditional discrimination (AB+ and CD+ vs AC‐ and BD‐). It is commonly assumed that the solution to these discriminations requires “configural” elements that are added to the compound of two stimuli. However, these discriminations can be solved by assuming that some elements of each stimulus are suppressed when two stimuli are presented in compound. Each of these approaches can solve patterning and biconditional discriminations because they allow some elements, as the arguments of associations, to have differential “presence” on reinforced versus nonreinforced trials, and thus differential associability and control over responding. The chapter then presents a more specific version of one of these models, describing how interactions between stimuli, particularly the competition for attention, provide a mechanism whereby some elements are more suppressed than others when stimuli are presented simultaneously as a compound

The acquisition of conditioned responding

This report analyzes the acquisition of conditioned responses in rats trained in a magazine approach paradigm. Following the suggestion by Gallistel, Fairhurst and Balsam (2004), Weibull functions were fitted to the trial-by-trial response rates of individual rats. These showed that the emergence of responding was often delayed, after which the response rate would increase relatively gradually across trials. The fit of the Weibull function to the behavioral data of each rat was equaled by that of a cumulative exponential function incorporating a response threshold. Thus the growth in conditioning strength on each trial can be modeled by the derivative of the exponential – a difference term of the form used in many models of associative learning (e.g., Rescorla & Wagner, 1972). Further analyses, comparing the acquisition of responding to a continuously reinforced stimulus (CRf) and a partially reinforced stimulus (PRf), provided further evidence in support of the difference term. In conclusion, the results are consistent with conventional models that describe learning as the growth of associative strength, incremented on each trial by an error-correction process.This work was supported by grant DP0771154 from the Australian Research Council

Changes in the distribution of response rates across the CS-US interval: Evidence that responding switches between two distinct states.

Two experiments used the peak procedure to examine timing of conditioned responses in a magazine approach paradigm with rats. A conditioned stimulus (CS) was reinforced with food on 50% of trials. Food was delivered at a fixed time, either 20 s, 30 s or 40 s into the CS presentation. Response rates were recorded during non-reinforced CS presentations that extended well beyond the scheduled time of food delivery. The mean response rate (averaged over many trials) increased during the CS, peaking at the expected time of reinforcement, and decreased again. Detailed analyses of the frequency distribution of response rates showed that responding was described by two distinct distributions, consistent with the rat being in a low response state on some trials and in a high response state on other trials. Modeling of these frequency distributions showed that the systematic rise and fall in response rate across a trial was primarily explained by a change in the proportion of time that the rat spent in the low versus high response state. However, the change in responding was also explained in part by a continuous shift in the high response state, such that responding in that state increased and then decreased gradually across the trial. These results support accounts that describe response timing as an abrupt change from low to high responding during the CS, but also provide evidence for a continuous change in conditioning strength across the duration of the CS. The implications of these findings for timing and associative theories of conditioning are discussed.Australian Research Counci

Response Rate and Reinforcement Rate in Pavlovian Conditioning

Four experiments used delay conditioning of magazine approach in rats to investigate the relationship between the rate of responding, R, to a conditioned stimulus (CS) and the rate, r, at which the CS is reinforced with the unconditioned stimulus (US). Rats were concurrently trained with four variable-duration CSs with different rs, either as a result of differences in the mean CS-US interval or in the proportion of CS presentations that ended with the US. In each case, R was systematically related to r, and the relationship was very accurately characterized by a hyperbolic function, R = Ar/(r+c). Accordingly, the reciprocal of these two variables – response interval, I (=1/R), and CS-US interval, i (=1/r) – were related by a simple affine (straight line) transformation, I = mi+b. This latter relationship shows that each increment in the time that the rats had to wait for food produced a linear increment in the time they waited between magazine entries. We discuss the close agreement between our findings and the Matching Law (Herrnstein, 1970), and consider their implications for both associative theories (e.g., Rescorla & Wagner, 1972) and non-associative theories (Gallistel & Gibbon, 2000) of conditioning.This work was supported by Grants DP0771154 and DP1092695 from the Australian Research Council

Elemental Representations of Stimuli in Associative Learning

This paper reviews evidence and theories concerning the nature of stimulus representations in Pavlovian conditioning. It focuses on the elemental approach developed in Stimulus Sampling Theory (Atkinson & Estes, 1963; Bush & Mosteller, 1951b) and extended by McLaren and Mackintosh (2000; 2002), and contrasts this with models that that invoke notions of configural representations that uniquely code for different patterns of stimulus inputs (e.g., Pearce, 1987, 1994; Rescorla & Wagner, 1972; Wagner & Brandon, 2001). The paper then presents a new elemental model that emphasizes interactions between stimulus elements. This model is shown to explain a range of behavioral findings, including those (e.g., negative patterning and biconditional discriminations) traditionally thought beyond the explanatory capabilities of elemental models. Moreover, the model offers a ready explanation for recent findings reported by Rescorla (2000; 2001; 2002b) concerning the way that stimuli with different conditioning histories acquire associative strength when conditioned in compoun

Possible attenuation of the G2 DNA damage cell cycle checkpoint in HeLa cells by extremely low frequency (ELF) electromagnetic fields

BACKGROUND: The issue remains unresolved as to whether low frequency magnetic fields can affect cell behaviour, with the possibility that they may be in part responsible for the increased incidence of leukaemia in parts of the population exposed to them. METHODS: Combined treatment of HeLa cells with gamma-irradiation (1, 3 and 5 Grays) and extra low frequency magnetic fields of ~50 Hz was carried out under rigorously controlled conditions. RESULTS: Synchronised cells progressing from S-phase arrived at mitosis on average marginally ahead of irradiation controls not exposed to ELF. In no instance out of a total of twenty separate experiments did this "double-insult" further delay entry of cells into mitosis, as had been anticipated. CONCLUSION: This apparently "non-genotoxic" agent (ELF) appears to be capable of affecting cells that would normally arrest for longer in G2, suggesting a weakening of the stringency of the late cycle (G2) checkpoint

Attentional Changes During Implicit Learning: Signal Validity Protects a Target Stimulus from the Attentional Blink

Participants in two experiments performed two simultaneous tasks: one, a dual-target detection task within a rapid sequence of target and distractor letters; the other, a cued reaction time task requiring participants to make a cued left/right response immediately after each letter sequence. Under these rapid visual presentation conditions, it is usually difficult to identify the second target when it is presented in close temporal proximity of the first target, a phenomenon known as the attentional blink. However, here, participants showed an advantage for detecting a target presented during the attentional blink if that target predicted which response cue would appear at the end of the trial. Participants also showed faster reaction times on trials with a predictive target. Both of these effects were independent of conscious knowledge of the target-response contingencies assessed by post-experiment questionnaires. The results suggest that implicit learning of the association between a predictive target and its outcome can automatically facilitate target recognition during the attentional blink, and therefore shed new light on the relationship between associative learning and attentional mechanisms.Australian Research Counci

An attention modulated associative network.

We present an elemental model of associative learning that describes interactions between stimulus elements as a process of competitive normalization. Building on the assumptions laid out in Harris (2006), stimuli are represented as an array of elements that compete for attention according to the strength of their input. Elements form associations among each other according to temporal correlations in their activation but restricted by their connectivity. The model moves beyond its predecessor by specifying excitatory, inhibitory, and attention processes for each element in real time and describing their interaction as a form of suppressive gain control. Attention is formalized in this model as a network of mutually inhibitory units that moderate the activation of stimulus elements by controlling the level to which the elements are suppressed by their own inhibitory processes. The model is applied to a range of complex discriminations and related phenomena that have been taken as evidence for configural-learning processes.Australian Research Council: DP077115

Summation of reinforcement rates when conditioned stimuli are presented in compound

Three experiments used delay conditioning of magazine approach in rats to examine the summation of responding when two conditioned stimuli (CSs) are presented together as a compound. The duration of each CS varied randomly from trial-to-trial around a mean that differed between the CSs. This meant that the rats’ response rate to each CS was systematically related to the reinforcement rate of that CS, but remained steady as time elapsed during the CS (Harris & Carpenter, in press; Harris, Gharaei, & Pincham, in press). When the rats were presented with a compound of two CSs that had been conditioned separately, they responded more during the compound than during either of the CSs individually. More significantly, however, in all three experiments, the rats responded to the compound at the same rate as they responded to a third CS that had been reinforced at a rate equal to the sum of the reinforcement rates of the two CSs in compound. We discuss the implications of this finding for associative models (e.g., Rescorla & Wagner, 1972) and rate-based models (Gallistel & Gibbon, 2000) of conditioning.Grant DP1092695 from the Australian Research Counci