Consequences of Base Time for Redundant Signals Experiments

We report analytical and computational investigations into the effects of base time on the diagnosticity of two popular theoretical tools in the redundant signals literature: (1) the race model inequality and (2) the capacity coefficient. We show analytically and without distributional assumptions that the presence of base time decreases the sensitivity of both of these measures to model violations. We further use simulations to investigate the statistical power model selection tools based on the race model inequality, both with and without base time. Base time decreases statistical power, and biases the race model test toward conservatism. The magnitude of this biasing effect increases as we increase the proportion of total reaction time variance contributed by base time. We marshal empirical evidence to suggest that the proportion of reaction time variance contributed by base time is relatively small, and that the effects of base time on the diagnosticity of our model-selection tools are therefore likely to be minor. However, uncertainty remains concerning the magnitude and even the definition of base time. Experimentalists should continue to be alert to situations in which base time may contribute a large proportion of the total reaction time variance.     

Superior discrimination between similar stimuli after simultaneous exposure

Human participants received unsupervised exposure to difficult-to-discriminate chequerboard stimuli (e.g., AX and BX), before learning a discrimination between them. Experiment 1 demonstrated that prior exposure enhanced later discrimination and that intermixed exposure (AX, BX, AX, BX ...) resulted in better subsequent discrimination than did blocked exposure (CY, CY ... DY, DY ...). Experiment 2 showed that simultaneous exposure to two similar stimuli (AX-BX, BX-AX ...) facilitated the later acquisition of a successive discrimination, more than successive exposure (AX-AX, BX-BX ...). These results parallel those observed by Mundy, Honey, and Dwyer (2007) who used pictures of human faces as stimuli and establish the generality of the fact that simultaneous exposure produces a particularly marked perceptual learning effect.     

Re-assessing causal accounts of learnt behavior in rats

Rats received either a common-cause (i.e., A→B, A→food) or a causal-chain training scenario (i.e., B→A, A→food) before their tendency to approach the food magazine during the presentation of B was assessed as a function of whether it was preceded by a potential alternative cause. Causal model theory predicts that the influence of an alternative cause should be restricted to the common-cause scenario. In Experiment 1, responding to B was reduced when it occurred after pressing a novel lever during the test phase. This effect was not influenced by the type of training scenario. In Experiment 2, rats were familiarized with the lever prior to test by training it as a potential cause of B. After this treatment, the lever now failed to influence test responding to B. In Experiment 3, rats given common-cause training responded more to B when it followed a cue that had previously been trained as a predictor of B, than when it followed another stimulus. This effect was not apparent in rats that received causal-chain training. This pattern of results is the opposite of that predicted by causal model theory. Thus, in three experiments, the presence of an alternative cause failed to influence test responding in manner consistent with causal model theory. These results undermine the application of causal model theory to rats, but are consistent with associative analyses.     

Grouping in short-term verbal memory: Is position coded temporally?

The nature of the mechanisms that code item position in serial short-term verbal recall was investigated with reference to temporal grouping phenomena--effects that arise when additional pauses are inserted in a presented list to form groups of items. Several recent models attempt to explain these phenomena by assuming that positional information is retained by associating items with contextual information. According to two of the models--the Phonological Loop model (Hitch, Burgess, Towse, & Culpin, 1996) and the OSCAR model (Brown, Preece, & Hulme, 2000)--contextual information depends critically on the timing of item presentation with reference to group onset. By contrast, according to the Start-End model (Henson, 1998) and a development from it, which we label the Oscillator-Revised Start-End model (Henson & Burgess, 1997), contextual information is independent of time from group onset. Three experiments examined whether coding of position is time dependent. The critical manipulation was to vary stimulus-onset asynchrony from one group to the next in the same list. Lists of consonants were presented visually, but with vocalization in Experiment 1, auditorily in Experiment 2, and auditorily with articulatory suppression in Experiment 3. The pattern of order errors consistently favoured the predictions of the time-independent models over those of the time-dependent models in that across-group transpositions reflected within-group serial position rather than time from group onset. Errors involving intrusions from previous lists also reflected within-group serial position, thereby extending support for the time-independent models.     

Impaired conditional task performance in a high schizotypy population: relation to cognitive deficits

Cognitive impairments in schizophrenia have been characterized as reflecting a core deficit in the maintenance or use of task-setting cues to mediate appropriate ongoing behaviour. This analysis suggests that cognitive deficits in schizophrenia will be particularly evident when different task-setting cues dictate when different responses are required by the same stimuli. One simple task in which task-setting cues are required is a biconditional discrimination. Here we examined the performance of participants with high and low schizotypy scores (Mason, Claridge, & Jackson, 1995) on a biconditional discrimination and an otherwise equivalent, control discrimination that did not require the use of task-setting cues. Participants scoring highly on the Introvertive Anhedonia subscale (which has been allied to the negative and cognitive symptoms of schizophrenia) performed poorly on the biconditional, but not on the control, discrimination. No other subscales demonstrated a significant influence on either biconditional or control performance.     

Associative change in connectionist networks: an addendum

The results of a recent study have provided direct support for the suggestion that conditional learning in rats is best characterized by a 3-layer connectionist network (M. J. Allman, J. Ward-Robinson, & R. C. Honey, 2004). In the 2 experiments reported here, rats were used to investigate the nature of the changes that occur when a stimulus compound is presented, whose components activate hidden units associated with food and no food, and either food or no food is presented. The results of both experiments, while controlling for the possible contribution of associations between these hidden units (within-layer links), provide evidence that the distribution of associative change between units in the hidden layer that are activated by the stimulus compound and those in the output layer (between-layer links) are unequal. They also indicate that associative change is more marked on trials on which no food was presented than on trials on which food was presented.     

Schizophrenia, ketamine and cannabis: evidence of overlapping memory deficits

Drug models of mental illness are considered useful if they provoke its characteristic symptoms. In this respect, ketamine and tetrahydrocannabinol (cannabis) are coming under increasing scrutiny as models for schizophrenia. However, although both undoubtedly produce psychotic symptoms characteristic of the disorder, we argue here that, because schizophrenia is also accompanied by cognitive deficits, a full understanding of the impact of these drugs on cognition will be crucial in taking these models further. Memory deficits are pronounced in schizophrenia and we focus upon patterns of working and episodic memory impairment produced by ketamine and cannabis, identifying overlaps between drug and illness. We suggest that close attention to these deficits can offer insights into core pathophysiology of schizophrenia.     

Transfer between contextual conditional discriminations: an examination of how stimulus conjuctions are represented

In 2 experiments, when rats were placed in 1 pair of contexts, A and B, 2 relationships were in force (X --> food and Y --> no food), and when they were placed in another pair of contexts, C and D, the complementary relationships were operative (Y --> food and X --> no food). In Experiment 1, rats then received a 2nd discrimination that was either contextually congruent (in A and B, Y --> food and X --> no food; in C and D, X --> food and Y --> no food) or contextually incongruent (in A and D, Y --> food and X --> no food; in C and B, X --> food and Y --> no food) with the 1st discrimination. In Experiment 2, the 1st discrimination, involving X and Y, was interleaved with a 2nd discrimination, involving V and W, that was again either congruent (in A and B, V --> food and W --> no food) or incongruent (in A and D, V --> food and W --> no food) with the 1st discrimination. The congruent discriminations were acquired more readily than were the incongruent discriminations.     

Analysis of potentiation and overshadowing effects in the instrumental performance of pigeons

In each of two experiments, pigeons were trained on a multiple VI (variable interval) schedule with a 3-s delay of reinforcement. Different components were associated with different key colors. Experiment 1 (stage 2) confirmed a previous finding that the response rate is higher in a component having a signal (illumination of the houselight) filling the delay interval than in a component lacking the signal. This potentiation effect was replaced by an overshadowing effect (i.e., the rate was low in the signaled component) when in stage 1 of Experiment 1 the birds received concurrent experience of a component containing houselight presentations not correlated with reinforcement. In Experiment 2 it was found that this overshadowing effect was abolished when the signal used was the presentation of a pattern on the response key rather than illumination of the houselight. These results are interpreted in terms of an interaction between the rate-enhancing properties of the signal (perhaps a consequence of its conditioned reinforcing power) and the tendency of the signal in some conditions to evoke behavior that competes with the response being recorded.