The compatibility effect found in the flanker task is usually explained in terms of response slowing due to incompatible flanker properties. It is as yet unclear whether the compatibility effect is also due to facilitative effects associated with compatible flanker properties. An extension of the Continuous Flow model put forward by Eriksen & Schultz (1979) namely the "Accumulated Activation Strength Difference" (AASD) model is proposed that maintains both response facilitation - due to compatible flanker properties, and response slowing - due to incompatible flanker properties, takes place under conditions where there is response competition. A series of flanker task experiments were conducted that presented 2, 4 or 6 flankers and combined flanker types in terms of their compatibility. It was found that flanker conditions that contained an equal number of compatible and incompatible flankers generated faster RTs than conditions that contained purely incompatible flankers. There was also no difference in RTs for incompatible flanker conditions or compatible flanker conditions whether there were 2, 4 or 6 flankers. This suggests that the effects of compatible and incompatible flankers cancel each other out. Furthermore, another experiment varied the ratio of compatible to incompatible flankers in a 4 flanker array. It was found that the ratio of compatible to incompatible flankers within a stimulus array mediates the magnitude of the compatibility effect. There is some evidence that this effect also occurs across different stimulus dimensions. The model also offers an explanation for why no response facilitation is observed for compatible trials compared to target alone trials and can account for how the patterns of errors in flanker tasks usually mirror the RT data. However, the model is complete, for example, it makes no attempt to account for how neutral flanker properties affect responding.

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Cognitive reserve is the name given to the latent variable that describes individual differences in the ability to offset cognitive decline in old age. This thesis attempts to provide mechanistic explanations for two major aspects of cognitive reserve. These are neural compensation and neural reserve. Furthermore, behavioural experiments carried out as part of this investigation have extended the knowledge of existing theories as to the age invariance of neural compensation and the relationship between language, other more traditional proxies of cognitive reserve, and executive control. The results of these studies carried out in this thesis have demonstrated a biologically viable mechanism for the monitoring of task demand with resultant control of interhemispheric communication as a method of compensation. Further, this aspect of neural compensation was not found in younger participants. The neural network model in this thesis demonstrated differences over age in the spacing of representations for bilingual and monolingual networks as well as demonstrating increased inhibition in the bilingual network as a result of a negative relationship between weights from the tags of each language to nodes in the hidden layer. Finally, regression analysis using data from two large scale behavioural experiments demonstrated a minimal influence of bilingual language use on performance in executive control tasks. The models in this thesis provide an insight into the mechanisms behind cognitive reserve whilst supporting empirical results. Further, the results from the neural network model allowed predictions to be made with regard to the performance of bilinguals in dual category retrieval tasks. The lack of a relationship between bilingualism and cognitive control is supported by emerging research in the area and suggests that the functionality underlying cognitive reserve may be better described by biological rather than cognitive processes.

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The acute effects of radiofrequency electromagnetic fields (RF EMF) on human cognitive performance were investigated. Cognitive tasks were identified from RF EMF cognitive research; tests used most extensively were psychomotor performance, attention, working and episodic memory tasks. Review of published imaging research enabled selection of a battery of cognitive tasks for use in the experimental studies. Effects of practice and the test-retest reliabilities on the tests were evaluated and the relationship between the tests within the battery and measures of ability and personality investigated. Three studies were conducted to identify whether RF EMF produced by occupational communication systems had reliable effects on cognitive performance and self-reports of mood, anxiety and workload. Double-blind repeated measures design was used to investigate the following fields in comparison to sham; a high frequency (HF) 29MHz continuous wave (CW) signal, a very high frequency (VHF) 75MHz CW field, an Ultra High Frequency (UHF) modulated 448MHz field, a UHF 1206MHz CW signal and a TETRA 388MHz field pulse modulated at 17.6Hz. The first study demonstrated no significant effects of the VHF signal on performance when compared to sham. The HF signal appeared to reduce response time to two of the cognitive tasks and affect error rate on one of them. Investigation of the two UHF signals showed no reliable differences between conditions on the cognitive measures. The third study showed reliable differences between TETRA exposure and sham conditions on two of the tasks. Overall the results indicate that RF EMF signals at the frequencies and power levels used in these studies are well tolerated in healthy subjects. In general, the signals do not appear to have reliable and robust effects on human cognitive performance. However, there may be subtle transitory effects of RF EMF that are not well understood at the present time.

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When we hear speech, besides the linguistic content, we may gain a great deal of information about the speaker from their voice, such as their identity, age, gender, or emotional state. No word is uttered the same way, even by the same talker, so one of the main challenges in spoken word recognition research is to understand the cognitive processes that underlie the processing of a complex signal like speech in the presence of high variability. Previous research has shown that listeners encode both linguistic and talker-related voice properties of the speech signal in their memory representations. Speaker variability is not the only variable we encounter; we frequently hear speech in varying auditory contexts as well. Recent evidence suggests that auditory background details, such as non-linguistic sounds co-occurring with spoken words, may be incorporated in our lexical memory. Here, I first test the hypothesis that the acoustic glimpses of words (left-overs) produced by masking from the associated sounds, rather than the sounds per se, are retained in memory. I then identify and examine the role of a novel element in the relationship between a spoken word and its associated sound, perceptual integrality, in the retention of sounds in memory. Last, I investigate the potential impact of the unique pairwise associations between words and sounds on the encoding of sounds in memory. My findings suggest that background sounds can be encoded in memory, but only in certain conditions. Specifically, this can happen when the auditory episode of the word(s) consists of highly contrasted acoustic glimpses of the same word(s), and when the sounds are made integral to, hence more difficult to perceptually segregate from the words, through intensity modulation.