Working memory is the ability to store relevant information temporarily to guide thoughts and behavior. As a basic executive function that is required for everyday tasks, it is essential to understand working memory to fully understand cognition. A neural correlate of working memory has been suggested in the persistent activity displayed by single neurons during the retention interval of working memory tasks performed by monkeys. Physiological and computational studies have proposed that the maintenance of this activity depends on NMDA receptor activation. Human homologues of persistent activity have been suggested using neuroimaging methods and electroencephalogram (EEG) recordings. To help bridge the gap between these studies within and between species, EEG signals in the form of event-related potentials (ERPs) were recorded while one female rhesus macaque monkey (Macaca mulatta) performed a series of tasks that involve visual working memory, in which a visual array presented for 500 ms must be maintained in working memory for a retention interval of 1 s to guide a subsequent saccade to a stimulus. In addition, the effect of NMDA blockade on ERPs was investigated by administering a sub-anesthetic dose of the NMDA antagonist ketamine. A neural correlate of visual working memory was identified in the ERP that was contralaterally-specific to the to-be-remembered target during the retention interval of the tasks. For the first time in the monkey, it was shown that the amplitude and polarity of this activity reflects the spatial location of the target stimulus, scales with the number of items that had to be remembered, and is predictive of trial outcome. The activity was less positive for contralaterally presented stimuli, greater memory loads, and correct trials. Furthermore, this contralateral activity was modulated by ketamine, primarily during the retention interval but also during memory item presentation at P100 and P300. Taken together, these properties support this contralateral activity as a candidate for an electrophysiological index of working memory that is similar to that of humans. These findings also provide a link to the single-neuron mechanisms of working memory in monkeys and further validate the monkey as a model of human visual working memory. / Thesis (Master, Neuroscience Studies) -- Queen's University, 2013-10-02 11:39:34.593

Chronic exposure to psychostimulants such as amphetamine (AMPH) can induce long-term disruptions in cognition via actions on prefrontal cortex dopamine. Previous work has shown that two types of executive functions, set shifting and working memory (WM), are disrupted by AMPH sensitization and that these cognitive domains are impaired in schizophrenics and stimulant abusers. We assessed the effects of AMPH sensitization on behavioural flexibility using a cross-maze set shifting task and a WM task using the delayed spatial win-shift (SWSh) task in Long Evans (LE) and Sprague Dawley (SD) rats. Rats were exposed to an AMPH sensitization regimen (15 AMPH or saline injections: 1-5 mg/kg every 2nd day, increasing the dose by 1 mg every 3rd injections) following habituation on the mazes. In experiment 1, LE and SD rats were initially trained on a visual cue discrimination. During the set shift, rats were required to shift from the previously acquired visual-cue-based strategy to a response strategy (e.g.; always turn left, ignore the visual cue). For the reversal, rats were trained to reverse their turn direction. AMPH treatment did not impair learning of the initial cue discrimination in either strain. However, AMPH treated rats learned the response discrimination faster than controls during the set shift and AMPH treated LE rats were faster than controls to reach acquisition criterion during the response reversal. AMPH treatment neither impaired nor improved reversal learning in SD rats. In experiment 2, rats were tested on the SWSh task in which spatial information acquired during a training phase was used 30 minutes later during the testing phase in order to retrieve food pellets on the maze. In this task, AMPH treated rats were faster to re-attain criterion than control rats. Correlational analysis further revealed that AMPH sensitized rats that required more days to reach criterion before AMPH treatment (i.e. slow learners) tended to make more errors during re-acquisition of the memory task. Viewed collectively, these results suggest that chronic AMPH treatment can enhance behavioural flexibility and WM assessed in this manner. However, repeated AMPH exposure may have exacerbated pre-existing cognitive deficits in slow learning rats.

Although the effect of working memory (WM) load on the magnitude of distractor interference has been studied extensively, a common characteristic in prior research is that the target and distractors belong to different objects. The present experiments investigate the effect of WM load on distractor interference when the relevant and irrelevant information is part of the same object. In two experiments, participants saw stimulus displays that consisted of a memory set followed by a Stroop color stimulus. The tasks were to respond to the color of the stimulus first and then to a memory probe. The principal manipulations were the relationship between the color and meaning of the Stroop stimulus (neutral vs. incongruent) and the level of WM load (high vs. low). The results show that WM load had little effect on the magnitude of Stroop interference. These results were consistent with previous research which shows that WM load plays a limited role in the efficiency of selective attention when the extent of attentional focus was held constant across different WM load conditions. They also emphasize the importance of stimulus structure in understanding selective attention in general, and distractor processing in particular.

To achieve goal-orientated behaviour, selective attention is often needed to filter out irrelevant information. Past research has shown that working memory (WM) plays a critical role in selective attention, with high WM load leading to more distractor interference than low WM load. However, because WM load is usually manipulated by requiring participants to hold in memory either one or several digits that were presented simultaneously while performing a selective attention task, the extent of attentional focus was not controlled. The present study examined the effect of WM load on distractor inhibition while keeping attentional focus constant by presenting one digit (low load condition) or six digits (high load condition) sequentially. The participants in the high-load condition demonstrated greater distractor interference than the participants in the low-load condition, suggesting that WM load influences distractor inhibition even when the extent of attentional focus was controlled. This result provides converging evidence to Lavie's (1995, 2005) load theory of attention and cognitive control.

Research has revealed a relation between phonological working memory (PWM) and language abilities, showing that children with language-learning impairments have poorer PWM than their typically-developing peers (Montgomery et al., 2010). Limited work has examined PWM in relation to sentence production abilities in school-aged children. We hypothesized that PWM abilities would predict performance on the Formulated Sentences (FS) subtest of the Clinical Evaluation of Language Fundamentals – IV, of typically-developing and language-learning impaired school-aged children (N=273). Additionally, we hypothesized that the children's PWM abilities would correlate with characteristics of their correct formulated sentences, measured by lexical diversity, length, and complexity (hypothesis 2), and that PWM would correlate with processing fluency, measured by mazes (hypothesis 3) and pauses (hypothesis 4). For our fifth hypothesis, we predicted there would be group differences in PWM and sentence characteristics when comparing the typically-developing children to children with language-learning impairment. We examined these hypotheses with data from a subset of 27 children with language-learning impairment and 27 typically-developing peers. Results support our first and third hypotheses and partially support our fifth hypothesis but not our second or fourth hypotheses. These results suggest that PWM can be used to predict FS scores and that PWM may be more closely related to the types of errors in children’s sentences. Furthermore, results from our fifth hypothesis suggest that deficits in PWM and sentence production may simply co-occur but not interact in children with language-learning impairment.

The purpose of the current investigation was to explore the effects of aphasia on working memory (WM) in bilingual adults. Available research supports the notion that there are strong positive correlations between WM capacity and language function in monolingual adults with aphasia and that aphasic adults' ability to comprehend language may be predicted by WM capacity. The relationship between WM capacity and auditory comprehension, as measured by the Token Test, was investigated in bilingual adults with and without aphasia. Additional areas of investigation included examination of the influence of aphasia on bilingualism and language proficiency as measured by differential performance in both languages on the Boston Naming Test (BNT); relationships between severity of aphasia, as measured by the Bilingual Aphasia Test (BAT); and WM, as measured by listening span. Sixteen participants between the ages of 26 and 91 were included in this study (mean age for women was 61.3; men was 52.8; 37% of the sample population was male, 63% was female). Eight participants were non-aphasic bilingual adults, the remaining 8 participants were bilingual aphasic adults. Results of the study indicated that both groups yielded relatively equivalent findings for the two languages on WM measures. Highly significant and strong positive correlations were observed between WM and auditory comprehension for both groups in both languages. There were no significant differences between English and Spanish results relative to auditory comprehension in the group with aphasia. There was, however, more variability on the BNT for the group with aphasia. Moreover, a significant difference between English and Spanish on the BNT was observed for the non-aphasic group. Significant relationships were found between language proficiency and aphasia severity for both languages; however, no significant differences were found between English and Spanish on the BAT. Nonetheless, moderate to strong positive linear relationships were observed between WM and aphasic severity (BAT) and strong positive relationships were found between language proficiency and aphasia severity for both languages for the group with aphasia. In conclusion, results suggest that the impact of bilingualism on WM for aphasic adults may be similar to what has been observed for monolingual aphasic individuals. Further research is needed relative to the nature of WM in bilingual adults with aphasia.

Chronic exposure to psychostimulants such as amphetamine (AMPH) can induce long-term disruptions in cognition via actions on prefrontal cortex dopamine. Previous work has shown that two types of executive functions, set shifting and working memory (WM), are disrupted by AMPH sensitization and that these cognitive domains are impaired in schizophrenics and stimulant abusers. We assessed the effects of AMPH sensitization on behavioural flexibility using a cross-maze set shifting task and a WM task using the delayed spatial win-shift (SWSh) task in Long Evans (LE) and Sprague Dawley (SD) rats. Rats were exposed to an AMPH sensitization regimen (15 AMPH or saline injections: 1-5 mg/kg every 2nd day, increasing the dose by 1 mg every 3rd injections) following habituation on the mazes. In experiment 1, LE and SD rats were initially trained on a visual cue discrimination. During the set shift, rats were required to shift from the previously acquired visual-cue-based strategy to a response strategy (e.g.; always turn left, ignore the visual cue). For the reversal, rats were trained to reverse their turn direction. AMPH treatment did not impair learning of the initial cue discrimination in either strain. However, AMPH treated rats learned the response discrimination faster than controls during the set shift and AMPH treated LE rats were faster than controls to reach acquisition criterion during the response reversal. AMPH treatment neither impaired nor improved reversal learning in SD rats. In experiment 2, rats were tested on the SWSh task in which spatial information acquired during a training phase was used 30 minutes later during the testing phase in order to retrieve food pellets on the maze. In this task, AMPH treated rats were faster to re-attain criterion than control rats. Correlational analysis further revealed that AMPH sensitized rats that required more days to reach criterion before AMPH treatment (i.e. slow learners) tended to make more errors during re-acquisition of the memory task. Viewed collectively, these results suggest that chronic AMPH treatment can enhance behavioural flexibility and WM assessed in this manner. However, repeated AMPH exposure may have exacerbated pre-existing cognitive deficits in slow learning rats. / Medicine, Faculty of / Graduate

The relationship between life stress and working memory capacity (WMC) has been documented in college students and older adults. It has been proposed that intrusive thoughts about life stress are the mechanism responsible for the impairments seen in WMC. To examine the mechanism responsible for these impairments the current study attempted to induce intrusive thoughts about personal events. The current study allowed for a test of predictions made by two theories of mind wandering regarding the impact of these intrusive thoughts on WMC task performance. One hundred fifty undergraduates were assigned to a control group, positive event group, or negative event group. Participants in the positive and negative event groups completed a short emotional disclosure about an imagined future positive or negative event, respectively, to induce positive or negative intrusive thoughts. WMC measures were completed prior to and following the emotional writing. Results indicated a significant relationship between WMC and mind wandering, however the writing manipulation did not result in any consistent changes in intrusive thoughts or WMC. The results suggest a causal relationship between WMC and mind wandering. The emotional valence of the intrusive thought altered the impact on WMC. No relationship was seen between the measures of stress and WMC. The results of the current study suggest that negative intrusive thoughts result in impaired WMC task performance but other types of off-task thoughts may not result in similar impairments.

Inattentive behavior is considered a core and pervasive feature of ADHD; however, an alternative model challenges this premise and hypothesizes a functional relationship between working memory and inattentive behavior. The current study investigated whether inattentive behavior in children with ADHD is functionally related to domain-general central executive and/or subsidiary storage/rehearsal components of working memory. Objective observations of children's attentive behavior by independent observers were conducted while children with ADHD (n=15) and typically developing children (n=14) completed 10 counterbalanced tasks that differentially manipulated central executive, phonological storage/rehearsal, and visuospatial storage/rehearsal demands. Results of latent variable and effect size confidence interval analyses revealed two conditions that completely accounted for the attentive behavior deficits in children with ADHD: (a) placing demands on central executive processing, the effe

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