The processing of information within the visual system is limited by several cognitive and neural bottlenecks. One critical bottleneck occurs in visual working memory (VWM), as the amount of information that can be maintained on-line is limited to three to four items. While numerous theories have addressed this limited capacity of VWM, it is unclear how processing bottlenecks in the initial selection and perception of visual information affect the number or precision of representations that can be maintained in VWM. The purpose of this dissertation was to examine whether early competition for resources within the visual system limits the number or precision of representation that can be maintained in VWM. To establish whether competitive interactions affect VWM, Chapters 1 – 4 tested whether performance on VWM tasks was related to the distance between memory items. The results of these experiments reveal that when objects are presented close together in space, VWM performance is impaired relative to when those same objects are presented further apart. Using a three-component model of continuous responses in a recall task, Chapters 3 – 4 demonstrated that the distance between objects primarily affects the precision of responses, and increases the number of non-target errors. Chapter 5 extended these findings to distractors, demonstrating that multiple distractors affect the precision and accuracy of VWM responses. Chapters 6 – 7 tested how attentional selection can bias memory representations, revealing that objects that are given high attentional priority were reported with greater precision. Finally, Chapters 8 and 9 examined bias-signals as a potential source of individual differences in VWM performance, revealing that high-performers have more precise representations of sub-capacity representations than low-performers. Together, these results reveal that VWM performance is limited by competition for representation within the visual system, and that attention plays a critical role in resolving competition and consequently, determining the contents of VWM.

The research focuses on the role of top-down influences on selective attention across the attentional visual field. The attentional visual field is the subset of the visual field in which attentional processes take place. The size of the attentional visual field is relatively large compared to the areas considered by most empirical studies of visual attention to date. Three possible forms of top-down influence are examined: 1) the expectation of the size of the area in which the target is likely to occur; 2) the expectation of the direction in which the target is likely to occur; and 3) existing unconscious bias in the spatial distribution of attention. Results from Experiment 1 suggest that participants modify the size of the attended area according to their expectation of the location of the target. Experiment 2 demonstrates that focus of attention can be oriented toward the expected target direction. Experiment 3 reveals that, even when no conscious control is involved, the distribution of attention is biased toward certain areas. Theoretical considerations are discussed, including the introduction of a simple statistical model to assist in conceptualizing the modifications of the distribution of attention over the attentional visual field. Practical applications of the results are also discussed.

It is often difficult for people to understand speech when there are other ongoing conversations in the background. This dissertation investigates how different background maskers interfere with our ability to comprehend speech and the reasons why older listeners have more difficulties than younger listeners in these tasks. An ecologically valid approach was applied: instead of words or short sentences, participants were presented with two fairly lengthy lectures simultaneously, and their task was to listen to the target lecture, and ignore the competing one. Afterwards, they answered questions regarding the target lecture. Experiment 1 found that both normal-hearing and hearing-impaired older adults performed poorer than younger adults when everyone was tested in identical listening situations. However, when the listening situation was individually adjusted to compensate for age-related differences in the ability to recognize individual words in noise, age-related difference in comprehension disappeared. Experiment 2 compared the masking effects of a single-talker competing lecture to a babble of 12 voices, and the signal-to-noise ratio (SNR) was manipulated so that the masker was either of similar volume as the target, or much louder. The results showed that the competing speech was much more distracting than babble. Moreover, increasing the masker level negatively affected speech comprehension only when the masker was babble; when it was a single-talker lecture, the performance plateaued as the SNR decreased from -2 to -12 dB. Experiment 3 compared the effects of semantic content and acoustic similarity on speech comprehension by comparing a normal speech masker with a time-reversed one (to examine the effect of semantic content) and a normal speech masker with an 8-band vocoded speech (to examine the effect of acoustic similarity). The results showed that both semantic content and acoustic similarity contributed to informational masking, but the latter seemed to play a bigger role than the former. Together, the results indicated that older adults’ speech comprehension difficulties with maskers were mainly due to declines in their hearing capacities rather than their cognitive functions. The acoustic similarity between the target and competing speech may be the main reason for informational masking, with semantic interference playing a secondary role.

This dissertation investigated different factors contributing to age differences in working memory (WM) performance. Younger and older adults participated in five experiments, four on visuospatial WM (VSWM) and one on verbal WM. All addressed methodological issues that may differentially lower older adults’ performance. Experiments 1a and 1b manipulated the administration of a VSWM span task, with participants performing the task under either an ascending format (shortest sets presented first), or an interference-reducing descending format (longest sets presented first). Older adults’ performed significantly better in the descending compared to ascending format, consistent with an age-related susceptibility to proactive interference (PI). By contrast, younger adults did better in the ascending compared to descending condition, possibly due to their ability to benefit from practice and strategy use when easier trials are presented first. Experiment 2 considered how the similarity of task material influenced the build-up of PI and whether or not the combination of two PI-reducing manipulations (i.e., descending format and distinct trials) would further improve older adults’ performance. Distinctiveness helped older adults on the ascending format; however, combining distinct trials and a descending format provided no additional benefit. Experiment 3 considered whether or not synchronizing a VSWM task with an individual’s circadian arousal pattern would moderate interference effects, with the task administered at a peak or off-peak time of day. Peak-time administration improved older adults’ performance on the descending, but not the ascending, condition. Experiment 4 investigated the possibility that the serial order requirement of many WM tasks contributes to age differences in performance. Younger and older adults participated in a verbal WM span task – Operation Span - under either serial order or free recall instructions. Typical age differences were found when order but not free recall was required. Further analyses of the order condition data revealed that older adults were, in fact, recalling the items just as well as young adults, only not in the correct order. Taken together, the findings strongly suggest that age differences found on typical WM span tasks are influenced by numerous factors, such as task presentation, individual circadian arousal patterns, material similarity, and recall instructions.

Listening to speech with competing speech in the background is challenging and becomes harder with age. Three experiments examined the auditory and cognitive aspects of auditory spatial attention in conditions in which the location of the target was uncertain. In all experiments, word identification was measured for target sentences presented with two competitor sentences. On each trial, the three sentences were presented with one from each of three spatially separated loudspeakers. A priori cues specified the location and identity callsign of the target. In Experiments I and II, sentences were also presented in conditions of simulated spatial separation achieved with the precedence effect. Participants were younger and older adults with normal hearing sensitivity below 4 kHz. For both age groups, the contributions of richer acoustic cues (those present when there was real spatial separation, but absent when there was simulated spatial separation) were most pronounced when the target occurred at “unlikely” spatial listening locations, suggesting that both age groups benefit similarly from richer acoustical cues. In Experiment II, the effect of time between the callsign cue and target word on word identification was investigated. Four timing conditions were tested: the original sentences (which contained about 300 ms of filler speech between the callsign cue and the onset of the target words), or modified sentences with silent pauses of 0, 150, or 300 ms replacing the filler speech. For targets presented from unlikely locations, word identification was better for all listeners when there was more time between the callsign cue and key words, suggesting that time is needed to switch spatial attention. In Experiment III, the effects of single and multiple switches of attention were investigated. The key finding was that, whereas both age groups performed similarly in conditions requiring a single switch of attention, the performance of older, but not younger listeners, was reduced when multiple switches of spatial attention were required. This finding suggests that difficulties disengaging attention may contribute to the listening difficulties of older adults. In conclusion, cognitive and auditory factors contributing to auditory spatial attention appear to operate similarly for all listeners in relatively simple situations, and age-related differences are observed in more complex situations.

Competing speech seems to pose a greater challenge to spoken language comprehension than does competing noise, especially for older adults. The difficulties of older adults may be due to declines in auditory and cognitive processing. However, evidence suggests that the use of top-down information processing to overcome this interference may be preserved in aging. This research investigated the effect of speech- and noise masking on language comprehension, as well as age-related differences in the use of top-down processing to overcome masking. Topic I examined whether younger and older adults gain the same release from masking given a partial preview of a target sentence in quiet (auditory prime) prior to hearing the full sentence in noise, and investigated the auditory factors contributing to the advantage provided by the primes. Results showed that despite age-related declines in overall performance, younger and older listeners benefited similarly from priming. This benefit was not attributable to cues about the target talker’s voice or fluctuations in the amplitude envelope of the target sentences. Topics II and III examined the effect of speech- and noise masking on the time-course of stream segregation. The analyses revealed that stream segregation takes time to build up when a speech target is masked by other speech, but not when it is masked by noise. Subsequent analyses showed that in younger adults, the delay in segregation under speech masking was primarily due to the vocal similarities between the talkers, with interference from the semantic content of the masker playing a secondary role in impeding performance. The results also showed that older listeners were less efficient than younger listeners in segregating speech from speech-like maskers. Furthermore, older listeners benefited less than younger listeners when the amplitude envelope modulations of maskers were limited. Overall, the findings indicate that some of the language comprehension difficulties experienced by older listeners in noisy environments may be due to age-related declines in stream segregation and a decreased ability to benefit from fluctuations in the amplitude envelopes of maskers. However, benefit from priming may help offset some of these age-related declines in auditory scene analysis.

There is growing evidence that young adult readers frequently fail to create exhaustive textbased representations as they read. Although there has been a significant amount of research devoted to age-related effects on text processing, there has been little research concerning this so-called shallow processing by older readers. This dissertation uses eye tracking to explore age-related effects in shallow processing across different levels of text representations. Experiment 1 investigated shallow processing by older readers at the textbase level by inserting semantic anomalies into passages read by participants. Older readers frequently failed to report the anomalies, but no more frequently than did younger readers. The eye-fixation behaviour revealed that older readers detected some of the anomalies sooner than did younger readers, but had to allocate disproportionately more processing resources to looking back to the anomalies to achieve comparable levels of detection success as their younger counterparts. Experiment 2 examined age-related effects of shallow processing at the surface form by inserting syntactic anomalies into passages read by older and younger adults. Older readers were less likely to detect syntactic anomalies when first encountering them relative to younger readers and engaged in increased regressive fixations to the anomalies. However, older readers with high iii reading comprehension skill were able to use their familiarity with text content to increase their likelihood of syntactic anomaly detection. Experiment 3 investigated the role of aging on shallow processing of the temporal dimension of the situation model. No age-related differences reporting the anomalies were found. The eye-fixation behaviour revealed that older readers with high working memory capacity detected some anomalies sooner than did younger readers; however, they had to allocate increased processing resources looking back to the anomalies to achieve comparable levels of detection as younger readers. Together, the results demonstrate that older readers are susceptible to shallow processing, but no more so than younger readers when they can rely on their linguistic skill or their existing knowledge to help reduce processing demands. However, older readers appear to require additional processing time to achieve comparable levels of anomaly detection as younger readers.

In four experiments on insight problem solving, we investigated the time course of the development of solution knowledge prior to response, as well as the impact of stimulus familiarity on task performance and eye movement measures. In each experiment, participants solved anagram problems while their eye movements were monitored. In Experiments 1a and 1b, each anagram problem consisted of a circular array of letters: a scrambled four-letter solution word containing three consonants and one vowel, and an additional randomly-placed distractor consonant. Viewing times on the distractor consonant compared to the solution consonants provided an online measure of knowledge of the solution. Viewing times on the distractor consonant and the solution consonants were indistinguishable early in the trial. In contrast, several seconds prior to the response, viewing times on the distractor consonant decreased in a gradual manner compared to viewing times on the solution consonants. Importantly, this pattern was obtained across both trials in which participants reported the subjective experience of insight and trials in which they did not. These findings are consistent with the availability of partial knowledge of the solution prior to such information being accessible to subjective phenomenal awareness. In Experiments 2 and 3, each anagram problem consisted of a centrally located three-letter string plus three additional individual letters located above and to the side of the central letter string. All the letters in the central letter string were members of the five-letter solution word, while one of the individual letters was a randomly placed distractor. In Experiment 2, we replicated our findings of the gradual development of solution knowledge using this more complex stimulus display. In Experiment 3, we manipulated the familiarity of the central letter string by presenting it either in the form of a three-letter word, or as a meaningless string of letters. Behavioural measures showed an overall negative impact of familiarity on task performance, while eye movement measures revealed a more complex pattern of effects, including both interference and facilitation. Critically, the effects of familiarity on problem solving did not interact with the development of solution knowledge prior to response.

New neurons are continually produced throughout adult life in the dentate gyrus of the hippocampus, in a process termed adult neurogenesis. Although there is a significant effort in the literature to understand the functional significance of hippocampal neurogenesis, conflicting experimental reports have left the role of neurogenesis unclear. Recently, computational modelling studies have hypothesized that neurogenesis may play a role in allowing association between event and context to be formed in memory. By using a novel odour task and a raised plus maze task, our work demonstrates that the reduction of hippocampal neurogenesis using focal irradiation impairs the ability of animal subjects to utilize contextual information to learn interfering information. The result of this work provides experimental evidence of a unique role neurogenesis may play in learning and memory.

When the eye rotates, switching from one fixation point to another, the perception of motion is strongly suppressed and rarely perceived. During these quick ‘saccadic’ eye movements, other aspects of visual perception become suppressed or compressed as well, with certain effects being stronger or weaker along the plane of the saccade - such differences can help identify the underlying neuronal pathways, since some exhibit directional tuning (e.g. neurons projecting from primate V1 to middle temporal area (MT)), and others do not (e.g. relay neurons linking the superior colliculus to area MT). A briefly presented motion probe was placed at a number of points relative to saccade to plot sensitivity to motion along different planes and directions. The results suggest that saccadic motion is suppressed before the eye begins to move, and is applied evenly across planes and directions.