These experiments explore the role of proactive interference in short-term memory using a probe recognition task (Monsell, 1974). Experiment 1 indicated that such tasks were particularly difficult for certain patients. Experiment 2 employed negative probes that were phonologically or semantically related to list items in the same or previous list. Subjects included patients with semantic and phonological short-term memory deficits, older adults and undergraduate subjects. Healthy subjects showed significant interference effects on both phonologically and semantically related probes, but only for probes related to items in the same list. A patient with a semantic short-term memory deficit showed interference effects for both phonologically and semantically related probes in both same and previous list conditions. A patient with a phonological short-term memory deficit showed no such effect. Results are discussed in context of theoretical implications for short-term memory models proposing distinctions between phonological and semantic capacities.

Attention deficit hyperactivity disorder (ADHD) is a prevalent neurobehavioral disorder that affects individuals in early childhood and is characterized by cognitive impairments associated with executive functioning. However, the exact nature of the impairment(s) is/are unclear. In a recent meta-analysis, Miyake, Friedman, Emerson, Witzki, Howerter, and Wager (2000) demonstrated that there are at least three separable frontal executive functions: set switching, working memory updating, and response inhibition. Here we used event-related potentials (ERPs) to investigate whether the cognitive impairments seen in ADHD are specific to one of these executive functions or rather represent a global executive functioning deficiency. The current studies examined the time course and scalp localization of executive functions in Combined Type ADHD and in comparison control children by implementing modified versions of three executive tasks used by Miyake et al. (2000), local-global, tone-monitoring, and Stroop; versions more appropriate for the ADHD population. An additional non-executive task (visual oddball) was included in order to demonstrate that the deficits in ADHD are specific to executive functioning. The goal was to determine if children with ADHD perform differently on these three executive tasks and if this difference can be attributed to a deficit associated with one or more of the executive functions. The current findings show that ADHD impacts only a subset of cognitive operations in the executive functions, leaving the other ERPs in the executive tasks and the visual oddball unaffected. Specifically, the ADHD group was impaired on tasks involving allocation of attention and response inhibition, the cognitive operations most closely related to the diagnostic criteria for this subtype of ADHD. These findings may extend our knowledge of the time course and localization of executive functions and provide a tool for studying the nature of disrupted executive functioning in ADHD.

Behavioral studies of decision-making in impulsive individuals demonstrate biases for immediate rewards. Decision-making includes evaluating motivational values of both options and actions. In the current studies, event related potentials (ERPs) and functional magnetic imaging (fMRI) were used to assess the temporal and spatial properties of the ventral tegmental (VTA) dopamine (DA) reward system during item evaluation and action/outcome-monitoring in impulsivity. A passive reward evaluation and an active action/outcome-monitoring task were used to test a model predicting that dysregulation of the VTA DA reward system leads to overestimations of reward value in the prefrontal cortex and underestimations of punishments in the anterior cingulate cortex (ACC) during reward evaluation and action/outcome-monitoring respectively. Passive reward evaluation ERP results showed a prefrontal P2a ERP index of reward expectation that did not differ with impulsivity. FMRI reward evaluation results indicated that the basal ganglia and left superior frontal cortex were sensitive to rewarding outcomes, whereas the medial ACC was sensitive to non-rewarding outcomes. Left superior frontal cortex fMRI activation showed greater differentiation between better than expected and worse than expected outcomes among high impulsive participants. Action/outcome-monitoring ERP results showed an ERN ERP index the ACC in the action/outcome-monitoring with the greatest response on error trials that led to punishments. The ERN was also larger among high impulsives, compared to the low impulsives, when errors led to missed rewards. The fMRI results showed that the basal ganglia responded like the ERN to errors resulting in punishments. The ACC and right middle frontal regions showed greater activation to correct responses, and a right ACC region showed larger activations when correct responses led to rewards. However, these activations did not differ with impulsivity. In conclusion, the current results suggest differential responsiveness of the mesotelencephalic reward system in impulsive individuals which may lead to reward hypersensitivity but not punishment hyposensitivity during item evaluation and action/outcome-monitoring among high impulsive participants, as seen in the left superior frontal fMRI activation during item evaluation and in the ACC ERP activity during outcome-monitoring. Differences between the ERP and fMRI results may reflect the different physiological processes that give rise to the signals.

This work is concerned with examining in a formal quantitative manner what human observers look at and what the objects of their gaze tell them. Three models designed to describe and predict the allocation of human attention in supervisory control tasks were investigated. A series of three experiments examined the relative influence of five factors on the sampling patterns of participants: the information generation rate of the information signal (bandwidth), the frequency of significant, i.e., task relevant, events on an information source (alarm frequency), the payoff matrix associated with missing or detecting critical events (value), the visual salience of the events, and the cost of making an observation. The paradigm employed is similar to that developed by Senders and colleagues (1964), in which observers were asked to monitor an array of four simulated ammeters and to press a button whenever the pointer of any ammeter entered an "alarm zone." Aspects of three mathematical models, Senders's constrained random sampler, Wickens and colleagues SEEV model, and Pirolli's and Card's Information Foraging Theory Model, were combined to form seven different models predicting performance in the task. The sampling patterns predicted by each model were compared against the eye movement data of participants. Results of the three experiments indicate that participants' sampling patterns were sensitive to the experimental manipulations. Comparisons of the model predicted patterns of attention allocation to those in the participant data indicated that different models described different participants. Participants who performed poorly at the task were best described by models incorporating bandwidth. Participants who performed well at the task were best described by models incorporating alarm frequency, and those who performed best at the task were not well-described by any of the models. Overall the models based on Information Foraging Theory were the most robust in predicting the attention allocation patterns of participants. Implications of each of the experimental manipulations and of the fit of the models to the participant data are discussed.

Eight experiments investigated the role of semantic and associative relatedness and short-term memory in speech planning. Subjects produced a conjoined noun phrase corresponding to picture pairs that were semantically related only, semantically and associatively related, and associatively related only. Interference effects, i.e., longer onset latencies were observed for neurologically intact undergraduates and elderly controls in both semantic conditions but were not apparent in the associative only condition. Interference effects only occurred under the following conditions: (1) The degree of semantic relatedness was high between picture pairs. (2) Each picture was presented several times. (3) Related and unrelated matched pairs occurred in the same block and (4) Several pairs appeared from the same category. A phonological STM patient showed interference effects for semantically related pairs within the range of elderly controls. However, a semantic STM patient showed mixed results across several experiments. His inconsistent performance pattern was attributed, in part, to long term priming effects resulting from repeated testing with the same materials. Overall, the results suggest that the lexical-semantic information for both pictures was planned prior to production, and that semantic and associative relatedness differentially affect planning in speech production.

Studies of hyperspecific transfer of processing in indirect memory tests are reviewed. A procedure for deriving a comprehensive assessment of priming of indirect memory is then proposed. The procedure is illustrated in Experiment 1, in which prior study of randomly selected words presented with no item-specific context and what could be construed as neutral instructions primed their identification more in a perceptual (fragment completion) task than in a conceptual (semantic cuing) task. Experiments 2 and 3 failed to provide evidence for hyperspecific (i.e., sublexical) transfer of processing in an indirect memory task that called for rapid identification of gradually presented words. Experiment 4 also failed to provide evidence of hyperspecific transfer of processing, despite following more closely the procedure of an experiment (Hayman & Tulving, 1989, Experiment 4) that has provided such evidence. It appears that hyperspecific transfer of processing may be more elusive than sometimes assumed.

The levels-of-processing effect is extraordinarily, even puzzlingly, robust. It occurs even when a memory test is expected, ample study time is given, and deep processing is encouraged regardless of orienting task. Thus, processing appears to be "fixed" by the requirements of the orienting task. This enigma is explored in a selective remembering procedure involving the recall of words of arbitrarily varying values. After verifying that selectivity is substantially localized at the encoding rather than the recall stage of the remembering process (Experiment 1), recall was found to be selective despite the imposition of an orienting task and regardless of whether item values were assigned according to the items' semantic category (Experiment 2) or entirely at random (Experiment 3). Indeed, no evidence was found for any effect at all of orienting tasks on selectivity (Experiment 4). Orienting tasks do not, after all, universally dominate the encoding process.

A series of three experiments were designed to test for Emergent Features, which, in Gestalt psychology, are indicative of grouping. By using basic elements such as dots or lines and building EFs sequentially, while maintaining the original discrimination constant, it was possible to compare the strength of different EFs. Proximity, Orientation, Symmetry, Linearity, Surroundedness, Inside/Outside relationship, Collinearity, Parallelism, Inflection Point and Closure were tested. A Configural Superiority Effect (CSE, where the RT in a composite condition is shorter than the RT in a singleton condition) was diagnostic of the presence of an EF in an Odd Quadrant task. Proximity, Orientation, Linearity, Inside/Outside and Collinearity produced CSEs and thus behave as basic in human vision. Mixed results were obtained for Symmetry and Parallelism. Further, Inflection Point and Closure were only suggested in the stimuli, so they cannot be ruled out as EFs.

Visual stability refers to the apparent stability of the visual world given the displacement of retinal images induced by eye movements. Phenomenally visual stability involves both a stable representation of visual space and reduced sensitivities to perceptual changes at the temporal proximity of eye movements. While the psychophysics of the perisaccadic perceptual changes have been studied extensively, how visual stability is implemented in the human visual system remains to be explored. This dissertation examines the cortical mechanisms of perceptual stability in spatial vision with four series of experiments. Series 1 established a paradigm to induce saccadic suppression of displacement (SSD) and examined how the direction of saccades and displacements influence the strength of SSD. Series 2 examined the consequence of disrupting the posterior parietal cortex (PPC) with transcranial magnetic stimulation (TMS) in perceiving perisaccadic displacements. Series 3 examined psychophysical factors influencing perisaccadic mislocalization. Finally, series 4 explored how TMS on PPC impacts perisaccadic mislocalization. These experiments conjointly illustrate how the PPC contributes to a stable visuospatial perception during saccades.

This investigation is concerned with the control and organization of the "psychological present," or primary memory, and specifically with the implications of such control and organization for the suffix effect. The suffix effect arises when a nominally irrelevant speech item, or "suffix," is appended to a spoken sequence of items, and it consists of an impairment in the recall of the most recent items in the sequence, especially the last item (e.g., Crowder, 1967; Dallett, 1965). The dominant explanation of the suffix effect has been in terms of "bottom-up" masking in general (e.g., Nairne, 1990) and precategorical acoustic masking in particular (e.g., Crowder & Morton, 1969; Crowder, 1978, 1983; Greene & Crowder, 1984). The current version of this explanation is "two-component" theory, wherein the precategorical masking explanation is confined to the terminal component of the suffix effect (i.e., at the last position of the sequence), with the preterminal component being open to influences of top-down or conceptually-based interpretation and strategy (see Greene, 1992 for a review). Reported here are 12 experiments, each of which provides evidence inconsistent with two-component theory. Experiments 1--4 failed to replicate the principal findings proffered in support of the theory; Experiments 5--11 extended some of the findings of the first four experiments by showing additional evidence of postcategorical influences on the terminal suffix effect; and Experiment 12 demonstrated a suffix effect with static visual presentation. These findings, and indeed those in the suffix effect literature in general, are interpreted along the lines of the now largely ignored perceptual grouping account proposed by Kahneman (1973; Kahneman & Henik, 1981).