Design Modifications from Pilot Study

The experience of running the pilot study, and subsequent feedback from colleagues, informed several changes to the procedure of the five main experiments in this study. First, multiple participants in the pilot study struggled with fatigue during the 64-trial visual task. Because the protocol for the main study used similar tasks over a much longer span of time, all five of the experimental tasks in the main study were broken up into four blocks separated by short breaks. At each break, text appeared on the computer monitor indicating the percentage of the task the participant had completed (25%, 50%, or 75%) and encouraging participants to take a moment to collect themselves and engage in physical activity or conversation before continuing if they felt it would be helpful.

Several other modifications were made to help participants adjust to the testing environment, as well as minimize stress and confusion. First, a task was added at the beginning of the Morning testing session for the sole purpose of familiarizing participants with the format of the computerized visual tasks. During this acclimation task, participants were asked to make the same sort of same/different judgments that they would be asked to make for all of the experimental tasks, but the stimuli that participants were asked to distinguish were photographs of families and landscapes that were highly discriminable. Participants used the same key mapping during the acclimation task as they would for all the experimental tasks throughout the day (“m” for same and “z” for different, or vice versa) in the hope that this would become habituated and minimize the attention required for proper responding (see section 3.4 for a full description of the procedure for the acclimation task). All participants completed the acclimation task irrespective of their experimental group. To further minimize stress and confusion in responding to the experimental tasks, the appropriate key mapping was displayed at the top of the screen every time a participant was asked to make a response. This was the case for both the

acclimation task and all experimental tasks.

For the main study, the short form of the Geriatric Depression Scale replaced the Beck

Depression Inventory as an indicator of depression in participants. While both scales are in common use and scores on both are strongly correlated, the Geriatric Depression Scale was designed specifically for individuals in the age range targeted for the current study, and there is evidence that the Geriatric Depression Scale is easier for older adults to complete (Olin et al., 1992). In addition, some research indicates that the Geriatric Depression Scale produces more accurate responses particularly in older women owing to its omission of questions related to sexual interest (Jefferson, Powers, & Pope, 2008).

Part of the goal for the main study was to determine what impaired perceptual process or processes was/were responsible for the relatively poor performance by individuals with PD on the pilot task. While previous research has indicated individuals with PD may display impairments on complex

visual discriminations and VWM of the type required to complete the pilot task (see section 1.5), the results of the pilot study could also be explained by individuals with PD having deficits in more fundamental visual processes, such as poor contrast sensitivity, or simply poor visual acuity. As such, Experiment 1 consisted of a line orientation discrimination task to assess the extent to which processes other than complex object discrimination might have caused the deficits observed in the pilot task. Additionally, a requirement has been added that all participants across groups must have been judged to have normal or corrected-to-normal visual acuity as determined by an eye appointment within the preceding year.

As noted previously, the rotation manipulation in the pilot study failed to produce any effect on task performance either for individuals with PD or their healthy counterparts. However, multiple other studies have documented impairments of mental rotation in individuals with PD. Examining the ability of individuals with PD to perform mental rotation, and the effect of levodopa on this process, therefore necessitated employing a different 3D manipulation. To this end, Experiment 5 is largely a replication of Lee et al. (1998), with the exceptions that a) the stimulus set used is one more recently developed by Ganis and Kievit (2015) and b) individuals with PD were tested in both ON-meds and OFF-meds states.

Finally, there is a body of research indicating that human vision is especially well-adapted for symmetry detection, so much so that this process may occur rapidly at a preattentional level (see Wagemans, 1997 for a review). Experiments 2a, 2b and 3 of the main study involve the simultaneous presentation of complex visual stimuli and ask participants to judge whether they are the same or different, and there is thus a risk that participants’ performance will be driven by preattentional processing if “same” stimuli presented side-by-side create a symmetrical display, while “different” stimuli do not. It should be noted that the stimuli used in these tasks are such that there is minimal risk of identical objects producing a symmetrical visual display: the face stimuli from Experiment 2b are adapted from real photographic images and unlikely to be perfectly symmetrical themselves, and

consequently unlikely to create a perfectly symmetrical visual display when placed side-by-side on the screen. Accidental display symmetry is even less likely for the abstract objects used in Experiments 2a and Experiment 3, since the methods for creating these stimuli tend to yield objects that are themselves highly asymmetrical. However, because the stimulus sets for Experiments 2a and 2b had not been tested in other studies and the author desired to show that performance on these tasks was a result of the visual processing of objects and not a heuristic judgment of symmetry, the stimuli presented on each trial of these two tasks were slightly offset from one another in the vertical direction. This should have disrupted any accidental display symmetry that might have resulted from the simultaneous presentation of two identical objects.

3.3 Participants

Given the efficacy of levodopa in treating a variety of PD symptoms, it is not uncommon to find studies testing the effects of drug modulation that use patient samples of fewer than 10 individuals (Lee et al., 1998; Stegemöller Simuni, & MacKinnon, 2009). It is also notable that significant between-subjects effects were observed in the pilot study using a sample of only six individuals with PD and five healthy older adults. To add a margin of safety, it was decided that the study would target 15 healthy older adults and 15 older adults who have received a clinical diagnosis of PD.

To be eligible for participation in this study, all individuals were required to be between 60 and 80 years of age and to have either normal or corrected-to-normal vision as determined by an eye appointment within the last 12 months. Additionally, individuals in the PD condition were required to have received a formal clinical diagnosis of the condition. The latter were further required to be currently taking levodopa as part of their treatment regimen, and their dosage must have been stable for at least the last 30 days. Accordingly, the researcher recruited 15 individuals for the PD condition. One of these individuals mentioned during the course of the study that she was receiving deep-brain stimulation as part of her treatment regimen, and her data have not been included in the analysis,

leaving 14 in the PD condition. Subsequently, 14 healthy individuals who were age (+/- 3 years) and gender matched to individuals in the PD group were recruited. Thus, the final sample included 14 individuals with a diagnosis of PD (Age: M = 71.1, S.D. = 5.8; 5 Female, 9 Male) and 14 healthy individuals (Age: M = 70.1, S.D. = 4.9; 5 Female, 9 Male). The two groups did not differ significantly in age (p = .81) or years of education (p = .51).

Individuals in the PD group were recruited in part from a laboratory database of known volunteers and in part from reaching out to a number of support groups across the state of Iowa. Individuals in the HOA group were recruited in part from the same laboratory database and in part from general community outreach.