Visual Object Discrimination Impairment as an Early Predictor of Mild Cognitive Impairment and Alzheimer's Disease

Abstract

Objective: Detection of cognitive impairment suggestive of risk for Alzheimer's disease (AD) progression is crucial to the prevention of incipient dementia. This study was performed to determine if performance on a novel object discrimination task improved identification of earlier deficits in older adults at risk for AD.

Method: In total, 135 participants from the 1Florida Alzheimer's Disease Research Center [cognitively normal (CN), Pre-mild cognitive impairment (PreMCI), amnestic mild cognitive impairment (aMCI), and dementia] completed a test of object discrimination and traditional memory measures in the context of a larger neuropsychological and clinical evaluation.

Results: The Object Recognition and Discrimination Task (ORDT) revealed significant differences between the PreMCI, aMCI, and dementia groups versus CN individuals. Moreover, relative risk of being classified as PreMCI rather than CN increased as an inverse function of ORDT score.

Discussion: Overall, the obtained results suggest that a novel object discrimination task improves the detection of very early AD-related cognitive impairment, increasing the window for therapeutic intervention. (JINS, 2019, 25, 688-698).

Keywords: Dementia; early diagnosis; memory; perirhinal cortex; temporal lobe; visual perception.

Figure I:. Representation of Standard Trials of the Object Recognition and Discrimination Task.

On each trial, participants were shown a horizontal array of four stimuli (Non-Objects, Shapes, Colors, Animals). Non-objects were photographs of models created from building blocks, resulting in entities that were not found in the real world. Task conditions included object type and difficulty level (Easy, Difficult). In the “easy” condition, the three non-target items were all presented in the same orientation. In the “difficult” condition, the three non-target items are rotated individually (horizontally or vertically) in picture plane. In this way, success on the “difficult” condition requires the participant to recognize that the three non-target items are the same entity even though their appearance is changed by rotation.

Figure II:. Graphical Representation of Performance of Clinical Diagnostic Subgroups Compared to the Cognitively Normal Group on Traditional Memory Measures and the Object Recognition and Discrimination Task.

There were statistically significant differences between diagnostic subgroups in performance on both traditional memory measures, and also the ORDT-DO (HVLT-R: F_[3,131] = 28.21, p < 0.001, ηp2 = 0.40; NACC-DR: F_[3,133] = 25.95, p < 0.001, ηp2 = 0.37; ORDT-DO: F_[3,134] = 8.74, p < 0.001, ηp2 = 0.17). Pairwise comparisons with Bonferroni correction revealed group mean differences between the CN and the aMCI and dementia groups [HVLT-R (CN: M = 7.35, SD = 3.06; aMCI: M = 1.96, SD = 2.81; Dementia: M = 0.31, SD = 1.25); NACC-DR (CN: M = 11.43, SD = 3.80; aMCI: M = 7.40, SD = 4.07; Dementia: M = 2.53, SD = 3.54); ORDT-DO (CN: M = 86.5, SD = 8.04; aMCI: M = 68.03, SD = 17.28; Dementia: M = 60.63, SD = 17.02)]. Importantly, when an independent samples t-test was performed comparing the CN and PreMCI groups, only the ORDT-DO revealed significant group mean difference in test scores (t_[23.93] = 2.18, p = 0.04, Cohen’s d = 0.61). The traditional neuropsychological measures failed to discriminate between these two groups, as would be expected based on diagnostic criteria (NACC-DR: t_[41] = 0.43, p = 0.93; HVLT-R: t_[28.29] = 1.33, p = 0.20). CN = Cognitively Normal; PreMCI = pre- Mild Cognitive Impairment; aMCI = amnestic Mild Cognitive Impairment; NACC Delayed Paragraph Recall (NACC-DR); Hopkins Verbal Learning Test-Revised (HVLT-R); Object Recognition and Discrimination Task Difficult Objects Trials (ORDT-DO). NOTE: Standard non-transformed scores compared to the cognitively normal group are presented, error bars equivalent to standard error of the mean.