Exploring Neural Efficiency in Multiple Sclerosis Patients during the Symbol Digit Modalities Test: A Functional Magnetic Resonance Imaging Study

Abstract

Background: Reduced information-processing speed (IPS) is a primary cognitive deficit of multiple sclerosis (MS) patients. The neural efficiency hypothesis describes an inverse relationship between cognitive performance in a task and the amount of cognitive resources devoted to it. Previous studies have shown that the neural efficiency hypothesis provides an appropriate framework to explore cognitive dysfunction in neurological patients. Objective: The aim of this study was to explore the neural efficiency hypothesis regarding IPS capabilities in cognitively preserved MS patients. Methods: 16 MS patients and 17 healthy controls (HCs) were enrolled and neuropsychologically assessed. All participants also performed a functional magnetic resonance imaging (fMRI)-adapted version of the Symbol Digit Modalities Test (SDMT) at different interstimulus intervals (ISI: 1.5, 2, and 2.5 s). Results: MS patients only displayed lower SDMT performance when the ISI was set at 1.5 s. However, MS patients' normal SDMT performance at larger ISIs was achieved at the cost of increased brain activation, hence revealing that they were less cognitively efficient than the HCs. Regression analyses confirmed this conclusion by showing an opposite relationship between SDMT performance and the amount of neural resources recruited in the HC and MS groups. Thus, while a positive relationship between both variables was observed in MS patients, this correlation was negative for the HC group. Conclusions: MS patients require more cognitive resources than HCs to achieve a normal SDMT performance, then revealing that they are less efficient regarding IPS capabilities.

Repeated Working Memory Training Improves Task Performance and Neural Efficiency in Multiple Sclerosis Patients and Healthy Controls

“…The concept of neural efficiency refers to achieving maximum performance levels while deploying a minimum amount of brain resources, and it has received experimental support from previous studies involving healthy volunteers [20, 30, 31] and MS patients [32]. The training program used in the present study increased both task performance and neural efficiency.…”

“…Conversely, the previous studies referred to above recruited patients with more advanced MS who exhibited moderate cognitive impairment. In these mildly impaired MS patients, increased cognitive performance is expected to be achieved only if additional brain resources can be recruited [32, 38, 40], which is exactly what was observed after they received neuropsychological training [8, 15, 33, 34, 36, 41].…”

“…Indeed, when this variable and the inverted-U relationship between brain activation and cognitive performance observed throughout MS progression [37, 38] are taken into account, all these results seem to conform a coherent picture. Thus, it should be noted that we recruited a homogeneous group of MS patients with the same clinical phenotype (RR) and no cognitive impairment and, even more importantly, we chose MS patients who did not achieve this normal cognitive performance through the early spontaneous adaptive neuroplasticity processes (e.g., increased brain activation) described in previous studies [4, 32, 39]. Therefore, the patients recruited in our study were very similar to HC, and the expected effects of n-back training should also be the same as in HC, namely, an increase in performance and neural efficiency [20, 32, 38].…”

“…Thus, it should be noted that we recruited a homogeneous group of MS patients with the same clinical phenotype (RR) and no cognitive impairment and, even more importantly, we chose MS patients who did not achieve this normal cognitive performance through the early spontaneous adaptive neuroplasticity processes (e.g., increased brain activation) described in previous studies [4, 32, 39]. Therefore, the patients recruited in our study were very similar to HC, and the expected effects of n-back training should also be the same as in HC, namely, an increase in performance and neural efficiency [20, 32, 38]. Conversely, the previous studies referred to above recruited patients with more advanced MS who exhibited moderate cognitive impairment.…”

Psychomotor slowing is associated with anomalies in baseline and prospective large scale neural networks in youth with epilepsy

“…As white matter abnormalities are appreciated in the epilepsies represented in this study ( Kim et al, 2014 ; Ciumas et al, 2014 ; Widjaja et al, 2013 ; Amarreh et al, 2013 ; Hutchinson et al, 2013 ), this is a relevant consideration. Other studies have shown that functional imaging evidence of neural efficiency are associated with improved processing speed in both healthy controls ( Rypma et al, 2006 ) and MS patients ( Fittipaldi-Márquez et al, 2017 ) and this represents a direction for future research.…”