Effectiveness Of TDCS At Improving Recognition And Reducing False Memories in Older Adults Part 1

Abstract: 

Background: 

False memories tend to increase in healthy and pathological aging, and their reduction could be useful in improving cognitive functioning. 

As we age, our bodies begin to show various symptoms of aging, the most common of which is pathological aging. Pathological aging refers to the phenomenon that the function of our organs gradually declines, leading to the appearance of various diseases. Although pathological aging is part of the natural process of life, it does not necessarily have a direct negative impact on memory.

Studies have shown that people's memory can deteriorate with age, but this is not entirely caused by pathological aging. Some studies have shown that factors such as menopause, retirement, and social detachment can also hurt memory. This means that even if we successfully avoid the effects of physical aging, we may still inevitably be affected by some other factors.

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However, although pathological aging does not directly affect memory, it can still hurt our health, and these effects will directly affect our mental and cognitive health. For example, some health problems can cause long-term pain, fatigue, and insomnia, which will directly affect our physical and mental state, and ultimately our memory and cognitive function.

Therefore, we need to minimize the impact of pathological aging on us. This includes maintaining a healthy diet and lifestyle, participating in appropriate sports and exercise activities, regular health checks, and maintaining good social relationships. In addition, we can also participate in cognitive training activities such as memory training to help us maintain control of memory and cognitive function.

Finally, we must make it clear that pathological aging is not an inevitable thing, and we can reduce its impact on us through an active lifestyle and healthy habits. Only in this way can we maintain a positive attitude, have good physical and cognitive health, and fully enjoy the beauty and excitement of life. It can be seen that we need to improve memory, and Cistanche can significantly improve memory because Cistanche is a traditional Chinese medicine with many unique effects, one of which is to improve memory. The efficacy of Cistanche comes from the various active ingredients it contains, including tannic acid, polysaccharides, flavonoid glycosides, etc., which can promote brain health in many ways.

The objective of this study was to use an active–placebo method to verify whether the application of transcranial direct current stimulation (tDCS) improved true recognition and reduced false memories in healthy older people. Method: Participants were 29 healthy older adults (65–78 years old) who were assigned to either an active or a placebo group; the active group received anodal stimulation at 2 mA for 20 min over F7. 

An experimental task was used to estimate true and false recognition. The procedure took place in two sessions on two consecutive days. Results: True recognition showed a significant main effect of sessions (p < 0.01), indicating an increase from before treatment to after it. 

False recognition showed a significant main effect of sessions (p < 0.01), indicating a decrease from before treatment to after it and a significant session × group interaction (p < 0.0001). 

Conclusions: Overall, our results show that tDCS was an effective tool for increasing true recognition and reducing false recognition in healthy older people, and suggest that stimulation improved recall by increasing the number of items a participant could recall and reducing the number of memory errors.

Keywords: transcranial direct current stimulation; true recognition; false recognition; aging; experiment.

1. Introduction

Human memory is susceptible to distortions, illusions, and false memories that tend to increase during both healthy and pathological aging [1], especially in the face of events that share perceptual or conceptual characteristics. 

In later life, it is important to minimize these false memories to carry out daily activities, such as remembering whether one took their medication, turned off the fire when cooking, closed the door before leaving, or just thought about it. 

Thus, maintaining a functional episodic memory system is vital for preserving a high quality of life with age, particularly for independent living [2]. Hence, there are obvious benefits if false memories can be reduced temporarily in certain circumstances. 

Evidence from injury studies has identified the medial temporal lobe (MTL), particularly the hippocampus and the prefrontal cortex (PFC), as critical brain structures for coding and retrieving episodic memory [3]. 

Decreased hippocampal volume is associated with reduced memory performance [4], and a decreased anterior, dorsolateral, and ventrolateral PFC is associated with reduced memory capacity [5]. 

Decreased structure and function of the MTL and PFC are also associated with increased susceptibility to false memories [6]. Older adults with smaller hippocampal volumes generate more false alarms on associative recognition tasks than older adults with larger volumes [7]. 

In addition, the connective integrity of these two regions is vital for accurate memory coding and retrieval, but there is a reduction in functional connectivity between the MTL and PFC regions with healthy aging [8]. Recently, there has been considerable interest in the use of transcranial direct current stimulation (tDCS) to improve cognition [9,10]. 

tDCS is a non-invasive technique that elicits constant weak electric currents through the cerebral cortex via electrodes placed on the scalp, flowing from the positively charged anode to the negatively charged cathode. This technique has been shown to modulate excitability in cortical and subcortical tissue and, therefore, may facilitate cell plasticity. 

The current is thought to modulate the resting membrane potential of neurons depending on the polarity of the electrode such that anodal stimulation induces depolarization of the membrane potential and increases cortical excitability and cathodal stimulation induces hyperpolarization and decreases cortical excitability [11]. 

Many experimental studies [12,13] have demonstrated the efficacy of tDCS in healthy subjects regarding different cognitive tasks, such as associative verbal learning, working memory, selective attention, visual memory, stimulus recall and recognition, and the reduction of false memories [14]. However, a lack of effectiveness of tDCS has been reported, which could be related to the heterogeneity of the parameters of the stimulation [13], such as the area of the stimulation (left lateral cortex, temporal-parietal lobe, etc.), the type of stimulation (anodal, cathodal, or without stimulation (sham)), the amount of current (1 mA, 2 mA, etc.), the type of session (single or repeated) and its duration (15 min, 20 min, 30 min, etc.), the interval between repetitions, the size of the electrode in square centimeters, or the type of design used (between subjects, intra-subject, with or without double-blind control, etc.). Regarding the effect of tDCS on true recall and recognition, Javadi and Walsh [15] administered anodal or cathodal tDCS over the left dorsolateral prefrontal cortex (dlPFC) during the encoding or recognition of words. About encoding, the data show that only anodal stimulation over the left dlPFC improved memory; in the case of recognition, anodal stimulation was associated with a trend toward improving recognition. 

These data essentially support the role of the left dlPFC during the encoding and retrieval of words. The effects of tDCS on associative memory have been measured with both recognition and recall tests. 

The results [16] indicate that significant increases were obtained on recall tests, indicating that tDCS improved the encoding of face-name associations; however, there were no significant effects of stimulation on recognition memory performance. Another study [17] assessed both immediate and delayed stimulation effects of the left dlPFC on associative memory, which was measured in terms of recall and recognition. 

The authors found no evidence of stimulation-induced recognition memory changes, but improved associative recall was observed. This recall advantage was evident even after a delay of 24 hours, suggesting that memory effects persist after a period of consolidation. 

The authors also point out that these results show that a single session of tDCS while studying (encoding) improved recall performance. In sum, these results seem to indicate that tDCS stimulation applied to the left dlPFC seems to improve true recall, but it does not affect true recognition.

However, the false memory literature contains few studies and little information. Several authors confirmed the notion that the modulating activity of the anterior temporal lobes (ATLs) with tDCS brain stimulation before or during a given cognitive task is an effective way to change memory processing [14]. They found evidence that anodal tDCS on the left anterior temporal lobes (placed over T3 using the Electroencephalography (EEG) International 10/20 System) is effective at reducing false memories while using a modified version of the Deese–Roediger–McDermott (DRM) paradigm. Anodal left and cathodal right ATLs resulted in a 73% decrease in the formation of false memories. 

A substantial reduction in false memories has been observed after anode stimulation (over site FT9, according to the International 10-10 System for EEG electrode placement), compared to sham, when using word lists composed of strong associates of the critical words; however, no effect at all emerged when lists were composed of exemplars belonging to the same taxonomic category as the critical lures (categorical lists) [18]. 

The authors suggest that the left ATL may function as an integration hub when processing associatively related verbal materials in the context of episodic learning. Given these inconclusive results, the objective of our study was to analyze whether tDCS, through the application of anodal stimulation, was effective at improving true recognition and reducing false memories in healthy older people when using a recognition task to elicit false phonological memories [19]. 

For the selection of the stimulation parameters, this investigation was based on the most commonly used criteria according to different reviews [13,20]. Traditionally, the study of false memories has been carried out through experimental procedures, where the studied stimuli are semantically related to each other (e.g., tiger, cougar, cat, etc.), which can provoke the false recognition of non-studied critical stimuli that are semantically related to the study list (e.g., panther). However, it is also possible to elicit false memories of critical words (e.g., chair) after studying words that are related to them phonologically rather than semantically (e.g., cheer, hair) [21]. 

These phonological false memories increase with healthy aging in a similar way to semantic false memories [22,23]. Thus, we proposed an experiment to elicit phonological false memories based on a perceptual manipulation of the stimuli that was implicit for the participants to increase the activation of critical words [19]. 

This adapted procedure [24] mainly consisted of presenting study words formed either from half of the letters in the alphabet (half condition) or from the entire alphabet (entire condition). 

On the subsequent recognition test, the new words could be formed either from the same letters as the ones studied in the half condition (or critical lures because they were phonologically related to the studied words), distractors formed from the other half of the letters in the alphabet, or distractors formed from the entire alphabet. 

Therefore, this experimental paradigm, which used a simple study and word recognition task, made it possible to obtain estimates of both true and false recognition (with the latter being operationalized from the false alarms elicited using the critical lures). 

We proposed applying this paradigm to healthy older people in two sessions. The materials used in both sessions were different for each subject (and counterbalanced between subjects). Participants were randomly assigned to either a treatment group that received two sessions of electrostimulation through tDCS or a control group that received two sham sessions. 

This procedure, therefore, allowed us to determine the effectiveness of tDCS applied over site F7 (International 10-20 System for EEG electrode placement) to stimulate the dlPFC by analyzing whether there was an improvement in true recognition or a reduction in false recognition in the treatment group.

2. Materials and Methods

2.1. Participants

The sample of older adults was composed of 29 people (18 women, 11 men) ranging from 65 to 78 years old (M = 68.79, SD = 3.33), who belonged to various leisure centers for older adults in the city of Valencia. 

The Ethical Committee on Human Research of the University of Valencia approved this study. All the participants voluntarily gave their consent to participate, and they reported being in good physical and mental health with no known memory impairments. 

In this regard, the mean for the older adults on the Mini-Mental State Examination [25] was 29.86 (SD = 0.35, range 29–30), revealing no memory impairment. Participants were randomly assigned to receive either tDCS or sham stimulation. 

The treatment group was composed of 16 older adults (10 women, 6 men) ranging from 65 to 77 years old (M = 68.93, SD = 3.35); the sham group was composed of 13 older adults (8 women, 5 men) ranging from 65 to 78 years old (M = 68.61, SD = 3.42). 

In addition, when comparing the scores for the MMSE [25] between the groups (treatment group = 29.94, sham group = 29.77), no significant differences were observed (t(27) = 1.3, p > 0.05).

2.2. Materials

The half condition included two lists of 50 words each, formed entirely from the following letters of the Spanish alphabet: a, e, u, b, d, g, j, n, r, and z (list A) or i, o, c, f, h, l, m, p, s, t, v, and y (list B). 

List C (entire condition) contained 50 words formed from the entire alphabet, with the only criterion being that each word had to contain at least one letter from list A and at least one letter from list B. Lists A, B, and C were balanced in terms of mean frequency per two million [26], 93.30 (SD = 166.69), 91.28 (SD = 129.87), and 92.40 (SD = 165.46), respectively, and length, 5.00 (SD = 1.20), 4.70 (SD = 1.30), and 4.95 letters (SD = 1.15), respectively.

2.3. Procedure

The experiment took place in two sessions on two consecutive days (one session each day). On day one, participants performed a first study and recognition task with no tDCS stimulation that would serve as a pre-test or baseline (the before condition in Table 1). 

They were then assigned either to the treatment group or the sham group, receiving either a tDCS or sham stimulation session for 20 min on the second day (24 hours later). 

The stimulation began five minutes before starting the experimental task and continued until the end of the recognition task, which would serve as a post-test (the after condition in Table 1). 

The experimental task was initiated five minutes after the stimulation started because three minutes of stimulation is the minimum time to induce significant after-effect changes in cortical excitability [11].

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