Cortical Hyper Excitability correlating with Visual Distortions and Hallucinations

Cortical Hyper Excitability correlating with Visual Distortions and Hallucinations

Nishtha Bakshi

2017

Background: The primary focus of our study is how the abnormalities in the visual experiences such as the visual distortions or hallucinations result in the increase in the cortical hyper excitability. The aberrant neural activity causes visual distortions. Susceptibility to such visual distortions reflects elevated levels of cortical hyper excitability.
Methods: Forty-eight individuals completed the "Pattern Glare Task" (where they viewed certain striped grating patterns with different spatial frequencies). Participants also completed the Cortical Hyperexcitability Index (Chi) and the Cambridge Depersonalization Scale (CDS).
Results: Pattern glare task showed that individuals experienced more visual distortions in the Medium Frequency (3 cpd). A very small sample of the population showed effects of depersonalisation disorder. Based on our results, we can say that individuals did show an elevated level of cortical hyperexcitability.
Conclusion: The study suggests that non-clinical population also experiences a certain level of increase in cortical hyper excitability. It only establishes the utility of pattern glare with regards to CHi and CDS to add to our existing knowledge.

visual distortions
cortical hyper excitability
Pattern Glare Task

Pattern Glare Test
The pattern glare task includes stripy patterns on three separate cards each with different spatial frequencies; low spatial frequency baseline grating (approx. 0.5 cycles per degree), high spatial frequency baseline grating (approx. 12 cpd), and the crucial medium spatial frequency grating (approx. 3 cpd). The computerised version of the pattern glare task was modified for this experiment, as we were using a paper-based version (Wilkins, 1995; Wilkins et al., 1984) for the same. The stimuli used in the experiment are given in FIGURE 1. The individuals are asked to stare at the white dot in the center of each pattern for approximately 10-15 seconds, while holding each pattern at arm's length. Following, a series of questions are asked to the participant i.e. if they experienced any blurring of lines, bending of lines, fading, shimmering, flickering or shadowy shapes. The participants on the basis of their experience on viewing each pattern, rate the above questions from a score of 0-7 where, 0-minimum and 7-maximum (Wilkins et al., 1984; Conlon et al., 1999). The score is obtained for each pattern and the difference between Pattern 2 and Pattern 3 is recorded, which is called as the '3-12 difference'; in other words, the difference between high frequency and the medium frequency (3cpd – 12cpd).
Cambridge Depersonalisation Scale
The CDS is a self-reporting questionnaire and is used to measure the duration and frequency of any depersonalisation symptoms that individual experiences in the time frame of the past six months. (Sierra and Berrios, 1999). The CDS is an instrument containing 29 items. Each of the items in the scale are rated on the basis of Likert-scale both for frequency (0-4; where, 0=never, 1=rarely, 2=often, 3=very often, and 4=all the time) and duration based on its average on how much the experiences last (1-6; where 1=few seconds, 2=few minutes, 3=few hours, 4=about a day, 5=more than a day, and 6=more than a week). Its global score is the sum of all items (0-290). Sierra et al., (2005) established four well determined factors to dictate the different symptoms of depersonalisation as single or underlying dimensions they were, ‘Anomalous Body Experience’, ‘Emotional Numbing’, ‘Anomalous Subjective Recall’, and ‘Alienation from Surroundings.’ This questionnaire addresses the complexity of depersonalisation and uncovers its symptoms, which can be directed towards distinct psychopathological domains.
Cortical Hyperexcitability Index
The CHi was designed to provide an index that discovers the visual irritability, discomfort and the associated visual distortions that individual’s experience (Braithwaite, Merchant, Dewe and Takahashi, 2015). The above-mentioned experiences are well linked to the increase of cortical hyperexcitability. A major advantage of the CHi’s design is that it unveils three broad factors which are (1) heightened visual sensitivity and discomfort, (2) negative aura-type visual aberrations, and (3) positive aura-type visual aberrations. The items present in the questionnaire picture a vast selection of visual experiences that have been previously reported through hallucinations based experimental studies on patients, control groups, non-clinical populations; aura and its underlying dimensions. The CHi uses a fine-grained 7-point Likert response scales, where in the test each question has two response scales i.e. frequency (1-7; where 1=not at all frequent and 7=very frequent) and intensity (1-7; where 1=not at all intense and 7=extremely intense). In terms of scoring, both the scales are summed to provide an overall CHi index for each question. However, a value of 1 is subtracted from each response on frequency and intensity, as the scale was transformed from 1-7 to a 0-6 Likert-scale. This was done for individuals who responded with 1 in every question would still have a score of 54.
Design and Procedure
All the participants were forwarded a brief explanation about the purpose of the study and how they can contribute to it. If the participants agree, later schedule a time for the voluntary study. The experiment was conducted in the Social Hub of the Graduate College, Lancaster University. The participants were seated comfortably on the right side of the researcher. The individuals were asked to read the Participant Information sheet carefully, later if they agree; they may sign their respective consent form. It was made clear to the participants that the confidentiality of their personal information will be ensured and that they could at any point (1) can ask questions during the experiment, (2) stop the experiment, if they are uncomfortable at any point during the conduction (3) participants have the right to withdraw themselves from the study with no further adverse consequences however, they need to inform the researcher about this via email. Participants were again asked if they suffered from any neurological disorder specially migraine, migraine (aura), or photo sensory epilepsy and if they had any severe incidences of alcohol and drug abuse. The first phase of the experiment included the pattern glare task. Individuals were handed over with the first pattern with low frequency (LF) and were asked to stare at the white dot in the centre of the pattern for 10-15 seconds. After this, they were asked to rate the questions based on their experience on a scale of 0-7 (0-minimun, 7-maximum). The questions included if they experienced any blurring of lines, bending of lines, shimmering or flickering, fading or if they could see any shadowy shapes. Before handing over the second pattern, it was made sure that the participant is comfortable with proceeding further with the experiment and is not experiencing any kind of visual stress. The same steps were repeated for both the other two patterns with medium frequency (MF) and high frequency (HF). The order in which the participants viewed the patterns was randomised for each one of them. Individuals who are prone to pattern glare can be quantified for such a criterion based on their sum of distortions in 3cpd (MF) or as the difference between 3 and 12 cpd, also called the '3-12 cpd difference'. After a two-minute break, the second phase of the experiment included participants to answer 29 questions on the Cambridge Depersonalisation Scale, which are based on the frequency and duration of any 'strange or funny experiences' that they felt in the past six months. Lastly, the third phase, the second questionnaire was introduced to the participants. The Cortical Hyper Excitability Index. Similar to the patterns, the questionnaires presented to the participants were also randomised in order to obtain a variety in the responses of the participants. The total time taken to conduct the experiment was about 20 minutes or less. Post conduction the individuals were thanked for their time and effort.

Lancaster University

Data/data spreadsheet.xlsx

Bakshi2017

John Towse

Open

None

English

Data

LA1 4YF

Jason Braithwaite

MSc

Perception

n=48

correlation
factor analysis