Persons using SCIT ('participants') are briefly shown a pair of lines on a computer screen and asked to indicate which side is the shortest by pressing a keyboard button. This process is repeated many times while we record their speed of response and the number of errors made. Most people can complete SCIT within 4 minutes.
The 'U' version is suitable for participants aged between 5-90 years. The 'H' version is more challenging and is often selected by participants aged between 15-70 years of age. However, both versions provide very similar results.
SCIT is sensitive to small changes in the speed of signal processing and in the effectiveness of signal processing, within subcortical and cortical pathways of the brain. SCIT can detect changes before they begin to noticeably affect an individual’s performance on everyday tasks.
Impaired SCIT performance indicates that your cognitive performance was not at the expected level for your age. SCIT does not indicate the reason for the impaired performance because it is a screening test, not a diagnostic test. If you get poorer performance than expected, and have concerns about your functioning, follow up with your doctor, so you can be clinically assessed.
SCIT has its origins in the ‘Inspection Time' task’ that has been used by psychologists since the 1970’s. The test has been refined in a number of significant ways to make SCIT much quicker and more sensitive than the inspection time task.
SCIT is one of the rare cognitive tests not influenced by practice, so it can be used to repeatedly test someone over a short period of time. This property makes SCIT useful for detecting the short-term effects of drugs or medications on cognition.
Most cognitively normal individuals display stable SCIT performance so that similar results are obtained when a person is retested months or even years later. This property makes SCIT useful for detecting decrements or improvements in cognition, such as happens in neurodegenerative diseases or after therapeutic interventions.
What to expect of the SCIT test
SCIT involves making a decision regarding a simple asymmetric shape ('stimulus') that is displayed on the computer screen. The participant is asked to decide which side is shorter (the left or the right side - Fig 1A) and then to indicate their decision by pressing the corresponding button (i.e. left or right) on a computer mouse or keyboard.
On any given trial of the SCIT the participant is asked to fixate on a crosshair in the middle of the screen, and then the stimulus is briefly presented on the screen, followed immediately by filled black circles on a white background (Fig 1B - 'mask'). The mask reduces any further processing of the stimulus by blocking any afterimage of the stimulus.
The participant receives repeated trials (crosshair, stimulus, mask), but the stimulus remains on the screen for periods that range from very short (approximately 16 milliseconds) to relatively long (approximately 128 milliseconds).
Each presentation is performed in a pseudo-random sequence, so that the participant cannot predict how long the stimulus will remain on the screen or on which side of the screen the stimulus will be the shortest. The stimulus is presented 64 times, at 8 different exposure durations, with eight trials at each duration (4 with the shortest side on the left, and 4 on the right). The responses of the participant are calculated for two parameters:
1. Number of errors made at each exposure duration (% Errors).
2. Time taken to respond as a function of exposure duration (Response Time).
Summary statistics can be compared to previous performance by the participant or to norms that have been obtained from other age-matched individuals.
Factors that DO NOT influence performance on SCIT
Sex Provided that they are cognitively normal at the time of testing, males and females perform equally well on SCIT.
Experience Unlike most other cognitive tests, performance on SCIT is not influenced by practice (the test-retest reliability is approximately 0.95), so SCIT can be used to test a participant repeatedly over a short period of time.
Emotional state Performance on SCIT is not influenced by mild or moderate mood states (anxiety, stress or depression). However, like all cognitive tests, performance on SCIT is affected by severe mood states.
Level of education Provided that age and IQ are accounted for, a participant's educational status does not affect their performance on SCIT.
Factors that CAN influence performance on SCIT
Visual impairment People with cataracts or other forms of visual impairment perform poorly on SCIT. Participants with myopia or other correctable visual disorders can perform normally if they wear their contact lenses or spectacles.
Age Performance on SCIT gradually declines with age, but this can be controlled for by referring to the normative charts or by comparing to data obtained from prior tests by the same participant.
Language skills Participants who lack a working knowledge of English can have difficulty understanding the written instructions for SCIT, and should not be tested unless a translator is available.
Intoxication Performance on SCIT can be impaired in individuals with a blood alcohol concentration of 0.05 or higher. It is likely that other recreational, prescribed or illicit drugs will affect performance on SCIT.
Sleepiness It is preferable to test individuals on SCIT when they feel rested and alert. Individuals with jetlag or acute sleep deprivation show impaired performance on SCIT. General tiredness does not affect performance.
Mild cognitive impairment Individuals with mild-moderate cognitive impairment (with an MMSE score of 24 or less out of 30) find SCIT difficult and are often unable to complete the test. The 'Mini Mental State Exam' (MMSE) is often administered by doctors to ascertain whether a person is likely to have dementia.
Movement disorders Individuals with severe osteoarthritis or Parkinsonian symptoms can find it difficult to press the keyboard buttons, which slows their recorded response times.
Systemic inflammation Individuals with untreated coeliac disease or Crohn's disease perform worse than healthy peers on SCIT. It is possible that other disorders involving systemic inflammation are also associated with impaired performance.