Sweep VEPPosted on Wednesday October 20, 2021
Sweep VEP is an objective method to estimate a person’s visual acuity. This test is useful for patients with unexplained vision loss or any adult / child who is unwilling or unable to perform a subjective acuity test.
How it works
Similar to a 3-channel VEP, Sweep VEP utilizes three active scalp electrodes: one on the midline of the visual cortex (Oz) and one on each hemisphere (O1) (O2). The patient then fixates on a low luminance checkerboard pattern onset stimulus that alternates between a light-gray checkerboard pattern and a uniform gray screen. Diagnosys’s LCD monitors are designed to be iso-luminant to ensure the visual system is responding to the pattern onset and not a flash artefact. The key to the Sweep VEP is the stepwise spatial frequency regression where the check-size (spatial frequency) gradually changes in size (stepwise regression). To obtain a visual acuity measurement, the protocol is designed to drive magnitude to zero (or regress the magnitude to zero). In other words, the patient loses the ability to differentiate between gray and white checks. The spatial frequency at which the patient loses this ability translates to the visual acuity.
Diagnosys software incorporates Michael Bach’s visual acuity method. With this method, the patient is presented with a series of check sizes on each eye, which creates three recordings from the three active electrodes. These three signals are converted into a fourth Laplace signal, which then undergoes a Fourier Transform to distinguish signal from noise in order to yield a single magnitude value. Each step of the protocol generates a magnitude value and is graphed on the Sweep Plot. An algorithm finds the tuning curve of linear fit that theoretically crosses the x-axis at zero, and this value is automatically translated into the final visual acuity estimate.
About the protocols
Because there is a very broad range of visual acuities across the human population, Diagnosys has divided the Sweep VEP protocols into three categories: Normal, Low Vision and Very Low Vision. Each category corresponds with its own spatial frequency / stimulus range.
An example of a response with a strong signal. Notice the consistency across the 3 occipital channels and the sharp appearance of the Laplace signal.
A response with more noise than signal. It is unclear whether the patient can reliably see the check size on the monitor. Notice the irregularities between the recordings for each of the 3 occipital channels. The Laplace signal is nearly flat and without consistent peaks.
A final Sweep VEP graph for each eye. There is a characteristic “notch” shape to the graph followed by significant data points. (Significant data points are those whose signal-to-noise ratio is above the minimum requirement.) Notice how the linear fit intersects the x-axis yielding an automatically calculated estimate of visual acuity.
Bach M, Farmer J (2020) Evaluation of the “Freiburg Acuity VEP” on Commercial Equipment. Doc Ophthalmol 140:139-145.
Bach M, Maurer JP, Wolf ME (2008) Visual evoked potential-based acuity assessment in normal vision, artificially degraded vision, and in patients. Br J Ophthalmol 92:396-403.