Deliberate alteration of the visual evoked potential

Effect of defocusing and of distracted attention upon recordings of the visual evoked potential

“…Moreover, the relatively young age of the subjects (21-47 years) studied [12] implied sufficient accommodation reserve, allowing the subjects intentional defocusing. The other two studies obtained results similar to ours [9,13]. In one of these studies, only one accommodation strength was tested using a relatively large (80 0 ) check size, while in the other study, no measurement of the amount of accommodation was done and the subjects reported focusing on a near imaginary point.…”

“…With this understanding, it is possible to differentiate young subjects suffering from disorders in the visual system (retina and visual cortex) from those having uncorrected refractive error, or are trying to defocus the target by recording the pattern VEPs using large and small checks (90 0 to 7.5 0 ). In order to differentiate between uncorrected refractive error and intentional defocusing we suggest recording a binocular pattern VEP before monocular testing, because intentional defocusing can not be detected by an observer during monocular viewing of the target [13]. If binocular testing results in normal recordings but monocular pattern VEPs are severely attenuated, we may assume the patient employed accommodation by utilizing convergence.…”

“…Another possible maneuver that may lead to abnormal pattern VEP in subjects with normal vision is intentional defocusing of the visual target [9][10][11][12]. The reports that subjects can intentionally alter the pattern of their pattern VEP, and few of them managed to make it non-recordable [12,13], prevented this test from becoming an acceptable one for objective assessment of visual function.…”

Assessment of patients with suspected non-organic visual loss using pattern appearance visual evoked potentials

“…Equally, monitoring of the raw EEG signal is essential to pick up the emergence of alpha rhythm, which may be a manifestation of poor concentration or drowsiness when the eyes are open. Reversal VEPs are more susceptible to voluntarily alteration [24,39] and are generally of lower amplitude than pattern onset VEPs thus reducing reliability at low contrasts [17]. The patient may also fail to fixate, attempt to defocus or try to prolong eye closure during blinking in order to alter the PVEP results.…”

Clinical Ocular Electrophysiology

“…Impaired VEP responses may be produced by deliberate poor fixation, defocusing to the fixation point, or conscious suppression. [61][62][63][64] This is an important issue in the evaluation of patients with functional visual loss. Voluntary flash VEP suppression is more difficult, because it does not require fixation.…”