Sir,

We welcome and thank Tan et al1 for their interest and observations regarding our previously published case report.

We are in agreement regarding Snellen acuity not being a diagnostic criteria for Charles–Bonnet Syndrome (CBS). Numerous reports have highlighted the presence of CBS-type hallucinations in individuals with good Snellen acuity with restricted visual fields. It appears, therefore, that the quality of visual function and its deviation from the previous norm for the individual are key elements in production of CBS hallucinations. It is for this reason that we agree with the observation by Holroyd et al2 that sudden and abrupt reduction in visual function may trigger CBS symptoms.

Tan et al1 comment on reduction of light perception in the fellow eye terminating CBS symptoms in our case. We agree that the proposed theory could account for resolution of CBS on eye closure. This theory, however, is similar to the other commonly perceived theories of deafferentation described by Bartlett3 and West.4 The visual sensory cortex, when deprived of normal afferent input may exhibit spontaneous independent activity with resultant conscious images. This hypothesis is supported by the observation that such hallucinations may be abolished by normal or excessive visual stimulation.3 The perceptual theory of West describes afferent input reduction below a threshold level (such as in disease of the eyes or visual pathway), resulting in the brain allowing previously registered subconscious perceptions to emerge into consciousness, resulting in a hallucinatory experience.4

Similarly, as the hallucinations of CBS have only been documented in acquired visual deprivation, it is possible that in-built higher centre mechanisms are in place so that eye closure may trigger intracerebral activity suggesting to the sensory visual cortex that visual stimulation has ended, rather than sensory input itself being detected by sensory mechanisms to be decreased. This may explain why darkness with the eyes open may still result in CBS but darkness as a result of eye closure may obliterate CBS hallucinations.