The relation between sensitivity changes and response time scale in the barnacle photoreceptor.

Responses to brief light flashes were recorded intracellularly from the photoreceptors of the barnacle in order to study the relation between changes in sensitivity and latency. Conditioning lights either increased the response to a test flash (facilitation) or reduced it (desensitization). In both cases there was a shortening of the response latency. Similar observations were made when background illumination was used instead of conditioning lights. These effects were not qualitatively altered when the extracellular Ca2+ concentration was lowered. The results suggest that facilitation is not the result of a reduction of intracellular Ca2+ concentration; i.e. it is not the reverse of desensitization. Since during facilitation there is an increase in photoresponse associated with latency shortening it can be suggested that the underlying mechanism is an enhancement of the effective light intensity. The experiments do not provide quantitative evidence for this idea.