Abstract

1. The frequency of efferent impulses in the optic nerves of the orb weaving spidersArgiope bruennichii andA. amoena increased by illumination of the brain. Addition of 2 mmol/l Co2+, a synaptic blocking agent, to the saline resulted in the generation of optic nerve impulses following cerebral illumination. Thus, efferent neurons within the brain appear to be sensitive to light. Maximum sensitivity of these neurons occurs at 420–440 nm. 2. The frequency of efferent optic nerve impulses decreased during illumination of the eyes. The spectral sensitivity for this inhibition was measured, a large peak occurring at 480–540 nm and a smaller peak at 360 nm. 3. The frequency of efferent optic nerve impulses increased transiently following a diminution in light intensity striking the eyes. Simultaneous diminution of light intensity striking both the eyes and the brain, however, increased the number of impulses from the optic nerve, suggesting that the cerebral photosensitive neurons play a role in increasing the response to dimming of light.

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