The neural basis of colour vision in the butterfly, Heliconius erato

Single unit recordings from visual interneurons in the protocerbrum of the neotropical butterfly Heliconius erato, revealed a variety of fibre types with wavelength specific patterns of activity. These ‘colour fibres’ were divided into four broad categories based upon the nature of their response to a flash of monochromatic light: (1) ‘simple’ colour fibres, which respond with excitation, when the eye is stimulated with some specific, narrow portion of the visible spectrum; (2) ‘on-off’ fibres, which produce ‘on’ bursts in response to certain portions of the spectrum, and ‘off’ effects when stimulated with other wavelengths; (3) ‘variable latency’ units which demonstrate dramatic shifts in latency, with comparatively small changes in the stimulating wavelength; and (4) ‘red-blue movement’ units which demonstrate bursts of activity when stimulated with flashes of red or blue light, and also serve as a non-directional movement detector. The wide variety of colour fibres appears to be correlated with the great behavioural significance of colour to this species.