Multimodal interneurones of locust optic lobe

Abstract

THE auditory interneurones which ascend the ventral cord of the migratory locust have been traced forward only as far as the neck connectives1,2. Fibres which give a similar response can occasionally be found in the lateral parts of the protocerebrum or in the optic lobes by probing with indium-filled platinum-plated microelectrodes. They have not been found, although sought, in the corpora pedunculata. However, by probing in the optic lobe, higher-order neurones which give a different set of responses to auditory stimuli are commonly encountered. These also respond to vibration or touch stimuli of all kinds, to a wide range of visual stimuli via either compound eye, but only to high-intensity stimulation of the ocelli. The most easily found example is in the outer layers of the medulla in the optic lobe. Typically they consist of units which are presumably large because they give large spikes, with a background spike discharge ranging from 1 impulse per 10 sec to 10 per sec. Sensitivities to all modalities are low, compared with other units which respond typically to only one of the modalities. No evidence of association between inputs has been found, although sought. When the stimulus is repeated regularly at intervals of up to 10 sec, the response almost always declines in number of impulses, occasionally down to no response with irregular return of the response to some of the stimuli. The upper limit of repetition frequency varies greatly between units, or between what appear to be the corresponding units in different animals. Also, the rate of decline of the response varies greatly as between units for a standard frequency of stimulus repetition. Responses to different modalities adapt at different rates. Adaptation to a stimulus of one modality does not usually influence the response to a different modality of stimulus newly introduced, demonstrating an independence of inputs, and separate adaptation to each.