Discriminatory approach to auditory stimuli in Guinea fowl (Numida meleagris) after hyperstriatal/hippocampal brain damage

Abstract

Introduction: The dorsomedial hyperstriatum accessorium (DMHA) appears to mediate some functions simialr to the mammalian hippocampus on tests in the visual modality ( Oades 1976a, 1976b). Here we investigate similar attention-related functions in the auditory modality after brain damage in and around the avian hippocampus. To what extent do these functions extend posterior to the posterior commissure (parahippocampus) and relate to the apparent hierarchical function reported from auditory areas that are adjacent posteriorly (e.g. Field L) ? Methods: Operation: Aspiration lesions and cuts were made to discrete parts of the hyperstriatum/hippocampus anterior and posterior to the posterior commissure in adult birds: the performance of these two lesion groups was compared with sham-controls.. Training/testing: Guinea fowl were trained to approach, feed and retreat from a food dish after hearing a species-specific food-trill. An ethogram including approach, search, locomotion and tension behaviour was recorded before and after operation for a training and test regime of variations of the natural calls and other sounds Histology is shown in figure 1 : Sonograms of fast-, slow, novel-, arousal-trills and "watch-winding" are shown in figure 2 : Photographs and Sketches of the behavioural responses are shown in figure 3. Results: 1/ Both groups with anterior and posterior lesions showed impaired recognition of the stimulus variations, shown a) by more search behaviour in both groups, and b) increased approach tendencies in the posterior-lesion group. [Nonetheless test stimuli were rcognized to be different - e.g. search after extinction > after filtered trills > after novel stimuli] 2/ Extended approach after posterior-lesions was followed by a long period of arousal / high tension. 3/ Transient tension behaviour after anterior damage habituated rapidly - search behaviour changed to low tension.. [The anterior lesioned group responded with search to filtered trills - there was message content still in the stimulus, whereas the posterior group changed behaviour.] Conclusions: It is proposed that after anterior hyperstriatal damage (hippocampus) there were changes in the thresholds for matching specifications of learned stimuli with new sensory input - initial activation was followed by disengagement. In contrast more posterior damage incurred an impairment to the general rules for the selection of sensory input - hence this led to more generalised approach and arousal responses. In conclusion there is a hierarchy of associative function extending anterior from the sensory field L, and this is discussed in terms of the function impaired after brain damage - perseveration (gross behavioural consequences) and persistence (application of recognition units to patterns of sensory stimulation).