Ž . Brain Research 856 2000 152–162 www.elsevier.comrlocaterbres Research report Organization of intratelencephalic projections to the visual Wulst of the chick Chao Deng ) , Lesley J. Rogers DiÕision of Zoology, School of Biological Sciences, Physiology Building, UniÕersity of New England, Armidale NSW 2351, Australia Accepted 23 November 1999 Abstract The avian visual Wulst, said to be the equivalent of the striate cortex in mammals, is the telencephalic visual area of the thalamofugal visual pathway. In this study, by means of retrograde labelling with fluorescent tracers injected into the Wulst regions in the left and right hemispheres, we have investigated the organization of the intratelencephalic projections to the visual Wulst in chicks. After injecting Ž . Ž . Fluorogold FG , True blue TB or rhodamine into the visual Wulst, fluorescent-labelled neurones were found in the ipsilateral Ž . Ž . neostriatum frontale, pars lateralis NFl , the ipsilateral neostriatum intermedium NI and the ipsilateral dorso-lateral neostriatum. Labelled neurones were also found in both the ipsilateral and contralateral archistriata. In addition, some neurones in the archistriatum were double-labelled, which indicates that these archistriatal neurones have axon collaterals projecting to the visual Wulst on both sides of the forebrain. Through these intratelencephalic afferents to the visual Wulst, visual information transmitted in the thalamofugal pathway may be modulated by other telencephalic areas. The possible roles of these connections in regulating behaviour are discussed. q 2000 Elsevier Science B.V. All rights reserved. Keywords: Visual Wulst; Intratelencephlic afferent; Double labelling; Fluorescent tracer; Chick 1. Introduction The visual Wulst, suggested to be the equivalent of the w x striate cortex in mammals 49,51 , is the telencephalic visual area of the avian thalamofugal visual pathway. Cytoarchitectonically, the visual Wulst is a multilayered structure with at least four layers, including the hyperstria- Ž . tum accessorium HA , the nucleus intercalatus of hypers- Ž . triatum accessorium IHA , the hyperstriatum intercalatum Ž . Ž . superior HIS and the hyperstriatum dorsale HD , ar- w x ranged in order from dorsal to ventral regions 28,55 . IHA and HD receive afferents from the nucleus geniculatus Ž . lateralis, pars dorsalis GLd , then IHA and HISrHD w x project predominantly to HA 13,49 . HA is the source of efferent projections of the visual Wulst to multiple telen- cephalic areas, including the neostriatum frontale and pale- ostriatal complex, and to extratelencephalic nuclei, includ- ing GLd, the pretectal nuclei and the optic tectum. The latter projections reach their targets via the tractus sep- w x tomesencephalicus 49 . Although there has been quite extensive investigation of the behavioural effects of lesions of the visual Wulst using adult pigeons, the function of the avian thalamofugal vi- ) Corresponding author. Fax: q61-2-67-733452; e-mail: cdeng3@metz.une.edu.au sual pathway is relatively unknown. Simple brightness, colour or pattern discriminations are not, or only slightly, w x impaired by lesions of the visual Wulst 25,43,44 . Lesions of the visual Wulst have been shown to cause some degree of impairment of visual discrimination of intensity differ- w x ence thresholds in the pigeon 39 . Furthermore, marked deficits in reversal learning are caused by lesioning the w x wx visual Wulst, as shown in Bobwhite quails 48 , chicks 5 w x and pigeons 35,50 . Marked deficits have also been found in delayed matching-to-sample performance after visual w x hyperstriatum lesions in pigeons 38 . These results suggest that tasks requiring higher information processing may be more effective in revealing the function of the avian visual w x Wulst. In fact, our recent study 12 found that the visual Wulst of the chick is involved in categorising food from non-food. In addition, the left and right visual Wulst regions have different contributions to this behaviour, since glutamate treatment of the left, but not the right, visual Wulst caused a deficit in the chick’s abilities of categoris- w x ing food from non-food 12 . This functional lateralization is consistent with the known structural asymmetry of the w x thalamo-Wulst projections 2,47 , but we do not know whether any telencephalic afferents to the visual Wulst also play a role in the lateralization of the visual Wulst. To understand fully how the visual Wulst functions, knowledge of the relationship between the visual Wulst 0006-8993r00r$ - see front matter q 2000 Elsevier Science B.V. All rights reserved. Ž . PII: S0006-8993 99 02403-8