There was clear evidence of a projection from 1C to the most caudal region of medial and lateral PN. Martin (1969) mentioned a projection to lateral PN in opossum but a similar projection has not been shown anatomically in monkey, rat or cat although auditory evoked responses have been recorded within this pontine region in cat (Aitken and Boyd, 1978).
In cats, the 10 projects primarily to the dorsolateral margin of middle parts of the ipsilateral dorsolateral pontine nuclei (Aitken and Boyd, 1978; Anderson etal., 1980; Hashikawa, 1983; Kawamura 1975; Kawamura and Brodai, 1973; Mower ef a/. 1979). It is not clear from which division of the 1C this projection arises. Using degeneration techniques Kawamura (1975) described a projection from ICC to the anterior two thirds of DLPN. But Anderson et al. (1980) injected HRP and tritiated leucine into ICC and found very little label in DLPN. Mower et al. (1979) injected HRP into middle regions of the lateral DLPN and found labelled cells in ail parts of the 1C. Hashikawa (1983) injected HRP, WGA-HRP and ^H-leucine into the rostral two thirds of DLPN and adjacent regions of the peduncular and lateral pontine nuclei and demonstrated a large projection from caudal ICX, a smaller projection from ICP and no projection from ICC. In the current study the primary injection site was not strictly confined to a single division of the 1C in any one subject. Nevertheless, in subjects IC I, IC2 and IC3 the injections were mostly restricted to ICC, ICP and ICX respectively. A projection from 1C to ventral and lateral PN was seen in all subjects, hence it is likely that all regions of the 1C project to PN.
There was no evidence of a direct projection from the 1C to dorsolateral parts of the DLPN as described in cats. There are several points which suggest that this pattern of collicular
DLPN in Figures 7A,B and C. There is a slight possibility that 1C projects to medial parts of DLPN, as in those cases with the largest injections of WGA-HRP there were labelled fibres in this region that may have obscured the presence of a terminal field. It is also possible that there are connections from the 10 through extended dendrites of DLPN cells in rabbits, as suggested by Bume et al., (1981) in rats. These possibilities are less likely in view of the clear evidence of termination within more caudal parts of VLPN.
162
subject, the sum of injections in this study show s that all parts of the 1C w ere filled, excep t for its extrem e caudal pole. Secondly, each major division of the 10 projected to the sa m e region of ventral and lateral PN but not to DLPN even though in no c a s e w as the primary site limited to a single collicular division. This su g g ests that no region w a s likely to have been omitted.
Finally, the other connections of the 10 were consistent with th o se s e e n in other sp e c ie s (Huffman and H enson, 1990) and in other studies in rabbits (Borg, 1973a,b).
The heavily labelled tract of fibres lying above the m ost dorsolateral asp ect of the ipsilateral DLPN throughout the caudal half of the PN is identical to that reported by Moore and Goldberg (1966) in m onkeys and by Burne etal. (1981) in rats. Both studies show ed that fibres from the 10 course adjacent to the ipsilateral DLPN and peduncular region. In both of th e se stu d ies it w as difficult to d ecid e if fibres of this tract terminated within PN. Termination s e e m s unlikely a s two further studies found no ev id en ce for a direct projection from 10 to PN in rats. Faye-Lund (1986) injected HRP into the DLPN of rats and found no ev id en ce of a direct projection from the 10. Mihailoff et al., (1989) found no ev id en ce for a projection from 10 to PN after ex ten siv e fills of the PN with WGA-HRP. However, Burne etal. (1981) su g g ested that collicular fibres could contact dorsally-extended dendritic branches of the DLPN cells. There w as no evid en ce for a projection from 10 to lateral DLPN in this current
c o n t study. This is clearly illustrated by the a b sen ce of anterograde label within lateral parts of the ^
10 projects Indirectly to PN via SC
A large projection from 10 to the d eep layers of the SO w as found, similar to that described in previous stu d ies (cat: Edwards et al., 1979; rat: Ooleman and Olerici, 1987; rabbit: Tarlov and Moore, 1966). Retrograde and anterograde WGA-HRP label within layers IV, V and VI of the SO and within the periaqueductal grey w as se e n to derive mainly from the extem al nucleus (subject 102 s e e Figure 9). Equivalent label detected in th e se areas following an injection into the central nucleus (subject 103, s e e Figure 9) w as probably due to spread of the primary site into lOX. lOX w as confirmed to be the major source of input from 10 to the d eep layers of SO by re-exam ination of c a s e s from the study by W ells e t al., (1989) in which injections of WGA- HRP were m ade in the d eep layers of the SO - only cells within lOX w ere labelled in th e se c a se s.
In several studies a projection from the d eep layers of the SO to PN has b een described (rat: Bum e etal., 1981; cat: Kawamura and Brodai, 1973; Mower e f a/., 1979). In the rabbit, there is a projection from the d e ep layers of the SO to caudal regions of the ipsilateral PN, including ventral PN, lateral PN and medial DLPN (W ells etal., 1989). Thus in the m ost caudal regions of ventrolateral PN there is overlap of projections from the 10 and d eep layers of the SO in the rabbit. Projections from the d eep layers of the SO to the PN (W ells et al., 1989) m ay be particularly important a s the direct projection of the 10 to PN appears to be much smaller.
the PN of inputs from ICC and ICP (see also Kamada et al., 1992). Secondly, the projection from 1C to the deep layers of the SC appears to be much larger than the projection from the 1C to PN.
The possibility that the cerebellum receives auditory and visual CS information from the deep layers of the SC has two implications. Firstly, because the deep layers of the 1C are involved in movement its output to the cerebellum may be some derivative of auditory and visual CS information. Secondly, the cerebellum can receive auditory information from regions of the PN that receive inputs from the SC including caudal, medial, parts of DLPN (Wells etal., 1989).
163