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The effect of dark-rearing, strobe-rearing and acute visual cortex removal on the visual responses in the superficial superior colliculus of the guinea-pig
ELS E V I E R
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Neuroscience Letters 213 (1996) 216-220
The effect of dark-rearing, strobe-rearing and acute visual cortex removal on the visual responses in the superficial superior colliculus of the guinea-pig S.K. Thornton*, D.J. Withington, D. McCrossan, N.J.
Ingham
Department of Physiology, Worsley Medical and Dental Building, University of Leeds, Leeds LS2 9NQ, UK Received 12 December 1995; revised version received 27 June 1996; accepted 27 June 1996
Abstract Extracellular multi-unit responses to visual stimuli were recorded in the cells of the superficial layers of the superior colliculus (SC) in four groups of adult guinea-pigs: a control group, a strobe-reared group, a dark-reared group and a group with the ipsilateral visual cortex removed acutely. Single unit visual responses were also recorded in a control and a dark-reared group. When guinea-pigs were either strobe or dark-reared from birth, the number of directionally selective responses in the superficial SC decreased significantly. Acute removal of the visual cortex had no affect on the number of directionally selective cells recorded in the SC. The correlation between azimuthal visual receptive field and rostrocaudal position of the recording electrode in the SC was not significantly different from the control group following strobe, dark-rearing or acute visual cortex removal. These data imply that, during early development, visual information is necessary for directional selectivity of the visual responses in the superficial SC. However, the map of visual azimuthal space is essentially unperturbed by visual restriction (in the form of dark or strobe-rearing) or acute visual cortex removal.
Keywords: Superior colliculus; Visual deprivation; Directional selectivity; Topography; Visual space map; Cortex removal
The superior colliculus (SC) is an important station in the mammalian midbrain for the processing of visual information and is thought to be involved in the integration of sensory and motor information for the orientation of the animal towards a visual stimulus [3]. It has been divided operationally into two parts, the superficial and deep layers, both of which contain maps of the visual world. In the guinea-pig, the developmental emergence of the visual space map has not been documented but in other species the retinotectal projection is ordered before or at eye-opening [7]. Since the eyes of the guinea-pig are open from birth it also likely that the visual receptive fields in the SC are topographically ordered at this time. The refinement of the map and development of other visual characteristics could take place postnatally since, despite the fact that the majority of optic nerve loss occurs prenatally,
* Corresponding author. Center for Neural Science, New York University, 4 Washington Place, Room 809, New York, NY 10003-6621, USA. Fax: +1 212 9954011.
gradual axonal loss still occurs during the first few weeks of life [16]. There is an abundance of physiological data which indicate that visual experience plays an important role in the normal development of visual responses in the mammalian SC [4,10,13]. The extent to which visual experience may affect normal physiological maturation of the SC depends not only on the type of deprivation encountered but also varies with the species studied. Dark-rearing and stroberearing are the two forms of visual deprivation commonly utilized in studies. Dark-rearing removes all aspects of visual perception, whereas stroboscopic illumination allows perception of visual patterns but abolishes that of movements. Species differences have been observed between the cat and the hamster. In the cat, both darkrearing [10] and strobe-rearing [13] affect visual responses in the SC, but the visual responses in the hamster SC are only influenced by strobe-rearing [4]. The deficits observed in the superficial SC after strobe-rearing in the hamster include a reduction in the number of directionally selective cells, an increase in the receptive field size, an
0304-3940/96/$12.00 © 1996 Elsevier Science Ireland Ltd. All rights reserved PII S 0 3 0 4 - 3 9 4 0 ( 9 6 ) 1285 1-2
S.K. Thornton et al. / Neuroscience Letters 213 (1996) 216-220
increase in the number of cells responding to slow movement and a decrease in the number of cells responding to visual stimuli [4]. Since the superficial SC receives visual input from the primary visual cortex, the removal of the visual cortex may influence the visual properties of the cells in the superficial SC. Lesions of the visual cortex result in deficits of visual response properties in the superficial SC of the cat [2,23] and hamster [22], whereas there were few discernible effects observed in the superficial SC in the monkey [24] or ground squirrel [20]. There is conflicting evidence on the effects of cortex ablation in the rabbit [8,18]. Little information is available on the effect of dark-rearing and strobe-rearing on the topography of the map of visual space in the SC. It has been shown that following strobe-rearing in Xeno,ous the distribution of the optic nerve terminals in the superficial SC was disrupted, but the topographic organization of the retinotectal projection remained unaltered [5]. Similar observations were also documented in Xenopus following dark-rearing [9]. In brief, the following series of experiments were performed to determine tl~te effects of dark-rearing, stroberearing and acute visual cortex removal on the topography and response properties of visual cells in the superficial SC. Preliminary data have already appeared in abstract form [26]. In the recording of vi:~ual multi-units, nineteen pigmented guinea-pigs (Cavia porcellus) were used for this study. Six were normal adult guinea-pigs, four were dark-reared from birth, four were strobe-reared from birth and five were adult guinea-pigs with their ipsilateral visual cortex aspirated acutely (extent of cortex removal has been documented in [28]). Single units were recorded in three normal adult animals and two dark-reared animals with their cortices intact. The strobe-reared animals were reared from birth in a stroboscopic environment, at a frequency of 2 Hz, in a 12 h strobe/dark cycle until they were at least 40 days old. The dark-reared animals we:re reared in a light-locked room from birth until they we:re at least 40 days old [29]. All animals were anaesthetized initially with Hypnorm® (Fentanyl citrate/Fluanisone, Janssen) (i.m.) and Hypnovel® (Midazolam, Roche) (i.p.) at a dose of 1 ml/ kg and 2 ml/kg, respectively. The left external jugular vein was cannulated to allow a 50% solution of Hypnorm® and Hypnovel® to be delivered intravenously as required (1.6 ml/kg, i.v). In the remaining animals, anaesthesia was maintained using hourly doses of a 50% solution of Hypnorm® and Hypnovel® (8 ml/kg, i.p.). In all animals a craniotomy was performed to expose the cortex overlying the right SC. The cortex was left intact for the majority of dark and strobe-reared animals and was aspirated at this point to reveal the right SC in the acute cortex removed animals. The animal was then transferred to a room where visual stimuli could be presenl:ed. Extracellular multi-unit and
217
single unit visual responses were recorded via glass-coated tungsten electrodes with resistances of < I Mfl and 2 Mfl, respectively. Visual multi-units and single units were usually recorded at a depth of 3.5-4.5 mm from the surface of the cortex. For recordings with the cortex intact, a current of approximately 10-30/~A was passed for 5 - 3 0 s at the bottom of the electrode track, so that the rostrocaudal electrode position in the SC could be verified after subsequent histological analysis. The animal was positioned so that the eye contralateral to the recording position was in the centre of a complete hemisphere of radius 35 cm. The front of the hemisphere was positioned directly in front of the animal (0 °) and the hemisphere was marked in 10° steps on the longitudinal axis and 15 ° radially. If the multi-units responded to moving stimuli, eight different directions were tested: up, down, to the right (tempo-nasally), to the left and four oblique directions. Any preference for these directions was recorded. The single units were only tested to visual stimuli moving in four perpendicular directions. It has been well documented that SC visual cells habituate rapidly. The interstimulus interval was at least 10 s. When single units were recorded, spikes were discriminated and counted electronically for a period of 2.5 s from the onset of the stimulus. The average of ten stimulus presentations was calculated and subtracted from the background activity. The azimuthal position of each receptive field was charted for both single and multi-units. The centre of the visual field was judged to be the area within the receptive field where the maximum response was elicited when projected stimuli were placed within it. For analysis of the topography data, normality and constant variance tests were performed on all residuals using the Kolnogorov-Smirnov and the Spearman Rank tests, respectively. Correlations between collicular position and peak angle were evaluated using regression analysis. All visual response characteristics of the units in the experimental groups were compared using the global Chi-squared test and significance was assumed when P < 0.05. The directional selectivity of individual experimental groups was compared with the control group and significance was assumed when P < 0.0125 (i.e. 0.05/ number of groups). Responses were classified according to the null hypothesis [1]. A multi-unit was classified as directionally selective if it preferentially responded to one direction significantly more than the opposing direction. The percentage of directionally selective multi-units was calculated in each of the four groups of animals and the percentages of directionally selective single units for control and dark-reared animals were also calculated and illustrated in the same figure (see Fig. 1). There was a significant decrease in the number of directionally selective multi-units in the superficial SC of animals which had been strobe-reared (0/65 visually responsive multi-units tested; Chi-squared = 12.63, P < 0.05) and dark-reared
218
S.K. Thornton et al. / Neuroscience Letters 213 (1996) 216-220
visual cortex removal. The results also suggest that following dark-rearing and strobe-rearing the number of directionally selective cells in the superficial layers of the SC is reduced. The percentage of directionally selective units found in the superficial layers of the SC is subject to species variation (ranging from 75% in the cat [23], 58% in the hamster [21], 9% in the rabbit [18], 7% in the mouse [6], to 5% in the monkey and 0% in the rat [11]). There have been no published data on the directional selectivity of visual responses in the superficial SC of the guinea-pig. The multi-unit data presented here suggest that in the superficial SC of the normal adult guinea-pig, 15% of visual responses are directionally selective. The value of multiunit recording for evaluating the response characteristics of cells is limited. Nevertheless, it may be reasonable to use this method if the cells which respond to a particular
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Fig. 1. Histogram illustrating the percentage of directionally selective multi-units for control group (cortex intact), dark-reared, strobe-reared and visual cortex removed groups. The percentage of directionally selective single units are also shown for the former two groups.
(1/68 multi-units and 0/3 single units; Chi-squared = 6.14, P < 0.05) compared with control animals (adult animals with their cortex intact; 8/53 multi-units and 2/9 single units). There was no significant difference in the number of directionally selective cells when comparing the normal group with the visual cortex removed group (11/52; Chisquared = 1.64, P > 0.05). The relationship between the rostrocaudal electrode recording positions in the SC and the positions of the visual receptive fields in azimuthal space is shown in Fig. 2 for all animal groups (single units for dark-reared and control animals are indicated by squares) Regression analysis revealed a significant correlation between recording position and azimuthal receptive field position: control (r = 0.88, P = 1.49 x 10-6), the group with the visual cortex acutely removed ( r = 0 . 7 2 , P = 1 x 10-4), the group dark-reared from birth (r = 0.86, P = 4.6 x 10 -8) and also for the group strobereared from birth (r = 0.91, P = 1 x 10-4). King and Carlile [14] showed that binocular eye-lid suture has no effect on the SC visuotopy. The data presented here indicate that superficial visuotopy is not significantly affected by dark-rearing, strobe-rearing or acute
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Fig. 2. The collicular representations of the contralateral visual hemifield for each of the four groups: A, control group; B, visual cortex removed; C, dark-reared; and D, strobe-reared. The ordinate represents the collicular recording position (0 mm = rostral, 4 mm = caudal). The abscissa represents the spatial location of the centre of the visual receptive feld: 0 ° represents the front of the animal and 1800 lies directly behind the animal. The line on each graph is drawn from the appropriate regression equation. Circles represent sites in the SC where multi-units were recorded and squares where single units were recorded.
S.K. Thornton et al. / Neuroscience Letters 213 (1996) 216-220
direction are clustered together. In this study, whether it was single or multi-units that were recorded, the results revealed that the directional selectivity was significantly reduced following dark-rearing from birth when compared to control animals. It is thought that the directional selectivity in the SC of the ground squirrel is primarily derived from the retinal input conferred upon the SC [20], whereas it is thought that directional selectivity in the SC of the cat is cortically mediated. Furthermore, contrary to previous documentation [2,23], evidence exists to indicate that directional selectivity in the superficial SC is not significantly affected following primary visual cortex ablation in either neonatal or adult cats [19]. It may be postulated that in the guineapig, directional selectivity results primarily from the organization of the retina rather than projections from the visual cortex, Following strobe-rearing and dark-rearing the topography o f the azimuthal map of visual space in the SC was not significantly disrupted. These results are in agreement with observations in X e n o p u s [5,9]. Many lines of evidence suggest that in mammals two distinct stages in the development of the retino-collicular projection take place post-conceptually [12,17,25]. Since the guinea-pig is a very precocial species and, unlike many mammals, eyes are open from birth, it seems reasonable to postulate that the arrival of optic axons at the SC and initiation of terminal formation within the structure, occur prenatally. The time course o f axonal loss from the optic nerve in the guinea-pig has also been documented and it may be postulated from these data that the refinement of the retinotectal projection occurs primarily prenatally in the guinea-pig [16]. In the guinea-pig, the peak of optic nerve degeneration occurs at approximately 42 days gestation but gradual axonal loss continues slowly, even postnatally. Therefore, what may be implicated from our data is that degradation o f the relatively mature visual map does not occur following visual deprivation since the retinocollicular projection is relatively hard-wired and as such is not shaped by postnatal visual experience. What may be further postulated is thai, although the visuotopy remains relatively unaffected by developmental restriction, the finer grained physiology (e.g. directional selectivity) could be affected since these features may not be mature in the neonate. The data also indicate that projections from the primary visual cortex (area 17) to the superficial SC do not play a role in the visual receptive field organization in the SC. It has been previously shown that both dark-rearing and strobe-rearing affect the spatial tuning (and topography in the case o f dark-rearing) o f the map of auditory space in the deep SC o f the guinea-pig [15,27,29]. Thus, it appears that movement detection during development is an important component for at least the refinement of the auditory map. Since the superficial visuotopy was not significantly affected by dark- or strobe-rearing it is possible that it is
219
the disruption of other visual response characteristics (e.g. directional selectivity) in the SC that influences the spatial tuning of the auditory receptive fields. Alternatively, the lack of, or aberrant input from, extrastriate visual cortical areas to visual cells in the deep layers could also be a possible route for auditory space map perturbation following strobe- or dark-rearing. [1] Bariow, H.B. and Levick, W.R., The mechanism of directionally selective units in the rabbits retina, J. Physiol. (London), 178 (1965) 477-504. [2] Berman, N. and Cynader, M., Receptive fields in cat superior colliculus after visual cortex lesions, J. Physiol. (London), 245 (1975) 261-270. [3] Carman, L.S. and Schneider, G.E., Orienting behavior in hamsters with lesions of superior colliculus, pretectum, and visual cortex, Exp. Brain Res., 90 (1992) 79-91. [4] Chalupa, L.M. and Rhoades, R.W., Directional selectivity in hamster superior colliculus is modified by strobe-rearing but not by dark-rearing, Science, 199 (1978)998-1001. [5] Chung, S.H., Gaze, R.M. and Stifling, R.V., Abnormal visual function in Xenopus following stroboscopic illumination, Nature (London), 246 (1973) 186-188. [6] Drager, U.C. and Hubel, D.H., Responses to visual stimulation and relationship between visual, auditory and somatosensory inputs in mouse superior colliculus, J. Neurophysiol., 38 (1975) 690-713. [7] Dreher, B. and Robinson, S.R., Development of the retinofugal pathway in birds and mammals: evidence for a common 'timetable', Brain Behav. Evol., 31 (1988) 369-390. [8] Graham, J., Berman, N. and Murphy, E.H., Effects of visual cortical lesions on receptive-field properties of single units in superior colliculus of the rabbit, J. Neurophysiol., 47 (1982) 272-286. [9] Grant, S. and Keating, M.J., Changing patterns of binocular visual connections in the intertectal system during development of the frog Xenopus laevis. II. Abnormalities following early visual deprivation, Exp. Brain Res., 75 (1989) 117-132. [10] Hoffmann, K.-P. and Sherman, S.M., Effect of early binocular deprivation on visual input to cat superior colliculus, J. Neurophysiol., 38 (1975) 1049-1059. [11] Humphrey, N.K., Responses to visual stimuli of units in the superior colliculus of rats and monkeys, Exp. Neurol., 20 (1968) 312340. [12] Jhaveri, S., Edwards, M.A. and Schneider, G.E., Initial stages of retinofugal axon development in the hamster: evidence for two distinct modes of growth, Exp. Brain. Res., 87 (1991) 371-382. [13] Kennedy, H., Flandrin, J.M. and Amblard, B., Afferent visual pathways and receptive field properties of superior colliculus neurons in stroboscopically reared cats, Neurosci. Lett., 19 (1980) 283-288. [14] King, A.J. and Carlile, S., Changes induced in the representation of auditory space in the superior colliculus by rearing ferrets with binocular eyelid suture, Exp. Brain Res., 94 (1993) 444-455. [15] King, A.J. and Palmer, A.R., Cells responsive to free-field auditory stimuli in guinea pig superior colliculus: distribution and response properties, J. Physiol., 342 (1983) 361-381. [16] Langford, C. and Sefton, A.J., The relative time course of axonal loss from the optic nerve of the developing guinea pig is consistent with that of other mammals, Vis. Neurosci., 9 (1992) 555-564. [17l Mark, R.F., Freeman, T.C.B., Ding, Y. and Marotte, L.R., Two stages in the development of a mammalian retinocollicular projection, NeuroReport, 5 (1993) 117-120. [18] Masland, R.H., Chow, K.L. and Stewart, D.L., Receptive-field characteristics of superior colliculus neurons in the rabbit, J. Neurophysiol., 34 (1971) 148-156. [19] Mendola, J.D. and Payne, B.R., Direction selectivity and physiological compensation in the superior colliculus following removal of areas 17 and 18, Vis. Neurosci., l0 (6)(1993) 1019-1026.
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[20] Michael, C.R., Functional organization of cells in superior colliculus of the ground squirrel, J. Neurophysiol., 35 (1972) 833-846. [21] Rhoades, R.W. and Chalupa, L.M., Directional selectivity in the superior colliculus of the golden hamster, Brain Res., 118 (1976) 334-338. [22] Rhoades, R.W. and Chalupa, L.M., Functional and anatomical consequences of neonatal cortical damage in superior colliculus of the golden hamster, J. Neurophysiol., 41 (6)(1978) 1466-1494. [23] Rosenquist, A.C. and Palmer, L.A, Visual receptive field properties of cells of the superior colliculus after cortical lesions in the cat, Exp. Neurol., 33 (1971) 629-652. [24] Schiller, P.H., Stryker, M., Cynader, M. and Berman, N., Response characteristics of single cells in the monkey superior colliculus following ablation or cooling of visual cortex, J. Neurophysiol., 37 (1974) 181-194. [25] Simon, D.K. and O'Leary, D.D.M., Development of Topographic order in the mammalian retinocoilicular projection, J. Neurosci., 12 (1992) 1212-1232.
[26] Thornton, S.K., Ingham, N.J. and Withington, D.J., The effects of visual deprivation (stroboscopic and dark-rearing) and removal of the visual cortex on visual responses in the guinea-pig superior colliculus, Ear. J. Neurosci. Suppl., 6 (1993) 132. [27] Thornton, S.K., lngham, N.J. and Withington, D.J., Visual movement and pattern are important for the development of a map of auditory space in the guinea-pig superior collieulus, Exp. Brain Res., 106 (1995) 257-264. [28] Withington, D.J., Binns, K.E. and Keating, M.J., The normal emergence of the superior collicular map of auditory space in the guinea-pig following developmental removal of visual cortex, Neurosci. Lett., 125 (1991) 209-211. [29] Withington-Wray, D.J., Binns, K.E. and Keating, M.J., The maturation of the superior collicular map of auditory space in the guinea pig is disrupted by developmental visual deprivation, Ear. J. Neurosci., 2 (1990) 682-692
li[gIg lfflTllB
Neuroscience Letters 213 (1996) 216-220
The effect of dark-rearing, strobe-rearing and acute visual cortex removal on the visual responses in the superficial superior colliculus of the guinea-pig S.K. Thornton*, D.J. Withington, D. McCrossan, N.J.
Ingham
Department of Physiology, Worsley Medical and Dental Building, University of Leeds, Leeds LS2 9NQ, UK Received 12 December 1995; revised version received 27 June 1996; accepted 27 June 1996
Abstract Extracellular multi-unit responses to visual stimuli were recorded in the cells of the superficial layers of the superior colliculus (SC) in four groups of adult guinea-pigs: a control group, a strobe-reared group, a dark-reared group and a group with the ipsilateral visual cortex removed acutely. Single unit visual responses were also recorded in a control and a dark-reared group. When guinea-pigs were either strobe or dark-reared from birth, the number of directionally selective responses in the superficial SC decreased significantly. Acute removal of the visual cortex had no affect on the number of directionally selective cells recorded in the SC. The correlation between azimuthal visual receptive field and rostrocaudal position of the recording electrode in the SC was not significantly different from the control group following strobe, dark-rearing or acute visual cortex removal. These data imply that, during early development, visual information is necessary for directional selectivity of the visual responses in the superficial SC. However, the map of visual azimuthal space is essentially unperturbed by visual restriction (in the form of dark or strobe-rearing) or acute visual cortex removal.
Keywords: Superior colliculus; Visual deprivation; Directional selectivity; Topography; Visual space map; Cortex removal
The superior colliculus (SC) is an important station in the mammalian midbrain for the processing of visual information and is thought to be involved in the integration of sensory and motor information for the orientation of the animal towards a visual stimulus [3]. It has been divided operationally into two parts, the superficial and deep layers, both of which contain maps of the visual world. In the guinea-pig, the developmental emergence of the visual space map has not been documented but in other species the retinotectal projection is ordered before or at eye-opening [7]. Since the eyes of the guinea-pig are open from birth it also likely that the visual receptive fields in the SC are topographically ordered at this time. The refinement of the map and development of other visual characteristics could take place postnatally since, despite the fact that the majority of optic nerve loss occurs prenatally,
* Corresponding author. Center for Neural Science, New York University, 4 Washington Place, Room 809, New York, NY 10003-6621, USA. Fax: +1 212 9954011.
gradual axonal loss still occurs during the first few weeks of life [16]. There is an abundance of physiological data which indicate that visual experience plays an important role in the normal development of visual responses in the mammalian SC [4,10,13]. The extent to which visual experience may affect normal physiological maturation of the SC depends not only on the type of deprivation encountered but also varies with the species studied. Dark-rearing and stroberearing are the two forms of visual deprivation commonly utilized in studies. Dark-rearing removes all aspects of visual perception, whereas stroboscopic illumination allows perception of visual patterns but abolishes that of movements. Species differences have been observed between the cat and the hamster. In the cat, both darkrearing [10] and strobe-rearing [13] affect visual responses in the SC, but the visual responses in the hamster SC are only influenced by strobe-rearing [4]. The deficits observed in the superficial SC after strobe-rearing in the hamster include a reduction in the number of directionally selective cells, an increase in the receptive field size, an
0304-3940/96/$12.00 © 1996 Elsevier Science Ireland Ltd. All rights reserved PII S 0 3 0 4 - 3 9 4 0 ( 9 6 ) 1285 1-2
S.K. Thornton et al. / Neuroscience Letters 213 (1996) 216-220
increase in the number of cells responding to slow movement and a decrease in the number of cells responding to visual stimuli [4]. Since the superficial SC receives visual input from the primary visual cortex, the removal of the visual cortex may influence the visual properties of the cells in the superficial SC. Lesions of the visual cortex result in deficits of visual response properties in the superficial SC of the cat [2,23] and hamster [22], whereas there were few discernible effects observed in the superficial SC in the monkey [24] or ground squirrel [20]. There is conflicting evidence on the effects of cortex ablation in the rabbit [8,18]. Little information is available on the effect of dark-rearing and strobe-rearing on the topography of the map of visual space in the SC. It has been shown that following strobe-rearing in Xeno,ous the distribution of the optic nerve terminals in the superficial SC was disrupted, but the topographic organization of the retinotectal projection remained unaltered [5]. Similar observations were also documented in Xenopus following dark-rearing [9]. In brief, the following series of experiments were performed to determine tl~te effects of dark-rearing, stroberearing and acute visual cortex removal on the topography and response properties of visual cells in the superficial SC. Preliminary data have already appeared in abstract form [26]. In the recording of vi:~ual multi-units, nineteen pigmented guinea-pigs (Cavia porcellus) were used for this study. Six were normal adult guinea-pigs, four were dark-reared from birth, four were strobe-reared from birth and five were adult guinea-pigs with their ipsilateral visual cortex aspirated acutely (extent of cortex removal has been documented in [28]). Single units were recorded in three normal adult animals and two dark-reared animals with their cortices intact. The strobe-reared animals were reared from birth in a stroboscopic environment, at a frequency of 2 Hz, in a 12 h strobe/dark cycle until they were at least 40 days old. The dark-reared animals we:re reared in a light-locked room from birth until they we:re at least 40 days old [29]. All animals were anaesthetized initially with Hypnorm® (Fentanyl citrate/Fluanisone, Janssen) (i.m.) and Hypnovel® (Midazolam, Roche) (i.p.) at a dose of 1 ml/ kg and 2 ml/kg, respectively. The left external jugular vein was cannulated to allow a 50% solution of Hypnorm® and Hypnovel® to be delivered intravenously as required (1.6 ml/kg, i.v). In the remaining animals, anaesthesia was maintained using hourly doses of a 50% solution of Hypnorm® and Hypnovel® (8 ml/kg, i.p.). In all animals a craniotomy was performed to expose the cortex overlying the right SC. The cortex was left intact for the majority of dark and strobe-reared animals and was aspirated at this point to reveal the right SC in the acute cortex removed animals. The animal was then transferred to a room where visual stimuli could be presenl:ed. Extracellular multi-unit and
217
single unit visual responses were recorded via glass-coated tungsten electrodes with resistances of < I Mfl and 2 Mfl, respectively. Visual multi-units and single units were usually recorded at a depth of 3.5-4.5 mm from the surface of the cortex. For recordings with the cortex intact, a current of approximately 10-30/~A was passed for 5 - 3 0 s at the bottom of the electrode track, so that the rostrocaudal electrode position in the SC could be verified after subsequent histological analysis. The animal was positioned so that the eye contralateral to the recording position was in the centre of a complete hemisphere of radius 35 cm. The front of the hemisphere was positioned directly in front of the animal (0 °) and the hemisphere was marked in 10° steps on the longitudinal axis and 15 ° radially. If the multi-units responded to moving stimuli, eight different directions were tested: up, down, to the right (tempo-nasally), to the left and four oblique directions. Any preference for these directions was recorded. The single units were only tested to visual stimuli moving in four perpendicular directions. It has been well documented that SC visual cells habituate rapidly. The interstimulus interval was at least 10 s. When single units were recorded, spikes were discriminated and counted electronically for a period of 2.5 s from the onset of the stimulus. The average of ten stimulus presentations was calculated and subtracted from the background activity. The azimuthal position of each receptive field was charted for both single and multi-units. The centre of the visual field was judged to be the area within the receptive field where the maximum response was elicited when projected stimuli were placed within it. For analysis of the topography data, normality and constant variance tests were performed on all residuals using the Kolnogorov-Smirnov and the Spearman Rank tests, respectively. Correlations between collicular position and peak angle were evaluated using regression analysis. All visual response characteristics of the units in the experimental groups were compared using the global Chi-squared test and significance was assumed when P < 0.05. The directional selectivity of individual experimental groups was compared with the control group and significance was assumed when P < 0.0125 (i.e. 0.05/ number of groups). Responses were classified according to the null hypothesis [1]. A multi-unit was classified as directionally selective if it preferentially responded to one direction significantly more than the opposing direction. The percentage of directionally selective multi-units was calculated in each of the four groups of animals and the percentages of directionally selective single units for control and dark-reared animals were also calculated and illustrated in the same figure (see Fig. 1). There was a significant decrease in the number of directionally selective multi-units in the superficial SC of animals which had been strobe-reared (0/65 visually responsive multi-units tested; Chi-squared = 12.63, P < 0.05) and dark-reared
218
S.K. Thornton et al. / Neuroscience Letters 213 (1996) 216-220
visual cortex removal. The results also suggest that following dark-rearing and strobe-rearing the number of directionally selective cells in the superficial layers of the SC is reduced. The percentage of directionally selective units found in the superficial layers of the SC is subject to species variation (ranging from 75% in the cat [23], 58% in the hamster [21], 9% in the rabbit [18], 7% in the mouse [6], to 5% in the monkey and 0% in the rat [11]). There have been no published data on the directional selectivity of visual responses in the superficial SC of the guinea-pig. The multi-unit data presented here suggest that in the superficial SC of the normal adult guinea-pig, 15% of visual responses are directionally selective. The value of multiunit recording for evaluating the response characteristics of cells is limited. Nevertheless, it may be reasonable to use this method if the cells which respond to a particular
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Fig. 1. Histogram illustrating the percentage of directionally selective multi-units for control group (cortex intact), dark-reared, strobe-reared and visual cortex removed groups. The percentage of directionally selective single units are also shown for the former two groups.
(1/68 multi-units and 0/3 single units; Chi-squared = 6.14, P < 0.05) compared with control animals (adult animals with their cortex intact; 8/53 multi-units and 2/9 single units). There was no significant difference in the number of directionally selective cells when comparing the normal group with the visual cortex removed group (11/52; Chisquared = 1.64, P > 0.05). The relationship between the rostrocaudal electrode recording positions in the SC and the positions of the visual receptive fields in azimuthal space is shown in Fig. 2 for all animal groups (single units for dark-reared and control animals are indicated by squares) Regression analysis revealed a significant correlation between recording position and azimuthal receptive field position: control (r = 0.88, P = 1.49 x 10-6), the group with the visual cortex acutely removed ( r = 0 . 7 2 , P = 1 x 10-4), the group dark-reared from birth (r = 0.86, P = 4.6 x 10 -8) and also for the group strobereared from birth (r = 0.91, P = 1 x 10-4). King and Carlile [14] showed that binocular eye-lid suture has no effect on the SC visuotopy. The data presented here indicate that superficial visuotopy is not significantly affected by dark-rearing, strobe-rearing or acute
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Fig. 2. The collicular representations of the contralateral visual hemifield for each of the four groups: A, control group; B, visual cortex removed; C, dark-reared; and D, strobe-reared. The ordinate represents the collicular recording position (0 mm = rostral, 4 mm = caudal). The abscissa represents the spatial location of the centre of the visual receptive feld: 0 ° represents the front of the animal and 1800 lies directly behind the animal. The line on each graph is drawn from the appropriate regression equation. Circles represent sites in the SC where multi-units were recorded and squares where single units were recorded.
S.K. Thornton et al. / Neuroscience Letters 213 (1996) 216-220
direction are clustered together. In this study, whether it was single or multi-units that were recorded, the results revealed that the directional selectivity was significantly reduced following dark-rearing from birth when compared to control animals. It is thought that the directional selectivity in the SC of the ground squirrel is primarily derived from the retinal input conferred upon the SC [20], whereas it is thought that directional selectivity in the SC of the cat is cortically mediated. Furthermore, contrary to previous documentation [2,23], evidence exists to indicate that directional selectivity in the superficial SC is not significantly affected following primary visual cortex ablation in either neonatal or adult cats [19]. It may be postulated that in the guineapig, directional selectivity results primarily from the organization of the retina rather than projections from the visual cortex, Following strobe-rearing and dark-rearing the topography o f the azimuthal map of visual space in the SC was not significantly disrupted. These results are in agreement with observations in X e n o p u s [5,9]. Many lines of evidence suggest that in mammals two distinct stages in the development of the retino-collicular projection take place post-conceptually [12,17,25]. Since the guinea-pig is a very precocial species and, unlike many mammals, eyes are open from birth, it seems reasonable to postulate that the arrival of optic axons at the SC and initiation of terminal formation within the structure, occur prenatally. The time course o f axonal loss from the optic nerve in the guinea-pig has also been documented and it may be postulated from these data that the refinement of the retinotectal projection occurs primarily prenatally in the guinea-pig [16]. In the guinea-pig, the peak of optic nerve degeneration occurs at approximately 42 days gestation but gradual axonal loss continues slowly, even postnatally. Therefore, what may be implicated from our data is that degradation o f the relatively mature visual map does not occur following visual deprivation since the retinocollicular projection is relatively hard-wired and as such is not shaped by postnatal visual experience. What may be further postulated is thai, although the visuotopy remains relatively unaffected by developmental restriction, the finer grained physiology (e.g. directional selectivity) could be affected since these features may not be mature in the neonate. The data also indicate that projections from the primary visual cortex (area 17) to the superficial SC do not play a role in the visual receptive field organization in the SC. It has been previously shown that both dark-rearing and strobe-rearing affect the spatial tuning (and topography in the case o f dark-rearing) o f the map of auditory space in the deep SC o f the guinea-pig [15,27,29]. Thus, it appears that movement detection during development is an important component for at least the refinement of the auditory map. Since the superficial visuotopy was not significantly affected by dark- or strobe-rearing it is possible that it is
219
the disruption of other visual response characteristics (e.g. directional selectivity) in the SC that influences the spatial tuning of the auditory receptive fields. Alternatively, the lack of, or aberrant input from, extrastriate visual cortical areas to visual cells in the deep layers could also be a possible route for auditory space map perturbation following strobe- or dark-rearing. [1] Bariow, H.B. and Levick, W.R., The mechanism of directionally selective units in the rabbits retina, J. Physiol. (London), 178 (1965) 477-504. [2] Berman, N. and Cynader, M., Receptive fields in cat superior colliculus after visual cortex lesions, J. Physiol. (London), 245 (1975) 261-270. [3] Carman, L.S. and Schneider, G.E., Orienting behavior in hamsters with lesions of superior colliculus, pretectum, and visual cortex, Exp. Brain Res., 90 (1992) 79-91. [4] Chalupa, L.M. and Rhoades, R.W., Directional selectivity in hamster superior colliculus is modified by strobe-rearing but not by dark-rearing, Science, 199 (1978)998-1001. [5] Chung, S.H., Gaze, R.M. and Stifling, R.V., Abnormal visual function in Xenopus following stroboscopic illumination, Nature (London), 246 (1973) 186-188. [6] Drager, U.C. and Hubel, D.H., Responses to visual stimulation and relationship between visual, auditory and somatosensory inputs in mouse superior colliculus, J. Neurophysiol., 38 (1975) 690-713. [7] Dreher, B. and Robinson, S.R., Development of the retinofugal pathway in birds and mammals: evidence for a common 'timetable', Brain Behav. Evol., 31 (1988) 369-390. [8] Graham, J., Berman, N. and Murphy, E.H., Effects of visual cortical lesions on receptive-field properties of single units in superior colliculus of the rabbit, J. Neurophysiol., 47 (1982) 272-286. [9] Grant, S. and Keating, M.J., Changing patterns of binocular visual connections in the intertectal system during development of the frog Xenopus laevis. II. Abnormalities following early visual deprivation, Exp. Brain Res., 75 (1989) 117-132. [10] Hoffmann, K.-P. and Sherman, S.M., Effect of early binocular deprivation on visual input to cat superior colliculus, J. Neurophysiol., 38 (1975) 1049-1059. [11] Humphrey, N.K., Responses to visual stimuli of units in the superior colliculus of rats and monkeys, Exp. Neurol., 20 (1968) 312340. [12] Jhaveri, S., Edwards, M.A. and Schneider, G.E., Initial stages of retinofugal axon development in the hamster: evidence for two distinct modes of growth, Exp. Brain. Res., 87 (1991) 371-382. [13] Kennedy, H., Flandrin, J.M. and Amblard, B., Afferent visual pathways and receptive field properties of superior colliculus neurons in stroboscopically reared cats, Neurosci. Lett., 19 (1980) 283-288. [14] King, A.J. and Carlile, S., Changes induced in the representation of auditory space in the superior colliculus by rearing ferrets with binocular eyelid suture, Exp. Brain Res., 94 (1993) 444-455. [15] King, A.J. and Palmer, A.R., Cells responsive to free-field auditory stimuli in guinea pig superior colliculus: distribution and response properties, J. Physiol., 342 (1983) 361-381. [16] Langford, C. and Sefton, A.J., The relative time course of axonal loss from the optic nerve of the developing guinea pig is consistent with that of other mammals, Vis. Neurosci., 9 (1992) 555-564. [17l Mark, R.F., Freeman, T.C.B., Ding, Y. and Marotte, L.R., Two stages in the development of a mammalian retinocollicular projection, NeuroReport, 5 (1993) 117-120. [18] Masland, R.H., Chow, K.L. and Stewart, D.L., Receptive-field characteristics of superior colliculus neurons in the rabbit, J. Neurophysiol., 34 (1971) 148-156. [19] Mendola, J.D. and Payne, B.R., Direction selectivity and physiological compensation in the superior colliculus following removal of areas 17 and 18, Vis. Neurosci., l0 (6)(1993) 1019-1026.
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[20] Michael, C.R., Functional organization of cells in superior colliculus of the ground squirrel, J. Neurophysiol., 35 (1972) 833-846. [21] Rhoades, R.W. and Chalupa, L.M., Directional selectivity in the superior colliculus of the golden hamster, Brain Res., 118 (1976) 334-338. [22] Rhoades, R.W. and Chalupa, L.M., Functional and anatomical consequences of neonatal cortical damage in superior colliculus of the golden hamster, J. Neurophysiol., 41 (6)(1978) 1466-1494. [23] Rosenquist, A.C. and Palmer, L.A, Visual receptive field properties of cells of the superior colliculus after cortical lesions in the cat, Exp. Neurol., 33 (1971) 629-652. [24] Schiller, P.H., Stryker, M., Cynader, M. and Berman, N., Response characteristics of single cells in the monkey superior colliculus following ablation or cooling of visual cortex, J. Neurophysiol., 37 (1974) 181-194. [25] Simon, D.K. and O'Leary, D.D.M., Development of Topographic order in the mammalian retinocoilicular projection, J. Neurosci., 12 (1992) 1212-1232.
[26] Thornton, S.K., Ingham, N.J. and Withington, D.J., The effects of visual deprivation (stroboscopic and dark-rearing) and removal of the visual cortex on visual responses in the guinea-pig superior colliculus, Ear. J. Neurosci. Suppl., 6 (1993) 132. [27] Thornton, S.K., lngham, N.J. and Withington, D.J., Visual movement and pattern are important for the development of a map of auditory space in the guinea-pig superior collieulus, Exp. Brain Res., 106 (1995) 257-264. [28] Withington, D.J., Binns, K.E. and Keating, M.J., The normal emergence of the superior collicular map of auditory space in the guinea-pig following developmental removal of visual cortex, Neurosci. Lett., 125 (1991) 209-211. [29] Withington-Wray, D.J., Binns, K.E. and Keating, M.J., The maturation of the superior collicular map of auditory space in the guinea pig is disrupted by developmental visual deprivation, Ear. J. Neurosci., 2 (1990) 682-692