Physiology & Behavior, Vol. 19, pp. 635-646. Pergamon Press and Brain Research Publ., 1977. Printed in the U.S.A.
Effects of Subcortical Lesions on Visual Intensity Discriminations in Rats C. R. LEGG 1 AND A. COWEY
Department o f Experimental Psychology, South Parks Road, Oxford OX1 3UD, England (Received 2 February 1977) LEGG, C. R. AND A. COWEY. Effects of subcortical lesions on visual intensity discrimination in rats. PHYSIOL. BEHAV. 19(5) 635-646, 1977. -- The role of several subcortical structures in visual intensity discrimination was examined by comparing the effects of localized lesions on a variety of intensity discriminations. In Experiment 1 light avoidance was unimpaired after lesions of the ventral lateral geniculate nucleus (LGNv), nucleus lateralis posterior (TLP), nucleus posterior of Gurdjian (NPG), dorsal pretectum (PTd), and ventral pretectum (PTv). The LGNv, TLP, NPG and PTv, but not the PTd, groups were impaired on a simultaneous black versus white (BW) discrimination in Experiment 2. None of these groups was impaired on a horizontal versus vertical discrimination (HV). The TLP group showed a transient impairment on a successive light versus dark discrimination, not present with the LGNv and NPG groups (Experiment 3). In Experiment 4 all three groups were impaired on a successive BW discrimination. In Experiment 5 rats with LGNv lesions but not with TLP lesions had elevated relative brightness thresholds. Both groups had normal absolute thresholds. The results are related to the possibility that information about intensity and pattern is coded in separate visual pathways. Visual intensity discrimination
Subcortical lesions
Although LGNv is the only primary optic centre known to be necessary for normal intensity discriminations similar impairments have been found after lesions in parts of the posterior thalamus not receiving direct visual input [25, 36, 46, 47, 4 8 ] . Horel [21] suggested that LGNv and posterior thalamus were directly connected, but recent autoradiographic studies indicate that they are not [17, 25, 38, 4 4 ] . However, the pretectal region was damaged in the experiments concerning the role of the posterior thalamus and the LGNv does project to the pretectal area [17, 25, 38, 44]. We therefore gave a variety of visual discrimination tasks to rats with discrete lesions in the posterior thalamus and pretectal area to determine (a) whether lesions restricted to posterior thalamus proper impaired intensity discrimination, (b) if so, whether the effects are dissociable from those of destroying the LGNv, and (c) whether damage to the pretectum mimics the effects of damaging LGNv. Five lesion groups were studied: (1) LGNv, (2) nucleus posterior of Gurdjian [18] (NPG), (3) nucleus lateralis posterior thalami (TLP), (4) ventro-medial pretectal area (PTv) and (5) dorso-medial pretectal area (PTd). A total of five experiments was performed and the rationale for each is given in the appropriate section.
INTENSITY discrimination is in many ways the poor relation of visual neuropsychology. In a period of extensive investigation of the role of the geniculostriate system in pattern vision and the superior colliculus in visuomotor integration or visual attention, little has been learned about the visual centres involved in intensity discrimination. Pasik and Pasik [33,34] have argued that the accessory optic system of the monkey is important, but in their experiments only animals with striate cortex ablation extending into areas 18 and 19, combined with accessory optic system lesions, showed any intensity discrimination impairment. Damaging the accessory optic system alone produced no deficit [32]. Recent studies [20, 25, 28] show that a previously neglected structure receiving a prominent input from the retina, the ventral lateral geniculate nucleus (LGNv), may be involved in intensity discrimination. Lesions of the LGNv impaired both the acquisition and retention of a black versus white discrimination, with little or no associated impairment on a discrimination between horizontal and vertical stripes. Despite this result it is premature to identify two complementary visual systems consisting of the geniculostriate system for pattern vision and the LGNv and its projections for intensity discrimination. Studies of animals with total geniculostriate system destruction show that they are not deprived of all pattern vision [33,49], instead they may be described as suffering from amblyopia [49], as may patients with striate cortex damage who display blindsight within their field defects [50]. Similarly lesions of LGNv may impair only certain aspects of intensity discrimination.
G E N E R A L METHOD
Animals and Surgery Thirty-three male hooded rats weighing approximately 3 5 0 g , and all less than six months old at the time of surgery were used. Using standard stereotaxic surgical
Present address: Department of Psychology, North East London Polytechnic, Three Mills, Abbey Lane, Stratford, London El5 2RP, England. 635
© r..) Z
Effects of Subcortical Lesions on Visual Intensity Discriminations in Rats C. R. LEGG 1 AND A. COWEY
Department o f Experimental Psychology, South Parks Road, Oxford OX1 3UD, England (Received 2 February 1977) LEGG, C. R. AND A. COWEY. Effects of subcortical lesions on visual intensity discrimination in rats. PHYSIOL. BEHAV. 19(5) 635-646, 1977. -- The role of several subcortical structures in visual intensity discrimination was examined by comparing the effects of localized lesions on a variety of intensity discriminations. In Experiment 1 light avoidance was unimpaired after lesions of the ventral lateral geniculate nucleus (LGNv), nucleus lateralis posterior (TLP), nucleus posterior of Gurdjian (NPG), dorsal pretectum (PTd), and ventral pretectum (PTv). The LGNv, TLP, NPG and PTv, but not the PTd, groups were impaired on a simultaneous black versus white (BW) discrimination in Experiment 2. None of these groups was impaired on a horizontal versus vertical discrimination (HV). The TLP group showed a transient impairment on a successive light versus dark discrimination, not present with the LGNv and NPG groups (Experiment 3). In Experiment 4 all three groups were impaired on a successive BW discrimination. In Experiment 5 rats with LGNv lesions but not with TLP lesions had elevated relative brightness thresholds. Both groups had normal absolute thresholds. The results are related to the possibility that information about intensity and pattern is coded in separate visual pathways. Visual intensity discrimination
Subcortical lesions
Although LGNv is the only primary optic centre known to be necessary for normal intensity discriminations similar impairments have been found after lesions in parts of the posterior thalamus not receiving direct visual input [25, 36, 46, 47, 4 8 ] . Horel [21] suggested that LGNv and posterior thalamus were directly connected, but recent autoradiographic studies indicate that they are not [17, 25, 38, 4 4 ] . However, the pretectal region was damaged in the experiments concerning the role of the posterior thalamus and the LGNv does project to the pretectal area [17, 25, 38, 44]. We therefore gave a variety of visual discrimination tasks to rats with discrete lesions in the posterior thalamus and pretectal area to determine (a) whether lesions restricted to posterior thalamus proper impaired intensity discrimination, (b) if so, whether the effects are dissociable from those of destroying the LGNv, and (c) whether damage to the pretectum mimics the effects of damaging LGNv. Five lesion groups were studied: (1) LGNv, (2) nucleus posterior of Gurdjian [18] (NPG), (3) nucleus lateralis posterior thalami (TLP), (4) ventro-medial pretectal area (PTv) and (5) dorso-medial pretectal area (PTd). A total of five experiments was performed and the rationale for each is given in the appropriate section.
INTENSITY discrimination is in many ways the poor relation of visual neuropsychology. In a period of extensive investigation of the role of the geniculostriate system in pattern vision and the superior colliculus in visuomotor integration or visual attention, little has been learned about the visual centres involved in intensity discrimination. Pasik and Pasik [33,34] have argued that the accessory optic system of the monkey is important, but in their experiments only animals with striate cortex ablation extending into areas 18 and 19, combined with accessory optic system lesions, showed any intensity discrimination impairment. Damaging the accessory optic system alone produced no deficit [32]. Recent studies [20, 25, 28] show that a previously neglected structure receiving a prominent input from the retina, the ventral lateral geniculate nucleus (LGNv), may be involved in intensity discrimination. Lesions of the LGNv impaired both the acquisition and retention of a black versus white discrimination, with little or no associated impairment on a discrimination between horizontal and vertical stripes. Despite this result it is premature to identify two complementary visual systems consisting of the geniculostriate system for pattern vision and the LGNv and its projections for intensity discrimination. Studies of animals with total geniculostriate system destruction show that they are not deprived of all pattern vision [33,49], instead they may be described as suffering from amblyopia [49], as may patients with striate cortex damage who display blindsight within their field defects [50]. Similarly lesions of LGNv may impair only certain aspects of intensity discrimination.
G E N E R A L METHOD
Animals and Surgery Thirty-three male hooded rats weighing approximately 3 5 0 g , and all less than six months old at the time of surgery were used. Using standard stereotaxic surgical
Present address: Department of Psychology, North East London Polytechnic, Three Mills, Abbey Lane, Stratford, London El5 2RP, England. 635
© r..) Z
