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No deficit in near-field visual acuity of pigeons after transection of the isthmo-optic tract
Physiology & Behavior,Vol. 21, pp. 813-816. PergamonPress and BrainResearch Publ., 1978. Printedin the U.S.A.
BRIEF COMMUNICATION No Deficit in Near-Field Visual Acuity of Pigeons After Transection of the Isthmo-Optic Tract I R O N A L D R. K N I P L I N G
University College, University o f Maryland, University Boulevard at Adelphi R o a d College Park, M D 20742 ( R e c e i v e d 2 F e b r u a r y 1978) KNIPLING, R. R. No deficit in near-field visual acuity of pigeons after transection of the isthmo-optic tract. PHYSIOL. BEHAV. 21(5) 813-816, 1978.--Five pigeons (Columbia livia) were trained on a visual discrimination task designed to measure near-field spatial resolution capacity (visual acuity) for a moderately bright target. After stabilization of discrimination performance over a number of sessions, bilateral radio-frequency coagulation lesions were performed. These lesions were intended to bilaterally transect the isthmo-optic tract (lOT), which is the pathway of the efferent fibers to the retina. The anatomical results revealed that two birds sustained complete bilateral transection of the lOT. One pigeon sustained partial bilateral transection, and two birds suffered no lOT damage. The latter two birds functioned as control cases. None of the experimental or control pigeons exhibited large behavioral deficits in near-field visual acuity postoperatively. These results indicate that the avian centrifugal system is not essentially involved in the processing of spatially detailed visual information. Visual acuity
Istho-optic tract
Pigeons
T H E basic anatomy of the avian visual centrifugal control " l o o p " has been known for over a decade [2]. Afferent optic nerve fibers from the retina project to the contralateral optic tectum. From the optic tectum a pathway arises that projects to the ipsilateral isthmo-optic nucleus (ION). This pathway is termed the tecto-isthmal tract (TIT). The ION contains the cell bodies of the efferent fibers that comprise the isthmooptic tract (IOT). The IOT merges with the optic tract, where efferent fibers constitute about 1% of the total number of myelinated fibers. The centrifugal fibers course through the chiasma and on to the contralateral retina, where they radiate uniformly to innervate the amacrine cells [11]. Thus the avian centrifugal system is a loop comprising the retina, optic nerve, optic tectum, TIT, ION, IOT, optic nerve, and finally again the retina. Figure 1 illustrates semischematically the gross anatomy of the system. Degeneration studies have indicated that a topographical correspondence exists for the fiber projections throughout the loop, i.e., the different regions of the ION project to the same retinal region from which they received their afferent input via the optic tectum [ 1]. Electrophysiological studies [5, 13, 15] have suggested that the centrifugal pathway plays a strictly modulatory role in retinal responsiveness to light, rather than an information-carrying role. Moreover, this modulation is facilitative rather than inhibitory. When the ION is electri-
LEFT
~ OPTIC~DCHIASMA
RIGHT EYE
FIG. 1. Semi-schematic iilustratioh of the centrifugal fiber system in the avian brain, also showing the site of the transections in the present experiment. Abbreviations: ION=isthmo-optic nucleus, TIT=tecto-isthmal tract, IOT=isthmo-optic tract.
ZThisresearch was supported in part by National Eye Institute grant number EY-00735. The author wishes to express special appreciation to Dr. William Hodos for technical assistance and for many helpful comments on earlier drafts of this paper.
C o p y r i g h t © 1978 B r a i n R e s e a r c h P u b l i c a t i o n s Inc.--0031-9384/78/110813-04502.00/0
814
KNIPLING
cally stimulated, retinal ganglion cell response to stationary light targets is increased due to an increase in effective receptive field size [13]. No specific modulatory effects on retinal responsiveness to moving stimuli have been reported, although the ION cells themselves responded vigorously to moving visual stimuli over a wide range of stimulus velocities [12]. Little is known about the behavioral functions of the centrifugal system. Coarse visual intensity and pattern discrimination performances by pigeons were unimpaired following bilateral ION ablations [6]. Deficits have been reported in the detection of slowly moving objects newly introduced into the peripheral visual fields of baby chicks following bilateral ION lesions [16]. Also, the ability of chicks with ION lesions to discriminate grain from pebbles was impaired under certain lighting conditions [16]. None of these behavioral studies have employed specific and sensitive psychophysical measurements of visual behavior before and after centrifugal fiber severance. The present report represents an attempt to further specify the behavioral functions of the centrifugal system by the fine measurement of visual acuity performance before and after the disruption of centrifugal influences on the retina. METHOD
Animals Five White Carneaux pigeons (Columbia livia) were housed individually for the duration of the experiment. They were maintained at approximately 80% ad lib body weight, with water always available in the home cages.
Apparatus and Procedure Five pigeons were trained on a discrimination task designed to measure near-field spatial resolution capacity (visual acuity) for a moderately bright (70 cd/m 2) target. A three-key operant conditioning apparatus was used in which the target viewing distance was estimated to be 62.0 mm. Using a variation of the method of constant stimuli, the pigeons were presented with either blank stimuli or squarewave line gratings which varied in spatial frequency from 1 to 20 lines/mm. The spatial frequencies of the series of line gratings were 1, 2, 4, 8, 12, and 20 lines/mm, and each line grating stimulus was matched for luminance with a blank stimulus to within 0.02 log units. Correct side key choice responses were reinforced by brief access to food or by conditioned reinforcers. Visual acuity at threshold (75% correct) was determined daily from psychometric functions of choice response accuracy. Sessions were conducted until five consecutive sessions occurred during which no daily threshold visual acuity exceeded -+ 25% of the mean threshold for the five sessions. The experimental apparatus and procedure have been described in more detail elsewhere [7,81.
Surgery and Histology Within 48 hr of reaching stability criterion, each pigeon was anesthetized with methoxyfluorane and xylocaine. Respiration was maintained with a mixture of 95% oxygen and 5% carbon dioxide. Bilateral lesions were produced by radio-frequency electrocoagulation via a stereotaxically guided electrode. These lesions were intended to bilaterally transect the lOT at the level A 4.0 according to the
stereotaxic atlas of the pigeon brain by Karten and Hodos [10]. The lesion site is semi-schematically illustrated in Fig. 1. After four days of postoperative recovery, each bird was retested until its acuity performance was again stable in five successive sessions. Upon completion of postoperative testing, each bird was sacrificed and perfused [6]. After fixation, the brains were removed, embedded in paraffin, and sectioned at 10 p.m. The lesions were reconstructed on standard drawings [10] of the pigeon brain from microscopic examination of Kliiver-Barrera-stained sections. In addition, the presence or absence of retrograde degeneration of the cells of the ION was noted for each case. The methods of perfusion and histological processing are described in more detail elsewhere [6]. RESULTS The five birds were assigned to three categories on the basis of histological reconstruction of the brain lesions. These categories were: (I) complete bilateral lOT transection (birds D--173 and I)--161); (2) partial bilateral lOT transection (Pigeon D-174---the right side lOT of this bird was completely severed, but only about 40% of the left side lOT fibers were severed); (3) control cases, in which no lOT damage was sustained (birds D-183 and D--168). Figure 2 illustrates the discrete and roughly spherical lesions effected through radio-frequency coagulation. In contrast, electrolytic lesions have been observed (in previous cases in this laboratory) to extended around the lOT without actually severing it. These observations support the findings of previous investigators [3] who found that radio-frequency lesions were clearly superior for severing myelinated fiber tracts such as the lOT. Moreover, the lOT appears to be a better target for stereotaxically placed lesions than the ION itself because a relatively small lesion can result in the complete removal of centrifugal influences. Lesions intended for the nucleus must be considerably larger and thus have a greater likelihood of damaging nearby cell groups and fiber pathways.
Behavior No gross behavioral changes were apparent following surgery. No pecking or feeding deficits were seen, and birds generally performed at a high steady rate on the discrimination problem both before and after surgery. In no case was pre- or postoperative choice response bias (left key vs. right key) greater than _+ 25%, and no large shifts in response bias were seen postoperatively. Table 1 presents the pre- and postoperative mean decimal visual acuities (for five stable sessions) for each of the five birds. The data are in the form of decimal acuity, which is the reciprocal of the threshold visual angle of the line grating stimuli. None of the experimental or control pigeons exhibited significant visual acuity deficits postoperatively. The two birds sustaining complete lOT transection (13-173 and D-161) both exhibited slight improvements in choice response accuracy. The partial bilateral transection case (D174) exhibited a slight deficit. The acuity performance of one of the control birds (D-183) was essentially unchanged postoperatively, whereas the second control bird (D--168) exhibited a substantial improvement in visual acuity. The visual acuity increment of this control subject is quite anomalous and at present inexplicable. No other birds in this laboratory have shown such dramatic increases in measured
IOT T R A N S E C T I O N S AND V I S U A L ACUITY
815 TABLE 1 PRE- AND POSTOPERATIVEDECIMALVISUALACUITIESAND PERCENT CHANGE FOR THE FIVE PIGEONS. PRE- AND POSTOPERATIVETHRESHOLDVISUALANGLESARE GIVENIN PARENTHESES Subject
Pre-Operative Acuity
Post-Operative Acuity
Percent Change
Category 1: C o m p l e t e Bilateral Transection D-173
0.18 (5.5 rain.)
0.20 (4.9 min.)
+11%
D-161
0.53 (1.9 rain.)
0.64 (1.6 rnin.)
+21%
Category 2: Partial Bilateral Transection D-174
0.28 (3.6 min.)
0.24 (4.2 rain.)
-14%
Cat egory 3: Control Cases
FIG. 2. Microphotograph of a transverse section (level A 4.0) of the right side of the brain of: (Top) normal control bird D--183, showing the intact lOT fiber bundle (dark region in center of photograph); and (Bottom) complete bilateral transection bird D-173, illustrating the clean and discrete lesions effected through radio-frequency coagulation. acuity postoperatively, or after a large number of sessions preoperatively. Pigeon D-168 sustained very small brain lesions (which completely spared the lOT), so a genuine facilitative behavioral effect of the surgical intervention is improbable. DISCUSSION The present results indicate that, under the experimental
D-183
0.27 (3.7 rain.)
0.26 (3.8 rain.)
- 4%
D-168
0.35 (2.9 rain.)
0.54 (1.9 rain.)
+54%
conditions used, spatial resolution capacity is unimpaired by the surgical removal of the centrifugal influences. Thus the centrifugal system is probably not essentially involved in the processing of spatially detailed visual information. This result is consistent with electron microscopic and electrophysioiogical studies of the vertebrate retina [4, 17, 18], which have suggested the existence of two anatomically and functionally distinguishable channels for spatial and temporal aspects of visual transmission in the retina. The amacfine cells, the sites of termination of the centrifugal axons, apparently belong to the temporal system [ 17]. Thus the centrifugal system is probably involved primarily in the modulation of the processing of temporal properties of stimuli rather t h a n spatial properties. However, there is no definitive electrophysiological or behavioral evidence that the centrifugal system modulates motion sensitivity. An important behavioral test of the centrifugal system would be to measure the behavioral movement detection capacities of birds before and after this transection of the centrifugal fibers. Sophisticated psychophysical methods for determining visual motion detection capacity have been reported [9,14], but these have not yet been employed to conclusively demonstrate a behavioral motion detection role for the centrifugal system.
REFERENCES 1. Cowan, W. M. Centrifugal fibers to the avian retina. Br. Med. Bull. 26: 112-118, 1970. 2. Cowan, W. M. and T. P. S. Powell. Centrifugal fibers in the avian visual system. Proc. R. Soc. B. 170: 205-226, 1963. 3. Dicara, L. V., L. Weaver and G. Wolf. Comparison of DC and RF for lesioning white and grey matter. Physiol. Behav. 12: 1087-1090, 1974. 4. Dowling, J. E. Organization of vertebrate retinas. Invest. Ophthalmol. 9: 655-680, 1970. 5. Galifret, Y., F. Conde-Courtine, J. Reperant and J. Serviere. Centrifugal control in the visual system of the pigeon. Vis. Res. Suppl. 3: 185-201, 1971.
6. Hodos, W. and H. J. Karten. Visual intensity and pattern discrimination deficits after lesions of the optic lobe in pigeons. Brain Behav. Evolut. 9: 165-194, 1974. 7. Hodos, W. and R. W. Leibowitz. Near-field visual acuity of pigeons: effects of scotopic adaptation and wavelength. Vision Res. 17: 463-467, 1977. 8. Hodos, W., R. W. Leibowitz and J. C. Bonbright Jr. Near-field visual acuity of pigeons: effects of head location and stimulus luminance. J. exp. anal. Behav. 26: 129-141, 1976. 9. Hodos, W., L. Smith and J. C. Bonbright Jr. Detection of the velocity of movement of visual stimuli by pigeons. J. exp. anal. Behav. 25: 143-156, 1975.
816 10. Karten, H. J. and W. Hodos. A Stereotaxic Atlas o f the Brain o f the Pigeon. Baltimore: Johns Hopkins, 1967. 11. Maturana, H. L. and S. Frenk. On the synaptic connections of the centrifugal fibers to the retina in the pigeon. Science 150: 359-361, 1965. 12. Miles, F. A. Centrifugal control of the avian retina: II. Receptive field properties of the cells in the isthmo-optic nucleus. Brain Res. 48: 93--113, 1972. 13. Miles, F. A. Centrifugal control of the avian retina: III. Effects of electrical stimulation of the isthmo-optic tract on the receptive field properties of retinal ganglion cells. Brain Res. 48: 115--127, 1972. 14. Mulvanny, P. J. Discrimination of stimulus velocity by the pigeon. Vision Res.. in press.
KNIPLING 15. Pearlman, A. L. and C. P. Hughes. Functional role of efferent fibers to the retina. Trans. Am. Neurol. Assoc. 98: 48-51, 1973. 16. Rogers, L. J. and F. A. Miles. Centrifugal control of the avian retina: V. Effects of lesions of the isthmo-optic nucleus on visual behavior. Brain Res. 48: 146-160, 1972. 17. Werblin, F. S. and J. E. Dowling. Organization of the retina of the mud-puppy, Necturus maculosus: II. Intracellular recording. J. Neurophysiol. 32: 339-355, 1969. 18. Wright, M. J. and H. Ikeda. Processing of spatial and temporal information in the visual system. In: The Neurosciences: Third Study Program, edited by F. O. Schmitt and F. G. Wordon, 1974, pp. 115-122.
BRIEF COMMUNICATION No Deficit in Near-Field Visual Acuity of Pigeons After Transection of the Isthmo-Optic Tract I R O N A L D R. K N I P L I N G
University College, University o f Maryland, University Boulevard at Adelphi R o a d College Park, M D 20742 ( R e c e i v e d 2 F e b r u a r y 1978) KNIPLING, R. R. No deficit in near-field visual acuity of pigeons after transection of the isthmo-optic tract. PHYSIOL. BEHAV. 21(5) 813-816, 1978.--Five pigeons (Columbia livia) were trained on a visual discrimination task designed to measure near-field spatial resolution capacity (visual acuity) for a moderately bright target. After stabilization of discrimination performance over a number of sessions, bilateral radio-frequency coagulation lesions were performed. These lesions were intended to bilaterally transect the isthmo-optic tract (lOT), which is the pathway of the efferent fibers to the retina. The anatomical results revealed that two birds sustained complete bilateral transection of the lOT. One pigeon sustained partial bilateral transection, and two birds suffered no lOT damage. The latter two birds functioned as control cases. None of the experimental or control pigeons exhibited large behavioral deficits in near-field visual acuity postoperatively. These results indicate that the avian centrifugal system is not essentially involved in the processing of spatially detailed visual information. Visual acuity
Istho-optic tract
Pigeons
T H E basic anatomy of the avian visual centrifugal control " l o o p " has been known for over a decade [2]. Afferent optic nerve fibers from the retina project to the contralateral optic tectum. From the optic tectum a pathway arises that projects to the ipsilateral isthmo-optic nucleus (ION). This pathway is termed the tecto-isthmal tract (TIT). The ION contains the cell bodies of the efferent fibers that comprise the isthmooptic tract (IOT). The IOT merges with the optic tract, where efferent fibers constitute about 1% of the total number of myelinated fibers. The centrifugal fibers course through the chiasma and on to the contralateral retina, where they radiate uniformly to innervate the amacrine cells [11]. Thus the avian centrifugal system is a loop comprising the retina, optic nerve, optic tectum, TIT, ION, IOT, optic nerve, and finally again the retina. Figure 1 illustrates semischematically the gross anatomy of the system. Degeneration studies have indicated that a topographical correspondence exists for the fiber projections throughout the loop, i.e., the different regions of the ION project to the same retinal region from which they received their afferent input via the optic tectum [ 1]. Electrophysiological studies [5, 13, 15] have suggested that the centrifugal pathway plays a strictly modulatory role in retinal responsiveness to light, rather than an information-carrying role. Moreover, this modulation is facilitative rather than inhibitory. When the ION is electri-
LEFT
~ OPTIC~DCHIASMA
RIGHT EYE
FIG. 1. Semi-schematic iilustratioh of the centrifugal fiber system in the avian brain, also showing the site of the transections in the present experiment. Abbreviations: ION=isthmo-optic nucleus, TIT=tecto-isthmal tract, IOT=isthmo-optic tract.
ZThisresearch was supported in part by National Eye Institute grant number EY-00735. The author wishes to express special appreciation to Dr. William Hodos for technical assistance and for many helpful comments on earlier drafts of this paper.
C o p y r i g h t © 1978 B r a i n R e s e a r c h P u b l i c a t i o n s Inc.--0031-9384/78/110813-04502.00/0
814
KNIPLING
cally stimulated, retinal ganglion cell response to stationary light targets is increased due to an increase in effective receptive field size [13]. No specific modulatory effects on retinal responsiveness to moving stimuli have been reported, although the ION cells themselves responded vigorously to moving visual stimuli over a wide range of stimulus velocities [12]. Little is known about the behavioral functions of the centrifugal system. Coarse visual intensity and pattern discrimination performances by pigeons were unimpaired following bilateral ION ablations [6]. Deficits have been reported in the detection of slowly moving objects newly introduced into the peripheral visual fields of baby chicks following bilateral ION lesions [16]. Also, the ability of chicks with ION lesions to discriminate grain from pebbles was impaired under certain lighting conditions [16]. None of these behavioral studies have employed specific and sensitive psychophysical measurements of visual behavior before and after centrifugal fiber severance. The present report represents an attempt to further specify the behavioral functions of the centrifugal system by the fine measurement of visual acuity performance before and after the disruption of centrifugal influences on the retina. METHOD
Animals Five White Carneaux pigeons (Columbia livia) were housed individually for the duration of the experiment. They were maintained at approximately 80% ad lib body weight, with water always available in the home cages.
Apparatus and Procedure Five pigeons were trained on a discrimination task designed to measure near-field spatial resolution capacity (visual acuity) for a moderately bright (70 cd/m 2) target. A three-key operant conditioning apparatus was used in which the target viewing distance was estimated to be 62.0 mm. Using a variation of the method of constant stimuli, the pigeons were presented with either blank stimuli or squarewave line gratings which varied in spatial frequency from 1 to 20 lines/mm. The spatial frequencies of the series of line gratings were 1, 2, 4, 8, 12, and 20 lines/mm, and each line grating stimulus was matched for luminance with a blank stimulus to within 0.02 log units. Correct side key choice responses were reinforced by brief access to food or by conditioned reinforcers. Visual acuity at threshold (75% correct) was determined daily from psychometric functions of choice response accuracy. Sessions were conducted until five consecutive sessions occurred during which no daily threshold visual acuity exceeded -+ 25% of the mean threshold for the five sessions. The experimental apparatus and procedure have been described in more detail elsewhere [7,81.
Surgery and Histology Within 48 hr of reaching stability criterion, each pigeon was anesthetized with methoxyfluorane and xylocaine. Respiration was maintained with a mixture of 95% oxygen and 5% carbon dioxide. Bilateral lesions were produced by radio-frequency electrocoagulation via a stereotaxically guided electrode. These lesions were intended to bilaterally transect the lOT at the level A 4.0 according to the
stereotaxic atlas of the pigeon brain by Karten and Hodos [10]. The lesion site is semi-schematically illustrated in Fig. 1. After four days of postoperative recovery, each bird was retested until its acuity performance was again stable in five successive sessions. Upon completion of postoperative testing, each bird was sacrificed and perfused [6]. After fixation, the brains were removed, embedded in paraffin, and sectioned at 10 p.m. The lesions were reconstructed on standard drawings [10] of the pigeon brain from microscopic examination of Kliiver-Barrera-stained sections. In addition, the presence or absence of retrograde degeneration of the cells of the ION was noted for each case. The methods of perfusion and histological processing are described in more detail elsewhere [6]. RESULTS The five birds were assigned to three categories on the basis of histological reconstruction of the brain lesions. These categories were: (I) complete bilateral lOT transection (birds D--173 and I)--161); (2) partial bilateral lOT transection (Pigeon D-174---the right side lOT of this bird was completely severed, but only about 40% of the left side lOT fibers were severed); (3) control cases, in which no lOT damage was sustained (birds D-183 and D--168). Figure 2 illustrates the discrete and roughly spherical lesions effected through radio-frequency coagulation. In contrast, electrolytic lesions have been observed (in previous cases in this laboratory) to extended around the lOT without actually severing it. These observations support the findings of previous investigators [3] who found that radio-frequency lesions were clearly superior for severing myelinated fiber tracts such as the lOT. Moreover, the lOT appears to be a better target for stereotaxically placed lesions than the ION itself because a relatively small lesion can result in the complete removal of centrifugal influences. Lesions intended for the nucleus must be considerably larger and thus have a greater likelihood of damaging nearby cell groups and fiber pathways.
Behavior No gross behavioral changes were apparent following surgery. No pecking or feeding deficits were seen, and birds generally performed at a high steady rate on the discrimination problem both before and after surgery. In no case was pre- or postoperative choice response bias (left key vs. right key) greater than _+ 25%, and no large shifts in response bias were seen postoperatively. Table 1 presents the pre- and postoperative mean decimal visual acuities (for five stable sessions) for each of the five birds. The data are in the form of decimal acuity, which is the reciprocal of the threshold visual angle of the line grating stimuli. None of the experimental or control pigeons exhibited significant visual acuity deficits postoperatively. The two birds sustaining complete lOT transection (13-173 and D-161) both exhibited slight improvements in choice response accuracy. The partial bilateral transection case (D174) exhibited a slight deficit. The acuity performance of one of the control birds (D-183) was essentially unchanged postoperatively, whereas the second control bird (D--168) exhibited a substantial improvement in visual acuity. The visual acuity increment of this control subject is quite anomalous and at present inexplicable. No other birds in this laboratory have shown such dramatic increases in measured
IOT T R A N S E C T I O N S AND V I S U A L ACUITY
815 TABLE 1 PRE- AND POSTOPERATIVEDECIMALVISUALACUITIESAND PERCENT CHANGE FOR THE FIVE PIGEONS. PRE- AND POSTOPERATIVETHRESHOLDVISUALANGLESARE GIVENIN PARENTHESES Subject
Pre-Operative Acuity
Post-Operative Acuity
Percent Change
Category 1: C o m p l e t e Bilateral Transection D-173
0.18 (5.5 rain.)
0.20 (4.9 min.)
+11%
D-161
0.53 (1.9 rain.)
0.64 (1.6 rnin.)
+21%
Category 2: Partial Bilateral Transection D-174
0.28 (3.6 min.)
0.24 (4.2 rain.)
-14%
Cat egory 3: Control Cases
FIG. 2. Microphotograph of a transverse section (level A 4.0) of the right side of the brain of: (Top) normal control bird D--183, showing the intact lOT fiber bundle (dark region in center of photograph); and (Bottom) complete bilateral transection bird D-173, illustrating the clean and discrete lesions effected through radio-frequency coagulation. acuity postoperatively, or after a large number of sessions preoperatively. Pigeon D-168 sustained very small brain lesions (which completely spared the lOT), so a genuine facilitative behavioral effect of the surgical intervention is improbable. DISCUSSION The present results indicate that, under the experimental
D-183
0.27 (3.7 rain.)
0.26 (3.8 rain.)
- 4%
D-168
0.35 (2.9 rain.)
0.54 (1.9 rain.)
+54%
conditions used, spatial resolution capacity is unimpaired by the surgical removal of the centrifugal influences. Thus the centrifugal system is probably not essentially involved in the processing of spatially detailed visual information. This result is consistent with electron microscopic and electrophysioiogical studies of the vertebrate retina [4, 17, 18], which have suggested the existence of two anatomically and functionally distinguishable channels for spatial and temporal aspects of visual transmission in the retina. The amacfine cells, the sites of termination of the centrifugal axons, apparently belong to the temporal system [ 17]. Thus the centrifugal system is probably involved primarily in the modulation of the processing of temporal properties of stimuli rather t h a n spatial properties. However, there is no definitive electrophysiological or behavioral evidence that the centrifugal system modulates motion sensitivity. An important behavioral test of the centrifugal system would be to measure the behavioral movement detection capacities of birds before and after this transection of the centrifugal fibers. Sophisticated psychophysical methods for determining visual motion detection capacity have been reported [9,14], but these have not yet been employed to conclusively demonstrate a behavioral motion detection role for the centrifugal system.
REFERENCES 1. Cowan, W. M. Centrifugal fibers to the avian retina. Br. Med. Bull. 26: 112-118, 1970. 2. Cowan, W. M. and T. P. S. Powell. Centrifugal fibers in the avian visual system. Proc. R. Soc. B. 170: 205-226, 1963. 3. Dicara, L. V., L. Weaver and G. Wolf. Comparison of DC and RF for lesioning white and grey matter. Physiol. Behav. 12: 1087-1090, 1974. 4. Dowling, J. E. Organization of vertebrate retinas. Invest. Ophthalmol. 9: 655-680, 1970. 5. Galifret, Y., F. Conde-Courtine, J. Reperant and J. Serviere. Centrifugal control in the visual system of the pigeon. Vis. Res. Suppl. 3: 185-201, 1971.
6. Hodos, W. and H. J. Karten. Visual intensity and pattern discrimination deficits after lesions of the optic lobe in pigeons. Brain Behav. Evolut. 9: 165-194, 1974. 7. Hodos, W. and R. W. Leibowitz. Near-field visual acuity of pigeons: effects of scotopic adaptation and wavelength. Vision Res. 17: 463-467, 1977. 8. Hodos, W., R. W. Leibowitz and J. C. Bonbright Jr. Near-field visual acuity of pigeons: effects of head location and stimulus luminance. J. exp. anal. Behav. 26: 129-141, 1976. 9. Hodos, W., L. Smith and J. C. Bonbright Jr. Detection of the velocity of movement of visual stimuli by pigeons. J. exp. anal. Behav. 25: 143-156, 1975.
816 10. Karten, H. J. and W. Hodos. A Stereotaxic Atlas o f the Brain o f the Pigeon. Baltimore: Johns Hopkins, 1967. 11. Maturana, H. L. and S. Frenk. On the synaptic connections of the centrifugal fibers to the retina in the pigeon. Science 150: 359-361, 1965. 12. Miles, F. A. Centrifugal control of the avian retina: II. Receptive field properties of the cells in the isthmo-optic nucleus. Brain Res. 48: 93--113, 1972. 13. Miles, F. A. Centrifugal control of the avian retina: III. Effects of electrical stimulation of the isthmo-optic tract on the receptive field properties of retinal ganglion cells. Brain Res. 48: 115--127, 1972. 14. Mulvanny, P. J. Discrimination of stimulus velocity by the pigeon. Vision Res.. in press.
KNIPLING 15. Pearlman, A. L. and C. P. Hughes. Functional role of efferent fibers to the retina. Trans. Am. Neurol. Assoc. 98: 48-51, 1973. 16. Rogers, L. J. and F. A. Miles. Centrifugal control of the avian retina: V. Effects of lesions of the isthmo-optic nucleus on visual behavior. Brain Res. 48: 146-160, 1972. 17. Werblin, F. S. and J. E. Dowling. Organization of the retina of the mud-puppy, Necturus maculosus: II. Intracellular recording. J. Neurophysiol. 32: 339-355, 1969. 18. Wright, M. J. and H. Ikeda. Processing of spatial and temporal information in the visual system. In: The Neurosciences: Third Study Program, edited by F. O. Schmitt and F. G. Wordon, 1974, pp. 115-122.