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Shapes of receptive field centers in optic tectum of goldfish
RESEARCH NOTE SHAPES OF RECEPTIVE FIELD CENTERS IN OPTIC TECTUM OF GOLDFISH NICO A. M. SCHELLMT and HESK SPEKREU~E Laboratory of iMedical Physics. University of Amsterdam, Amsterdam, The Netherlands (&ceiced 29 December 19753
ISTRODUmIOX In fish the lobes of the optic tectum receiv-e visual information directly from the retinal ganglion cells. Tectal units of goldfish and carp are generally reported to have receptive Iiefd centers with shapes varying from circular to elliptical, and sizes ranging from a few to tens of degrees of visual angle (Jacobson and Gaze, 1964: Guthrie and Banks. 1974; Niida and Sato. 1975; Sutterlin and Prosser. 1970: Wartzock and Marks. 1973). On the other hand. in the isolated retina of goldfish the receptive field centers of ganglion cell responses appear to be mainly circular and measure about 10’ in diameter (Easter, _1963: Spekreijse. Wagner and Wolbarsht. 1972). It cannot be excluded that this discrepancy may be due to differences in experimental conditions (isolated retina vs anaesthetized animal). For example directional selectivity has been found in optic nerve recordings in sitrt, but not for ganglion cell recordings in isolated retina (Daw and Beauchamp, 1972; Scheilart, 1973). Furthermore, shape and size of receptive field centers of tectal units of goldfish have never been determined in a quantitative way. In general only the sign of the response was plotted to obtain an indication of receptive field extension. In the present paper a constant response criterion is used to define a receptive field center. The origin of single unit activity recorded extra-cellularly from the optic tectum is frequently uncertain. Some authors claim to record both from tectal somata and from optic nerve fibre endings (Sutteriin and Prosser. 1970; Guthrie and Banks. 1974), others consider they record exclusively the latter (Jacobson and Gaze, 1964; Wartzock and Marks, 1973). Criteria like depth of recording, spike waveform and spike duration are used to establish the origin of tectal recordings. One of the purposes of the present paper is to investigate whether shape and size of receptive fieId centers determined from tectal and optic nerve recordings can be used as criteria to distinguish between recordings of nerve fibre endings and tectal neurons.
METHODS
Experimental details have been described etsewhere (Regan, Schellart, Spekreijse, Van den Berg. 1975). To restrict loss of blood. cerebra-spinal fluid and lymph, recordings from the optic nerve and chiasm were generally made
’ It should be noted that the air water interface causes a compression of the visual world on the retina which increases with eccentricity.
by piercing the telencephalon with the micro-electrode. Extracellular activity of the right optic tectum and the left optic nerve was recorded with low impedance (about 4 Ma) glass pipettes tilled with 1.5 41 NaCl. Occasionally the position of the electrode tip in the tectum was marked by pontamine sky blue. Histological esamination showed that the dye was located in the stratum opticum or in the stratum fibrosum et griseum superficiale (see Vanegas. Laufer and .&mat. 1971). Stimuli were projected upon ,I verticalj. positioned cylindrical white screen. placed at a distance of I.3 m of the submerged leti eye of the fish. With a motor controlled mirror placed outside the visual ticld of the fish a O&-j-’ circular spot could be positioned anywhere on the screen The spot was flashed on and off at I.4 Hz ijO”, duty cyclcl and its position was plotted by an S-Y recorder. The background intensity of the screen gave a retinal illumination of 25pW’cm2 (incandescent light). The stimulus intensity was 9.S-9g /tW,‘cm’: i.e. about 2-3 log units photopic. These numbers are not corrected for absorption in the dioptric media of the etc. To prevent response saturation affecting the estimate of the receptive field size. the maximal response was determined in the most sensitive point of the field. Next stimulus diameter and’or intensity was so reduced that the number of spikes per stimulus fell to about half. This stimulus setting was used for the receptive field plotting. Red light (630 nm peak wavelength. 50 nm halfwidth) was used since in isolated retina all ganglion cells have a red process (Spekreijse rr a!.. 1971) and about 9O?, of the tectal recordings show antagonistic colour coding (present study). Care was taken to prevent optical artifacts of the stimulus like reflection at the meniscus of the water. The spikes of the on and off response, elicited by the square wave modulated stimulus, were counted separately during 10 stimulus periods. These two numbers are taken as a measure for the strength of the on and off responses. The line that connects the points in the visual field where this stimulus elicited a response equal to 4 I 2 the response in the most sensitive point, defines the border of the receptive tield center.
RESULTS
?&D
DISCUSSIOS
The shapes of the receptive field centers at tectal level can vary substantially as Fig. I shows. They can be distinguished into four groups: (L) Circzdar jielris Of this group the longest diameter within the 0.71 (i \‘2) contour is at most twice the longest diameter perpendicular to the first. These diameters form the diagonals of a quadrangle. Each side of this quadrangle and the diagonals should lie for at least 5096 of their lengths within the constant response contour.
I013
1019
Research Note mceptimii&l
cnners
_--
,‘Y h
!
1’) v
f
w
;&o_
.\
: ,,,l -
15
,-
1
--__
.._
‘..._ _.i
inesular
-once4 ---.-
off cell
Fig. 1. This figure shows examples of various types of receptive field centers in the optic tectum of goldfish. For all, except the multi-center type, more than one typical field is depicted. The crosses indicate the position of the optical axis in the visual field. The circles give the points of highest sensitivity. In the upper left part of the figure the projection of the experimental screen on the X-Y plotter is shown. Each square represents a visual angIe of 5” by 5”.
(2) Elongaredfields
As (I), but both diameters differ more than a factor 2. (3) Irregular fields All fields with a single contour that do not fulfill the requirements of class (1) and (1). (4) “Multi-center” fields This group consists of units that have two or more areas of the same response sign enclosed by 0.71 contours. Table 1 gives the occurrence and the mean size of the four field types for tectal as well as optic nerve recordings. The longest diameter sets the size of the first three types. For the multi-center fields the maxi-
mal distance between two points of a contour is taken as measure, irrespective of whether these points belong to the same contour or not. From the table it can be seen that the elongated, irregular and multi-center tectal fields are often much larger than the circular tectal fields. For circular and elongated fields the field size scarcely changes with the intensity, diameter of the stimulus and the (photopit) background intensity. This is in contrast to the multi-center fields which vary more with stimulus parameters. The same also holds with regard to the criterion chosen. When instead of the 0.71 criterion a 0.50 criterion is used then the classification of the circular, elongated and irregular fields changes 11, 8 and 18% respectively, but 70% of the multi-center fields becomes single contoured fields (mainly irregular) due to fusion of the contours. Since in about 90% of the cases our stimulus situation reveals hardly or not at all a surround process, for most of the units the constant response contour may be considered as an iso-sensitivity contour of the receptive field center. The mean size of the circular tectal off-fields is 9.8”, whereas the on-fields measure 6.9”. This difference appears to be significant (t-test, P < 0.05). The same holds for the circular fields of the optic nerve recordings, which have a mean size of 13” and 7” respectively. Since the size of circular fields of the appropriate class (on or off) do not differ significantly between determinations in the optic nerve and tecturn, it cannot be excluded that part of the tectal circular fields are from optic nerve fibre recordings. Elongated and irregular receptive fields of goldfish have not been reported before. This might be due to stimulation of the fish eye in air (Jacobson and Gaze, 1964; Sutterlin and Presser, 1970; Guthrie and Banks, 1974). For small specimens this gives a refractive error of about 65 D resulting in an out-of-focus blurring disk of about 15”. However, even in our experiments where the eye is submerged, a blurring disk of ca. 2” remains due to hypermetropia (ca. 9 D, 16-22 cm fish). On the other hand, since occasionally in the same experiment both circular and non-circular fields could be recorded from about the same region in the visual field, it seems unlikely that the non-circular fields are caused by errors .in the optics external or internal to the eye. Estimation of the incidence of receptive field types is hampered by experimental limitations since no information can be obtained about receptive field areas
Table 1. Shape, size and frequency of occurrence of receptive field centers for tectal and optic nerve recordings. For the classification of the shape a larger number (numbers in parentheses) of units is used than for the determination of size. For the latter, if even a small portion of the contour was located outside the screen, the data were excluded Tectum
Type Circular Elongated Irregular Multi-center
iMean degree
S.D. degree
7.6 16.5 30.8 32.3
3.3 8.3_ 10.8 13.2
Optic nerve Frequency of occurrence (per cent) N = 51 (71)
Mean degree
SD. degree
Frequency of occurrence (per cent) .V = 13 (27)
53 (50) 20 (23) g( 8) 20 (19)
8.1 8.5 20 -
4.3 3.5 1.5 -
62 (67) 15 (11) 23 (15) 0( 7)
1020
Research Note
located outside the projection screen. Fields located partly outside the screen were rejected in Table 1, resulting in an underestimation of the numbers of large field cells. Furthermore, since multi-center units are generally rather insensitive. it cannot be ruled out that sometimes the large centra of multi-center fields are in fact the surround of a center process located outside the screen. With the visual field projecting more or less point to point upon the optic tectum, the electrode can be positioned such that these problems are less serious for tectal than for optic nerve recordings. About 150/, of the optic nerve recordings are not classified since they showed the same sensitivity over half or more of the screen. These fields belong therefore either to a surround process. to the “periphery” of a center process or they represent a fifth type of “diffuse” receptive fields. On the basis of the ratio of on and off cells in isolated retina, optic nerve and tectal recordings it can be calculated which part of the tectal recordings may belong to intrinsic cells. In the retina the ratio of on:off is 0.72 (sample of 234 cells, Schellart, 1973). In the optic nerve it is 0.76 (44 units) and in the tectum 2.2 (113 units). If the assumption is made that on and off terminals are recorded with the same ease then there remains an excess of on responses, which must be from tectal neurons, even if all the off units recorded would be optic nerve terminals, which is unlikely to be true. It has therefore to be* concluded that in a part of tectal neurons no regrouping takes place of receptive fields of lower order neurons into receptive field centers with a high degree of complexity of shape. ~cknowledgrments-We are grateful to Hans Meek and Frans Riemslag for their assistance in the experiments.
This research was supported by a grant from the Netherlands Organization for the Advancement of Pure Research (Z.W.O.).
REFERESCES Daw N. W. and Beauchamp R. D. (1972) Unusual units in the goldfish optic nerve. vision Res. 12, 1849-1856. Easter S. S. (1968) Excitation in the goldfish retina: evidence for a non-linear intensity code. J. Physiol., Lord. 195, X3-271.
Guthrie D. M. and Banks J. R. (1974) Input characteristics of the intrinsic cells of the optic tectum of teleost fish. Camp. Biochem. Phpiol. 47A, 83-92. Jacobson M. and Gaze R. M. (1964) Types of visual response from single units in the optic tectum and optic nerve of the goldfish. Q. JI np. Physiol 19, 199-209. Niida A. and Sato Y. (1972) An analysis of visual responses in the optic tract and tectum of the crucian carp. J. Fat. Sci. Hoi&do
Unit.. S VI, Zool. 18. 371-386.
Regan D., Schellart N. A. M.. Spekreijse H. and Van den Berg T. J. T. P. t 1975) Photometry in goldfish by electrophysiological recording: comparison oi criterion response method with heterochromatic flicker photometry. Vision Res. 15, 799-807. Schellart N. A. M. (1973) Dynamics and statistics of photopit ganglion cell responses in isolated goldfish retina. Ph.D. Thesis, Univ. of Amsterdam. Spekreijse H., Wagner H. G. and Wolbarsht M. L. (1971) Spectral and spatial coding of ganglion cell responses in goldfish retina. /. Neurophysiol. 35, 73-86. Sutterlin A. M. and Prosser C. L. (1970) Electrical properties of goldfish optic tectum. J. .Veuroph_vsiol. 33, 36-45. Vanegas H., Laufer M. and Amat J. (1974) The optic tecturn of a perciform teleost. I. General configuration and cytoarchitecture. J. camp. ,Veurol. 154, 43-60. Wartzock D. and Marks W. B. (1973) Directionally selective visual units recorded in optic tectum of goldfish. J. :l’europh~sio/. 36, 588-604.
ISTRODUmIOX In fish the lobes of the optic tectum receiv-e visual information directly from the retinal ganglion cells. Tectal units of goldfish and carp are generally reported to have receptive Iiefd centers with shapes varying from circular to elliptical, and sizes ranging from a few to tens of degrees of visual angle (Jacobson and Gaze, 1964: Guthrie and Banks. 1974; Niida and Sato. 1975; Sutterlin and Prosser. 1970: Wartzock and Marks. 1973). On the other hand. in the isolated retina of goldfish the receptive field centers of ganglion cell responses appear to be mainly circular and measure about 10’ in diameter (Easter, _1963: Spekreijse. Wagner and Wolbarsht. 1972). It cannot be excluded that this discrepancy may be due to differences in experimental conditions (isolated retina vs anaesthetized animal). For example directional selectivity has been found in optic nerve recordings in sitrt, but not for ganglion cell recordings in isolated retina (Daw and Beauchamp, 1972; Scheilart, 1973). Furthermore, shape and size of receptive field centers of tectal units of goldfish have never been determined in a quantitative way. In general only the sign of the response was plotted to obtain an indication of receptive field extension. In the present paper a constant response criterion is used to define a receptive field center. The origin of single unit activity recorded extra-cellularly from the optic tectum is frequently uncertain. Some authors claim to record both from tectal somata and from optic nerve fibre endings (Sutteriin and Prosser. 1970; Guthrie and Banks. 1974), others consider they record exclusively the latter (Jacobson and Gaze, 1964; Wartzock and Marks, 1973). Criteria like depth of recording, spike waveform and spike duration are used to establish the origin of tectal recordings. One of the purposes of the present paper is to investigate whether shape and size of receptive fieId centers determined from tectal and optic nerve recordings can be used as criteria to distinguish between recordings of nerve fibre endings and tectal neurons.
METHODS
Experimental details have been described etsewhere (Regan, Schellart, Spekreijse, Van den Berg. 1975). To restrict loss of blood. cerebra-spinal fluid and lymph, recordings from the optic nerve and chiasm were generally made
’ It should be noted that the air water interface causes a compression of the visual world on the retina which increases with eccentricity.
by piercing the telencephalon with the micro-electrode. Extracellular activity of the right optic tectum and the left optic nerve was recorded with low impedance (about 4 Ma) glass pipettes tilled with 1.5 41 NaCl. Occasionally the position of the electrode tip in the tectum was marked by pontamine sky blue. Histological esamination showed that the dye was located in the stratum opticum or in the stratum fibrosum et griseum superficiale (see Vanegas. Laufer and .&mat. 1971). Stimuli were projected upon ,I verticalj. positioned cylindrical white screen. placed at a distance of I.3 m of the submerged leti eye of the fish. With a motor controlled mirror placed outside the visual ticld of the fish a O&-j-’ circular spot could be positioned anywhere on the screen The spot was flashed on and off at I.4 Hz ijO”, duty cyclcl and its position was plotted by an S-Y recorder. The background intensity of the screen gave a retinal illumination of 25pW’cm2 (incandescent light). The stimulus intensity was 9.S-9g /tW,‘cm’: i.e. about 2-3 log units photopic. These numbers are not corrected for absorption in the dioptric media of the etc. To prevent response saturation affecting the estimate of the receptive field size. the maximal response was determined in the most sensitive point of the field. Next stimulus diameter and’or intensity was so reduced that the number of spikes per stimulus fell to about half. This stimulus setting was used for the receptive field plotting. Red light (630 nm peak wavelength. 50 nm halfwidth) was used since in isolated retina all ganglion cells have a red process (Spekreijse rr a!.. 1971) and about 9O?, of the tectal recordings show antagonistic colour coding (present study). Care was taken to prevent optical artifacts of the stimulus like reflection at the meniscus of the water. The spikes of the on and off response, elicited by the square wave modulated stimulus, were counted separately during 10 stimulus periods. These two numbers are taken as a measure for the strength of the on and off responses. The line that connects the points in the visual field where this stimulus elicited a response equal to 4 I 2 the response in the most sensitive point, defines the border of the receptive tield center.
RESULTS
?&D
DISCUSSIOS
The shapes of the receptive field centers at tectal level can vary substantially as Fig. I shows. They can be distinguished into four groups: (L) Circzdar jielris Of this group the longest diameter within the 0.71 (i \‘2) contour is at most twice the longest diameter perpendicular to the first. These diameters form the diagonals of a quadrangle. Each side of this quadrangle and the diagonals should lie for at least 5096 of their lengths within the constant response contour.
I013
1019
Research Note mceptimii&l
cnners
_--
,‘Y h
!
1’) v
f
w
;&o_
.\
: ,,,l -
15
,-
1
--__
.._
‘..._ _.i
inesular
-once4 ---.-
off cell
Fig. 1. This figure shows examples of various types of receptive field centers in the optic tectum of goldfish. For all, except the multi-center type, more than one typical field is depicted. The crosses indicate the position of the optical axis in the visual field. The circles give the points of highest sensitivity. In the upper left part of the figure the projection of the experimental screen on the X-Y plotter is shown. Each square represents a visual angIe of 5” by 5”.
(2) Elongaredfields
As (I), but both diameters differ more than a factor 2. (3) Irregular fields All fields with a single contour that do not fulfill the requirements of class (1) and (1). (4) “Multi-center” fields This group consists of units that have two or more areas of the same response sign enclosed by 0.71 contours. Table 1 gives the occurrence and the mean size of the four field types for tectal as well as optic nerve recordings. The longest diameter sets the size of the first three types. For the multi-center fields the maxi-
mal distance between two points of a contour is taken as measure, irrespective of whether these points belong to the same contour or not. From the table it can be seen that the elongated, irregular and multi-center tectal fields are often much larger than the circular tectal fields. For circular and elongated fields the field size scarcely changes with the intensity, diameter of the stimulus and the (photopit) background intensity. This is in contrast to the multi-center fields which vary more with stimulus parameters. The same also holds with regard to the criterion chosen. When instead of the 0.71 criterion a 0.50 criterion is used then the classification of the circular, elongated and irregular fields changes 11, 8 and 18% respectively, but 70% of the multi-center fields becomes single contoured fields (mainly irregular) due to fusion of the contours. Since in about 90% of the cases our stimulus situation reveals hardly or not at all a surround process, for most of the units the constant response contour may be considered as an iso-sensitivity contour of the receptive field center. The mean size of the circular tectal off-fields is 9.8”, whereas the on-fields measure 6.9”. This difference appears to be significant (t-test, P < 0.05). The same holds for the circular fields of the optic nerve recordings, which have a mean size of 13” and 7” respectively. Since the size of circular fields of the appropriate class (on or off) do not differ significantly between determinations in the optic nerve and tecturn, it cannot be excluded that part of the tectal circular fields are from optic nerve fibre recordings. Elongated and irregular receptive fields of goldfish have not been reported before. This might be due to stimulation of the fish eye in air (Jacobson and Gaze, 1964; Sutterlin and Presser, 1970; Guthrie and Banks, 1974). For small specimens this gives a refractive error of about 65 D resulting in an out-of-focus blurring disk of about 15”. However, even in our experiments where the eye is submerged, a blurring disk of ca. 2” remains due to hypermetropia (ca. 9 D, 16-22 cm fish). On the other hand, since occasionally in the same experiment both circular and non-circular fields could be recorded from about the same region in the visual field, it seems unlikely that the non-circular fields are caused by errors .in the optics external or internal to the eye. Estimation of the incidence of receptive field types is hampered by experimental limitations since no information can be obtained about receptive field areas
Table 1. Shape, size and frequency of occurrence of receptive field centers for tectal and optic nerve recordings. For the classification of the shape a larger number (numbers in parentheses) of units is used than for the determination of size. For the latter, if even a small portion of the contour was located outside the screen, the data were excluded Tectum
Type Circular Elongated Irregular Multi-center
iMean degree
S.D. degree
7.6 16.5 30.8 32.3
3.3 8.3_ 10.8 13.2
Optic nerve Frequency of occurrence (per cent) N = 51 (71)
Mean degree
SD. degree
Frequency of occurrence (per cent) .V = 13 (27)
53 (50) 20 (23) g( 8) 20 (19)
8.1 8.5 20 -
4.3 3.5 1.5 -
62 (67) 15 (11) 23 (15) 0( 7)
1020
Research Note
located outside the projection screen. Fields located partly outside the screen were rejected in Table 1, resulting in an underestimation of the numbers of large field cells. Furthermore, since multi-center units are generally rather insensitive. it cannot be ruled out that sometimes the large centra of multi-center fields are in fact the surround of a center process located outside the screen. With the visual field projecting more or less point to point upon the optic tectum, the electrode can be positioned such that these problems are less serious for tectal than for optic nerve recordings. About 150/, of the optic nerve recordings are not classified since they showed the same sensitivity over half or more of the screen. These fields belong therefore either to a surround process. to the “periphery” of a center process or they represent a fifth type of “diffuse” receptive fields. On the basis of the ratio of on and off cells in isolated retina, optic nerve and tectal recordings it can be calculated which part of the tectal recordings may belong to intrinsic cells. In the retina the ratio of on:off is 0.72 (sample of 234 cells, Schellart, 1973). In the optic nerve it is 0.76 (44 units) and in the tectum 2.2 (113 units). If the assumption is made that on and off terminals are recorded with the same ease then there remains an excess of on responses, which must be from tectal neurons, even if all the off units recorded would be optic nerve terminals, which is unlikely to be true. It has therefore to be* concluded that in a part of tectal neurons no regrouping takes place of receptive fields of lower order neurons into receptive field centers with a high degree of complexity of shape. ~cknowledgrments-We are grateful to Hans Meek and Frans Riemslag for their assistance in the experiments.
This research was supported by a grant from the Netherlands Organization for the Advancement of Pure Research (Z.W.O.).
REFERESCES Daw N. W. and Beauchamp R. D. (1972) Unusual units in the goldfish optic nerve. vision Res. 12, 1849-1856. Easter S. S. (1968) Excitation in the goldfish retina: evidence for a non-linear intensity code. J. Physiol., Lord. 195, X3-271.
Guthrie D. M. and Banks J. R. (1974) Input characteristics of the intrinsic cells of the optic tectum of teleost fish. Camp. Biochem. Phpiol. 47A, 83-92. Jacobson M. and Gaze R. M. (1964) Types of visual response from single units in the optic tectum and optic nerve of the goldfish. Q. JI np. Physiol 19, 199-209. Niida A. and Sato Y. (1972) An analysis of visual responses in the optic tract and tectum of the crucian carp. J. Fat. Sci. Hoi&do
Unit.. S VI, Zool. 18. 371-386.
Regan D., Schellart N. A. M.. Spekreijse H. and Van den Berg T. J. T. P. t 1975) Photometry in goldfish by electrophysiological recording: comparison oi criterion response method with heterochromatic flicker photometry. Vision Res. 15, 799-807. Schellart N. A. M. (1973) Dynamics and statistics of photopit ganglion cell responses in isolated goldfish retina. Ph.D. Thesis, Univ. of Amsterdam. Spekreijse H., Wagner H. G. and Wolbarsht M. L. (1971) Spectral and spatial coding of ganglion cell responses in goldfish retina. /. Neurophysiol. 35, 73-86. Sutterlin A. M. and Prosser C. L. (1970) Electrical properties of goldfish optic tectum. J. .Veuroph_vsiol. 33, 36-45. Vanegas H., Laufer M. and Amat J. (1974) The optic tecturn of a perciform teleost. I. General configuration and cytoarchitecture. J. camp. ,Veurol. 154, 43-60. Wartzock D. and Marks W. B. (1973) Directionally selective visual units recorded in optic tectum of goldfish. J. :l’europh~sio/. 36, 588-604.