Electroretinogram and Visual-evoked Potential Measurements in Sheep George M. Strain, Marjorie S. Claxton, Judith S. Prescott-Mathews and Dayatra J. LaPhand
ABSTRACT
Electroretinogram (ERG) and visual-evoked potential (VEP) recordings were taken from ten Suffolkcross sheep. Stimuli for VEP were 1.5 flashes of white light/s; ERG stimuli were single flashes. The ERG measurements of the a and b wave latencies and a-to-b amplitude were measured between the lower eyelid and the vertex, with ground on the nuchal crest. The VEP after monocular stimulation were measured between the nuchal crest and the interorbital line, with ground on the vertex. Measurements consisted of the latencies to seven alternating positive and negative peaks Pl, NI, P2, N2, P3, N3 and P4, and six amplitudes, PI-NI, N1-P2 ,P2-N2, N2-P3, P3-N3 and N3-P4. Average latencies for the a and b waves were 13.6 and 28.2 ms; the mean ab amplitude was 131.68 AV. Average latencies for the seven VEP peaks were 35.0, 43.1, 52.8, 64.1, 74.5, 90.4 and 112.2 ms. Mean amplitudes ranged from 3.90 to 8.29 1IAV.
deux yeux, alors qu'elle fut reiteree a toutes les 1.5 s et limitee a un seul oeil dans le cas des PCVEs. Pour obtenir les ERGs, les electrodes sous-dermales captrices en platine (resistence inferieur a 5000 ohms) furent placees a la paupiere inferieure (+) de chaque oeil, et au vertex du crane (-), tandis qu'une autre au centre de la crete de la neque servait d'electrode indifferente ou de masse. Les electrodes decelant les PCVEs etaient situees a la crete de la neque (+), au centre de la ligne interorbitale, et au vertex (masse). Les ondes alpha (negative) et bata (positive) de l'ERG survinrent en moyenne 13.6 et 28.2 ms apres la stimulation, et l'amplitude moyenne de alpha a bata se situa vers 131.68 uv. Les sept oscillations du PCVE, i.e., les quatre ondes positives (P1,P2,P3 et P4) separees par trois ondes negatives (N1,N2 et N3), apparurent 35.0, 43.1, 52.8, 64.1, 74.5, 90.4, et 112.2 ms. L'amplitude moyenne des oscillations
While these electrodiagnostic tests of visual function undoubtedly have the same neural generators in different mammalian species, it is necessary to document normative values for each species in which they are to be used, since recordings will differ due to differences in head, eye and brain size, orientation of the generators with respect to the recording electrodes, presence or absence of horns and recording protocol. The VEP has previously been recorded from sheep in both clinical (5) and experimental (3,4) applications, but its use and interpretation have been made difficult by the lack of normative data. The purpose of the present report was to document normative data for electrodiagnostic tests of vision in sheep.
P1-Nl, N1-P2, P2-N2, N2-P3, P3-N3, N3-P4 des PCVEs varia entre 3.90 et
The ERG and VEP were recorded from ten Suffolk-cross sheep (eight male, two female) that were three to four months of age. Mean weight was 27.9 ± 5.7 kg (range 20-38 kg). All were from the same Suffolk sire; the dams were various Merino-Louisiana Native-Suffolk crosses and were all half sisters. The subjects consisted of one set of triplets and 3.5 sets of twins. All sheep appeared to have normal vision based on behavioral responses to the proximity of the investigators and locomotion around fixed objects. Halters and manual restraint were
8.29 uv. (Traduit par Dr L.P. Phaneuf)
RESUME
INTRODUCTION
A l'aide de stimuli lumineux (eclairs d'une duree de 10 us), on enregistra chez dix jeunes ovins (8 males, 2 femelles) les potentiels retiniens (electroretinogrammes ou ERG) et les potentiels corticaux visuels evoques (PCVE). Pour les ERGs, une stimulation lumineuse unique interessa les
The visual-evoked potential (VEP) is a noninvasive measure of activity in postretinal visual pathways with demonstrated utility in human (1,2) and veterinary (3-10) applications. Likewise, the electroretinogram (ERG) provides a noninvasive measure of retinal function (6,11,12).
MATERIALS AND METHODS SHEEP
Department of Veterinary Physiology, Pharmacology and Toxicology (Strain, Prescott-Mathews, LaPhand) and Department of Veterinary Clinical Sciences (Claxton), School of Veterinary Medicine, Louisiana State University, Baton Rouge, Louisiana 70803-8420. Supported by the LSU School of Veterinary Medicine Veterinary Medical Student Honors Program (JSPM) and Summer Research Apprenticeship Program (DJL). Submitted March 20, 1990.
Can J Vet Res 1991; 55: 1-4
I
~
averaged for data analysis. Measurements consisted of the latencies to the a and b wave peaks and the ab amplitude; measurements of each parameter were taken from 20 eyes. Each VEP was the average of responses to 100 flashes of light; two RECORDINGS were collected for each eye. Low and The recording protocol essentially high filter settings were 1 and 100 Hz, duplicated that used in a previous without 60 Hz filtering. The two study (6). Subdermal platinum alloy recordings for each eye were comneedle electrodes (Model E2, Grass bined with a computer-based proInstruments Co., Quincy, Massa- gram, resulting in a composite chusetts) were used to record the average of 200 responses from which responses; electrode impedances measurements were taken. Measurewere kept below 5 Kohms. The ERG ments consisted of the latencies to a were recorded between the lower lid series of seven positive and negative of the stimulated eye (positive peaks (PI, NI, P2, N2, P3, N3 and P4), electrode) and C, (vertex; negative and the resultant six peak-to-peak electrode), with ground at Q, (mid- amplitudes (P1-NI, N1-P2, P2-N2, N2point of the nuchal crest). The VEP P3, P3-N3 and N3-P4). were recorded between O (positive Summary statistics consisting of electrode) and Fpz (mid-point of the the mean, SD, and range for each interorbital line; negative electrode), latency and amplitude measure were with ground at Cz. calculated. Stimuli were 10 ,s flashes of white light in a dark room from a flash unit RESULTS (Model PS33, Grass Instruments Co., intensity setting 16) held ELECTRORETINOGRAMS Clear ERG were recorded from approximately 25 cm from the eye; manufacturer-specified light inten- both eyes of all sheep (Fig. 1). Peak sity at that distance was approxi- latencies were highly consistent from mately 1,500,000 candlepower (19,000,000 lumens). The ERG were recorded from single flashes; VEP Ovine were averaged from flashes at 1.5/s. were ERG Left and right eye recorded simultaneously, with the lamp held equidistant from each eye. Sequential VEP were recorded monocularly from both eyes, with the unstimulated eye covered. Electroretinograms were recorded within -l 30s after lights were extinguished, so that dark adaptation was not
employed during recording as dictated by animal behavior. No mydriatic or restraint drugs were used. All relevant guidelines for the care and use of experimental animals were followed.
eye to eye, while amplitudes were more variable. Latencies to the a and b wave peaks were 13.6 ± 1.1 ms (mean ± SD) and 28.2 ± 0.9 ms, respectively (Table I), while the ab peak-to-peak amplitudes averaged 131.68 ± 44.43 ,uV. Later peaks were frequently observed in the first 100 ms (Fig. 1, bottom two recordings), occurring at latencies consistent with VEP peaks (see below). VISUAL-EVOKED POTENTIALS
The VEP contained four positive and three negative intervening peaks within the first 150 ms after stimulation (Fig. 2). Latencies (mean + SD) to peaks P1, N1, P2, N2, P3, N3 and P4 were 35.0 + 1.9, 43.1 + 1.9, 52.8 ± 2.0 64.1 +3.8, 74.5 ±6.1, 90.4+ 6.3 and 112.2 ± 12.0 ms, respectively (Table II). Variability of the latencies was progressively greater for each subsequent peak. Mean peak-to-peak amplitudes ranged from 3.90 to 8.29 ,AV (Table III). Positive and negative peaks preceding P1 in many recordings occurred at latencies consistent with ERG a and b peak latencies.
DISCUSSION
The ovine ERG and VEP reported in this study were highly similar to
Electroretinograms
performed. DATA
Single ERG were recorded simultaneously from both eyes with an electrodiagnostic signal averaging system (Compact Four, Nicolet Biomedical Instrument Co., Madison, Wisconsin) and stored on disk for later analysis; two recordings were taken to assure repeatability. Low and high filter settings were 1 and 100 Hz; 60 Hz filtering was not required. The two recordings were 2
0
Q) 4--) -S
0
50
100
150
200
250
Latency (ms)
Fig. 1. Typical ovine electroretinograms, showing identification of the a and b waves. Far-field conducted VEP peaks were detected in some ERG records (bottom two tracings).
Ovine Visual Evoked Potential S _
*
_
0
50
250
200
150
100
Latency (ms) Fig. 2. Typical ovine visual-evoked potentials, showing identification of peaks Pi, NI, P 2, N2, P3, N3 and P4. Latencies to later peaks were clearly more variable than those of early peaks.
those reported for other species (6,9,13) and those reported for sheep in other investigations (3-5). However, a greater number of peaks were routinely present than are seen in cattle (6) or dogs (10). The recordings were repeatable for a given sheep and similar between sheep, indicating that ERG and VEP can be reliably recorded for either clinical or experimental purposes.
The ERG amplitudes rec-orded in sheep considerably exceed ed those reported for cows, although a similar protocol was utilized. This rce.sulted in part from the use of a vertex location for the negative electrode in both studies. The smaller head size of young sheep compared to adult cow in less signal attenuation, which is of the directly related to the squa re ofth distance separating the elect rode and
soresulted reic
TABLE I. Latency and amplitude measurements of ovine electroretinogramsa
Latency (ms) Measurement Mean ± SD Range n an
=
a 13.6 ± 1.1 12 - 15 20
Amplit
udea(V)
b 28.2 ± 0.9 27 - 30 20
i
131.68 ± 44.43 62.50 -234.37
20
VEP.
No. of measurements
TABLE II. Latency measurements of ovine visual-evoked potentialsa
PI
N2 P2 N' 64.1 43.1 52.8 35.0 ± 1.9 ± 1.9 ±2.0 ±3.8 ±SD 57 - 73 49 -57 31 - 38 39 -46 Range 20 20 n 20 20 aLatencies expressed in milliseconds; n = No. of measurements
Measurement Mean
P3 74.5
±6.1 65 - 87 20
N3 90.4
the source (14). In addition, ERG were recorded in response to single flashes in this study, while bovine ERG were averages of responses to 50 flashes. Repeated flashes, even at the low rate of 1.5/s, produce photobleaching of the retina, reducing the amplitude of subsequent responses and producing a smaller amplitude average. Peak VEP amplitudes in this study were similar to bovine amplitudes, presumably due to the probable cortical source of the response (see below). The sheep used in this study were only three to four months of age, but the ovine visual system by most measures is mature at birth. Visual cortex microelectrode recordings taken from lambs within hours of birth have demonstrated neurons with simple, complex and hypercomplex receptive field properties with essentially adult response patterns (15), and histological development of the cortex in ungulates is complete at birth (16). The menace response, a learned reaction to threatening gestures requiring an intact visual cortex, is generally present by one to two weeks (17). Although VEP peak latencies were not mature in the dog until after 100 days (10), bovine latencies were within the adult range by seven days (8). Thus, the latencies of this study are unlikely to differ from adult sheep, although peak amplitudes in older sheep will likely be smaller due to larger head sizes, as was seen for both and cow (8,10). dogThe retinal origins of the peaks in the ERG are accepted to be the rods and cones for the a wave, and the bipolar cell layer, primarily the Muller glial cells, for the b wave (12). Later waves following the b wave were consistent in latency with early peaks of the VEP, although inverted in polarity. It is likely, therefore, that these peaks reflect a far-field simultaneous recording of early peaks of the
P4
112.2 ± 12.0 ±6.3 78 - 108 89- 133 20 20
The generators of the peaks of the VEP are less established. Studies in rats (18), monkeys (19) and humans (20) have suggested that the origin of peak P1 is either the thalamocortical radiations or primary visual cortex, while the later peaks are generated in primary and secondary visual cortex. However, prominent early peaks in the canine VEP occur simultaneously
TABLE III. Amplitude measurements of ovine visual-evoked potentialsa
N2-P3 Measurement P2-N2 PI-N,N1-P2 6.98 3.90 Mean 7.36 8.29 ± SD ± 1.99 ± 2.62 ± 2.88 ± 3.18 Range 1.46 - 12.81 4.76 - 11.96 1.46 - 11.47 0.00 - 9.27 n 20 20 20 20 = aAmplitudes expressed in microvolts; n No. of measurements
with ERG peaks, suggesting a retinal source for the PI and N, of the canine response (9). Peaks suggested to be far-field conducted ERG were present in some sheep VEP, suggesting that the peaks identified in the sheep VEP in this study are generated in sites similar to those of the rat, monkey and human (18-20). REFERENCES 1. CHIAPPA KH. Evoked Potentials in Clinical Medicine. New York: Raven Press, 1990: 37-171. 2. SOKOL S. Visual evoked potentials. In: Aminoff MJ, ed. Electrodiagnosis in Clinical Neurology. New York: Churchill Livingstone, 1980: 348-369. 3. DALY CC, GREGORY NG, WOTTON SB, WHITTINGTON PE. Concussive methods of pre-slaughter stunning in sheep: assessment of brain function using cortical evoked responses. Res Vet Sci 1986; 41: 349352.
P3-N3
N3-P4
7.34 5.26 ± 4.14 ± 4.55 1.46 - 15.74 0.97 - 17.57 20 20
4. GREGORY NG, WOTTON SB. Studies on the central nervous system: visually evoked cortical responses in sheep. Res Vet Sci 1983; 34: 315-319. 5. STRAIN GM, OLCOTT BM, BRAUN WF Jr. Electroencephalogram and evoked potentials in naturally occurring scrapie in sheep. Am J Vet Res 1986: 47: 828-836. 6. STRAIN GM, OLCOTT BM, HOKETT LD. Electroretinogram and visual-evoked potential measurements in Holstein cows. Am J Vet Res 1986; 47: 1079-1081. 7. STRAIN GM, CLAXTON MS, TURNQUIST SE, KREEGER JM. Evoked potential and electroencephalographic assessment of central blindness due to brain abscesses in a steer. Cornell Vet 1987; 77: 374-382. 8. STRAIN GM, GRAHAM MC, CLAXTON MS, OLCOTT BM. Postnatal development of brainstem auditory-evoked potentials, electroretinograms, and visualevoked potentials in the calf. J Vet Intern Med 1989; 3: 231-237. 9. STRAIN GM, JACKSON RM, TEDFORD BL. Visual evoked potentials in the clinically normal dog. J Vet Intern Med 1990; 4: 222-225. 10. STRAIN GM, JACKSON RM, TEDFORD BL. Postnatal development of the canine visual-evoked potential. Am J Vet Res 1991; 52: (in press).
11. ARMINGTON JC. Electroretinography. In: Aminoff MJ, ed. Electrodiagnosis in Clinical Neurology. New York: Churchill Livingstone, 1980: 305-347. 12. FISHMAN GA. The Electroretinogram and Electro-oculogram in Retinal and Choroidal Disease. San Francisco: American Academy of Ophthalmology and Otolaryngology, 1975: 1-45. 13. RUBIN LF. Clinical electroretinography in dogs. J Am Vet Med Assoc 1967; 151: 14561469. 14. GEDDES LA, BAKER LE. Principles of Applied Biomedical Instrumentation. New York: Wiley, 1968: 278-291. 15. CLARKE PGH, MARTIN KAC, RAMACHANDRAN VS, RAO VM, WHITTERIDGE D. Development and plasticity of neuronal connections in the lamb visual system. In: Freeman RD, ed. Developmental Neurobiology of Vision. New York: Plenum Press, 1979: 403-410. 16. LATSHAW WK. Veterinary Developmental Anatomy. Toronto: B.C. Decker, 1987: 256-257. 17. DE LAHUNTA A. Veterinary Neuroanatomy and Clinical Neurology. 2nd ed. Philadelphia: W.B. Saunders, 1983: 289290. 18. DYER RS, JENSEN KF, BOYES WK. Focal lesions of visual cortex - effects on visual evoked potentials in rats. Exp Neurol 1987; 95: 100-115. 19. KRAUT MA, AREZZO JC, VAUGHAN HG Jr. Intracortical generators of the flash VEP in monkeys. Electroencephalogr Clin Neurophysiol 1985; 62: 300-312. 20. DUCATI A, FAVA E, MOTTI EDF. Neuronal generators of the visual evoked potentials: intracerebral recordings in awake humans. Eleetroencephalogr Clin Neurophysiol 1988; 71: 89-99.
ABSTRACT
Electroretinogram (ERG) and visual-evoked potential (VEP) recordings were taken from ten Suffolkcross sheep. Stimuli for VEP were 1.5 flashes of white light/s; ERG stimuli were single flashes. The ERG measurements of the a and b wave latencies and a-to-b amplitude were measured between the lower eyelid and the vertex, with ground on the nuchal crest. The VEP after monocular stimulation were measured between the nuchal crest and the interorbital line, with ground on the vertex. Measurements consisted of the latencies to seven alternating positive and negative peaks Pl, NI, P2, N2, P3, N3 and P4, and six amplitudes, PI-NI, N1-P2 ,P2-N2, N2-P3, P3-N3 and N3-P4. Average latencies for the a and b waves were 13.6 and 28.2 ms; the mean ab amplitude was 131.68 AV. Average latencies for the seven VEP peaks were 35.0, 43.1, 52.8, 64.1, 74.5, 90.4 and 112.2 ms. Mean amplitudes ranged from 3.90 to 8.29 1IAV.
deux yeux, alors qu'elle fut reiteree a toutes les 1.5 s et limitee a un seul oeil dans le cas des PCVEs. Pour obtenir les ERGs, les electrodes sous-dermales captrices en platine (resistence inferieur a 5000 ohms) furent placees a la paupiere inferieure (+) de chaque oeil, et au vertex du crane (-), tandis qu'une autre au centre de la crete de la neque servait d'electrode indifferente ou de masse. Les electrodes decelant les PCVEs etaient situees a la crete de la neque (+), au centre de la ligne interorbitale, et au vertex (masse). Les ondes alpha (negative) et bata (positive) de l'ERG survinrent en moyenne 13.6 et 28.2 ms apres la stimulation, et l'amplitude moyenne de alpha a bata se situa vers 131.68 uv. Les sept oscillations du PCVE, i.e., les quatre ondes positives (P1,P2,P3 et P4) separees par trois ondes negatives (N1,N2 et N3), apparurent 35.0, 43.1, 52.8, 64.1, 74.5, 90.4, et 112.2 ms. L'amplitude moyenne des oscillations
While these electrodiagnostic tests of visual function undoubtedly have the same neural generators in different mammalian species, it is necessary to document normative values for each species in which they are to be used, since recordings will differ due to differences in head, eye and brain size, orientation of the generators with respect to the recording electrodes, presence or absence of horns and recording protocol. The VEP has previously been recorded from sheep in both clinical (5) and experimental (3,4) applications, but its use and interpretation have been made difficult by the lack of normative data. The purpose of the present report was to document normative data for electrodiagnostic tests of vision in sheep.
P1-Nl, N1-P2, P2-N2, N2-P3, P3-N3, N3-P4 des PCVEs varia entre 3.90 et
The ERG and VEP were recorded from ten Suffolk-cross sheep (eight male, two female) that were three to four months of age. Mean weight was 27.9 ± 5.7 kg (range 20-38 kg). All were from the same Suffolk sire; the dams were various Merino-Louisiana Native-Suffolk crosses and were all half sisters. The subjects consisted of one set of triplets and 3.5 sets of twins. All sheep appeared to have normal vision based on behavioral responses to the proximity of the investigators and locomotion around fixed objects. Halters and manual restraint were
8.29 uv. (Traduit par Dr L.P. Phaneuf)
RESUME
INTRODUCTION
A l'aide de stimuli lumineux (eclairs d'une duree de 10 us), on enregistra chez dix jeunes ovins (8 males, 2 femelles) les potentiels retiniens (electroretinogrammes ou ERG) et les potentiels corticaux visuels evoques (PCVE). Pour les ERGs, une stimulation lumineuse unique interessa les
The visual-evoked potential (VEP) is a noninvasive measure of activity in postretinal visual pathways with demonstrated utility in human (1,2) and veterinary (3-10) applications. Likewise, the electroretinogram (ERG) provides a noninvasive measure of retinal function (6,11,12).
MATERIALS AND METHODS SHEEP
Department of Veterinary Physiology, Pharmacology and Toxicology (Strain, Prescott-Mathews, LaPhand) and Department of Veterinary Clinical Sciences (Claxton), School of Veterinary Medicine, Louisiana State University, Baton Rouge, Louisiana 70803-8420. Supported by the LSU School of Veterinary Medicine Veterinary Medical Student Honors Program (JSPM) and Summer Research Apprenticeship Program (DJL). Submitted March 20, 1990.
Can J Vet Res 1991; 55: 1-4
I
~
averaged for data analysis. Measurements consisted of the latencies to the a and b wave peaks and the ab amplitude; measurements of each parameter were taken from 20 eyes. Each VEP was the average of responses to 100 flashes of light; two RECORDINGS were collected for each eye. Low and The recording protocol essentially high filter settings were 1 and 100 Hz, duplicated that used in a previous without 60 Hz filtering. The two study (6). Subdermal platinum alloy recordings for each eye were comneedle electrodes (Model E2, Grass bined with a computer-based proInstruments Co., Quincy, Massa- gram, resulting in a composite chusetts) were used to record the average of 200 responses from which responses; electrode impedances measurements were taken. Measurewere kept below 5 Kohms. The ERG ments consisted of the latencies to a were recorded between the lower lid series of seven positive and negative of the stimulated eye (positive peaks (PI, NI, P2, N2, P3, N3 and P4), electrode) and C, (vertex; negative and the resultant six peak-to-peak electrode), with ground at Q, (mid- amplitudes (P1-NI, N1-P2, P2-N2, N2point of the nuchal crest). The VEP P3, P3-N3 and N3-P4). were recorded between O (positive Summary statistics consisting of electrode) and Fpz (mid-point of the the mean, SD, and range for each interorbital line; negative electrode), latency and amplitude measure were with ground at Cz. calculated. Stimuli were 10 ,s flashes of white light in a dark room from a flash unit RESULTS (Model PS33, Grass Instruments Co., intensity setting 16) held ELECTRORETINOGRAMS Clear ERG were recorded from approximately 25 cm from the eye; manufacturer-specified light inten- both eyes of all sheep (Fig. 1). Peak sity at that distance was approxi- latencies were highly consistent from mately 1,500,000 candlepower (19,000,000 lumens). The ERG were recorded from single flashes; VEP Ovine were averaged from flashes at 1.5/s. were ERG Left and right eye recorded simultaneously, with the lamp held equidistant from each eye. Sequential VEP were recorded monocularly from both eyes, with the unstimulated eye covered. Electroretinograms were recorded within -l 30s after lights were extinguished, so that dark adaptation was not
employed during recording as dictated by animal behavior. No mydriatic or restraint drugs were used. All relevant guidelines for the care and use of experimental animals were followed.
eye to eye, while amplitudes were more variable. Latencies to the a and b wave peaks were 13.6 ± 1.1 ms (mean ± SD) and 28.2 ± 0.9 ms, respectively (Table I), while the ab peak-to-peak amplitudes averaged 131.68 ± 44.43 ,uV. Later peaks were frequently observed in the first 100 ms (Fig. 1, bottom two recordings), occurring at latencies consistent with VEP peaks (see below). VISUAL-EVOKED POTENTIALS
The VEP contained four positive and three negative intervening peaks within the first 150 ms after stimulation (Fig. 2). Latencies (mean + SD) to peaks P1, N1, P2, N2, P3, N3 and P4 were 35.0 + 1.9, 43.1 + 1.9, 52.8 ± 2.0 64.1 +3.8, 74.5 ±6.1, 90.4+ 6.3 and 112.2 ± 12.0 ms, respectively (Table II). Variability of the latencies was progressively greater for each subsequent peak. Mean peak-to-peak amplitudes ranged from 3.90 to 8.29 ,AV (Table III). Positive and negative peaks preceding P1 in many recordings occurred at latencies consistent with ERG a and b peak latencies.
DISCUSSION
The ovine ERG and VEP reported in this study were highly similar to
Electroretinograms
performed. DATA
Single ERG were recorded simultaneously from both eyes with an electrodiagnostic signal averaging system (Compact Four, Nicolet Biomedical Instrument Co., Madison, Wisconsin) and stored on disk for later analysis; two recordings were taken to assure repeatability. Low and high filter settings were 1 and 100 Hz; 60 Hz filtering was not required. The two recordings were 2
0
Q) 4--) -S
0
50
100
150
200
250
Latency (ms)
Fig. 1. Typical ovine electroretinograms, showing identification of the a and b waves. Far-field conducted VEP peaks were detected in some ERG records (bottom two tracings).
Ovine Visual Evoked Potential S _
*
_
0
50
250
200
150
100
Latency (ms) Fig. 2. Typical ovine visual-evoked potentials, showing identification of peaks Pi, NI, P 2, N2, P3, N3 and P4. Latencies to later peaks were clearly more variable than those of early peaks.
those reported for other species (6,9,13) and those reported for sheep in other investigations (3-5). However, a greater number of peaks were routinely present than are seen in cattle (6) or dogs (10). The recordings were repeatable for a given sheep and similar between sheep, indicating that ERG and VEP can be reliably recorded for either clinical or experimental purposes.
The ERG amplitudes rec-orded in sheep considerably exceed ed those reported for cows, although a similar protocol was utilized. This rce.sulted in part from the use of a vertex location for the negative electrode in both studies. The smaller head size of young sheep compared to adult cow in less signal attenuation, which is of the directly related to the squa re ofth distance separating the elect rode and
soresulted reic
TABLE I. Latency and amplitude measurements of ovine electroretinogramsa
Latency (ms) Measurement Mean ± SD Range n an
=
a 13.6 ± 1.1 12 - 15 20
Amplit
udea(V)
b 28.2 ± 0.9 27 - 30 20
i
131.68 ± 44.43 62.50 -234.37
20
VEP.
No. of measurements
TABLE II. Latency measurements of ovine visual-evoked potentialsa
PI
N2 P2 N' 64.1 43.1 52.8 35.0 ± 1.9 ± 1.9 ±2.0 ±3.8 ±SD 57 - 73 49 -57 31 - 38 39 -46 Range 20 20 n 20 20 aLatencies expressed in milliseconds; n = No. of measurements
Measurement Mean
P3 74.5
±6.1 65 - 87 20
N3 90.4
the source (14). In addition, ERG were recorded in response to single flashes in this study, while bovine ERG were averages of responses to 50 flashes. Repeated flashes, even at the low rate of 1.5/s, produce photobleaching of the retina, reducing the amplitude of subsequent responses and producing a smaller amplitude average. Peak VEP amplitudes in this study were similar to bovine amplitudes, presumably due to the probable cortical source of the response (see below). The sheep used in this study were only three to four months of age, but the ovine visual system by most measures is mature at birth. Visual cortex microelectrode recordings taken from lambs within hours of birth have demonstrated neurons with simple, complex and hypercomplex receptive field properties with essentially adult response patterns (15), and histological development of the cortex in ungulates is complete at birth (16). The menace response, a learned reaction to threatening gestures requiring an intact visual cortex, is generally present by one to two weeks (17). Although VEP peak latencies were not mature in the dog until after 100 days (10), bovine latencies were within the adult range by seven days (8). Thus, the latencies of this study are unlikely to differ from adult sheep, although peak amplitudes in older sheep will likely be smaller due to larger head sizes, as was seen for both and cow (8,10). dogThe retinal origins of the peaks in the ERG are accepted to be the rods and cones for the a wave, and the bipolar cell layer, primarily the Muller glial cells, for the b wave (12). Later waves following the b wave were consistent in latency with early peaks of the VEP, although inverted in polarity. It is likely, therefore, that these peaks reflect a far-field simultaneous recording of early peaks of the
P4
112.2 ± 12.0 ±6.3 78 - 108 89- 133 20 20
The generators of the peaks of the VEP are less established. Studies in rats (18), monkeys (19) and humans (20) have suggested that the origin of peak P1 is either the thalamocortical radiations or primary visual cortex, while the later peaks are generated in primary and secondary visual cortex. However, prominent early peaks in the canine VEP occur simultaneously
TABLE III. Amplitude measurements of ovine visual-evoked potentialsa
N2-P3 Measurement P2-N2 PI-N,N1-P2 6.98 3.90 Mean 7.36 8.29 ± SD ± 1.99 ± 2.62 ± 2.88 ± 3.18 Range 1.46 - 12.81 4.76 - 11.96 1.46 - 11.47 0.00 - 9.27 n 20 20 20 20 = aAmplitudes expressed in microvolts; n No. of measurements
with ERG peaks, suggesting a retinal source for the PI and N, of the canine response (9). Peaks suggested to be far-field conducted ERG were present in some sheep VEP, suggesting that the peaks identified in the sheep VEP in this study are generated in sites similar to those of the rat, monkey and human (18-20). REFERENCES 1. CHIAPPA KH. Evoked Potentials in Clinical Medicine. New York: Raven Press, 1990: 37-171. 2. SOKOL S. Visual evoked potentials. In: Aminoff MJ, ed. Electrodiagnosis in Clinical Neurology. New York: Churchill Livingstone, 1980: 348-369. 3. DALY CC, GREGORY NG, WOTTON SB, WHITTINGTON PE. Concussive methods of pre-slaughter stunning in sheep: assessment of brain function using cortical evoked responses. Res Vet Sci 1986; 41: 349352.
P3-N3
N3-P4
7.34 5.26 ± 4.14 ± 4.55 1.46 - 15.74 0.97 - 17.57 20 20
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