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Presaccadic spike potentials with large horizontal eye movements
Electroencephalography and clinical Neurophysiology, 1989, 73: 260- 263
260
Elsevier Scientific Publishers Ireland, Ltd. EEG 89506
Short communication
Presaccadic spike potentials with large horizontal eye movements H.R. Doig and C. Boylan Department of Vision Sciences, University of Aston, Aston Triangle, Birmingham B4 7ET ( U.KI) (Accepted for publication: 20 May 1989)
Summary, Presaccadic spike potentials were recorded from electrodes at the inner canthus and below the eye in 10 normal subjects for a range of horizontal saccades (5 o, 10 o, 20 ° and 40 o ). An eye movement trace was recorded for 20 abducting and 20 adducting saecades to determine the beginning of the saccade, and the spike potential back-averaged from this point. The latencies of the spike potential onset and peak were found with respect to the start of the eye movement and the amplitude from the onset to the peak was measured for each saccade size. The latency values remained constant throughout the range of eye movement sizes, although adducting saccades showed an earlier onset and peak latency than abducting saccades. The amplitude data, however, showed a definite relationship between saccade size and spike potential amplitude, with a significant increase in the amplitude for saccades between 10 ° and 400 (inner electrode abduction, lower electrode abduction and adduction P < 0.01; inner electrode adduction P < 0,05). A possible explanation of this increase in amplitude is hypothesised from a computer model of the action potential activity that may occur in the extraocular muscles before an eye movement. Key words: Saccade; Presaccadic spike; Amplitude; Latency; Motoneurone
Immediately before the onset of saccadic eye movements a large amplitude negative potential can be recorded from electrodes placed around the eyes with Pz as a reference (Thickbroom and Mastaglia 1985, 1986; Boylan and Doig 1988; Riemslag et al. 1988). The onset of this presaccadic spike potential (SP) occurs 4.4-12.0 msec before the start of the saccade, peaking at or within 7.8 msec after the start of the saccade (Boylan and Doig 1988). The SP is generally considered to originate in the extra-ocular muscle motor units (Thickbroom and Mastaglia 1985) or, more specifically, the ocular motoneurones innervating these motor units (Riemslag et al. 1988). All studies of the SP have shown that it has a very consistent wave form, with little difference in the parameters found between either the two eyes, or different electrode sites around the eyes. Abducting and adducting saccades show a consistent latency difference, with addueting saccades having an earlier onset and peak than abducting eye movements, but no amplitude difference for the saccade types has been reported (Boylan and Doig 1988). In an early study of the SP Thickbroom and Mastaglia (1985) showed that as well as the large anterior negative potential a smaller synchronous posterior positivity could be
Correspondence to." Mr. H.R. Doig, Department of Vision Sciences, University of Aston, Aston Triangle, Birmingham B4 7ET (U.K.).
recorded and they suggested that this positive potential may be due to current flow over the scalp from the anterior source. A similar positive potential has been recorded by some workers over the parietal region of the head with the same latency with regard to the onset of the eye movement as the large anterior spike potential (Kurtzberg and Vaughan 1982; lgnocheck et al. 1986). The amplitude of this posterior potential appears to be dependent upon the instructions given to the subjects and the saccade direction (Ignocheck et at. 1986). Although the specific origin of this positive spike potential is unknown it has been suggested that it may represent activity at multiple central sites reflecting both the performance and the attentional/motivational value of rapid eye movements (Ignocheck et al. 1986). Thickbroom and Mastagha (1985), in this early investigation of the SP, found that the onset to peak of the amplitude is unaffected by the saccade size for eye movements of between 10 ° and 40 ° (Thickbroom and Mastaglia 1985). However, more recently it has been reported that the SP amplitude does alter with saccade size, the amplitude increasing with increasing saccade size to reach a maximum with eye movements of approximately 8-10 °, with no further increase in the SP amplitude for larger saccades (Riemslag et al. 1988). No changes in the onset and peak latencies have been reported with different saccade sizes. The present study was designed to examine the SP parameters over a wide range of saccade sizes, with statistical analysis of the recorded amplitude and latency values. It is important that the results are analysed statistically, particularly when
0013-4649/89/$03.50 c~'~1989 Elsevier Scientific Publishers Ireland. Ltd.
SPIKE POTENTIALS W I T H LARGE EYE MOVEMENTS
261
considering amplitudes, as this is known to be a variable parameter in SP recordings (Boylan and Doig 1988).
Inner electrode / Abduction
Methods
80-
60-
60-
50,
Ten normal subjects, aged 22-30 (mean 25.8) years, with no known ophthalmological or neurological defects were examined. From each subject recordings were made from either the right or left eye, randomly chosen, to produce a set of results from 10 eyes, 5 right and 5 left. In accordance with earlier work, electrodes placed at the inner canthus and below the eye were used to record the SP, with Pz as the reference (Thickbroom and Mastaglia 1985, 1986; Boylan and Doig 1988). The saccades were visually triggered by two, alternately flashing, red LEDs. Electro-oculography (EOG) was used to monitor the eye position using electrodes at the inner and outer canthi. Forty SP and eye movement traces were recorded from each subject, 20 abducting and 20 adducting for horizontal
Lower electrode / Abduction 15-
Inner electrode / Abduction 15Peak I 10-
s"
-1[
~ -10
-15
-15 •
i
•
!
•
i
•
0
lO
IPeak
gs
{
~ -10 1
Onset •
0
i
•
i
•
=
•
=
10 2o 30 4o Saccade size (degrees)
3o"
20-
20
10-
10 •
,
-
10
i
-
20
•
.
30
=
-
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40
-
O
Saccade size (degrees)
Inner electrode / Adduction
t
•
=
•
=
-
=
-
10 20 30 40 Saccade size (degrees)
Lower electrode / Adduction
8O
80-
70. 60'
E <
5O
50'
40
40.
30 <
20 "
30. 20. 10
•
i
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Lower electrode / Adduction
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-20
~
40-
30-
O
_
Saccade size (degrees)
Inner electrode / Adduction
-15
e
10
15-
--~-10
~
>~ 50"
/
Fig. 2. The amplitude ( + 1 S.D.) of the SP onset-peak plotted against the saccade size. There is an initial increase in the amplitude from 5 o to 10 o saccades, little change from 10 o to 20 o eye movements, but with 40 o saccades larger amplitudes are recorded.
-20
!
1 0 20 30 40 Saccade size (degrees)
5
*
Saccade size (degrees)
g-5
0
~
40' ¢x
0
E0
-20
70-
10"
10-
I
Lower electrode / Abduction
8070 2
•
0
10 20 30 40 Saccadesize (degrees)
Fig. 1. The latency ( + 1 S.D.) of the onset and peak of the SP plotted against the saccade size. A negative value indicates a latency before the start of the saccade, and a positive value indicates a latency after the beginning of the saccade. The onset and peak occur earlier with adducting saccades than with abducting saccades for the 4 different saccade sizes, but there is no consistent change in the latency values with saccade size.
saccades of 5 o, 10 o, 20 o and 40 o, each saccade size being recorded separately. A Nicolet Pathfinder II was used to record both the EOG and SPs, with the individual eye position traces and the SPs recorded on separate channels. Each individual SP and EOG trace was stored on hard disc. A time sweep of 500 msec was used, and the bandpass filter was set with a low cut-off of 0.50 Hz and a high cut-off of 100 Hz. Following the recording, the EOG traces were recalled and the beginning of the saccade identified using the averager's internal cursor. This was manually performed so that any traces in which this point was not easily recognisable could be rejected. The computer then back-averaged the SP traces from the beginning of the saccade and, for display purposes, the traces were aligned at a predetermined and fixed latency. Using the averager's internal cursor the latencies of the onset and peak were determined with respect to the beginning of the eye movement, a negative value indicating a latency before the start of the saccade and a positive value indicating a latency after the start of the saccade. The onset to peak amplitude was also measured for each of the averaged traces.
262
H.R. DOIG, C. BOYLAN
Results Clear SPs were recorded from all subjects for all saccade sizes and at both electrode sites. The group means and standard deviations for the latency and amplitude data were calculated separately for abducting and addueting saccades. The SP onset and peak latencies had consistent values across the range of saccade sizes at both electrodes. The onset began between 4.6 and 12.0 msec before the start of the eye movement at both electrode sites for both abducting and addueting saccades peaking between the start of the saccade to 8.3 msec after the saccade had started. In each recording condition the SPs recorded with adducting saccades bad earlier onset and peak latencies than those recorded with abducting saccades. These results are shown graphically in Fig. 1. The amplitude values did, however, show a change with the different saccade sizes. Fig. 2 shows that when the amplitudes are plotted against saccade size there is an initial increase in the SP amplitude between 5 o and 10 o with little change from
Inner eleetrooe Abduction
10 o to 20 o saccades. For 5 o saccades the amplitude measured 19.3-28.6 ~V, for 10 ° saccades 31.8-38.0 p,V and for 20 ° saccades 37.1-48.3 p.V. However, with the larger saccades of 40 o the amplitude was seen to increase again to 47.0-59.2 p.V. To determine whether the amplitude changes found between 10 ° and 40 o saccade,s were statistically significant a Wilcoxon T test was used. Parametric statistical tests were not suitable because, as shown in Fig. 2, the variance of the data was not consistent within the range of saccade sizes with a generally reduced standard deviation for the smaller eye movements. In each case it was found that the amplitude differences were significant at both the electrode sites both for abduction and adduction (inner electrode abduction n = 10, T = 3, P < 0.01, adduction n =10, T = 7.5, P < 0.05; lower electrode abduction and adduction n = 10, T ~ 0, P < 0.01). The change in the SP amplitude with saccade size can be seen in Fig. 3 which shows the group average traces from the 2 electrode sites for the 4 saccade sizes. The traces show that the SP wave form remained constant throughout, but the onset to peak amplitude was larger with 40 ° saccades than for the smaller sized eye movement,~,
Inner electrode Ad(:luct.ion
Discussion /, / 40 =
i
i/
/
/
20*
10"
5o
[-
i
L
I
[
~
i
I
I
I
I
v
•i20.
Lower electroOe Abductiotl
Lower electrode Adductien
20 °
l i I l t i l Ll L l J t " [~-r~'r i 1 Fig. 3. The SPs recorded at the lower and inner electrodes during abducting and adducting saccades for 4 different saccade sizes. The onset and peak latencies and the wave form of the SP are little affected by the saccade magnitude, but the amplitude is larger with the larger saccade sizes.
The results of the present study have demonstrated that the SP latency values appear to be little affected by the size of the eye movement, with a consistent onset and peak regardless of the magnitude of the saccade. Adducting saccades were found to have an earlier onset and peak than abducting ones, confirming earlier findings concerning latency differences between the different movements (Boylan and Doig 1988). The change in the SP amplitude as the saccade size varies for saccades smaller than 10 ° is not a new finding. Previous studies of the effect of saccade size upon the SP amplitude have reported that the SP amplitude appears to be dependent upon saccade size only for eye movements of up to approximately 10 ° (Riemslag et al. 1988). This is confirmed by our results which show that the SP amplitude increases between 5 o and 10 ° saccades, but varies little between 10 ° and 20 ° saccades. The increase in the SP amplitude between 10 o and 4 0 ° saccades, however, is in contrast to earlier studies which showed that the SP amplitude remained constant for saccades of these sizes (Thickbroom and Mastaglia 1985; Riemslag et al. 1988). It must be observed, however, that Thickbroom and Mastaglia (1985) used a different recording montage when they reported no variation in the SP amplitude with saccade size, a sternoclavicular reference being used and 30 electrodes across the scalp to record the EEG activity. The apparent discrepancy between the present study and the results reported by Thickbroom and Mastaglia (1985) may be accounted for by the different recording techniques. The apparent saturation in SP amplitude with 8-10 o eye movements has been attributed to the supposed origin of the SP in the ocular motoneurones, which increase their firing rate with increasing saccade size to a maximum for saccades of about 10 o (Riemslag et al. 1988). Our results suggest that if we accept this suggested site of
S P I K E P O T E N T I A L S W I T H L A R G E EYE M O V E M E N T S origin of the SP, then a further change in the motoneurone activity must occur with larger saccades. Although information regarding experimental recording of motoneurone activity with large saccades is sparse, a possible explanation m a y be hypothesised from a computer model of the motor unit activity that precedes saccades (Thickbroom and Mastaglia 1987). Within this model the wave form of the action potential to the motor units is assumed to be biphasic, with an initial negative phase followed by a positive phase. By increasing the duration of the second (positive) component of the action potential, the SP amplitude is predicted to increase by up to 120%. It is possible that with the larger 40 o saccades there is an increase in the duration of the second positive component of the action potential resulting in the increase in the SP amplitude that we have recorded. Another factor that must be considered is the dependence of the cortically recorded SP to motivational factors (Ignocheck et al. 1986). If the two SP recordings are related it is conceivable that the increase in the SP amplitude with 40 ° saccades m a y reflect the extra effort required in making saccades of such an unnaturally large size (Bahill et al. 1975). A n y comparisons, however, between these two recordings must be treated with caution until their specific origins are better known. Assuming that the SP we have recorded is related to the ocular motoneurone behaviour, further work is required to confirm the action potential activity that has been suggested above and to show whether the surface recorded SP does indeed represent the motoneurone behaviour.
References Bahill, A.T., Adler, D. and Stark, L. Most naturally occurring h u m a n saccades have magnitudes of 15 degrees or less. Invest. Ophthalmol., 1975, 14: 468-469.
263 Boylan, C. and Doig, H.R. Presaccadic spike potential to horizontal eye movements. Electroenceph. clin. Neurophysiol., 1988, 70: 559-562. Ignocheck, A., Weinstein, J.M. and Balaban, C.D. H u m a n spike potentials prior to saccades and optokinetic nystagm u s fast phases: effects of instructions, eye movement direction and electrode laterality. Brain Res., 1986, 384: 94-100. Kurtzberg, D. and Vaughan, H.C. Topographic analysis of h u m a n cortical potentials preceding self-initiated and visually triggered saccades. Brain Res., 1982, 243: 1-9. Riemslag, F.C.C., Van der Heijde, G.L., Van Dongen, M.M.M.M. and Ottenhoff, F. On the origin of the presaccadic spike potential. Electroenceph. clin. Neurophysiol., 1988, 70: 281-287. Thickbroom, G.W. and Mastaglia, F.L. Pre-saccadic 'spike' potential: investigation of topography and source. Brain Res., 1985, 339: 271-280. Thickbroom, G.W. and Mastaglia, F.L. Presaccadic spike potential. Relation to eye movement direction. Electroenceph. clin. Neurophysiol., 1986, 64: 211-214. Thickbroom, G.W. and Mastaglia, F.L. Presaccadic spike potential: a computer model based upon motor unit recruitment patterns in the extraocular muscles. Brain Res., 1987, 422: 377-380.
260
Elsevier Scientific Publishers Ireland, Ltd. EEG 89506
Short communication
Presaccadic spike potentials with large horizontal eye movements H.R. Doig and C. Boylan Department of Vision Sciences, University of Aston, Aston Triangle, Birmingham B4 7ET ( U.KI) (Accepted for publication: 20 May 1989)
Summary, Presaccadic spike potentials were recorded from electrodes at the inner canthus and below the eye in 10 normal subjects for a range of horizontal saccades (5 o, 10 o, 20 ° and 40 o ). An eye movement trace was recorded for 20 abducting and 20 adducting saecades to determine the beginning of the saccade, and the spike potential back-averaged from this point. The latencies of the spike potential onset and peak were found with respect to the start of the eye movement and the amplitude from the onset to the peak was measured for each saccade size. The latency values remained constant throughout the range of eye movement sizes, although adducting saccades showed an earlier onset and peak latency than abducting saccades. The amplitude data, however, showed a definite relationship between saccade size and spike potential amplitude, with a significant increase in the amplitude for saccades between 10 ° and 400 (inner electrode abduction, lower electrode abduction and adduction P < 0.01; inner electrode adduction P < 0,05). A possible explanation of this increase in amplitude is hypothesised from a computer model of the action potential activity that may occur in the extraocular muscles before an eye movement. Key words: Saccade; Presaccadic spike; Amplitude; Latency; Motoneurone
Immediately before the onset of saccadic eye movements a large amplitude negative potential can be recorded from electrodes placed around the eyes with Pz as a reference (Thickbroom and Mastaglia 1985, 1986; Boylan and Doig 1988; Riemslag et al. 1988). The onset of this presaccadic spike potential (SP) occurs 4.4-12.0 msec before the start of the saccade, peaking at or within 7.8 msec after the start of the saccade (Boylan and Doig 1988). The SP is generally considered to originate in the extra-ocular muscle motor units (Thickbroom and Mastaglia 1985) or, more specifically, the ocular motoneurones innervating these motor units (Riemslag et al. 1988). All studies of the SP have shown that it has a very consistent wave form, with little difference in the parameters found between either the two eyes, or different electrode sites around the eyes. Abducting and adducting saccades show a consistent latency difference, with addueting saccades having an earlier onset and peak than abducting eye movements, but no amplitude difference for the saccade types has been reported (Boylan and Doig 1988). In an early study of the SP Thickbroom and Mastaglia (1985) showed that as well as the large anterior negative potential a smaller synchronous posterior positivity could be
Correspondence to." Mr. H.R. Doig, Department of Vision Sciences, University of Aston, Aston Triangle, Birmingham B4 7ET (U.K.).
recorded and they suggested that this positive potential may be due to current flow over the scalp from the anterior source. A similar positive potential has been recorded by some workers over the parietal region of the head with the same latency with regard to the onset of the eye movement as the large anterior spike potential (Kurtzberg and Vaughan 1982; lgnocheck et al. 1986). The amplitude of this posterior potential appears to be dependent upon the instructions given to the subjects and the saccade direction (Ignocheck et at. 1986). Although the specific origin of this positive spike potential is unknown it has been suggested that it may represent activity at multiple central sites reflecting both the performance and the attentional/motivational value of rapid eye movements (Ignocheck et al. 1986). Thickbroom and Mastagha (1985), in this early investigation of the SP, found that the onset to peak of the amplitude is unaffected by the saccade size for eye movements of between 10 ° and 40 ° (Thickbroom and Mastaglia 1985). However, more recently it has been reported that the SP amplitude does alter with saccade size, the amplitude increasing with increasing saccade size to reach a maximum with eye movements of approximately 8-10 °, with no further increase in the SP amplitude for larger saccades (Riemslag et al. 1988). No changes in the onset and peak latencies have been reported with different saccade sizes. The present study was designed to examine the SP parameters over a wide range of saccade sizes, with statistical analysis of the recorded amplitude and latency values. It is important that the results are analysed statistically, particularly when
0013-4649/89/$03.50 c~'~1989 Elsevier Scientific Publishers Ireland. Ltd.
SPIKE POTENTIALS W I T H LARGE EYE MOVEMENTS
261
considering amplitudes, as this is known to be a variable parameter in SP recordings (Boylan and Doig 1988).
Inner electrode / Abduction
Methods
80-
60-
60-
50,
Ten normal subjects, aged 22-30 (mean 25.8) years, with no known ophthalmological or neurological defects were examined. From each subject recordings were made from either the right or left eye, randomly chosen, to produce a set of results from 10 eyes, 5 right and 5 left. In accordance with earlier work, electrodes placed at the inner canthus and below the eye were used to record the SP, with Pz as the reference (Thickbroom and Mastaglia 1985, 1986; Boylan and Doig 1988). The saccades were visually triggered by two, alternately flashing, red LEDs. Electro-oculography (EOG) was used to monitor the eye position using electrodes at the inner and outer canthi. Forty SP and eye movement traces were recorded from each subject, 20 abducting and 20 adducting for horizontal
Lower electrode / Abduction 15-
Inner electrode / Abduction 15Peak I 10-
s"
-1[
~ -10
-15
-15 •
i
•
!
•
i
•
0
lO
IPeak
gs
{
~ -10 1
Onset •
0
i
•
i
•
=
•
=
10 2o 30 4o Saccade size (degrees)
3o"
20-
20
10-
10 •
,
-
10
i
-
20
•
.
30
=
-
O
40
-
O
Saccade size (degrees)
Inner electrode / Adduction
t
•
=
•
=
-
=
-
10 20 30 40 Saccade size (degrees)
Lower electrode / Adduction
8O
80-
70. 60'
E <
5O
50'
40
40.
30 <
20 "
30. 20. 10
•
i
•
!
20
•
i
30
•
!
40
•
0
•
0
i
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•
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•
10 20 30 40 Saccade size (degrees)
• , • , • , • , 10 2O 30 40
Lower electrode / Adduction
10-
-20
~
40-
30-
O
_
Saccade size (degrees)
Inner electrode / Adduction
-15
e
10
15-
--~-10
~
>~ 50"
/
Fig. 2. The amplitude ( + 1 S.D.) of the SP onset-peak plotted against the saccade size. There is an initial increase in the amplitude from 5 o to 10 o saccades, little change from 10 o to 20 o eye movements, but with 40 o saccades larger amplitudes are recorded.
-20
!
1 0 20 30 40 Saccade size (degrees)
5
*
Saccade size (degrees)
g-5
0
~
40' ¢x
0
E0
-20
70-
10"
10-
I
Lower electrode / Abduction
8070 2
•
0
10 20 30 40 Saccadesize (degrees)
Fig. 1. The latency ( + 1 S.D.) of the onset and peak of the SP plotted against the saccade size. A negative value indicates a latency before the start of the saccade, and a positive value indicates a latency after the beginning of the saccade. The onset and peak occur earlier with adducting saccades than with abducting saccades for the 4 different saccade sizes, but there is no consistent change in the latency values with saccade size.
saccades of 5 o, 10 o, 20 o and 40 o, each saccade size being recorded separately. A Nicolet Pathfinder II was used to record both the EOG and SPs, with the individual eye position traces and the SPs recorded on separate channels. Each individual SP and EOG trace was stored on hard disc. A time sweep of 500 msec was used, and the bandpass filter was set with a low cut-off of 0.50 Hz and a high cut-off of 100 Hz. Following the recording, the EOG traces were recalled and the beginning of the saccade identified using the averager's internal cursor. This was manually performed so that any traces in which this point was not easily recognisable could be rejected. The computer then back-averaged the SP traces from the beginning of the saccade and, for display purposes, the traces were aligned at a predetermined and fixed latency. Using the averager's internal cursor the latencies of the onset and peak were determined with respect to the beginning of the eye movement, a negative value indicating a latency before the start of the saccade and a positive value indicating a latency after the start of the saccade. The onset to peak amplitude was also measured for each of the averaged traces.
262
H.R. DOIG, C. BOYLAN
Results Clear SPs were recorded from all subjects for all saccade sizes and at both electrode sites. The group means and standard deviations for the latency and amplitude data were calculated separately for abducting and addueting saccades. The SP onset and peak latencies had consistent values across the range of saccade sizes at both electrodes. The onset began between 4.6 and 12.0 msec before the start of the eye movement at both electrode sites for both abducting and addueting saccades peaking between the start of the saccade to 8.3 msec after the saccade had started. In each recording condition the SPs recorded with adducting saccades bad earlier onset and peak latencies than those recorded with abducting saccades. These results are shown graphically in Fig. 1. The amplitude values did, however, show a change with the different saccade sizes. Fig. 2 shows that when the amplitudes are plotted against saccade size there is an initial increase in the SP amplitude between 5 o and 10 o with little change from
Inner eleetrooe Abduction
10 o to 20 o saccades. For 5 o saccades the amplitude measured 19.3-28.6 ~V, for 10 ° saccades 31.8-38.0 p,V and for 20 ° saccades 37.1-48.3 p.V. However, with the larger saccades of 40 o the amplitude was seen to increase again to 47.0-59.2 p.V. To determine whether the amplitude changes found between 10 ° and 40 o saccade,s were statistically significant a Wilcoxon T test was used. Parametric statistical tests were not suitable because, as shown in Fig. 2, the variance of the data was not consistent within the range of saccade sizes with a generally reduced standard deviation for the smaller eye movements. In each case it was found that the amplitude differences were significant at both the electrode sites both for abduction and adduction (inner electrode abduction n = 10, T = 3, P < 0.01, adduction n =10, T = 7.5, P < 0.05; lower electrode abduction and adduction n = 10, T ~ 0, P < 0.01). The change in the SP amplitude with saccade size can be seen in Fig. 3 which shows the group average traces from the 2 electrode sites for the 4 saccade sizes. The traces show that the SP wave form remained constant throughout, but the onset to peak amplitude was larger with 40 ° saccades than for the smaller sized eye movement,~,
Inner electrode Ad(:luct.ion
Discussion /, / 40 =
i
i/
/
/
20*
10"
5o
[-
i
L
I
[
~
i
I
I
I
I
v
•i20.
Lower electroOe Abductiotl
Lower electrode Adductien
20 °
l i I l t i l Ll L l J t " [~-r~'r i 1 Fig. 3. The SPs recorded at the lower and inner electrodes during abducting and adducting saccades for 4 different saccade sizes. The onset and peak latencies and the wave form of the SP are little affected by the saccade magnitude, but the amplitude is larger with the larger saccade sizes.
The results of the present study have demonstrated that the SP latency values appear to be little affected by the size of the eye movement, with a consistent onset and peak regardless of the magnitude of the saccade. Adducting saccades were found to have an earlier onset and peak than abducting ones, confirming earlier findings concerning latency differences between the different movements (Boylan and Doig 1988). The change in the SP amplitude as the saccade size varies for saccades smaller than 10 ° is not a new finding. Previous studies of the effect of saccade size upon the SP amplitude have reported that the SP amplitude appears to be dependent upon saccade size only for eye movements of up to approximately 10 ° (Riemslag et al. 1988). This is confirmed by our results which show that the SP amplitude increases between 5 o and 10 ° saccades, but varies little between 10 ° and 20 ° saccades. The increase in the SP amplitude between 10 o and 4 0 ° saccades, however, is in contrast to earlier studies which showed that the SP amplitude remained constant for saccades of these sizes (Thickbroom and Mastaglia 1985; Riemslag et al. 1988). It must be observed, however, that Thickbroom and Mastaglia (1985) used a different recording montage when they reported no variation in the SP amplitude with saccade size, a sternoclavicular reference being used and 30 electrodes across the scalp to record the EEG activity. The apparent discrepancy between the present study and the results reported by Thickbroom and Mastaglia (1985) may be accounted for by the different recording techniques. The apparent saturation in SP amplitude with 8-10 o eye movements has been attributed to the supposed origin of the SP in the ocular motoneurones, which increase their firing rate with increasing saccade size to a maximum for saccades of about 10 o (Riemslag et al. 1988). Our results suggest that if we accept this suggested site of
S P I K E P O T E N T I A L S W I T H L A R G E EYE M O V E M E N T S origin of the SP, then a further change in the motoneurone activity must occur with larger saccades. Although information regarding experimental recording of motoneurone activity with large saccades is sparse, a possible explanation m a y be hypothesised from a computer model of the motor unit activity that precedes saccades (Thickbroom and Mastaglia 1987). Within this model the wave form of the action potential to the motor units is assumed to be biphasic, with an initial negative phase followed by a positive phase. By increasing the duration of the second (positive) component of the action potential, the SP amplitude is predicted to increase by up to 120%. It is possible that with the larger 40 o saccades there is an increase in the duration of the second positive component of the action potential resulting in the increase in the SP amplitude that we have recorded. Another factor that must be considered is the dependence of the cortically recorded SP to motivational factors (Ignocheck et al. 1986). If the two SP recordings are related it is conceivable that the increase in the SP amplitude with 40 ° saccades m a y reflect the extra effort required in making saccades of such an unnaturally large size (Bahill et al. 1975). A n y comparisons, however, between these two recordings must be treated with caution until their specific origins are better known. Assuming that the SP we have recorded is related to the ocular motoneurone behaviour, further work is required to confirm the action potential activity that has been suggested above and to show whether the surface recorded SP does indeed represent the motoneurone behaviour.
References Bahill, A.T., Adler, D. and Stark, L. Most naturally occurring h u m a n saccades have magnitudes of 15 degrees or less. Invest. Ophthalmol., 1975, 14: 468-469.
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