Unit
Activity
in Posterior
Association
Cortex
of Cat
R. T. ROBERTSON, K. S. MAYERS, T. J. TEYLER, L. A. BETTINGER, H. BIRCH, J. L. DAVIS, D. S. PHILLIPS, AND R. F. THOMPSON Department of Medical Psychology, University of Oregon Medical School, Portland, Oregon 97201; Department of Psychobiology, Uni-iJersity of California, Iwine, California 92664; and Department of Psychology and Social Relations, Haruard University, Cambridge, Massachusetts 02138
AMASSIAN (3) first described nonspecific cortical association responses in animals anesthetized with chloralose. A number of subsequent studies have been concerned with the organization and distribution of these responses to auditory, visual, and somatic stimulation. Al though varying degrees of modality specificity have been reported for gross evoked potentials in the association response areas (12, 13), it now appears that these regions are polysensory in terms of evoked field potential measures (2, 3, 9, 32, 36). Tfie deg-ree of polysensory convergence c on neurons in the association response areas must, however, be deter tmined at the (3) level of single-unit responses. Amassian was first to note that single neurons displayed polysensory response properties. This was later confirmed by Bental and Bihari (5) and Shimazono et al. (25). These studies, as well as others (6, 15-20), have suggested that although many of the c neurons in these areas are polysensory, one tends to act as a modality (often visual) more potent stimulus than the others. It that the high has been fur ther suggested degree of sensory convergence reported in earlier studies of evoked field potentials (2, 32, 36) may be a function of high levels of chloralose anesthesia (19). Sin ce the associa tion responses recorded cat are from the waking, - unrestrained quite dependent on the particular parameters of stimulus presentation and on the behavioral state of the animal (30, 34), these factors may also be relevant in the situation. ’This study is acu te experiment-al invest igation of the putative an empirical Received
for
publication
June
24, 1974.
stimulus-coding properties of single neurons in posterior association cortex of cat, and is directed toward three goals: 1) to determine the degree of polysensory convergence at the unit level and the relative “potency” of auditory, visual, and somatic stimuli to evoke responses of these cells; 2) to investigate the effects of varying stimulus characteristics on the individual response”s and response interactions of the cells; and 3) to compare response characteristics under chloralose with those observed in the unanesthetized state. METHODS
A total of 71 adult cats was used in this study; 63 animals were anesthetized with chloralose, initial dose of 70 “g/kg, intraperitoneally, with subsequent doses of 15 mg/kg given as needed for the maintenance of anesthetic level. After tracheotomy, the animals were placed in a rigid head holder leaving the eyes and ears free of obstruction. In addition, eight animals were prepared under ether, fixed in an atraumatic head holder (26) and paralyzed with gallamine triethiodide. All wound edges and pressure points were periodically infused with a long-lasting local anesthetic (Zyljectin). At least 2 h intervened between the termination of ether and the start of unit recording. In both preparations, body temperature was regulated to 36-38OC. Skull overlying the posterior middle suprasylvian gyrus (PMSA) was removed and dura reflected, or a small hole was drilled in rthe skull and the dura penetrated with the microelectrode. Cell activity was recorded in the crown of the middle suprasylvian gyrus, generally 2-7 mm anterior to stereotaxic zero, corresponding to area PMSA as defined electrophysiologically by Thompson et al. (32). Cortical depth at which unit activity was encountered was measured by a finely cali-
780
Downloaded from www.physiology.org/journal/jn at Macquarie Univ (137.111.162.020) on February 14, 2019.
CORTICAL
UNIT
brated microdrive and, in some instances, verified histologically. ’ Recording apparatus included glass or Bakelite-coated tungsten or stainless steel microelectrodes with tip diameters of l-3 pm and impedance of 0.5-10 l’W2. Evoked field potentials and unit data were recorded simultaneously from the microelectrode. Using ground as a reference, signals were delivered to a preamplifier with band-pass filter settings of 0% 10,000 Hz. The total activity was then displayed on the upper beam of a dual-beam oscilloscope, simultaneously filtered by an electronic high-pass filter, and reamplified to display the unit activity on the lower beam. Constant comparisons of unit activity and field potentials could be made since the filtering systems did not introduce any significant phase shifts in the evoked responses. Data were photographed on-line or transcribed on magnetic tape. Averages of gross evoked potentials and poststimulus time histograms of unit discharge were obtained from a Fabritek model 1062 computer, either on-line (evoked potentials) or from tape (unit time histograms). The auditory stimulation was provided by a free field click delivered binaurally from a loudspeaker driven by a 0.25-ms pulse. The somatic stimulus was a 0.25-ms pulse delivered through an isolation transformer to two needle electrodes in the pads of the ipsilateral forepaw. The visual stimulus was provided by a single flash to the atropinized eyes from a neon strobe light placed 25 cm in front of the animal. All stimuli were of variable intensity. For part I of the RESULTS, stimulus intensity was not varied. The auditory stimulus was an SOto 85-dB click over a 60-dB background level; visual was the neon strobe light driven by a Grass SD-Z photostimulator set at intensity 8; and somatic was the 0.25-ms pulse at an amplitude of 40 V. These intensities evoked a maximum amplitude field potential in the association response area. When the intensities were varied for parts II and III of RESULTS, the intensities were, in most cases, reduced. In any recording session, the microelectrode was lowered through the cortex in 5+rn steps while the animal was presented with a combination of auditory, visual, and somatic stimuli at intervals of Z-10 s. Because we were primarily interested in the response properties of association-area neurons, it was necessary to define operationally a response. In general agreement with Dubner (17) a neuron was considered responsive to a given stimulus modality if any one of the following criteria were met: I) if spike discharge was
ACTIVITY
781
evoked at a constant latency with a consistent temporal relationship with the gross evoked potential-generally discharge probabilities of 0.30 were accepted, although probabilities as low as 0.10 were considered as responsive if the background activity was very low (less than 0.1/s) and if activity was monitored in at least 30 stimulus presentations; 2) if inhibition of spontaneous cell discharge was consistently observed; or 3) if a stimulus had no observable effect when presented alone, the cell was judged as responsive if, when that stimulus was presented with other stimuli, the discharge to the latter was consistently altered by at least 20(%. Relative potency of the three stimuli for any given cell was of critical importance to this study and was defined by two measures. The first was simply to determine probability of firing of the cell to the stimuli at a given stimulus repetition rate (Z-s intervals) and under the standard stimulus conditions described above. One stimulus was defined as more potent than another if it evoked cell discharge at least 20% more than the other stimulus. The second method used to determine stimulus potency was to analyze the relative interaction of two stimuli paired sequentially, determining the probability of cell discharge to the second stimulus after it had discharged to the first. The probability of cell discharge to the second stimulus varies both with the interval between the two stimuli and with the particular characteristics of each stimulus. We define the stimulus that results in the longest blocking period as being most potent. On the other hand, by giving a standard first stimulus and varying the second, we define a more potent second stimulus as that one which will discharge the cell with a shorter interval than another stimulus. It should be pointed out that potency as operationally defined by these methods may not be a unitary construct; that is, two different aspects of stimulus-response interactions may be measured by these two methods. RESULTS
I. General characteristics of unit response in chloraloseanesthetized preparation A. total of approximately 700 individual neurons were examined in the various aspects of this experiment. Figure 1 presents data on the cortical depth distribution of 339 of these cells selected for stability and absence of injury in the chloralose-anesthetized p reparation Figure 1 also shows the mean response tencies to
Downloaded from www.physiology.org/journal/jn at Macquarie Univ (137.111.162.020) on February 14, 2019.
782
ROBERTSON
--
1
?‘I
?
1
Median t response latency (m set)
J
2400-1 YO population units
P
‘I& trimodal units
FIG. 1. Depth distribution of units from chloralose-anesthetized preparation in percent total sample. Percent trimodall y responsive median trimodal response latency as a function depth.
the of and of
a combined auditory, visual, and somatic stimulus and percent trimodally responsive cells for each depth category. Statistical analysis did not suggest any specific cortical layering of these units (~2 = 3.27, df = 3). Twenty-five cells from each depth group were randomly selected and subjected to further analyses. These data indicate no significant differences in latency (F = 2.24, response characterdf = 3,96) or trimodal istics (x2 = 0.52, df = 3) with depth. As shown in Fig. 2, 82% of all neurons investigated responded to all three modalities of stimulation. Of the remaining IS%, 14.8’:1, responded to two modalities, leav-
ET
AL.
ing only 3.201, of the cells unimodally responsive. Visual stimulation influenced more cells (99.2%) than auditory (94.87,) or somatic stimulation (84.8%). No cells were found unimodally responsive to somatic stimulation. Most cells discharged with a short-latency response, though others tended to discharge later (75-100 ms), perhaps corresponding to the “secondary” response which is characteristic of some cortical evoked potentials (8, 21, 35). In addition, some cells were observed to respond with two bursts, corresponding in time to each of the field potentials. The late discharge of these cells is not due to artifact of movement in response to the stimulus as these effects were &also observed in the paralyzed preparation. Figure 3 presents histograms of latency to first discharge for all cells to auditory, visual, and somatic stimuli. As can be seen, responses to visual stimulation tended to occur with shorter latencies (mode, 30 ms; median, 35 ms) than auditory (mode, 30 ms; median, 40 ms) or somatic (mode, 40 ms; median, 60 ms). Modal pattern of response to each modality of stimulation was a single spike, although multiple discharges were also ob30
Auditory
20 10
.-!
0
0 -3
Visual
AVS / Lb
AV L--
AS ._v_-
--J vs
-.-A
v -.-/-___J
S n ”
Trimodal
Bimodal
Unimodal
Fro. 2. Percents of total unit population responsive to different stimulus modalities or combinations of modali ties. Chloralose-anesthetized preparation. Categories are exclusive. In this and subsequent figures and tables: A, auditory stimulation; V, visual stimulation; S, somatic stimulation.
2030
30-u 40
4050
5060
Response
6070
7080
latency
8090
90lOO- 110100 110 120
( msec
120+
)
Fro. 3. Percents of total population of units in chloralose-anesthetized. preparation exhibiting responses at different latencies following unimodal auditory, visual, or somatic stimulation.
Downloaded from www.physiology.org/journal/jn at Macquarie Univ (137.111.162.020) on February 14, 2019.
CORTICAL
UNIT
Soma tic 76.2%
Auditory 30.8%
-Visual 53%
FIG. 4. Percents of trimodal cells in the chloralose-anesthetized preparation showing relative potency effects. A: all three modalities equally potent; B: two modalities of equal potency: C: one modality most potent. In C are shown the percent of unimodal cells responding to each stimulus modality.
served. Mean number of evoked spikes was 1.6 for auditory, 1.9 for visual, and 1.7 for somatic stimulation. Probability of unit discharge in response to a series of stimulus presentations sometimes varied with the stimulus modality as shown in Fig. 4. Of those units determined to be trimodal, almost half (46y0) did not display significant differences in probto different ability of response (cf. METHODS) stimulus modalities; 18% responded less to one stimulus modalitv than to the other two, and 36y0 had one “best stimulus.” Of those cells which responded with a significantly greater probability to one of the three stimuli, 53.0% responded best to visual stimulation, 30.8% to auditory stimulation, and only 16.2y0 to somatic stimulation. Table lA -presents data for stimulus modality potency comparisons for all trimodal cells. Probability of discharge of units varied greatly from animal to animal depending on the excitability of the preparation, and also between cells in any one preparation. 1. Stimulus modality poten.cy chloralose-anesthetized preparation
TABLE
in
Stimulus-Potency
Relationship
A.
Serial presentation yO of population
B.
Sequential presentation u]O of population
A > V
ACTIVITY
783
Occasionally, responsivity varied over time for any one cell, even to the extent of showing no evoked discharge to one modality for several minutes. In-addition to the response produced by the standard brief, discrete, stimuli, it was noted late in the data collection process that if the stimulus remained on for a period of more than about 500 ms, some cells (12 of 19 tested) also discharged to stimulus offset. This effect was particularly true for visual stimulation, however, due in part to difficulty achieving a rapid offset without introducing other stimulus qualities, the effect was less clear for auditory and somatic stimulation. No cells were observed to discharge tonically during the time the stimulus was being presented. Inhibition of spontaneous activity by all modalities of stimulation was observed for two cells. Length of inhibition of spontaneous discharge ranged from 175 to 450 ms, with no obvious differences between stimuli. Only 2 cells out of a population of 339 is, of course, a very small number, but this is probably due to the difficulty in observing the effects of inhibition when the spontaneous discharge rates were so low (mean, 0.6/s) rather than lack of any inhibitory influences. Suppression of discharges evoked by a different modality of stimulation was also observed and will be discussed below (section 11). To assess the relationship between unitary discharge and evoked lfield potential, simultaneous multiple-unit recordings and local evoked potentials were obtained by high-pass (600-10 kHz) and low-pass (0.8600 Hz) filtering of the activity recorded by a single, large microelectrode (tip diameter 2-3 pm). Spikes exceeding 30 PV were included in the multiple-unit responses which typically consisted of 4-10 units. Figure 5 illustrates in the relationship between the poststimulus time histogram for
comparisons
for v>s
trimodally
A = V
A < V
v=s
46.7
37.8
56.2
36.5
35.7
45.5
36.4
vS
A=S
39.1
47.1
13.8
45.5
36.4
18.0
A
Activity
in Posterior
Association
Cortex
of Cat
R. T. ROBERTSON, K. S. MAYERS, T. J. TEYLER, L. A. BETTINGER, H. BIRCH, J. L. DAVIS, D. S. PHILLIPS, AND R. F. THOMPSON Department of Medical Psychology, University of Oregon Medical School, Portland, Oregon 97201; Department of Psychobiology, Uni-iJersity of California, Iwine, California 92664; and Department of Psychology and Social Relations, Haruard University, Cambridge, Massachusetts 02138
AMASSIAN (3) first described nonspecific cortical association responses in animals anesthetized with chloralose. A number of subsequent studies have been concerned with the organization and distribution of these responses to auditory, visual, and somatic stimulation. Al though varying degrees of modality specificity have been reported for gross evoked potentials in the association response areas (12, 13), it now appears that these regions are polysensory in terms of evoked field potential measures (2, 3, 9, 32, 36). Tfie deg-ree of polysensory convergence c on neurons in the association response areas must, however, be deter tmined at the (3) level of single-unit responses. Amassian was first to note that single neurons displayed polysensory response properties. This was later confirmed by Bental and Bihari (5) and Shimazono et al. (25). These studies, as well as others (6, 15-20), have suggested that although many of the c neurons in these areas are polysensory, one tends to act as a modality (often visual) more potent stimulus than the others. It that the high has been fur ther suggested degree of sensory convergence reported in earlier studies of evoked field potentials (2, 32, 36) may be a function of high levels of chloralose anesthesia (19). Sin ce the associa tion responses recorded cat are from the waking, - unrestrained quite dependent on the particular parameters of stimulus presentation and on the behavioral state of the animal (30, 34), these factors may also be relevant in the situation. ’This study is acu te experiment-al invest igation of the putative an empirical Received
for
publication
June
24, 1974.
stimulus-coding properties of single neurons in posterior association cortex of cat, and is directed toward three goals: 1) to determine the degree of polysensory convergence at the unit level and the relative “potency” of auditory, visual, and somatic stimuli to evoke responses of these cells; 2) to investigate the effects of varying stimulus characteristics on the individual response”s and response interactions of the cells; and 3) to compare response characteristics under chloralose with those observed in the unanesthetized state. METHODS
A total of 71 adult cats was used in this study; 63 animals were anesthetized with chloralose, initial dose of 70 “g/kg, intraperitoneally, with subsequent doses of 15 mg/kg given as needed for the maintenance of anesthetic level. After tracheotomy, the animals were placed in a rigid head holder leaving the eyes and ears free of obstruction. In addition, eight animals were prepared under ether, fixed in an atraumatic head holder (26) and paralyzed with gallamine triethiodide. All wound edges and pressure points were periodically infused with a long-lasting local anesthetic (Zyljectin). At least 2 h intervened between the termination of ether and the start of unit recording. In both preparations, body temperature was regulated to 36-38OC. Skull overlying the posterior middle suprasylvian gyrus (PMSA) was removed and dura reflected, or a small hole was drilled in rthe skull and the dura penetrated with the microelectrode. Cell activity was recorded in the crown of the middle suprasylvian gyrus, generally 2-7 mm anterior to stereotaxic zero, corresponding to area PMSA as defined electrophysiologically by Thompson et al. (32). Cortical depth at which unit activity was encountered was measured by a finely cali-
780
Downloaded from www.physiology.org/journal/jn at Macquarie Univ (137.111.162.020) on February 14, 2019.
CORTICAL
UNIT
brated microdrive and, in some instances, verified histologically. ’ Recording apparatus included glass or Bakelite-coated tungsten or stainless steel microelectrodes with tip diameters of l-3 pm and impedance of 0.5-10 l’W2. Evoked field potentials and unit data were recorded simultaneously from the microelectrode. Using ground as a reference, signals were delivered to a preamplifier with band-pass filter settings of 0% 10,000 Hz. The total activity was then displayed on the upper beam of a dual-beam oscilloscope, simultaneously filtered by an electronic high-pass filter, and reamplified to display the unit activity on the lower beam. Constant comparisons of unit activity and field potentials could be made since the filtering systems did not introduce any significant phase shifts in the evoked responses. Data were photographed on-line or transcribed on magnetic tape. Averages of gross evoked potentials and poststimulus time histograms of unit discharge were obtained from a Fabritek model 1062 computer, either on-line (evoked potentials) or from tape (unit time histograms). The auditory stimulation was provided by a free field click delivered binaurally from a loudspeaker driven by a 0.25-ms pulse. The somatic stimulus was a 0.25-ms pulse delivered through an isolation transformer to two needle electrodes in the pads of the ipsilateral forepaw. The visual stimulus was provided by a single flash to the atropinized eyes from a neon strobe light placed 25 cm in front of the animal. All stimuli were of variable intensity. For part I of the RESULTS, stimulus intensity was not varied. The auditory stimulus was an SOto 85-dB click over a 60-dB background level; visual was the neon strobe light driven by a Grass SD-Z photostimulator set at intensity 8; and somatic was the 0.25-ms pulse at an amplitude of 40 V. These intensities evoked a maximum amplitude field potential in the association response area. When the intensities were varied for parts II and III of RESULTS, the intensities were, in most cases, reduced. In any recording session, the microelectrode was lowered through the cortex in 5+rn steps while the animal was presented with a combination of auditory, visual, and somatic stimuli at intervals of Z-10 s. Because we were primarily interested in the response properties of association-area neurons, it was necessary to define operationally a response. In general agreement with Dubner (17) a neuron was considered responsive to a given stimulus modality if any one of the following criteria were met: I) if spike discharge was
ACTIVITY
781
evoked at a constant latency with a consistent temporal relationship with the gross evoked potential-generally discharge probabilities of 0.30 were accepted, although probabilities as low as 0.10 were considered as responsive if the background activity was very low (less than 0.1/s) and if activity was monitored in at least 30 stimulus presentations; 2) if inhibition of spontaneous cell discharge was consistently observed; or 3) if a stimulus had no observable effect when presented alone, the cell was judged as responsive if, when that stimulus was presented with other stimuli, the discharge to the latter was consistently altered by at least 20(%. Relative potency of the three stimuli for any given cell was of critical importance to this study and was defined by two measures. The first was simply to determine probability of firing of the cell to the stimuli at a given stimulus repetition rate (Z-s intervals) and under the standard stimulus conditions described above. One stimulus was defined as more potent than another if it evoked cell discharge at least 20% more than the other stimulus. The second method used to determine stimulus potency was to analyze the relative interaction of two stimuli paired sequentially, determining the probability of cell discharge to the second stimulus after it had discharged to the first. The probability of cell discharge to the second stimulus varies both with the interval between the two stimuli and with the particular characteristics of each stimulus. We define the stimulus that results in the longest blocking period as being most potent. On the other hand, by giving a standard first stimulus and varying the second, we define a more potent second stimulus as that one which will discharge the cell with a shorter interval than another stimulus. It should be pointed out that potency as operationally defined by these methods may not be a unitary construct; that is, two different aspects of stimulus-response interactions may be measured by these two methods. RESULTS
I. General characteristics of unit response in chloraloseanesthetized preparation A. total of approximately 700 individual neurons were examined in the various aspects of this experiment. Figure 1 presents data on the cortical depth distribution of 339 of these cells selected for stability and absence of injury in the chloralose-anesthetized p reparation Figure 1 also shows the mean response tencies to
Downloaded from www.physiology.org/journal/jn at Macquarie Univ (137.111.162.020) on February 14, 2019.
782
ROBERTSON
--
1
?‘I
?
1
Median t response latency (m set)
J
2400-1 YO population units
P
‘I& trimodal units
FIG. 1. Depth distribution of units from chloralose-anesthetized preparation in percent total sample. Percent trimodall y responsive median trimodal response latency as a function depth.
the of and of
a combined auditory, visual, and somatic stimulus and percent trimodally responsive cells for each depth category. Statistical analysis did not suggest any specific cortical layering of these units (~2 = 3.27, df = 3). Twenty-five cells from each depth group were randomly selected and subjected to further analyses. These data indicate no significant differences in latency (F = 2.24, response characterdf = 3,96) or trimodal istics (x2 = 0.52, df = 3) with depth. As shown in Fig. 2, 82% of all neurons investigated responded to all three modalities of stimulation. Of the remaining IS%, 14.8’:1, responded to two modalities, leav-
ET
AL.
ing only 3.201, of the cells unimodally responsive. Visual stimulation influenced more cells (99.2%) than auditory (94.87,) or somatic stimulation (84.8%). No cells were found unimodally responsive to somatic stimulation. Most cells discharged with a short-latency response, though others tended to discharge later (75-100 ms), perhaps corresponding to the “secondary” response which is characteristic of some cortical evoked potentials (8, 21, 35). In addition, some cells were observed to respond with two bursts, corresponding in time to each of the field potentials. The late discharge of these cells is not due to artifact of movement in response to the stimulus as these effects were &also observed in the paralyzed preparation. Figure 3 presents histograms of latency to first discharge for all cells to auditory, visual, and somatic stimuli. As can be seen, responses to visual stimulation tended to occur with shorter latencies (mode, 30 ms; median, 35 ms) than auditory (mode, 30 ms; median, 40 ms) or somatic (mode, 40 ms; median, 60 ms). Modal pattern of response to each modality of stimulation was a single spike, although multiple discharges were also ob30
Auditory
20 10
.-!
0
0 -3
Visual
AVS / Lb
AV L--
AS ._v_-
--J vs
-.-A
v -.-/-___J
S n ”
Trimodal
Bimodal
Unimodal
Fro. 2. Percents of total unit population responsive to different stimulus modalities or combinations of modali ties. Chloralose-anesthetized preparation. Categories are exclusive. In this and subsequent figures and tables: A, auditory stimulation; V, visual stimulation; S, somatic stimulation.
2030
30-u 40
4050
5060
Response
6070
7080
latency
8090
90lOO- 110100 110 120
( msec
120+
)
Fro. 3. Percents of total population of units in chloralose-anesthetized. preparation exhibiting responses at different latencies following unimodal auditory, visual, or somatic stimulation.
Downloaded from www.physiology.org/journal/jn at Macquarie Univ (137.111.162.020) on February 14, 2019.
CORTICAL
UNIT
Soma tic 76.2%
Auditory 30.8%
-Visual 53%
FIG. 4. Percents of trimodal cells in the chloralose-anesthetized preparation showing relative potency effects. A: all three modalities equally potent; B: two modalities of equal potency: C: one modality most potent. In C are shown the percent of unimodal cells responding to each stimulus modality.
served. Mean number of evoked spikes was 1.6 for auditory, 1.9 for visual, and 1.7 for somatic stimulation. Probability of unit discharge in response to a series of stimulus presentations sometimes varied with the stimulus modality as shown in Fig. 4. Of those units determined to be trimodal, almost half (46y0) did not display significant differences in probto different ability of response (cf. METHODS) stimulus modalities; 18% responded less to one stimulus modalitv than to the other two, and 36y0 had one “best stimulus.” Of those cells which responded with a significantly greater probability to one of the three stimuli, 53.0% responded best to visual stimulation, 30.8% to auditory stimulation, and only 16.2y0 to somatic stimulation. Table lA -presents data for stimulus modality potency comparisons for all trimodal cells. Probability of discharge of units varied greatly from animal to animal depending on the excitability of the preparation, and also between cells in any one preparation. 1. Stimulus modality poten.cy chloralose-anesthetized preparation
TABLE
in
Stimulus-Potency
Relationship
A.
Serial presentation yO of population
B.
Sequential presentation u]O of population
A > V
ACTIVITY
783
Occasionally, responsivity varied over time for any one cell, even to the extent of showing no evoked discharge to one modality for several minutes. In-addition to the response produced by the standard brief, discrete, stimuli, it was noted late in the data collection process that if the stimulus remained on for a period of more than about 500 ms, some cells (12 of 19 tested) also discharged to stimulus offset. This effect was particularly true for visual stimulation, however, due in part to difficulty achieving a rapid offset without introducing other stimulus qualities, the effect was less clear for auditory and somatic stimulation. No cells were observed to discharge tonically during the time the stimulus was being presented. Inhibition of spontaneous activity by all modalities of stimulation was observed for two cells. Length of inhibition of spontaneous discharge ranged from 175 to 450 ms, with no obvious differences between stimuli. Only 2 cells out of a population of 339 is, of course, a very small number, but this is probably due to the difficulty in observing the effects of inhibition when the spontaneous discharge rates were so low (mean, 0.6/s) rather than lack of any inhibitory influences. Suppression of discharges evoked by a different modality of stimulation was also observed and will be discussed below (section 11). To assess the relationship between unitary discharge and evoked lfield potential, simultaneous multiple-unit recordings and local evoked potentials were obtained by high-pass (600-10 kHz) and low-pass (0.8600 Hz) filtering of the activity recorded by a single, large microelectrode (tip diameter 2-3 pm). Spikes exceeding 30 PV were included in the multiple-unit responses which typically consisted of 4-10 units. Figure 5 illustrates in the relationship between the poststimulus time histogram for
comparisons
for v>s
trimodally
A = V
A < V
v=s
46.7
37.8
56.2
36.5
35.7
45.5
36.4
vS
A=S
39.1
47.1
13.8
45.5
36.4
18.0
A
