JOURNAL OF NEUROPHYSIOLOGY Vol. 39, No. 5, September 1976. Printed
in U.S.A.
Activity and Excitability to Electrical Current of Cortical Auditory Receptive Neurons of Awake Cats as Affected by Stimulus Association C. D. WOODY, J. D. KNISPEL, P. A. BLACK-CLEWORTH
T. J. CROW,
AND
Brain Research Institute, Departments of Anatomy and Psychiatry, UCLA Center, and Laboratory of Neurophysiology, Mental Retardation Research University of California, Los Angeles, Los Angeles, California 90024
Medical Center,
AND CONCLUSIONS 4. The mechanism mediating the enhanced exI. Unit activity and excitability were studied citability to extracellular current injection in the at the midlateral and suprasylvian cortex of CS-responsive units in the blink-conditioned cats is thought to be pre- or intrasynaptic as naive, blink-conditioned and “randomization” cats. The latter received the same CS and US as opposed to postsynaptic since no differences in did the conditioned animals, but in random tem- excitability between these and other cells were poral order and with random intertrial intervals found in further studies employing intracellular with mean comparable to that used for condi- current injection. Postsynaptic changes in unit tioning. The randomization group failed to de- excitability have been found previously at more rostral, cortical motor areas after conditioning. velop a blink CR. 2. With conditioning, spontaneous and They are thought to subserve performance of the evoked unit discharges were increased above specific conditioned blink movement, whereas the present changes at sensory association corlevels found in naive animals. Correspondingly, levels of extracellularly injected current required tex are thought to subserve the specificity of the to elicit a spike discharge were lower in con- CS in eliciting the conditioned response. Short-latency conditioned blinking resulting ditioned than in naive animals. In addition, in the from pairing click CS with glabella tap US in the conditioned animals, the degree of enhancement of evoked activity and excitability was found to cat is characterized by I) the performance of a be greatest in the units that responded to the CS, specific conditioned motor response, an eye blink as opposed to some other movement, and as opposed to units that responded to another 2) the selective ability of a stimulus of conditionauditory stimulus of equal intensity but of no special behavioral significance vis-a-vis the con- ing significance, the CS, to elicit the CR (41, 43, 45). Altered excitability to electrical current inditioned reflex. 3. In the randomization group, spontaneous jection has been found after conditioning, in rosand evoked unit activity and the excitability of tral cortical areas that project ultimately to the units to extracellularly injected current were target muscle of the CR. It is thought that inalso increased with respect to the levels found in creased excitability in neurons projecting to naive animals, but evoked activity and unit ex- target musculature occurs postsynaptically and citability were not increased to the degree found facilitates performance of the specific, learned motor response. Details supporting this view in the CS-responsive units in the conditioned animals. Other, transient changes in rates of have been presented elsewhere (39). The present study explores the possibility that spontaneous unit discharge were found to accompany serial presentation of auditory stimuli c.hanges in excitability and unit activity might in all animals. The rates of discharge increased also occur at more caudal, association, or senin randomization animals and decreased in naive sory receptive regions of the cortex and contriband conditioned animals. The observations indi- ute to the selective ability of the CS to evoke the c.ate that, despite lack of CR development, the conditioned response. There is reason to believe randomization procedure is by no means a neu- that auditory receptive and association areas of cortex are involved in mediating discrimination tral control for conditioning. learning (5, 6), although not exclusively (21, 25). Receivedfor publication October 1, 1975. The selectivity of the CS in eliciting the CR may SUMMARY
1045
Downloaded from www.physiology.org/journal/jn by ${individualUser.givenNames} ${individualUser.surname} (129.186.138.035) on January 13, 2019
1046
WOODY,
KNISPEL,
CROW, AND
BLACK-CLEWORTH
be viewed in the context of discrimination learning or, equivalently, in terms of whatever cellular changes in the CNS would enable the same physical stimulus which fails to elicit a consistent motor response before c.onditioning to do so, selectively, afterward. This change in neural fun&ion could only derive from the phenomenon t.of stimulus association, that is, the order, intert*+ val, frequency, and duration of presenting stimuli such as CS and US. Previous attempts to find neural correlates of behavior at sensory regions of the cortex have sometimes been less than satisfactory because I) Click Tap I Hiss t-+ 200 msec the preparations or anatomical systems studied Training inter-trial interval = 10 set Testing inter-trial interval = 5 set were too complex, 2) neural activity was asFIG. 1. Schemata of training and testing paradigms sessed too indirectly, or3) the behavior was too complicated to be well controlled. We have for conditioned (C), randomization (R), and naive (N) animals. The conditioning training paradigm employs a minimized the first two problems by studying fixed intertrial interval (ITI) of 10 s, the randomization isolated unit activity in relation to short-latency training paradigm, an IT1 averaging 10 s. During testconditioned blinking. Here, the CR ordinarily ing, ITIs were reduced to 5 s. The click continued to be follows the CS by as little as 20 ms (41). reinforced or associated in C and R animals. Separate, The third problem is far more complicated unpaired hisses were presented during testing in all than might be thought. Part of the complexity animal groups as a control or reference stimulus. derives from previously unrecognized “cellular” consequences of the stimulus associations val (ITI) was 10 s and 150 stimulus associations themselves. Some behavioral consequences have, were presented in daily training sessions reof course, been previously recognized in peated 5 times/wk. The animals achieved better the context of sensory preconditioning or than 70% blink performance to the click-CS over lo-21 training sessions with the exception of two second-order conditioning; however, commonly, effects of stimulus association on brain cats. One reached 60% performance a.fter 16 and behavior have been defined in terms of the sessions; the other reached 35% performance level after 16 training sessions, but better than development or not of a peripheral, conditioned motor response (14). As this study will demon- 50% performance when tested (45). These animals constituted the conditioned group. strate, this fails to take into consideration Testing was performed by presenting a series changes in cellular function that do not lead directly to immediate peripheral motor conse- of clicks associated with taps, as shown in Fig. 1. In addition, a series of behaviorally neutral quences. Such changes could be of considerable significance to conditioning, particularly to me- hisses (no previous association) of comparable diation of the selective ability of the CS to loudness to the click was presented separately. The physical c.haracteristics of the click and the elicit the CR. hiss are shown later in Fig. 5. The click was METHODS delivered through a loudspeaker placed 3-5 ft Unit activity was recorded from the midlateral from the ears of the animal. The hiss was proand suprasylvian cortex of 17 cats. Unit re- duced by air (vacuum pulse) passing through an sponses to auditory stimuli and the amount of orifice l-2 ft away from the animal. The interinjected current required to initiate a spike dis- trial interval during testing was 5 s. The animals c.harge were studied a) in naive cats, h) in cats showed blink CRs to clicks but not to the hisses. As reported previously, for the type of conconditioned to blink by pairing click with glabella tap, and c) in cats subjected to pairings ditioned blinking resulting from pairing click-CS with glabella tap US, the specificity of the CS in of the same stimuli but in a random temporal order in which conditioned blink responses eliciting the CR develops regardless of whether there was prior discrimination training with the failed to develop. The training and testing procedures are dia- “neutral” or “indifferent” stimulus (cf. Fig. 6 of ref 4 1 and Fig. 3 of ref 45). grammed in Fig. 1.
Conditioned
group (C)
Seven cats were trained by repeatedly pairing 70-dB click (Cs) with glabella tap (US). The CS- US interval was 400 ms. The intertrial inter-
Randomization
group (R)
Three cats were subjected to click and tap, paired in random temporal order (cf. ref 27) in 21 training sessions otherwise similar in frequency
Downloaded from www.physiology.org/journal/jn by ${individualUser.givenNames} ${individualUser.surname} (129.186.138.035) on January 13, 2019
NEURAL
EFFECTS
OF
STIMULUS
and duration to the training sessions for the conditioned group. The randomization “training” procedure was designed in such a way that the mean interval between stimulus repetition was 10 S. These animals failed to develop a conditioned blink response to the click (cf. ref 45). During testing, a series of randomly paired clicks and taps was presented with mean IT1 of 5. s. A series of neutral hisses was presented separately, as shown in Fig. 1. Otherwise stimulus variables were comparable to those in the conditioned group. Naive group (N) Seven cats were not subjected to any initial training or stimulus association. During testing, series of clicks (no taps) and series of hisses were presented separately (see Fig. 1). Otherwise, stimulus variables during testing were c.omparable to those of the other two groups. The purpose of this design was to allow comparison of effects of three different click series and three similar hiss series in the three experimental groups. The hiss was always of neutral behavioral significance (no prior associations during training period). In the C group, the click was of behavioral significance (a CS). In the R group, the click was of “associational” significance (as a randomly associated stimulus). And in the N group, the click was of neutral behavioral significance. 1 Unit activity was recorded in awake, unanesthetized, unparalyzed cats. The animals were previously prepared, aseptically, so that microelectrodes and a stainless steel guide tube could be inserted through a 1.5-mm bur hole in the skull, into the auditory association cortex. The bodies of the animals were loosely restrained in cloth sleeves. The heads were held in a fixed position by metal screws, previously implanted in the skull under pentobarbital anesthesia. Another ljermanent screw in the glabella allowed attachment of an electromagnetic tapper used for delivery of the US. Additional braces were placed between the animal’s teeth and the floor of the frame supporting the head screws and the hydraulic microdrive assembly. Animals tolerate this procedure well, occasionally having to be kept from falling asleep during experiments. I l The terminologies neutral behavioral significance
ASSOCIATION
1047
The regions of cortex penetrated were established by direct postmortem examination of the brains, with the exception of two cats that are being used in other experiments. A composite picture is shown in Fig. 2. The area studied generally corresponds to association and secondary visual cortex (ASSOC and V II) described by Woolsey (Fig. 128 of ref 46) and is said to be distinct from auditory receptive cortex receiving direct projections from the medial geniculate body (36). Comparable areas were penetrated bilaterally and in the different experimental groups. No differences in the data depending on laterality were distinguished, and results from both sides were grouped together in our analyses. The angle of insertion of the guide tube and microelectrode was within t 30” perpendicular to the surface of the cortex. At the end of each session the electrode and guide tube were removed and the hole closed with bone wax. Penicillin was administered intramuscularly as needed. Microelectrodes of lo- to 50-M fl resistance were pulled from l-mm OD borosilicate capillaries and filled with 1.5 M K + citrate. The tips of some of the electrodes in the lower resistance range were bumped. All electrodes were led through Pt-lr wire to a Mentor, lOl*-a input resistance amplifier and thence to an FM tape recorder. One of the four chronically implanted screws served as reference electrode. Upper band pass of data recorded through the FM tape system was 2,500 Hz. Higher band pass (up to 20 kHz) was obtained on an on-line oscilloscope display. Low-frequency resolution with the polarizable electrode system was adequate for measuring abrupt base-line shifts seen on penetrating cells (see Fig. 4).
and apciational significance are used for lack of more adequate terminologies to describe still obscure con-
sequencesof various patterns of stimulus association. As indicated by our experimental results, these matters are more complicated than has generally been recognized. For example, once a neutral stimulus is presented during testing, it is no longer neutral, and its presentation can have cellular consequences, such as changes in spontaneous unit activity.
FIG. 2. Shaded area is a composite of cortical regions studied in these experiments. Midlateral and suprasylvian gyri and a small portion of ectosylvian gyrus are included.
Downloaded from www.physiology.org/journal/jn by ${individualUser.givenNames} ${individualUser.surname} (129.186.138.035) on January 13, 2019
1048
WOODY,
KNISPEL,
CROW,
Methodology concerning delivery of electricalstimuli through the microelectrode and criteriafor subsequent classification of studied units as “intracellular” or “extracellular” will now be considered. Cells were classified as intracellular if the size of recorded action potential (AP) was > 20 mV We cannot exclude the possibility that some of these recordings may have been quasiintracellular, but the following considerations support the likelihood that most were intracellular and differed from the extracellular recordings: 1) Abrupt negative shifts in base-line potential of equivalent or greater magnitude normally accompanied the appearance of 220 mV action potentials, which were larger than any APs seen immediately preceding the base-line shift. 2) The size and frequency of action potentials of this magnitude can usually be altered appropriately by depolarizing and hyperpolarizing currents (40, 42). 3) As will be shown later, the latency of spike initiation by an intracellularly injected current pulse was significantly shorter on the average for such neurons than that by extracellular current for neurons of smaller AP size that were classified as extracellular. 4) Many properties of cells with 220 mV AP correspond to the range measured by others in units with APs of 70-l 10 mV, including membrane resistance and the IR drop required to initiate an action potential (40). The small size of the action potential could either reflect damage to the neuron or other factors such as recording in a dendritic process with APs initiated remotely from the recording electrode. Whatever damage was present, including progressive decrease with time in the size of the intracellularly recorded action potential, was not enough to appreciably affect the neural response to natural stimuli. The PST histogram responses to click and hiss of these and comparable units were generally similar to those of extracellularly recorded #units (see Fig. 8; also ref 28 and 40). Cells were classified as extracellular if the size of recorded action potential and base-line variation was ~6 mV or if the action potential was
in U.S.A.
Activity and Excitability to Electrical Current of Cortical Auditory Receptive Neurons of Awake Cats as Affected by Stimulus Association C. D. WOODY, J. D. KNISPEL, P. A. BLACK-CLEWORTH
T. J. CROW,
AND
Brain Research Institute, Departments of Anatomy and Psychiatry, UCLA Center, and Laboratory of Neurophysiology, Mental Retardation Research University of California, Los Angeles, Los Angeles, California 90024
Medical Center,
AND CONCLUSIONS 4. The mechanism mediating the enhanced exI. Unit activity and excitability were studied citability to extracellular current injection in the at the midlateral and suprasylvian cortex of CS-responsive units in the blink-conditioned cats is thought to be pre- or intrasynaptic as naive, blink-conditioned and “randomization” cats. The latter received the same CS and US as opposed to postsynaptic since no differences in did the conditioned animals, but in random tem- excitability between these and other cells were poral order and with random intertrial intervals found in further studies employing intracellular with mean comparable to that used for condi- current injection. Postsynaptic changes in unit tioning. The randomization group failed to de- excitability have been found previously at more rostral, cortical motor areas after conditioning. velop a blink CR. 2. With conditioning, spontaneous and They are thought to subserve performance of the evoked unit discharges were increased above specific conditioned blink movement, whereas the present changes at sensory association corlevels found in naive animals. Correspondingly, levels of extracellularly injected current required tex are thought to subserve the specificity of the to elicit a spike discharge were lower in con- CS in eliciting the conditioned response. Short-latency conditioned blinking resulting ditioned than in naive animals. In addition, in the from pairing click CS with glabella tap US in the conditioned animals, the degree of enhancement of evoked activity and excitability was found to cat is characterized by I) the performance of a be greatest in the units that responded to the CS, specific conditioned motor response, an eye blink as opposed to some other movement, and as opposed to units that responded to another 2) the selective ability of a stimulus of conditionauditory stimulus of equal intensity but of no special behavioral significance vis-a-vis the con- ing significance, the CS, to elicit the CR (41, 43, 45). Altered excitability to electrical current inditioned reflex. 3. In the randomization group, spontaneous jection has been found after conditioning, in rosand evoked unit activity and the excitability of tral cortical areas that project ultimately to the units to extracellularly injected current were target muscle of the CR. It is thought that inalso increased with respect to the levels found in creased excitability in neurons projecting to naive animals, but evoked activity and unit ex- target musculature occurs postsynaptically and citability were not increased to the degree found facilitates performance of the specific, learned motor response. Details supporting this view in the CS-responsive units in the conditioned animals. Other, transient changes in rates of have been presented elsewhere (39). The present study explores the possibility that spontaneous unit discharge were found to accompany serial presentation of auditory stimuli c.hanges in excitability and unit activity might in all animals. The rates of discharge increased also occur at more caudal, association, or senin randomization animals and decreased in naive sory receptive regions of the cortex and contriband conditioned animals. The observations indi- ute to the selective ability of the CS to evoke the c.ate that, despite lack of CR development, the conditioned response. There is reason to believe randomization procedure is by no means a neu- that auditory receptive and association areas of cortex are involved in mediating discrimination tral control for conditioning. learning (5, 6), although not exclusively (21, 25). Receivedfor publication October 1, 1975. The selectivity of the CS in eliciting the CR may SUMMARY
1045
Downloaded from www.physiology.org/journal/jn by ${individualUser.givenNames} ${individualUser.surname} (129.186.138.035) on January 13, 2019
1046
WOODY,
KNISPEL,
CROW, AND
BLACK-CLEWORTH
be viewed in the context of discrimination learning or, equivalently, in terms of whatever cellular changes in the CNS would enable the same physical stimulus which fails to elicit a consistent motor response before c.onditioning to do so, selectively, afterward. This change in neural fun&ion could only derive from the phenomenon t.of stimulus association, that is, the order, intert*+ val, frequency, and duration of presenting stimuli such as CS and US. Previous attempts to find neural correlates of behavior at sensory regions of the cortex have sometimes been less than satisfactory because I) Click Tap I Hiss t-+ 200 msec the preparations or anatomical systems studied Training inter-trial interval = 10 set Testing inter-trial interval = 5 set were too complex, 2) neural activity was asFIG. 1. Schemata of training and testing paradigms sessed too indirectly, or3) the behavior was too complicated to be well controlled. We have for conditioned (C), randomization (R), and naive (N) animals. The conditioning training paradigm employs a minimized the first two problems by studying fixed intertrial interval (ITI) of 10 s, the randomization isolated unit activity in relation to short-latency training paradigm, an IT1 averaging 10 s. During testconditioned blinking. Here, the CR ordinarily ing, ITIs were reduced to 5 s. The click continued to be follows the CS by as little as 20 ms (41). reinforced or associated in C and R animals. Separate, The third problem is far more complicated unpaired hisses were presented during testing in all than might be thought. Part of the complexity animal groups as a control or reference stimulus. derives from previously unrecognized “cellular” consequences of the stimulus associations val (ITI) was 10 s and 150 stimulus associations themselves. Some behavioral consequences have, were presented in daily training sessions reof course, been previously recognized in peated 5 times/wk. The animals achieved better the context of sensory preconditioning or than 70% blink performance to the click-CS over lo-21 training sessions with the exception of two second-order conditioning; however, commonly, effects of stimulus association on brain cats. One reached 60% performance a.fter 16 and behavior have been defined in terms of the sessions; the other reached 35% performance level after 16 training sessions, but better than development or not of a peripheral, conditioned motor response (14). As this study will demon- 50% performance when tested (45). These animals constituted the conditioned group. strate, this fails to take into consideration Testing was performed by presenting a series changes in cellular function that do not lead directly to immediate peripheral motor conse- of clicks associated with taps, as shown in Fig. 1. In addition, a series of behaviorally neutral quences. Such changes could be of considerable significance to conditioning, particularly to me- hisses (no previous association) of comparable diation of the selective ability of the CS to loudness to the click was presented separately. The physical c.haracteristics of the click and the elicit the CR. hiss are shown later in Fig. 5. The click was METHODS delivered through a loudspeaker placed 3-5 ft Unit activity was recorded from the midlateral from the ears of the animal. The hiss was proand suprasylvian cortex of 17 cats. Unit re- duced by air (vacuum pulse) passing through an sponses to auditory stimuli and the amount of orifice l-2 ft away from the animal. The interinjected current required to initiate a spike dis- trial interval during testing was 5 s. The animals c.harge were studied a) in naive cats, h) in cats showed blink CRs to clicks but not to the hisses. As reported previously, for the type of conconditioned to blink by pairing click with glabella tap, and c) in cats subjected to pairings ditioned blinking resulting from pairing click-CS with glabella tap US, the specificity of the CS in of the same stimuli but in a random temporal order in which conditioned blink responses eliciting the CR develops regardless of whether there was prior discrimination training with the failed to develop. The training and testing procedures are dia- “neutral” or “indifferent” stimulus (cf. Fig. 6 of ref 4 1 and Fig. 3 of ref 45). grammed in Fig. 1.
Conditioned
group (C)
Seven cats were trained by repeatedly pairing 70-dB click (Cs) with glabella tap (US). The CS- US interval was 400 ms. The intertrial inter-
Randomization
group (R)
Three cats were subjected to click and tap, paired in random temporal order (cf. ref 27) in 21 training sessions otherwise similar in frequency
Downloaded from www.physiology.org/journal/jn by ${individualUser.givenNames} ${individualUser.surname} (129.186.138.035) on January 13, 2019
NEURAL
EFFECTS
OF
STIMULUS
and duration to the training sessions for the conditioned group. The randomization “training” procedure was designed in such a way that the mean interval between stimulus repetition was 10 S. These animals failed to develop a conditioned blink response to the click (cf. ref 45). During testing, a series of randomly paired clicks and taps was presented with mean IT1 of 5. s. A series of neutral hisses was presented separately, as shown in Fig. 1. Otherwise stimulus variables were comparable to those in the conditioned group. Naive group (N) Seven cats were not subjected to any initial training or stimulus association. During testing, series of clicks (no taps) and series of hisses were presented separately (see Fig. 1). Otherwise, stimulus variables during testing were c.omparable to those of the other two groups. The purpose of this design was to allow comparison of effects of three different click series and three similar hiss series in the three experimental groups. The hiss was always of neutral behavioral significance (no prior associations during training period). In the C group, the click was of behavioral significance (a CS). In the R group, the click was of “associational” significance (as a randomly associated stimulus). And in the N group, the click was of neutral behavioral significance. 1 Unit activity was recorded in awake, unanesthetized, unparalyzed cats. The animals were previously prepared, aseptically, so that microelectrodes and a stainless steel guide tube could be inserted through a 1.5-mm bur hole in the skull, into the auditory association cortex. The bodies of the animals were loosely restrained in cloth sleeves. The heads were held in a fixed position by metal screws, previously implanted in the skull under pentobarbital anesthesia. Another ljermanent screw in the glabella allowed attachment of an electromagnetic tapper used for delivery of the US. Additional braces were placed between the animal’s teeth and the floor of the frame supporting the head screws and the hydraulic microdrive assembly. Animals tolerate this procedure well, occasionally having to be kept from falling asleep during experiments. I l The terminologies neutral behavioral significance
ASSOCIATION
1047
The regions of cortex penetrated were established by direct postmortem examination of the brains, with the exception of two cats that are being used in other experiments. A composite picture is shown in Fig. 2. The area studied generally corresponds to association and secondary visual cortex (ASSOC and V II) described by Woolsey (Fig. 128 of ref 46) and is said to be distinct from auditory receptive cortex receiving direct projections from the medial geniculate body (36). Comparable areas were penetrated bilaterally and in the different experimental groups. No differences in the data depending on laterality were distinguished, and results from both sides were grouped together in our analyses. The angle of insertion of the guide tube and microelectrode was within t 30” perpendicular to the surface of the cortex. At the end of each session the electrode and guide tube were removed and the hole closed with bone wax. Penicillin was administered intramuscularly as needed. Microelectrodes of lo- to 50-M fl resistance were pulled from l-mm OD borosilicate capillaries and filled with 1.5 M K + citrate. The tips of some of the electrodes in the lower resistance range were bumped. All electrodes were led through Pt-lr wire to a Mentor, lOl*-a input resistance amplifier and thence to an FM tape recorder. One of the four chronically implanted screws served as reference electrode. Upper band pass of data recorded through the FM tape system was 2,500 Hz. Higher band pass (up to 20 kHz) was obtained on an on-line oscilloscope display. Low-frequency resolution with the polarizable electrode system was adequate for measuring abrupt base-line shifts seen on penetrating cells (see Fig. 4).
and apciational significance are used for lack of more adequate terminologies to describe still obscure con-
sequencesof various patterns of stimulus association. As indicated by our experimental results, these matters are more complicated than has generally been recognized. For example, once a neutral stimulus is presented during testing, it is no longer neutral, and its presentation can have cellular consequences, such as changes in spontaneous unit activity.
FIG. 2. Shaded area is a composite of cortical regions studied in these experiments. Midlateral and suprasylvian gyri and a small portion of ectosylvian gyrus are included.
Downloaded from www.physiology.org/journal/jn by ${individualUser.givenNames} ${individualUser.surname} (129.186.138.035) on January 13, 2019
1048
WOODY,
KNISPEL,
CROW,
Methodology concerning delivery of electricalstimuli through the microelectrode and criteriafor subsequent classification of studied units as “intracellular” or “extracellular” will now be considered. Cells were classified as intracellular if the size of recorded action potential (AP) was > 20 mV We cannot exclude the possibility that some of these recordings may have been quasiintracellular, but the following considerations support the likelihood that most were intracellular and differed from the extracellular recordings: 1) Abrupt negative shifts in base-line potential of equivalent or greater magnitude normally accompanied the appearance of 220 mV action potentials, which were larger than any APs seen immediately preceding the base-line shift. 2) The size and frequency of action potentials of this magnitude can usually be altered appropriately by depolarizing and hyperpolarizing currents (40, 42). 3) As will be shown later, the latency of spike initiation by an intracellularly injected current pulse was significantly shorter on the average for such neurons than that by extracellular current for neurons of smaller AP size that were classified as extracellular. 4) Many properties of cells with 220 mV AP correspond to the range measured by others in units with APs of 70-l 10 mV, including membrane resistance and the IR drop required to initiate an action potential (40). The small size of the action potential could either reflect damage to the neuron or other factors such as recording in a dendritic process with APs initiated remotely from the recording electrode. Whatever damage was present, including progressive decrease with time in the size of the intracellularly recorded action potential, was not enough to appreciably affect the neural response to natural stimuli. The PST histogram responses to click and hiss of these and comparable units were generally similar to those of extracellularly recorded #units (see Fig. 8; also ref 28 and 40). Cells were classified as extracellular if the size of recorded action potential and base-line variation was ~6 mV or if the action potential was
