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Slow Cortical Potentials (SCP) during habituation conditioning and extinction in rabbit cortex
Neuropsychologia, 1969,Vol.7, pp. 335to 347. PergamonPress.Printedin E&and
SLOW CORTICAL CONDITIONING
POTENTIALS (SCP) DURING HABITITATION AND EXTINCTION IN RABBIT CORTEX*
TERESA PINTO-HAMUY~, HUMBERTO BRACCHITTA and ISABEL LAGARRIGUJZ University of Chile, Santiago, Chile and Stanford University, Palo Alto, California, U.S.A. (Received 22 February 1969)
Abstract-The evolution of DC cortical shifts during learning, as well as its association with behavioral responses, were studied in thirteen rabbits using unpolarizable electrodes chronically implanted. The animals were trained to an avoidance CR, ten of them with a light and three with a tone, as CS. A possible relationship between SCP and emotional activation was sought through a correlation analysis run between DC responses and heart rate changes, obtaining a negative answer. The hypothesis that SCP are necessary for the establishment of a temporary connection at cortical level, was forwarded by the results of the individual and group correlation analysis. Twelve out of thirteen Ss showed a positive and significant correlation between CR and this electrical sign. Although SCP are not unique to the learning process, they appear as an electrical sign of an essential process for the establishment of a temporary connection. INTRODUCTION RUSINOV [l] was the first to observe the appearance of a CR by pairing a sensory stimulus with anodal polarization of the motor cortex. He demonstrated that a previously ineffective sound or light became effective in eliciting limb movement when combined with simultaneous, constant current, anodal polarization of the appropriate portion of motor cortex. RUSINOV postulated that a “dominant focus” [2] would be artificially created by anodal polarization thus establishing a temporary connection between the sensory analyzer (auditory cortex) and the motor cortex focus. No further evidences concerning the intimate neurophysiological mechanisms which underlie the functional connections were presented. However, the author proposed the hypothesis that polarization induced field effects which would influence a neuronal population electrotonically, increasing their excitability. These basic observations were confirmed by MORRELL [3], who explored the precise values of current density necessary to produce the aformentioned effect. Single-unit recordings were also obtained, indicating that the potential gradients established during such polarization were effective in altering the firing rates of single neurons. Similar values had been shown by TERZUOLO and BULLOCK [4] as adequate to modulate neuronal firing rates. Another line of evidence was provided by the use of direct-current (d.c.) or chopperstabilized amplifiers, which allowed recording of long-lasting potential changes in the cerebral cortex to sensory or direct subcortical electrical stimulation [5-81. Since the gradients so measured were equivalent to those artificially imposed in the earlier cited studies, it seemed reasonable to assume that similar mechanisms were involved in both cases.
* This research was supported by the Faculty of Medicine, University of Chile, Project No. 2,1074275, by the “Comisi6n National de Investigaci6n Cientlfica y Tecnol6gica”, Project No. 23; and USPHS Grant NB-03543 to Dr. Frank Morrell. IThis study was initiated while the senior author had a Guggenheim Fellowship at the Division of Neurology, Stanford Univetsity. Her present address is Institute of Physiology-Casilla 6524, Santiago, Chile. 335
336
TERE.SA PINTO-HAMUY, HUMBERTO BRACCHITTAand ISABELLAGARRIGUE
In fact, measurement of slow cortical potentials (SCP), which occurred naturally during learning situations were taken and found to be related to the establishment of the CR [9, lo]. ROWLANDand GOLDSTONE[I!] studied direct current (d.c.) cortical shifts developed in cats while they were learning an instrumental response (IR). In each session, the changes in intensity of SCP varied according to the level of animal’s satiation. No consistent trend in the evolution, locus or polarity of SCP was reported throughout training. It appeared worth while to make a detailed study of the evolution of SCP during learning to determine to what extent both, behavorial and electrical responses were associated through conditioning. ROWLAND’Sresults suggested the convenience of using shock as reinforcement, since in each session it has a more stable motivational value. Rabbits were selected because they have a smooth cortex which allows an easier interpretation of the recorded potential fields
WI. The following hypotheses were explored : (a) The SCP represent electrocortical correlates of emotional activation. Heart rate changes were considered as an emotional activation index. They should be correlated with the electrocortical response. (b) The SCP are related to a state of alertness and not specifically to the conditional stimulus (CS). Reinforcement, which heightens this state, should increase the frequency of electrical responses from the very first session; no correlation should necessarily be present between the conditional response (CR) and SCP. (c) The SCP is a necessary condition for the establishment of the CR. In this case a significant correlation should be found between learned motor responses and SCP. Our results allow the conclusion that slow potential shifts represent an electrical sign of a process which appears to be essential for the establishment of the instrumental response. METHOD The experiments
were performed with thirteen male rabbits, weighing 2 kg each
Surgical procedure Aseptic precautions were taken. Three non-polarizable electrodes (3 mm id.) [13] were chronically implanted in each animal: one as reference over the bone of the nasal sinus, a second one in the area corresponding to the sensory modality of the CS (specific electrode), and a third one in another sensory area (control electrode). WOOLSEY’S[14] evoked potential maps were employed as reference for electrode placements. Animals 10, 12, and 13 were implanted with the specific sensory electrode plus another one placed in the motor area. Electrodes for EMG and EKG recording were also chronically implanted. Training apparatus Training was carried out in a sound-proof chamber, as the animal was observed through a one-way vision window. Animals were held in a restraining apparatus consisting of an ad hoc wicker bodice which surrounded the posterior part of the subjects’ (Ss’) body (Fig. 1). The animal sat on a board, a leather belt strapped to its thorax to avoid displacement of the trunk. This arrangement permitted the animal to move its head and forelegs freely. The latter rested on a grid which could be electrified (Fig. 1). The animal’s electrical resistance was indicated at all times, allowing a perfect adjustment of the shock intensity delivered. In the last group, shock intensity was monitored to compensate automatically for variations in the Ss’ resistance. Training procedure After the operation the animals went through adaptation, lasting about ten days, followed by habituation, conditioning and extinction periods of the learning process. Each training session consisted of ten to fifteen trials, the intertrial interval being 1-3 min. Three. Ss were trained with tone and ten with light. The CS lasted 3 set-it was either a continuous light (an increment of the dim base light) or a tone of 3500 cps. The US was an electric discharge of 0.5 set, varying in strength with the resistance. The instrumental CR was a head flexion. A record of motor responses, SCP, EMG, and EKG frequency changes were taken in each trial during the habituation, conditioning and extinction periods.
SLOW CORTICAL CONDITIONING
POTENTIALS (SCP) DURING HABITITATION AND EXTINCTION IN RABBIT CORTEX*
TERESA PINTO-HAMUY~, HUMBERTO BRACCHITTA and ISABEL LAGARRIGUJZ University of Chile, Santiago, Chile and Stanford University, Palo Alto, California, U.S.A. (Received 22 February 1969)
Abstract-The evolution of DC cortical shifts during learning, as well as its association with behavioral responses, were studied in thirteen rabbits using unpolarizable electrodes chronically implanted. The animals were trained to an avoidance CR, ten of them with a light and three with a tone, as CS. A possible relationship between SCP and emotional activation was sought through a correlation analysis run between DC responses and heart rate changes, obtaining a negative answer. The hypothesis that SCP are necessary for the establishment of a temporary connection at cortical level, was forwarded by the results of the individual and group correlation analysis. Twelve out of thirteen Ss showed a positive and significant correlation between CR and this electrical sign. Although SCP are not unique to the learning process, they appear as an electrical sign of an essential process for the establishment of a temporary connection. INTRODUCTION RUSINOV [l] was the first to observe the appearance of a CR by pairing a sensory stimulus with anodal polarization of the motor cortex. He demonstrated that a previously ineffective sound or light became effective in eliciting limb movement when combined with simultaneous, constant current, anodal polarization of the appropriate portion of motor cortex. RUSINOV postulated that a “dominant focus” [2] would be artificially created by anodal polarization thus establishing a temporary connection between the sensory analyzer (auditory cortex) and the motor cortex focus. No further evidences concerning the intimate neurophysiological mechanisms which underlie the functional connections were presented. However, the author proposed the hypothesis that polarization induced field effects which would influence a neuronal population electrotonically, increasing their excitability. These basic observations were confirmed by MORRELL [3], who explored the precise values of current density necessary to produce the aformentioned effect. Single-unit recordings were also obtained, indicating that the potential gradients established during such polarization were effective in altering the firing rates of single neurons. Similar values had been shown by TERZUOLO and BULLOCK [4] as adequate to modulate neuronal firing rates. Another line of evidence was provided by the use of direct-current (d.c.) or chopperstabilized amplifiers, which allowed recording of long-lasting potential changes in the cerebral cortex to sensory or direct subcortical electrical stimulation [5-81. Since the gradients so measured were equivalent to those artificially imposed in the earlier cited studies, it seemed reasonable to assume that similar mechanisms were involved in both cases.
* This research was supported by the Faculty of Medicine, University of Chile, Project No. 2,1074275, by the “Comisi6n National de Investigaci6n Cientlfica y Tecnol6gica”, Project No. 23; and USPHS Grant NB-03543 to Dr. Frank Morrell. IThis study was initiated while the senior author had a Guggenheim Fellowship at the Division of Neurology, Stanford Univetsity. Her present address is Institute of Physiology-Casilla 6524, Santiago, Chile. 335
336
TERE.SA PINTO-HAMUY, HUMBERTO BRACCHITTAand ISABELLAGARRIGUE
In fact, measurement of slow cortical potentials (SCP), which occurred naturally during learning situations were taken and found to be related to the establishment of the CR [9, lo]. ROWLANDand GOLDSTONE[I!] studied direct current (d.c.) cortical shifts developed in cats while they were learning an instrumental response (IR). In each session, the changes in intensity of SCP varied according to the level of animal’s satiation. No consistent trend in the evolution, locus or polarity of SCP was reported throughout training. It appeared worth while to make a detailed study of the evolution of SCP during learning to determine to what extent both, behavorial and electrical responses were associated through conditioning. ROWLAND’Sresults suggested the convenience of using shock as reinforcement, since in each session it has a more stable motivational value. Rabbits were selected because they have a smooth cortex which allows an easier interpretation of the recorded potential fields
WI. The following hypotheses were explored : (a) The SCP represent electrocortical correlates of emotional activation. Heart rate changes were considered as an emotional activation index. They should be correlated with the electrocortical response. (b) The SCP are related to a state of alertness and not specifically to the conditional stimulus (CS). Reinforcement, which heightens this state, should increase the frequency of electrical responses from the very first session; no correlation should necessarily be present between the conditional response (CR) and SCP. (c) The SCP is a necessary condition for the establishment of the CR. In this case a significant correlation should be found between learned motor responses and SCP. Our results allow the conclusion that slow potential shifts represent an electrical sign of a process which appears to be essential for the establishment of the instrumental response. METHOD The experiments
were performed with thirteen male rabbits, weighing 2 kg each
Surgical procedure Aseptic precautions were taken. Three non-polarizable electrodes (3 mm id.) [13] were chronically implanted in each animal: one as reference over the bone of the nasal sinus, a second one in the area corresponding to the sensory modality of the CS (specific electrode), and a third one in another sensory area (control electrode). WOOLSEY’S[14] evoked potential maps were employed as reference for electrode placements. Animals 10, 12, and 13 were implanted with the specific sensory electrode plus another one placed in the motor area. Electrodes for EMG and EKG recording were also chronically implanted. Training apparatus Training was carried out in a sound-proof chamber, as the animal was observed through a one-way vision window. Animals were held in a restraining apparatus consisting of an ad hoc wicker bodice which surrounded the posterior part of the subjects’ (Ss’) body (Fig. 1). The animal sat on a board, a leather belt strapped to its thorax to avoid displacement of the trunk. This arrangement permitted the animal to move its head and forelegs freely. The latter rested on a grid which could be electrified (Fig. 1). The animal’s electrical resistance was indicated at all times, allowing a perfect adjustment of the shock intensity delivered. In the last group, shock intensity was monitored to compensate automatically for variations in the Ss’ resistance. Training procedure After the operation the animals went through adaptation, lasting about ten days, followed by habituation, conditioning and extinction periods of the learning process. Each training session consisted of ten to fifteen trials, the intertrial interval being 1-3 min. Three. Ss were trained with tone and ten with light. The CS lasted 3 set-it was either a continuous light (an increment of the dim base light) or a tone of 3500 cps. The US was an electric discharge of 0.5 set, varying in strength with the resistance. The instrumental CR was a head flexion. A record of motor responses, SCP, EMG, and EKG frequency changes were taken in each trial during the habituation, conditioning and extinction periods.