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Effects of cooling inferotemporal cortex on performance of visual memory tasks
EXPERIMENTAL
71,398-409
NEUROLOGY
Effects of Cooling
JOAQUIN Department
lnferotemporal Cortex on Performance of Visual Memory Tasks
M. FUSTER,
of Psychiatry
Received
(1981)
RICHARD
H. BAUER,
AND JOHN P. JERVEY’
and Brain Research Institute. School of Medicine, California, Los Angeles, California 90024 June 23, 1980; revision
received
University
of
July 29, 1980
Monkeys with implanted cooling probes on the surface of the inferotemporal cortex were tested on performance of three visual tasks: simultaneous matching to sample, delayed matching to sample, and delayed response. Bilateral cooling of the inferotemporal cortex induced a pronounced deficit in delayed matching and a lesser deficit in simultaneous matching, but no significant deficit in delayed response. Reaction times were longer during cooling in the three tasks. Unilateral cooling induced performance deficits and prolongations of reaction time of less magnitude than those induced by bilateral cooling. The effects of cooling on performance and on patterns of ocular motility indicate that the functional depression of the inferotemporal cortex results in impaired attention and impaired provisional memory of visual information.
INTRODUCTION Ablations of the inferotemporal (IT) cortex impair learning of visual discriminations in primates-see reviews by Gross (11) and Dean (7). Accordingly, the IT cortex is presumed to play a role in visual memory. This hypothesis received considerable support from evidence that IT lesions severely retard the formation of stimulus-reward associations Abbreviations: IT-inferotemporal; DMS, SMS-delayed, simultaneous matching to sample; DR-delayed response. ’ Supported by grants BNS76-16984 from the National Science Foundation and AA3513 from the National Institute on Alcohol Abuse and Alcoholism. The first author held a Research Scientist Award (MH 25082) from the National Institute of Mental Health. The technical assistance of William Bergerson, Mordecai Dunst, and Darrell Riley is gratefully acknowledged. Phillip Schroth provided valuable statistical help. Data were analyzed with the IBM 3032 computer of the Health Sciences Computing Facility, using a program (“Proc Anova”) from the Statistical Analysis System Institute, Inc., Gary, NC. 398 0014-4886/81/020398-12$02.00/O Copyright 0 1981 by Academic Press, Inc. All rights of reproduction in any form reserved.
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during visual learning that involves compound stimuli (22) or multiple pairs of stimuli (15). Further support came from demonstration that IT lesions impair performance of tasks requiring short-term retention of visual cues (6, 20). Nonetheless, the memory concept remains in question because, given sufficient training, animals subjected to IT ablation eventually establish stimulus-reward associations and, having done so, retain them .without difficulty (3, 22). They may also overcome their deficit in short-term memory performance (6). However, the eventual formation of memories and the recovery of performance can be the result of compensatory functional changes developing after ablation. Following ablation, alternate brain mechanisms may come into play and the animal may resort to alternative behavioral strategies, thus compensating for and masking the IT deficit. This study examined the effects of cooling the IT cortex on performance of two visual memory tasks, one requiring retention of color cues and the other of spatial cues. Because of the reversibility of cortical depression by cold, the cryogenic method partly circumvents the problems of ablation, among them the development of compensatory changes. Furthermore, inasmuch as each experimental animal can be repeatedly used as its own control, the cryogenic method allows a more satisfactory assessment than ablation allows of the relationships between dysfunction of the IT cortex and certain relevant variables, such as the length of the retention interval in a memory task and the reaction times of the animal. These variables bear on the question of mnemonic function and the aspects of this function that may be affected by IT lesion. METHODS Subjects. The experiments were conducted on four male macaque monkeys (three M. mulutta and one M. assamensis) weighing about 7.0 kg on the average. They were housed in individual cages and maintained on a normal diet, except for some limitation of fluid intake-no water for 20 h prior to each testing session, during which they received liquid reinforcement. Apparatus. All training and testing was conducted in a sound-resistant room with low-level ambient illumination and continuous white noise. The monkey was in a primate chair facing a panel with three translucid stimulus-response buttons (diameter 2.5 cm) forming an upright isosceles triangle (base 11.0 cm, sides 7.5 cm). Each button could be illuminated by rear projection with red, green, or white light. The buttons were within arm’s reach, at about 18 cm from the eyes. By restricting right-arm movements all animals were obliged to press the buttons with the left hand.
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BAUER,
AND
JERVEY
Pressing a button activated a microswitch. An electronic timer allowed measurement of reaction times to the nearest 0.1 s. A metal spigot in contact with the animal’s lips allowed delivery of reinforcement (grape juice). Stimulus presentations and reinforcement were controlled by solid-state programming apparatus. Training. The animals were first trained to perform a simultaneous matching to sample @MS) task. A trial began by presentation of the sample: a red or green light in the top button. Pressing that button turned on the two lower buttons, one red and the other green. Pressing the lower button with the sample color led to automatic juice reward (about 1 .O ml). Pressing the incorrect button, or failure to make a choice within 4 s, resulted in extinction of all three buttons and termination of the trial. The intertrial interval was about 30 s. The color of each button was changed quasirandomly between trials. Following SMS training, the animals were trained in delayed matching to sample (DMS). This task was identical to SMS, except that-in DMS-pressing the top button turned the sample off and initiated a preset delay with unlit buttons, at the end of which the two colors appeared in the lower buttons. The monkey was then rewarded for choosing the sample color. DMS training involved the imposition of progressively longer delays. Finally the animals were trained in spatial delayed response (DR). Here a trial began with white light in one of the lower buttons. By pressing it, the animal turned it off and initiated the delay. At the end of delay, both lower buttons were illuminated with white light and, for reward, the monkey had to press the one lit at the beginning of the trial. The correct button changed randomly from trial to trial, DR training also involved progressively longer delays. A monkey was considered fully trained when its performance on each of the tasks attained a stable level without evidence of further improvement, and remained at that level for five or more consecutive sessions. Having reached that level, the four animals performed SMS in the 95 to 100% correct range, and DMS and DR-with 32-s delay-in the 80 to 95% correct range. Surgery. After training, cooling probes were surgically implanted bilaterally on the surface of the IT cortex. The implantation was made in one or two stages (both probes in one operation or each probe in a separate operation) with the animal under Nembutal anesthesia. A cooling probe was an elongated gold-plated piece of copper, one end to be placed in contact with cortex and the other exposed outside for connection to a heat-extracting device. The portion of probe to be in contact with the brain had the shape of a curved blade (8 mm wide and 2 mm thick); its curvature
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conformed to the inferior convexity of the temporal lobe. An opening was made in the skull above the zygomatic arch at a location determined by stereotaxic and cranial measurements. The probe was introduced obliquely through the opening and slid between the dura and the base of the skull in the middle cranial fossa. It was then fixed in position and attached to the bone around the opening by means of screws and dental acrylic. The exposed portion of the probe was thermically insulated with dental acrylic
DMS-8
DMS-20
DMS- 22
FIG. 1. Brain diagrams showing, in shading, the position of the cortical cooling probes in the four experimental animals.
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except for the distal end, a flat platform with a threaded socket for eventual attachment of a thermoelectric cooler. Thermistors for temperature control were implanted between dura and cortex directly under the probes. In two animals, four Ag/AgCl electrode cups were implanted in the bone around the orbits for recording eye position and movements. All leads were connected to plugs affixed to the skull with dental acrylic. The position of the cooling probes was verified postmortem in two animals. The other two animals are now being used in a single-unit study and the position of their probes was ascertained by X-ray. Figure 1 shows the portions of cortex covered by cooling probes in the four animals. As seen, all probes covered a large portion of area TE of von Bonin and Bailey (2). Cooling. Before a testing session, two thermoelectric (Peltier) coolers were attached with bolts to the probes. The cooling procedure was described elsewhere (1,9, 10). In the present experiments IT temperature was lowered to 20°C. At this temperature setting, no abnormalities could be observed in the overt behavior of the animal. Sensorium, manual dexterity, and motivation appeared undisturbed. On the basis of previous determinations (10) it is reasonable to assume that, while the IT cortical surface was at 2o”C, the entire cortex covered by the cooling probe was at 20 to 25°C. It can also be assumed that a subjacent portion of cortex lining the lower bank of the superior temporal sulcus was also at subnormal temperature (about 30°C). Testing. Behavioral testing was resumed 2 weeks at the earliest after implantation of the two probes. After a few testing sessions with coolers attached but inoperative, the experiments were initiated. The animals were subjected to daily testing sessions of 110 trials each: 10 SMS trials, and 10 trials each of DM and DR, with each offive delays (1,4,8,16, and 32 s). The trials of any given task and delay were administered in blocks of 10 trials in counterbalanced order of tasks and delays. In the first 5 trials of each block, the reaction time to one stimulus (sample/cue or choice lights) was recorded; in the second 5 trials, to the other. Cooling sessions alternated with noncooling sessions. The animals were tested in three IT cooling conditions: bilateral 2O”C, right side 20°C) and left side 20°C. During a given cooling session, the entire behavioral testing was conducted in one of these conditions. Electrooculogram. The potentials from two pairs of periorbital electrodes, one recording horizontal and the other vertical changes of eye position, were led through DC amplifiers to the two axes of an oscilloscope and concomitantly recorded on FM magnetic tape. The oscilloscope displayed a continuous vectogram of eye movements. The taped electrooculograms were graphically displayed by means of a chart recorder.
INFEROTEMPORAL
COOLING
DELAYED MATCHING-TO-SAMPLE
403 DELAY ED RESPONSE
FIG. 2. Average measures of performance-from aU subjects-at normal temperature and during bilateral cooling of the inferotemporal cortex. Delayed matching to sample data are plotted at left and delayed response data at right; simultaneous matching to sample far left (in DMS graphs). Top-percentage of correct responses, middle-sample/cue reaction time, bottom-choice reaction time. Delay time in seconds is on the abscissa.
RESULTS Correctness
The pooled correct-response data from bilateral IT cooling and noncooling sessions are displayed in Fig. 2 (upper graphs), where mean percentage is plotted for each task, delay, and temperature condition. It is
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evident from the graphs that cooling induced a marked deficit in performance of DMS at all delays. Minor deficits in SMS (two isolated points in the left graph) and in DR are also apparent. Mean correctness values for lo-trial blocks of DMS and DR were subjected to arcsine transformation-because of heterogeneity of variance-and to a complete 4 (animal) x 2 (task) x 2 (temperature) x 5 (delay) factorial analysis of variance: the last three terms were treated as repeated-measure variables. Some results of the analysis are shown in Table 1. The main effects for task, temperature, and delay are highly significant. The task x temperature interaction is also significant. Comparisons of overall means-Tukey’s (a) test with P < 0.01 was used for these and subsequent comparisons-showed that cooling had no significant effect on the DR at any delay, whereas it produced a significant deficit in the DMS at all delays. Such comparisons also showed that correctness levels in the two tasks did not differ at normal temperature and that the task main effect was principally due to differential effects of cooling on the tasks. In the absence of a significant interaction between temperature and delay, the DMS data were combined in two groups-cooling and noncooling-without regard for delay. An analysis of variance was then performed on DMS and SMS correctness data: 4 (animal) x 2 (task) x 2 (temperature), the last two terms as repeated-measure variables. This analysis showed significant main effects for task and temperature. In addition, it showed a significant interaction between task and temperature, indicating that cooling caused a larger deficit in the DMS than in the SMS. TABLE Extract
from
Analysis of Variance of Delayed Matching and Delayed Response Data Correctness
Source
1
df
of variance
Animal Task Delay Temperature Animal x temperature Task x delay Task x temperature Delay x temperature ” RT-Reaction time. * P < 0.05. ** P < 0.01,
3. 1, 4, 1, 3, 4, 1, 4.
44 44 176 44 44 176 44 176
*** P < 0.001.
(F) 3.7* 128.9*** 155.4*** 97.4*** 2.0 1.0 61.8*** 1.1
Sample/cue
(F) 43.1*** 7.1* 16.2*** 53.1*** 3.2* 0.4 1.2 1.1
to Sample
RT”
Choice
RT”
(F) 96.4*** 33.6*** 127.1*** 81.1*** 12.0*** 1.8 15.4*** 1.0
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TABLE 2 Extract from Analysis of Variance of Delayed and Simultaneous Matching to Sample Data Correctness Source of variance
df
(F)
Animal Task Temperature Animal x temperature Task x temperature
3,44 1, 44 1, 44 3, 44 1, 44
15.0*** 113.5*** 87.5*** 7.9*x* 11.0**
Choice RT”
CF.1 194.4*** 190.4*** 89.5*** 4.7** 5.3*
a RT-reaction time. * P < 0.05, ** P < 0.01, *** P < 0.001.
After completion of these experiments, all animals became subjects of another study (cooling on single-unit activity). That allowed replication of the IT cooling effects on many occasions and during a long period. Thus, the fully reversible DMS deficit was repeatedly elicited for several months in all animals. The deficit can be reliably reproduced in the two animals presently being used in that study. Reaction Time
The middle graphs in Fig. 2 indicate a prolongation of reaction time to both the DMS sample and the DR cue under IT cooling. Measures of sample/cue reaction time were submitted to square-root transformation and to analysis of variance in a similar manner as correctness data. This analysis (Table 1) revealed significant main effects for temperature, task, and delay. Mean comparisons showed that cooling significantly lengthened sample/cue time in both the DMS and DR tests (sample reaction time was not measured in the SMS test). Choice reaction time was also longer, in the three tasks, under IT cooling (Fig. 2, lower graphs). Analysis of variance (on transformed data) showed significant main effects for temperature, task, and delay (Tables 1 and 2). There were significant temperature x task interactions in the DMS-DR pairing (Table 1) and in the SMS-DMS pairing (Table 2). Mean comparisons showed that the prolongations of choice reaction time during cooling were highly significant in all tasks and after all delays, and that the task main effect for the DMS-DR tests was attributable to the differential cooling effect. Ocular Motility
Under normal conditions a monkey fixates its gaze on the stimulus button immediately after onset of the sample in a DMS trial (Fig. 3,
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AND
JERVEY
“normal”). Fixation is usually maintained until the stimulus disappears, and then persists beyond that moment for 0.5 s or longer. During the delay, the gaze ordinarily wanders to and fro between buttons in more or less regular oscillation. As soon as the two lower buttons are illuminated for choice, one or two rapid shifts of eye direction between the two buttons can be commonly observed before the animal makes its choice. Under IT cooling, fixation on the sample button was generally more laggard and less stable (Fig. 3, “cool”). Ocular motility during the delay became fitful and its range increased. Before the instrumental choice, the animal showed more alternating movements of the eyes between right and left button than in the noncooling condition. This was particularly evident after long delays. Unilateral
Cooling
Unilateral IT cooling induced a considerably smaller deficit in DMS correctness, at all delays, than that induced by bilateral IT cooling. A slight deficit in SMS was also observed. Reaction times were slightly prolonged in the three tasks, but generally not as much as during bilateral cooling. These differences were particularly obvious in choice reaction time. No clear and systematic differences were observed between the effects of right and left cooling, with one exception: sample/cue reaction time was longer under right than left cooling. No such asymmetry of cooling effects was evident in choice reaction time. DISCUSSION Since IT cooling induced only a small deficit in the SMS task, we may conclude that visual perception and discrimination were relatively unimpaired by the procedure. However, cooling induced a significantly larger deficit in DMS performance, indicating an impairment of the kind of short-term memory that is required in a visual delay task. A similar effect was previously obtained by prefrontal cooling, but not by parietal cooling (1,9, 10). The similarity of cooling effects on the DMS task suggests that the prefrontal and IT cortices are jointly involved in provisional memory of visual information. Reciprocal connections between the two cortical regions (4, 12, 13, 16, 17) may be part of the anatomic substrate for such a joint function. The cooling effect on choice reaction time provides additional evidence for involvement of the IT cortex in short-term memory processes. During cooling, the animals took longer to make the choice at the end of the retention period. A comparable lengthening of reaction time was observed as a result of IT ablation (19). The higher incidence of shifts of gaze between the choice stimuli, noted by us with cooling, may simply reflect the greater
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difficulty of the monkey to remember the correct cue and, consequently, to decide. It is unclear why IT cooling affected correct performance of one visual memory task--MS-and not the other-DR. This dissociation may be due to one important difference between tasks: whereas DMS can only be correctly performed by use of visual memory, DR is a spatial task that could be performed by nonvisual (e.g., proprioceptive) memory. Thus, in DR, the animal during IT cooling might overcome a visual memory deficit by resorting to a nonvisual strategy. Nevertheless, the longer cue and choice reaction times suggest that the DR also presents some difficulty for the IT-cooled animal. The deficit may be overcome not only by use of a nonvisual strategy but by more self-imposed exposure to the cue and by longer deliberation at the choice. Some investigators (5, 15, 21), on the basis of ablation results, have concluded that the IT cortex is functionally distinguishable from the cortex lying directly behind it and around the inferior occipital sulcus (“posterior inferotemporal” and “prestriate” cortex). Posterior cortex is deemed essential for neural processes related to visual attention, perception, and discrimination, whereas the anterior cortex-inferotemporal cortex proper (5,7)-would be somehow involved in visual memory (5,15,18,19, 21,22). The present results of reversible lesion support a role of the anterior cortex in provisional memory of visual stimuli. Other researchers (8, 14), delivering a disruptive electrical stimulus to the IT cortex at various times during the DMS trial, noted that performance was maximally impaired when that stimulus was given during the matching period. They concluded that the IT cortex plays a critical role in information retrieval. Our results are consistent with this conclusion. However, both cooling and stimulation (14) data are also consistent with a role of the IT cortex in retention of visual information. In fact, all three principal component operations of provisional memory-encoding, retention, and retrieval-may be to some degree compromised in dysfunction of the IT cortex. A disruption of visual encoding processes, attention in particular, is suggested by stimulation results (8, 14) and also, by our cooling results (effects on SMS performance and eye motility during sample). Both methods, however, reveal a relatively greater disruption of the mnemonic processes of retention and retrieval. It is a goal for future research to clarify the relationship between the role of the IT cortex in provisional memory and its demonstrated importance for the learning of discriminations. One reasonable hypothesis is that the formation of long-term visual memory depends on the same mechanisms for encoding, temporarily retaining, and retrieving visual information that operate during a visual delay-task trial and for which the functional integrity of the IT cortex appears essential.
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REFERENCES 1. BAUER, R. H., AND J. M. FUSTER. 1976. Delayed-matching and delayed-response deficit from cooling dorsolaterai prefrontal cortex in monkeys. J. Comp. Physiof. Psychol. 90: 293-302. 2.
3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13.
14.
BONIN, G. VON, AND P. BAILEY. 1947. The Neocortex of Macaca Mulatta. Univ. of Illinois Press, Urbana. BUTLER, C. R. 1%9. Is there a memory impairment in monkeys after inferior temporal lesions? Brain Res. 13: 383-393. CHAVIS, D. A., AND D. N. PANDYA. 1976. Further observations on corticofrontal connections in the rhesus monkey. Brain Res. 117: 369-386. COWEY, A., AND C. G. GROSS. 1970. Effects of foveai prestriate and inferotemporai lesions on visual discriminations by rhesus monkeys. Exp. Brain Res. 11: 128- 144. DEAN, P. 1974. The effect of inferotemporal lesions on memory for visual stimuli in rhesus monkeys. Brain Res. 77: 451-469. DEAN, P. 1976. Effects of inferotemporal lesions on the behavior of monkeys. Psychol. Bull. 83: 41-71. DELACOUR, J. 1977. Cortex inferotemporal et memoire visuelle ii court terme chez le singe. Nouveiles don&es. Exp. Brain Res. 28: 301-310. FUSTER, J. M., AND G. E. ALEXANDER. 1970. Delayed response deficit by cryogenic depression of frontal cortex. Brain Res. 20: 85-90. FUSTER, J. M., AND R. H. BAUER. 1974. Visual short-term memory deficit from hypothermia of frontal cortex. Brain Res. 81: 393-400. GROSS, C. G. 1973. Inferotemporai cortex and vision. Pages 77-123 in E. STELLAR and J. M. SPRAGUE, Eds., Progress in Physiological Psychology, Vol. 5. Academic Press, New York. JACOBSON, S., AND J. Q. TROJANOWSKI. 1977. Prefrontal granular cortex of the rhesus monkey. I. Intrahemispheric cortical afferents. Brain Res. 132: 209-233. JONES, E. G., AND T. P. S. POWELL. 1970. An anatomical study of converging sensory pathways within the cerebral cortex of the monkey. Brain 93: 793-820. KOVNER, R., AND J. S. STAMM. 1972. Disruption of short-term visual memory by electrical stimulation of inferotemporai cortex in the monkey. J. Comp. Physiol. Psychol.
15.
16. 17. 18.
19. 20.
81: 163- 172.
MISHKIN, M. 1972. Cortical visual areas and their interactions. Pages 187-208 in A. G. KARCZMAR and J. C. ECCLES, Eds.,BrainandHuman Behavior. Springer, New York. PANDYA, D. N., P. DYE, AND N. BUTTERS. 1971. Efferent cortico-corticaiprojections of the prefrontal cortex in the rhesus monkey. Brain Res. 31: 35-46. PANDYA, D. N., AND H. G. J. M. KUYPERS. 1969. Corticocortical connections in the rhesus monkey. Brain Res. 13: 13-36. SAHGAL, A., AND S. D. IVERSEN. 1978. Categorization and retrieval after selective inferotemporal lesions in monkeys. Brain Res. 146: 341-350. SAHGAL, A., AND S. D. IVERSEN. 1978. The effects offoveaiprestriate andinferotemporai lesions on matching to sample behaviour in monkeys. Neuropsychologia 16: 391-406. STEPIE& L. S., J. P. CORDEAU, AND T. RASMUSSEN. 1960. The effect of temporal lobe and hippocampai lesions on auditory and visual recent memory in monkeys. Brain 83: 470-489.
21. WILSON, M., H. M KAUFMAN, R. E. ZIELER, AND J. P. LIEB. 1972. Visual identification and memory in monkeys with circumscribed inferotemporai lesions. J. Comp. Physiol. Psychol. 78: 173- 183. 22. WILSON, M., W. A. WILSON, JR., AND H. S. SUNENSHINE. 196%.Perception, learning and retention of visual stimuli by monkeys with inferotemporai lesions. J. Comp. Physiol. Psychol.
65: 404-412.
71,398-409
NEUROLOGY
Effects of Cooling
JOAQUIN Department
lnferotemporal Cortex on Performance of Visual Memory Tasks
M. FUSTER,
of Psychiatry
Received
(1981)
RICHARD
H. BAUER,
AND JOHN P. JERVEY’
and Brain Research Institute. School of Medicine, California, Los Angeles, California 90024 June 23, 1980; revision
received
University
of
July 29, 1980
Monkeys with implanted cooling probes on the surface of the inferotemporal cortex were tested on performance of three visual tasks: simultaneous matching to sample, delayed matching to sample, and delayed response. Bilateral cooling of the inferotemporal cortex induced a pronounced deficit in delayed matching and a lesser deficit in simultaneous matching, but no significant deficit in delayed response. Reaction times were longer during cooling in the three tasks. Unilateral cooling induced performance deficits and prolongations of reaction time of less magnitude than those induced by bilateral cooling. The effects of cooling on performance and on patterns of ocular motility indicate that the functional depression of the inferotemporal cortex results in impaired attention and impaired provisional memory of visual information.
INTRODUCTION Ablations of the inferotemporal (IT) cortex impair learning of visual discriminations in primates-see reviews by Gross (11) and Dean (7). Accordingly, the IT cortex is presumed to play a role in visual memory. This hypothesis received considerable support from evidence that IT lesions severely retard the formation of stimulus-reward associations Abbreviations: IT-inferotemporal; DMS, SMS-delayed, simultaneous matching to sample; DR-delayed response. ’ Supported by grants BNS76-16984 from the National Science Foundation and AA3513 from the National Institute on Alcohol Abuse and Alcoholism. The first author held a Research Scientist Award (MH 25082) from the National Institute of Mental Health. The technical assistance of William Bergerson, Mordecai Dunst, and Darrell Riley is gratefully acknowledged. Phillip Schroth provided valuable statistical help. Data were analyzed with the IBM 3032 computer of the Health Sciences Computing Facility, using a program (“Proc Anova”) from the Statistical Analysis System Institute, Inc., Gary, NC. 398 0014-4886/81/020398-12$02.00/O Copyright 0 1981 by Academic Press, Inc. All rights of reproduction in any form reserved.
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during visual learning that involves compound stimuli (22) or multiple pairs of stimuli (15). Further support came from demonstration that IT lesions impair performance of tasks requiring short-term retention of visual cues (6, 20). Nonetheless, the memory concept remains in question because, given sufficient training, animals subjected to IT ablation eventually establish stimulus-reward associations and, having done so, retain them .without difficulty (3, 22). They may also overcome their deficit in short-term memory performance (6). However, the eventual formation of memories and the recovery of performance can be the result of compensatory functional changes developing after ablation. Following ablation, alternate brain mechanisms may come into play and the animal may resort to alternative behavioral strategies, thus compensating for and masking the IT deficit. This study examined the effects of cooling the IT cortex on performance of two visual memory tasks, one requiring retention of color cues and the other of spatial cues. Because of the reversibility of cortical depression by cold, the cryogenic method partly circumvents the problems of ablation, among them the development of compensatory changes. Furthermore, inasmuch as each experimental animal can be repeatedly used as its own control, the cryogenic method allows a more satisfactory assessment than ablation allows of the relationships between dysfunction of the IT cortex and certain relevant variables, such as the length of the retention interval in a memory task and the reaction times of the animal. These variables bear on the question of mnemonic function and the aspects of this function that may be affected by IT lesion. METHODS Subjects. The experiments were conducted on four male macaque monkeys (three M. mulutta and one M. assamensis) weighing about 7.0 kg on the average. They were housed in individual cages and maintained on a normal diet, except for some limitation of fluid intake-no water for 20 h prior to each testing session, during which they received liquid reinforcement. Apparatus. All training and testing was conducted in a sound-resistant room with low-level ambient illumination and continuous white noise. The monkey was in a primate chair facing a panel with three translucid stimulus-response buttons (diameter 2.5 cm) forming an upright isosceles triangle (base 11.0 cm, sides 7.5 cm). Each button could be illuminated by rear projection with red, green, or white light. The buttons were within arm’s reach, at about 18 cm from the eyes. By restricting right-arm movements all animals were obliged to press the buttons with the left hand.
400
FUSTER,
BAUER,
AND
JERVEY
Pressing a button activated a microswitch. An electronic timer allowed measurement of reaction times to the nearest 0.1 s. A metal spigot in contact with the animal’s lips allowed delivery of reinforcement (grape juice). Stimulus presentations and reinforcement were controlled by solid-state programming apparatus. Training. The animals were first trained to perform a simultaneous matching to sample @MS) task. A trial began by presentation of the sample: a red or green light in the top button. Pressing that button turned on the two lower buttons, one red and the other green. Pressing the lower button with the sample color led to automatic juice reward (about 1 .O ml). Pressing the incorrect button, or failure to make a choice within 4 s, resulted in extinction of all three buttons and termination of the trial. The intertrial interval was about 30 s. The color of each button was changed quasirandomly between trials. Following SMS training, the animals were trained in delayed matching to sample (DMS). This task was identical to SMS, except that-in DMS-pressing the top button turned the sample off and initiated a preset delay with unlit buttons, at the end of which the two colors appeared in the lower buttons. The monkey was then rewarded for choosing the sample color. DMS training involved the imposition of progressively longer delays. Finally the animals were trained in spatial delayed response (DR). Here a trial began with white light in one of the lower buttons. By pressing it, the animal turned it off and initiated the delay. At the end of delay, both lower buttons were illuminated with white light and, for reward, the monkey had to press the one lit at the beginning of the trial. The correct button changed randomly from trial to trial, DR training also involved progressively longer delays. A monkey was considered fully trained when its performance on each of the tasks attained a stable level without evidence of further improvement, and remained at that level for five or more consecutive sessions. Having reached that level, the four animals performed SMS in the 95 to 100% correct range, and DMS and DR-with 32-s delay-in the 80 to 95% correct range. Surgery. After training, cooling probes were surgically implanted bilaterally on the surface of the IT cortex. The implantation was made in one or two stages (both probes in one operation or each probe in a separate operation) with the animal under Nembutal anesthesia. A cooling probe was an elongated gold-plated piece of copper, one end to be placed in contact with cortex and the other exposed outside for connection to a heat-extracting device. The portion of probe to be in contact with the brain had the shape of a curved blade (8 mm wide and 2 mm thick); its curvature
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401
conformed to the inferior convexity of the temporal lobe. An opening was made in the skull above the zygomatic arch at a location determined by stereotaxic and cranial measurements. The probe was introduced obliquely through the opening and slid between the dura and the base of the skull in the middle cranial fossa. It was then fixed in position and attached to the bone around the opening by means of screws and dental acrylic. The exposed portion of the probe was thermically insulated with dental acrylic
DMS-8
DMS-20
DMS- 22
FIG. 1. Brain diagrams showing, in shading, the position of the cortical cooling probes in the four experimental animals.
402
FUSTER,
BAUER,
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except for the distal end, a flat platform with a threaded socket for eventual attachment of a thermoelectric cooler. Thermistors for temperature control were implanted between dura and cortex directly under the probes. In two animals, four Ag/AgCl electrode cups were implanted in the bone around the orbits for recording eye position and movements. All leads were connected to plugs affixed to the skull with dental acrylic. The position of the cooling probes was verified postmortem in two animals. The other two animals are now being used in a single-unit study and the position of their probes was ascertained by X-ray. Figure 1 shows the portions of cortex covered by cooling probes in the four animals. As seen, all probes covered a large portion of area TE of von Bonin and Bailey (2). Cooling. Before a testing session, two thermoelectric (Peltier) coolers were attached with bolts to the probes. The cooling procedure was described elsewhere (1,9, 10). In the present experiments IT temperature was lowered to 20°C. At this temperature setting, no abnormalities could be observed in the overt behavior of the animal. Sensorium, manual dexterity, and motivation appeared undisturbed. On the basis of previous determinations (10) it is reasonable to assume that, while the IT cortical surface was at 2o”C, the entire cortex covered by the cooling probe was at 20 to 25°C. It can also be assumed that a subjacent portion of cortex lining the lower bank of the superior temporal sulcus was also at subnormal temperature (about 30°C). Testing. Behavioral testing was resumed 2 weeks at the earliest after implantation of the two probes. After a few testing sessions with coolers attached but inoperative, the experiments were initiated. The animals were subjected to daily testing sessions of 110 trials each: 10 SMS trials, and 10 trials each of DM and DR, with each offive delays (1,4,8,16, and 32 s). The trials of any given task and delay were administered in blocks of 10 trials in counterbalanced order of tasks and delays. In the first 5 trials of each block, the reaction time to one stimulus (sample/cue or choice lights) was recorded; in the second 5 trials, to the other. Cooling sessions alternated with noncooling sessions. The animals were tested in three IT cooling conditions: bilateral 2O”C, right side 20°C) and left side 20°C. During a given cooling session, the entire behavioral testing was conducted in one of these conditions. Electrooculogram. The potentials from two pairs of periorbital electrodes, one recording horizontal and the other vertical changes of eye position, were led through DC amplifiers to the two axes of an oscilloscope and concomitantly recorded on FM magnetic tape. The oscilloscope displayed a continuous vectogram of eye movements. The taped electrooculograms were graphically displayed by means of a chart recorder.
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DELAYED MATCHING-TO-SAMPLE
403 DELAY ED RESPONSE
FIG. 2. Average measures of performance-from aU subjects-at normal temperature and during bilateral cooling of the inferotemporal cortex. Delayed matching to sample data are plotted at left and delayed response data at right; simultaneous matching to sample far left (in DMS graphs). Top-percentage of correct responses, middle-sample/cue reaction time, bottom-choice reaction time. Delay time in seconds is on the abscissa.
RESULTS Correctness
The pooled correct-response data from bilateral IT cooling and noncooling sessions are displayed in Fig. 2 (upper graphs), where mean percentage is plotted for each task, delay, and temperature condition. It is
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evident from the graphs that cooling induced a marked deficit in performance of DMS at all delays. Minor deficits in SMS (two isolated points in the left graph) and in DR are also apparent. Mean correctness values for lo-trial blocks of DMS and DR were subjected to arcsine transformation-because of heterogeneity of variance-and to a complete 4 (animal) x 2 (task) x 2 (temperature) x 5 (delay) factorial analysis of variance: the last three terms were treated as repeated-measure variables. Some results of the analysis are shown in Table 1. The main effects for task, temperature, and delay are highly significant. The task x temperature interaction is also significant. Comparisons of overall means-Tukey’s (a) test with P < 0.01 was used for these and subsequent comparisons-showed that cooling had no significant effect on the DR at any delay, whereas it produced a significant deficit in the DMS at all delays. Such comparisons also showed that correctness levels in the two tasks did not differ at normal temperature and that the task main effect was principally due to differential effects of cooling on the tasks. In the absence of a significant interaction between temperature and delay, the DMS data were combined in two groups-cooling and noncooling-without regard for delay. An analysis of variance was then performed on DMS and SMS correctness data: 4 (animal) x 2 (task) x 2 (temperature), the last two terms as repeated-measure variables. This analysis showed significant main effects for task and temperature. In addition, it showed a significant interaction between task and temperature, indicating that cooling caused a larger deficit in the DMS than in the SMS. TABLE Extract
from
Analysis of Variance of Delayed Matching and Delayed Response Data Correctness
Source
1
df
of variance
Animal Task Delay Temperature Animal x temperature Task x delay Task x temperature Delay x temperature ” RT-Reaction time. * P < 0.05. ** P < 0.01,
3. 1, 4, 1, 3, 4, 1, 4.
44 44 176 44 44 176 44 176
*** P < 0.001.
(F) 3.7* 128.9*** 155.4*** 97.4*** 2.0 1.0 61.8*** 1.1
Sample/cue
(F) 43.1*** 7.1* 16.2*** 53.1*** 3.2* 0.4 1.2 1.1
to Sample
RT”
Choice
RT”
(F) 96.4*** 33.6*** 127.1*** 81.1*** 12.0*** 1.8 15.4*** 1.0
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TABLE 2 Extract from Analysis of Variance of Delayed and Simultaneous Matching to Sample Data Correctness Source of variance
df
(F)
Animal Task Temperature Animal x temperature Task x temperature
3,44 1, 44 1, 44 3, 44 1, 44
15.0*** 113.5*** 87.5*** 7.9*x* 11.0**
Choice RT”
CF.1 194.4*** 190.4*** 89.5*** 4.7** 5.3*
a RT-reaction time. * P < 0.05, ** P < 0.01, *** P < 0.001.
After completion of these experiments, all animals became subjects of another study (cooling on single-unit activity). That allowed replication of the IT cooling effects on many occasions and during a long period. Thus, the fully reversible DMS deficit was repeatedly elicited for several months in all animals. The deficit can be reliably reproduced in the two animals presently being used in that study. Reaction Time
The middle graphs in Fig. 2 indicate a prolongation of reaction time to both the DMS sample and the DR cue under IT cooling. Measures of sample/cue reaction time were submitted to square-root transformation and to analysis of variance in a similar manner as correctness data. This analysis (Table 1) revealed significant main effects for temperature, task, and delay. Mean comparisons showed that cooling significantly lengthened sample/cue time in both the DMS and DR tests (sample reaction time was not measured in the SMS test). Choice reaction time was also longer, in the three tasks, under IT cooling (Fig. 2, lower graphs). Analysis of variance (on transformed data) showed significant main effects for temperature, task, and delay (Tables 1 and 2). There were significant temperature x task interactions in the DMS-DR pairing (Table 1) and in the SMS-DMS pairing (Table 2). Mean comparisons showed that the prolongations of choice reaction time during cooling were highly significant in all tasks and after all delays, and that the task main effect for the DMS-DR tests was attributable to the differential cooling effect. Ocular Motility
Under normal conditions a monkey fixates its gaze on the stimulus button immediately after onset of the sample in a DMS trial (Fig. 3,
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“normal”). Fixation is usually maintained until the stimulus disappears, and then persists beyond that moment for 0.5 s or longer. During the delay, the gaze ordinarily wanders to and fro between buttons in more or less regular oscillation. As soon as the two lower buttons are illuminated for choice, one or two rapid shifts of eye direction between the two buttons can be commonly observed before the animal makes its choice. Under IT cooling, fixation on the sample button was generally more laggard and less stable (Fig. 3, “cool”). Ocular motility during the delay became fitful and its range increased. Before the instrumental choice, the animal showed more alternating movements of the eyes between right and left button than in the noncooling condition. This was particularly evident after long delays. Unilateral
Cooling
Unilateral IT cooling induced a considerably smaller deficit in DMS correctness, at all delays, than that induced by bilateral IT cooling. A slight deficit in SMS was also observed. Reaction times were slightly prolonged in the three tasks, but generally not as much as during bilateral cooling. These differences were particularly obvious in choice reaction time. No clear and systematic differences were observed between the effects of right and left cooling, with one exception: sample/cue reaction time was longer under right than left cooling. No such asymmetry of cooling effects was evident in choice reaction time. DISCUSSION Since IT cooling induced only a small deficit in the SMS task, we may conclude that visual perception and discrimination were relatively unimpaired by the procedure. However, cooling induced a significantly larger deficit in DMS performance, indicating an impairment of the kind of short-term memory that is required in a visual delay task. A similar effect was previously obtained by prefrontal cooling, but not by parietal cooling (1,9, 10). The similarity of cooling effects on the DMS task suggests that the prefrontal and IT cortices are jointly involved in provisional memory of visual information. Reciprocal connections between the two cortical regions (4, 12, 13, 16, 17) may be part of the anatomic substrate for such a joint function. The cooling effect on choice reaction time provides additional evidence for involvement of the IT cortex in short-term memory processes. During cooling, the animals took longer to make the choice at the end of the retention period. A comparable lengthening of reaction time was observed as a result of IT ablation (19). The higher incidence of shifts of gaze between the choice stimuli, noted by us with cooling, may simply reflect the greater
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difficulty of the monkey to remember the correct cue and, consequently, to decide. It is unclear why IT cooling affected correct performance of one visual memory task--MS-and not the other-DR. This dissociation may be due to one important difference between tasks: whereas DMS can only be correctly performed by use of visual memory, DR is a spatial task that could be performed by nonvisual (e.g., proprioceptive) memory. Thus, in DR, the animal during IT cooling might overcome a visual memory deficit by resorting to a nonvisual strategy. Nevertheless, the longer cue and choice reaction times suggest that the DR also presents some difficulty for the IT-cooled animal. The deficit may be overcome not only by use of a nonvisual strategy but by more self-imposed exposure to the cue and by longer deliberation at the choice. Some investigators (5, 15, 21), on the basis of ablation results, have concluded that the IT cortex is functionally distinguishable from the cortex lying directly behind it and around the inferior occipital sulcus (“posterior inferotemporal” and “prestriate” cortex). Posterior cortex is deemed essential for neural processes related to visual attention, perception, and discrimination, whereas the anterior cortex-inferotemporal cortex proper (5,7)-would be somehow involved in visual memory (5,15,18,19, 21,22). The present results of reversible lesion support a role of the anterior cortex in provisional memory of visual stimuli. Other researchers (8, 14), delivering a disruptive electrical stimulus to the IT cortex at various times during the DMS trial, noted that performance was maximally impaired when that stimulus was given during the matching period. They concluded that the IT cortex plays a critical role in information retrieval. Our results are consistent with this conclusion. However, both cooling and stimulation (14) data are also consistent with a role of the IT cortex in retention of visual information. In fact, all three principal component operations of provisional memory-encoding, retention, and retrieval-may be to some degree compromised in dysfunction of the IT cortex. A disruption of visual encoding processes, attention in particular, is suggested by stimulation results (8, 14) and also, by our cooling results (effects on SMS performance and eye motility during sample). Both methods, however, reveal a relatively greater disruption of the mnemonic processes of retention and retrieval. It is a goal for future research to clarify the relationship between the role of the IT cortex in provisional memory and its demonstrated importance for the learning of discriminations. One reasonable hypothesis is that the formation of long-term visual memory depends on the same mechanisms for encoding, temporarily retaining, and retrieving visual information that operate during a visual delay-task trial and for which the functional integrity of the IT cortex appears essential.
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