Spatial Learning and Memory as a Function of Age in the Dog

Copyright 1995 by the American Psychological Association, Inc. 0735-7044/95/S3.00

Behavioral Neuroscience 1995, Vol. 109, No. 5, 851-8.

Spatial Learning and Memory as a Function of Age in the Dog E. Head, R. Mehta, J. Hartley, and M. Kameka University of Toronto

W. W. Ruehl

B. J. Cummings and C. W. Cotman

Deprenyl Animal Health, Inc.

University of California, Irvine

This document is copyrighted by the American Psychological Association or one of its allied publishers. This article is intended solely for the personal use of the individual user and is not to be disseminated broadly.

N. W. Milgram University of Toronto Spatial learning and memory were studied in dogs of varying ages and sources. Compared to young dogs, a significantly higher proportion of aged dogs could not acquire a spatial delayed nonmatching-to-sample task. A regression analysis revealed a significant age etfect during acquisition. Spatial memory was studied by comparing performance at delay intervals of 20,70, and 110 s. At short delays aged and young dogs were similar; at longer delays, errors increased to a greater extent in old than in young dogs; however this was not statistically significant. It was possible to identify 2 groups of aged animals, age-impaired and age-unimpaired. Several of the dogs were also tested on an object recognition memory task, which was more difficult to learn than the spatial task. The possibility that these findings are confounded by breed differences is considered. Overall, the present results provide further evidence of the value of a canine model of aging-

Milgram et al.'s (1994) study examined object recognition memory because this had been studied before in nonhuman primates. Aged monkeys show deficits both in acquisition (Moss, Rosene, & Peters, 1988; Presty et al., 1987; Rapp & Amaral, 1989) and in performance when the memory demands are high (Arnsten & Goldman-Rakic, 1985; Bartus, Fleming, & Johnson, 1978; Moss et al., 1988; Presty et al., 1987; Walker et al., 1988). In Milgram et al.'s study, although there were age differences in acquisition of an object recognition memory task, but they were unable to demonstrate additional age differences with increasing memory demands. The task was simply too difficult; only 1 of 7 aged dogs and 5 of 8 young dogs were able to meet the acquisition criterion within 400 trials. In order to further understand cognitive deterioration during aging in the dog, we have developed a spatial version of a nonmatching-to-sample paradigm. The task is analogous to the delayed-response (DR) task commonly used in primates except that the DR task uses a matching-to-sample procedure. Spatial learning and memory, with the DR methodology, have been measured in pups (Fox, 1971) and in adult dogs (Konorski, 1961) and there is evidence showing the dog is able to retain spatial information for up to a 5-min delay (Hunter, 1913; Walton, 1915). Another reason for studying spatial learning and memory is because they are sensitive to age in several mammalian species, including humans. Aged humans, patients with Parkinson's disease, and patients with Alzheimers' disease have all been found to be impaired on DR tests (Freedman & OscarBerman, 1986; Irel, Kesler, Markowitsch, & Hofmann, 1987; Oscar-Berman & Bonner, 1985; Oscar-Berman, Hutner, & Bonner, 1992) and on other tests of spatial memory (Blennow, Wallin, & Gottfries, 1991; Craik & Jennings, 1992; Craik, Morris, & Gick, 1990; Freedman & Oscar-Berman, 1989;

Several neuropathological changes found in both aged humans and in patients suffering from Alzheimer's disease can also be found in aged dogs (Cummings, Honsberger, et al., 1993; Cummings, Su, Cotman, White, & Russel, 1993; Dayan, 1971; Ferrer et al., 1993; Giaccone et al., 1990; Osmand & Switzer, 1992; Suzuki, Akiyama, & Suu, 1978; Mervis, 1978; Wisniewski, Johnson, Raine, Kay, & Terry, 1970; Wisniewski et al., 1990; Wisniewski & Terry, 1973). It is widely believed that dogs also show age-dependent cognitive impairments (Mosier, 1989). However, the only experimental evidence of this is Milgram, Head, Weiner, and Thomas's (1994) work, which showed impairment in aged dogs in both object reversal learning and object recognition memory. These studies suggest that the dog could serve as an animal model of aging and is well suited for studies of neurobiological correlates of agedependent cognitive dysfunction. E. Head, R. Mehta, J. Hartley, M. Kameka, and N. W. Milgram, Division of Life Sciences, University of Toronto, Scarborough, Ontario, Canada; B. J. Cummings and C. W. Cotman, Irvine Research Unit in Brain Aging, University of California, Irvine; W. W. Ruehl, Deprenyl Animal Health, Inc., Overland Park, Kansas. We would like to thank B. Moloo, R. Renlund, and Karen Parisien at the Division of Comparative Medicine, University of Toronto, for their excellent care of the dogs. We are also grateful to Dina Brooks and E. A. Phillipson for their input and for providing us with subjects. Funding for this research was provided by Deprenyl Animal Health of Overland Park, Kansas; Medical Research Council of Canada (Operating Grant MT-4606); and Natural Sciences and Engineering Research Council of Canada Grant A7659. Correspondence concerning this article should be addressed to N. W. Milgram, University of Toronto, Scarborough Campus, Division of Life Sciences, 1265 Military Trail, Scarborough, Ontario, Canada MIC 1A4. Electronic mail may be sent via Internet to milgram@psych. toronto.edu.

851

This document is copyrighted by the American Psychological Association or one of its allied publishers. This article is intended solely for the personal use of the individual user and is not to be disseminated broadly.

852

HEAD ET AL.

Mildworf, Globus, & Melamed, 1986; Salthouse, 1992; Verma, Yusko, Beranek-McClung, & Williams, 1991). Both aged rats (Barnes, 1988; Barnes & McNaughten, 1980; Campbell, Krauter, & Wallace, 1980; de Toledo-Morrell, Morrell, & Fleming, 1984; Gage, Dunnett, & Bjorklund, 1984,1989; Gage, Kelly, & Bjorklund, 1984; Gallagher & Burwell, 1989; Gold & McGaugh, 1975; Stewart, Mitchell, & Kalant, 1989; Willig et al., 1987) and nonhuman primates (Arnsten & GoldmanRakic, 1985; Bachevalier, 1993; Bachevalier et al., 1991; Bartus, Fleming, & Johnson, 1978; Dean & Bartus, 1988; Rapp & Amaral, 1989; Riopelle & Rogers, 1965; Voytko, 1993; Walker et al., 1988) are deficient in their ability to learn and recall spatial information. Indeed, deterioration in spatial memory ability in the monkey occurs earlier than deterioration in object-recognition memory (Bachevalier et al.; Walker et

al.). Age-dependent deficits in spatial learning in the dog would also be consistent with evidence of neuropathology in both frontal cortex and hippocampus (e.g., Cummings, Honsberger, et al., 1993). Work with dogs (Konorski, 1961; Lawicka & Konorski, 1959; Stasiak & Lawicka, 1987, 1989), nonhuman primates (Fuster, 1989; Goldman & Rosvold, 1970; Mishkin & Pribram, 1956; Mishkin, Vest, Waxier, & Rosvold, 1969) and humans (Freedman & Oscar-Berman, 1986) demonstrates that spatial memory is impaired by dysfunction in prefrontal cortex. The hippocampus also plays a role in spatial learning and memory both in rats (e.g., de Toledo-Morrell et al., 1984; Morris, Garraud, Rawlins, & O'Keefe, 1982; O'Keefe & Nadel, 1978) and monkeys (e.g., Dean & Bartus, 1988; Zola-Morgan & Squire, 1985). Another component of the spatial learning network is the posterior parietal cortex (e.g., Van Essen, Anderson, & Felleman, 1992). However, we know of no studies that have looked at this region in the dog. In the present experiment, we examined both spatial learning and memory in dogs of varying ages. Dogs were initially trained to perform a spatial version of a delayed nonmatchingto-sample task with a 10-s delay. Dogs that were able to learn to respond accurately at a 30-s delay were then tested with a variable-delay paradigm in order to assess the effect of age on performance with increased memory demands. Method The subjects included 8 aged (ranging from 88 to 148 months), purebred beagles of both sexes (Marshall Farms, North Rose, NY), which were obtained as retired breeders. Two middle-aged beagles aged 88 and 92 months were also tested. Thirteen mongrel dogs obtained from Ontario pounds included 5 young dogs (8 months-2.5 years), 3 middle-aged dogs (5-8 years) and 5 aged dogs (> 10 years). The chronological age of the pound-source dogs was based on the mean of three estimates made by the supervisor of the facility and two veterinarians from teeth markings (Kirk & Bistner, 1975) at the start of the study. The cutoff age of 5 years for the young group corresponds roughly to the time when age-dependent decreases in brain metabolism are first observed in beagles (London, Ohata, Takei, French, & Rapoport, 1983). The cutoff point between the middle-aged and old groups was based partly on analogy with humans and was intended to correspond to the approximate time point for the appearance of age-associated memory impairment (McEntee & Crook, 1990). Additionally, reports of amyloid plaques in dogs have all involved dogs older than 8 years (e.g., Giaccone et al., 1990).

The previous backgrounds of the pound-source dogs was unknown, but given the ease of handling and general behavior it was likely that they had all served as companion dogs. An adaptation period of at least 30 days preceded the start of behavioral testing. Because our young dogs in this study were all pound-source dogs, we had to deal with the possibility of breed differences. Therefore, data obtained from 10 young beagles in a previous study were used for the purpose of comparison (Milgram et al., 1994). The dogs were housed individually in pens (1.07 x 1.22 m) with continual access to fresh water through a drinking spout. The humidity was kept at 40-60%, the temperature was kept at 22-24 °C, and the dogs were maintained on a 12-hr light-dark cycle. They were fed approximately 300 g of dog food daily, between 2:30 and 3:30 p.m. On entrance into the facility, all of the dogs were given veterinary examinations and comprehensive testing of blood chemistry. All dogs were determined to be in good health at the start of the studies, and health checks occurred monthly for monitoring purposes. Behavioral testing occurred in the morning and early afternoon, prior to feeding. The dogs were exercised daily in the early morning for approximately 15 min, while their pens were cleaned. Apparatus The test apparatus has been described previously (Milgram et al., 1994); it consisted of a wooden box equipped with a sliding Plexiglas tray containing two laterally placed food wells and a medial food well. Adjustable vertical stainless steel bars at the front of the box allowed the size of the opening to each food well to be individually fixed for each dog. The experimenter was separated visually from the dog by a screen with a one-way mirror and a hinged door at the bottom. The only source of lighting was from a lamp attached to the front of the box, which was projected onto the tray and which assured that the dogs could not see the experimenter when the screen was lowered. The hinged door was opened for the presentation and withdrawal of the food tray. Data acquisition was controlled by a customized program developed in the ASYST programming language, which controlled all timing, randomization procedures, reward locations, and stimulus objects. Just prior to the start of each trial, the computer emitted a tone that served as a cue for the dog and instructed the experimenter to deliver the food tray. Each trial was started when the experimenter pressed a key and simultaneously presented the tray to the subject. The dogs' responses were recorded by presses of a mouse button (left or right), which also indicated the end of the trial and signaled the beginning of the intertrial interval. Lean ground beef rolled into small, 1.5-g balls was used as reward in the spatial learning and memory task used in this study. Other Test Experience The subjects used in this study had previously been tested in reward approach learning, object approach learning, visual discrimination learning, and reversal learning, as described in Milgram et al., 1994. Five of the aged beagles and 3 of the 5 aged pound-source dogs had also been trained on an object recognition memory task (Milgram et al.). Four dogs used in this study (2 old and 2 middle-aged beagles and 1 old pound-source dog) were tested on object-recognition memory task after being tested on the spatial task in this experiment. The methods used were identical to those used in Milgram et al. (1994). Acquisition Each trial started with the sample presentation of a red Lego block covering a food reward in either the left or right well. After the dog

853

This document is copyrighted by the American Psychological Association or one of its allied publishers. This article is intended solely for the personal use of the individual user and is not to be disseminated broadly.

SPATIAL LEARNING AND MEMORY IN YOUNG AND AGED DOGS displaced the object and obtained the reward, the tray was withdrawn for a specified time interval. The test presentation following the delay used identical red plastic Lego blocks covering both the left and right wells with the reward in the well on the side opposite to that of the sample. Training on the spatial task consisted of 10 trials per day, separated by a 1.5-min intertrial interval. During the acquisition phase the dogs were first tested with a 10-s delay. On reaching a criterion of either 9/10 correct in one session or 8/10 correct in two consecutive sessions, they were trained at a 20-s delay, and if they were successful at 20 s, the delay was increased to 30 s. At each delay, dogs were tested for a maximum of 40 daily test sessions. If a dog failed to achieve the criterion at one delay, it was not tested at any longer delays. If a dog was able to achieve the criterion at the 30-s delay interval, it was then placed into a variable-delay paradigm. The testing procedures were identical to those in the acquisition phase except that (a) there were 12 trials per day, and (b) the delay on each trial could be 20, 70, or 110 s. Each of the three delays occurred four times per test session in a quasi-random order. Dogs were tested for 480 trials.

BOO -r

8»

O 48O 0> 360 -

2*0 0)

*

0 120 , u (£ fc

_*

* ! *** I Young Middle

n -

0 0

oBj

Old

B T

B o

*o

6 --

»

«••

Data Analysis Statistical Analysis System (SAS) was used to calculate one-way and two-way analyses of variance (ANOVAs), repeated-measures ANOVA, regression and correlation analyses, t tests, and chi-square. Post hoc tests were carried out using the Scheffe method with a .05 level of significance. If data did not satisfy the assumption of homogeneity of variance, they were submitted to a log transformation. A spatial acquisition score was calculated by combining the total errors made at each of the three delays. Dogs that were not tested on 20 or 30-s delays because of a failure to meet criterion at a lower delay were assigned a score of 200 for each delay with which they were not tested. This procedure was decided on an a priori basis and was based on the assumption that if a dog was unable to achieve criterion at any given delay, it would also be unable to achieve criterion at a longer delay. Evaluations of group comparisons were done in two ways: (a) comparing all of the aged with the middle-aged and young dogs, and (b) restricting the comparisons to pound-source dogs only. The second procedure was intended to control for possible confounding effects of source (beagles vs. pound source).

Results Acquisition of Spatial Memory Task The acquisition scores of this test in all three age groups were variable, as shown in Figure 1A. Among the aged animals, 2/5 pound-source and 3/8 beagles did not achieve the criterion at a 30-s delay. Of these, the 2 aged pound-source dogs and 1 of the aged beagles did not learn when tested with a delay of 10 s, 1 aged beagle was unsuccessful at a 20-s delay after learning the task with a 10-s delay, and the last aged beagle was unsuccessful at the 30-s delay. All of the young and middle-aged dogs learned the task with a 30-s delay. A chi-square test was used to determine whether the probability of failure was greater in old dogs when compared to the young. The result was found to be highly significant, x 2 0> N = 18) = 1.1,p < .01. Aged pound-source dogs did not differ significantly from the aged beagles in acquisition, t(\\) = 0.3679 p < .72, and subsequently the data from the two groups were combined in an ANOVA based on log-transformed acquisition scores. No significant main effect of age was found, F(2, 20) = 1.98 p
4-1 •d 6 O

2 -r=.54

40

irj

life

lib

Chronological Age (tenths) Figure 1. A: The individual scores and mean number of errors to criterion (as indicated by histogram) on spatial acquisition show that the aged animals as a group performed more poorly and showed greater individual variability than young and middle-aged animals. B: There is a significant linear relationship between the error scores during acquisition and the chronological age of the dog. Pound-source dogs are indicated by asterisks and beagle dogs by circles.

.1637, although the aged dogs committed more errors, as shown in Figure 1A. Figure 1A also shows one outlier, Whiskey, in the young group. This dog was the youngest of all the dogs at 8 months and was easily distracted from performing the task. When this young animal was removed from the analysis, the age effect approached significance, F(2, 19) = 3.04/? < .0718. An analysis using only pound-source dogs also yielded nonsignificant results, F(2,10) = 1.17 p < .3490. Regression analysis of the acquisition scores using chronological age as a dependent measure indicated a linear relationship between chronological age and spatial acquisition, F(l, 21) = 7.705/7 < .0113, Figure IB;F(l, 20) = 14.548,p < .0011, with Whiskey removed. Removal of the beagles from the analysis gave similar results, F(l, 10) = l.llOp < .0196, with Whiskey removed. An analysis of the error scores during the initial 10-s delay training also indicated that the effect of age was significant, F(l, 20) = 7.266/> < .0139, Whiskey removed.

Variable Delay Test Only dogs reaching criterion at the 30-s delay during acquisition were tested on the variable delay task; this included

854

HEAD ET AL.

100 -r-

Young Dogs —

—

—

-Middle Aged Dogs Old Dogs

90 - -

This document is copyrighted by the American Psychological Association or one of its allied publishers. This article is intended solely for the personal use of the individual user and is not to be disseminated broadly.

80 --

01

u 70 --

60 - -

50

20

70

.5931, whereas accuracy was significantly affected by age at 70 s, F(l, 14) = 5.697, p < .0317, and at 110 s, F(l, 14) = 7.296, p < .0172 (see Figure 3). If the beagles are removed from the analysis, the effects at the 20-s, F( 1,9) < 0.246, p < .6319, and at the 110-s delay, F(l, 9) = 8.034, p < .0196, are similar; however, the age effect on performance at the 70-s delay was not significant, F(l, 9) = 2.538, p < .1456. Although there was no significant age effect, we could not rule out the existence of a subpopulation of aged dogs being deficient at long delays. To test this possibility, we used procedures described by Rapp and Amaral (1991) to separate the aged animals into impaired and unimpaired groups. The criterion for impairment was an average percentage correct across all delays of less than 77%. We based this on the data from the young dogs, which showed a range of 77% to 90% average correct. This procedure led to 7 middle-aged and old dogs being classified as impaired and 4 being classified as unimpaired. Among the aged dogs, the impaired group included 2 beagles and 2 pound-source dogs; 2 beagles and 1 pound-source dog were classified as unimpaired. Of the middle-aged dogs only 1 dog, a pound-source dog, was not impaired. The data from the middle-aged and old dogs were combined because middle-aged impaired were not significantly different from aged impaired dogs, /(4) = 1.6241,/? < .18, and middle-aged unimpaired were no different from aged unimpaired dogs, t(3) = 1.4757, p < .24. As expected, an overall

11O

Delay Interval (seconds) 100

Figure 2. The accuracy of young, middle-aged, and old dogs as a function of increasing delay interval on a spatial delayed nonmatchingto-sample task. At the two longer delay intervals, middle-aged and old dogs perform more poorly than young dogs. Bars indicate standard errors of the mean.

-T-

90 -80 -70 --

60 -50 - —

16 dogs. Five of the young, 4 of the middle-aged, and 3 of the 5 aged pound-source dogs were tested. The beagles tested on the variable-delay task included one of the middle-aged dogs and 4 of the 8 aged beagles. One middle-aged beagle was tested with different delays in a pilot study, and the results are not reported here. An independent t test of the average performance on all delays revealed no significant differences, t(5) = 0.3782, p < .7208, between aged pound-source dogs and beagles, and the following analysis was based on the two groups combined. Increasing the delay interval resulted in a significant decrease in accuracy, F(2, 26) = 29.74, p < .0001. A repeated measures ANOVA revealed a close to significant age effect, F(2, 13) = 3.12, p < .0782, but no significant Age x Delay interaction, F(4, 26) = .66, p < .6223, as shown in Figure 2. If the beagles are removed from the analysis, the effect of age is smaller, F(2, 8) = 2.24, p < .1687, with the effect of delay, F(2,16) = 30.46, p < .0001, and the Age x Delay interaction, F(4, 16) = 0.33, p < .8535, being similar. A regression analysis of the effect of chronological age on performance at each separate delay showed that the correlation depended on delay interval; higher correlations were obtained at the longer delays. Thus, at the 20-s delay there was no effect of age, F(l, 14) = 0.299, p
< .0002, and a significant effect of delay, F(2,26) = 34.77,p < .0001. As shown in Figure 4, groupwise comparisons indicated that impaired aged were significantly different from both young and aged unimpaired dogs, and the latter two groups did not differ from each other. When the data from the aged unimpaired and young dogs were combined and compared to the aged impaired dogs, the Age x Delay interaction approached significance, F(2, 28) = 2.71, p < .08. Post hoc analyses indicated that aged impaired dogs were significantly different from young and aged unimpaired dogs at delays of 70 and 110s, but not at 20 s. Comparison of Spatial Memory With Object-Recognition Memory Twelve of the dogs used in this study were also tested on an object-recognition memory task (Milgram et al, 1994). A paired / test revealed that the dogs reached criterion on the initial spatial memory task at a 10-s delay with fewer errors (M = 86, SD = 75.9) than on the object recognition memory task (M = 155.7, SD = 49.7), f(ll) = 3.88, p < .003. If the pound-source dogs are excluded from the analysis, removing any possible effect of breed, the effect becomes larger, r(8) = 100

—

-r

Young Dogs Impaired Aged Dogs

— — — Unimpaired Aged Dogs 90 --

g 80

5

£

01 70 --

£

60 --

50

2O

7O

110

Delay Interval (seconds) Figure 4. Accuracy scores of age-impaired, age-unimpaired, and young dogs on the variable delay procedure for the three delay intervals of 20, 70, and 110 s.

855

4.41, p < .003. The above analysis was only done with old and middle-aged dogs because none of the young dogs were tested on both tasks. However, in our previous study a group of 9 young beagles were tested on the object-recognition memory task (M = 237.1, SD - 353). When these scores were compared with the spatial acquisition scores of the young dogs used in this study (M = 29, SD = 43.2), an independent t test using log-transformed data indicated a highly significantly difference, t(l2) = 3.54, p < .004. This comparison, unfortunately, is confounded by breed and source (young beagles were tested on object recognition, whereas young pound-source dogs were tested on the spatial task). Discussion The results of the present study reveal first that spatial learning and memory are sensitive to age in the dog. When compared to young dogs, aged dogs showed slower acquisition of a nonmatching version of a delayed response task. The aged group also showed considerable individual variability, and the standard ANOVA did not achieve statistical significance. A regression analysis, however, did show significant age effects in both acquisition and in performance at long delay intervals. Individual variability in aged subjects is not unexpected and is seen in aged humans (Albert, 1993), monkeys (Rapp, 1993; Rapp & Amaral, 1992), and rats (Gage, Kelly, & Bjorklund, 1984; Gallagher, 1993). We also observed individual differences in our young group. In particular, one dog (the youngest) had attentional difficulties, which led to his performing more poorly than his peers. Poorer performance of very young dogs relative to adults in a spatial delayed-response task has been reported previously (Fox, 1971). With respect to the increased variability seen in the aged dogs, factors such as neuropathology (e.g., beta-amyloid accumulation, cell loss, or synaptic loss) may better account for individual differences than chronological age (Cummings et al., 1995). Aged dogs are impaired during acquisition of a spatialmemory task and this is consistent with the nonhuman primate literature. Aged monkeys are reported to show impaired acquisition of a delayed response task with a 5-s delay (Bachevalier et al., 1991; Presty et al., 1987; Walker et al., 1988). On the other hand, aged monkeys (Bartus et al., 1978; Rapp & Amaral, 1989) and rats (Dunnett, Evendon, & Iversen, 1988) are not impaired on a delayed-response task when the delay is very short (less than 1 s). Therefore, deficient performance during spatial acquisition in aged animals is dependent on delay, which suggests that we observed a deficit in acquisition in dogs, because the initial testing was at a 10-s delay. We found evidence of age differences in performance at long delays; as we increased delay intervals up to 70 and 110 s in the variable delay condition, young dogs were able to maintain a high degree of accuracy even at the longest delay, whereas middle-aged and old dogs performed more poorly. However, the differences were small and were not detected by a repeated measures ANOVA. In interpreting the absence of a significant effect, it should be kept in mind that the sample of aged dogs used was selective, being limited to those dogs that were able to solve the problem with a 30-s delay. Our findings with dogs parallel results obtained from aged

This document is copyrighted by the American Psychological Association or one of its allied publishers. This article is intended solely for the personal use of the individual user and is not to be disseminated broadly.

856

HEAD ET AL.

monkeys with variable delay procedures. Some studies find an Age x Delay interaction. For example, old monkeys are more severely affected by increasing delays than young monkeys (Bartus et al., 1978; Rapp & Amaral, 1989). Other studies find a trend in this direction but the interaction is not significant (Bachevalieret al., 1991; Walker et al., 1988). Such inconsistencies may be a result of which delay intervals are included in the testing procedures; had we included a 0-s delay, it is more likely that we would have found a statistically significant interaction (Bachevalier et al.; Walker et al.). The large amount of individual variability in accuracy within all age groups would also contribute to a lack of statistical significance and possibly, with a larger sample size, we would be able to demonstrate a statistically significant effect in the dog. The existence of greater variability in the aged group when compared to young and middle-aged dogs creates problems for statistical analysis; an ANOVA procedure with grouped animals is sensitive to variability and less likely to yield significant results when the variability is large. We dealt with this problem in two ways: by using regression analysis and by comparing impaired with unimpaired groups. The regression analysis took into consideration differences in chronological age within the different age groups. The separation of impaired and unimpaired groups, on the other hand, took into account individual differences in performance. The neuroanatomical basis of spatial and object recognition memory has been extensively studied, and separate anatomical systems have been described for each (e.g., Van Essen et al., 1992). Lesion and electrophysiological studies have implicated the prefrontal cortex in both forms of memory (Wilson, Scalaidhe, & Goldman-Rakic, 1993). Our studies indicate that for the aged dog, object-recognition learning can be dissociated from spatial learning because many of the aged dogs that were unable to learn the former were able to meet criterion on the spatial version of the task. On the other hand, 3 of our aged dogs were severely deficient in both tasks, which suggests the presence of global cognitive deficiency. The dissociation between object recognition and spatial memory was not unique to our aged dogs; young dogs also performed more poorly on the object recognition memory task. Obviously, for the dog, acquisition of a visual object recognition task is much more difficult than a spatial memory task. The difficulty in learning an object recognition in the dog contrasts with the monkey, which learns quite rapidly. One factor that may contribute to such species differences is differences in visual system organization. At the level of the lateral geniculate nucleus, the dog's visual system differs significantly from the primate's; the dog has one parvocellular layer and three magnocellular layers (Howard & Breazile, 1973; Kaas, Guillery, & Allman, 1972); the primate has three layers of each. The difference in the number of parvocellular layers may be important because the primate parvocellular system is believed to be particularly important for tasks requiring high visual acuity (e.g., Van Essen et al., 1992). One difficulty with this type of research is the existence of breed differences. Milgram et al. (1994) have discussed this issue in some detail in a previous article, in which we reported that pound-source dogs were superior to beagles on two pretraining tasks, reward and object approach learning, which

we regarded as procedural learning tasks. Aged beagles, but not aged pound-source dogs, were impaired on these tasks. We also found breed differences in visual discrimination learning and reversal learning; our aged pound-source dogs performed more poorly than our aged beagles. Because the aged poundsource dogs were, on average, older than the aged beagles, we could not determine whether these effects were due to breed differences, differential rearing environments, or age. In Milgram et al., the young group consisted only of beagle dogs. In the present study, we were able to obtain a sample of young pound-source dogs; we did not have an appropriate young control group for the aged beagles. When we compared the young pound-source dogs used in this study with the young beagles used in Milgram et al., we found only one difference; the pound-source dogs showed faster discrimination learning. We can not rule out, however, the possibility that young beagles will perform differently from young pound-source dogs on our spatial task. We have measured age-dependent cognitive dysfunction in dogs on a variety of behavioral tasks to date, including reward and object approach learning, visual discrimination learning, reversal learning, object recognition memory, and spatial learning and memory. We also have preliminary evidence of a robust age-dependent impairment on olfactory discrimination learning. In the case of many of these tasks, the underlying neuroanatomical substrate is largely understood. Clearly, aged dogs are deficient on tasks measuring both medial temporal lobe and prefrontal cortical functioning, and the extent of the dysfunction varies. Some dogs are impaired on an object recognition memory task, others on a spatial task, and others on both tasks. The latter case is probably indicative of a more global cognitive dysfunction and most likely involves global brain dysfunction. We mentioned previously that age per se was not the only predictor of spatial learning and memory in the dog. We are currently studying neurobiological correlates that would better account for the individual variability that Milgram et al. (1994) observed in their behavioral study and that we observed in the current study. One neuropathological marker of Alzheimer's disease is beta-amyloid, the constituent of senile plaques, and several studies have demonstrated an accumulation in aged dog brain with frontal cortex and hippocampus being most affected (Cummings, Su, et al., 1993; Cummings, Honsberger, et al., 1993). We have recently found that beta amyloid accumulation is highly correlated with individual behavioral deficits (Cummings et al., 1995).

References Albert, M. (1993). Neuropsychological and neurophysiological changes in healthy adult humans across the age range. Neurobiology of Aging, 14, 623-625. Arnsten, A. F. T., & Goldman-Rakic, P. S. (1985). Alpha-adrenergic mechanisms in prefrontal cortex associated with cognitive decline in aged nonhuman primates. Science, 230, 1273-1276. Bachevalier, J. (1993). Behavioral changes in aged rhesus monkeys. Neurobiology of Aging, 14, 619-621. Bachevalier, J., Landis, L. S., Walker, L. C., Brickson, M., Mishkin, M., Price, D. L., & Cork, L. C. (1991). Aged monkeys exhibit behavioral

This document is copyrighted by the American Psychological Association or one of its allied publishers. This article is intended solely for the personal use of the individual user and is not to be disseminated broadly.

SPATIAL LEARNING AND MEMORY IN YOUNG AND AGED DOGS deficits indicative of widespread cerebral dysfunction. Neurobiology of Aging, 12, 99-111. Barnes, C. A. (1988). Selectivity of neurological and mnemonic deficits in aged rats. In T. L. Petit & G. O. Ivy (Eds.), Neumplasticity: A lifespan approach (pp. 235-264). New York: Liss. Barnes, C. A., & McNaughten, B. L. (1980). Spatial memory and hippocampal synaptic plasticity in senescent and middle-aged rats. In D. O. Stein (Ed.), The psychobiology of aging: Problems and perspectives (pp. 253-272). New York: Elsevier/North-Holland. Bartus, R. T., Fleming, D. L., & Johnson, H. R. (1978). Aging in the rhesus monkey: Debilitating effects on short term memory. Journal Of Gerontology, 33, 858-871. Blennow, K., Wallin, A., & Gottfries, C. G. (1991). Heterogeneity of 'probable Alzheimer's disease'. In K. Iqbal, D. R. C. McLachlan, B. Winblad, & H. M. Wisniewski (Eds.), Alzheimer's disease: Basic mechanisms, diagnosis and therapeutic strategies (pp. 21-26). New York: Wiley. Campbell, B. A., Krauter, E. E., & Wallace, J. E. (1980). Animal models of aging: Sensory-motor and cognitive function in the aged rat. In D. O. Stein (Ed.), The psychobiology of aging: Problems and perspectives (pp. 201-226). New York: Elsevier/North-Holland. Craik, F. I. M., & Jennings, J. (1992). Human memory. In F. I. M. Craik & T. A. Salthouse (Eds.), The handbook of aging and cognition (pp. 86-87). Hillsdale, NJ. Erlbaum. Craik, F. I. M., Morris, R. G., & Gick, M. L. (1990). Adult age differences in working memory. In G. Vallar & T. Shallice (Eds.), Neuropsychological impairments of short term memory (pp. 247-267). Cambridge, England: Cambridge University Press. Cummings, B. J., Head, E., Afagh, A. J., Ivy, G. O., Milgram, N. W., & Cotman, C. W. (1995). Beta-amyloid accumulation in the aged canine brain is predicted by cognitive deficits on some tasks but not others. Manuscript in preparation. Cummings, B. J., Honsberger, P. E., Afagh, A. J., Head, E., Ivy, G. O., Milgram, N. W., & Cotman, C. W. (1993). Cognitive function and Alzheimer's like pathology in the aged canine: In Neuropathology. Society for Neuroscience Abstracts, 19, 1046. Cummings, B. J., Su, J. H., Cotman, C. W., White, R., & Russel, M. J. (1993). B-amyloid accumulation in aged canine brain: A model of plaque formation in Alzheimer's disease. Neurobiology of Aging, 14, 547-560. Dayan, A. D. (1971). Comparative neuropathology of ageing. Studies on the brains of 47 species of vertebrates. Brain, 94, 31^*2. Dean, R. L., & Bartus, R. T. (1988). Behavioral models of aging in nonhuman primates. In L. L. Iverson, S. D. Iverson, & S. H. Snyder (Eds.), Handbook of psychopharmacology: Vol 20. Psychopharmacology of the aging nervous system (pp. 325-392). New York: Plenum Press. de Toledo-Morrell, L., Morrell, F., & Fleming, S. (1984). Agedependent deficits in spatial memory are related to impaired hippocampal kindling. Behavioral Neuroscience, 98, 902-907. Dunnett, S. B., Evendon, J. L., & Iversen, S. D. (1988). Delaydependent short term memory deficits in rats. Psychopharmacology, 96, 174-180. Ferrer, I., Pumarola, M., Rivera, R., Zujar, M. J., Cruz-Sanchez, F., & Vidal, A. (1993). Primary central white matter degeneration in old dogs.ActaNeuropathologica, 86, 172-175. Fox, M. W. (1971). Integrative development of brain and behavior in the dog. Chicago: University of Chicago Press. Freedman, M., & Oscar-Herman, M. (1986). Selective delayed response deficits in Parkinson's and Alzheimer's disease. Archives of Neurology, 43, 886-890. Freedman, M., & Oscar-Berman, M. (1989). Spatial and visual learning deficits in Alzheimer's and Parkinson's disease. Brain and Cognition, 11, 114-126. Fuster, J. M. (1989). The prefrontal cortex. Anatomy, physiology and

857

neuropsychology of the frontal lobe (2nd ed., pp. 51-82). New York: Raven Press. Gage, F. H., Dunnett, S. B., & Bjorklund, A. (1984). Spatial learning and motor deficits in aged rats. Neurobiology of Aging, 5, 43^8. Gage, F. H., Dunnett, S. B., & Bjorklund, A. (1989). Age-related impairments in spatial memory are independent of those in sensorimotor skills. Neurobiology of Aging, 10, 347-352. Gage, F. H., Kelly, P. A. T., & Bjorklund, A. (1984). Regional changes in brain glucose metabolism reflect cognitive impairments in aged Tats. Journal of Neuroscience, 4, 2856-2865. Gallagher, M. (1993). Issues in the development of models for cognitive aging across primate and nonprimate species. Neurobiology of Aging, 14, 631-633. Gallagher, M., & Burwell, R. D. (1989). Relationship of age-related decline across several behavioral domains. Neurobiology of Aging, 10, 691-708. Giaccone, G., Verga, L., Finazzi, M., Polio, B., Tagliavini, F., Frangione, B., & Bugiani, O. (1990). Cerebral preamyloid deposits and congophilic angiopathy in aged dogs. Neuroscience Letters, 114, 178-183. Gold, P. E., & McGaugh, J. L. (1975). Changes in learning and memory during aging. In J. M. Ordy & K. R. Brizee (Eds.), Neurobiology of aging: Vol 16. An interdisciplinary life-span approach (pp. 145-148). New York: Plenum Press. Goldman, P. S., & Rosvold, H. E. (1970). Localization of function within the dorsolateral prefrontal cortex of the rhesus monkey. Experimental Neurology, 27, 291-304. Howard, D. R., & Breazile, J. E. (1973). Optic fiber projections to dorsal lateral geniculate nucleus in the dog. American Journal of Veterinary Research, 34, 419—424. Hunter, W. S. (1913). The delayed reaction in animals and children. Behavioral Monographs, 2, 1-86. Irle, E., Kessler, J., Markowitsch, J. H., & Hofmann, W. (1987). Primate learning tasks reveal strong impairments in patients with presenile or senile dementia of the Alzheimer type. Brain and Cognition, 6, 429-449. Kaas, J. H., Guillery, R. W., & Allman, J. M. (1972). Some principles of organization in the dorsal lateral nucleus. Brain, Behavior and Evolution, 6, 253-299. Kirk, R. W., & Bistner, S. I. (1975). Handbook of veterinary procedures and emergency treatment (2nd ed.). Philadelphia: Saunders. Konorski, J. (1961). The physiological approach to the problem of recent memory. In A. Fessard, R. W. Gerard, & J. Konorski (Eds.), Brain mechanisms and learning, a symposium (pp. 115-132). Springfield, IL: Charles C. Thomas. Lawicka, W., & Konorski, J. (1959). The physiological mechanism of delayed reactions: III. The effects of prefrontal ablations on delayed reactions in dogs. Acta Biologica Experimental (Warsaw), 19, 221-231. London, E. D., Ohata, M., Takei, H., French, A. W. M., & Rapoport, I. (1983). Regional cerebral metabolic rate for glucose in beagle dogs of different ages. Neurobiology of Aging, 4, 121-126. McEntee, W. J., & Crook, T. H. (1990). Age-associated memory impairment: A role for catecholamines. Neurology, 40, 526-530. Mervis, R. (1978). Structural alterations in neurons of aged canine neocortex: A Golgi study. Experimental Neurology, 62, 417-432. Mildworf, B., Globus, M., & Melamed, E. (1986). Patterns of cognitive impairment in patients with Alzheimer's disease and Parkinson's disease: Are they similar or different? In A. Fisher, I. Hanin, & C. Lachman (Eds.), Alzheimer's disease and Parkinson's disease— Strategies for research and development (pp. 135-140). New York: Plenum Press. Milgram, N. W., Head, E., Weiner, E., & Thomas, E. (1994). Cognitive functions and aging in the dog: Acquisition of nonspatial visual tasks. Behavioral Neuroscience, 108, 57-68.

This document is copyrighted by the American Psychological Association or one of its allied publishers. This article is intended solely for the personal use of the individual user and is not to be disseminated broadly.

858

HEAD ET AL.

Mishkin, M., & Pribram, K. H. (1956). Analysis of the effects of frontal lesions in monkey: II. Variations of delayed response. Journal of Comparative and Physiological Psychology, 49, 36-40. Mishkin, M., Vest, B., Waxier, M., & Rosvold, H. E. (1969). A re-examination of the effects of frontal lesions on object alternation. Neuropsychologia, 7, 357-363. Morris, R. G. M., Garraud, P., Rawlins, J. W. P., & O'Keefe, J. (1982). Place navigation impaired in rats with hippocampal lesions. Nature, 297, 681-683. Mosier, J. E. (1989). Effects of aging on body systems in the dog. Veterinary clinics of north america: Small animal practice, 19, 1-12. Moss, M. B., Rosene, D. L., & Peters, A. (1988). Effects of aging on visual recognition memory in the rhesus monkey. Neurobiology of Aging, 9, 495-502. O'Keefe, J., & L. Nadel (1978). The hippocampus as a cognitive map. London: Clarendon Press. Oscar-Berman, M., & Bonner, R. T. (1985). Matching- and delayed matching-to-sample performance as measures of visual processing, selective attention, and memory in aging and alcoholic individuals. Neuropsychologia, 23, 639-651. Oscar-Berman, M., Hutner, N., & Bonner, R. T. (1992). Visual and auditory spatial and nonspatial delayed-response performance by Korsakoff and non-Korsakoff alcoholic and aging individuals. Behavioral Neuroscience, 160, 613-622. Osmand, A. P., & R. C. Switzer (1992). Diffuse plaques and cerebrovascular amyloidosis are accompanied by entorhinal perineural and hippocampal terminal zone deposition of amyloid in the aging dog. Evidence for a neuronal origin of amyloid. Neurobiology of Aging, 13 (Suppl. 1), p. S104. Presty, S. K., Bachevalier, J., Walker, L. C., Struble, R. G., Price, D. L., Mishkin, M., & Cork, L. C. (1987). Age differences in recognition memory of the rhesus monkey (Macaca mulatto). Neurobiology of Aging, 8, 435-440. Rapp, P. R. (1993). Neuropsychological analysis of learning and memory in aged nonhuman primates. Neurobiology of Aging, 14, 627-629. Rapp, P. R., & Amaral, D. G. (1989). Evidence for task-dependent memory dysfunction in the aged monkey. Journal of Neuroscience, 9, 3568-3576. Rapp, P. R., & Amaral, D. G. (1991). Recognition memory deficits in a subpopulation of aged monkeys resemble the effects of medial temporal lobe damage. Neurobiology of Aging, 12, 481^86. Rapp, P. R., & Amaral, D. G. (1992). Individual differences in the cognitive and neurobiological consequences of normal aging. Trends in Neuroscience, 15, 340-345. Riopelle, A. J., & Rogers, C. M. (1965). Age changes in chimpanzees. In A. M. Schrier, H. F. Harlow, & F. Stollnitz (Eds.), Behavior of nonhuman primates (Vol. 2, pp. 449-462). New York: Academic Press. Salthouse, T. A. (1992). Reasoning and spatial abilities. In F. I. M. Craik & T. A. Salthouse (Eds.), The handbook of aging and cognition (pp. 167-213). Hillsdale, NJ: Erlbaum. Stasiak, M., & Lawicka, W. (1987). Proreal lesion effects in dogs

trained using Konorski short-term memory test with auditory location cues.^cra Physiologogica Polonica, 38, 111. Stasiak, M., & Lawicka, W. (1989). Behavioral recovery on a spatial variant of the Konorski test following prefrontal damage in dogs. Acta Neurobiologica Experimentalis (Warsaw) 50, 201-212. Stewart, J., Mitchell, J., & Kalant, N. (1989). The effects of life-long food restriction on spatial memory in young and aged Fischer 344 rats measured in the eight-arm radial and the Morris water mazes. Neurobiology of Aging, 10, 669-675. Suzuki, Y., Akiyama, K., & Suu, S. (1978). Lafora-like inclusion bodies in the CNS of aged dogs. Acta Neuropathologica (Berl), 44, 217-222. Van Essen, D. C., Anderson, C. H., & Felleman, D. J. (1992). Information processing in the primate visual system: An integrated systems perspective. Science, 255, 419-423. Verma, N. P., Yusko, M. J., Beranek-McClung, B. J., & Williams, L. A. (1991). Multidisciplinary validation of two dementia categories. In K. Iqbal, D. R. C. McLachlan, B. Winblad, & H. M. Wisniewski (Eds.), Alzheimer's disease: Basic mechanisms, diagnosis and therapeutic strategies (pp. 13-19). New York: Wiley. Voytko, M. L. (1993). Cognitive changes during normal aging in monkeys assessed with an automated test apparatus. Neurobiology of Aging, 14, 643-644. Walker, L. C, Kitt, C. A., Struble, R. J., Wagster, M. V., Price, D. L., & Cork, L. C. (1988). The neural basis of memory decline in aged monkeys. Neurobiology of Aging, 9, 657-666. Walton, A. C. (1915). The influence of diverting stimuli during delayed reaction in dogs. Journal of Animal Behavior, 5, 259-291. Willig, F., Palacios, A., Monmaur, P., M'Harzi, M., Laurent, J., & Delacour, J. (1987). Short-term memory, exploration and locomotor activity in aged rats. Neurobiology of Aging, 8, 393^02. Wilson, F. A. W., O. Scalaidhe, S. P., & Goldman-Rakic, P. S. (1993). Dissociation of object and spatial processing domains in the prefrontal cortex. Science, 260, 1955-1958. Wisniewski, H. M., Johnson, A. B., Raine, C. S., Kay, W. J., & Terry, R. D. (1970). Senile plaques and cerebral amyloidosis in aged dogs. Laboratory Investigations, 23, 287-296. Wisniewski, H. M., & Terry, R. D. (1973). Morphology of the aging brain, human and animal. Progress in Brain Research, 40, 167-186. Wisniewski, H. M., Wegiel, J., Morys, J., Bancher, C, Soltysiak, Z., & Kim, K. S. (1990). Aged dogs: An animal model to study betaprotein amyloidogenesis. In K. Maurer, P. Riederer, & H. Beckmann (Eds.), Alzheimer's disease. Epidemiology, neuropathology, neurochemistty and clinics (Key topics in brain research) (pp. 151167), Berlin: Springer-Verlag. Zola-Morgan, S., & L. R. Squire (1985). Medial temporal lesions in monkeys impair memory on a variety of tasks sensitive to human amnesia. Behavioral Neuroscience, 99, 22-34.

Received December 8, 1994 Revision received May 18, 1995 Accepted June 1,1995