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A new psychometric test of attention-related behavior in rats; its validity in the aging process
Arch. Gerontol. Geriatr., 8 (1989) 29-36
29
Elsevier
A G G 00234
A new psychometric test of attention-related behavior in rats; its validity in the aging process Gy~Srgy Nrmeth *, Giinter Mayer and Siegfried Hoyer Institute of Pathochemistry and General Neurochemistry, University of Heidelberg, Im Neuenheimer Feld 220- 221, Postfach 104340, D-6900 Heidelberg, FRG (Received 15 July 1987; revised version received 19 April 1988; accepted 20 April 1988)
Summary The ability to distinguish relevance from irrelevance has been attributed to an attention-related mechanism and may be supposed to be disturbed in aging. The reaction to low electrical stimuli which causes neither pain nor escape behavior was investigated by means of a newly developed test in adult and aged rats. The animals' reaction was classified into two different responses depending on the intensity of the electrical stimuli. The first reaction related to sensitivity, the second reaction contained two components, an orienting response and a cognition-controlled type of discriminative behavior. There was no significant difference between the amperage values of the two reactions in adult rats. With respect to aged rats, the amperage values of both reactions are significantly increased as compared with the adult rats. The sensitivity reaction and the attention-related behaviors diverged considerably. These findings show very precisely that certain behavioral reactions may decline differently or even independently with age indicating different age-related changes in the underlying neuroanatomical systems of attention. The results demonstrate the sensitivity of the test used as a model for studying some types of attention-related mechanisms in the aging process. The use of a relatively simple test of animals' reactivity to sensory stimuli may reveal changes that are critical to understanding not only of the aging brain, but of different types of brain lesions and disorders, as well as of drug treatments. Aging process; Attention-related mechanisms; Electrical stimulus intensity; Psychometric test; Rats: Reduced orientation response of the aging brain
Introduction Like most biological systems in the body, the central nervous system undergoes a significant degeneration and displays a reduced biological plasticity in old age
Correspondence to Gy/Srgy Nrmeth, MD, Institute of Pathochemistry and General Neurochemistry, University of Heidelberg, Im Neuenheimer Feld 220-221, Postfach 104340, D-6900 Heidelberg, FRG. * On leave from the Department of Neurology and Psychiatry, Medical School of the University of Debrecen, Hungary. 0167-4943/89/$03.50 © 1989 Elsevier Science Publishers B.V. (Biomedical Division)
30 (Brody et al., 1975: Hoffmeister and Miiller, 1979: Hoyer, 1985: ttoyer and Krier, 1986), which eventually produce functional impairments. One of the most important higher functions of the central nervous system involves the regulation and control of behavior. It seems to be reasonable, therefore, to assume specific alterations ~n the central nervous system to be manifested ultimately in particular behaxioral deficit~,. Given this, it logically follows that those models of the central nervous ~,}steln concerned with functional impairments must include, or be concerned with specific, age-dependent deficits in behavior. However, before this principle can be implemented in the development of ne'x psychometric tests, it is first neces~,ar\, to characterize the behavioral deficits to be measured of the aged subjects being used. There has been a large number of attempts to develop psychometric tests measuring the attention-related behaviors. Many of these have been criticall', reviewed rccentl\' by Oades (1981a, 1982) and Mason (1984). Few experimental designs in use, apart from the "hole-board search task', arc suitable to test the attention-related functions which involved the decision betwecn relevant and irrelevant stimuli (Oades, 1981b, 1982). A limitation for the "relevance" in this test is that any indicator based on repeated choices of animals will in,,olve a memory component. However, it is proposed that, in the light of the differences seen in terms of the parameter 'relevance', this may reflect an impairment of the selective characteristics associated with any attention-related mechanism. Here we report results from a partly attention-related cognitivc discrimination test. Using low amperage values of electrical stimulation, we demonstrate an apparatus for the quantitative evaluation of sensory and attentional control in rats. We have proposed a specific and objective indicator for the pcrfornmnce of attention-related mechanisms, conscious of the fact that there are functional interactions between the neural mechanisms underlying attention, motivation and learning.
Method
Animals Twentyfive adult, (1-year-old and 25 aged (2-year-old) male Wistar rats (breeder: Zentralmstitut fiir Versuchstierzucht Hannover, FRGL weighing between 370 520 g, served as experimental objects. The animals arrived in the laboratory at least 1 week prior to testing: they were housed singly in Macrolon type 111 cages in a temperature-controlled animal room on a reversed 12-h light/dark (8 : 00 2 0 : 0 0 h) schedule and were handled daily. According to Hollander et al., 2-years-old male Wistar rats may be designated as aged (Hollander et al., 1983).
Behavioral apparatus and procedure A large rectangular box (70 × 45 cm) with 40 cm high wooden walls was used. The rats' behavior could be observed from above as there was no top to the box.
31 The floor consisted of a grid of parallel running steel bars (at 1 cm intervals). The box was divided into two parts by a wooden wall in which there was a gate of a size adequate for a rat to be able to pass into the other compartment of the box. One chamber had black walls and the other had white walls and the illumination of the latter was provided by a 60-W bulb placed at the center of the illuminated field. The grid of the black chamber was electrified with the help of a transformer to produce source voltages in the range from 0 to 150 V (AC). The transformer output was connected with a voltmeter across a regulatable resistor to the grid. The amperage values were registered with the help of a digital amperemeter (TEL DM 1000 B). The measuring range was between 0 and 800 t~A. The sensitivity of the amperemeter was 1 #A. The animal was put into the center of the dark compartment with the gate opened to the illuminated and currency-free compartment. After a minimum of 10 s adaptation, a continuously increasing electrical stimulation at the rate of 1 /~A amperage value of the observed animals was administered via the grid of the black compartment. The specifity of the electrical procedure used might be criticized. Since the behavioral effects of the electrical currents depend on the current density in tissue, the main source of variability is the specific skin resistance. It is well documented that skin resistance (Thomas and Norr, 1957; Venables and Martin, 1967) and skin conductance (Lacey and Siegel, 1949) are primarily located in the epidermis itself. Measurement of this parameter shows a high dependence on the density and activity of the sweat glands. To reduce the interindividual variations in skin conductance leading to density variations in electrical power ( # A / c m 2) (Edelberg et al., 1960; Edelberg, 1967) during testing, the electrified floor was moistened. With regard to skin resistance, Lykken (1968) noted that extrasomatic measurements using currents with fixed voltages enable direct and reliable measurement. To diminish dependence of the measured stimulus intensity on inter-individual skin resistance variations, we connected a variable pre-resistance (2 M~2-0 ~2) in front of the amperemeter. The voltage was fixed at 60 V during testing. In this way, the amperage level was independent of the skin resistance of the animal. Our variable preresistance enables us to give the animals a sensory input of a well-defined unit of electrical power (Watt). Finally, a neurological examination was performed on the animals whibh included the orienting response towards the tactile (on five different spots), visual, auditory and olfactory stimuli applied on both sides (Ljungberg and Ungerstedt, 1976). The righting and equilibrium reflexes, the flexion, toe spreading and grasping reflexes, and the visual placing reaction were also investigated according to Tupper and Wallace (1980) and Bure~ at al. (1983). Statistically significant differences of the electrical measurements were calculated by means of Student's t test ( p < 0.05) The Statistical Analysing System (SAS) was used in the URZ (Heidelberg University Computing Center)
32
Results In the apparatus described above two easily distinguishable behavioral reactions could be observed depending on the amperage value of the electrical stimulation. The first reaction was that the animal showed fast lifting movements of their forelimbs which was always followed by grooming. The rats did not leave the black (electrically stimulated) chamber. The currency threshold of the first observable phenomenon of the 1-year-old group was between 21 and 81 ~tA and of the 2-years-old group between 28 and 87/~A. The animals were electrically stimulated continuously. The second reaction ~.as observed at a higher amperage and it comprises two distinct types of behavior: in the beginning, the downward orientation with the rat's snout a n d Dater, after a maximum of 10 s, slow egress from the black compartment. The animal, however. was in the illuminated compartment only for a maximum of 10 s, and then turned around and re-entered the black compartment, in contrast to the initial first reaction, the two following types of behavior of the second reaction x~ere carried out
TABLE 1
Individual first and second reaction amperage values presented in ~,A ill 50 animals (25 :aduh a n d 25 aged rats in each group) 1-year-c,ld rats
1st reaction
2nd reaction
2-years-old rats
1st reaction
No.
(,aA)
(/cA)
No.
(ktA)
64 59 38 31 32 43 81 32 57 33 34 34 30 26 36 37 53 3O 31 24 2l 22 25 24 27
64 81 39 35 35 59 81 38 75 33 51 64 56 26 36 37 76 51 31 28 24 25 26 43 34
26 27 2S 29 30 31 32 33 34 35 ~6 37 38 39 40 41 42 43 44 45 46 47 48 49 50
36 48 28 44 48 44 47 87 81 58 59 76 43 67 87 78 48 51 36 6g 61 41 52 39 50
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25
2nd reaction (#A) 56 I1~ 1(i2 54 136 98 154 113
114 124 124 128 117 160 134 129 128 12a 87 125 13~ 98 1o2 127 89
33
consciously by the animals, as is proved by the orientation reaction and the leaving of the black compartment. On the other hand, the electrical stimulation was not aversive because the animals spontaneously re-entered the black compartment. With regard to the second reaction, the stimulation values were between 24 and 81 /zA and from 54 to 160 ~A, respectively. Our findings are summarized in Table I. Subsequent inspection of the individual data indicated considerable variability among the animals. As can be seen in Fig. 1, the amperage values between the 1- and 2-years-old rats show a significant difference ( p < 0.05) with regard to both the first and the second reaction. In the 1-year-old rats the first and second amperage values revealed no significant difference to the extent that in two animals at the first amperage values both the first and the second reaction could be observed. A high dissociation between the amperage values of the two reaction types of the 2-years-old rats ( p < 0.05) could be observed. The detailed neurological examination which included the response of the rat to different sensory stimuli, revealed a normal neurological status in all animals of both groups. +
JA
#
125-120 115 IlO 105
]0095 90 85 80 7570
65 6O 55
50-
4O 35 3O 252O 15 I0 5 0 One-year-oil rats reactions t 2. mean values 36,96 45,92 ltd. dev. _+14,83 4-18.58
Two-year-old rail I.
2.
54,88 114.72 "1"16.47 _+25.32
Fig. 1. Comparison of the first and second reaction amperage values between the group of 1- and 2-years-old rats. Mean values are presented as columns and the standard deviations are shown as bars. *, significantly different from the 1st reaction of the 1-year-old rats ( p < 0.05); ~ , significantly different from the 2nd reaction of the 1-year-old rats ( p < 0.05); + , significantly different from the 1st reaction of the 2-years-old rats ( p < 0.05); for details, see text.
34 Discussion
One aspect of selective attention that has a long history since the last century and which is applicable to behavioral studies is the ability to distinguish relevance from irrelevance (James, 1890; Mackintosh, 1975: Mountcastle, 1978: Salomon. 1979). Precise psychometric studies unequivocally clarifying the measurement of attention with the aim of separating it completely from memory and learning functions, have, so far, not been published, although many attempts have been made (Donovick et al., 1978; Rickert et al., 1978: Simon et al., 1980; Oades, 1982). In our test procedure the response of the animals was classified into two different reactions, depending on the intensity of the electrical stimuli. The first reaction of the response relates to sensitivity as an index of stimulus registration in the central nervous system. In the second reaction of the response the downward orientation is associated with the specific period of attention-related mechanisms, where the signals, which can be equated with the process of cognition, gain in weight. This reflects the ability to determine relevance and the extent to which stimuli control behavior. The second part of this reaction, characterized by the slow exit from the electrically stimulated compartment, may be interpreted as a cognition- but not as memory-controlled type of discriminative behavior. Both parts of this reaction could be observed one after the other without any exception. By contrast to the first reaction, considering these two types of the second reaction, it is important to notice that any goal-directed movement requires consciousness and can only be put into effect through conscious intention. The effects of the electrical stimulation had been investigated before in many ways (Campbell and Teghtsoonian, 1958; Campbell and Church, 1969; Masterson and Campbell, 1972; Crow~ 1973; Price and Fibiger, 1975; Crow and Wendlandt, 1976; London and Buterbaugh, 1978; for a review see Mason, 1984). However, in this study, it has to be emphasized that the currency threshold in both the first and second reaction was much lower than those which produced a footshock, the effect of which is generally investigated in the literature. With the help of the test used in this study the possibility arises of investigating the effect of the electrical stimulation at such a low level that there is no pain and, therefore, no escape behavior. It is now generally accepted that memory capacity and some cognitive functions decrease with normal aging (Gold and McGaugh, 1975; Finch, 1985: Bahes and Kliegl, 1985; Haxby et al., 1986). To our knowledge, the present study has demonstrated for the first time also an impairment of the attention-related behaviors in the normal aging process in aged rats. The difference between the two age groups studied was significant for the amperage values of both the first and the second reaction. Consequently, both types of the investigated behavior undergo a change in the aging process. As regards the adult rats, the first and second reaction amperage values did not differ considerably, which means that there is a narrow range between the sensitivity and the attentional control. Surprisingly, the first and second reaction amperage values of the attention test diverged considerably in the aged rats. Consequently, the impairment of the attention-related behavior is more pronounced in aging than the sensitivity reaction.
35 I n the light of the definition of the first a n d second reaction, the a b o v e - m e n t i o n e d data may reflect the aging process. Firstly, in animals, there are parallels b e t w e e n the decreased stimulus selection a n d the n o r m a l aging process. Secondly, the m e m o r y a n d learning deficits which develop in old age can at least be partly explained b y a t t e n t i o n a l deficits. These results give rise to the plausible hypothesis that different measures reflect the neuropsychological status of different n e u r o a n a t o m i c a l substrates which m a y show a different or even an i n d e p e n d e n t decline with age. Thus, for example, biochemical a n d a n a t o m i c a l markers of the central catecholaminergic a n d cholinergic systems decline with age, b u t it is u n k n o w n whether these are covariant or reflect i n d e p e n d e n t i m p a i r m e n t s . W i t h the help of substrates which inhibit acetylcholine n e u r o t r a n s m i s s i o n or involve some of the c a t e c h o l a m i n e systems, and using biochemical measures c o m b i n e d , a m o n g others, with the a t t e n t i o n test m e n t i o n e d above, this test may be very useful for further studies o n some aspects of aging. Therefore, the a t t e n t i o n test provides an o p p o r t u n i t y to analyse different c o m p o n e n t s of attention- a n d cognition-related m e c h a n i s m s a n d to separate the attentional and memorial c o m p o n e n t s of a n i m a l behavior.
Acknowledgements Ge~Srgy N r m e t h has received H u m b o l d t - f o u n d a t i o n , Bonn, F R G .
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References BaRes, P.B. and Kliegl, R. (1985): On the dynamics between growth and decline in the aging of intelligenceand memory. In: Neurology, Proceedingsof the XIIIth World Congress of Neurology, pp. 1-17. Editors: K. Poeck, H.-J. Freund and H. G~inshirt.Springer, Berlin-Heidelberg-NewYork-Tokyo. Brody, H., Harman, D. and Ordy, J.M. (1975): Aging, Vol. I: Clinical and morphological and neurochemical aspects in the aging nervous system. Raven Press, New York. Burel, J., Buregov~t, O. and Huston, J.P. (1983): Techniques and Basic Experiments for the Study of Brain and Behaviour, pp. 77-133. Elsevier Science Publ., Amsterdam, New York. Campbell, B.A. and Church, R.M. (1969): Punishment and Aversive Behavior, Appleton-Century-Crafts, New York. Campbell, B.A. and Teghtsoonian, R. (1958): Electrical and behavioral effects of different types of shock stimuli on the rat. J. Comp. Psychol., 51,185-192. Crow, T.J. (1973): Catecholamine containing neurons and electrical self-stimulation. 1. A review of some data. Psychol. Med., 2, 414-417. Crow, T.J. and Wendlandt, S. (1976): Impaired acquisition of a passive avoidance response after lesions induced in the locus coeruleus by 6-OH dopamine. Nature, 259, 42-44. Donovick, P.J., Burright, R.G., Sikorszky, R.D., Stamato, N.J. and Maclaughlin, W.W. (1978): Cue elimination effects on discrimination behavior of rats with septal lesions. Physiol. Behav., 20, 71-75. Edelberg, R. (1967): Electrical properties of the skin. In: Methods in Psychophysiology. Editor: C.C. Brown. Williams and Williams, Baltimore.
36 Edelberg, R., Greiner, T. and Burch, N.R. (1960): Some membrane properties of the effector in the galvanic skin response. J. Appl. Physiol.. 15. 691 696, Finch, C.A. (1985): Modulation of aging process in the brain. In: Thresholds in Aging, pp. 175 188. Editors: M. Bergener, M. Ermini, H.B. St~ihelin, Academic Press, London. Gold, P.E. and McGaugh, J.L. (1975): Changes in learning and memory during aging. Adv. Behav. Biol.. 16, 145 158. Haxby, V.J., Grady, C.L., Duara, R., Robertson-Chabo, E.A., Koviarz, B., Cutler, R.N. and Rapoport, S.I. (19861: Relations among age, visual memory and resting cerebral metabolism in 40 healthy men. Brain Cogn., 5,412-427. Hoffmeister, F. and Miiller, C. (1979): Brain Function in Old Age: Evaluation of Changes and I)isorders. Springer, New York. Hollander, C.F., Van Zwieten, M.J. and Zurcher C. (1983): The aged animal. In: Aging of the Brain, pp. 187 196. Editors: W.H. Gispen and J. Traber. Elsevier Science Publ., Amsterdam. Hoyer, S. (1985): The effect of age on glucose and energy metabolism in brain cortex of rats. Arch. Gerontol. Geriatr., 4, 193--203. Hoyer, S. and Krier, C. (1986): Ischemia and the aging brain. Studies on glucose and energy metabolism in rat cerebral cortex. Neurobiol. Aging, 7, 23 29. James, W. (1890): Principles of Psychiatry. Holt. New York. Lacey, O.L. and Siegel, P.S. (1949): An analysis of measurement of the galvanic skin response, J. f-xp. Psychol., 39, 122 127. Ljungberg, T. and Ungerstedt, U. (1976): Sensory inattention produced by 6-hydroxydopamine-induced degeneration of ascending dopamine neurons in the brain. Exp. Neurot.. 53, 585- 600. London, E.D. and Buterbaugh, G.G. (1978): Modification of electroshock convulsive responses and thresholds in neonatal rats after brain monoamine reduction. J. Pharmacol. Exp. Ther.. 2(/6. 81 90. Lykken, O.T. (1968): Neuropsychology and psychophysiology in personality research. In: Handbook of Personality Theory and Research, pp. 50 128. Editors: E.F. Borgetta and W.W. Lambert. Rand McNally, Chicago. Mackintosh, N.J. (19751: A theory of attention: variations in the associability of stimuli with reinforcement. Psychol. Rev., 82, 276-298. Mason, S.T. (1984): Catecholamines and Behavior. Cambridge University Press. Cambridge, London, New York, Rochelle, Melbourne, Sydney. Masterson, F.A. and Campbell, B.A. (1972): Techniques of electrical shock motivation. In: Methods m Psychobiology, Vol. II., pp. 21-85. Editor: R.D. Myers. Academic Press, London. New York. Mountcastle, U.B. (1978): Brain mechanisms for directed attention. J. Roy, Soc. Med.. 71, 14-28. Oades, R.D. (1981a): Impairments of search behavior in rats after haloperidol treatment, hippocampal or neocortical damage suggest a mesocorticolimbic role in cognition. Biol. Psychol., 12. 77-85. Oades, R.D. (1981b): Types of memory or attention? Impairments after lesions of the hippocampus and limbic ventral tegmentum. Brain Res. Bull., 7, 221-226. Oades, R.D. (1982): Search strategies on a hole-board are impaired in rats with ventral tegmental damage: animal modell for tests of thought disorder. Biol. Psychiat., 17, 243 258. Price, M.T.C. and Fibiger, H.C. (19751: Discriminated escape learning and response to electric shock after 6-hydroxydopamine lesions of the nigro-neostriatial dopaminergic projection. Pharmacol. Biochem. Behav., 3, 285-290. Rickert, E.J.. Bennett, T.L., Lane, P. and French, J. (1978): Hippocampectomy and the attcnuation of blocking. Behav. Biol., 22, 147. Salomon, P.R. (1979): Temporal versus spatial information processing theories of hippocampal function. Psychol. Bull., 86. 1272-1279. Simon, H., Scatton, B. and Le Moal, M. (1980): Dopaminergic A-10 neurons are inw)lved in cognitive functions. Nature, 286, 150-151. Thomas, P.E. and Norr, J.M. (1957): Relationship between sweat gland activity and electrical resistance of the skin. J. Appl. Physiol., 10, 505-510. Tupper, D.E. and Wallace, R.B. (1980): Utility of the neurological examination in rats. Acta Neurobiol. Exp., 40, 999-1003. Venables, P.H. and Martin, J. (1967): Skin resistance and skin potential. In: Manual of Psychophysiological Methods, pp. 30-51. Editors: P.H. Venables and J. Martin. North-Holland Publ. Co., Amsterdam.
29
Elsevier
A G G 00234
A new psychometric test of attention-related behavior in rats; its validity in the aging process Gy~Srgy Nrmeth *, Giinter Mayer and Siegfried Hoyer Institute of Pathochemistry and General Neurochemistry, University of Heidelberg, Im Neuenheimer Feld 220- 221, Postfach 104340, D-6900 Heidelberg, FRG (Received 15 July 1987; revised version received 19 April 1988; accepted 20 April 1988)
Summary The ability to distinguish relevance from irrelevance has been attributed to an attention-related mechanism and may be supposed to be disturbed in aging. The reaction to low electrical stimuli which causes neither pain nor escape behavior was investigated by means of a newly developed test in adult and aged rats. The animals' reaction was classified into two different responses depending on the intensity of the electrical stimuli. The first reaction related to sensitivity, the second reaction contained two components, an orienting response and a cognition-controlled type of discriminative behavior. There was no significant difference between the amperage values of the two reactions in adult rats. With respect to aged rats, the amperage values of both reactions are significantly increased as compared with the adult rats. The sensitivity reaction and the attention-related behaviors diverged considerably. These findings show very precisely that certain behavioral reactions may decline differently or even independently with age indicating different age-related changes in the underlying neuroanatomical systems of attention. The results demonstrate the sensitivity of the test used as a model for studying some types of attention-related mechanisms in the aging process. The use of a relatively simple test of animals' reactivity to sensory stimuli may reveal changes that are critical to understanding not only of the aging brain, but of different types of brain lesions and disorders, as well as of drug treatments. Aging process; Attention-related mechanisms; Electrical stimulus intensity; Psychometric test; Rats: Reduced orientation response of the aging brain
Introduction Like most biological systems in the body, the central nervous system undergoes a significant degeneration and displays a reduced biological plasticity in old age
Correspondence to Gy/Srgy Nrmeth, MD, Institute of Pathochemistry and General Neurochemistry, University of Heidelberg, Im Neuenheimer Feld 220-221, Postfach 104340, D-6900 Heidelberg, FRG. * On leave from the Department of Neurology and Psychiatry, Medical School of the University of Debrecen, Hungary. 0167-4943/89/$03.50 © 1989 Elsevier Science Publishers B.V. (Biomedical Division)
30 (Brody et al., 1975: Hoffmeister and Miiller, 1979: Hoyer, 1985: ttoyer and Krier, 1986), which eventually produce functional impairments. One of the most important higher functions of the central nervous system involves the regulation and control of behavior. It seems to be reasonable, therefore, to assume specific alterations ~n the central nervous system to be manifested ultimately in particular behaxioral deficit~,. Given this, it logically follows that those models of the central nervous ~,}steln concerned with functional impairments must include, or be concerned with specific, age-dependent deficits in behavior. However, before this principle can be implemented in the development of ne'x psychometric tests, it is first neces~,ar\, to characterize the behavioral deficits to be measured of the aged subjects being used. There has been a large number of attempts to develop psychometric tests measuring the attention-related behaviors. Many of these have been criticall', reviewed rccentl\' by Oades (1981a, 1982) and Mason (1984). Few experimental designs in use, apart from the "hole-board search task', arc suitable to test the attention-related functions which involved the decision betwecn relevant and irrelevant stimuli (Oades, 1981b, 1982). A limitation for the "relevance" in this test is that any indicator based on repeated choices of animals will in,,olve a memory component. However, it is proposed that, in the light of the differences seen in terms of the parameter 'relevance', this may reflect an impairment of the selective characteristics associated with any attention-related mechanism. Here we report results from a partly attention-related cognitivc discrimination test. Using low amperage values of electrical stimulation, we demonstrate an apparatus for the quantitative evaluation of sensory and attentional control in rats. We have proposed a specific and objective indicator for the pcrfornmnce of attention-related mechanisms, conscious of the fact that there are functional interactions between the neural mechanisms underlying attention, motivation and learning.
Method
Animals Twentyfive adult, (1-year-old and 25 aged (2-year-old) male Wistar rats (breeder: Zentralmstitut fiir Versuchstierzucht Hannover, FRGL weighing between 370 520 g, served as experimental objects. The animals arrived in the laboratory at least 1 week prior to testing: they were housed singly in Macrolon type 111 cages in a temperature-controlled animal room on a reversed 12-h light/dark (8 : 00 2 0 : 0 0 h) schedule and were handled daily. According to Hollander et al., 2-years-old male Wistar rats may be designated as aged (Hollander et al., 1983).
Behavioral apparatus and procedure A large rectangular box (70 × 45 cm) with 40 cm high wooden walls was used. The rats' behavior could be observed from above as there was no top to the box.
31 The floor consisted of a grid of parallel running steel bars (at 1 cm intervals). The box was divided into two parts by a wooden wall in which there was a gate of a size adequate for a rat to be able to pass into the other compartment of the box. One chamber had black walls and the other had white walls and the illumination of the latter was provided by a 60-W bulb placed at the center of the illuminated field. The grid of the black chamber was electrified with the help of a transformer to produce source voltages in the range from 0 to 150 V (AC). The transformer output was connected with a voltmeter across a regulatable resistor to the grid. The amperage values were registered with the help of a digital amperemeter (TEL DM 1000 B). The measuring range was between 0 and 800 t~A. The sensitivity of the amperemeter was 1 #A. The animal was put into the center of the dark compartment with the gate opened to the illuminated and currency-free compartment. After a minimum of 10 s adaptation, a continuously increasing electrical stimulation at the rate of 1 /~A amperage value of the observed animals was administered via the grid of the black compartment. The specifity of the electrical procedure used might be criticized. Since the behavioral effects of the electrical currents depend on the current density in tissue, the main source of variability is the specific skin resistance. It is well documented that skin resistance (Thomas and Norr, 1957; Venables and Martin, 1967) and skin conductance (Lacey and Siegel, 1949) are primarily located in the epidermis itself. Measurement of this parameter shows a high dependence on the density and activity of the sweat glands. To reduce the interindividual variations in skin conductance leading to density variations in electrical power ( # A / c m 2) (Edelberg et al., 1960; Edelberg, 1967) during testing, the electrified floor was moistened. With regard to skin resistance, Lykken (1968) noted that extrasomatic measurements using currents with fixed voltages enable direct and reliable measurement. To diminish dependence of the measured stimulus intensity on inter-individual skin resistance variations, we connected a variable pre-resistance (2 M~2-0 ~2) in front of the amperemeter. The voltage was fixed at 60 V during testing. In this way, the amperage level was independent of the skin resistance of the animal. Our variable preresistance enables us to give the animals a sensory input of a well-defined unit of electrical power (Watt). Finally, a neurological examination was performed on the animals whibh included the orienting response towards the tactile (on five different spots), visual, auditory and olfactory stimuli applied on both sides (Ljungberg and Ungerstedt, 1976). The righting and equilibrium reflexes, the flexion, toe spreading and grasping reflexes, and the visual placing reaction were also investigated according to Tupper and Wallace (1980) and Bure~ at al. (1983). Statistically significant differences of the electrical measurements were calculated by means of Student's t test ( p < 0.05) The Statistical Analysing System (SAS) was used in the URZ (Heidelberg University Computing Center)
32
Results In the apparatus described above two easily distinguishable behavioral reactions could be observed depending on the amperage value of the electrical stimulation. The first reaction was that the animal showed fast lifting movements of their forelimbs which was always followed by grooming. The rats did not leave the black (electrically stimulated) chamber. The currency threshold of the first observable phenomenon of the 1-year-old group was between 21 and 81 ~tA and of the 2-years-old group between 28 and 87/~A. The animals were electrically stimulated continuously. The second reaction ~.as observed at a higher amperage and it comprises two distinct types of behavior: in the beginning, the downward orientation with the rat's snout a n d Dater, after a maximum of 10 s, slow egress from the black compartment. The animal, however. was in the illuminated compartment only for a maximum of 10 s, and then turned around and re-entered the black compartment, in contrast to the initial first reaction, the two following types of behavior of the second reaction x~ere carried out
TABLE 1
Individual first and second reaction amperage values presented in ~,A ill 50 animals (25 :aduh a n d 25 aged rats in each group) 1-year-c,ld rats
1st reaction
2nd reaction
2-years-old rats
1st reaction
No.
(,aA)
(/cA)
No.
(ktA)
64 59 38 31 32 43 81 32 57 33 34 34 30 26 36 37 53 3O 31 24 2l 22 25 24 27
64 81 39 35 35 59 81 38 75 33 51 64 56 26 36 37 76 51 31 28 24 25 26 43 34
26 27 2S 29 30 31 32 33 34 35 ~6 37 38 39 40 41 42 43 44 45 46 47 48 49 50
36 48 28 44 48 44 47 87 81 58 59 76 43 67 87 78 48 51 36 6g 61 41 52 39 50
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25
2nd reaction (#A) 56 I1~ 1(i2 54 136 98 154 113
114 124 124 128 117 160 134 129 128 12a 87 125 13~ 98 1o2 127 89
33
consciously by the animals, as is proved by the orientation reaction and the leaving of the black compartment. On the other hand, the electrical stimulation was not aversive because the animals spontaneously re-entered the black compartment. With regard to the second reaction, the stimulation values were between 24 and 81 /zA and from 54 to 160 ~A, respectively. Our findings are summarized in Table I. Subsequent inspection of the individual data indicated considerable variability among the animals. As can be seen in Fig. 1, the amperage values between the 1- and 2-years-old rats show a significant difference ( p < 0.05) with regard to both the first and the second reaction. In the 1-year-old rats the first and second amperage values revealed no significant difference to the extent that in two animals at the first amperage values both the first and the second reaction could be observed. A high dissociation between the amperage values of the two reaction types of the 2-years-old rats ( p < 0.05) could be observed. The detailed neurological examination which included the response of the rat to different sensory stimuli, revealed a normal neurological status in all animals of both groups. +
JA
#
125-120 115 IlO 105
]0095 90 85 80 7570
65 6O 55
50-
4O 35 3O 252O 15 I0 5 0 One-year-oil rats reactions t 2. mean values 36,96 45,92 ltd. dev. _+14,83 4-18.58
Two-year-old rail I.
2.
54,88 114.72 "1"16.47 _+25.32
Fig. 1. Comparison of the first and second reaction amperage values between the group of 1- and 2-years-old rats. Mean values are presented as columns and the standard deviations are shown as bars. *, significantly different from the 1st reaction of the 1-year-old rats ( p < 0.05); ~ , significantly different from the 2nd reaction of the 1-year-old rats ( p < 0.05); + , significantly different from the 1st reaction of the 2-years-old rats ( p < 0.05); for details, see text.
34 Discussion
One aspect of selective attention that has a long history since the last century and which is applicable to behavioral studies is the ability to distinguish relevance from irrelevance (James, 1890; Mackintosh, 1975: Mountcastle, 1978: Salomon. 1979). Precise psychometric studies unequivocally clarifying the measurement of attention with the aim of separating it completely from memory and learning functions, have, so far, not been published, although many attempts have been made (Donovick et al., 1978; Rickert et al., 1978: Simon et al., 1980; Oades, 1982). In our test procedure the response of the animals was classified into two different reactions, depending on the intensity of the electrical stimuli. The first reaction of the response relates to sensitivity as an index of stimulus registration in the central nervous system. In the second reaction of the response the downward orientation is associated with the specific period of attention-related mechanisms, where the signals, which can be equated with the process of cognition, gain in weight. This reflects the ability to determine relevance and the extent to which stimuli control behavior. The second part of this reaction, characterized by the slow exit from the electrically stimulated compartment, may be interpreted as a cognition- but not as memory-controlled type of discriminative behavior. Both parts of this reaction could be observed one after the other without any exception. By contrast to the first reaction, considering these two types of the second reaction, it is important to notice that any goal-directed movement requires consciousness and can only be put into effect through conscious intention. The effects of the electrical stimulation had been investigated before in many ways (Campbell and Teghtsoonian, 1958; Campbell and Church, 1969; Masterson and Campbell, 1972; Crow~ 1973; Price and Fibiger, 1975; Crow and Wendlandt, 1976; London and Buterbaugh, 1978; for a review see Mason, 1984). However, in this study, it has to be emphasized that the currency threshold in both the first and second reaction was much lower than those which produced a footshock, the effect of which is generally investigated in the literature. With the help of the test used in this study the possibility arises of investigating the effect of the electrical stimulation at such a low level that there is no pain and, therefore, no escape behavior. It is now generally accepted that memory capacity and some cognitive functions decrease with normal aging (Gold and McGaugh, 1975; Finch, 1985: Bahes and Kliegl, 1985; Haxby et al., 1986). To our knowledge, the present study has demonstrated for the first time also an impairment of the attention-related behaviors in the normal aging process in aged rats. The difference between the two age groups studied was significant for the amperage values of both the first and the second reaction. Consequently, both types of the investigated behavior undergo a change in the aging process. As regards the adult rats, the first and second reaction amperage values did not differ considerably, which means that there is a narrow range between the sensitivity and the attentional control. Surprisingly, the first and second reaction amperage values of the attention test diverged considerably in the aged rats. Consequently, the impairment of the attention-related behavior is more pronounced in aging than the sensitivity reaction.
35 I n the light of the definition of the first a n d second reaction, the a b o v e - m e n t i o n e d data may reflect the aging process. Firstly, in animals, there are parallels b e t w e e n the decreased stimulus selection a n d the n o r m a l aging process. Secondly, the m e m o r y a n d learning deficits which develop in old age can at least be partly explained b y a t t e n t i o n a l deficits. These results give rise to the plausible hypothesis that different measures reflect the neuropsychological status of different n e u r o a n a t o m i c a l substrates which m a y show a different or even an i n d e p e n d e n t decline with age. Thus, for example, biochemical a n d a n a t o m i c a l markers of the central catecholaminergic a n d cholinergic systems decline with age, b u t it is u n k n o w n whether these are covariant or reflect i n d e p e n d e n t i m p a i r m e n t s . W i t h the help of substrates which inhibit acetylcholine n e u r o t r a n s m i s s i o n or involve some of the c a t e c h o l a m i n e systems, and using biochemical measures c o m b i n e d , a m o n g others, with the a t t e n t i o n test m e n t i o n e d above, this test may be very useful for further studies o n some aspects of aging. Therefore, the a t t e n t i o n test provides an o p p o r t u n i t y to analyse different c o m p o n e n t s of attention- a n d cognition-related m e c h a n i s m s a n d to separate the attentional and memorial c o m p o n e n t s of a n i m a l behavior.
Acknowledgements Ge~Srgy N r m e t h has received H u m b o l d t - f o u n d a t i o n , Bonn, F R G .
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