(Olson & Pettigrew, 1974; Cynader, Berman & Hein, 1973; 1976; Cynader &

Transcription

1 J. Phy8iol. (1978), 274, pp With 10 textftgurem Printed in Great Britain MODIFICATION OF VISUAL RESPONSE PROPERTIES IN THE SUPERIOR COLLICULUS OF THE GOLDEN HAMSTER FOLLOWING STROBOSCOPIC REARING BY LEO M. CHALUPA AND ROBERT W. RHOADES From the Department of Psychology, University of California, Davis, California 95616, U.S.A. (Received 29 June 1977) SUMMARY 1. Visual response properties of superior collicular neurones were investigated in golden hamsters reared from birth to adulthood in a stroboscopic environment. 2. In comparison to normally reared animals, there was a marked decrease in the incidence of directionally selective cells in the colliculus of the strobereared hamsters. This effect was apparent when directional selectivity was determined by either the null criterion or a statistical measure. The reduction in directionally selective cells was found in both superficial and the deep layers of the colliculus. 3. Neurones in strobereared hamsters also exhibited a different speed preference distribution from that obtained for normal animals, in that more cells in the restricted hamsters responded only to slow velocities, and less were broadly tuned with regard to the speed of moving stimuli. 4. In addition to the effects obtained in dynamic response properties, there were also changes in the static response properties of superior collicular neurones. These were an increase in the proportion of cells whose responses were not affected by changing the size of a stationary flashed stimulus, and a concomitant decrease in the number of cells demonstrating either partial or complete suppression when the size of a flashed stimulus exceeded the boundaries of the receptive field activating region. Furthermore, while all cells which responded to stationary stimuli in normal animals yielded only phasic responses to stimulus onset and/or offset, in the strobereared hamsters eight cells were encountered which responded in a sustained fashion to stationary spots. 5. There was no indication of an increased responsivity in the restricted animals to strobe stimulation, even when a strobe rate identical to that employed in the rearing environment was employed. 6. The results were interpreted as indicating a disruption of normal visual functional organization in the hamster's superior colliculus by an aberrant visual input during development. INTRODUCTION A stroboscopic environment permits experience with visual patterns, but abolishes the perception of moving visual stimuli. Rearing cats in such an environment has been shown to produce changes in the functional organization of the visual cortex (Olson & Pettigrew, 1974; Cynader, Berman & Hein, 1973; 1976; Cynader &

2 572 L. M. CHALUPA AND R. W. RHOADES Chernenko, 1976) as well as the superior colliculus (Flandrin & Jeannerod, 1975; Flandrin, Kennedy & Amblard, 1976). Since in the cat, visual deprivation also markedly alters the response properties of many cortical and superior collicular neurones (see Barlow, 1975, for a review) the interpretation of the results of stroberearing, as well as other visual restriction experiments is complicated. One problem is in the determination of whether or not the changes resulting from visual restriction are due to the 'active' incorporation of the abnormal environmental input into the developing visual system or the degradation of a genetically predetermined functional state. Furthermore, it is also unclear whether visual system plasticity demonstrated with restriction paradigms is dependent upon the same synaptic subsets and/or mechanisms which are modified as a consequence of visual deprivation. A recent study by Berman & Daw (1977) has shown that in the cat, the critical period for unidirectional restriction terminates earlier than that for monocular deprivation. These findings were interpreted as suggesting that different sets of synapses are involved in the functional consequences of these two types of manipulations. The most clearcut method of differentiating the mechanisms underlying visual restriction from those responsible for the effects of visual deprivation would be to demonstrate the effects of some type of visual restriction in a visual system which is not dependent upon experience for its normal development. Recently, we (Rhoades & Chalupa, 1977a) have found that the visual response properties of superior collicularneurones in thegolden hamster (Mesocricetus auratus) develop relatively normally in animals raised from birth to adulthood in total darkness. It thus became of considerable interest to determine the effects of visual restriction, specifically stroberearing, upon collicular functional organization in this species. Brief accounts of some of these findings have been reported elsewhere (Chalupa & Rhoades, 1977b, c). METHODS Rearing conditions. All hamsters used in the present study were born and raised in a specially constructed lighttight chamber which was ventilated by a forced air system through a series of baffled ducts. For 12 h each day the chamber was illuminated twice every second by a 4 psec flash from a General Radio (Model 1543) strobe unit. This flash rate has been demonstrated to alter the receptive field properties of both cortical (Olson & Pettigrew, 1974) and collicular (Flandrin, Kennedy & Amblard, 1976) neurones in the cat. The restriction chamber was placed in a lighttight room and all animal maintenance was carried out under stroboscopic illumination. All of the experimental hamsters were weaned at 28 days of age, divided according to sex and housed in group cages with a diet of laboratory chow and tap water until the day of the behavioural or electrophysiological experiment. The animals were between 6 and 8 months old at the time of testing and weighed between 175 and 220 g. Animal preparation and recordings. The procedures employed in this study have been described in detail elsewhere (Rhoades & Chalupa, 1977a, b) and they will be outlined only briefly here. Sixteen hamsters were used in the recording experiments. Immediately following removal from the restriction chamber each animal was anaesthetized with an i.p. injection of sodium pentobarbitone (53 mg/kg) and atropine (0 10 ml. 15% solution) was also administered. A single inverted bolt was then fixed to the animal's nasal bone and a craniotomy was made overlying the left superior colliculus. A tracheotomy was also performed and a Y shaped cannula inserted to permit artificial respiration. The hamster's eyelids were cut away bilaterally and fine stainlesssteel wires were sewn into the animal's chest for heart rate monitoring. The hamster was then mounted in the stereotaxic instrument using a special headholder which attached to the bolt on the animal's head. All wound edges were infiltrated with a long lasting local anaesthetic (Nupercaine, CIBA) and the hamster was paralysed with Flaxedil (25 mg/kg) delivered

3 STROBE REARING AFFECTS HAMSTER'S COLLICULUS intraperitoneally. Artificial respiration was provided by a Harvard small animal respirator. A state of light anaesthesia and paralysis was maintained throughout the course of the experiment with supplemental doses of pentobarbitone and Flaxedil; the hamster was also sedated with a single I.M. injection of chlorprothixene (0.50 mg). Pupils were dilated with homatropine hydrobromide and the corneas were protected with clear silicone fluid. Rectal temperature was maintained at about 37 0C and e.k.g. was monitored in all experiments. Retinoscopy was not used in these experiments since it has been demonstrated by Glickstein & Millodot (1970) that this procedure results in systematic errors of hypermetropia with an eye as small as that of the hamster. Furthermore, it has been noted by others (Tiao & Blakemore, 1976a; Drager & Hubel, 1976) that the depth of focus in the eye of a small rodent is quite large and thus the retinal image should not be degraded by small refractive errors. In order to assure ourselves that stroberearing did not induce a major change in the retinal image, collicular neurones were recorded from one strobereared and one normal animal while a series of corrective lenses were placed in front of the hamster's eye. It was found that 8 positive or negative diopters (D) of correction did not cause any changes in responsivity of collicular cells to the types of stimuli employed in the present experiments, nor were there any changes in receptive field size. Parametric investigation of receptive field properties was accomplished with the use of a visual movement stimulator controlled by a function generator. The function generator also triggered a dual beam storage oscilloscope which displayed both the unit's activity and appropriate event markers. Tests for directional selectivity were carried out by moving a light spot across the activating region of the receptive field in 8 directions (up, down, temporal, nasal, and the oblique axes dividing the four quadrants). For each direction a minimum of five stimulus presentations were employed with a intertrial interval of about 10 sec. Each cell was tested at the speed which was qualitatively judged to yield the most vigorous discharges. Speed selectivity was tested quantitatively in a similar fashion, with each stimulus velocity being repeated for a minimum of five presentations (10 sec intertrial interval). If the cell had a preferred direction, that direction was used for the speed tests, otherwise a direction was selected at random. Responses to various standing spots were also investigated by presenting a minimum of three different spot sizes (all 15 log units above background) for at least five trials each. Additionally, cells were tested with stroboscopic flashes of various frequencies, including that employed during the rearing period. Spike counts and latencies were read directly off the oscilloscope face, and photographs of selected traces were taken with a 35 mm camera. The responses of some cells were recorded on magnetic tape and subsequently analysed with an Ortec computer (Model 4620) and histogram analyser (Model 4621). Electrolytic lesions (35,#A for 2 sec, electrode positive) were used to mark the loci of representative cells. At the termination of an experiment, the animal was administered an overdose of barbiturate, and perfused through the heart with physiological saline followed by 10% buffered formalin. The brain was removed, sectioned in paraffin at 10/sm, and every tenth section was stained for cells and fibres (Kluver & Barrera, 1953). Behavioural testing. Seven strobereared and ten normal hamsters were tested for visual orientating abilities using the procedure developed by Schneider (1967, 1969). Individual sunflower seeds, held with fine forceps, were slowly introduced with a jiggling motion into various portions of the hamster's visual field. As soon as the animal orientated toward the sunflower seed it was permitted to consume it. Twentyfive trials were carried out for each hamster. In all cases care was taken to avoid touching the animal's vibrassae with the seed. The experimenter conducting these tests did not have knowledge of the animal's reading history. Additional details of the behavioural testing procedures are provided elsewhere (Rhoades & Chalupa, 1977a). It should be noted that none of the animals used for behavioural testing were employed in the recording experiments. RESULTS One hundred and eightythree units, judged to be cells according to standard criteria (Hubel, 1960) were recorded from the superior colliculus of sixteen hamsters reared from birth to adulthood in a stroboscopic environment. For purposes of comparison, data obtained from 313 visual cells recorded from normally reared hamsters 573

4 574 L. M. CHALUPA AND R. W. RHOADES (Rhoades & Chalupa, 1976, 1977b; Chalupa & Rhoades, 1977a), and 184 collicular neurones from darkreared hamsters (Rhoades & Chalupa, 1977a) are also included. The procedures in these experiments were identical to the ones employed in the present study, with two exceptions. Approximately 50 % of the visual cells recorded from normal hamsters (Rhoades & Chalupa, 1976; Chalupa & Rhoades, a) were isolated in anaesthetized, but nonparalysed preparations, and response properties were tested using handheld visual stimuli. The remaining portion of the control data from normal animals were obtained in paralysed and anaesthetized animals employing the visual movement stimulator previously described (Rhoades & Chalupa, 1977a). Since no appreciable differences were apparent between the two samples, these data have, in some instances, been combined in the present paper. The results to be reported will first be discussed in terms of the dynamic receptive field properties, where the major effects of the restriction paradigm were observed. Following this, data regarding the static aspects of receptive field organization, where more subtle effects were noted, will be presented. Lastly, the behavioural observations obtained with the visual orientating tests will be discussed. Almost all of our data were obtained from the monocular portion of the superior colliculus, and the few cells recorded from the small binocular segment (Tiao & Blakemore, 1976b) do not permit a meaningful discussion of binocularity in the restricted animals. Directional selectivity. One hundred and twentythree cells in the superior colliculus of the strobereared hamsters were tested for directional selectivity and the responses of each cell were analysed according to two criteria. The first of these was the traditional 'null' criterion established by Barlow & Levick (1965), and the second was a statistical measure in which responses to opposing directions of movement were compared with a simple t test. According to the null criterion a cell was judged to be selective if it possessed a preferred direction in which movement yielded responses and an opposing null direction where movement resulted in either no reliable discharges or a suppression of spontaneous activity. Fig. 1A illustrates the responses of a collicular cell recorded from a strobereared hamster which was judged to be selective according to the null criterion. With the statistical measure a cell was considered to be selective if the magnitude and variability of the responses to opposing directions of movement were such that the results of a t test comparing Fig. 1. In A the responses of a cell judged to be directionally selective according to the null criterion which was recorded from the superficial collicular laminae of a strobereared hamster. Each point on the polar diagram represents the sum of the action potentials obtained in five stimulus presentations. Representative oscilloscope traces for each direction are also shown. The activating region of the receptive field measured 9 x 6' and the stimulus was a 3. 6 light spot (1.5 log units above background) which was swept across the field at 25 '/sec. In B, the responses of another superficial layer cell considered to be selective according to the statistical, but not the null criterion. This cell had no spontaneous activity and responded reliably to each of the directions tested. The polar diagram and oscilloscope traces indicate a clear preference for movement in the upper temporal as opposed to lower nasal direction. Here the activating region measured 11 x 8 and the stimulus was a 2. 7 light spot (1.5 log units above background) which was moved at a rate of 12 '/sec. In C, the responses of a superficial layer cell (activating region 40 x 70) which was not directionally selective. The stimulus here was a 20 light spot (1*5 log units above background) swept across the receptive field at 50/sec.

5 STROBE REARING AFFECTS HAMSTER'S COLLICULUS 575 La2 88 LO~~~~1 La~~~~~~~ U)~~~~~~~b 06El~~~~~~~~~~~~~~~~ /~~~~4 8,! 4':~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~( to CL~~~~~ 4. ills p4~~~c v~~~~~

6 576 L. M. CHALUPA AND R. W. RHOADES these responses proved to be significant (P < 0.05). Fig. 1 B depicts the responses of a cell judged as selective according to the statistical but not the null criterion. Typically, cells exhibiting selectivity were not tightly tuned for a single direction of movement and in most cases a fairly wide range of adjacent directions yielded reliable discharges (e.g. Fig. 1 A). This was true for all three groups (normal, darkreared and strobereared) of hamsters that we have tested. Furthermore, the directional selectivity which we have observed for collicular neurones in normal, darkreared and strobereared hamsters has not been found to be dependent upon the contrast of the stimulus employed, or within the range of velocities over which the cell was responsive, the speed of that stimulus. In contrast to the directionally selective cells, Fig. 1 C shows the responses of a cell which responded in an approximately equal fashion to movement in each of the four pairs of opposing directions which were tested. The most clearcut consequence of stroberearing upon the functional organization of the hamster's superior colliculus was a marked reduction in the number of cells which exhibited directional selectivity as defined by either the null or statistical criterion. In normal animals, 572 % (n = 99) of 173 cells tested were directionally selective according to the statistical measure, while 266 % (n = 46) exhibited selectivity as defined by the more rigorous null criterion. It should be noted that cells which were selective by the null criterion were also considered to be directionally selective by the statistical measure. In contrast, in the strobereared hamsters, only 26 % (n = 32) of the 123 cells tested were statistically selective, while 65 0/ (n = 8) had a preferred and opposing null direction. The reliability of the observed change in the collicular functional organization of the strobereared animals is further substantiated by the fact that in ten of the sixteen restricted hamsters we tested, no ' null' cells were encountered. The minimum number of cells tested for directional selectivity in any of the ten animals was five. In normal (n = 17) and darkreared (n = 12) hamsters in which at least five cells were similarly tested there was only one case (a darkreared animal) in which a 'null' cell was not isolated. We have previously reported (Chalupa & Rhoades, 1977a) that ablations of visual cortex significantly reduce the incidence of directional selectivity in the superior colliculus of the hamster. Further, this effect appeared to be limited to the superficial collicular laminae, that is those dorsal to the stratum griseum intermediate. Thus it was of interest to determine whether or not the effects of stroboscopic rearing were also confined to the superficial layers. Fig. 2 shows the percentage of directionally selective cells encountered in the superficial and deep collicular laminae of normally reared (A) and strobereared hamsters (B). As is evident from Fig. 2B, the effects of stroboscopic rearing, appear to be the same for the superficial and deep laminae. Eightyseven of the cells tested for directional selectivity were in the superficial layers and of these 287 0% (n = 25) and 5 7 % (n = 5) were selective according to the statistical and null criteria, respectively. In the deeper layers thirtysix cells were tested and here the respective percentages of 'statistical' and 'null' cells were 194 0/ (n = 7) and 83 0/ (n = 3). As has been noted by others (Tiao & Blakemore, 1976b) it is difficult to localize cells to specific laminae in the hamster's colliculus, but the assignment of neurones to either the superficial layers (stratum opticum and above) or deeper laminae (stratum griseumn intermediate and below) can be made with reasonable certainty utilizing

7 STROBE REARING AFFECTS HAMSTER'S COLLICULUS 577 a combination of histological and electrophysiological indices. Histologically, the stratum opticum can be identified with a fibre stain. Further, with our electrodes, in each penetration a region was encountered in which the vast majority of the units recorded were almost undoubtedly fibres since they exhibited monophasic action potentials of very brief duration and showed either no or very short injury discharges. This area is approximately um from the collicular surface and corresponds to the 8tratum opticum. In addition, the visual response properties of cells in the superficial laminae differ along several dimensions from those recorded in the deeper layers (Tiao & Blakemore, 1976b; Chalupa & Rhoades, 1977a). A 3 Superficial layers B 100 _ Deep layers 100 *~ * _ 90 _ Statistical Null Statistical Null criterion criterion criterion criterion Fig. 2. In A, the percentage ofdirectionally selective neurones as judged by the statistical and null criteria, encountered in the superficial and deep collicular laminae of normally reared hamsters, and in B, the same data for the strobereared animals. Note that the effect of stroboscopic rearing upon the incidence of directionally selective cells appears to be the same regardless of the layers sampled or the criterion employed. For those cells which did exhibit directional selectivity a distribution of preferred directions was computed. For cells judged as selective by the statistical measure the preferred direction was defined as that which accounted for the most variability in its respective t test. For the null cells the direction chosen was that which yielded the greater number of discharges. Fig. 3 depicts the results of this analysis for both the normal (A) and strobereared (B) hamsters. In the normal animals a significant preference for upward movement is evident both for the statistically selective cells (X2 = 3312, P < 0.01) and also for that subset of these neurones exhibiting selectivity as defined by the null measure (X2 = 2983, P < 0.01). In considering the distribution of preferred directions for the strobereared hamsters caution must be exercised since the number of directionally selective cells was so small as to preclude 19 PHY 274

8 578 L. M. CHALUPA AND R. W. RHOADES meaningful statistical analysis. It can, however, be said that the 4 to 1 preference for upward as opposed to downward moving stimuli which is evident in the distribution for the normal hamsters appears to be considerably reduced for the strobereared animals. Thus, no one direction appeared to be preferred predominantly by those cells which demonstrated a directional preference. Speed preferences. One hundred and nineteen cells were examined for responses to 271 A p * Statistical criterion A Null criterion 211 w Z 12 9 _ 3,1 6 _ I I I I N UN U UT T LT D LN 27 B 21 (, %.i3 0 z I I I N UN U UT T LT D LN Preferred direction Fig. 3. In A, the distribution of preferred directions for directionally selective neurones recorded from normal hamsters. Note the clear preponderance of cells preferring upward as opposed to downward movement. This is the case when selectivity was judged by either the statistical or null criterion. In B, the same distribution for the directional cells recorded from the strobereared animals.

9 STROBE REARING AFFECTS HAMSTER'S COLLICULUS 579 different stimulus velocities. Cells were classified into one of three types: (a) those which responded only to slowly moving stimuli, less than 50'/sec; (b) those which preferred slow velocities but did respond reliably to rapidly moved stimuli; and (c) cells which exhibited a clear preference for stimulus velocities in excess of 50 0/sec. An example of each of these cell types is shown in Fig. 4 (A, B, and C respectively). The response measure which we employed was the total number of spikes evoked per stimulus sweep. It is likely that many cells which were classified as broadly tuned but exhibiting a preference for slowly moved stimuli would be considered to show a preference for higher stimulus velocities if either the peak frequency of the response (Pettigrew, Nikara & Bishop, 1968) or mean response frequency (Movshon, 1975) were employed. However, the primary aim here was to compare the speed preferences of cells in normal and strobereared hamsters. Thus, while the measure chosen for determining speed selectivity may certainly have affected the overall distribution of preferences (Movshon, 1975), it would do so equally for both groups of animals. Fig. 4 shows the percentages of cells falling into each of the three categories for the normal (D) and strobereared (E) hamsters. In the normal (paralysed and anaesthetized) animals sixtythree cells were tested for speed selectivity and 19 % (n = 12) responded only to slowly moving stimuli, while 651 % (n = 41) were classified as broadly tuned, and / (n = 10) yielded maximum discharges to rapidly moved spots or bars. In the strobereared hamsters 353 % (n = 42) of the neurones tested were discharged only by slowly moving stimuli, while 454 % (n = 54) and 193% (n = 23) were respectively classified as broadly tuned or preferring rapidly moving stimuli. These data then indicate that the stroboscopic rearing paradigm produces a shift in the distribution of speed preferences towards cells which respond only to slowly moving stimuli. Static receptive field characteristics. In normal hamsters it has been reported (Tiao & Blakemore, 1976b; Chalupa & Rhoades, 1977a) that receptive field size increases as a function of depth in the superior colliculus. This point is illustrated in Fig. 5A which depicts the data obtained from a series of normally reared hamsters (paralysed and anaesthetized preparations). A relatively monotonic increase in receptive field size as a function of depth in the colliculus was also evident in the strobereared hamsters as shown in Fig. 5B. It is important to note that these measurements refer only to that portion of the receptive field from which a response could be evoked. We have termed this area the activating region of the receptive field, and it corresponds to the minimum discharge field defined by Henry & Bishop (1972) with reference to striate cortical neurones in the cat. For the majority of cells in the hamster's colliculus, stimulation of an area greater than the activating region results in either attenuation or complete suppression of responses (Tiao & Blakemore, 1976b; Rhoades & Chalupa, 1977b). Sixtysix neurones in the strobereared hamsters were tested for responses to flashed spots of various sizes, the intensity of the spot being equal for all sizes tested. Of these cells 31*8 % (n = 21) showed a similar pattern and magnitude of the poststimulus time histogram regardless of the spot size employed (Fig. 6A). In normal hamsters 19 % (n = 19) of the cells studied exhibited such an insensitivity to the size of a flashed stimulus. A smaller number of cells, 91 % (n = 6) in the strobereared hamsters and 6 % (n = 6) in normal animals, responded to increases in stimulus size in a manner I92

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11 STROBE REARING AFFECTS HAMSTER'S COLLICULUS 581 similar to that depicted in Fig. 6B. Here a spot which exceeded the boundaries of the cell's activating region evoked a greater response than a small spot centered on that region. The majority of cells in both the strobe and normally reared hamsters exhibited clear decreases in discharge magnitudes when large stimuli were employed. In 22*7 % (n = 15) of the cells tested in the strobereared animals and 29% (n = 29) of those examined in normals, large spots were completely ineffective in eliciting responses from neurones which yielded brisk discharges to smaller spots of light (Fig. 6C). In 364% (n = 24) of the cells in the strobereared animals and 46% (n = 46) of the cells investigated in normal hamsters the suppression observed with large stimuli was asymmetric, in that it affected the 'on' and 'off' discharges unequally. An example of this partial suppression is shown in Fig. 6D. These findings suggest that stroboscopic rearing results in some changes in the processing of stationary stimuli by the hamster's colliculus. Cells whose responses are relatively unaffected by increments in stimulus size appear to increase in number and there is also a concomitant decrease in the proportion of cells manifesting either partial or complete suppression of their responses with stimuli whose dimensions exceed the activating region of the receptive field. A further indication of some alteration in the processing of static stimuli by collicular neurones in the strobereared animals are the data from eight cells which yielded sustained discharges to flashed spots of one or more sizes. The responses of four such cells are shown in Fig. 7. Of the 100 cells tested systematically with spot stimuli of various sizes in normal hamsters, all exhibited only phasic responses to light onset and/or offset. In normal animals the majority of cells in the superficial layers of the colliculus can be discharged by stroboscopic flashes although these responses are typically much less vigorous than those obtained with moving stimuli of appropriate size, direction and velocity. In the restricted hamsters we observed no clear increase in responsiveness to stroboscopic or other flashed stimuli relative to moving spots of light. Additionally, there was no obvious preference for stimuli flashed at 2 Hz as compared to other strobe rates. In the deeper collicular layers of the restricted animals many cells did not respond to strobe illumination, but in most instances these could be activated with moving stimuli. Overall twentytwo of the 159 visually responsive cells (13.8 %) Fig. 4. In A, the responses of a superficial collicular neurone, which responded reliably only to relatively slowly moving stimuli. Each point on the graph represents the mean (and standard error) of at least five stimulus presentations. The duration of the oscilloscope traces is 10 sec. The activating region of this receptive field measured 6 x 50 and the stimulus was a 20 light spot (1.5 log units above background). In B, the responses of a cell (activating region 11 x 8 ) from the superficial collicular layers which was broadly tuned with respect to stimulus velocity, but still exhibited some preference for slowly moving stimuli. This cell had no spontaneous activity and thus all the data points reflect discharges evoked by the stimulus (a 4.50 light spot). All other conventions and parameters are the same as in A. In C, a graph which depicts the responses of a deeper layer cell which exhibited a clear preference for rapidly moving stimuli. Here the activating region measured 12 x 1950 and the stimulus was a 50 light spot. All other conventions the same as in A. All three cells were recorded from strobereared hamsters. In D and E the percentage of each of the above three cell types are shown for the normal (D) and strobereared (E) animals. Note the increase in the number ofcells which responded only to slowly moved stimuli in the strobereared hamsters and the concomitant decrease in cells which were broadly tuned.

12 582 L. M. CHALUPA AND R. W. RHOADES in the strobereared hamsters could not be activated reliably by flashed spots of light. This compares with thirtyfive of the 214 neurones tested with flashed stimuli (11*2 %) in the normal hamsters. Thus there was no indication of an increase in the effectiveness of either stroboscopic or other flashed stimuli in driving tectal neurones in the restricted hamsters. Two other effects of stroboscopic rearing upon the functional organization of the hamster's superior colliculus should be noted. First, there was a tendency for neurones in the superficial layers of the colliculus to have considerably more response x 0 E c 10._ 0) L C._ c; C._._ , C 0 B Depth in superior colliculus (pm) Fig. 5. In A, histograms depicting length of the major axis of the receptive field activating region as a function of depth in the colliculus for 124 cells recorded from normal hamsters (anaesthetized and paralysed preparations), and in B, the comparable data for the 151 fields plotted for neurones recorded from the strobereared animals. For each 100 jam class interval the mean major axis length and standard error are shown (an exception is the jgm class in the normal hamsters where only one field was plotted). Note in both cases the fairly monotonic increase in axis length with increasing depth in the colliculus.

13 A STROBE REARING AFFECTS HAMSTER'S COLLICULUS B C i_4.l~~~ L LIL 2 ~~~~~3,PLY s...~ D La Fig. 6. In A, the poststimulus time histograms (PSTHs) summarizing the responses of a cell from the superficial collicular laminae of a strobereared hamster which was relatively unaffected by the size of a flashed spot stimulus. The activating region of the receptive field measured 4 x 40 and the spot sizes were 1, 3 and 250. All stimuli were 15 log units above background and were flashed for 500 msec. In B, the PSTHs depict the responses of a deeper layer cell (activating region 24 x 190) which exhibited clear summation with increasing stimulus size. The spots employed measured 15, 8, and 270. All other conventions and parameters the same as in A. In a, the PSTHs of a superficial layer cell whose responses were completely suppressed by a spot which exceeded the boundaries of the activating region (9 x 50). The stimuli were 17, 45 and 30 spots. All other conventions and parameters the same as in A. In D, the PSTHs illustrate the responses of a superficial layer neurone (activating region 19 x 110) which exhibited partial suppression when large stimuli were employed. Note the attenuation of the phasic off discharge with the largest spot. The stimuli measured 28, 8, and 35'. For all histograms the bin width was 20 msec. Ab

14 584 A L. M. CHALUPA AND R. W. RHOADES I B 0 'I l.l, C w IL D I1 100 Fig. 7. The PSTHs offour cells which yielded sustained responses to standing spots are shown. In A, the activating region of this deeper layer cell measured 24 x 1450, and the spot size was 190. The stimulus duration was 3 sec and the intensity was 15 log units above background. Histogram bin width was 50 msec. The calibration is 25 spikes/bin and 1 sec, and the dark bar below the histogram indicates the period during which the stimulus was on. In B, the cell was recorded from the superficial laminae and the receptive field activating region measured 11 x 8. The spot size was 6, stimulus duration 5 sec. All other conventions and parameters the same as in A except for the bin width which was 100 msec. In C, the receptive field activating region measured 26 x 110 and the stimulus was a 30 spot, duration 5 sec. All other parameters and conventions were the same as those in A. In D, a cell which yielded relatively sustained on and offresponses to a 14' spot (5 see duration). This cell had no spontaneous activity. The receptive field activating region measured 7 x 90, and the bin width employed for the PSTH was 100 msec. All other conventions and stimulus parameters were the same as those in A.

15 STROBE REARING AFFECTS HAMSTER'S COLLICULUS 585 variability than that evidenced in normally reared animals. Secondly, there was an increase in the number of neurones in the superficial laminae which were completely unresponsive to visual or extravisual (somatosensory and auditory) stimuli. Of the 183 cells isolated in the superior colliculus of these animals. seventeen neurones localized to the superficial layers were classified as unresponsive. These were noted only because of their spontaneous activity levels. In contrast, in normal hamsters of the 313 cells isolated, only five neurones from the superficial layers could not be activated by visual stimuli. The cells classified as unresponsive were thoroughly A EF a 10'e B ti' F ' I~~~~~~~~~A C G J D H Fig. 8. The responses of an unusual cell recorded in the superficial collicular laminae of a strobereared hamster. The cell exhibited a relatively high degree of axis selectivity as is evidenced by a comparison of the responses in A and C with those in E and G. Specificity for stimulus size is also indicated when A and C are compared with B and D. The receptive field activating region measured 22 x 180 and the stimuli employed were dark 'tongues' (1 0 log units below background illumination) which measured 2*3 and 8.50 in width. These were moved into the receptive field at a velocity of approximately 10 '/sec. The duration of the oscilloscope traces is 10 sec.

16 586 L. M. CHALUPA AND R. W. RHOADES tested with a wide variety of visual and extravisual stimuli, including stroboscopic stimulation with the frequency at which the animals were raised. It is thus unlikely that the failure to activate these neurones was due to inadequate stimulation. We did in fact, encounter several cells which could be activated only by a limited set of 20 15s 10l 5 In (A c._ 0 B I I I I I I I 0 z C Cells isolated/1 00,pm Fig. 9. The number of visually responsive neurones isolated per 100 jsm of electrode travel through the superior colliculus is shown for fortyfour penetrations in paralysed and anaesthetized normal hamsters (A), thirtysix electrode penetrations in similarly prepared hamsters reared from birth to adulthood in total darkness (B), and for thirtyeight penetrations in strobereared animals (C). The mean number of responsive cells isolated per 100 jm (calculated from the grouped data) was 158 in the normal hamsters, 094 in the darkreared animals, and 109 in the strobereared group. stimuli. Examples of the responses of one such neurone recorded in the superficial collicular layers of a strobereared animal are shown in Fig. 8. This cell was completely insensitive to spots or bars of light. It also failed to respond to diffuse

17 STROBE REARING AFFECTS HAMSTER'S COLLICULUS 587 illumination or strobing. It was however, highly responsive to a narrow tongue moved slowly either upward or downward into the receptive field. In addition to some axis selectivity (compare for example, 8A and C with 8E and G), this cell also exhibited a high degree of specificity for stimulus size (compare 8A and C with B and D). A stimulus larger than the receptive field activating region failed to evoke any response from this cell (A cn c 0 a1) 0(n Strobe reared Normals Fig. 10. The mean percentage (and standard error) of trials on which an orientating response to a sunflower seed was obtained in normal (n = 10) and strobereared (n = 7) hamsters. While the percentage for the normal animals was greater than that for the strobereared group this difference was not statistically significant. Another variable which has been examined by several investigators following restricted rearing is the number of responsive cells encountered in a given electrode penetration (e.g. Blakemore & Cooper, 1970; Pettigrew, Olson & Hirsch, 1973). A substantial decrease in the number of cells isolated might suggest that the changes seen following restriction result from some failure to 'maintain' neurones whose response properties do not 'match' the restricted environment. The data shown in Fig. 9 suggest that the functional changes observed in the hamster's colliculus following stroberearing were not accompanied by any dramatic reduction in the number of cells encountered in a given electrode track. In A and B of this Figure are

18 588 L. M. CHALUPA AND R. W. RHOADES the number of visually responsive cells isolated per 100,m in the colliculus of normal (A) and darkreared (B) hamsters. We have previously reported that darkrearing (Rhoades & Chalupa, 1977a; Chalupa & Rhoades, 1977b) results in only relatively subtle changes in the functional organization of the hamster's colliculus. It is readily apparent that the data from the strobe reared animals (Fig. 9C) fall well within the range defined by these two other groups. Visual orientating behaviour. In an effort to determine whether or not the functional changes which we observed in the colliculus of the strobereared hamsters were reflected in their visually guided behaviour, seven strobereared and ten normal animals were tested using the procedures developed by Schneider (1967, 1969). The results of these tests are shown in Fig. 10. The strobereared animals as a group failed to orientate to visual stimuli more often than the normal animals, however, these differences were not statistically significant. While Schneider (1967, 1969) has shown that the visual orientating test we employed was adequate to differentiate hamsters with lesions of the superior colliculus from normal and cortically ablated animals, it may be necessary to use more elaborate behavioural methods to demonstrate any effects of stroberearing. This problem is now being pursued in our laboratory. DISCUSSION Rearing hamsters in a stroboscopic environment produced several changes in the functional organization of the superior colliculus. Dynamic response properties, that is, those which can be investigated only with moving stimuli, showed the most clearcut effects. These consisted of a reduction in the incidence of directionally selective cells, as well as a change in the distribution of the speed preferences of collicular neurones. There were in addition, strong indications that static response properties also changed as a result of this restriction paradigm. This was suggested by a decrease in the number of cells whose responses were suppressed by flashed spots larger than the activating region of the receptive field as well as the presence of cells which responded in a sustained fashion to stationary stimuli. The effects of stroboscopic rearing on collicular as well as cortical functional organization have been previously investigated in the cat. At both the collicular (Flandrin, Kennedy & Amblard, 1976) and cortical (Cynader, Berman & Hein, 1973; Olson & Pettigrew, 1974; Cynader & Chernenko, 1976) levels this manipulation has been shown to reduce the incidence of directional selectivity and also to induce changes in the distribution of ocular dominance. Stroboscopic rearing has also been demonstrated to decrease the number of orientation selective cortical neurones and to increase the proportion of these cells responsive to stroboscopic illumination (Cynader et al. 1973; Olson & Pettigrew, 1974). The interpretation of these changes in strobereared cats, especially those regarding orientation and directional selectivity, is complicated by the fact that visual deprivation results in a similar pattern of changes in the cat's visual system (see Barlow, 1975 for a review). At cortex binocular visual deprivation has been shown by a number of investigators (e.g. Pettigrew, 1974; Imbert & Buisseret, 1975; Blakemore & Van Sluyters, 1975; Buisseret & Imbert, 1976; Cynader, Berman & Hein, 1976; Singer & Tretter, 1976) to reduce the incidence of both orientation and direction selectivity,

19 STROBE REARING AFFECTS HAMSTER'S COLLICULUS 589 while similar rearing procedures have been demonstrated by Sterling & Wickelgren (1970), Hoffmann & Sherman (1975), and Flandrin & Jeannerod (1975, 1977) to reduce the number of directionally selective cells encountered in the superior colliculus. In the cat the effects of stroberearing may be the result of any one or some combination of three processes: (a) the active incorporation of an abnormal environmental input into the functional organization of the visual structure in question; (b) the disruption of a genetically prespecified state by the aberrant input; or (c) a failure to maintain that state as a result of some insufficiency in the roaring environment. Unequivocal evidence for active incorporation would be provided by the de novo synthesis of neuronal response properties which reflected some stimulus characteristics) of the restricted environment. That such a process may have been functional in strobereared cats is suggested by the presence of units in visual cortex which responded only to stroboscopic illumination (Cynader et al. 1973). Several other restriction studies reporting abnormal visual response properties that reflected the environmental input during development also support the conclusion that such inputs may be actively incorporated into the developing visual system (Hirsch & Spinelli, 1970; Pettigrew & Freeman, 1973; Van Sluyters & Blakemore, 1973). In contrast, a purely disruptive effect would entail a departure from the normally observed functional organization without the concomitant occurrence of new receptive field characteristics or types. This may be viewed as an interruption of or interference with a genetically determined organization which develops normally in the absence of any visual input. Conceptually, this mechanism can be differentiated from a 'failure to maintain' which might also underlie some of the effects of restrictive rearing. Here insufficient environmental input is unable to support or validate an initially specified but labile functional organization. Empirically it is not possible in a single restriction experiment to differentiate the latter two mechanisms. Such a differentiation can, however, be accomplished by comparing the consequences of restriction with comparable periods of visual deprivation. In the hamster we have shown (Rhoades & Chalupa, 1977 a) that visual experience is not necessary for the establishment and/or maintenance of the dynamic response properties characteristic of superior collicular neurones in this species. Directional selectivity as judged by either the statistical or null criterion and the distribution of speed preferences were the same in the animals reared from birth to adulthood in total darkness as for normal hamsters. The only differences detected between normal and darkreared animals was the occurrence of a few cells which responded in a sustained fashion to large spots of light. Thus for the most part the functional organization of the hamster's colliculus is innately specified and that organization requires no environmental input for its maintenance. The data which we have presented here best support the disruption hypothesis. This is indicated by the changes observed in the dynamic response properties, that is, directional selectivity and speed preferences. Both of these characteristics of collicular functional organization in the hamster develop normally in the absence of visual experience, thus environmental insufficiency or failure to maintain could not provide the basis for these effects of stroboscopic rearing. Further, the lack of an increase in responsivity to strobe illumination or flashed spots seems to minimize the possibility

20 590 L. M. CHALUPA AND R. W. RHOADES that the effects we observed in the strobereared animals were due to an active incorporation of the rearing environment. On the basis of the findings presented in the present study, and those dealing with the darkreared animals (Rhoades & Chalupa, 1977a) the following view may be put forth regarding the role of environmental influences upon the functional organization of the hamster's superior colliculus. This system has a strong genetic basis (as compared to the cat) in that it develops relatively normally without visual experience. At the same time, an aberrant visual input, such as strobe illumination, can disrupt and perhaps induce some active adaptation at the neuronal level. Since our animals were raised from birth to adulthood in a stroboscopic environment we do not know if there is an optimal or critical period for producing these effects. The adaptive significance, if any, for the existence of a developmental period during which aberrant stimulation can result in a certain degree of functional reorganization in the hamster's superior colliculus is also unclear. The behavioural results we have obtained indicate little or no impairment in the strobereared animals' ability to orientate to visual stimuli. This suggests that the functional changes which we have described may play a relatively minor role in the hamster's visually guided behaviour. However, more stringent testing procedures may uncover some behavioural correlates of these effects. For purposes of comparison, we employed data obtained from two studies from this laboratory dealing with normal animals (Chalupa & Rhoades, 1977a; Rhoades & Chalupa, 1977b), and one which examined the effects of darkrearing upon the functional organization of the superior colliculus in this species (Rhoades & Chalupa, 1977 a). All of these animals were tested under conditions comparable to these in the present study (see Methods). To our knowledge, the only other published paper dealing with the hamster's superior colliculus is that of Tiao & Blakemore (1976b). In most respects the data which we have obtained from normal animals are in agreement with the results of those investigators. There are however, two discrepancies which are germane to the present study. First, Tiao & Blakemore (1976b) reported that only about 12 % of the cells in the normal hamster's superior colliculus are directionally selective as defined by the null criterion. Secondly, they found that very few of the cells which they recorded would respond at all beyond velocities of /sec. The reasons for these differences are unclear since we have consistently observed a higher incidence of directional selectivity in normal and darkreared hamsters, and have found in all groups which we have tested that the majority of visual cells respond reliably to stimuli whose velocities exceed 50 '/sec. Differences in microelectrode characteristics, type and depth of anaesthesia employed and levels of background illumination might account for some of these discrepancies. We wish to thank Mark Diaz, Mark Leary and Nolan Rosenbaum for their assistance in various phases of this study. Supported in part by the Chancellor's Patent Fund, and R03 MH from N.I.M.H. REFERENCES BARLOW, H. B. (1975). Visual experience and cortical development. Nature, Lond. 258, BARLOW, H. B. & LEVICK, W. R. (1965). The mechanism of directionally selective unit in rabbit's retina. J. Phy8iol. 178, BERMAN, N. & DAW, N. W. (1977). Comparison of the critical periods for monocular and directional deprivation in cats. J. Physiol. 265,

21 STROBE REARING AFFECTS HAMSTER'S COLLICULUS 591 BTLAEMORE, C. & COOPER, G. F. (1970). Development of the brain depends on the visual environment. Nature, Lond. 228, BLAKEmORE, C. & VAN SLUYTERS, R. C. (1975). Innate and environmental factors in the development of the kitten's visual cortex. J. Physiol. 248, BUIsSERET, P. & IMBERT, M. (1976). Visual cortical cells: their developmental properties in normal and dark reared kittens. J. Phy8iol. 255, CHALTUPA, L. M. & RHOADES, R. W. (1977a). Responses of visual, somatosensory and auditory neurones in the golden hamster's superior colliculus. J. Physiol. 270, CHALUPA, L. M. & RHOADES, R. W. (1977b). Directional selectivity in hamster's superior colliculus is modified by stroberearing but not by darkrearing. Science, N.Y. (in the Press). CHALUPA, L. M. & RHOADES, R. W. (1977c). Environmental influences on the fictional organization of the superior colliculus in the golden hamster. Soc. Neuroscience. 3, 423. CYNADER, M., BERMAN, N. & HEIN, A. (1973). Cats reared in stroboscopic illumination: effects on receptive fields in visual cortex. Proc. natn. Acad. Sci. U.S.A. 70, CYNADER, M., BERMAN, N. & HEIN, A. (1976). Recovery of function in cat visual cortex following prolonged deprivation. Expl Brain Re8. 25, CYNADER, M. & CHERNENKO, G. (1976). Abolition of direction selectivity in the visual cortex of the cat. Science, N.Y. 193, DRAGER, U. C. & HUBEL, D. H. (1975). Responses to visual stimulation and the relationship between, visual, auditory and somatosensory inputs in mouse superior colliculus. J. Neurophy8iol. 38, FLANDRIN, J. M. & JEANNEROD, M. (1975). Superior colliculus: environmental influences on the development of directional responses in the kitten. Brain Re8. 89, FLANDRIN, J. M. & JEANNEROD, M. (1977). Lack of recovery in collicular neurons from the effects of early deprivation or neonatal cortical lesion in kitten. Brain Re8. 120, FLANDRIN, J. M., KENNEDY, H. & AMBLARD, B. (1976). Effects of stroboscopic rearing on the binocularity and directionality of cat superior colliculus neurons. Brain Rea. 101, GLIcKsTEIN, M. & MILLODOT, M. (1970). Retinoscopy and eye size. Science, N.Y. 168, HENRY, G. H. & BISHOP, P. 0. (1972). Striate neurons: receptive field organization. Inve8t Ophth. 11, HIRsCH, H. V. B. & Spnqsaij, D. N. (1970). Visual experience modifies distribution of horizontally and vertically oriented receptive fields in cats. Science, N.Y. 168, HOFFMANN, K.P. & SHERMAN, S. M. (1975). Effects of early binocular deprivation on visual input to cat superior colliculus. J. Neurophysiol. 38, HuBEL, D. H. (1960). Single unit activity in lateral geniculate body and optic tract of unrestrained cats. J. Phyaiol. 150, IMBERT, M. & BUISSERET, P. (1975). Receptive field characteristics and plastic properties of visual cortical cells in kittens reared with and without visual experience. Expl Brain Re". 22, KLUVER, H. & BAmaiRA, E. (1953). A method for the combined staining of cells and fibers in the nervous system. J. Neuropath. exp. Neurol. 12, MOVSHON, J. A. (1975). The velocity tuning of single units in cat striate cortex. J. Phy8iol. 249, OLSON, C. R. & PETrIGREW, J. D. (1974). Single units in visual cortex of kittens reared in stroboscopic illumination. Brain Re8. 70, PETTIGREW, J. D. (1974). The effect of visual experience on the development of stimulus specificity by kitten cortical neurones. J. Phyiiol. 237, PETTIGREW, J. D., NIxARA, T. & BISHOP, P. 0. (1968). Responses to moving slits by single units in cat striate cortex. Expl Brain Re8. 6, PETTIGREW, J. D., OLSON, C. & HISCH, H. V. B. (1973). Cortical effect of selective visual experience: degeneration or reorganization? Brain Re8. 51, PETTIGREW, J. D. & FREEMAN, R. D. (1973). Visual experience without lines: effect on developing cortical neurones. Science, N.Y. 182, RHOADES, R. W. & CHALUPA, L. M. (1976). Directional selectivity in the superior colliculus of the golden hamster. Brain Me&. 118, RHOADES, R. W. & CHALUPA, L. M. (1977 a). Receptive field characteristics of superior colliculus neurons and visually guided behaviour in darkreared hamsters. J. comp. Neurol. (in the Press).

22 592 L. M. CHALUPA AND R. W. RHOADES RHOADES, R. W. & CHALUPA, L. M. (1977 b). Differential effects of stimulus size on 'on' and 'off responses of superior collicular neurons. Expl Neurol. 57, SINGER, W. & TREETER, F. (1976). Receptivefield properties and neuronal connectivity in striate and parastriate cortex of contourdeprived cats. J. Neurophy8iol. 39, SCHNEIDER, G. E. (1967). Contrasting visuomotor functions of tectum and cortex in the golden hamster. Psychol. Forsch. 31, SCHNEIDER, G. E. (1969). Two visual systems. Science, N.Y. 136, STERLING, P. & WICKELGUEN, B. G. (1970). Function of the projection from the visual cortex to the superior colliculus. Brain Behav. & Evol. 3, TIAO, Y.C. & BLAKEMORE, C. (1976a). Functional organization in the visual cortex of the golden hamster. J. comp. Neurol. 168, TIAO, Y.C. & BLAxEMoRE, C. (1976b). Functional organization in the superior colliculus of the hamster. J. comp. Neurol. 168, VAN SLUTERS, R. C. & BTAKMORE, C. (1973). Experimental creation of unusual neuronal properties in visual cortex of kittens. Nature, Lond. 246,