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INTRODUCTION |
In normal hearing cats, an unexpected sound elicits a rapid, short-duration (<1.0 s) head-orienting response (OR) that points the eyes and ears toward the sound source (acoustical orientation). Because the accuracy of the OR varies directly with the duration of the acoustic stimulus, we have proposed that the trajectory of an OR initiated during the stimulus may be modified by response-produced auditory feedback (Beitel and Kaas 1993
). If ORs were executed simply reflexively on the basis of cues available at stimulus onset, the alignment of the head with the sound source should be independent of the stimulus duration (Beitel 1991
; Beitel and Kaas 1993
; Knudsen et al. 1979
). Once an OR is initiated, however, the stimulus impinging at the ears changes as the head rotates in the sound field, and cues provided by the continuing portion of the acoustic stimulus may be used to determine final head position. An OR, in other words, may be modified "on-line" during its execution (Guitton et al. 1990
).
A useful theoretical framework for describing the effects of response-produced auditory feedback, or its distortion, on ORs is provided by the concept of reafference (von Holst 1954
; von Holst and Mittelstaedt 1950
). Reafference refers to sensory input that is a consequence of self-produced movement, in contrast to sensory input that depends on activity external to the organism. For example, an animal receives reafferent auditory, somatosensory, vestibular, and visual inputs when it moves its head in a sound field, and normally these reafferent inputs are harmonious.
To evaluate the effect of distorting auditory reafference on acoustical orientation by normal hearing cats and by cats with auditory cortex ablated bilaterally, in this study, an experimental procedure was adapted from Wallach (1939)
so that an OR produced a corresponding movement of the sound source, i.e., produced an isogonal rotation of the head and sound field. Specifically, the azimuthal location of the sound source (
a = 85°) relative to the median plane of the head (0°) was invariant during an OR. This procedure produced sensorimotor discordance or discordant reafference (Howard 1968
), a term used in this report to refer to unnatural auditory stimulation that is a consequence of self-produced movement. Conceptually, this experimental arrangement is analogous to the stabilized image paradigm used in oculomotor research (Robinson 1965
).
Two predictions were tested with this artificial procedure. First, ORs executed during the stimulus by a normal hearing cat should terminate in large overshoots (hypermetria:OR >
a) as the animal attempts to orient toward the constantly receding sound source. In contrast, ORs elicited by stationary sound sources usually terminate in small undershoots (Beitel and Kaas 1993
). Second, sound localization acuity is reduced significantly in mammals after bilateral ablation of auditory cortex (Beitel and Kaas 1993
; Heffner and Heffner 1990
; Kavanagh and Kelly 1987
). If processing of reafferent acoustic stimuli also is affected by bilateral cortical lesions, performance should be essentially the same as that observed during acoustical orientation toward stationary sources of sound (Beitel and Kaas 1993
), i.e., ORs executed by cats with cortical lesions should typically terminate in large undershoots (hypometria: OR <
a).
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METHODS |
Four normal hearing cats (normal group) and four cats with bilateral ablation of auditory cortex (lesion group) were used in this study. Each of the cats had been tested previously in an investigation of orientation to stationary sources of sound; a detailed description of the surgical and behavioral procedures common to the two studies has been published elsewhere (Beitel and Kaas 1993
). The unique feature of the present study was the manner in which stimuli were delivered on each trial. Stimuli were produced from two small receivers that moved isogonally with every movement of an animal's head (Fig. 1B, inset). On each trial, a stimulus duration was chosen so that the offset of the stimulus occurred before, during, or after an OR. The ORs were recorded on moving film, and the data were extracted subsequently by quantitative analysis of the film. All procedures followed National Institutes of Health guidelines for care and use of laboratory animals.

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| FIG. 1.
Trajectories of head-orienting responses (ORs) evoked by sources of sound that rotated with the head. Parameter is stimulus duration (1.0, 0.3, and 0.1 s). Estimated reaction times (RTs) were >0.1 s for all illustrated trials. Zero values on the abscissas correspond to the frame for each trial that occurred before the onset of head movement. A and B: head position and head velocity as functions of movement duration in a normal hearing cat. *, secondary movement of the head. C and D: head position and head velocity as functions of movement duration in a cat with auditory cortex ablated bilaterally. - - -, in A and C, are a = 85°. B, inset: top view of the receivers attached to a cat's head. Receivers were located in the horizontal plane at an azimuthal angle ( a) of 85° to the left and right of straight ahead with respect to the median and interaural planes of the head (0°, 0°).
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Surgical and histological procedures
Cats were anesthetized with pentobarbital sodium (40 mg/kg), and a percutaneous post was cemented to the midline of the skull over the frontal sinus. Sterile surgical procedures were used to ablate the auditory cortex bilaterally by subpial aspiration in the lesion group. The cortical ablations removed or undercut all cortical fields between the suprasylvian sulcus and the rhinal fissure. Histological evidence on the extent of the lesions in the cats used in this study has been summarized in a previous report (Beitel and Kaas 1993
).
Behavioral procedures
TESTING PROCEDURE.
The wire-mesh testing enclosure was lined with 20-cm-thick fiberglass and contained a platform on which a cat was harnessed in prone position during testing. At the beginning of a testing session, an aluminum frame (10 g) was attached to the percutaneous post. The frame consisted of a midline indicator that was aligned with the median plane of the head to provide a reference for subsequent analysis of the film and two rigid 7-cm lengths of 2-mm-diam aluminum rod. A miniature receiver (Knowles BK-1600) was attached at the end of each rod; the rods were aligned so that the receivers were located in the horizontal plane at an azimuthal angle (
a) of 85° to the left and right of straight ahead with respect to the median and interaural planes of the head (0°,0°; cf. Fig. 1B, inset). A thin shielded cable attached to the frame connected the receivers to the stimulus generating system. A testing session consisted of six trials presented with a 2-min minimum intertrial interval. One unique pairing of stimulus duration and left or right sound source position occurred on each trial. Stimulus durations and positions were counterbalanced over trials.
ACOUSTIC STIMULI.
On each trial, a single burst of noise was delivered from a noise generator (Grason-Stadler 901-A) to an electronic switch (Grason-Stadler 829-C; rise-fall = 10 ms), to a matched impedance transformer and then through attenuators to one of the receivers. The output of the receivers was uniform (±7 dB) from ~1 to 3.5 kHz (rolloff >20 dB/octave for frequencies > 4 kHz). Stimulus level settings were adjusted to 50 dB-A SPL (re 20 µN/m2) at the region occupied by a cat's head during testing. Stimulus durations (0.1, 0.3, and 1.0 s) were timed by a Hunter Decade Timer.
CINEMATOGRAPHY AND ANALYSIS OF FILM.
ORs were photographed at 10 frames/s with a constant-speed 35-mm movie camera enclosed in a cabinet and mounted on a stand above the testing enclosure. On each trial, the first frame was produced 100 ms before the onset of the acoustic stimulus. Two neon bulbs mounted near the restraining platform were pulsed at 120 pulses/s and produced pulse-marks on the film representing the trial and stimulus durations. On each trial the camera operated for 1 s after the offset of the acoustic stimulus.
Data were extracted from the developed film by projecting images onto sheets of paper. The position of the midline indicator in the frame preceding the onset of the acoustic stimulus was drawn, and the angular rotation of the midline indicator then was drawn frame by frame and measured with simple drafting tools. In some trials, a head movement consisted of an initial and secondary components (cf. Fig. 1, A and B). In this study, an OR was defined as the initial component of a head movement, and its magnitude in degrees was measured as the difference between the position of the midline indicator before stimulus onset and the position of the indicator when the head movement either terminated or paused in a fixed position of
100 ms duration. Measurement error was <1°. However, because the ORs were filmed at 10 frames/s, any changes in the trajectory of an OR that occurred within 100 ms intervals were not recorded on film. Reaction time (RT) to the onset of an OR was estimated for each trial by counting the number of frames from stimulus onset to the frame in which displacement of the midline indicator was first captured. Estimated RT (±50 ms) was defined as the time from stimulus onset to the midpoint between the frame that preceded the onset of head movement and the frame in which head movement first occurred. Movement duration was measured by counting the number of frames between the onset and termination of head movement.
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RESULTS |
A total of 48 trials (6 trials per cat) were obtained from the two groups of cats. However, in the normal group there was no response on two trials (1 at 0.1-s duration; 1 at 0.3-s duration), and in the lesion group, there was no response on five trials (3 at 0.1-s duration; 2 at 0.3-s duration). The seven no-response trials have been excluded from the analyses of results presented below.
Habituation
Because the unreinforced head-orienting response habituates with repetition of acoustic stimuli (Beitel and Kaas 1993
; Thompson and Welker 1963
), it is important to determine whether the results were confounded by habituation of the OR during the testing session. To test for habituation, two analyses were conducted. First, the magnitudes of ORs on trials during the first and second halves of the testing session were matched for stimulus duration and then compared by a binomial test. The difference scores for first and second halves of the testing session were not significant (P > 0.05) in either group of cats. Second, there was no increase in RTs between first and second halves of the testing session in either group of cats (P > 0.05). These results indicate that habituation of the OR, as measured by the magnitude and latency of response, did not occur during the testing session.
General features of ORs
Every OR was initiated in the correct right or left direction in both the normal and lesion groups. The overall median RT between stimulus onset and initiation of an OR was 150 ms, and there were no significant differences in RTs among stimulus durations or between the two groups of animals (P > 0.05). RTs were generally longer than those reported previously (Beitel and Kaas 1993
); the difference may be due to the reduced amplitude of the stimuli in the present study (50 vs. 70 dB SPL). It is noteworthy that on all trials in which the stimulus duration was 0.1 s, the ORs in both groups of cats were initiated after the offset of the stimulus. The overall mean movement duration of ORs was 505 ± 176 (SD) ms. There were no significant differences in movement durations among stimulus durations or between the two groups of animals (P > 0.05).
Kinematic analysis of OR trajectories
Figure 1, A and B, shows examples of head position and velocity as functions of movement duration in a normal cat. The stimulus was delivered from a receiver that was located at 85° on the cat's right. Stimulus durations were 1.0, 0.3, or 0.1 s; estimated RT on each of the three trials was 150 ms. In each trial, the OR rapidly rotated the cat's head rightward, and the magnitude of the response varied directly with the stimulus duration (Fig. 1A). For the trial with a stimulus duration of 0.1 s, head movement was initiated after the offset of the stimulus, and the magnitude of the OR was 83°, i.e., final head position undershot the initial position of the acoustic target by 2°. For the trials with stimulus durations of 0.3 and 1.0 s, the sound field was rotated by the head movement. The magnitudes of the ORs were 130.5 and 136.5°, corresponding to overshoots of 45.5 and 51.5°, respectively. Although each OR was initiated toward the target, for trials in which head movement overlapped the continuing portion of the signal, response-produced change in target location decisively influenced the magnitude of the ORs.
The velocity curves shown in Fig. 1B are monophasic functions, and although peak velocities vary directly with the magnitudes of the ORs, this relationship is nonlinear. OR magnitudes at stimulus durations of 0.3 and 1.0 s are similar, whereas peak velocities at stimulus durations of 0.1 and 0.3 s are similar and much lower than peak velocity for the 1.0-s duration stimulus. For the data combined across stimulus durations, the magnitudes and peak velocities of ORs were positively correlated in the normal group (r = 0.53; n = 22; P < 0.02).
The trajectories shown in Fig. 1, C and D, are from a lesioned cat, and the conditions of stimulation for the three trials depicted were identical to those described above. Stimulus durations were 1.0, 0.3, or 0.1 s; and estimated RTs on the three trials were 150, 150, and 250 ms, respectively. In contrast to the results for the normal cat, each of the ORs terminated in an undershoot, although the sound field was rotated by a head movement on trials with stimulus durations of 0.3 and 1.0 s. For trials with stimulus durations of 0.1, 0.3, and 1.0 s, the magnitude of ORs was 50, 42.5, and 60°, and the corresponding undershoots were 35, 42.5, and 25°.
The velocity curves shown in Fig. 1D are monophasic functions, and for the data combined across stimulus durations, the magnitudes and peak velocities of ORs were correlated positively in the lesion group (r = 0.58; n = 19; P < 0.01).
Figure 2A shows mean head position during the first 400 ms after the onset of head movement in the two groups of cats. There are several notable features in this graph. The curves diverge from one another 100 ms after the initiation of movement, and the values of mean head position at each stimulus duration in the normal group exceed all values of mean head position in the lesion group. The magnitude of mean head position is directly proportional to the stimulus duration in the normal group at movement durations >100 ms. A similar but much weaker trend occurs also in the lesion group. The trajectories for stimulus durations of 0.3 and 1.0 s in the normal group show a distinctive increase in rate of change in head position (velocity) between 100 and 200 ms. The significance of this observation is that on-line modifications of ORs may occur during the initial 200 ms of the orienting response. Finally, mean head position overshoots the initial position of the sound source between 200 and 300 ms at stimulus durations of 0.3 and 1.0 s in the normal group, whereas the magnitude of the remaining trajectories approach asymptotes at 400 ms that in each case undershoot the initial position of the sound source (i.e., mean ORs < 85°).

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| FIG. 2.
A: mean head position as a function of movement duration. Parameter is stimulus duration (1.0, 0.3, and 0.1 s). Normal group, open symbols; lesion group, closed symbols. , estimated average time of offset of 0.3-s duration stimuli (0.3 s-median RT = 150 ms). - - -, a = 85°. B: mean magnitude of ORs as a function of stimulus duration. - - -, a = 85°. Bars represent standard errors.
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With the exception of one trial in the lesion group, all ORs reached peak velocity within 400 ms after the onset of head movement. The results of an analysis of variance (ANOVA) showed that mean peak velocity was 515 and 256°/s for the normal and lesion groups, respectively (F = 34.63; df = 1/35; P < 0.0001). Neither the duration variable nor the interaction involving the lesion and duration variables were significantly different (P > 0.05).
In a previous study that included the cats used in the present study (Beitel and Kaas 1993
), we reported that ORs evoked by stationary sources of sound are stepwise movements with classical saccade-like features. In the present study, ORs were rapid stepwise movements, velocity profiles were monophasic, and the magnitudes and peak velocities of ORs were positively correlated in both the normal and the lesion groups. These results are characteristic for saccadic head movements (Guitton et al. 1984
, 1990
; Zangmeister et al. 1981
), suggesting that the saccade-like profile of the OR was not perturbed when response-produced auditory feedback was distorted.
Magnitude of ORs
In the normal group, the minimum and maximum magnitudes of ORs were 66 and 188°. In the lesion group, the corresponding magnitudes were 8 and 92°; in this group, 18 of 19 ORs undershot the initial position of the sound source. To summarize the effects of the lesion and stimulus duration variables on the magnitude of ORs, an ANOVA was conducted, and the results of the analysis are depicted in Fig. 2B, which shows mean OR for the two groups of cats as a function of stimulus duration. The F ratios for main effects (lesion and duration) were significant (P < 0.0001 and P < 0.002, respectively). For the normal and lesion groups, mean OR was 105.2 and 54.3°, and mean OR varied directly with stimulus duration (60.3, 85.5, and 94.4°, respectively). However, the interaction involving the lesion and duration variables was not statistically significant (P > 0.05).
Orthogonal comparisons of the 0.1- and 0.3-s duration conditions showed that mean OR was significantly larger at the longer stimulus duration in the normal group (F = 8.359; df = 1/35; P < 0.01) but not in the lesion group (P > 0.05). The overall stimulus duration effect is primarily a result of differences in performance at 0.1-s duration versus performance at the longer stimulus durations in the normal group, indicating that the magnitude of ORs may be dramatically affected when the head movement produces an isogonal rotation of the sound field.
Effects of stationary versus rotating sound sources on the magnitude of ORs in normal hearing cats
Because ORs executed toward stationary sound sources typically terminate in small undershoots in normal hearing cats (Beitel and Kaas 1993
) and ORs elicited during isogonal rotation of the sound field typically terminate in large overshoots (this study), a direct comparison of these conditions should provide useful information for assessing the functional significance of response-produced auditory feedback in acoustical orientation. To make this comparison, data for ORs to stationary sources were obtained from protocols used in experiment 1 in Beitel and Kaas (1993)
. The data include all trials (n = 13 trials from 6 cats) at stimulus durations of 0.3 and 1.5 s in which 70°
a
90°.
Figure 3A illustrates the trajectories for head position on two single trials in cat 123, one in response to a stationary sound source (
a = 83.5°) and one in response to a rotating sound source (
a = 85°). In the stationary trial, head movement was initiated 100 ms after the onset of a 0.3-s duration burst of noise. Normal auditory feedback was available during the trajectory, and final head position is aligned with the acoustic target. On the rotating trial, head movement was initiated 150 ms after the onset of the stimulus. Distorted response-produced auditory feedback was introduced on this trial by isogonal rotation of the sound source, resulting in an exaggerated OR. On rotating trials with stimulus durations of 0.3 or 1.5 s (n = 15 trials from 4 cats), five ORs were more hypermetric than the OR illustrated in Fig. 3A.

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| FIG. 3.
Comparison of ORs evoked by rotating versus stationary sources of sound in normal hearing cats. A: trajectories for head position on 2 single trials in cat 123, one in response to a stationary sound source( ; sampling rate = 20 frames/s; a = 83.5°) and one in response to a rotating sound source (- - -; sampling rate = 10 frames/s; a = 85°). Horizontal lines indicate estimated onset and offset of acoustic stimuli (0.3 s noise bursts). B: mean normalized magnitude of head-orienting responses (OR/ a) as a function of stimulus duration in the control group. - - -, OR = a. Bars represent standard errors. Values of an amplification factor (å) are shown on the upper abscissa (cf. DISCUSSION). In rotating trials, a = 85°; in stationary trials, 70° a 90°. Sound level: 70 dB SPL for stationary trials; 50 dB SPL for rotating trials. Longer stimulus duration was 1.0 s for rotating trials and 1.5 s for stationary trials.
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To compare statistically the magnitude of ORs elicited by stationary sound sources with ORs elicited by rotating sound sources, an ANOVA was conducted using the normalized magnitude of responses (OR/
a) as the data. The results of the analysis are depicted in Fig. 3B, which shows the mean normalized magnitude of ORs to stationary and rotating sources of sound as a function of stimulus duration. For the stationary and rotating conditions, the overall means were 0.979 and 1.404, respectively (F = 14.058; df = 1/24;P < 0.001); however, the duration variable and the interaction term were not significant (P > 0.05). For the stationary source condition, the means for stimulus durations of 0.3 and 1.5 s are located below and close to the dashed line in Fig. 3B that represents alignment of the head with the sound source (OR/
a = 1). For the rotating condition, however, the means for stimulus durations of 0.3 and 1.0 s are located above the dashed line (hypermetria).
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DISCUSSION |
The results of this study confirm the two predictions made in INTRODUCTION regarding the likely effects of isogonal rotation of the head and sound field on ORs. When head movements were initiated before termination of the noise burst, ORs were typically hypermetric (OR >
a) in the normal hearing cats. Normal reafference is a feedback system that has a limiting effect on movement (von Holst 1954
). If, however, the head movement produces an isogonal rotation of the sound field, normal reafferent feedback is distorted, and the head may continue to rotate until, in an extreme case, it is limited by the mobility of the neck. A situation this extreme actually occurred in a normal hearing cat that executed an OR of 188° during a 1.0-s duration burst of noise.
In contrast, performance by cats with bilateral ablation of auditory cortex was essentially the same as that observed previously during acoustical orientation toward stationary sources of sound (Beitel and Kaas 1993
). Isogonal rotation of the head and sound field appeared to have little effect on the magnitude of ORs, i.e., responses in these animals terminated in large undershoots (hypometria: OR <
a).
In natural listening conditions, animals typically are confronted with more complex acoustic situations than those investigated in this study. The cat has mobile pinnae, and efference copy information about head and pinna position must be combined with concordant sensory inputs to point the head accurately at an acoustic target (Wise and Irvine 1983
). Furthermore the head and the acoustic target may be in motion simultaneously, requiring an animal to distinguish between acoustic stimuli resulting from its own movements and acoustic stimuli resulting from movements of the sound source. The results of this study suggest that the vital ability to process either kind of acoustic stimuli may be affected after bilateral ablation of auditory cortex.
Normal hearing cats
An on-line modification of the head-orienting response has been observed previously in cats orienting to visual targets (Guitton et al. 1990
). In the present study, the performance of the normal hearing cats under discordant reafferent conditions is consistent with the hypothesis that ORs elicited by stationary acoustic targets may be modified on-line by auditory reafference. Presumably normal auditory reafference has a limiting effect on orienting head movements (von Holst 1954
), resulting in final head positions that are typically hypometric (OR <
a). If this feedback mechanism is distorted by isogonal rotation of the head and sound field, the animals would be expected to execute ORs that overshoot the initial position of the acoustic target.
For the normal hearing cats, the main results may be succinctly summarized with an algorithm introduced by Robinson (1965; also cf. Fuchs 1967
) to describe the behavior of the oculomotor system under various conditions of visual feedback. If a proportion of the OR is added to the azimuthal position of the sound source,
a, the resulting target position
t is given by
where å (an amplification factor) is the proportion of the OR added to the initial azimuthal angle. The error angle
e is the difference between the angular positions of the target and the magnitude of the OR
where K = å
1. In the artificial condition in which an isogonal rotation of the head and sound field occurs, å = 1, K = 0, and
e =
a. The amplification factor, å, is entirely discordant in this case. This condition is analogous to the stabilized image paradigm in oculomotor research, and the error remains constant at the magnitude of the azimuthal angle of the sound source. However, for the normal reafferent condition in which an OR is executed in a stationary sound field, å = 0, K =
1, and
e =
a
OR. The error is the simple difference between the magnitudes of the azimuthal angle and the orienting response.
This argument is summarized graphically in Fig. 3B, which identifies å = 0 and å = 1 with the stationary source and rotating source conditions, respectively. The results of this study offer compelling evidence that discordant auditory reafference (å = 1) profoundly perturbed the OR in the normal hearing cats during isogonal rotation of the head and sound field. The mean trajectories of head movements showed a marked increase in rate of change in head position (velocity) between 100 and 200 ms (Fig. 2A), mean head positions were hypermetric 300 ms after the onset of head movement (Fig. 2A), and ORs terminated in large overshoots (Fig. 3B). These observations suggest that during an isogonal rotation of the head and sound field, the distortion of normal auditory reafference results in a motor command that increases the velocity and magnitude of the head movement within 200 ms of the onset of the OR. For the normal reafferent condition (å = 0), however, ORs terminated in small undershoots that essentially aligned the median plane of the head with the acoustic target.
None of the results obtained with the isogonal rotation condition would be expected if an OR was executed exclusively on the basis of information available at stimulus onset because final head position presumably would be independent of the stimulus duration or the extent of temporal overlap between the stimulus and the response (Beitel 1991
; Beitel and Kaas 1993
; Knudsen et al. 1979
).
Effects of bilateral ablation of auditory cortex
It is not surprising that the ORs executed by cats in the lesion group consistently terminated in large undershoots. Auditory cortex is a necessary component of the auditory forebrain for consistently accurate acoustical orientation (Beitel and Kaas 1993
; Thompson and Masterton 1978
), thresholds for right-left discrimination are elevated, and ability to discriminate sound direction within a lateral hemifieldis abolished after bilateral ablation of auditory cortex (Heffner and Heffner 1990
; Kavanagh and Kelly 1987
). Furthermore, bilateral lesions of the auditory cortex have been reported to impair the ability of dogs to detect simulated movement of a sound source (Altman and Kalmykova 1986
), and, in cat, small bilateral lesions restricted to the primary auditory cortex impaired saccadic-pursuit head tracking of moving acoustic targets (unpublished observations). Thus it seems reasonable to suggest that processing of normal and distorted reafferent acoustic stimuli also is affected by bilateral ablation of auditory cortex.
The large cortical ablations in this study (cf. Beitel and Kaas 1993
) probably disrupted descending pathways to the superior colliculus (Meredith and Clemo 1989
), to the basal ganglia (Reale and Imig 1983
), and to the pons (Diamond et al. 1969
); one or more of these pathways may be components of a premotor system contributing to head-orienting responses to acoustic stimuli. For example, the ablations removed all gray matter in the caudal half of the anterior ectosylvian sulcus where an auditory field (Field AES) (Clarey and Irvine 1986
) is located that sends a robust projection to the superior colliculus (Meredith and Clemo 1989
). Electrical stimulation of the deep layers of the superior colliculus in the cat elicits orienting head and pinna movements (Harris 1980
; Roucoux et al. 1980
; Stein and Clamann 1981
), and brain stem lesions that destroy or isolate the superior colliculus unilaterally may affect the accuracy (Sprague and Meikle 1965
) or the latency (Thompson and Masterton 1978
) of ORs evoked by acoustic stimuli. A question arises, therefore, as to whether the cortical ablations in this study affected predominantly premotor components of the sensorimotor systems involved in the head -orienting response.
The available evidence suggests a negative answer to this question. A detailed neurological examination of the cats in the lesion group found no evidence that the surgical ablation of auditory cortex impaired a cat's ability to execute large magnitude ORs to tactile stimuli, and each of the animals previously produced at least one large OR (>90°) to stationary sources of sound (Beitel and Kaas 1993
). There were no differences in the mean reaction times or the mean durations of ORs between the two groups, and the results indicate that the saccade-like profile of the response trajectories was not perturbed after bilateral ablation of auditory cortex. Collectively, these observations suggest that the premotor and motor systems for the head orienting response were not affected by the lesions.
Limitations of the study
Three issues that were not addressed by this study are noteworthy. First, the sampling rate (10 frames/s) was too slow to determine whether an OR executed during isogonal rotation of the sound field consisted of only one or several head saccades. Recently we have examined this issue in normal hearing cats using a search coil procedure (Beitel and Jenkins 1995
). Preliminary results indicate that ORs executed during isogonal rotation of the sound field may exhibit complex velocity profiles with multiple peaks, suggesting an integrated succession of saccadic components rather than a single exaggerated head saccade. Second, only one azimuthal angle (
a = 85°) was investigated in the present study. The perturbing effects (hypermetria) produced by discordant auditory reafference in normal hearing cats do not occur if the rotating sound source is located in the frontal sound field (±30°), i.e., the effects may be restricted to acoustic targets that are located eccentrically in the lateral sound field near the interaural axis (Beitel and Jenkins 1995
). Finally, the cat has mobile pinnae, and no attempt was made to determine whether pinna movements occurred before or during head rotation.