Biomedical subjects

T R Stanford

Publications and source records attributed to T R Stanford.

11 recordsLinked to original sources

PubMed1997

Intracellular recordings in response to monaural and binaural stimulation of neurons in the inferior colliculus of the cat.

The inferior colliculus (IC) is a major auditory structure that integrates synaptic inputs from ascending, descending, and intrinsic sources. Intracellular recording in situ allows direct examination of synaptic inputs to the IC in response to acoustic stimulation. Using this technique and monaural or binaural stimulation, responses in the IC that reflect input from a lower center can be distinguished from responses that reflect synaptic integration within the IC. Our results indicate that many IC neurons receive synaptic inputs from multiple sources. Few, if any, IC neurons acted as simple relay cells. Responses often displayed complex interactions between excitatory and inhibitory sources, such that different synaptic mechanisms could underlie similar response patterns. Thus, it may be an oversimplification to classify the responses of IC neurons as simply excitatory or inhibitory, as is done in many studies. In addition, inhibition and intrinsic membrane properties appeared to play key roles in creating de novo temporal response patterns in the IC.

Acoustic Stimulation↗

PubMed1997

A neuronal population code for sound localization.

The accuracy with which listeners can locate sounds is much greater than the spatial sensitivity of single neurons. The broad spatial tuning of auditory neurons indicates that a code based on the responses of ensembles of neurons, a population code, must be used to determine the position of a sound in space. Here we show that the tuning of neurons to the most potent localization cue, the interaural time difference in low-frequency signals (< approximately 2kHz), becomes sharper as the information ascends through the auditory system. We also show that this sharper tuning increases the efficiency of the population code, in the sense that fewer neurons are required to achieve a given acuity.

Animals↗

PubMed1996

Combined eye-head gaze shifts produced by electrical stimulation of the superior colliculus in rhesus monkeys.

1. We electrically stimulated the intermediate and deep layers of the superior colliculus (SC) in two rhesus macaques free to move their heads both vertically and horizontally (head unrestrained). Stimulation of the primate SC can elicit high-velocity, combined, eye-head gaze shifts that are similar to visually guided gaze shifts of comparable amplitude and direction. The amplitude of gaze shifts produced by collicular stimulation depends on the site of stimulation and on the parameters of stimulation (frequency, current, and duration of the stimulation train). 2. The maximal amplitude gaze shifts, produced by electrical stimulation at 56 sites in the SC of two rhesus monkeys, ranged in amplitude from approximately 7 to approximately 80 deg. Because the head was unrestrained, stimulation-induced gaze shifts often included movements of the head. Head movements produced at the 56 stimulation sites ranged in amplitude from 0 to approximately 70 deg. 3. The relationships between peak velocity and amplitude and between duration and amplitude of stimulation-induced head movements and gaze shifts were comparable with the relationships observed during visually guided gaze shifts. The relative contributions of the eyes and head to visually guided and stimulation-induced gaze shifts were also similar. 4. As was true for visually guided gaze shifts, the head contribution to stimulation-induced gaze shifts depended on the position of the eyes relative to the head at the onset of stimulation. When the eyes were deviated in the direction of the ensuing gaze shift, the head contribution increased and the latency to head movement onset was decreased. 5. We systematically altered the duration of stimulation trains (10-400 ms) while stimulation frequency and current remained constant. Increases in stimulation duration systematically increased the amplitude of the evoked gaze shift until a site specific maximal amplitude was reached. Further increases in stimulation duration did not increase gaze amplitude. There was a high correlation between the end of the stimulation train and the end of the evoked gaze shift for movements smaller than the site-specific maximal amplitude. 6. Unlike the effects of stimulation duration on gaze amplitude, the amplitude and duration of evoked head movements did not saturate for the range of durations tested (10-400 ms), but continued to increase linearly with increases in stimulation duration. 7. The frequency of stimulation was systematically varied (range: 63-1,000 Hz) while other stimulation parameters remained constant. The velocity of evoked gaze shifts was related to the frequency of stimulation; higher stimulation frequencies resulted in higher peak velocities. The maximal, site-specific amplitude was independent of stimulation frequency. 8. When stimulating a single collicular site using identical stimulation parameters, the amplitude and direction of stimulation-induced gaze shifts, initiated from different initial positions, were relatively constant. In contrast, the amplitude and direction of the eye component of these fixed vector gaze shifts depended upon the initial position of the eyes in the orbits; the endpoints of the eye movements converged on an orbital region, or "goal," that depended on the site of collicular stimulation. 9. When identical stimulation parameters were used and when the eyes were centered initially in the orbits, the gaze shifts produced by caudal collicular stimulation when the head was restrained were typically smaller than those evoked from the same site when the head was unrestrained. This attenuation occurred because stimulation drove the eyes to approximately the same orbital position when the head was restrained or unrestrained. Thus movements produced when the head was restrained were reduced in amplitude by approximately the amount that the head would have contributed if free to move. 10. When the head was restrained, only the eye component of the intended gaze shift

Animals↗

PubMed1996

Site and parameters of microstimulation: evidence for independent effects on the properties of saccades evoked from the primate superior colliculus.

1. Microstimulation is used to investigate how activity in the superior colliculus (SC) contributes to determining the properties of primate saccadic eye movements. The site of collicular stimulation, the duration of the stimulation train, and the frequency of the stimulation train are each varied to examine the relative contributions of the locus, duration, and level of collicular activity to determining saccade amplitude, direction, duration, and velocity. 2. For any given site of stimulation, a relationship between movement amplitude and train duration can be demonstrated. Movement amplitude is a monotonically increasing, but saturating, function of increasing train duration. The size of the largest movement is dictated by the site of stimulation. Within the range over which amplitude can be modulated, movement offset is linked to the offset of the stimulation train. As a result, each decrement or increment in train duration produces a corresponding decrement or increment in movement duration. 3. The peak velocity of an evoked movement is influenced by the frequency of stimulation; a higher frequency of stimulation produces a movement of higher velocity. 4. The effects of train duration and frequency can be traded to produce movements that have comparable amplitudes but different dynamic characteristics; high-velocity movements of short duration and low-velocity movements of long duration can be produced by stimulating with high-frequency, short-duration, and low-frequency, long-duration trains, respectively. Across stimulation frequencies, the amplitude of an evoked movement is best related to the total number of pulses in the stimulation train. 5. Because it is possible to compensate for reduced velocity by increasing the duration of the stimulation train, the same site-specific maximum amplitude can be attained with different frequencies of stimulation. 6. Small, but significant, changes in movement direction occur as a result of varying train duration or train frequency. 7. The latency to movement onset (i.e., interval from stimulation onset to movement onset) depends upon the frequency of stimulation. A higher frequency of stimulation produces a movement of shorter latency. 8. These data demonstrate that both the site of stimulation and the parameters of stimulation contribute to determining the properties of a movement evoked from the primate SC. In doing so, they contradict the results of early microstimulation studies that suggest that the properties of eye movements evoked from the primate SC are determined solely by the site of stimulation. The findings conflict with the traditional view of collicular function that suggests that the collicular motor representation is purely anatomic. Rather, these data support a revised view whereby the locus, duration, and level of collicular activity contribute to determining the properties of a primate saccadic eye movement. According to this view, independent information relating to desired displacement and saccade velocity are extracted from the spatiotemporal profile of collicular activity.

Animals↗

PubMed1994

Saccades to remembered target locations: an analysis of systematic and variable errors.

We studied the effects of varying delay interval on the accuracy and velocity of saccades to the remembered locations of visual targets. Remembered saccades were less accurate than control saccades. Both systematic and variable errors contributed to the loss of accuracy. Systematic errors were similar in size for delay intervals ranging from 400 msec to 5.6 sec, but variable errors increased monotonically as delay intervals were lengthened. Compared to control saccades, remembered saccades were slower and the peak velocities were more variable. However, neither peak velocity nor variability in peak velocity was related to the duration of the delay interval. Our findings indicate that a memory-related process is not the major source of the systematic errors observed on memory trials.

Animals↗

PubMed1994

Systematic errors for saccades to remembered targets: evidence for a dissociation between saccade metrics and activity in the superior colliculus.

Pronounced systematic errors are a consistent feature of saccades to remembered target locations. In an effort to determine their neural basis, we recorded from saccade-related burst neurons (SRBNs) in the superior colliculus and compared the discharges associated with saccades to visual and remembered targets. For all recorded SRBNs, the relationship between discharge rate and saccade metrics differed for saccades to visual and remembered targets; movement fields constructed from saccades to remembered targets were shifted relative to those for visual targets. The shifts of the remembered movement fields were consistent with the direction and magnitude of the systematic errors observed on memory trials. These findings suggest that the superior colliculus is not the source of the systematic errors observed on memory trials. Systematic errors are likely due to the addition or omission of signals downstream from the superior colliculus.

Action Potentials↗

PubMed1993

High-frequency neurons in the inferior colliculus that are sensitive to interaural delays of amplitude-modulated tones: evidence for dual binaural influences.

1. Localization of sounds has traditionally been considered to be performed by a duplex mechanism utilizing interaural temporal differences (ITDs) at low frequencies and interaural intensity differences at higher frequencies. More recently, it has been found that listeners can detect ITDs at high frequencies if the amplitude of the sound varies and an ITD is present in the envelope. Here we report the responses of neurons in the inferior colliculi of unanesthetized rabbits to ITDs of the envelopes of sinusoidally amplitude-modulated (SAM) tones. 2. Neurons were studied extracellularly with glass-coated Pt-Ir or Pt-W microelectrodes. Their sensitivity to ITDs in the envelopes of high-frequency sounds (> or = 2 kHz) was assessed using SAM tones that were presented binaurally. The tones at the two ears had the same carrier frequency but modulation frequencies that differed by 1 Hz. This caused a cyclic variation in the ITD produced by the envelope. In this "binaural SAM" stimulus, the carriers caused no ITD because they were in phase. In addition to the binaural SAM stimulus, pure tones were used to investigate responses to ipsilateral and contralateral stimulation and the nature of the interaction during binaural stimulation. 3. Neurons tended to display one of two kinds of sensitivity to ITDs. Some neurons discharged maximally at the same ITD at all modulation frequencies > 250 Hz (peak-type neurons), whereas others were maximally suppressed at the same ITD (trough-type neurons). 4. At these higher modulation frequencies (> 250 Hz), the characteristic delays that neurons exhibited tended to lie within the range that a rabbit might normally encounter (+/- 300 microseconds). The peak-type neurons favored ipsilateral delays, which correspond to sounds in the contralateral sound field. The trough-type neurons showed no such preference. 5. The preference of peak-type neurons for a particular delay was sharper than that of trough-type neurons and was comparable to that observed in neurons of the inferior colliculus that are sensitive to delays of low-frequency pure tones. 6. At lower modulation frequencies (< 150 Hz) characteristic delays often lay beyond +/- 300 microseconds. 7. Increasing the ipsilateral intensity tended to shift the preferred delay ipsilaterally at lower (< 250 Hz), but not at higher, modulation frequencies. 8. When tested with pure tones, a substantial number of peak-type neurons were found to be excited by contralateral stimulation but inhibited by ipsilateral stimulation.(ABSTRACT TRUNCATED AT 400 WORDS)

Acoustic Stimulation↗

PubMed1992

A comparison of the interaural time sensitivity of neurons in the inferior colliculus and thalamus of the unanesthetized rabbit.

The localization of low-frequency sounds (less than 3 kHz) along the azimuth involves comparing the ongoing difference in the time of arrival of a sound at the two ears. Information about interaural time differences (ITDs) is derived from an initial comparison performed in the superior olivary complex. However, little is known about which aspects of this information are transformed as it ascends the brainstem. To address this issue, we compared the ITD sensitivity of neurons in the inferior colliculus (IC) and auditory thalamus, successive stations in the auditory pathway. We found ITD sensitivity in the IC and thalamus to be similar in several respects. At both levels, the large majority of neurons responded maximally to ITDs within the range that a rabbit would normally encounter (+/- 300 microseconds) and preferred ipsilateral delays, delays that would be created by sounds in the contralateral sound field. The range of frequencies over which ITD sensitivity was expressed was also similar in the midbrain and thalamus. Several differences were also apparent. In comparison to IC neurons, neurons in the thalamus responded over more restricted ranges of ITD, responded at lower rates, and, in response to monaural stimulation, showed an increased influence of inhibition. Finally, a greater proportion of thalamic units had characteristic delays corresponding to intermediate discharge rates. The preservation of a bias for ipsilateral delays from IC to thalamus suggests that a representation of contralateral azimuths is present at both levels. Similarities between the two levels suggest that information about ITDs is faithfully transmitted from midbrain to thalamus. Differences in ITD sensitivity, such as the sharper tuning for ITDs, suggest that the thalamus is not a simple relay. Enhanced sensitivity to ITDs should translate to better-defined azimuthal receptive fields, and therefore may be a step toward achieving an optimal representation of azimuth within the auditory pathway.

Acoustic Stimulation↗

PubMed1989

Temporal coding of envelopes and their interaural delays in the inferior colliculus of the unanesthetized rabbit.

1. The difference in the time of arrival of a sound at the two ears can be used to locate its source along the azimuth. Traditionally, it has been thought that only the on-going interaural temporal disparities (ITDs) produced by sounds of lower frequency (approximately less than 2 kHz) could be used for this purpose. However, ongoing ITDs of low frequency are also produced by envelopes of amplitude-modulated (AM) tones. These ITDs can be detected and used to lateralize complex high-frequency sounds (1, 8, 12, 15, 22, 24, 26). Auditory neurons synchronize to the modulation envelope, but do so at progressively lower modulation frequencies at higher levels of the auditory pathway. Some neurons of the cochlear nucleus synchronize best to frequencies as high as 700 Hz, but those of the inferior colliculus (IC) exhibit their best synchrony below 200 Hz. Even though synchrony to higher modulation frequencies is reduced at higher levels of the auditory pathway, is information about ITDs retained? 2. We answered this question by extracellularly recording the responses of neurons in the IC of the unanesthetized rabbit. We used an unanesthetized preparation because anesthesia alters the responses of neurons in the IC to both monaurally presented tones and ITDs. The unanesthetized rabbit is ideal for auditory research. Recordings can be maintained for long periods, and the acoustic stimulus to each ear can be independently controlled. 3. We studied the responses of 89 units to sinusoidally AM tones presented to the contralateral ear. For each unit, we recorded the response at several modulation frequencies. The degree of phase locking to the envelope at each frequency was measured using the synchronization coefficient. Two measures were used to assess the range of modulation frequencies over which phase locking occurred. The "best AM frequency" was the frequency at which we observed the greatest phase locking. The "highest AM frequency" was the highest frequency at which significant phase locking (0.001 level) was observed. We could not assess synchrony to ipsilateral AM tones directly, because most units did not respond to ipsilateral stimulation. 4. We studied the sensitivity of 63 units to ITDs produced by the envelopes of AM tones. Sensitivity to ITDs was tested by presenting AM tones to the two ears that had the same carrier frequency, but modulation frequencies that differed by 1 Hz. Units that were sensitive to ITDs responded to this stimulus by varying their response rate cyclically at the difference frequency, i.e., 1 Hz.(ABSTRACT TRUNCATED AT 400 WORDS)

Animals↗

PubMed1989

Monaural and binaural response properties of neurons in the inferior colliculus of the rabbit: effects of sodium pentobarbital.

1. We studied the effects of sodium pentobarbital on 22 neurons in the inferior colliculus (IC) of the rabbit. We recorded changes in the sensitivity of these neurons to monaural stimulation and to ongoing interaural time differences (ITDs). Monaural stimuli were tone bursts at or near the neuron's best frequency. The ITD was varied by delivering tones that differed by 1 Hz to the two ears, resulting in a 1-Hz binaural beat. 2. We assessed a neuron's ITD sensitivity by calculating three measures from the responses to binaural beats: composite delay, characteristic delay (CD), and characteristic phase (CP). To obtain the composite delay, we first derived period histograms by averaging, showing the response at each stimulating frequency over one period of the beat frequency. Second, the period histograms were replotted as a function of their equivalent interaural delay and then averaged together to yield the composite delay curve. Last, we calculated the composite peak or trough delay by fitting a parabola to the peak or trough of this composite curve. The composite delay curve represents the average response to all frequencies within the neuron's responsive range, and the peak reflects the interaural delay that produces the maximum response. The CD and CP were estimated from a weighted fit of a regression line to the plot of the mean interaural phase of the response versus the stimulating frequency. The slope and phase intercept of this regression line yielded estimates of CD and CP, respectively. These two quantities are thought to reflect the mechanism of ITD sensitivity, which involves the convergence of phase-locked inputs on a binaural cell. The CD estimates the difference in the time required for the two inputs to travel from either ear to this cell, whereas the CP reflects the interaural phase difference of the inputs at this cell. 3. Injections of sodium pentobarbital at subsurgical dosages (less than 25 mg/kg) almost invariably altered the neuron's response rate, response latency, response pattern, and spontaneous activity. Most of these changes were predictable and consistent with an enhancement of inhibitory influences. For example, if the earliest response was inhibitory, later excitation was usually reduced and latency increased. If the earliest response was excitatory, the level of this excitation was unaltered or slightly enhanced, and changes in latency were minimal. 4. The neuron's response pattern also changed in a predictable way. For example, a response with an inhibitory pause could either change to a response with a longer pause or to a response with an onset only.(ABSTRACT TRUNCATED AT 400 WORDS)

Animals↗

PubMed1987

Interaural phase-sensitive units in the inferior colliculus of the unanesthetized rabbit: effects of changing frequency.

We studied the interaural phase sensitivity of 85 units in the inferior colliculus (IC) of the unanesthetized rabbit. We assessed this sensitivity at several frequencies within each unit's responsive range. The interaural phase disparity was varied by delivering tones that differed by 1 Hz to the two ears, resulting in a 1-Hz binaural beat. We analyzed each unit's response to different frequencies by calculating four measures: characteristic delay (CD), characteristic phase (CP), composite peak delay, and mean peak delay. We estimated the CD and CP from the slope and phase intercept, respectively, of the regression line fitted to a plot of the mean interaural phase against stimulating frequency. The composite peak delay was estimated from the peak of a composite delay curve. This was generated by replotting the response to changes in interaural phase, as a function of the equivalent interaural delay and averaging the resultant interaural delay curves. The composite delay curve reflects the unit's average response to interaural delays across frequencies. Last, we calculated a mean peak delay, derived by converting the mean interaural phase of the response at each frequency to an equivalent delay and then averaging these delays. Interaural phase sensitivity was observed to frequencies as high as 2,150 Hz. However, the majority of units showed such sensitivity below 1,500 Hz. For most units, the interaural delay curves measured at several frequencies coincided near the peak discharge. This result is consistent with a neural model, where excitatory inputs from each ear converge upon a binaural cell, evoking maximum discharge only when the two inputs arrive simultaneously. As a first approximation, our data fit this model, indicating that IC neurons can act like coincidence detectors or cross-correlators. The distributions of CD, composite peak delay, and mean peak delay showed that most units preferred ipsilateral stimulus delays, which in the natural situation corresponds to sounds emanating from the contralateral field. Moreover, most units preferred delays that were within the estimated physiological range of the rabbit. These results support the viewpoint that neurons in the IC participate in sound localization. The distributions of CP and CD differ substantially from those found in the IC of the anesthetized cat. These differences may reflect species differences, the effects of anesthesia, or a difference in the population of units sampled. For each unit, we assessed the linearity of the plot of mean interaural phase against frequency of stimulation using a chi 2 method. For most units the plots were significantly nonlinear.(ABSTRACT TRUNCATED AT 400 WORDS)

Animals↗