Biomedical subjects

A R Palmer

Publications and source records attributed to A R Palmer.

At least 19 recordsLinked to original sources

PubMed2001

The ability of inferior colliculus neurons to signal differences in interaural delay.

Sound localization in humans depends largely on interaural time delay (ITD). The ability to discriminate differences in ITD is highly accurate. ITD discrimination (Delta ITD) thresholds, under some circumstances, are as low as 10-20 micros. It has been assumed that thresholds this low could only be obtained if the outputs from many neurons were combined. Here we use Receiver Operating Characteristic analysis to compute neuronal Delta ITD thresholds from 53 cells in the inferior colliculus in guinea pigs. The Delta ITD thresholds of single neurons range from several hundreds of micros down to 20-30 micros. The lowest single-cell thresholds are comparable to human thresholds determined with similar stimuli. This finding suggests that the highly accurate sound localization of human observers is consistent with the resolution of single cells and need not reflect the combined activity of many neurons.

Animals↗

PubMed2001

Precise tuning of barnacle leg length to coastal wave action.

Both spatial and temporal variation in environmental conditions can favour intraspecific plasticity in animal form. But how precise is such environmental modulation? Individual Balanus glandula Darwin, a common northeastern Pacific barnacle, produce longer feeding legs in still water than in moving water. We report here that, on the west coast of Vancouver Island, Canada, the magnitude and the precision of this phenotypic variation is impressive. First, the feeding legs of barnacles from protected bays were nearly twice as long (for the same body mass) as those from open ocean shores. Second, leg length varied surprisingly precisely with wave exposure: the average maximum velocities of breaking waves recorded in situ explained 95.6-99.5% of the variation in average leg length observed over a threefold range of wave exposure. The decline in leg length with increasing wave action was less than predicted due to simple scaling, perhaps due to changes in leg shape or material properties. Nonetheless, the precision of this relationship reveals a remarkably close coupling between growth environment and adult form, and suggests that between-population differences in barnacle leg length may be used for estimating differences in average wave exposure easily and accurately in studies of coastal ecology.

Adaptation, Physiological↗

PubMed2001

A neural code for low-frequency sound localization in mammals.

We report a systematic relationship between sound-frequency tuning and sensitivity to interaural time delays for neurons in the midbrain nucleus of the inferior colliculus; neurons with relatively low best frequencies (BFs) showed response peaks at long delays, whereas neurons with relatively high BFs showed response peaks at short delays. The consequence of this relationship is that the steepest region of the function relating discharge rate to interaural time delay (ITD) fell close to midline for all neurons irrespective of BF. These data provide support for a processing of the output of coincidence detectors subserving low-frequency sound localization in which the location of a sound source is determined by the activity in two broad, hemispheric spatial channels, rather than numerous channels tuned to discrete spatial positions.

Action Potentials↗

PubMed2001

Functional magnetic resonance imaging measurements of sound-level encoding in the absence of background scanner noise.

Effects of sound level on auditory cortical activation are seen in neuroimaging data. However, factors such as the cortical response to the intense ambient scanner noise and to the bandwidth of the acoustic stimuli will both confound precise quantification and interpretation of such sound-level effects. The present study used temporally "sparse" imaging to reduce effects of scanner noise. To achieve control for stimulus bandwidth, three schemes were compared for sound-level matching across bandwidth: component level, root-mean-square power and loudness. The calculation of the loudness match was based on the model reported by Moore and Glasberg [Acta Acust. 82, 335-345 (1996)]. Ten normally hearing volunteers were scanned using functional magnetic resonance imaging (tMRI) while listening to a 300-Hz tone presented at six different sound levels between 66 and 91 dB SPL and a harmonic-complex tone (F0= 186 Hz) presented at 65 and 85 dB SPL. This range of sound levels encompassed all three bases of sound-level matching. Activation in the superior temporal gyrus, induced by each of the eight tone conditions relative to a quiet baseline condition, was quantified as to extent and magnitude. Sound level had a small, but significant, effect on the extent of activation for the pure tone, but not for the harmonic-complex tone, while it had a significant effect on the response magnitude for both types of stimulus. Response magnitude increased linearly as a function of sound level for the full range of levels for the pure tone. The harmonic-complex tone produced greater activation than the pure tone, irrespective of the matching scheme for sound level, indicating that bandwidth had a greater effect on the pattern of auditory activation than sound level. Nevertheless, when the data were collapsed across stimulus class, extent and magnitude were significantly correlated with the loudness scale (measured in phons), but not with the intensity scale (measured in SPL). We therefore recommend the loudness formula as the most appropriate basis of matching sound level to control for loudness effects when cortical responses to other stimulus attributes, such as stimulus class, are the principal concern.

Adult↗

PubMed2000

Phase-locked responses to pure tones in guinea pig auditory cortex.

Phase-locked responses to pure tones are a characteristic of most auditory cells at the level of the brain stem and allow sophisticated analyses based on coincidence detection. Phase-locking to tones has not previously been shown at the level of the auditory cortex in single unit studies. We have now identified phase-locked responses in 10% of low-frequency (< 1 kHz) units in the ventrorostral belt, a strip of cortex immediately ventral to the primary auditory area. All of these units showed phase-locking in their response to binaural tone pips of 60-200 Hz and showed narrow band pass characteristics within this range.

Acoustic Stimulation↗

PubMed2000

Time-course of the auditory BOLD response to scanner noise.

It is a concern for auditory fMRI studies that acoustic noise generated by the scanner produces an auditory response that can confound stimulus-induced activation. To establish how to minimize this problem, the present study mapped the time-course of the auditory response to a burst of acoustic scanner noise by employing a single-event method. Recorded bursts of scanner noise were interspersed with clustered-volume acquisitions at a range of stimulus-to-imaging delays to map the response with a temporal resolution of 1 sec. There were strong responses (1.5% signal change) to scanner noise in primary and secondary auditory cortex. In both cortical areas, the mean response rose to a peak by 4-5 sec after stimulus onset and decayed after a further 5-8 sec. The time course indicates that noise contamination in auditory fMRI can be substantially reduced by using a 9-12-sec repetition time, thus maximizing the dynamic range available for displaying the response to acoustical stimuli of interest.

Adult↗

PubMed2000

Sound-level measurements and calculations of safe noise dosage during EPI at 3 T.

This paper describes systematic methods for measuring and controlling sound levels within a magnetic resonance scanner. The methods are illustrated by application to the acoustic noise generated by a 3 T scanner during echoplanar imaging (EPI). Across five measurement sessions, sound pressure levels at the center of the head gradient coil ranged from 122 to 131 dB SPL [123 to 132 dB(A)]. For protection against damaging noise exposure, UK and US industrial guidelines stipulate that the maximum permitted daily noise dosage is equivalent to 90 dB(A) for 8 hours, where noise dosage is a function of the level of an acoustic signal and the length of exposure to it. Without hearing protection, this equivalent level would be exceeded by less than 5 seconds of exposure to the measured levels of scanner acoustic noise. These findings highlight the importance of noise reduction and hearing protection for those exposed to the acoustic noise generated during EPI.

Auditory Threshold↗

PubMed2000

Identification and localisation of auditory areas in guinea pig cortex.

The organisation of guinea pig auditory cortex was studied by combining histological methods with microelectrode mapping. This allowed the location of seven auditory areas to be determined in relation to the visual and primary somatosensory areas. The auditory areas were identified by single-unit recordings and their borders defined by evoked potential mapping. The visual areas were identified by their relatively high densities of myelinated fibres, while the primary somatosensory cortex was identified by its characteristic barrels of high cytochrome oxidase (CYO) activity in layer IV. The auditory region had moderate levels of CYO and myelin staining. When staining was optimal, there was a clear edge to the moderate CYO activity, which apparently corresponds to the dorsal border of the primary auditory area (AI) and the other core field that lies dorsocaudal to it (DC). Thus the primary somatosensory area and the visual and auditory regions were separated from each other by a region with lower levels of CYO and myelin staining. The ventral borders of AI and DC could not be determined histologically as there were no sharp transitions in the levels of CYO or myelin staining. The two core areas were partially surrounded by belt areas. The dorsorostral belt and most of the belt around DC responded more strongly to broad-band stimuli than pure tones, while the ventrorostral belt, small field and a belt zone ventral to the rostral part of DC responded better to pure tones. Units in the small field (S) typically had higher thresholds and broader tuning to pure tones than AI, while units in the ventrorostral belt typically had longer onset latencies and gave more sustained responses than units in AI.

Animals↗

PubMed2000

Organisation of binaural interactions in the primary and dorsocaudal fields of the guinea pig auditory cortex.

This study investigated the nature and topography of binaural interactions in the primary auditory field (AI) and dorsocaudal field (DC) of the urethane anaesthetised guinea pig auditory cortex. Single and multi-units were classified by their responses to monaural and binaural stimulation. In both AI and DC, units displayed binaural facilitation, binaural inhibition, or a level dependent mixture of facilitation and inhibition. There was a significant difference in the distribution of binaural response types between the two fields. Facilitated units predominated in DC (facilitated: 58%; inhibited: 24%; mixed: 6%; non-interacting: 12%), while inhibited units were the most common class in AI (facilitated: 15%; inhibited: 44%; mixed: 18%; non-interacting: 22%). It has previously been suggested that inhibited and facilitated units are concerned with processing different areas of space suggesting a possible separation of function between the two core fields. Topographically, the binaural response properties in AI and DC varied along isofrequency bands, with neurones displaying similar interactions aggregating in clusters. These clusters were similar in size for the two fields and often overlapped neighbouring isofrequency bands. However, their shape and position varied between different animals. This clustered organisation of binaural interactions is similar to that reported in recent studies of AI in other mammals.

Acoustic Stimulation↗

PubMed2000

Modelling convergent input onto interaural-delay-sensitive inferior colliculus neurones.

Convergent input from cells in the medial superior olive (MSO) and lateral superior olive (LSO) onto a single inferior colliculus (IC) cell explains many findings that are not compatible with a simple coincidence detector mechanism. Here this explanation is tested using a physiologically accurate computer model of the binaural pathway in which the input to the IC cell is either from two MSO cells or a MSO and a LSO cell. Auditory nerve (AN) spike trains are formed by a stochastic hair cell model following a basilar membrane simulation using a gammatone filter. In subsequent cells input spikes cause post-synaptic potentials (PSPs) which are summed causing the cell to fire when the sum crosses a threshold. The individual cells are matched to the physiology by varying the number of inputs, the magnitude and duration of the PSPs and the firing threshold. Non-linear best-phase-versus-frequency functions arise if the two IC inputs have different best frequencies and different characteristic delays. One input can be selectively suppressed by turning on an additional tone at the worst phase of that input. Non-zero characteristic phases arise if the characteristic frequencies of the AN fibres feeding into a single superior olive cell are mismatched.

Ear↗

PubMed2000

Responses of neurons in the inferior colliculus to dynamic interaural phase cues: evidence for a mechanism of binaural adaptation.

Responses to sound stimuli that humans perceive as moving were obtained for 89 neurons in the inferior colliculus (IC) of urethan-anesthetized guinea pigs. Triangular and sinusoidal interaural phase modulation (IPM), which produced dynamically varying interaural phase disparities (IPDs), was used to present stimuli with different depths, directions, centers, and rates of apparent motion. Many neurons appeared sensitive to dynamic IPDs, with responses at any given IPD depending strongly on the IPDs the stimulus had just passed through. However, it was the temporal pattern of the response, rather than the motion cues in the IPM, that determined sensitivity to features such as motion depth, direction, and center locus. IPM restricted only to the center of the IPD responsive area, evoked lower discharge rates than when the stimulus either moved through the IPD responsive area from outside, or up and down its flanks. When the stimulus was moved through the response area first in one direction and then back in the other, and the same IPDs evoked different responses, the response to the motion away from the center of the IPD responsive area was always lower than the response to the motion toward the center. When the IPD was closer at which the direction of motion reversed was to the center, the response to the following motion was lower. In no case did we find any evidence for neurons that under all conditions preferred one direction of motion to the other. We conclude that responses of IC neurons to IPM stimuli depend not on the history of stimulation, per se, but on the history of their response to stimulation, irrespective of the specific motion cues that evoke those responses. These data are consistent with the involvement of an adaptation mechanism that resides at or above the level of binaural integration. We conclude that our data provide no evidence for specialized motion detection involving dynamic IPD cues in the auditory midbrain of the mammal.

Acoustic Stimulation↗

PubMed2000

Neural responses in the inferior colliculus to binaural masking level differences created by inverting the noise in one ear.

We have measured the responses of inferior colliculus neurons in the anesthetized guinea pig to signals which in human psychophysical experiments reveal a release of masking as a result of binaural processing (the binaural masking level difference: BMLD). More specifically we have used diotic tones at 500 Hz (So) masked by noise that is either identical at the two ears (No) or inverted in one ear (Npi). This combination of signals and noise maskers produces a prominent masking release in humans such that the So signal is about 6-12 dB more detectable in the presence of the Npi noise than the No noise. Low-frequency inferior colliculus neurons are sensitive to the interaural delay of the masking noise and generally respond most to the components nearest their best frequency. Since most inferior colliculus neurons have peaks in their delay functions close to zero interaural time delay this means that while No noise is effective in driving the unit, Npi noise is much less effective. As the level of an So tone was progressively increased in the presence of No and Npi noises, the first response could be either an increase or a decrease in the activity due to the noise. However, because Npi generated little or no activity itself, the predominant response to the So tone was an increase in discharge in this condition. Masked thresholds were defined as the point at which the standard separation D (related to the d' of signal detection theory) = 1 in either direction. BMLDs were measured in single neurons and in the majority of units were in a direction consistent with the psychophysical observations irrespective of the direction of the discharge rate change that occurred at threshold. The lowest masked thresholds always occurred at or near the signal frequency of 500 Hz. An average value of the single unit BMLD around 500 Hz was 3.6 dB (NoSo vs. NpiSo) compared with 6.6 dB for the NoSo versus NoSpi BMLD we had previously reported. This lower magnitude is consistent with the hierarchy of human psychophysical BMLDs.

Acoustic Stimulation↗

PubMed2000

Modulation and task effects in auditory processing measured using fMRI.

Active listening has been reported to elicit a different sensory response from passive listening and is generally observed as an increase in the magnitude of activation. Sensory activation differences may therefore be masked by the effect of attention. The present study measured activation induced by static and modulated tones, while controlling attention by using target-discrimination and passive listening tasks. The factorial design enabled us to determine whether the stimulus-induced activation in auditory cortex was independent of the information-processing demands of the task. Contrasted against a silent baseline, listening to the tones induced widespread activation in the temporal cortex, including Heschl's gyrus (HG), planum temporale, superior temporal gyrus (STG), and superior temporal sulcus. No additional auditory areas were recruited in the response to modulated tones compared to static tones, but there was an increase in the response in the STG, anterior to HG. Relative to passive listening, the active task increased the response in the STG, posterior to HG. The active task also recruited regions in the frontal and parietal cortex and subcortical areas. These findings indicate that preferential responses to the changing spectro-temporal properties of the stimuli and to the target-discrimination task involve distinct, non-overlapping areas of the secondary auditory cortex. Thus, in the present study, differences in sensory activation were not masked by the effects of attention.

Acoustic Stimulation↗

PubMed1999

A ventrorostral belt is adjacent to the guinea pig primary auditory cortex.

The previously defined anterior area (A) of guinea pig auditory cortex has been divided into a large dorsal portion identified as the primary area (AI) and a smaller ventrorostral belt (VRB). This division is based on: (1) the much longer response latency of units in VRB (21.7 ms) than AI (14.1 ms); (2) the absence of pure onset units in VRB (i.e. units that lacked a sustained response), which are common in AI; (3) the weakness of noise-induced evoked potentials in VRB compared to AI; (4) units in VRB had lower thresholds and stronger phase locking to amplitude modulated stimuli than in AI.

Animals↗

PubMed1999

"Sparse" temporal sampling in auditory fMRI.

The use of functional magnetic resonance imaging (fMRI) to explore central auditory function may be compromised by the intense bursts of stray acoustic noise produced by the scanner whenever the magnetic resonance signal is read out. We present results evaluating the use of one method to reduce the effect of the scanner noise: "sparse" temporal sampling. Using this technique, single volumes of brain images are acquired at the end of stimulus and baseline conditions. To optimize detection of the activation, images are taken near to the maxima and minima of the hemodynamic response during the experimental cycle. Thus, the effective auditory stimulus for the activation is not masked by the scanner noise. In experiment 1, the course of the hemodynamic response to auditory stimulation was mapped during continuous task performance. The mean peak of the response was at 10.5 sec after stimulus onset, with little further change until stimulus offset. In experiment 2, sparse imaging was used to acquire activation images. Despite the fewer samples with sparse imaging, this method successfully delimited broadly the same regions of activation as conventional continuous imaging. However, the mean percentage MR signal change within the region of interest was greater using sparse imaging. Auditory experiments that use continuous imaging methods may measure activation that is a result of an interaction between the stimulus and task factors (e.g., attentive effort) induced by the intense background noise. We suggest that sparse imaging is advantageous in auditory experiments as it ensures that the obtained activation depends on the stimulus alone.

Acoustic Stimulation↗

PubMed1999

Desynchronizing responses to correlated noise: A mechanism for binaural masking level differences at the inferior colliculus.

We examined the adequacy of decorrelation of the responses to dichotic noise as an explanation for the binaural masking level difference (BMLD). The responses of 48 low-frequency neurons in the inferior colliculus of anesthetized guinea pigs were recorded to binaurally presented noise with various degrees of interaural correlation and to interaurally correlated noise in the presence of 500-Hz tones in either zero or pi interaural phase. In response to fully correlated noise, neurons' responses were modulated with interaural delay, showing quasiperiodic noise delay functions (NDFs) with a central peak and side peaks, separated by intervals roughly equivalent to the period of the neuron's best frequency. For noise with zero interaural correlation (independent noises presented to each ear), neurons were insensitive to the interaural delay. Their NDFs were unmodulated, with the majority showing a level of activity approximately equal to the mean of the peaks and troughs of the NDF obtained with fully correlated noise. Partial decorrelation of the noise resulted in NDFs that were, in general, intermediate between the fully correlated and fully decorrelated noise. Presenting 500-Hz tones simultaneously with fully correlated noise also had the effect of demodulating the NDFs. In the case of tones with zero interaural phase, this demodulation appeared to be a saturation process, raising the discharge at all noise delays to that at the largest peak in the NDF. In the majority of neurons, presenting the tones in pi phase had a similar effect on the NDFs to decorrelating the noise; the response was demodulated toward the mean of the peaks and troughs of the NDF. Thus the effect of added tones on the responses of delay-sensitive inferior colliculus neurons to noise could be accounted for by a desynchronizing effect. This result is entirely consistent with cross-correlation models of the BMLD. However, in some neurons, the effects of an added tone on the NDF appeared more extreme than the effect of decorrelating the noise, suggesting the possibility of additional inhibitory influences.

Acoustic Stimulation↗

PubMed1998

Convergent input from brainstem coincidence detectors onto delay-sensitive neurons in the inferior colliculus.

Responses of low-frequency neurons in the inferior colliculus (IC) of anesthetized guinea pigs were studied with binaural beats to assess their mean best interaural phase (BP) to a range of stimulating frequencies. Phase plots (stimulating frequency vs BP) were produced, from which measures of characteristic delay (CD) and characteristic phase (CP) for each neuron were obtained. The CD provides an estimate of the difference in travel time from each ear to coincidence-detector neurons in the brainstem. The CP indicates the mechanism underpinning the coincidence detector responses. A linear phase plot indicates a single, constant delay between the coincidence-detector inputs from the two ears. In more than half (54 of 90) of the neurons, the phase plot was not linear. We hypothesized that neurons with nonlinear phase plots received convergent input from brainstem coincidence detectors with different CDs. Presentation of a second tone with a fixed, unfavorable delay suppressed the response of one input, linearizing the phase plot and revealing other inputs to be relatively simple coincidence detectors. For some neurons with highly complex phase plots, the suppressor tone altered BP values, but did not resolve the nature of the inputs. For neurons with linear phase plots, the suppressor tone either completely abolished their responses or reduced their discharge rate with no change in BP. By selectively suppressing inputs with a second tone, we are able to reveal the nature of underlying binaural inputs to IC neurons, confirming the hypothesis that the complex phase plots of many IC neurons are a result of convergence from simple brainstem coincidence detectors.

Acoustic Stimulation↗

PubMed1997

Detectability index measures of binaural masking level difference across populations of inferior colliculus neurons.

In everyday life we continually need to detect signals against a background of interfering noise (the "cocktail party effect"): a task that is much easier to accomplish using two ears. The binaural masking level difference (BMLD) measures the ability of listeners to use a difference in binaural attributes to segregate sound sources and thus improve their discriminability against interfering noises. By computing the detectability of tones from rate-versus-level functions in the presence of a suprathreshold noise, we previously demonstrated that individual low-frequency delay-sensitive neurons in the inferior colliculus are able to show BMLDs. Here we consider the responses of a population of such neurons when the noise level is held constant (as conventionally in psychophysical paradigms). We have sampled the responses of 121 units in the inferior colliculi of five guinea pigs to identical noise and 500 Hz tones at both ears (NoSo) and to identical noise but with the 500 Hz tone at one ear inverted (NoSpi). The result suggests that the neurons subserving detection of So tones in No (identical noise at the two ears) noise are those neurons with best frequencies (BFs) close to 500 Hz that respond to So tones with an increase in their discharge rate from that attributable to the noise. The detection of the inverted (Spi) signal is also attributable to neurons with BFs close to 500 Hz. However, among these neurons, the presence of the Spi tone was indicated by an increased discharge rate in some neurons and by a decreased discharge rate in others.

Animals↗