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Biomedical subjects

D P Phillips

Publications and source records attributed to D P Phillips.

At least 19 recordsLinked to original sources
PubMed2003

Acoustic hemifields in the spatial release from masking of speech by noise.

The Hearing-in-Noise Test (HINT) is able to measure the benefit to speech intelligibility in noise conferred when the noise masker is displaced 90 degrees in eccentricity from a speech source located at zero degrees azimuth. Both psychoacoustic and neurophysiological data suggest that the perceptual benefit of the 90-degree azimuth separation would be greatest if the speech and noise were presented in different acoustic hemifields, and would be smallest if the two sources were in the same acoustic hemifield. The present study tested this hypothesis directly in ten normal-hearing adult listeners. Using the HINT stimuli, we confirmed the hypothesis. Release from masking scores averaged 8.61 dB for "between-hemifield" conditions, 6.05 dB for HINT conditions, and 1.27 dB for "within-hemifield" conditions, even though all stimulus configurations retained a 90-degree angular separation of speech and noise. These data indicate that absolute separation of speech and noise alone is insufficient to guarantee a significant release from masking, and they suggest that what matters is the location of the stimulus elements relative to the left and right spatial perceptual channels.

Adult↗
PubMed2002

Central auditory onset responses, and temporal asymmetries in auditory perception.

Historically, central auditory responses have been studied for their sensitivity to various parameters of tone and noise burst stimulation, with response rate plotted as a function of the stimulus variable. The responses themselves are often quite brief, and locked in time to stimulus onset. In the stimulus amplitude domain, it has recently become clear that these responses are actually driven by properties of the stimulus' onset transient, and this has had important implications for how we interpret responses to manipulations of tone (or noise) burst plateau level. That finding was important in its own right, but a more general scrutiny of the available neurophysiological and psychophysical evidence reveals that there is a significant asymmetry in the neurophysiological and perceptual processing of stimulus onsets and offsets: sound onsets have a more elaborate neurophysiological representation, and receive a greater perceptual weighting. Hypotheses about origins of the asymmetries, derived independently from psychophysics and from neurophysiology, have in common a response threshold mechanism which adaptively tracks the ongoing level of stimulation.

Acoustic Stimulation↗
PubMed2002

Auditory temporal gap detection for noise markers with partially overlapping and non-overlapping spectra.

Temporal gap detection thresholds were obtained from six listeners using an adaptive tracking method and constant spectrum-level noises. In separate blocks of trials, the markers bounding the gap were systematically varied in their spectral overlap or separation (expressed in equivalent rectangular bandwidths, ERBs). In the same listeners, gap thresholds were also obtained for noises of the same bandwidths as those constituting the overlap in the overlap conditions (in the presence of a wideband notched noise masker: 'mask' conditions). For the spectral overlap/separation conditions, gap thresholds were a systematic, linear function of spectral dissimilarity in four of six listeners. In the mask conditions, gap thresholds were inversely related to bandwidth in all listeners. For the three-, four- and five-ERB conditions, gap thresholds in the same listeners for the spectral overlap conditions were higher than those for mask stimuli with the same available within-channel bandwidth and spectrum levels. These data suggest that the spectral dissimilarity between the markers over-rode the availability of within-channel information in the recovery of the temporal gap.

Adult↗
PubMed2001

Sensitivity of unanesthetized chinchilla auditory system to noise burst onset, and the effects of carboplatin.

The gross near-field responses of the auditory nerve and inferior colliculus to noise burst stimuli were recorded through intracranially implanted electrodes in six unanesthetized chinchillas. Responses were studied as a function of stimulus plateau amplitude and rise time, both before and after a systemic dose of 75 mg/kg of carboplatin. Both recording sites showed sensitivity to stimulus level and rise time. Increases in stimulus level and decreases in stimulus rise time each produced increases in the response magnitude, and decreases in response latency. When the stimuli were re-specified as rate of pressure change at sound onset (Pa/s), the amplitude and latency of responses at each site were found to be a direct function of rate of sound pressure change. These data provide the first confirmation in unanesthetized animals of previous single unit observations in barbiturate-anesthetized cats. Carboplatin treatment resulted in a 20-80% loss of inner hair cells, a modest threshold elevation, and a 50-75% reduction in peak response amplitudes. The general patterns of sensitivity to stimulus level and rise time were not markedly affected by carboplatin, nor was the fashion in which response parameters (amplitude and latency) were ruled by rate of pressure change at sound onset.

Acoustic Stimulation↗
PubMed2001

Spatial and temporal factors in auditory saltation.

This report describes three experiments on auditory saltation, studied with click stimuli presented at interclick intervals (ICIs) from 30 to 240 ms. In experiment 1, subjects rated the strength of the saltation illusion evoked by trains of six monaural clicks (i.e., three presented to one ear followed by three to the other ear), and six dichotic clicks on which were imposed either a 500-micros interaural time difference or a 9-dB interaural level difference (ILD). The interaural disparity of theclicks was reversed at the midpoint of the train. Subjects reported equivalent strengths of saltation for the dichotic clicks, but weaker saltation for the monaural ones. These data indicated that saltation is supported by dichotic clicks, regardless of the stimulus manipulation used to generate the lateralized images. In experiment 2, subjects rated the strength of the saltation evoked by six click trains lateralized, in separate trials, by ILDs ranging from 9 to 30 dB, and by a train of monaural clicks. In all ILD conditions, the level of the clicks at the "quiet" ear was above click-detection threshold when presented monaurally. Saltation weakened with increasing ILD, and approximated that seen with monaural clicks when the ILD was 30 dB. These data indicated that for the range of ICIs used here, saltation weakened as the stimuli became more strongly lateralized. In experiment 3, the number of dichotic clicks preceding the disparity reversal was, in separate trials, varied from 3 to 10, and subjects were asked to detect the presence of spatial stationarity in the click train. By plotting the subjective ratings as a function of the length of the leading click train, it was shown that the temporal window within which the saltation effect operates varies between listeners, but is usually less than about 350-400 ms.

Acoustic Stimulation↗
PubMed2000

Independence of frequency channels in auditory temporal gap detection.

The ability of listeners to detect a temporal gap in a 1600-Hz-wide noiseband (target) was studied as a function of the absence and presence of concurrent stimulation by a second 1600-Hz-wide noiseband (distractor) with a nonoverlapping spectrum. Gap detection thresholds for single noisebands centered on 1.0, 2.0, 4.0, and 5.0 kHz were in the range from 4 to 6 ms, and were comparable to those described in previous studies. Gap thresholds for the same target noisebands were only modestly improved by the presence of a synchronously gated gap in a second frequency band. Gap thresholds were unaffected by the presence of a continuous distractor that was either proximate or remote from the target frequency band. Gap thresholds for the target noiseband were elevated if the distractor noiseband also contained a gap which "roved" in time in temporal proximity to the target gap. This effect was most marked in inexperienced listeners. Between-channel gap thresholds, obtained using leading and trailing markers that differed in frequency, were high in all listeners, again consistent with previous findings. The data are discussed in terms of the levels of the auditory perceptual processing stream at which the listener can voluntarily access auditory events in distinct frequency channels.

Adult↗
PubMed1999

An increase in the number of deaths in the United States in the first week of the month--an association with substance abuse and other causes of death.

BACKGROUND AND METHODS: There are regular changes in mortality rates, such as increased rates of death from influenza in the winter and from motor vehicle accidents on long holiday weekends. Previous research has shown that among persons with schizophrenia, the rates of cocaine use and hospital admissions increase at the beginning of the month, after the receipt of disability payments. Using computerized data from all death certificates in the United States between 1973 and 1988, we compared the number of deaths in the first week of the month with the number of deaths in the last week of the preceding month. RESULTS: The average number of deaths was about 5500 per day, or about 165,000 in a 30-day month. There were 100.9 deaths (95 percent confidence interval, 100.8 to 101.0) in the first week of the month for every 100 deaths in the last week of the preceding month. This was equivalent to about 4320 more deaths in the first week of each month than in the last week of the preceding month in an average year. Between 1983 and 1988, for deaths involving substance abuse and an external cause (such as suicides, accidents, and homicides), there were 114.2 deaths (95 percent confidence interval, 110.5 to 117.9) in the first week of the month for every 100 in the last week of the preceding month. There were significant increases in the number of deaths in the first week of the month for many causes of death, including substance abuse, natural causes, homicides, suicides, and motor vehicle accidents. CONCLUSIONS: In the United States, the number of deaths is higher in the first week of the month than in the last week of the preceding month. The increase at the beginning of the month is associated with substance abuse and other causes of death.

Aged↗
PubMed1999

What's in a name: mortality and the power of symbols.

One's attitude about oneself, and the treatment one receives from others, might be affected, in some small but measurable way, by stigmatic or salutary labeling due to one's name. If names affect attitudes and attitudes affect longevity, then individuals with "positive" initials (e.g., A.C.E., V.I.P.) might live longer than those with "negative" initials (e.g., P.I.G., D.I.E.). Using California death certificates, 1969-1995, we isolated 2287 male decedents with "negative" initials and 1200 with "positive" initials. Males with positive initials live 4.48 years longer (p<0.0001), whereas males with negative initials die 2.80 years younger (p<0.0001) than matched controls. The longevity effects are smaller for females, with an increase of 3.36 years for the positive group (p<0.0001) and no decrease for the negative. Positive initials are associated with shifts away from causes of death with obvious psychological components (such as suicides and accidents), whereas negative initials are associated with shifts toward these causes. However, nearly all disease categories display an increase in longevity for the positive group and a decrease for the negative group. These findings cannot be explained by the effects of death cohort artifacts, gender, race, year of death, socioeconomic status, or parental neglect.

Aged↗
PubMed1999

Response magnitude and timing of auditory response initiation in the inferior colliculus of the awake chinchilla.

Recent single-unit studies in anesthetized cats have revealed that the latency and strength of transient responses to tone burst stimuli are determined largely by stimulus events in the first few ms of the signal. The present study sought to extend these findings by studying the inferior colliculus potential (ICP) in unanesthetized chinchillas. The ICP magnitude and latency were studied as a function of the plateau amplitude and rise time of noise burst stimuli. ICP amplitude increased with stimulus amplitude and decreased with stimulus rise time. ICP latency decreased with stimulus amplitude and increased with stimulus rise time. The absolute values of the ICP latencies confirmed that it is only the first few ms of the stimulus which determine the timing of response initiation, and therefore, that it is not the plateau level of the stimulus that directly determines the latent period. These data constitute a direct link between earlier single-unit studies in anesthetized animals and brainstem-evoked potential data in animals and man.

Animals↗
PubMed1999

Azimuthal tuning of human perceptual channels for sound location.

Human sound localization is acute for frontal locations, but relatively poor in the lateral hemifields. Previous studies in man have not, however, provided evidence on the tuning of the perceptual channels for auditory space that subserve this pattern of acuity. The spatial tuning of perceptual channels used in human azimuthal sound localization was determined using a between-channel auditory temporal gap detection paradigm. In this paradigm, gap thresholds are low when the markers bounding the silent period (gap) activate the same perceptual channel but are elevated when the two markers activate different channels. To determine the tuning of spatial channels, gap thresholds were obtained in an anechoic room with white noise markers coming from each combination of 12 leading marker locations and 18 trailing marker locations throughout the full 360 degrees of azimuth in the horizontal plane through the interaural axis. Gap thresholds remained low (2-4 ms) for all combinations of leading and trailing markers between 30 degrees and 150 degrees in both lateral hemifields. When the leading marker was located deep in one hemifield, and the trailing marker was in the opposite hemifield, gap thresholds rose to 8-16 ms. For leading marker locations at 30 degrees from the midline, gap thresholds were low for all trailing marker locations in the ipsilateral hemifield and locations near the midline in the contralateral hemifield, and were elevated (6-8 ms) only near the contralateral pole. Finally, for leading marker locations at 0 degree or 180 degrees, gap thresholds were low for any trailing location within 30 degrees of the midline at the front or back, and thresholds were elevated for trailing locations at the lateral poles. These data are accountable in terms of two broadly tuned perceptual channels, occupying the left and right auditory hemifields, respectively, each extending 30 degrees across the midline. These channels have widths and locations similar to the spatial receptive fields previously described for central auditory neurons in animals. The data suggest a model of spatial acuity based on the rates of activation of two spatially overlapping channels, rather than the selective activation of members of a large population of finely tuned channels.

Adult↗
PubMed1999

The influence of medical and legal authorities on deaths facilitated by physicians.

This study investigated whether two widely publicized cases of deaths facilitated by physicians were followed by significant peaks in mortality. In March, 1991, Timothy Quill, MD, published a controversial editorial describing the physician-assisted suicide (PAS) of his 45-year-old, female leukemia patient. In a landmark decision in December 1990, the Missouri Supreme Court allowed removal of life support for Nancy Cruzan, a comatose accident victim. Correcting for trends and seasonal fluctuations, the authors examined: (1) U.S. leukemia mortality in the period centered on Quill's editorial, and (2) mortality from accident/coma combinations in the period centered on the Missouri Supreme Court's decision on Cruzan. Female leukemia deaths rose 11.3% above the expected rate (p < .01) just after Quill's article was published. The more closely the decedent matched Quill's patient, the greater the peak, with the largest peak (33.9%) evident for female leukemia patients in their 40s, who were long-term residents of smaller communities (p < .05). Five possible explanations for the findings were assessed, leading to the conclusion that Quill's editorial elicited an increase in mortality. The involvement of physicians in this increase is supported by analysis of the Cruzan case. This showed a mortality peak of 57% for accident/coma patients following the court decision.

Accidents↗
PubMed1999

Auditory gap detection, perceptual channels, and temporal resolution in speech perception.

This article overviews some recent advances in our understanding of temporal processes in auditory perception. It begins with the premise that hearing is the online perceptual elaboration of acoustic events distributed in time. It examines studies of gap detection for two reasons: first, to probe the temporal acuity of auditory perception in its own right and, second, to show how studies of gap detection have provided new insights into the processes involved in speech perception and into the architecture of auditory spatial perceptual mechanisms. The implications of these new data for our comprehension of some central auditory processing disorders are examined.

Auditory Diseases, Central↗
PubMed1998

Factors shaping the response latencies of neurons in the cat's auditory cortex.

This article addresses two issues. Firstly, the hypothesis that response latency might be a neural code for tone frequency was examined in single-neuron data from the primary auditory cortex of anesthetized cats. Minimal response latencies for characteristic frequency (CF) tones were independent of neural CF. Mean response latencies for a constant amplitude CF tone were also independent of CF. These data, and the fact that cortical neurons do not have an obvious independent referent for stimulus onset time, do not support the view that latency is a code for frequency. Secondly, to investigate a simple threshold model of spike initiation time, we describe the prolongations of response latency with increases in stimulus rise time and their dependence on the peak amplitude of the stimulus. These data show that in cortical neurons, it is not the peak stimulus intensity which determines first-spike latency, and second, that the response latencies are systematically not those expected on the basis of simple threshold model.

Acoustic Stimulation↗
PubMed1998

"Central" auditory gap detection: a spatial case.

Normal listeners were tested for their temporal auditory gap detection thresholds using free-field presentation of white-noise stimuli delivered from the left (L) and right (R) poles of the interaural axis. The noise bursts serving as the leading and trailing markers for the silent period were presented in either the same (LL,RR) or different (LR,RL) auditory locations. The duration of the leading marker was a second independent variable. Gap thresholds for stimuli in which the markers had the same location were low, and usually were independent of the duration of the leading marker. Gap thresholds for the LR and RL conditions were longer. These gap thresholds were sensitive to the duration of the leading marker, and increased as the leading marker duration decreased. This finding is consistent with the hypothesis that a relative timing operation mediates gap detection when the markers activate different perceptual channels. The present data suggest that this timing process can operate on perceptual channels emerging from central nervous system processing.

Adult↗
PubMed1998

Recognition of temporally distorted words by listeners with and without a simulated hearing loss.

In separate experiments, word recognition performance of two groups of 12 normal-hearing young adults was investigated as a function of temporal distortion (i.e., time compression ratio or reverberation time) with and without a simulated high-frequency hearing loss (i.e., low-pass filtered at 2000 Hz). Performance decreased significantly as a function of increasing time compression, reverberation, and with the simulated hearing loss (p < .05). A statistically significant interaction between each of time compression ratio and reverberation time with the filtered listening condition was found (p < .05). This finding of an interactive, as opposed to a simple additive, effect of multiple distortions of the speech on word recognition performance suggests that loss of audibility alone cannot account for decrements in word recognition performance with time-altered speech. It is suggested that this multiplicative effect for combined acoustic distortions is a consequence of the functional loss of the high-frequency region of the cochlea. Not only is there a loss of audibility, but there is also a loss in temporal resolution capacity since it is perception mediated through the population of high-frequency auditory channels that has the best temporal resolution.

Adult↗