Psychophysical Detection Thresholds Research Articles

Stimulus features affecting psychophysical detection thresholds for electrical stimulation of the cochlea. I: Phase duration and stimulus duration

The shapes and levels of psychophysical detection threshold versus frequency functions for sinusoidal electrical stimulation of the deafened cochlea vary from subject to subject. These variations have been shown previously to be correlated with nerve-survival patterns. The shapes of these functions are critical in the design and calibration of analog processors for auditory prostheses. This paper examines two stimulus features that may contribute to the shapes of these functions: phase duration and stimulus duration. Psychophysical experiments were performed with unilaterally deafened and implanted macaque monkeys. Effects of phase duration were studied by measuring thresholds for single symmetric biphasic pulses presented at a rate of 1 per trial. Psychophysical detection thresholds for these pulses decreased, in the region of maximum slope, at a rate averaging -6.2 dB per doubling of phase duration, which is steeper than the rate of decrease in thresholds reported for single auditory-nerve fibers. Slope was weakly correlated with threshold level. Thresholds for long-duration sinusoids were consistently lower and the slopes of the threshold versus phase-duration functions were consistently steeper than those for single pulses. Thresholds for sinusoids and pulse trains decreased as a function of stimulus duration for durations up to at least 300 ms. The rate of decrease as a function of stimulus duration depended on the phase duration of the stimulus, and for long phase-duration signals, it depended on frequency. The rate of decrease was correlated with the absolute-detection-threshold level. We conclude that phase duration contributes significantly to, but does not completely account for, the steep slopes of the threshold versus frequency functions for long-duration sinusoids. Temporal integration is greater for longer phase-duration signals, giving rise to steeper slopes of threshold functions for long-duration sinusoids as compared to those for single pulses in the 1- to 5-ms/phase range. Current neural data and models do not account for the steep slopes of psychophysical threshold functions.

Changes over time in thresholds for electrical stimulation of the cochlea

The purpose of this paper is to better characterize changes over time that occurred in psychophysical detection thresholds for electrical stimulation of the cochlea. Threshold changes observed in nonhuman primates implanted with cochlear electrode arrays can be divided into at least three types based on the patterns of change over time. Short-term increases and subsequent decreases in threshold were commonly observed during the first months after implantation and were often followed by periods of long-term threshold stability. Long-term slow increases in thresholds and more rapid increases after a period of threshold stability have also been observed. The threshold changes may be divided into at least two classes based on their dependence on the waveforms used for the threshold measurements. Some changes occurred primarily in thresholds for long phase-duration signals while other changes were equal in magnitude (in decibels) for all tested stimuli. This suggests that at least two mechanisms underlay these threshold changes. The observed changes in thresholds have implications for experimental studies of electrical stimulation and for clinical application of auditory prostheses.

Cortical and Retinal Refractory Periods in the Human Visual System

Refractory periods of the visual system were investigated in 12 healthy subjects by simultaneously recording retinal (ERG) and cortical (VEP) evoked electrical activity. Double-flash stimuli were presented at different interstimulus intervals, and response components evoked by the second flash were analyzed in detail, and related to psychophysical detection thresholds. With short interstimulus intervals ERG b-wave peak latencies were increased and b-wave amplitudes were significantly reduced, while P100 component latencies of the VEP were significantly influenced only at long interstimulus intervals. Regression analysis of the individual data as well as analysis of the retinocortical transmission times showed that the cortical latency changes were not simply caused by changes on the level of the retina. Additional influences of the interstimulus interval on nonretinal structures of the human visual system must be assumed. The subjective psychophysical detection thresholds were significantly higher than the threshold values at which reliable electrical or cortical response components could be elicited.

Amplitude and phase characteristics of the steady-state visual evoked potential

The amplitude and phase characteristics of the steady-state visual evoked potential (VEP) and grating perception were studied for an unbiased group of fifteen healthy female subjects. The variability of VEP data, as obtained by using a digital sweep technique, was high between subjects but relatively low within them. Earlier claims that psychophysical detection thresholds can be predicted from VEP amplitude values were confirmed, whereas no correlation could be established between amplitude values and the perception of suprathreshold contrast. By using a principle of minimum phase difference the importance of VEP phase as an indicator of data reliability and of perceptual encoding processes could also be established.

Effects of temporal frequency on the contrast sensitivity of the human accommodation system

Contrast thresholds were determined for the initiation of accommodative responses to drifting sine-wave stimuli (4.1 c/deg) over a range of temporal frequencies (0–14.0 Hz). Accommodation was monitored with a dynamic infrared optometer. Psychophysical detection thresholds were also determined for the same stimuli. A comparison of the contrast thresholds for detection with those for the initiation of accommodation indicates that these two tasks are mediated by mechanisms of comparable sensitivity. Examination of the effects of temporal frequency on the contrast sensitivity of these two visual tasks reveals modest differences in the tuning properties of the underlying sensory mechanisms. Transient mechanisms tuned to intermediate temporal frequencies appear to have a greater role in the initiation of accommodative responses to grating stimuli than in the detection of such stimuli.

The effects of skin temperature on the psychophysical responses to vibration on glabrous and hairy skin.

Psychophysical detection thresholds for vibration were measured at the thenar eminence and volar forearm using a 0.008-cm2 (1.0-mm-diam) contactor. Measurements were made at 14 sinusoidal frequencies between 12 and 500 Hz at six skin temperatures between 15 degrees and 40 degrees C. The results are consistent with the hypothesis that three functionally discrete non-Pacinian afferent systems mediate vibrotactile responses. It was possible to identify the response characteristic of the rapidly adapting (Meissner) system, but it was not possible to isolate the responses of two slowly adapting (SAI, SAII) systems.

Temporal summation by Pacinian corpuscles precludes entrainment at the detection threshold.

For rapidly adapting mechanoreceptor afferents, entrainment is defined as a response of one spike per stimulus cycle. For Pacinian corpuscles, the entrainment threshold and psychophysical detection threshold are similar. The psychophysical threshold, however, exhibits temporal summation, which proves that the psychophysical threshold and the physiological entrainment threshold cannot be identical.

Effects of skin temperature and body site on psychophysical characteristics of cutaneous mechanoreceptor subsystems

Psychophysical detection thresholds were obtained at the thenar eminence and volar forearm using 16 sinusoidal frequencies between 15 and 700 Hz. A small (0.008 cm2) contactor was used, which preferentially excites non-Pacinian (NP) subsystems. Measurements were made using carefully controlled (± 0.5 °C) skin surface temperatures of 15°, 20°, 25°, 30°, 35°, and 40 °C to enhance or degrade the subsystem responses. The results show several frequency characteristics that cannot be ascribed to activity in the Pacinian system, but do conform to electrophysiological measurements of Ruffini capsules, Meissner corpuscles, and Merkel cells. When threshold values are averaged across temperatures, the resultant curve is approximately flat. The results suggest that the NP system is probably composed of more than a single receptor type and that well-controlled skin temperature is necessary for the precise measurement of vibrotactile sensation.