Haptic Mental Rotation: More Consistent in Blind Subjects?

Four adventitiously blind subjects and four sighted subjects with vision occluded examined a tangible letter J, presented at various orientations, and stated whether the stimulus was in normal or mirror-reversed form. Although the pattern of response-latency measurements suggested a higher rate of rotation of mental images in the sighted subjects, it indicated a greater degree of intersession variability in the orientation of the frame of reference they used. When the effect of this variability was eliminated from the data, the mental rotation functions for the two groups of subjects were virtually identical. These results support the hypothesis of earlier research that shifts in the orientation (defined with respect to external coordinates) of the frame of reference are responsible, at least in part, for apparent differences in mental rotation of blind and sighted subjects.

Field reversed configuration confinement enhancement through edge biasing and neutral beam injection

Field reversed configurations (FRCs) with high confinement are obtained in the C-2 device by combining plasma gun edge biasing and neutral beam injection. The plasma gun creates an inward radial electric field that counters the usual FRC spin-up. The n = 2 rotational instability is stabilized without applying quadrupole magnetic fields. The FRCs are nearly axisymmetric, which enables fast ion confinement. The plasma gun also produces E × B shear in the FRC edge layer, which may explain the observed improved particle transport. The FRC confinement times are improved by factors 2 to 4, and the plasma lifetimes are extended from 1 to up to 4 ms.

Dynamic formation of a hot field reversed configuration with improved confinement by supersonic merging of two colliding high-β compact toroids

A hot stable field-reversed configuration (FRC) has been produced in the C-2 experiment by colliding and merging two high-β plasmoids preformed by the dynamic version of field-reversed θ-pinch technology. The merging process exhibits the highest poloidal flux amplification obtained in a magnetic confinement system (over tenfold increase). Most of the kinetic energy is converted into thermal energy with total temperature (T{i}+T{e}) exceeding 0.5 keV. The final FRC state exhibits a record FRC lifetime with flux confinement approaching classical values. These findings should have significant implications for fusion research and the physics of magnetic reconnection.

Human vibrotactile frequency discriminative capacity after adaptation to 25 Hz or 200 Hz stimulation

A two-interval forced-choice (2-IFC) tracking procedure was used to evaluate the effects of a 15-s pre-exposure to either 25 Hz or 200 Hz stimulation ("25 Hz or 200 Hz adaptation") on human vibrotactile frequency discrimination threshold (frequency DL/Weber fraction). Three subjects were studied. All stimuli (standard and comparison) were delivered to a central location on the thenar eminence of the hand. The frequency DL/Weber fraction was determined for each subject under the following conditions: (1) no recent prior exposure to vibrotactile stimulation ("unadapted"); (2) after 15 s adaptation to 25 Hz stimulation; and (3) after 15 s adaptation to 200 Hz stimulation. The results demonstrate that the effects of frequency of adaptation on frequency discriminative capacity when the standard stimulus is 25 Hz are not the same as when the standard stimulus is 200 Hz. The differential changes in the capacity of subjects to discriminate frequency of cutaneous flutter (10-50 Hz) or vibratory (>200 Hz) stimulation that occur subsequent to a 15-s exposure of the thenar to 25 Hz or 200 Hz stimulation are proposed to reflect frequency-specific, adaptation-induced modification of the response of contralateral primary somatosensory cortex (SI and SII) to skin mechanoreceptor afferent drive.

Vibrotaction and texture perception

Several recent studies support Katz's hypothesis that vibrotaction plays a role in the perception of tactile textures with elements too small and closely spaced to be processed spatially. For example, eliminating vibration by preventing movement of a stimulus surface across the skin compromises psychophysical scaling and discrimination of fine, but not coarse, textures. Fine-texture discrimination is also impaired when vibrotactile channels are desensitized by adaptation. A role for vibrotaction in texture perception is plausible, given the keenness of this submodality: the sensory qualities produced by a sinusoidal vibration uniquely specify its frequency and amplitude, and subjects can distinguish some complex vibrations that differ in waveform but have the same spectral components. Finally, imposed vibration can modify the perceived texture of a haptically-examined surface. Taken together, these lines of evidence support the view that vibrotaction is both necessary and sufficient for the perception of fine tactile textures.

Vibrotactile adaptation impairs discrimination of fine, but not coarse, textures

The effect of vibrotactile adaptation on the ability to discriminate textured surfaces was examined in three experiments. The surfaces were rectilinear arrays of pyramids produced by etching of silicon wafers. Adaptation to 100-Hz vibration severely hampered discrimination of surfaces with spatial periods below 100 microm (Experiment 1), but had little effect on the discrimination of coarser textures (Experiment 2). To determine which vibrotactile channel--Rapidly Adapting or Pacinian--plays the larger role in mediating the discrimination of fine textures, widely separated adapting frequencies (10 and 250 Hz) were used in Experiment 3. The fact that high- but not low-frequency adaptation interfered with discrimination suggests that the Pacinian system contributes importantly to this ability. Taken as a whole, the results of this study strongly support the duplex theory of tactile texture perception, according to which different mechanisms--spatial and vibrotactile--mediate the perception of coarse and fine textures, respectively.

Imposed vibration influences perceived tactile smoothness

According to the duplex theory of tactile texture perception, detection of cutaneous vibrations produced when the exploring finger moves across a surface contributes importantly to the perception of fine textures. If this is true, a vibrating surface should feel different from a stationary one. To test this prediction, experiments were conducted in which subjects examined two identical surfaces, one of which was surreptitiously made to vibrate, and judged which of the two was smoother. In experiment 1, the vibrating surface was less and less often judged smoother as the amplitude of (150 Hz) vibration increased. The effect was comparable in subjects who realized the surface was vibrating and those who did not. Experiment 2 showed that different frequencies (150-400 Hz) were equally effective in eliciting the effect when equated in sensation level (dB SL). The results suggest that vibrotaction contributes to texture perception, and that, at least within the Pacinian channel, it does so by means of an intensity code.

Local Vibrotactile and Pain Sensitivities Are Negatively Related in Temporomandibular Disorders

Earlier research has shown that cutaneous experimental pain can elevate the vibrotactile threshold at the same skin locus. The purpose of this study was to determine whether vibrotactile and pain thresholds in a clinical (temporomandibular disorders [TMD]) population are consistent with the hypothesis that chronic pain causes a similar elevation. Specifically, we predicted that TMD subjects with soreness (low palpation-pain threshold) at a given skin site would have relatively high vibrotactile thresholds at the same location. Measurements on the skin overlying the masseter in 18 individuals with TMD showed that pain sensitivity was negatively correlated with sensitivity to 20-Hz vibration (presumed to activate a rapidly adapting mechanoreceptive channel), but not with sensitivity to 200-Hz vibration (thought to activate primarily a slowly adapting channel, because the Pacinian channel is lacking in the orofacial region). There was no relationship between vibration thresholds over the masseter and pain threshold at other orofacial sites, including the contralateral masseter. Vibrotactile and pain thresholds were uncorrelated in control participants without chronic pain (n = 18). The results indicate that in TMD, a localized relationship exists between pain sensitivity and the sensitivity of a low-frequency vibrotactile channel.

Individual differences in perceptual space for tactile textures: evidence from multidimensional scaling

Ratio scaling was used to obtain from 5 subjects estimates of the subjective dissimilarity between the members of all possible pairs of 17 tactile surfaces. The stimuli were a diverse array of everyday surfaces, such as corduroy, sandpaper, and synthetic fur. The results were analyzed using the multidimensional scaling (MDS) program ALSCAL. There was substantial, but not complete, agreement across subjects in the spatial arrangement of perceived textures. Scree plots and multivariate analysis suggested that, for some subjects, a two-dimensional space was the optimal MDS solution, whereas for other subjects, a three-dimensional space was indicated. Subsequent to their dissimilarity scaling, subjects rated each stimulus on each of five adjective scales. Consistent with earlier research, two of these (rough/smooth and soft/hard) were robustly related to the space for all subjects. A third scale, sticky/slippery, was more variably related to the dissimilarity data: regressed into three-dimensional MDS space, it was angled steeply into the third dimension only for subjects whose scree plots favored a nonplanar solution. We conclude that the sticky/slippery dimension is perceptually weighted less than the rough/smooth and soft/hard dimensions, materially contributing to the structure of perceptual space only in some individuals.

Complex tactile waveform discrimination

Complex vibrotactile waveforms consisting of two superimposed sinusoids at varying phases were presented to the fingertip, and observers made "same-different" judgments. It was found that the low-frequency (10Hz+30Hz) waveforms were discriminable from one another while discrimination of the high-frequency (100Hz+300Hz) vibrations was poor. High-frequency adaptation did not impair discrimination of the low-frequency waveforms, suggesting that the RA channel mediated discrimination. Low-frequency adaptation impaired discrimination of the high-frequency stimuli, suggesting that the RA channel likewise mediated the modest level of performance observed in the absence of an adapting stimulus. The results indicate that this channel encodes complex waveforms temporally. A simple model for low-frequency waveform discrimination is proposed. The results obtained with the high-frequency complex waveforms are compatible with the hypothesis that the PC channel integrates stimulus energy over time.

Evidence for the duplex theory of tactile texture perception

Three experiments are reported bearing on Katz's hypothesis that tactile texture perception is mediated by vibrational cues in the case of fine textures and by spatial cues in the case of coarse textures. Psychophysical responses when abrasive surfaces moved across the skin were compared with those obtained during static touch, which does not provide vibrational cues. Experiment 1 used two-interval forced-choice procedures to measure discrimination of surfaces. Fine surfaces that were readily discriminated when moved across the skin became indistinguishable in the absence of movement; coarse surfaces, however, were equally discriminable in moving and stationary conditions. This was shown not to result from any inherently greater difficulty of fine-texture discrimination. Experiments 2 and 3 used free magnitude estimation to obtain a more comprehensive picture of the effect of movement on texture (roughness) perception. Without movement, perception was seriously degraded (the psychophysical magnitude function was flattened) for textures with element sizes below 100 microns; above this point, however, the elimination of movement produced an overall decrease in roughness, but not in the slope of the magnitude function. Thus, two components of stimulation (presumably vibrational and spatial) contribute to texture perception, as Katz maintained; mechanisms for responding to the latter appear to be engaged at texture element sizes down to 100 microns, a surprisingly small value.

A ratio code for vibrotactile pitch

Subjective impressions of pitch for 80 different sinusoidal vibrotactile stimuli delivered to the index finger were measured by free magnitude estimation in four subjects. In three of the subjects, pitch at a given frequency decreased as stimulus amplitude increased. The data of these subjects were well described by a model of pitch based on the relative levels of activation of the three major tactile channels. The main element in this model was a ratio of P channel activity to the sum of the activity levels of the P, NPI, and NPIII channels. Activity levels of the channels were estimated on the basis of the psychophysical literature, including a study of vibrotactile loudness using the same subjects and stimuli as those employed here. A fourth subject, whose pattern of loudness judgments had previously been shown to differ from those of the other subjects, did not conform to this pitch model: her data revealed significant increases in pitch with increases in amplitude, and appear to reflect an inability to combine signals across vibrotactile channels. Pitch changes resulting from vibrotactile adaptation were directionally consistent with our ratio model: pitch was slightly increased by adaptation to a 25 Hz stimulus, and slightly decreased by 200 Hz adaptation.

Adaptation-induced enhancement of vibrotactile amplitude discrimination: the role of adapting frequency

Two-interval forced-choice tracking was used to measure amplitude discrimination for 20-Hz vibrotactile test stimuli presented to the thenar eminence of three human observers. For all observers, relative difference threshold could be decreased by adaptation to a 20- or 100-Hz stimulus. Maximal enhancement of discrimination occurred when the amplitude of the adapting stimulus was such that it excited the NP I system to approximately the same degree that the test stimuli did. A signal detection analysis determined that shifts in the observers' criteria could not consistently account for the enhancement of amplitude discrimination. A more likely explanation, in view of recent physiological discoveries, is that under optimal conditions of adaptation test stimuli differing slightly in amplitude become more distinctive because CNS events underlying the resultant sensory experiences become more refined and stimulus specific.

Perceived intensity of vibrotactile stimuli: the role of mechanoreceptive channels

The perceived intensity of vibrotactile stimuli was studied by means of free magnitude estimation. Eighty different sinusoidal stimuli ranging in frequency from 10 to 200 Hz, and in amplitude from 2.4 to 154 microns, were presented to the left index fingerpad of psychophysical observers through a 5-mm-diameter contactor. Estimates at a given frequency increased with amplitude in all four subjects, and estimates at a given amplitude increased with frequency in three. For the fourth subject, however, intermediate frequencies (25-75 Hz) produced the most intense sensations; the relative sensory effectiveness of different frequencies suggested that in her case, perceived vibrotactile intensity was determined largely by signals in Meissner afferents. From the data of this unusual subject, and from high-frequency (200-Hz) measurements on the normal subjects, quantitative descriptions were derived of the signals in Meissner and Pacinian channels, respectively, that could contribute to subjective intensity. Candidate algorithms by which the signals from the two channels might interact were then evaluated by comparison of modeled and empirically determined subjective intensity values. It was found that subjective intensity is given by the sum of (1) the stronger of the two channels' signals, and (2) half the weaker signal, the latter apparently being reduced by cross-channel suppression that occurs only at suprathreshold levels. Adapting to 25-Hz vibration selectively reduces the perceived intensity of low frequencies, whereas adapting to 200-Hz vibration has a corresponding effect at high frequencies. It is concluded that an understanding of perceived vibrotactile intensity requires knowledge of the signals in vibrotactile channels, and of the interactions between those channels.

Vibrotactile adaptation enhances frequency discrimination

Human vibrotactile frequency discrimination (with respect to a 25-Hz standard stimulus, 20 dB above unadapted detection threshold) was measured on the thenar eminence and index fingerpad, using two-interval forced-choice tracking. Measurements were made in the unadapted state and following exposure to 25-Hz adapting stimuli of various amplitudes. The standard and all comparison stimuli were equated for perceived intensity, on the basis of matching experiments that were carried out separately under each adapting condition. Frequency difference thresholds were lowest when the amplitude of the adapting stimulus was equal to the amplitude of the standard. This result complements the earlier finding [A. K. Goble and M. Hollins, J. Acoust. Soc. Am. 93, 418-424 (1993)] that adaptation sharpens amplitude discrimination of supraliminal stimuli that are similar to the adapting stimulus. Taken together, these discoveries suggest that somatosensory mechanisms that are engaged by extended stimulation serve to enhance detection of changes in the properties, both quantitative and qualitative, of that stimulation.

The tactile movement aftereffect

The existence of a tactile movement aftereffect was established in a series of experiments on the palmar surface of the hand and fingers of psychophysical observers. During adaptation, observers cupped their hand around a moving drum for up to 3 min; following this period of stimulation, they typically reported an aftereffect consisting of movement sensations located on and deep to the skin, and lasting for up to 1 min. Preliminary experiments comparing a number of stimulus materials mounted on the drum demonstrated that a surface approximating a low-spatial-frequency square wave, with a smooth microtexture, was especially effective at inducing the aftereffect; this adapting stimulus was therefore used throughout the two main experiments. In Experiment 1, the vividness of the aftereffect produced by 2 min of adaptation was determined under three test conditions: with the hand (1) remaining on the now stationary drum; (2) in contact with a soft, textured surface; or (3) suspended in air. Subjects' free magnitude estimates of the peak vividness of the aftereffect were not significantly different across conditions; each subject experienced the aftereffect at least once under each condition. Thus the tactile movement aftereffect does not seem to depend critically on the ponditions of stimulation that obtain while it is being experienced. In Experiment 2, the vividness and duration of the aftereffect were measured as a function of the duration of the adapting stimulus. Both measures increased steadily over the range of durations explored (30-180 sec). In its dependence on adapting duration, the aftereffect resembles the waterfall illusion in vision. An explanation for the tactile movement aftereffect is proposed, based on the model of cortical dynamics of Whitsel et al. (1989, 1991). With assumed modest variation of one parameter across individuals, this application of the model is able to account both for the data of the majority of subjects, who experienced the aftereffect as opposite in direction to the adapting stimulus, and for those of an anomalous subject, who consistently experienced the aftereffect as being in the same direction as the adapting stimulus.

Perceptual dimensions of tactile surface texture: a multidimensional scaling analysis

The purpose of this study was to examine the subjective dimensionality of tactile surface texture perception. Seventeen tactile stimuli, such as wood, sandpaper, and velvet, were moved across the index finger of the subject, who sorted them into categories on the basis of perceived similarity. Multidimensional scaling (MDS) techniques were then used to position the stimuli in a perceptual space on the basis of combined data of 20 subjects. A three-dimensional space was judged to give a satisfactory representation of the data. Subjects' ratings of each stimulus on five scales representing putative dimensions of perceived surface texture were then fitted by regression analysis into the MDS space. Roughness-smoothness and hardness-softness were found to be robust and orthogonal dimensions; the third dimension did not correspond closely with any of the rating scales used, but post hoc inspection of the data suggested that it may reflect the compressional elasticity ("springiness") of the surface.

Vibrotactile adaptation enhances amplitude discrimination

Human psychophysical detection and amplitude discrimination thresholds for 25-Hz sinusoidal vibrations were measured on the thenar eminence using two-interval forced-choice tracking, in the unadapted state and following exposure to 25-Hz adapting stimuli representing a range of amplitudes (5-25 dB SL). As expected, detection threshold was elevated 6 to 7 dB for each 10-dB increase in the adapting stimulus. In contrast, amplitude difference thresholds for 10 and 20 dB SL standard stimuli were generally lowest when the amplitude of the adapting stimulus was equal to the amplitude of the standard. The results indicate that while adaptation impairs detection of a liminal vibrotactile stimulus, it improves intensity discrimination of supraliminal stimuli that are close in amplitude to the adapting stimulus. The compatability between these results and a recently proposed model of cortical dynamics (Whitsel et al., 1989) suggests that cortical events may contribute significantly to the physiological basis of vibrotactile adaptation.

Vibrotactile adaptation on the face

Threshold amplitude for vibration is elevated if testing is preceded by extended exposure to a vibratory adapting stimulus of appropriate amplitude and frequency. This phenomenon, previously studied almost exclusively on the hand, is here shown for the first time to occur on the face as well. Adaptation is then used analytically to determine that the two-branched threshold-versus-frequency function obtained on the face by Verrillo and Ecker (1977) represents the activity of two distinct mechanisms. Action spectra of vibrotactile adaptation reveal the presence of both mechanisms even in subjects whose unadapted threshold function (like that reported by Barlow, 1987) shows no sign of duplexity. Finally, the data suggest that on the face (unlike the hand), cross-channel adaptation may occur at high adapting amplitudes.

Time course and action spectrum of vibrotactile adaptation

In a series of experiments designed to explore the processes underlying adaptation of the sense of flutter-vibration, vibrotactile threshold was measured on the pad of the index finger, using Békésy tracking. Unadapted thresholds were first measured, for a number of frequencies (4-90 Hz) and contactor sizes (1-8 mm diameter). As expected, these measurements indicated the presence of (1) a Pacinian system possessing spatial summation and increasing in sensitivity, as frequency was raised, at the rate of 12 dB/octave; and (2) a non-Pacinian system showing little spatial summation, and with a frequency characteristic matching that of the NP I mechanism of Bolanowski et al. (1988). These baseline data of Experiment 1 guided the selection of stimulus parameters for subsequent experiments, in which threshold for a test stimulus was measured before, during, and after periods of vibrotactile adaptation. In Experiment 2, test stimuli of 10 Hz and 50 Hz were combined factorially with 30-dB SL adapting stimuli of the same two frequencies. When the test stimulus was 10 Hz, the two adapting frequencies were equally effective in raising threshold; however, when the 50-Hz test stimulus was used, the 50-Hz adapting stimulus raised threshold by a greater amount than did the 10-Hz adapter. These results confirm on the finger the independence of adaptation in Pacinian and non-Pacinian channels, a result previously established on the thenar by other workers. For all four frequency combinations, threshold rose exponentially with a time constant of 1.5-2 min. In Experiment 3, an action spectrum was determined, showing the adapting amplitude needed at each of a series of frequencies to raise the threshold of a 10-Hz stimulus by 10 dB; this spectrum was essentially flat from 30 to 90 Hz. The results, taken in conjunction with what is known about rapidly adapting cutaneous mechanoreceptors, imply that the effectiveness of an adapting stimulus is not determined solely by the amount of activity it generates in first-order afferents.

Perception of the length of voluntary movements

Two experiments were performed to study the ability of blindfolded subjects to estimate distance on the basis of proprioceptive cues. In the first experiment, subjects judged the length of metal rods that they were allowed to explore freely. With this access to positional as well as other cues, subjects' estimates were a nearly linear function of actual length. These data closely paralleled control measurements obtained under conditions of visual, rather than haptic, inspection. In the second experiment, each subject slid his or her index finger laterally along a straight path delimited by the apparatus, and then gave a magnitude estimate of the distance through which the finger had moved. Velocity of movement was manipulated by asking subjects, on each trial, to move at one of five speeds ranging from "very slow" to "very fast"; these instructions elicited velocities spanning a 100-to-1 range. Magnitude estimates of distance in this second experiment increased as a function of actual distance, but decreased as a function of velocity. This latter phenomenon resembles the dependence of perceived distance on velocity that has been shown by other investigators to occur when a stimulus object is drawn across the skin. The data of the present study are consistent with the hypothesis that the perceived length of an active movement depends on a combination of movement and position signals from primary and secondary sensory fibers in muscle spindles.

Dependence of subjective traverse length on velocity of moving tactile stimuli

Two series of experiments were performed to assess the effects of stimulus velocity on human subjects' perception of the distance traversed by a moving tactile stimulus. In all experiments, constant-velocity stimuli were applied to the dorsal surface of the left forearm; velocities ranging between 1.0 and 256 cm/sec were used. In some experiments the stimuli moved from distal to proximal over the skin, and in others they moved from proximal to distal. The length of skin contacted by the moving stimulus was defined by a plate having an aperture of 4.0 X 0.5 cm. In the first series of experiments, subjects were required to compare the distance traversed by a test stimulus delivered 2 sec after a standard stimulus, and also to report the on-locus and the off-locus of the brushing stimulus. In the second series of experiments, the subjects rated the perceived distance on the skin using a free-magnitude-estimation procedure. The data from both series of experiments defined the same relationship between stimulus velocity and perceived stimulus distance. More specifically, although the length of skin contacted by the stimulus was the same at all velocities, subjects' estimates of stimulus distance decreased with increasing stimulus velocity. In addition, the function relating estimates of stimulus distance to velocity was flat for velocities between 5 and 20 cm/sec, but possessed an appreciable negative slope at lower and higher velocities. It is interesting that the plateau of the relationship between perceived stimulus distance and velocity occurred within the range of velocities that human subjects employ to scan textured surfaces; it also corresponded precisely with the range of stimulus velocities at which the directional sensitivity of somatosensory cortical neurons and human subjects is optimal.

Styles of mental imagery in blind adults

Two tests of mental imagery were administered to six blind and four sighted subjects. One test, involving pictorial imagery, required subjects to imagine a checkerboard of which only certain squares, specified by the experimenter, were filled in; the other test was similar but emphasized nonpictorial imagery, requiring subjects to imagine a three-dimensional assemblage of small cubes. In both cases, the subject's task was to name a common object which the imaged pattern resembled. It was found that the greater the proportion of life for which each of the blind subjects had been without sight, the less was his or her pictorial score, expressed as a proportion of total imagery score. This suggests that the nature of mental imagery slowly changes following the loss of sight. Relative facility with the two types of imagery also varied from one subject to another within the sighted group; hand movements used by some of them as an aid to imagery may have been a factor in producing this variation.

Clinical and pharmacokinetic evaluation of parenteral moxalactam in infants and children

Thirty-four infants and children ranging in age from 2.5 to 180 months (mean, 40 months) were treated with parenteral moxalactam (150 mg/kg per day) for suspected or proved bacterial infections outside the central nervous system. Six patients infected with Haemophilus influenzae b, nine infected with Staphylococcus aureus, three infected with Streptococcus pneumoniae, one infected with Streptococcus pyogenes, one infected with Enterobacter aerogenes, one infected with Fusobacterium nucleotum, and one infected with Staphylococcus epidermidis, microaerophilic streptococcus, and Propionibacterium sp. were clinically and bacteriologically cured. One patient with polymicrobial pansinusitis did not respond to moxalactam. No patients developed meningitis. All of the isolates tested were inhibited by less than or equal to 5 micrograms of moxalactam per ml, except for one Staphylococcus epidermidis isolate which was resistant to greater than 20 micrograms/ml. Five patients had transient neutropenia which resolved after the drug was discontinued. The mean peak serum level was 106 micrograms/ml at 15 min after a 50-mg/kg dose. The mean elimination half-life was 91.2 min. These data indicate that this dosage of moxalactam is a safe and effective treatment for bacterial infections outside the central nervous system.

Is the binocular rivalry mechanism tritanopic?

Binocular rivalry for a series of colored targets was measured in three trichomats and two red-green dichromats by cumulating those times when ony one target or the other was perceived (exclusive visibility time). Targets were black and colored 3 c/deg square-wave gratings, 1 degree in diameter. For trichromats, exclusive visibility time increased as a function of color difference between the targets, but for dichromats there was no effect of color on binocular rivalry. Taken together, these data indicate that the binocular rivalry mechanism is tritanopic: it is responsive to color signals only from the medium- and long-wave-sensitive cones.

Adaptation of the binocular rivalry mechanism

The completeness of binocular rivalry suppression was measured by recording the fraction of a trial during which targets were perceived as alternating in their entirety, rather than blending together into a composite. It was found that the completeness of rivalry declines with prolonged stimulation. A control condition in which subjects saw the same grating targets, but without rivalry, resulted in no decrement. The phenomenon was spatial-frequency specific. The results suggest that binocular rivalry suppression is mediated by a cooperative mechanism which is susceptible to adaptation.

Corticopontine visual projections in macaque monkeys

These experiments were designed to study the projections to the pons from visual and visual association cortex of monkeys by degeneration staining and horseradish peroxidase (HRP) methods. When lesions were made in these cortical visual areas, degenerated fibers were found in the rostral dorsolateral area of the pontine nuclei. When HRP was injected among visually responsive cells in this region of the pons, layer V cortical pyramidal cells were labeled. These labeled cells were concentrated most heavily on both banks of the superior temporal and intraparietal fissures, and on the rostral bank of the parieto-occipital fissure. The efferent targets and receptive field properties of these cortical regions are consistent with their possible role in visual guidance of movement.

Representation of moving stimuli by somatosensory neurons

The findings obtained in neurophysiological and psychophysical investigations using tactile stimuli that move at constant velocity across the skin are reviewed. For certain neurons in the postcentral gyrus of the cerebral cortex (S-I) of macaque monkeys, direction of stimulus motion is a "trigger feature"" i.e., moving tactile stimuli evoke vigorous discharge activity in these neurons only if the stimuli are moved in a particular direction across the receptive field. This directional selectivity is maximal when stimulus velocity is between 5 and 50 cm/sec, and falls off rapidly at lower or higher velocities. The capacity for human subjects to correctly identify the direction of stimulus motion on the skin exhibits a similar dependence on stimulus velocity. The similar effects of velocity on neural and psychophysical measures of directional sensitivity support the idea that direction of stimulus motion on the skin can only be recognized if the moving stimulus optimally activates the group of S-I neurons for which that directions of simulus motion is the trigger feature.

Does accommodative micropsia exist?

Subjects matched the size of a test target seen under various conditions of accommodation and convergence to the size of a reference target seen under constant conditions. Convergence, as expected, had a substantial effect on perceived size, while accommodation by itself had little or none.

Dark adaptation and visual pigment regeneration in human cones

Foveal threshold elevation and red-green cone pigment regeneration have been studied in the dark after a wide range of bleaches in normal man with a view to probing the limits of the application of the Dowling-Rushton relation: i.e., the direct proportionality between log threshold elevation and fraction of unregenerated pigment. Cone pigment regeneration (and threshold recovery) is much faster after short bleaches than expected from the kinetics of a simple monomolecular reaction. Recovery is faster after a fixed (short) duration bleach the weaker it is. Except for the first 30 s after relatively weak bleaches and the entire recovery after a very brief (<0.001 s) saturating bright flash which bleaches a little more than 50%, the results are accurately fit by the Dowling-Rushton relation over the entire range tested with only one arbitrary constant (the proportionality factor). Theory predicts too low threshold in comparison with what is obtained, for both of these exceptions