Central neural mechanisms for detecting second-order motion

Single-unit neurophysiology and human psychophysics have begun to reveal distinct neural mechanisms for processing visual stimuli defined by differences in contrast or texture (second-order motion) rather than by luminance (first-order motion). This processing begins in early visual cortical areas, with subsequent extrastriate specialization, and may provide a basis for form-cue invariant analyses of image structure, such as figure-ground segregation and detection of illusory contours.

Motion perception over long interstimulus intervals

Recent studies using moving arrays of textured micropatterns have suggested that motion perception can be supported by two mechanisms, one quasilinear and sensitive to the motion of luminance-defined local texture, the other nonlinear and coding motion of contrast-defined envelopes of texture (Baker & Hess, 1998; Boulton & Baker, 1993b). Here we used similar patterns to study motion perception under conditions previously shown to isolate the nonlinear mechanism (low micropattern densities and positive interstimulus intervals [ISIs]. We measured direction discrimination for two-flash apparent motion over a much larger range of ISIs, and susceptibility to masking by incoherently moving "distractor" micropatterns. The results suggest that two nonlinear mechanisms can support motion perception under these conditions. One operates only for relatively short ISIs (less than c. 100 msec), is sensitive to small spatial displacements, and is relatively insensitive to distractor masking. The other operates over much longer ISIs, is insensitive to small spatial displacements, and is highly disrupted by distractor masking. These results are in line with previous studies suggesting that three mechanisms support motion perception.

Comparison of motion and stereopsis: linear and nonlinear performance

To address the issue of whether the luminance-dependent (linear) and contrast-dependent (nonlinear) processes in stereo and motion have a common computational basis, we compare both carrier-dependent and envelope-dependent performance for these two modalities by using the same stimulus and task: two-flash apparent motion/depth for a wide range of displacements. We do this for different densities, bandwidths, contrasts, spatial frequencies, and exposure durations. The results suggest that there is concordance not only between the luminance-dependent (linear) processes of motion and stereo but also between the envelope-dependent (nonlinear) processes of both modalities. Only one exception was found, but we show this to be amenable to an explanation based on a different contrast dependence for the nonlinear mechanisms of stereo and motion. This suggests that the computational basis of linear and nonlinear processes may be similar for stereopsis and motion.

Cortical processing of second-order motion

Neurons in the mammalian visual cortex have been found to respond to second-order features which are not defined by changes in luminance over the retina (Albright, 1992; Zhou & Baker, 1993, 1994, 1996; Mareschal & Baker, 1998a,b). The detection of these stimuli is most often accounted for by a separate nonlinear processing stream, acting in parallel to the linear stream in the visual system. Here we examine the two-dimensional spatial properties of these nonlinear neurons in area 18 using envelope stimuli, which consist of a high spatial-frequency carrier whose contrast is modulated by a low spatial-frequency envelope. These stimuli would fail to elicit a response in a conventional linear neuron because they are designed to contain no spatial-frequency components overlapping the neuron's luminance defined passband. We measured neurons' responses to these stimuli as a function of both the relative spatial frequencies and relative orientations of the carrier and envelope. Neurons' responses to envelope stimuli were narrowband to the carrier spatial frequency, with optimal values ranging from 8- to 30-fold higher than the envelope spatial frequencies. Neurons' responses to the envelope stimuli were strongly dependent on the orientation of the envelope and less so on the orientation of the carrier. Although the selectivity to the carrier orientation was broader, neurons' responses were clearly tuned, suggesting that the source of nonlinear input is cortical. There was no fixed relationship between the optimal carrier and envelope spatial frequencies or orientations, such that nonlinear neurons responding to these stimuli could perhaps respond to a variety of stimuli defined by changes in scale or orientation.

Arthroscopic release for lateral epicondylitis: a cadaveric model

At least 10 different surgical approaches to refractory lateral epicondylitis have been described, including an arthroscopic release of the extensor carpi radialis brevis tendon. The advantages of an arthroscopic approach include an opportunity to examine the joint for associated pathology, no disruption of the extensor mechanism, and a rapid return to premorbid activities with possibly fewer complications. A cadaveric study was performed to determine the safety of this procedure. Ten fresh-frozen cadaveric upper extremities underwent arthroscopic visualization of the extensor tendon and release of the extensor carpi radialis brevis tendon. The specimens were randomized with regard to the use of either a 2.7-mm or a 4.0-mm 30 degree arthroscope through modified medial and lateral portals. Following this, the arthroscope remained in the joint, and the portal, cannula track, and surgical release site were dissected to determine the distance between the cannula and the radial, median, ulnar, lateral antebrachial, and posterior antebrachial nerves, and the brachial artery and the ulnar collateral ligament. No direct lacerations of neurovascular structures were identified; however, the varying course of the lateral and posterior antebrachial nerves place these superficial sensory nerves at risk during portal placement. As in previous reports, the radial nerve was consistently in close proximity to the proximal lateral portal (3 to 10 mm: mean, 5.4 mm). The ulnar collateral ligament was not destabilized. Arthroscopic release of the extensor carpi radialis brevis tendon appears to be a safe, reliable, and reproducible procedure for refractory lateral epicondylitis. Cadaveric dissection confirms these findings.

Arthroscopic treatment of transchondral talar dome fractures: a long-term follow-up study

Arthroscopic treatment of transchondral talar dome fractures allows accurate visualization and debridement of the lesion with less postoperative morbidity and earlier mobilization than arthrotomy. Although many studies document the results of open treatment, no reports of long-term results of arthroscopic treatment of these fractures have been published. We reviewed the results in 12 patients who had arthroscopic excision and curettage of a transchondral talar dome fracture and had an average of 10.1 years of follow up (range, 8.1 to 13.9 years). There were 5 medial and 7 lateral lesions. According to the four-stage classification of Berndt and Harty, 2 were stage II; 8 were stage III; and 2 were stage IV. All patients were evaluated with a subjective questionnaire, ankle radiographs, and a physical examination. The long-term subjective and objective results were good or excellent in 10, fair in 1, and poor in 1 patient. All patients' radiographs showed residual subchondral changes at the site of the original lesion, but minimal to no degenerative changes. Patients who are treated with arthroscopic debridement and curettage for transchondral talar dome fractures achieve a predictably high percentage of successful results with low morbidity.

The quality of medical textbooks: bladder cancer diagnosis as a case study

PURPOSE

This study was conducted to determine whether there is an inherent bias in medical texts that influences physicians to favor certain signs and symptoms over others when evaluating patients with bladder cancer.

MATERIALS AND METHODS

Numerous sources, primarily textbooks from various medical specialties, were reviewed (italics are added for emphasis). Attention was paid to the criteria the authors suggested for use in the diagnosis of bladder cancer.

RESULTS

Although most authors agree on a core of presenting signs and symptoms, some concentrate strongly on hematuria with the result of down playing the importance of other findings such as irritative voiding symptoms. The concern is that this approach may lead to delay in the diagnosis of tumors, some of which may be highly aggressive, which present with dysuria and pyuria but no hematuria.

CONCLUSIONS

Inconsistencies do exist in the literature regarding which diagnostic criteria to use in selecting those patients to be evaluated for bladder cancer. Recommendations are made for ways to improve standardization.

Temporal and spatial response to second-order stimuli in cat area 18

Temporal and spatial response to second-order stimuli in cat area 18. J. Neurophysiol. 80: 2811-2823, 1998. Approximately one-half of the neurons in cat area 18 respond to contrast envelope stimuli, consisting of a sinewave carrier whose contrast is modulated by a drifting sinewave envelope of lower spatial frequency. These stimuli should fail to elicit a response from a conventional linear neuron because they are designed to contain no spatial frequency components within the cell's luminance-defined frequency passband. We measured neurons' responses to envelope stimuli by varying both the drift rate and spatial frequency of the contrast modulation. These data were then compared with the same neurons' spatial and temporal properties obtained with luminance-defined sinewave gratings. Most neurons' responses to the envelope stimuli were spatially and temporally bandpass, with bandwidths comparable with those measured with luminance gratings. The temporal responses of these neurons (temporal frequency tuning and latency) were systematically slower when tested with envelope stimuli than with luminance gratings. The simplest kind of model that can accommodate these results is one having separate, parallel streams of bandpass processing for luminance and envelope stimuli.

A cortical locus for the processing of contrast-defined contours

Object boundaries in the natural environment are often defined by changes in luminance; in other cases, however, there may be no difference in average luminance across the boundary, which is instead defined by more subtle 'second-order' cues, such as changes in the contrast of a fine-grained texture. The detection of luminance boundaries may be readily explained in terms of visual cortical neurons, which compute the linear sum of the excitatory and inhibitory inputs to different parts of their receptive field. The detection of second-order stimuli is less well understood, but is thought to involve a separate nonlinear processing stream, in which boundary detectors would receive inputs from many smaller subunits. To address this, we have examined the properties of cortical neurons which respond to both first- and second-order stimuli. We show that the inputs to these neurons are also oriented, but with no fixed orientational relationship to the neurons they subserve. Our results suggest a flexible mechanism by which the visual cortex can detect object boundaries regardless of whether they are defined by luminance or texture.

Two mechanisms underlie processing of stochastic motion stimuli

We have constructed "limited lifetime" stochastic motion stimuli using Gabor functions instead of dots, thereby controlling the local attributes of spatial frequency and orientation. Human psychophysical data for direction discrimination using these stimuli reveal two qualitatively distinct kinds of processing. For small displacements, direction discrimination performance as a function of displacement is scaled with spatial frequency in a manner consistent with a linear filtering motion mechanism. Motion perception for relatively large displacements is not directly related to the spatial frequency, and is consistent with a nonlinear process which signals motion of contrast envelopes.

Patellofemoral arthrosis: the treatment options

Patellofemoral arthritis is a relatively common and frequently complex problem encountered by those who treat patients with anterior knee pain. Because the pathologic conditions that precipitate this pain are so varied, the treatment options must also be varied. Using the Insall classification system for chondral injuries, we explored the treatment options available. Special emphasis was placed on grade IV changes and the surgical treatment of this condition. We reviewed the following options: spongialization, tibial tubercle elevation, patellectomy, patellar resurfacing, patellofemoral arthroplasty, and autologous chondrocyte transplantation. Regardless of the procedure favored, proper patellar tracking must be restored. Without such attention to patellar alignment, patients cannot achieve pain-free function.

A nonlinear chromatic motion mechanism

Previous research has demonstrated two categorically distinct mechanisms mediating apparent motion of kinematograms composed of eccentricity-confined, randomly placed Gabor micropatterns: a quasi-linear mechanism operating for high micropattern densities and short time separations, and a nonlinear mechanism operating at low micropattern densities or longer time separations. Here we compare the performance of these two mechanisms using color (isoluminant) and luminance-defined stimuli. When these stimuli are defined only by their color contrast, the response of the quasi-linear mechanism is severely impaired, while the nonlinear mechanism remains fully operative. This result further strengthens the dichotomy between the two kinds of motion perception, and suggests that when color vision supports motion perception it does so primarily, or perhaps entirely, via a nonlinear mechanism.

Second-order motion perception in peripheral vision: limits of early filtering

Spatial and temporal analysis of contrast-modulated sine-wave gratings reveals that the second-order motion stimulus contains two sidebands, with equal energy but moving in opposite directions, flanking a stationary carrier. Any early linear spatial filtering process in the visual system that attenuates one sideband more than the other will be detrimental to the balance between the two sidebands, so that the perceived direction of the carrier might be opposite to that of the envelope motion. We tested this hypothesis by using contrast-modulated gratings presented centrally or at 20 deg in the horizontal nasal field with a two-alternative forced-choice staircase paradigm. We found that when the envelope frequency was close to that of the carrier, a second-order stimulus whose envelope motion direction was correctly identified in the fovea appeared to drift in the opposite direction in the periphery. Further increasing the envelope spatial frequency resulted in a reversed motion percept in both central and peripheral viewing conditions. For subjects to identify correctly the direction of motion of the envelope, the spatial frequency ratio of the carrier to the envelope had to be more than 2 in the fovea and more than 6 in the periphery. These phenomena in second-order motion perception can be explained by a linear model of motion detection with an early spatial filtering process. Further experiments and computer simulation show that undersampling of the carrier has little effect on second-order motion perception in the periphery, as long as the carrier is detectable.

Lower extremity problems in female athletes

Women are participating in sports in greater numbers than ever before. As physicians, our job is to help them enjoy and benefit from their participation by treating injuries quickly and appropriately when they occur and preventing them wherever possible. Although certain injuries do occur more in female athletes than in male athletes, taking the proper precautions to prevent injury (e.g., using the right equipment, training properly, warming up and stretching before activity) can lessen the risk of injuries for all athletes.

The effects of distractor elements on direction discrimination in random Gabor kinematograms

For both Fourier and non-Fourier moving patterns, models have been proposed which detect motion based on either the net orientation of energy in the stimulus (after nonlinear stage for non-Fourier motion stimuli) or on the changes in the relative locations of spatial primitives in the image. Both approaches have been successful in accounting for detection of simple translational displacements, but we examined how such models coped with more demanding stimuli. We examined direction discrimination using two-flash random Gabor kinematograms which selectively reveal Fourier and non-Fourier motion mechanisms. In addition to target elements, multiple distractor elements were added, either static or randomly moving. It was found that detection of Fourier motion was relatively unaffected by the distractors unless they were of orthogonal orientation. Detection of non-Fourier motion was possible, but with a slightly higher error rate, even with many distractors and was not at all affected by orthogonal distractors. The results for distractors of the same orientation as targets are in better agreement with predictions of energy than with edge-matching models. The differing effects of orthogonal distractors further strengthen the proposed dichotomy of quasi-linear and nonlinear motion mechanisms, but indicate that the latter operates on a more complex representation than a simple contrast envelope.

Arthroscopy of the elbow

As our understanding of the anatomy and function of the elbow joint continues to grow and technology continues to advance, our ability to correct disorders of the elbow with arthroscopic techniques will expand. Today, we are at the brink of major advances in the arthroscopic evaluation and treatment of elbow ailments. Many open surgical procedures currently being performed will undoubtedly be adapted for an arthroscopic approach, as we are already seeing in the treatment of radiocapitellar arthrosis, tennis elbow, arthrofibrosis, and ulnohumeral arthroplasty. Elbow procedures, such as ligamentous tightening, fracture treatment with bioabsorbable devices, and biologic joint replacement will be commonly performed in the future with the aid of the arthroscope. Although elbow arthroscopy is technically demanding, it is a highly effective surgical technique in treating many intra-articular disorders with minimal morbidity. Most of the complications associated with elbow arthroscopy can be avoided by adhering to strict and proper surgical technique. Successful elbow arthroscopy requires a thorough understanding of local gross and arthroscopic anatomy. To maintain proper orientation at all times, the skin should be properly marked before starting the procedure. The joint should be kept distended during initiation of portals to move the neurovascular structures away from the arthroscopic instruments. Nonvented cannulas with blunt trocars should be used to allow for safe passage of instruments and to avoid multiple capsular punctures. Finally, the elbow should remain flexed to 90 deg during most of the procedure, thus keeping the neurovascular structures in the antecubital fossa relaxed. If these techniques are followed, the surgical morbidity should remain low, and surgeon and patient will find elbow arthroscopy tremendously effective.

Spatial properties of envelope-responsive cells in area 17 and 18 neurons of the cat

1. Many neurons in areas 17 and 18 respond to spatial contrast envelope stimuli whose Fourier components fall outside the cell's spatial-frequency-selective range. The spatial properties of such envelope responses are investigated here and compared with responses to conventional luminance-defined gratings to explore the underlying receptive-field mechanism. 2. Three spatial properties of envelope responses are reported more extensively in this paper. First, the envelope responses were selective to the carrier spatial frequency in a narrow range of frequencies higher than a given cell's luminance spatial frequency selective range (luminance passband). Second, a given cell's dependence on envelope spatial frequency often differed from its luminance passband. Last, the optimal carrier spatial frequency did not shift systematically with the envelope spatial frequency, supporting the hypothesis that the carrier and envelope spatial-frequency dependencies were mediated by distinct mechanisms. 3. In contrast to the direction selectivity to the envelope motion in many envelope-responsive cells, no direction preference to carrier motion was found for envelope responses. The direction of carrier motion did not alter the direction selectivity for envelope motion, further supporting the hypothesis that the carrier and envelope temporal properties were mediated by separate mechanisms. 4. The distributions of the optimal carrier and luminance spatial frequencies among envelope-responsive cells were analyzed. The optimal carrier spatial frequencies were randomly distributed from five times the cell's optimal luminance spatial frequency to the upper resolution limit of the X-retinal ganglion cells at the same retinal eccentricity, suggesting that the selective ranges of envelope responses and luminance responses are not strongly correlated over the population of envelope-responsive cells. 5. Our data support a "two-stream" receptive-field model for envelope-responsive cells. One stream is a conventional, spatially linear receptive-field mechanism, mediating luminance responses for the cell; the other mediates envelope responses and consists of a two-stage processing: a set of spatially small and distributed nonlinear neural subunits whose outputs are spatially pooled at the second stage. 6. In conclusion, this study indicates that envelope responses in area 17 and 18 neurons cannot be due to a nonlinearity that is common to all visual stimuli before narrowband spatial-frequency-selective filtering; instead, a specialized processing stream, parallel to the conventional luminance response stream, is needed to supplement the traditional luminance processing stream in these cells. This specialized stream responds to the envelope stimuli and is selective to their carrier and envelope spatial frequencies. The distributions of the optimal luminance and carrier spatial frequencies indicate a rich variety of possible integration between luminance and envelope information.

Comparison of open and arthroscopically assisted rotator cuff repairs

Open rotator cuff repair has shown reliable results in terms of pain relief and improved shoulder function. Recently, however, arthroscopically assisted rotator cuff repair has shown promising preliminary results. We compared the results of these two procedures with regard to pain, function, range of motion, strength, patient satisfaction, and return to previous activity. Thirty-seven rotator cuff repairs were evaluated in 36 patients with a minimum followup of 2 years. The open repair group comprised 20 shoulders with an average followup of 3.3 years; the arthroscopically assisted repair group comprised 17 shoulders with an average followup of 3.2 years. Overall, the open repair group had 80% good-to-excellent results and 88% patient satisfaction, and the arthroscopically assisted repair group had 85% good-to-excellent results and 92% patient satisfaction. Shoulder flexion and abduction strength, the size of the tear repaired, and the functional outcome did not differ significantly between the two groups. In general, however, small and moderate-sized tears (< 3 cm) had better functional outcome with arthroscopically assisted repair. The arthroscopically assisted repair group was hospitalized 1.2 days less and returned to previous activity an average of 1 month earlier. In the surgical treatment of symptomatic complete rotator cuff tears, arthroscopically assisted rotator cuff repair is as effective as open repair.

Envelope-responsive neurons in areas 17 and 18 of cat

1. Single cortical neurons are known to respond to visual stimuli containing Fourier components only in a narrow range of spatial frequencies. This investigation demonstrates that some neurons in cat area 17 and 18 can also respond to certain stimuli that have no Fourier components inside the cell's luminance spatial frequency passband. 2. To study such "non-Fourier" responses, we used envelope stimuli that consisted of a high-spatial-frequency sinusoidal luminance grating (carrier) whose contrast was modulated by a low-spatial frequency sine wave (envelope). There was no Fourier component at the apparent periodicity of the envelope spatial frequency. However, some cells responded to such a "phantom" component of the envelope modulation when it fell inside the cell's luminance spatial frequency passband while all the real Fourier components in the stimuli were outside. 3. We conducted extensive control experiments to eliminate the possibility of producing artifactual responses to the envelope stimuli due to any small residual nonlinearity of the z-linearized CRT screen. The control experiments included 1) testing of screen linearity to ensure that the effect from the residual screen nonlinearity was no larger than the sensitivity level of visual responses and 2) comparing the responses to envelope stimuli with the responses to the equivalent contrast of the artifact produced by the screen nonlinearity. All these control experiments indicated that any effect of screen nonlinearity did not contribute significantly to the neural envelope responses. 4. We performed a statistical analysis to obtain an index of relative strength of envelope responses for each cell and to objectively classify cells as "envelope-responsive" or "non-envelope-responsive."(ABSTRACT TRUNCATED AT 250 WORDS)

A sustained input to the direction-selective mechanism in cat striate cortex neurons

Direction-selective neurons in cat striate cortex were tested with bar-shaped stimuli, sequentially flashed at spatially displaced positions chosen to elicit maximal direction selectivity. Temporally overlapping flash exposures of prolonged duration (400-1000 ms) were employed at a series of onset asynchronies to explore the nature of temporal tuning of the direction-selective mechanism. In most neurons studied, direction selectivity was found to be supported by a surprisingly broad range of stimulus onset asynchronies, which was greater for longer exposure durations. These findings imply the existence of a sustained input to the direction-selective mechanism, in spite of the relatively transient nature of most cortical neurons' step responses. A model is described to illustrate how different front-end temporal filters can affect the dependence of two-flash direction selectivity on stimulus onset asynchrony. The versions of the model which successfully predict the form of the observed responses are those which combine inputs from sustained and transient filters.

Comparison of lateral ankle ligamentous reconstruction procedures

The static restraints of various surgical procedures for chronic lateral ankle instability were compared. Forty cadaveric ankles were divided equally into the following five groups: 1) ankles with intact anterior talofibular and calcaneofibular ligaments, 2) ankles with incised anterior talofibular and calcaneofibular ligaments, 3) ankles with Chrisman-Snook procedure, 4) ankles with Watson-Jones procedure, or 5) ankles with modified Broström procedure. All ankles were placed in a mechanical apparatus for anterior drawer stress and inversion stress tests. After each application of force, a radiograph of the ankle joint was taken, and the anterior talar displacement and the talar tilt angle were measured. All procedures reduced anterior drawer and talar tilt when compared with the ankles with incised anterior talofibular and calcaneofibular ligaments. Significant differences were found among the groups for both inversion and anterior drawer stress at all forces, except for the third and fourth groups. The modified Broström group had the least amount of anterior talar displacement and talar tilt angle at all forces. There were no significant differences between the Watson-Jones and the Chrisman-Snook procedures in anterior talar displacement and talar tilt. The modified Broström procedure produced a greater mechanical restraint than either of the other procedures.

Direction identification thresholds for second-order motion in central and peripheral vision

Evidence bearing on the question of whether first-order and second-order motion are detected by use of the same or different principles has been sought. This question was approached by measuring thresholds for correctly identifying the direction of motion of various second-order motion patterns. The patterns used were contrast-modulated noise patterns in which the contrast of a carrier was modulated sinusoidally in one dimension, and the modulating waveform drifted smoothly while the carrier itself remained stationary. The carrier used was in most cases static two-dimensional noise; other carriers gave similar results. Thresholds were measured in terms of amplitude of contrast modulation (modulation depth) for each of a range of envelope drift speeds and spatial frequencies in the fovea and at several viewing eccentricities. Along with direction-identification thresholds, thresholds for either simple detection of the modulation or for correctly identifying the orientation of the modulation were simultaneously measured. Thresholds for direction identification were generally somewhat higher than those for simple detection. However, they were in most cases very similar to thresholds for identification of orientation, as found for conventional luminance gratings. Contrary to some reports, sensitivity to contrast-modulated patterns declines with eccentricity at a similar rate to that found with luminance gratings. The results suggest that first-order and second-order motion are either detected by a common motion-detection mechanism or are detected by different mechanisms that use a common principle of motion detection.

Arthroscopically assisted rotator cuff repair: correlation of functional results with integrity of the cuff

Thirty-three patients (35 full-thickness rotator cuff tears) who underwent arthroscopically assisted mini open repair between June 1987 and January 1990 were evaluated for shoulder function and cuff integrity. The study population was composed of 19 women and 14 men with an average age of 63 years (range 35-76) and an average follow-up of 3.7 years (range 2.5-5.1). Functional results were obtained using the UCLA Shoulder Rating Scale. Integrity of the rotator cuff was established by shoulder arthrography at a minimum 2 years postoperatively. UCLA Shoulder Rating Scale showed 86% good/excellent results with 92% patient satisfaction. The shoulder arthrography showed 12 (34%) full-thickness tears, seven (20%) partial tears, and 16 (46%) no tears. The size of the arthrographic defect correlated well with the size of the intraoperative tear. Seventy percent of the large tears had follow-up full-thickness defects and 80% of the small tears had no defect. Eighty percent and 88% good/excellent functional results were achieved in patients with full-thickness defects and no defects, respectively, without significant differences. The size of the tear at the time of the repair is a major determinant of the integrity of the cuff after repair. The integrity of the cuff at follow-up does not determine the functional outcome of the operated shoulder.