Biomedical subjects

S J Hamstra

Publications and source records attributed to S J Hamstra.

12 recordsLinked to original sources

PubMed2001

A novel approach to endourological training: training at the Surgical Skills Center.

PURPOSE: We investigated the effects of didactic teaching and supervised hands-on practice on endourological skills using high fidelity genitourinary bench models at a surgical skills laboratory. We also validated a global rating scale and checklist designed specifically for endourological tasks. MATERIALS AND METHODS: We assessed 17 urology residents for the ability to remove a mid ureteral stone using a high fidelity genitourinary model on 3 occasions, including a pre-test at the beginning of the study to assess baseline skills, after a didactic teaching session and after a supervised practice session on high fidelity models. Performance was graded according to a global rating scale, checklist, pass rating and time needed to complete task. RESULTS: Senior residents achieved significantly higher pre-test global rating scores than junior residents (p <0.01). One-way repeated measures analysis of variance revealed a significant effect of training on the endoscopic global rating score (p <0.001). Post-hoc tests demonstrated significant improvement in the global rating scores from the pre-test to the post-didactic session (p <0.05) and from the post-didactic to the post-practice session (p <0.01). Interrater reliability using the global rating scale was high (Pearson's r = 0.82, p <0.01). Significant but less powerful results were observed in the checklist score, pass rating and time. CONCLUSIONS: There was a positive effect of training at the surgical skills laboratory on endourological skills. The global rating scale showed good construct validity and reliability for assessing endourological tasks, more so than the checklist, pass rating or time.

Adult↗

PubMed2001

The joint contributions of saccades and ocular drift to repeated ocular fixations.

We measured the joint contributions of different sized saccades and ocular drift to overall performance in an alternating fixation task. Subjects repeatedly shifted fixation between the centres of a pair of bars of width 2 degrees -2 arc min, either mentally selected or electronically cut from a stationary sine grating display. Eye movement patterns exhibited consistent features across all displays, and pairs of widely separated bars were studied most. Variability (S.D.) and relative accuracy (under/overshooting bias) were estimated from the concentration of eye positions over the two target bars. Overall variability, i.e. for eye movements as a whole, reached a minimum of 5 min for bar widths less than 20 min across subjects, displays and tasks. This was consistent, as were several other aspects of the study, with a constant 20-min diameter goal zone hypothesis. For wide bars, overall variability increased nearly as the square root of bar width, and for narrow bars, was independent of bar separation. A typical between-bar crossing saccade was tightly constrained in departure point but widely scattered in landing position, the associated variability increasing with bar separation. The final high overall precision was achieved largely by within-bar saccades of greater than 7.5 min effective amplitude that were present at a rate of 1 (range 0.3-3) per crossing saccade. This is consistent with views that very small saccades (the smaller microsaccades) make little obvious contribution to oculomotor performance.

Adult↗

PubMed2001

Fiberoptic orotracheal intubation on anesthetized patients: do manipulation skills learned on a simple model transfer into the operating room?

BACKGROUND: With increasing pressure to use operating room time efficiently, opportunities for residents to learn fiberoptic orotracheal intubation in the operating room have declined. The purpose of this study was to determine whether fiberoptic orotracheal intubation skills learned outside the operating room on a simple model could be transferred into the clinical setting. METHODS: First-year anesthesiology residents and first- and second-year internal medicine residents were recruited. Subjects were randomized to a didactic-teaching-only group (n = 12) or a model-training group (n = 12). The didactic-teaching group received a detailed lecture from an expert bronchoscopist. The model-training group was guided, by experts, through tasks performed on a simple model designed to refine fiberoptic manipulation skills. After the training session, subjects performed a fiberoptic orotracheal intubation on healthy, consenting, anesthetized, paralyzed female patients undergoing elective surgery with predicted "easy" laryngoscopic intubations. Two blinded anesthesiologists evaluated each subject. RESULTS: After the training session, the model group significantly outperformed the didactic group in the operating room when evaluated with a global rating scale (P < 0.01)and checklist (P0.05). Model-trained subjects completed the fiberoptic orotracheal intubation significantly faster than didactic-trained subjects (P < 0.01). Model-trained subjects were also more successful at achieving tracheal intubation than the didactic group (P < 0.005). CONCLUSION: Fiberoptic orotracheal intubation skills training on a simple model is more effective than conventional didactic instruction for transfer to the clinical setting. Incorporating an extraoperative model into the training of fiberoptic orotracheal intubation may greatly reduce the time and pressures that accompany teaching this skill in the operating room.

Adult↗

PubMed2000

Visual-spatial abilities in surgical training.

As surgery continues to advance, we will need to better understand the role visual-spatial abilities play in the acquisition of technical skills. Many universities have established surgical skills centers with specific curricula to teach residents technical skills as adjuncts to operating room learning. Yet, as educators we do not fully understand the role visual-spatial abilities plays in the acquisition of surgical skills. This paper summarizes the research to date on the relevance of visual-spatial abilities to surgical training.

Canada↗

PubMed1999

Gap detection thresholds as a function of tonal duration for younger and older listeners.

Twenty normal hearing younger and twenty older adults in the early stages of presbycusis, but with relatively normal hearing at 2 kHz, were asked to discriminate between the presence versus absence of a gap between two equal-duration tonal markers. The duration of each marker was constant within a block of trials but varied between 0.83 and 500 ms across blocks. Notched-noise, centered at 2 kHz, was used to mask on- and off-transients. Gap detection thresholds of older adults were markedly higher than those of younger adults for marker durations of less than 250 ms but converged on those of younger adults at 500 ms. For both age groups, gap detection thresholds were independent of audiometric thresholds. These results indicate that older adults have more difficulty detecting a gap than younger adults when short marker durations (i.e., durations characteristic of speech sounds) are employed. It is shown that these results cannot be explained by linear models of temporal processing but are consistent with differential adaptation effects in younger and older adults.

Adult↗

PubMed1996

Evidence for a neural mechanism that encodes angles.

We measured the discrimination threshold (delta theta)Th for angle theta, where theta was either the angle of a Vee composed of two straight lines contained within the frontoparallel or the angle intersection of two straight lines contained within the frontoparallel plane. The two-line pattern was rotated bodily through a random angle between trials with the aim of eliminating the absolute orientation of one or the other line as a reliable cue to the task. We report evidence that this aim was achieved. Our main conclusion is that the ability to discriminate a change in angle theta cannot entirely be explained in terms of the ability to discriminate changes in the orientations of the individual lines that comprise the Vee. We propose that the human visual pathway contains a neural mechanism that encodes the difference in the orientations of two simultaneously-presented straight lines. Discrimination threshold for angle (delta theta)Th is roughly twice orientation discrimination threshold for an isolated line. When subjects cannot use the orientation of one or another line as a cue to the task, the plot of (delta theta)Th vs theta is approximately flat between the delta = 20 and 160 deg.

Cues↗

PubMed1995

Orientation discrimination in cyclopean vision.

Using a dynamic random-noise display we measured orientation discrimination threshold for two kinds of cyclopean bar and for a cyclopean edge. Ocular vergence was monitored by means of nonious lines. Orientation discrimination threshold for a cyclopean bar or cyclopean edge fell to a minimum at some disparity between about 2 and 50 min arc. For the three subjects tested with bars, the minimum value of discrimination threshold lay between 0.6 and 1.5 deg, and for the two subjects tested with the edge between 0.7 and 2.0 deg. The lowest discrimination thresholds for the cyclopean bars were similar for crossed and uncrossed disparities for all three subjects tested. Matched depth increased smoothly with increasing disparity through a range over which orientation discrimination threshold fell and then levelled out. We conclude that the processing of the depth of a cyclopean form is dissociated from the processing of the orientation of that same cyclopean form. We suggest that orientation discrimination of cyclopean form is determined by the relative activity of binocular, disparity-sensitive, orientation-tuned neurons.

Adult↗

PubMed1995

Visual processing of the motion of an object in three dimensions for a stationary or a moving observer.

A rate of change of relatively disparity is a sufficient binocular stimulus for the perception of motion in depth. For motion within the meridian that contains the eyes, disparity change associated with approaching motion is processed through four channels, each tuned to a different direction of motion in depth. Directional discrimination is most acute but detection sensitivity falls to a minimum for a trajectory passing approximately midway between the eyes. This can be explained if discrimination depends on the relatively of the four channels. Two binocular retinal-image correlates of the direction of the motion of an object in depth are the ratio between the velocities of the retinal images of the object in the left and right eyes [(d phi/dt)R/(d phi/dt)L], and the ratio between the translational velocity of the binocularly fused images and the rates of change of disparity [(d phi/dt)/(d gamma/dt)]. Directional discrimination is possible by using the second cue alone. An isotropic rate of expansion of the retinal image is a sufficient monocular stimulus for the perception of motion in depth. There is no evidence that expansion is processed through channels tuned to the direction of motion in depth. Two monocular correlates of the direction of the motion of an object in depth are the ratio between the translational velocity and the rate of expansion of the retinal image of the object [(d phi/dt)/(d theta/dt)], and the ratio between the velocities of opposite edges of the retinal image [(d alpha 1/dt)/(d alpha 2/dt)]. Subjects are able monocularly to discriminate the direction of motion in depth with high acuity (better than 0.1 deg threshold) in the vertical, horizontal, or oblique meridians, even when the direction and the speed of translational motion are removed as cues. Visual discrimination of time to contact with an approaching object can be disconfounded from discrimination of its rate of expansion and vice versa with a threshold separation ratio of as much as 100:1.

Cues↗

PubMed1994

Shape discrimination for rectangles defined by disparity alone, by disparity plus luminance and by disparity plus motion.

We measured the just-noticeable difference in aspect ratio a/b for dotted rectangles defined entirely by disparity (DD rectangles), dotted rectangles defined by both disparity and luminance contrast (DD+LD rectangles) and rectangles defined by both disparity and relative motion (DD+MD rectangles) over a wide range of uncrossed (0-126 min arc) and crossed (0-126 min arc) disparities. The rectangle's height and width were, respectively, a and b. Vergence was monitored by nonious lines, fusion was monitored by a fusion line, and subjects were instructed to look at the rectangle. We conclude that for DD rectangles, aspect ratio discrimination for crossed and uncrossed disparities cannot be compared accurately unless (a) ocular vergence is monitored, (b) a wide range of crossed and uncrossed disparities are investigated and (c) trials are repeated until threshold has stabilized for the entire range of both crossed and uncrossed disparities. Intersubject variations in fixation disparity is the reason for (a). The reason for (b) is that the effect of disparity on aspect ratio discrimination threshold is different for crossed and uncrossed disparities. The reason for (c) is that, for some subjects, the lowest discrimination threshold continues to fall with practice for uncrossed disparities beyond the point at which the lowest threshold has effectively stabilized for crossed disparities. We report that aspect ratio discrimination threshold for a DD rectangle first decreased and then levelled out as its disparity was progressively increased from zero, while the perceived depth of the rectangle increased smoothly and approximately linearly. We found that the lowest value of aspect ratio discrimination threshold was the same for both crossed and uncrossed disparities. This lowest value occurred at or just before the disparity at which fusion was lost for the dots surrounding the rectangle. The lowest value of discrimination threshold was lower for DD+LD and for DD+MD rectangles than for DD rectangles. The lowest value of discrimination threshold for DD rectangles was 4.0, 3.4, 7.4 and 3.1% for our four subjects. If we assume that a and b are encoded directly, then a 3.1% discrimination threshold implies that the precision of encoding a and b is better than 1 min arc--considerably better than the 9 min arc mean dot separation, and considerably better than the 3-5 c/deg estimate for cyclopean grating acuity.

Adult↗

PubMed1993

Dissociation of discrimination thresholds for time to contact and for rate of angular expansion.

It is well known that, if a rigid sphere is moving at constant speed towards the eye along the line of sight then, for small values of theta, T = theta/theta, where T is the time to contact, theta is the instantaneous angular size and theta is the rate of increase of angular size. We describe a rationale and an experimental procedure for demonstrating empirically when subjects base discrimination of time to contact on trial-to-trial variations of (theta/theta) rather than on variations of theta or on variations of delta theta (the change of angular size during a presentation). Discrimination threshold for the ratio (theta/theta) was 0.070-0.13, and was independent of mean time to contact over a range of at least T = 1.0-4.0 sec. We conclude that the human visual pathway contains a mechanism that is sensitive to the ratio (theta/theta) rather independently of the values of theta and theta. Using a different procedure we demonstrated empirically that subjects based discriminations of rate of expansion on trial-to-trial variations of theta rather than variations of time to contact or on variations of delta theta. Discrimination threshold for rate of expansion was 0.85-0.14, and was independent of mean time to contact over a range of at least T = 1.0-4.0 sec. We conclude that the human visual pathway contains a mechanism that is sensitive to rate of expansion rather independently of time to contact or absolute change in size. When rate of expansion and time to contact were both available as cues, discrimination threshold was on average lower than when only one of the two cues was present. We conclude that there is some summation of the two cues. Our data can be explained by a small modification to a model previously put forward to account for data on threshold elevations and aftereffects caused by adapting to changing-size stimulation. This model incorporates a filter that is strongly activated by isotropic, homogeneous two-dimensional expansion of the retinal image and whose output is inversely proportional to time to contact (i.e. the more urgent the demand for evasive action, the stronger the output of the filter).

Discrimination, Psychological↗

PubMed1992

Shape discrimination and the judgement of perfect symmetry: dissociation of shape from size.

We measured the accuracy with which subjects judged that a square or circle was perfectly symmetrical i.e. that aspect ratio (a/b) was exactly unity (where a and b were, respectively, the vertical and horizontal dimensions). Errors were remarkably small, ranging from 0.7 to 0.4% for the judgement of squareness and from 1.4 to < 0.1% for the judgement of circularity. Precision in judging aspect ratio was measured by requiring subjects to judge whether the aspect ratio (a/b)TEST of a test rectangle was greater or less than the aspect ratio (a/b)REF of a reference rectangle. Similar measurements were made for elliptical targets. To ensure that subjects based judgements on aspect ratio rather than a, b or (a-b), the area of each successive presentation was varied randomly. The just-discriminable percentage change of aspect ratio was as low as 1.6% at (a/b)REF = 1.0 (i.e. for a square or circular reference), and rose progressively as (a/b)REF was made progressively larger or smaller than 1.0. Aspect ratio discrimination threshold was independent of mean area over a sixteen-fold range of 0.25-4.0 deg2. For both rectangles and ellipses, the best value of aspect ratio discrimination threshold corresponded to a precision of encoding a and b of 14 sec arc or better. In further experiments, the method of constant stimuli was used to measure an aspect ratio aftereffect produced by adapting separately to rectangles of (a/b)ADAPT equal to 1.5, 1.0 and (1/1.5). Similar aftereffects were obtained whether the area of the test stimulus was fixed or varied randomly from trial to trial, and whether the test stimulus was rectangular or elliptical. The aftereffect could not be explained in terms of fatigue of neurons sensitive to linear dimension a or b. Nor could the aftereffect be explained in terms of the "contour repulsion" hypothesis, or in terms of orientation discrimination. We conclude (1) that the same neural mechanism determines aspect ratio discrimination threshold for rectangles and ellipses and (2) that this mechanism is sensitive to aspect ratio independently of linear dimensions. We propose that aspect ratio perception is determined by the balance of excitation of two pools of neurons that are selectively sensitive to different, but overlapping ranges of (a/b). One pool prefers aspect ratios > 1.0 and the others prefer aspect ratios < 1.0. We suppose that the two pools respond identically to changes in area (a * b).(ABSTRACT TRUNCATED AT 400 WORDS)

Adaptation, Ocular↗

PubMed1992

Dissociation of orientation discrimination from form detection for motion-defined bars and luminance-defined bars: effects of dot lifetime and presentation duration.

A stationary bar-shaped area that was perfectly camouflaged within a dot pattern was rendered visible by moving the dots inside and outside the bar at equal and opposite speeds. Orientation discrimination for this motion-defined (MD) bar was compared with orientation discrimination for a luminance-defined (LD) bar created by switching off all dots outside the bar. The best values of orientation discrimination threshold were similar for MD and LD bars at long presentation durations and long dot lifetimes. But, as presentation duration or dot lifetime was reduced below 1.0 sec, orientation discrimination threshold for MD bars increased at an accelerating rate, while discrimination for LD bars was comparatively unaffected. However, these effects of presentation duration and dot lifetime were largely due to changes in bar visibility. When bar visibility was normalised relative to the relevant bar detection threshold, the effect of presentation duration upon orientation discrimination was abolished for both MD and LD bars, and the effect of dot lifetime was abolished or even reversed. These observations dissociate detection and discrimination for MD and LD form. We suggest that orientation discrimination for MD and LD bars is determined by opponent-orientation mechanisms whose performance is not directly affected by presentation duration, nor degraded by reducing dot lifetime.

Adolescent↗