Brain region-dependent effects of dexamethasone on counterregulatory responses to hypoglycemia in conscious rats

The aim of this study was to determine whether activation of central type II glucocorticoid receptors can blunt autonomic nervous system counterregulatory responses to subsequent hypoglycemia. Sixty conscious unrestrained Sprague-Dawley rats were studied during 2-day experiments. Day 1 consisted of either two episodes of clamped 2-h hyperinsulinemic (30 pmol x kg(-1) x min(-1)) hypoglycemia (2.8 +/- 0.1 mM; n = 12), hyperinsulinemic euglycemia (6.2 +/- 0.1 mM; n = 12), hyperinsulinemic euglycemia plus simultaneous lateral cerebroventricular infusion of saline (24 microl/h; n = 8), or hyperinsulinemic euglycemia plus either lateral cerebral ventricular infusion (n = 8; LV-DEX group), fourth cerebral ventricular (n = 10; 4V-DEX group), or peripheral (n = 10; P-DEX group) infusion of dexamethasone (5 microg/h), a specific type II glucocorticoid receptor analog. For all groups, day 2 consisted of a 2-h hyperinsulinemic (30 pmol x kg(-1) x min(-1)) or hypoglycemic (2.9 +/- 0.2 mM) clamp. The hypoglycemic group had blunted epinephrine, glucagon, and endogenous glucose production in response to subsequent hypoglycemia. Consequently, the glucose infusion rate to maintain the glucose levels was significantly greater in this group vs. all other groups. The LV-DEX group did not have blunted counterregulatory responses to subsequent hypoglycemia, but the P-DEX and 4V-DEX groups had significantly lower epinephrine and norepinephrine responses to hypoglycemia compared with all other groups. In summary, peripheral and fourth cerebral ventricular but not lateral cerebral ventricular infusion of dexamethasone led to significant blunting of autonomic counterregulatory responses to subsequent hypoglycemia. These data suggest that prior activation of type II glucocorticoid receptors within the hindbrain plays a major role in blunting autonomic nervous system counterregulatory responses to subsequent hypoglycemia in the conscious rat.

The effects of dehydroepiandrosterone sulfate on counterregulatory responses during repeated hypoglycemia in conscious normal rats

We previously determined that both antecedent hypoglycemia and elevated cortisol levels blunt neuroendocrine and metabolic responses to subsequent hypoglycemia in conscious, unrestrained rats. The adrenal steroid dehydroepiandrosterone sulfate (DHEA-S) has been shown in several studies to oppose corticosteroid action. The purpose of this study was to determine if DHEA-S could preserve counterregulatory responses during repeated hypoglycemia. We studied 40 male Sprague-Dawley rats during a series of 2-day protocols. Day 1 consisted of two 2-h episodes of 1) hyperinsulinemic (30 pmol. kg(-1). min(-1)) euglycemia (6.2 +/- 0.2 mmol/l; n = 12; ANTE EUG), 2) hyperinsulinemic euglycemia (6.0 +/- 0.1 mmol/l; n = 8) plus simultaneous intravenous infusion of DHEA-S (30 mg/kg; ANTE EUG + DHEA-S), 3) hyperinsulinemic hypoglycemia (2.8 +/- 0.1 mmol/l; n = 12; ANTE HYPO), or 4) hyperinsulinemic hypoglycemia (2.8 +/- 0.1 mmol/l; n = 8) with simultaneous intravenous infusion of DHEA-S (30 mg/kg; ANTE HYPO + DHEA-S). Day 2 consisted of a single 2-h hyperinsulinemic hypoglycemic (2.8 +/- 0.1 mmol/l) clamp. During the final 30 min of day 2, hypoglycemia norepinephrine levels were significantly lower in the ANTE HYPO group versus the ANTE HYPO + DHEA-S group (2.0 +/- 0.2 vs. 3.3 +/- 0.6 nmol/l; P < 0.05). In addition, epinephrine (8 +/- 1 vs. 17 +/- 2, 14 +/- 3, and 15 +/- 3 nmol/l), glucagon (91 +/- 8 vs. 273 +/- 36, 231 +/- 42, and 297 +/- 48 ng/l), and corticosterone (1,255 +/- 193 vs. 1,915 +/- 212, 1,557 +/- 112, and 1,668 +/- 119 pmol/l) were significantly lower in the ANTE HYPO group versus the ANTE EUG, ANTE EUG + DHEA-S, and ANTE HYPO + DHEA-S groups (P < 0.05). Endogenous glucose production was also significantly less in the ANTE HYPO group versus the ANTE EUG, ANTE EUG + DHEA-S, and ANTE HYPO + DHEA-S groups (13 +/- 5 vs. 32 +/- 3, 38 +/- 7, and 29 +/- 8 micro mol/l. kg(-1). min(-1); P < 0.05). Consequently, the amount of exogenous glucose needed to maintain the glycemic level during the clamp studies was significantly higher in the ANTE HYPO versus the ANTE EUG, ANTE EUG + DHEA-S, and ANTE HYPO + DHEA-S groups (57 +/- 8 vs. 22 +/- 5, 18 +/- 6, and 18 +/- 3 micro mol/l. kg(-1). min(-1); P < 0.05). In summary, day-1 antecedent hypoglycemia blunted neuroendocrine and metabolic responses to next-day hypoglycemia. However, simultaneous DHEA-S infusion during antecedent hypoglycemia preserved neuroendocrine and metabolic counterregulatory responses during subsequent hypoglycemia in conscious rats.

Effect of morning exercise on counterregulatory responses to subsequent, afternoon exercise

The aim of this study was to determine whether a bout of morning exercise (EXE(1)) can alter neuroendocrine and metabolic responses to subsequent afternoon exercise (EXE(2)) and whether these changes follow a gender-specific pattern. Sixteen healthy volunteers (8 men and 8 women, age 27 +/- 1 yr, body mass index 23 +/- 1 kg/m(2), maximal O(2) uptake 31 +/- 2 ml x kg(-1) x min(-1)) were studied after an overnight fast. EXE(1) and EXE(2) each consisted of 90 min of cycling on a stationary bike at 48 +/- 2% of maximal O(2) uptake separated by 3 h. To avoid the confounding effects of hypoglycemia and glycogen depletion, carbohydrate (1.5 g/kg body wt po) was given after EXE(1), and plasma glucose was maintained at euglycemia during both episodes of exercise by a modification of the glucose-clamp technique. Basal insulin levels (7 +/- 1 microU/ml) and exercise-induced insulin decreases (-3 microU/ml) were similar during EXE(1) and EXE(2). Plasma glucose was 5.2 +/- 0.1 and 5.2 +/- 0.1 mmol/l during EXE(1) and EXE(2), respectively. The glucose infusion rate needed to maintain euglycemia during the last 30 min of exercise was increased during EXE(2) compared with EXE(1) (32 +/- 4 vs. 7 +/- 2 micromol x kg(-1) x min(-1)). Although this increased need for exogenous glucose was similar in men and women, gender differences in counterregulatory responses were significant. Compared with EXE(1), epinephrine, norepinephrine, growth hormone, pancreatic polypeptide, and cortisol responses were blunted during EXE(2) in men, but neuroendocrine responses were preserved or increased in women. In summary, morning exercise significantly impaired the body's ability to maintain euglycemia during later exercise of similar intensity and duration. We conclude that antecedent exercise can significantly modify, in a gender-specific fashion, metabolic and neuroendocrine responses to subsequent exercise.

Effects of antecedent prolonged exercise on subsequent counterregulatory responses to hypoglycemia

In the present study the hypothesis tested was that prior exercise may blunt counterregulatory responses to subsequent hypoglycemia. Healthy subjects [15 females (f)/15 males (m), age 27 +/- 1 yr, body mass index 22 +/- 1 kg/m(2), hemoglobin A(Ic) 5.6 +/- 0.5%] were studied during 2-day experiments. Day 1 involved either 90-min morning and afternoon cycle exercise at 50% maximal O2 uptake (VO2(max)) (priorEXE, n = 16, 8 m/8 f) or equivalent rest periods (priorREST, n = 14, 7 m/7 f). Day 2 consisted of a 2-h hypoglycemic clamp in all subjects. Endogenous glucose production (EGP) was measured using [3-3H]glucose. Muscle sympathetic nerve activity (MSNA) was measured using microneurography. Day 2 insulin (87 +/- 6 microU/ml) and plasma glucose levels (54 +/- 2 mg/dl) were equivalent after priorEXE and priorREST. Significant blunting (P < 0.01) of day 2 norepinephrine (-30 +/- 4%), epinephrine (-37 +/- 6%), glucagon (-60 +/- 4%), growth hormone (-61 +/- 5%), pancreatic polypeptide (-47 +/- 4%), and MSNA (-90 +/- 8%) responses to hypoglycemia occurred after priorEXE vs. priorREST. EGP during day 2 hypoglycemia was also suppressed significantly (P < 0.01) after priorEXE compared with priorREST. In summary, two bouts of exercise (90 min at 50% VO2(max)) significantly reduced glucagon, catecholamines, growth hormone, pancreatic polypeptide, and EGP responses to subsequent hypoglycemia. We conclude that, in normal humans, antecedent prolonged moderate exercise blunts neuroendocrine and metabolic counterregulatory responses to subsequent hypoglycemia.

Journal article content as a predictor of commitment to change among continuing medical education respondents

BACKGROUND

Journal reading is a time-tested means of continuing medical education (CME) among physicians. Asking physicians to make a written statement of commitment to change has been shown to increase the likelihood of eventual change in practice behavior when used in conjunction with CME lectures. We describe the type and medical content of articles responsible for commitment to change comments among readers of a large circulation primary care journal.

METHODS

Response to the question "what change(s) do you plan to make in your practice as a result of reading the articles in this issue?" were analyzed from CME response cards associated with six issues (1 year's publication) of the Archives of Family Medicine. Responses indicating a commitment to change were analyzed to characterize the type and content of articles responsible for their generation.

RESULTS

During the 1-year study period, original contributions (reports of research trials) dealing with medicine and preventive medicine content accounted for the greatest number of comments. After adjusting for the frequency of article type and content, special articles and those dealing with complementary medicine accounted for the highest number of comments on a per-article basis.

FINDINGS

Family physicians make commitment to change statements on the basis of a broad range of journal articles. Certain articles are more likely to generate statements of commitment to planned changes in practice.

Effects of differing durations of antecedent hypoglycemia on counterregulatory responses to subsequent hypoglycemia in normal humans

The aim of this study was to determine whether the duration of antecedent hypoglycemia regulates the magnitude of subsequent counterregulatory failure. A total of 31 lean healthy overnight-fasted individuals (16 men/15 women) were studied. There were 15 subjects (8 men/7 women) who underwent two separate 2-day randomized experiments separated by at least 2 months. On day 1, 2-h hyperinsulinemic (9 pmol x kg(-1) x min(-1)) euglycemic (5.2 +/- 0.1 mmol/l) or hypoglycemic (2.9 +/- 0.1 mmol/l) glucose clamps (prolonged hypoglycemia) were carried out in the morning and afternoon. Of the other subjects, 16 participated in a 2-day study in which day 1 consisted of morning and afternoon short-duration hypoglycemia experiments (hypoglycemic nadir of 2.9 +/- 0.1 mmol for 5 min), and 10 of these individuals underwent an additional 2-day study in which day 1 consisted of morning and afternoon intermediate-duration hypoglycemia (hypoglycemic nadir of 2.9 +/- 0.1 mmol for 30 min). The next morning (day 2) all subjects underwent an additional 2-h hyperinsulinemic-hypoglycemic clamp (2.9 +/- 0.1 mmol/l). The rate of fall of glucose (0.07 mmol/min) was carefully controlled during all hypoglycemic studies so that the glucose nadir was reached at 30 min. Despite equivalent day 2 plasma glucose and insulin levels, there were significant differences in counterregulatory physiological responses. Steady-state epinephrine, glucagon, growth hormone, cortisol, and pancreatic polypeptide levels were similarly significantly blunted (P < 0.01) by the differing duration day 1 hypoglycemia compared with day 1 euglycemia. Muscle sympathetic nerve activity and endogenous glucose production were also similarly blunted (P < 0.01) by day 1 hypoglycemia (relative to day 1 euglycemia). Day 2 hypoglycemic symptoms were significantly reduced (P < 0.01) after day 1 prolonged intermediate- but not short-duration hypoglycemia. In summary, two episodes of short-duration moderate hypoglycemia can produce significant blunting of key neuroendocrine and metabolic counterregulatory responses. Hypoglycemic symptom scores are reduced by prolonged but not short-duration prior hypoglycemia. We conclude that in healthy overnight fasted humans, 1) neuroendocrine, autonomic nervous system, and metabolic counterregulatory responses are sensitive to the blunting effects of even short-duration prior hypoglycemia, and 2) the duration of antecedent hypoglycemia results in a hierarchy of blunted physiological responses with hypoglycemic symptom awareness less vulnerable than neuroendocrine responses.

Illustrations and implications of current models of HIV health service provision in rural areas

Despite recent evidence of faster than average increases in HIV/AIDS cases in rural areas across the U.S., there is still a generally poor understanding of successful models of rural HIV/AIDS health-care delivery. Past research in rural Kentucky suggested several barriers to care resulting in most rural HIV-positive patients traveling from rural to urban areas for care. Patients sought urban areas for care for reasons including patient confidentiality, a perceived lack of expertise on the part of rural physicians in caring for HIV-positive patients, and outright referral from rural to urban areas. Case histories are used to illustrate a variety of models of care used by rural HIV-positive patients. These include splitting and sharing care between rural primary care physicians and urban medical specialists, as well as patients receiving all their care in urban areas. Implications of these models for quality of care are discussed.

Frequency of human immunodeficiency virus testing among rural US residents and why it is done

OBJECTIVE

To examine the frequency with which rural residents' undergo human immunodeficiency virus (HIV) antibody testing and the reasons why.

DESIGN

Data are from the 1991 National Health Interview Survey's supplemental questions on knowledge and attitudes about acquired immunodeficiency syndrome.

SUBJECTS

The respondents were 42,725 adults (aged > or = 18 years), representing a nationwide sample of the civilian, noninstitutionalized population of the United States. Rural (n = 9903) and urban (n = 32,822) respondents were compared.

RESULTS

Although 7.1% of rural and 7.9% of urban respondents are at high risk for contracting HIV (P = .06), 25.2% of rural and 33.0% of urban respondents had been tested for HIV (P = .001). Excluding blood donations, 10.7% of rural and 17.2% of urban respondents had been tested for HIV (P = .001). The primary reason between the two groups for not getting tested was a belief of being at low risk for contracting HIV. Rural respondents were less likely than urban respondents (6.6% vs 10.4%) (P = .001) to be tested for HIV in the next 12 months. Rural respondents were less informed about HIV risks than were urban respondents. Urban residence is a significant predictor of having had an HIV test even after controlling for actual risk status, perceived risk status, age, education, income, sex, perceived health status, and a scale of knowledge of acquired immunodeficiency syndrome risk factors (odds ratio, 1.54; 95% confidence interval, 1.37 to 1.73).

CONCLUSIONS

Rural residents are less knowledgeable about HIV risk factors and are less likely to have been tested for HIV. With the increasing rates of infection in rural areas, specific and focused efforts for counseling and testing for HIV antibodies in rural areas might prevent and control HIV infection and acquired immunodeficiency syndrome.

The utility of electronic mail as a medium for patient-physician communication

OBJECTIVE

To examine patients' knowledge, use, and attitudes regarding electronic mail communication with their family physicians.

DESIGN

Mail survey.

SETTING

A university-based family practice center.

PARTICIPANTS

Adult patients (18 years and older) of full-time faculty physicians at a university-based family practice center who had seen their physicians at lease once in the 10-month period between July 1, 1992, and April 30, 1993 (N = 4094). Subjects were eligible for participation if both they and their physicians had an electronic mail address at the University of Kentucky, Lexington (n = 117).

INTERVENTION

None.

MAIN OUTCOME MEASURES

Patient-reported knowledge, use, and attitudes regarding the utility of electronic mail as a means of patient-physician communication.

RESULTS

The response rate to the survey was 74% (n = 87). Patient-physician communication via electronic mail was positively perceived by patients for whom electronic mail was accessible Patient-physician communication via electronic mail was perceived to increase speed, convenience, and access to medical care. Electronic mail communication was perceived to be good for simple and nonurgent problems, such as refilling prescriptions, communicating laboratory results, and making appointments. Ninety percent of the patients who had corresponded with their physicians via electronic mail used the medium to discuss a medical problem.

CONCLUSIONS

Although electronic mail is not presently in wide use for patient-physician communication, there is great potential for its widespread acceptance.

Temporal relationships between induced changes in c-myc mRNA abundance, proliferation, and differentiation in HL60 cells

Changes in the relative abundances of c-myc mRNA have been related to changes in other parameters of differentiation (histochemical, clonogenic) during the course of the differentiation of HL60 cells to monocytes/macrophages or to granulocytes. Induction of differentiation to monocytes/macrophages was marked by a rapid rate of appearance of committed cells (80 to 90% in 24 hours) and a concomitant rapid loss of c-myc mRNA. Induction of granulocytic differentiation resulted in a much slower rate of appearance of committed cells (50% in 48 hours), and a much faster rate of loss of c-myc mRNA (tenfold in 1 hour). These data are consistent with there being a direct link between down-regulation of the expression of c-myc and the onset of proliferation arrest and monocytic differentiation, but show there is no such association of c-myc mRNA abundance and proliferation or differentiation during the maturation of HL60 granulocytes.

Effects of flicker modulation depth on the detection of changes in target location

Response times to a change in location of a small, low contrast target in a uniform flickering field were investigated under a range of flicker modulation depths. Increasing depth of modulation gave an increased response time for modulation depths from 0% (no flicker, to approx. 35%. Further increases in modulation depth had only a marginal effect on response time. It is shown that this effect is not due to flicker adaptation. The functional form of the modulation dependence is qualitatively similar to that reported by Badcock and Smith (1989, Vision Research, 29, 803-808), but shows a large difference in the modulation at which a levelling of performance occurred. It is shown that this difference is consistent with the presence of a third temporal filter, as proposed by Mandler and Makous (1984, Vision Research, 24, 1881-1887).

Discrimination of motion in depth trajectory following acute alcohol ingestion

Evoked potentials were recorded at four scalp sites O1, O2, T5, T6; before and after the consumption of one of two alcohol dose levels, to the occurrence of disparate stimuli in dynamic random dot stereograms. Twenty young adult subjects, all of whom had vision which was normal or corrected to normal, were randomly assigned to the two dosage groups. Subjects viewed stimuli which embodied crossed disparity and followed one of three trajectories in depth. The task was to distinguish between and identify, by specific button presses, these trajectories. One half of the stimuli were presented with monocular cues to motion in depth (dark condition); in the remainder only stereoscopic depth information was available (uniform condition). Responses to both conditions showed alcohol-related reduction of evoked responses across all sites. It was concluded that the visual systems associated with the processing of motion in depth were susceptible to alcohol effects, as revealed by reductions in the amplitude of evoked responses.

Detecting target shifts under uniform field flicker

An experiment was performed in which a small target, viewed foveally, underwent 1 degree lateral shifts at quasi-random intervals. Subjects' response latency to the shift was investigated under various square-wave flicker conditions and compared to non-flickering conditions. Flicker was found to raise latencies, the effect peaking between 8 and 14 Hz. Under constant luminance difference between target and background the high target contrast phase was found to dominate the response cueing. The results are discussed in terms of a simple model of cumulative signal strength and adaptation processes.

Component enhancement through temporal compression and expansion of event-related potentials

A technique is described in which wave forms are temporally compressed or expanded to provide optimal correlation with a superaveraged template. In certain instances, this filter can be applied more appropriately than other filters which shift or rotate wave forms. A 2-part experiment was carried out in which the efficacy of the filter was evaluated for responses of (a) moderate size and consistent latency, and (b) small amplitude and variable latency. The filter was effective in enhancing data for the small, variable responses while it had no significant effect on more consistent data. Possible functional interpretations of latency adjustment techniques are presented.

Stereoscopic evoked responses to crossed and uncrossed disparity accompanying simulated refractive error

Evoked potentials were recorded to the occurrence of a disparate stimulus in dynamic random dot stereograms. Seven adult males, all of whom had vision which was normal or corrected to normal, participated in the experiment. Subjects viewed 100 ms duration stimuli which embodied 30 arc min of either crossed or uncrossed disparity under four conditions of spherical overcorrection: -0.25, +1.0, +2.0, +3.0 dioptres. The first condition, essentially normal refraction, yielded reliable behavioural reports of the stimulus and clear evoked potentials to both crossed and uncrossed disparity. With increasing overcorrection the behavioural reports became less reliable, and the evoked potentials were degraded for both conditions of disparity. The responses to the crossed disparity condition, however, showed significantly less degradation in both behavioural and electrophysiological measures. The implications of this finding may be that there are separate cortical subsystems for the processing of crossed and uncrossed disparity and that the former is more robust under non-ideal viewing conditions.

The determinants of the non-motoric CNV in a complex, variable foreperiod, information processing paradigm

The aim of the present study was to examine the determinants of the non-motoric CNV or E-wave elicited in a complex, variable foreperiod information processing task. Subjects were required to determine whether two sets of tones, one set presented back-to-back (S1/S2), the other separated by a variable foreperiod (S3/S4), were either matching or mismatching. Data were collected over two recording sessions; a baseline and an experimental run. The experimental session comprised three conditions; a match, a mismatch and no-response conditions. Mean amplitude and factor score comparisons showed the parietally-dominant E-wave to be dependent on the task relevance of the stimulus which it precedes. Differing midline distributions for the negative afterwave and the E-wave with increased processing requirements was taken to suggest that the two responses reflect functionally distinct phenomena.

Scalp response topography to dynamic random dot stereograms

Responses were recorded to dynamic random dot stereograms from 32 sites (acquired in separate 8-channel montages). The disparate stimulus comprised a central, 3 degrees square of 30 arc min crossed disparity, superimposed on a 10.7 degrees square background field. Stimulus onset occurred 100 msec into a 500 msec recording epoch, with a 100 msec stimulus duration. Wave forms for 8 subjects were normalised across the 100 msec pre-stimulus period. Average amplitudes were calculated for the post-onset intervals: 120-140, 140-180, 180-220, 220-260 and 260-300 msec. Contour lines of evoked response amplitude were plotted from these data. These plots are compared with theoretical plots generated from a '3 concentric sphere' model of the head. The results lend support to the proposal that preliminary stereoscopic processing takes place in the primary visual cortex, followed by secondary processing in more anterior cortical regions.

Effects of movement in the background field on long-range apparent motion

Two experiments are reported using time-till-breakdown as a measure of apparent motion (AM) in untrained subjects. In Experiment 1 centrally viewed dynamic random dot stereograms and non-disparate stimuli in dynamic random dot fields yielded higher optimal frequencies than did a standard binocular condition. In Experiment 2 a higher optimal frequency was observed for disparate dynamic random dot stimuli compared with both standard binocular and non disparate stimuli presented on a static random dot field. Interaction between short- and long-range AM processes is considered in the interpretation of the findings.

Detection threshold differences to crossed and uncrossed disparities

Using a sample of 85 subjects measurements were made of minimum stimulus durations necessary for detection of crossed and uncrossed disparity stimuli which were presented in five positions in the visual field: centre, lower, upper, right, and left field. The results indicated large detection duration differences between the two disparity conditions, with a marked superiority for crossed disparity detection at all positions. A left-right visual field anisotropy was demonstrated for crossed disparity stimuli.

Evoked potentials to dynamic random dot stimuli with varying dot density ratios of disparity to background

A systematic study was made of visual evoked responses to dynamic random dot stimuli containing controllable, monocularly visible contrast cues. Ratios of dot densities for the centrally presented, disparate figure and the background were varied in steps of 1/8 maximum density between 0/8 and 8/8. The figure was either a square or an equivalent area of nebulous shape. A 30-arc min disparity was compared with binocular nondisparate and monocular conditions. Evoked responses (scalp sites 02, 01, T6, T5) were averaged for each of 24 disparity-contrast-shape conditions. At all contrast levels, response amplitudes and latency over the left hemisphere was significantly greater than over the right hemisphere. For 30 arc min disparity, amplitudes in the 8/8 condition were significantly smaller than in conditions where stimulus/background contrast could afford monocular depth cues. Hemisphere amplitude differences diminished as contrast decreased. Factor analyses isolated two overlapping components in the response to disparate stimuli. The earlier, at 236 ms latency, may index the stereoscopic stimulus features. The later, at 295 ms, peaking at maximum contrast and present in all suprathreshold nondisparity conditions, may index contrast features of the stimulus. The results indicate the importance of controlling dot density ratios in electrophysiological studies of the stereoscopic response to random dot stimuli.

Evoked potentials to dynamic random dot stereograms in upper, center and lower fields

Dynamic random dot stereograms consisting of square, crossed disparities subtending 3 degrees, exposure duration 100 ms, were presented in the midplane centrally (surrounding the fixation point) and in the upper and lower fields at 3.5 degrees vertical displacement from fixation. Stereoscopic cerebral evoked potentials (EPs) recorded from scalp sites 02, 01, T6 and T5 (10-20 system) were analyzed in a group of normal young adults (N = 28). Fourteen were able to perceive and report the stereophenomenon (perceivers); the other half were unable to do so (nonperceivers). Mean response amplitudes were significantly larger to center field stimuli than to those in the upper field. Responses were larger in perceivers than nonperceivers, maximal at T5, and larger to stimuli with definite than indefinite boundaries. Peak EP latencies were shortest to lower field stimuli (243 ms) but not significantly different than to center (247 ms) or upper (253 ms) field stimuli. A separate experiment comparing EP amplitudes and latencies with crossed and uncrossed disparities of varying magnitudes confirmed greater responses to the crossed.

Stereoscopic cerebral evoked potentials of Air Force pilots and civilian comparison groups

Dynamic random-dot stereograms were presented to Air Force jet pilots (N = 14) through a new visual display system. Games which simulated target-detection exercises were included in the test sequence. Event-related potentials were recorded at scalp sites 01, 02, T5, T6, referenced to an anterior midline site. Comparisons were made where possible with the data from recent studies on three civilian groups. A pattern of response was revealed in the event-related potential measures. In general, left-hemisphere amplitudes exceeded right and the predominant response was recorded at the left-temporal site. Amplitudes of responses in subjects able to describe the stimuli in subjective report (perceivers) exceeded those of non-perceivers. Stimuli with definite boundaries evoked stronger and earlier-latency responses than stimuli with nebulous boundaries. The response patterns of subjects make a contribution to normative data on the cerebral electrical indices of stereoscopic vision.

Evoked responses to distinct and nebulous stereoscopic stimuli

When geometric or contoured shapes are generated as disparity regions in dynamic random-dot stereograms, the question of whether the subject's response is evoked largely by boundaries and corners of the disparate region arises. To examine this question a disparate square was shifted randomly within a focal region at one millisecond intervals, resulting in a nebulous stimulus which "welled up" in the centre of the field of view. The technique may be regarded as an analogue of the defocusing technique used to resolve similar questions about evoked potentials to patterned stimuli. Average visual evoked potentials (VEP) were recorded from homolateral pairs of occipital and posterior temporal electrodes referred to a common midline prefrontal site. The latencies and amplitudes of VEP to distinct and nebulous stimuli were compared in a group of young normal adults, of whom 14 succeeded in perceiving and 14 failed to perceive the presented stimuli. Latencies of maximal response to distinct and nebulous stimuli were closely similar in the two groups (range N242-N252). Mean amplitudes of the peak response differed between perceivers and non-perceivers. The responses to nebulous stimuli were smaller than those to distinct stimuli in site-for-site comparisons in both groups. Even in short averaging runs a substantial VEP (about 3 microvolt) was recorded to nebulous stimuli in the perceiver group. It is suggested that both disparity and boundary features contribute to the VEP when this particular display system is used.

Measurement of dynamic stereoacuity and global stereopsis

Quantification of global stereopsis in addition to stereoacuity has been achieved by using dynamic random dot test patterns generated by inexpensive computer controlled integrated electronics. A pilot study revealed marked differences between the responses of a variety of clinically defective and normal subjects.