The P50 evoked potential component and mismatch detection in normal volunteers: implications for the study of sensory gating.

Sensory gating is a complex, multistage, multifaceted physiological function believed to be protecting higher cortical centers from being flooded with incoming irrelevant sensory stimuli. Failure of such mechanisms is hypothesized as one of the mechanisms underlying the development of psychotic states. Attenuation of the amplitude of the P50 evoked potential component with stimulus repetition is widely used to study sensory gating. In the current study, we investigated the responsiveness of the P50 component to changes in the physical characteristics of ongoing trains of auditory stimuli. Forty normal volunteers were studied in a modified oddball paradigm. At all cerebral locations studied, P50 amplitudes were higher in response to infrequent stimuli. We postulate that the increase in P50 amplitude reflects the system's recognition of novel stimuli or "gating in" of sensory input. The ratio of the amplitude of the responses to the infrequent stimuli to those of the frequent stimuli was significantly higher for the posterior temporal regions. This finding provides further evidence that the temporal lobes may be significantly involved in sensory gating processes. Although this study only included normal subjects, the data generated contribute to the understanding of sensory gating mechanisms that may be relevant to psychotic states.

Low resolution spin echo: a simple timesaving technique for MRI liver exams.

MR evaluation of the liver at mid-field strength requires relatively lengthy T2-weighted sequences (T2W) for differentiation of benign cavernous hemangiomas from malignant lesions. A short duration T2W, which maintains standard signal-to-noise ratio and also contrast relationships, can be easily implemented by increasing the pixel size in the phase-encoding direction (thus reducing spatial resolution) and proportionally decreasing the number of phase-encoding steps in the matrix (thus reducing acquisition time). Blinded interpretations of a quick (4 min), low resolution (3.4 mm x 1.7 mm pixel) T2W sequence (matrix 64 x 256, FOV 21.7 cm phase x 43.5 cm frequency) were compared to the 17 min standard resolution (1.7 mm x 1.7 mm) T2W sequence (256 x 256 matrix, FOV 43 x 43 cm) in 25 patients suspected of having liver metastasis. Lesions felt to be cavernous hemangiomas showed a 100% (24/24) agreement for interpreter "A" and 96% (22/23) agreement for interpreter "B" when 4 min low resolution T2W was compared to the standard 17 min sequence. Sensitivity (for all types of lesions) of the low resolution T2W sequence ranged from 100% (31/31) for interpreter "A" to 80% (28/35) for interpreter "B." Missed lesions (interpreter "B") were either partially obscured by excessive fat (wrap around) (N = 4), less than 1 cm in size (N = 2), or degraded by motion artifact (N = 1). Thus in many situations low resolution T2 may provide a substantial timesaving alternative to standard T2W particularly where T2W is used primarily for lesion classification in normal sized patients.

Fast short-tau inversion-recovery MR imaging.

To enhance the versatility of the short-tau inversion-recovery (STIR) sequences, the authors determined a range of repetition time (TR) and inversion time (TI) combinations that suppress signal intensity from fat by study of both patient and phantom images. To make fast STIR images, variations in the following pulsing conditions were studied with use of an interactive computer program: decreasing the TR, limiting the number of excitations, and limiting the number of phase-encoding steps. The authors found that (a) STIR imaging need not be time consuming, (b) fat suppression can be accomplished at shorter TR by using shorter TI, and (c) short-TR fast STIR imaging is sensitive to enhancement with gadopentetate dimeglumine.

Histopathology of human keratorefractive lenticules.

Eight human corneal lenticules (three from keratophakia, two from hypermetropic keratomileusis, and three from myopic keratomileusis) were examined by light and electron microscopy. The keratophakic lenticules were removed for optical reasons 3, 3.5, and 6 months after surgery. Microscopically, all displayed hypocellularity, mature collagen fibrils and microfibrils, keratocyte ghosts, and keratocytic debris. The hypermetropic keratomileusis specimens were removed at 13 and 14 months postoperatively, the first because of opacities from enzymatic digestion of the cornea, and the second due to contact lens-induced erosion of Bowman's layer and decreased lacrimal secretion. Ultrastructurally, both lenticules exhibited fractures in Bowman's layer, and the 14-month specimen showed multilayered squamous epithelia. The myopic keratomileusis specimens were removed at 4, 8, and 48 months postoperatively due to opacification of the interface caused by delayed epithelial healing, detergent trauma, and previous epithelization, respectively. Electron microscopy revealed fractures in Bowman's layer, subepithelial fibrocellular growth, sparse keratocyte populations of the anterior stroma, porous collagen bundles, keratocytic debris, and regions of epithelial ingrowth.

Urine screening for leucocytes and bacteria by dipstick and reflectance spectrophotometry.

In order to develop a rapid and reliable method for screening large numbers of urines for leucocytes and bacteria a dipstick method was used with colorimetric reading of colour changes with a reflectance spectrophotometer. One thousand consecutive urine specimens were tested by that method using a single dipstick for leucocyte esterase, nitrite, blood and protein in parallel with routine methods for enumeration of leucocytes by microscopy and quantitative culture for bacteria. Results of the four dipstick tests taken together had predictive values of 83.7% for a positive test for evidence of infection and 97.2% for a negative result. The false negative rate was 1%. Of the 191 urines with a significant bacteriuria (greater than 10(5) organisms/ml), 10 were false negatives by dipstick testing, representing 5% of all infected urines. Elimination of the need for culture on specimens that were negative by all four tests would reduce the workload by 36%.

Epithelial ion transport in rabbit corneas following myopic keratomileusis.

In isolated rabbit corneas that had undergone lamellar keratectomy or myopic keratomileusis, the stimulation of chloride transport by 10(-5) M epinephrine was completely inhibited at 1 week following surgery. At 28 days following surgery, both groups responded to 10(-7) M epinephrine. The response to 10(-5) M amphotericin B was normal both at 1 week and at 28 days following surgery. We conclude that, although the Na-K pump was not affected by the lamellar keratectomy and cryolathing, that either the epithelial beta receptors and/or the cAMP pathway were temporarily inhibited for at least 1 week following surgery. A lamellar keratectomy, therefore, can have an adverse effect on the epithelial transport system of the corneal epithelium even though the epithelium may appear normal clinically.

Prospective evaluation of myopic keratomileusis.

The initial results of a prospective evaluation of myopic keratomileusis are reported. The procedure was attempted on 42 eyes, with an average follow-up of 10 months. The average reductions of myopia were 7.05 D and 11.59 D when measured by keratometry and refraction, respectively. Regular astigmatism increased by 0.50 D on keratometry, but there was no change in the refractive cylinder. The majority of patients had some irregular astigmatism on corneoscopy. The average percent correction, based on refraction, was 94.1 +/- 24%, and the correlation coefficient was 0.43. Visual rehabilitation was rapid. No patient followed for 1 year or more had a decrease in the best-corrected acuity, and 63% of this group had an average improvement of 1.5 lines with as much as five lines. The final refraction may take up to 6 months to stabilize. Complications consisted of epithelium and debris in the interface and focal necrosis of Bowman's membrane. The technique is difficult but may have application in the visual rehabilitation of the high myope.