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Biomedical subjects

M Di Stefano

Publications and source records attributed to M Di Stefano.

At least 109 records · Page 6Linked to original sources
PubMed1991

Effect of octreotide (SMS 201-995) on meal-stimulated pancreatic secretion in three patients with external pancreatic fistula.

Octreotide (SMS 201-995) is a long-acting somatostatin analogue that inhibits both basal and secretin-cholecystokinin-stimulated pancreatic secretion. This study assesses the effect of this compound on pancreatic secretion induced by ordinary meals. Three patients with stable secretion of pure pancreatic juice from high output pancreatic fistula were studied. In all three, meal ingestion caused a marked and prolonged increase in pancreatic juice flow, and in bicarbonate and protein output. The subcutaneous injection of 50 micrograms octreotide before meals almost totally prevented (by about 90%) this increase. The inhibitory effect of octreotide on postprandial pancreatic secretion appeared soon, and persisted for the duration of the study period (8 h).

Food↗
PubMed1989

Spatial distribution of the inhibitory effect of peripheral non-informative cues on simple reaction time to non-fixated visual targets.

It is known that reaction time (RT) for the detection of a light target at extrafoveal locations is lengthened by a previous non-informative light cue at the same location. We describe an additional inhibitory effect from cues remote from the target but occurring within the same lateral or altitudinal visual hemifield. Subjects made a speeded key-press response to the second of two successive light flashes in a pair while maintaining fixation. Each of the two flashes could appear at random in one of four positions, two in the right and two in the left visual fields, or two in the upper and two in the lower visual fields. We found an RT prolongation not only for cued over uncued positions, but also for within-field non-coincident cue-target pairs over between-fields cue-target pairs. The within-field inhibitory effect, though smaller than the same-location effect, was fully apparent even when the target occurred at 1 degree of visual angle from the midline and at 29 degrees from the cue. Both effects were seen with cue-target asynchronies ranging from 0.2 to 1.5 sec. The results are relevant to the understanding of the neural mechanisms for covert shifts of attention across the main meridians of the visual field.

Adult↗
PubMed1989

Thrombosis of portal vein in absence of hepatic disease.

Of 602 patients referred for portal hypertension during a 20 year period, 18 children (mean age of 5.0 years) and 29 adults (mean age of 26.5 years) had thrombosis in the portal bed with a healthy liver (7.8 per cent incidence). In 29 of the 47 patients we studied, the causative factor remained obscure. Variceal hemorrhage occurred in all and was the indication for our observation and treatment. In 11 adult patients (mean age of 22.1 years), surgical treatment was not performed because of reluctance of the patient or because of technical reasons: nine of the patients rebled and one patient died because of massive bleeding. Direct procedures on esophageal varices had a high rate of rebleeding (seven of 11 patients) independent of the age of the patient when the operation was performed. In 13 patients, shunting offered a good chance of long term protection: three of the 13 rebled at a mean follow-up study of 5.7 years. No recurrence of bleeding was observed in 15 patients when variceal eradication had been obtained after chronic endoscopic sclerotherapy at one to eight years of follow-up study (mean of 3.9 years). The data from this study indicate that sclerotherapy should be the first choice of treatment to control recurrent bleeding in extrahepatic portal hypertension caused by thrombotic obstruction, particularly in the young patient.

Adolescent↗
PubMed1987

Binocular interactions measured by choice reaction times in pigeons.

In 7 pigeons binocular interactions were studied by recording monocular and binocular choice reaction times (RTs) to a pattern discrimination task. In all the animals binocular responses were found to be significantly faster than monocular RTs. In order to assess whether binocular advantage might be ascribed to a neural interaction between the two eyes, monocular and binocular performances were evaluated according to a probability summation model. The results indicate that in the great majority of animals, probability summation alone can account for binocular superiority.

Animals↗
PubMed1986

Spatial and temporal properties of neurons of the lateral suprasylvian cortex of the cat.

Neurons in the posteromedial lateral suprasylvian cortex (PMLS) of cats were recorded extracellularly to investigate their response to stimulation by bars and by sinusoidal gratings. Two general types of cells were identified: those that modulated in synchrony with the passage of drifting bars and gratings and those that responded with an unmodulated increase in discharge. Both types responded to contrast reversed gratings with a modulation of activity: the cells that modulated to drifting gratings modulated to the first harmonic of contrast reversed gratings (at appropriate spatial phase and frequency), whereas those that did not modulate to drifting gratings always modulated to the second harmonic of contrast reversed gratings. No cell had a clear null point. Nearly all cells were selective for spatial frequency. The preferred frequency ranged from 0.1 to 1 cycles per degree (cpd), and selectivity bandwidths (full width at half height) were around two octaves. Preferred spatial frequency was not correlated with receptive field size, but bandwidth and receptive field size were positively correlated. Preferred spatial frequency decreased with eccentricity, at about 0.05 octaves/deg. The response of all cells increased as a function of grating contrast up to a saturation level. The contrast threshold for response to a grating of optimal parameters was approximately 1% for most cells and the saturation contrast approximately 10%. The contrast gain was approximately 25 spikes/s per log unit of contrast. All cells were tuned for temporal frequency, preferring frequencies from approximately 3 to 10 Hz, with a selectivity bandwidth approximately 2 octaves. For some cells, the spatial selectivity did not depend on the temporal frequency and vice versa. Others were spatiotemporally coupled, with the preferred temporal frequency being lower at high than at low spatial frequencies, and the preferred spatial frequency lower at high than at low temporal frequencies. Previous results showing broad velocity tuning to a bar were replicated and found to be predictable from the combined spatial and temporal tuning of PMLS cells and the Fourier spectrum of a bar. Preferred temporal frequency steadily decreased with eccentricity, at 0.025 octaves/deg. The results for PMLS cells are compared with those of other visual areas. Acuity and spatial preference and selectivity bandwidth is comparable to all areas except area 17, where they are a factor of about two higher. Temporal selectivity in PMLS is as fine as observed in other areas. The possibility that PMLS cells may be involved with motion detection and detection of motion in depth is discussed.

Animals↗
PubMed1985

Visual acuity of neurones in the cat lateral suprasylvian cortex.

The spatial acuity was measured for cells of the posteromedial lateral suprasylvian area (PMLS) of the cat. Acuities were found to be 2 cycles/degree (15 mins arc) at best, and 1 cycle/degree (30 mins arc) on average. Both best acuity and average acuity remained constant with receptive field eccentricity within 20 degrees of the area centralis, and then fell gradually with eccentricity. Acuity was good, given receptive field size, and was not correlated with receptive field size. Comparisons are drawn with other visual structures.

Animals↗
PubMed1985

Interhemispheric influences on area 19 of the cat.

Anatomical studies have shown an extensive network of homotopic and heterotopic interhemispheric connections in area 19 of the cat visual cortex (Segraves and Rosenquist 1982a; 1982b). We have investigated their functional organization by recording visual responses in area 19 of cats following a midsagittal section of the optic chiasm. This operation interrupts all crossed optic fibers coming both from the nasal and the temporal retinae; as a result, each hemisphere receives optic fibers only from the lateral hemiretina of the ipsilateral eye which conveys information from the contralateral visual field. Visual information transmitted to the same hemisphere from the contralateral retina and the ipsilateral visual field must be attributed to an indirect, interhemispheric pathway. We found that a rather high proportion of neurons (31.8%) in area 19 of seven split-chiasm cats responded to visual stimuli presented to the contralateral eye. All neurons receiving this interhemispheric activation were also driven by the ipsilateral eye via an intrahemispheric pathway. The property of binocularity was significantly related to the visuotopic map in that both receptive fields of each binocular neuron adjoined or were in the immediate vicinity of the vertical meridian. Due to the small size of receptive fields in area 19, the contribution of the interhemispheric pathway to the representation of the visual field is rather limited and it is certainly less extensive than that predicted by anatomical studies. The representation of the ipsilateral visual field in area 19 of intact cats, as assessed electrophysiologically, was comparable to that found in split-chiasm cats. Recordings in areas 17-18 of split-chiasm cats showed that the visual field represented through the corpus callosum in these visual areas is certainly not less and probably more, extensive than that found in area 19. The results support the conclusion that the relation to the vertical meridian and the receptive field size can explain the organization of the interhemispheric connections in the visual areas studied so far.

Animals↗
PubMed1984

Lack of binocular activation of cells in area 19 of the Siamese cat.

Single cells were recorded in area 19 of 8 Siamese cats. Receptive fields (RFs) were typical for this area in terms of size, directional specificity and type. However, 69 out of the 70 units found were monocularly driven through the contralateral eye. Moreover, the amount of excursion of RFs into the ipsilateral visual field was more limited than that generally demonstrated for areas 17 and 18, extending to a maximum of 5 degrees with very few cells having RFs situated completely within the ipsilateral hemifield.

Animals↗
PubMed1982

The contribution of the corpus callosum to receptive fields in the lateral suprasylvian visual areas of the cat.

In both ordinary cats and 'Boston' Siamese cats the visual areas in the lateral parts of the middle and posterior suprasylvian gyri (LSA) contain an extensive representation of the ipsilateral half of the visual field. In addition, in both groups of cats the overwhelming majority of neurons in LSA can be driven from both eyes. In Siamese cats this binocular interaction is in marked contrast with what is found in area 17 where neurons are almost exclusively activated through the contralateral eye. Transection of the posterior 1/3 to 1/2 of the corpus callosum had a different effect on the physiological organization of LSA in the two types of cats. In ordinary cats it caused the loss of the ipsilateral hemifield representation in the eye ipsilateral to the side of recording and reduced this representation in the other eye. However, after the section of the corpus callosum LSA neurons remained binocular. In Siamese cats the callosal transection left the representation of the ipsilateral hemifield in LSA unaffected, both totally abolished the input from the ipsilateral eye. These findings suggest that the visual callosal input to LSA has a different functional significance in ordinary and Siamese cats. In the former cats it may be related to perceptual equivalence across the vertical meridian of the visual field, whereas in the latter cats it may subserve interocular equivalence.

Animals↗
PubMed1981

Differences in binocular interactions between cortical areas 17 and 18 and superior colliculus of Siamese cats.

In Siamese cats most retinal projections are crossed and almost all neurons in cortical areas 17 and 18 respond exclusively to the contralateral eye. However, neurons in visual areas in the suprasylvian sulci (LSA) can be activated from both eyes, since the input from the ipsilateral eye is relayed to them by the corpus callosum (Marzi et al., '80). This study shows that the superior colliculus (SC) is also the site of binocular interactions in Siamese cats with a predominantly monocular organization of areas 17 and 18. In 8 unanesthetized, brainstem-sectioned Siamese cats, identified as "Boston" on the basis of the pattern of visual field representation in areas 17 and 18 (Hubel and Wiesel, '71; Shatz, '77a), only 25 out of 140 neurons in these areas could be driven from both eyes, whereas as many as 124 out of 143 SC neurons showed a clear binocular input. The input from each eye to the ipsilateral SC and the resulting binocular interactions in our Siamese cats might have depended on corticotectal projections from LSA. If so, since a posterior callosal section abolishes the input from each eye to ipsilateral LSA, the SC should similarly be affected by callosotomy. The posterior half of the corpus callosum was sectioned acutely in three Siamese cats after recording from SC; the input from the ipsilateral eye to SC was suppressed by this operation in one cat (which, however, also had an unintended interruption of the posterior, habenular, and anterior intertectal commissures), and drastically reduced in the other two. In a fourth cat, in which callosotomy was performed three weeks before recording from SC, the number of SC neurons responsive to the ipsilateral eye was significantly inferior to that of Siamese cats with an intact corpus callosum. These findings indicate the importance of the corpus callosum for ipsilateral eye-SC relations, as well as for SC binocularity, in Siamese cats. At the same time they imply that noncallosal routes can also transmit information from each eye to the ipsilateral SC in callosotomized Siamese cats.

Animals↗
PubMed1979

Iconic storage in the two hemispheres.

Three experiments have tested for the existence of laterality effects in iconic storage by employing a Sperling partial-report paradigm and lateralized presentations of alphabetical or pattern material. Even though an overall laterality effect was found in favor of the right visual field for the alphabetical material and in favor of the left visual field for the pattern material, the amount and time decay of partial-report advantage was similar in the two visual fields. These results indicate that hemispheric asymmetries occur beyond the iconic stage of visual information processing.

Discrimination Learning↗
PubMed1978

Role of Siamese cat's crossed and uncrossed retinal fibres in pattern discrimination and interocular transfer.

Siamese and ordinary cats were monocularly tested in pattern discriminations and visual perimetry following unilateral optic tract section. While ordinary cats exhibited similar performances when tested with either eye, in Siamese cats the eye contralateral to the sectioned optic tract was practically blind. Thus, the Siamese cat's uncrossed visual pathways are not able to subserve either form or ambient vision. The nature of such a loss is discussed.

Animals↗
PubMed1977

Influence of spatial stimulus-response compatibility on reaction time of ipsilateral and contralateral hand to lateralized light stimuli.

In a simple reaction time task, key-pressing responses to unpatterned light stimuli presented in the right and left visual fields were faster for the hand ipsilateral to the visual stimulus than for the hand contralateral to the stimulus. The superiority of the ipsilateral reactions was seen also when responses were made with the hands crossed, so that such a superiority cannot be attributed to spatial compatibility between the side of the stimulus and the position of the response device. Similar results were obtained in a reaction time task of the Donders's c-type. It is argued that these laterality effects in visuomotor reaction time tasks are best explained by the anatomical relationships between the visual fields, the cerebral hemispheres, and the hands. The possible interaction between these effects and the effects of spatial compatibility between side of stimulus and side of response device are discussed in relation to reaction time tasks of the Donder's b-type.

Adult↗