David Finlay

Evidence for disparity change as the primary stimulus for stereoscopic processing

Subjects were able to respond to a lens-induced stereoscopic slant more quickly and more accurately when it was imposed on only part of a surface rather than on the whole surface. This shows that the presence of a stereoscopic boundary, where disparity is discontinuous, increases the efficiency of stereoscopic processing. This finding is not consistent with many current models of stereopsis.

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.

Motion Perception in the Peripheral Visual Field

Literature dealing with the peripheral retina and its ‘specialization for motion detection’ is reviewed. The data at hand seem to indicate that the central retina is more ‘specialized’ for motion perception than the peripheral retina. It is clear that motion improves vision for stimuli presented peripherally.

Determination of melatonin and monoamines in rat pineal using reversed-phase ion-interaction chromatography with fluorescence detection

A method is reported for the ion-interaction, reversed-phase separation of 24 compounds (chiefly monoamines) arising from the metabolism of tyrosine and tryptophan. These compounds were separated as two groups. The first group comprised 3,4-dihydroxyphenylethylene glycol, tyrosine, 3-methoxy-4-hydroxyphenyl glycol, 5-hydroxytryptophan, norepinephrine, 3,4-dihydroxyphenylacetic acid, epinephrine, 5-hydroxyindole-3-acetic acid, homovanillic acid, 5-hydroxytryptophol, dopamine, tryptophan. N-acetylserotonin, N-acetyltryptophan, 5-methoxytryptophan and serotonin. The mobile phase consisted of a 6.8:93.2 (v/v) mixture of acetonitrile and an aqueous solution containing 0.16 M ammonium phosphate, 0.06 M citric acid, 0.15 mM disodium EDTA, 10 mM dibutylamine and 6 mM sodium 1-octanesulphonate at pH 4.50. The second group of compounds comprised 6-hydroxymelatonin, 5-methoxyindole-3-acetic acid, indole-3-acetic acid, 5-methoxytryptamine, tryptamine, 5-methoxytryptophol, melatonin and tryptophol. The mobile phase consisted of a 16:84 (v/v) mixture of acetonitrile and an aqueous solution containing 0.05 M ammonium phosphate, 0.05 M citric acid, 0.15 mM disodium EDTA, 25 mM dibutylamine and 5 mM sodium 1-octanesulphonate at pH 5.30. Detection was by fluorescence measurement (lambda ex = 280 nm, lambda em = 340 nm). The proposed method exhibited linear calibration over the biochemically significant concentration range, with detection limits in the 10-200 pg range. Excellent precision for peak areas and retention times was observed, even over a period of 24 h. The applicability of amperometric detection (at 0.72V) is also demonstrated. The method is applied to the determination of monoamines in individual rat pineals. Low nanogram levels of tyrosine, norepinephrine, 5-hydroxyindole-3-acetic acid, tryptophan, serotonin and 6-hydroxymelatonin, and picogram levels of 5-hydroxytryptophan, 5-hydroxytryptophol, 5-methoxyindole-3-acetic acid, indole 3-acetic acid, 5-methoxytryptophol and melatonin were indicated in most of the samples.

Perceptual integration of tertiary taste mixtures

Integration psychophysics was used to explore the taste perception of mixtures of sucrose, fructose, and citric acid. Three levels of each stimulus were varied in a 3 × 3 × 3 factorial design. Subjects rated total intensity, sweetness, and acidity of the 27 mixtures on graphic rating scales. Consistent with earlier work, the perceived total intensity of the tertiary mixtures was found to be dictated by the intensity of the (subjectively) stronger component alone (i.e., either the integrated sweetness or the acidity, whichever was the more intense). In contrast, the sweetness and acidity of the mixture were susceptible to mutual suppression: Sweetness suppressed acidity, acidity suppressed sweetness. There was, however, a difference between sucrose and fructose in their interactions with citric acid, fructose being the more susceptible to suppression. This selectivity of suppression indicates that the two sweetnesses could not have been inextricably integrated. Implications for taste coding are discussed, and the findings are reconciled in terms of two separate coding mechanisms: one for taste intensity, another for taste quality.

Some experiments on the breakdown effect in apparent motion.

This study was concerned with the breakdown of long-range stroboscopic apparent motion, that is, the cessation of the experience of movement between two sources previously seen in continuous apparent motion. In the first three experiments, the conditions under which breakdown occurs were delimited. The variables employed in these studies were temporal frequency, spatial separation, stimulus duration, interstimulus interval (ISI), stimulus size, and direction of motion. The results indicated that temporal frequency described the data better than did either duration or ISI, that breakdown occurred more readily for greater spatial separations between stimuli, and that within the limits tested, size was not important. Furthermore, continuous motion was more effective than interrupted unidirectional motion in producing breakdown. The final two experiments dealt with the time course of the breakdown for one temporal frequency over a prolonged observation period, and with adaptation effects within the temporal frequency envelope defining the breakdown effects. The limited amount of temporal-frequency-specific adaptation that could be demonstrated suggested that this envelope could best be thought of as describing a single process.

Frequency, Phase, and Colour Coding in Apparent Motion: 2

We present some results which indicate that the known spatiotemporal limits for apparent motion are consistent with the motion being sinusoidal or a result of filtering. Given this we investigated how two such motions interact as a function of their relative temporal phase differences. This was accomplished by inducing two independent motions from complementary coloured event pairs. Results indicated critical phase limits for perceiving the two motions (red and green) which were consistent with the frequency specificity of the effect. The results are discussed within the framework of a filtering process for the perception of apparent motion.

Interhemispheric visual integration in three cases of familial callosal agenesis.

Three cases of callosal agenesis (a 39-year-old woman and her 11- and 12-year-old daughters) were tested on their ability to integrate visual information between the visual hemifields. They were all able to name colors and digits in either hemifield with high accuracy and were able to decide whether letters or digits in opposite hemifields were the same or different. They had greater difficulty deciding whether colors in opposite hemifields were the same or different. When shown 6-letter words made up of pairs of 3-letter words that straddled the midline (e.g., MANAGE, ROTATE), they responded to them as whole words and never as 3-letter words, suggesting perceptual continuity across the midline, at least for verbal material. The most likely interpretation is that the integration of form, but not color, is achieved through the intact anterior commissure in these participants.

The Waggon-Wheel Effect

The waggon-wheel effect was studied by use of three wheels with different numbers of spokes (4, 8, 16) and a wide range of strobe temporal frequencies. The results obtained are discussed in terms of a model in which: (i) nearest-neighbour relationships predict the direction and speed of movement, (ii) persistence and masking occur over ranges consistent with values reported in the literature, (iii) apparent motion is generated between currently illuminated spokes and persisting images of spokes, (iv) duration of spoke illumination (sweep) is a determining factor and, (v) a top-down process finds the best fit (a sort of ‘simple structure’) for the complex spatiotemporal display.

Visual field differences in a facial recognition task using signal detection theory.

Abstract A signal detection theory model was used to ascertain whether reported differences in facial recognition was due to particular characteristics of the two hemifaces or was due to the visual field to which the hemiface was presented. The results indicated that the right hemisphere was more efficient in handling facial information than the left and that facial characteristics as presented in photographic form were not important.