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.

A study of three cases of familial related agenesis of the corpus callosum.

This paper provides data on a family in which three members, all female, have been diagnosed as having agenesis of the corpus callosum. That all three acallosal individuals came from the same family and showed relatively uniform neuropsychological impairment and could be compared in similar terms with their "callosal" siblings, also female, provides a unique sample. Inter-hemispheric transfer, psychometric measures, and motor and cognitive function were examined in the acallosal individuals, all of whom had borderline to low-average intelligence, with results compared to their non-acallosal siblings. The data indicated that all acallosal individuals exhibited deficits with the cognitive tests indicating difficulties of inter-hemispheric transfer of tactile information, difficulties in some areas of memory and, at least as far as the children are concerned, a marked difference in Verbal IQ and Performance IQ.

Event-related potential response to attended and unattended locations in an interference task.

Event-related potentials were measured in response to an interference task in which unattended stimulus items were compatible, incompatible a neutral with regard to the attended stimulus items. Two stimulus items were presented simultaneously and bilaterally--one in each visual field. This allowed examination of the event-related potential waveform according to whether recording sites were contralateral to the attended or unattended location. The first experiment used sustained cueing with 3.5 degrees separation between attended and unattended locations. Attentional modulation of the N1 was observed but not for the P1. In the second experiment, separation between attended and unattended locations was increased to 11.5 degrees. In both experiments, the hemisphere contralateral to the unattended material (unattended hemisphere) showed a greater negativity in the N2 latency range in the temporal regions to compatible and incompatible conditions compared to a neutral condition. These data are inconsistent with findings suggesting the filtering of material early in visual processing.

Time-till-breakdown and scalp electrical potential maps of long-range apparent motion.

A series of psychophysical and electrophysiological experiments is reported using the apparent motion (AM) breakdown effect. Breakdown describes an effect in AM in which, during continuous viewing, the percept of smooth of a single stimulus alternates with the percept of two discrete alternating stimuli. Visual evoked potentials (VEPs) were recorded during periods of motion or breakdown ("nonmotion") in horizontal and vertical displays. VEPs were compared with synthetic VEPs ("composite-flash") produced by adding VEPs to each element of the display recorded in isolation. Subtraction of VEPs was used in an attempt to compare the electrical responses with the processing of information relating to the form of the stimulus, subthreshold motion processing, and suprathreshold motion processing. The results, presented as scalp electrical potential distribution maps, were interpreted as consistent with a central adaptation process underlying the breakdown effect. The results also indicated that the hemispheric asymmetries in AM VEPs described by Manning, Finlay, and Fenelon (1988) were most likely due to the position of the stimuli in the visual field, rather than as a lateralization of motion processes per se. The results also provided evidence that the subthreshold and suprathreshold motion responses to the display were the product of different populations of motion units.

An event-related potential examination of attended and unattended stimuli in visual selection using bilateral stimulus presentation.

The experiment reported here examined an interference paradigm using a bilateral stimulus presentation in which stimuli were presented simultaneously in the left and right visual fields. The lateralization of the early visual components allowed an ERP examination of material presented in each field. Attention was directed to one field or the other on each trial by a 100% valid cue. Two letters were nominated as targets and the simultaneous presentation allowed presentation of material compatible, incompatible or neutral with reference to the target. A negative peak was observed at 230 ms post stimulus at occipital and temporal sites. There was a variation in this peak for unattended stimuli, with compatible and incompatible target letters being significantly different to non-target letters. Contrary to previous research, this finding suggests that material is not filtered out at an early stage as proposed by early selection. It was found that the response to unattended target material also varied according to the type of item presented at the attended location. These findings were discussed in relation to previous studies which found no difference in the processing of unattended target and non-target material, and also in relation to suggestions that automatic processing of unattended material occurs only when there is controlled processing occurring simultaneously.

Detection duration thresholds and evoked potential measures of stereosensitivity.

Visual evoked potentials have been proposed by some researchers to be more useful than behavioral techniques to evaluate stereo performance in children and certain clinical populations. Stimulus duration detection thresholds, visual evoked potentials, and scalp electrical potential distribution maps to dynamic random dot stereograms were studied. A high degree of correspondence was found between visual evoked potential amplitudes and behaviorally determined detection thresholds. Upper field stimuli had higher detection thresholds and generated lower-amplitude visual evoked potential responses than did centrally presented stimuli. For the most eccentrically presented stimuli, lower detection thresholds were found for stimuli presented in the right visual field than the left visual field. This finding was consistent with the pattern of VEP responses to be lateralized, with higher-amplitude responses recorded over left-hemisphere sites. The study examined a proposal that the major negative component of the stereoscopic visual evoked potential originates in cortical area V1. The results failed to support the proposal and were consistent with the main negative component of the VEP being generated in V2, rather than V1.

Differential motion thresholds to sinusoidal gratings at two eccentricities.

Differential motion thresholds were measured at eccentricities of 9 degrees and 16.6 degrees using computer-generated sinusoidal gratings. Three spatial frequencies (0.51, 0.25, and 0.13 cycles/deg) were examined at reference velocities of 2, 4, 8, 16, 32, and 48 deg/sec. Minimum differential velocity thresholds were between 20 and 30% of the reference velocities for the three spatial frequencies at both eccentricities. Increasing eccentricity produced an increase in the velocity at which minimum velocity discrimination occurred. Temporal frequency tuning was between 4 and 8 Hz, regardless of eccentricity.

Motion thresholds in infants to sinusoidal gratings.

Motion thresholds were determined at 9 degrees eccentricity in infants (mean = 14 weeks old). The stimuli used were computer-generated sinusoidal gratings presented through a 7.45 degrees aperture at a contrast ratio of .83. The range of velocities (.5, 1, 2, 4, and 6 degrees per s) was examined at only one spatial frequency (1 cycle per degree). At low velocities (less than 2 degrees per s), the infants showed no clear preference for the moving stimulus over the stationary stimulus. At faster velocities (2-6 degrees per s), the infants exhibited a clear preference for the moving stimulus. The results were interpreted as indicating that infants at 3 months of age are relatively insensitive to slow motions for low spatial frequency stimuli.

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.

Time-till-breakdown and VEP measures of short-range apparent motion.

Varying short-range apparent motion (AM) stimulus displacements from 2.7 to 21.6 min it was found that VEP amplitudes varied as a function of the limits for short-range AM described using time-till-breakdown as a behavioural measure of AM strength. This VEP amplitude difference was, however, in the reverse direction to that predicted as the "motion" condition elicited lower VEP amplitude responses than the "non-motion" conditions (which did not significantly differ from each other). This direction of VEP amplitude difference was supported by an intensive study of a single subject. The "breakdown effect" enabled VEPs to be gathered during periods in which the subjective experience was of either coherent lateral motion, or breakdown (incoherent motion) without changing any stimulus parameter. The VEP component identified in expt 2, as predicted, was of lower amplitude during motion with respect to periods of breakdown. The results of these experiments are discussed in terms of describing motion and breakdown in short-range AM displays as "coherent" and "incoherent" motion, rather than as "motion" and "non-motion".

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 x 3 x 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.

Difficulties in the definition of 'stereoscotoma' using temporal detection of thresholds of dynamic random dot stereograms.

Temporal detection thresholds are reported from two subjects to dynamic random dot stereograms (DRDSs) of both crossed and uncrossed disparity. The stimuli were 1 degree square, of 0.25 degrees disparity and were presented at 81 positions in a 9 degree square central region. In both subjects crossed disparity stimuli were detected at shorter durations. One subject displayed particularly acute stereoscopic vision and presented a pattern of temporal thresholds increasing gradually with eccentricity. The other subject displayed evidence, particularly in response to uncrossed stimuli, of a 'stereo-scotoma' which was not evident in monocular testing or ophthalmologic examination. Criteria used to define 'stereo-scotoma' are discussed. It was concluded that, although left-right field differences may be found with a large subject sample, individual factors may be more important in the distribution of stereo sensitivity throughout the portion of visual field tested.

Motion components of VEPs to the waggon wheel effect.

Visual evoked potentials (VEPs) from eight subjects are presented from scalp sites O1, O2, T5, T6, P3 and P4. A spoked wheel was illuminated by a constantly flashing strobe. By adjusting the real angular speed of the wheel the subjective impression was given of; a stationary wheel, a wheel spinning slowly or spinning rapidly clockwise. Two visually identified components, a negativity and a positivity, were found to be of larger amplitude in response to motion. The results were interpreted as consistent with a model of AM proposing motion information to be processed within the visual system.

Visual evoked potentials to stimuli in apparent motion.

Following an extensive single subject pilot study 12 Ss viewed a continuously cycling (3 Hz), two stimulus, apparent motion (AM) display. The "AM breakdown effect" was utilised to gather visual evoked potentials (VEPs) from sites O1, O2, T5, T6, P3, and P4 during periods in which the display elicited either: the percept of motion; or the percept of two discrete alternating stimuli. VEPs displayed components of larger amplitude during motion periods in all right (but no left) hemisphere sites during periods 60-82 msec post left field stimulus onset (positive component) and 100-126 msec post right field stimulus onset (negative component). Results were interpreted as indicating initial extraction of motion information within the occipital lobe, with further motion processing taking place in temporal and parietal lobes.

Individual differences in responses of untrained observers to stroboscopic apparent motion.

Two large groups of inexperienced subjects (n = 208 and n = 50) and a small group of experienced subjects (n = 5) were tested using time-till-breakdown as a measure of long-range apparent motion across a range of temporal frequencies. One group of inexperienced subjects was retested after one week and demonstrated quite stable patterns of response. Large intersubject variability was observed in terms of the amount of motion seen, with most inexperienced subjects reporting very little apparent motion. A raster display produced a peak frequency 1 Hz higher than a standard tachistoscope display. The role of experience was also examined with a small group of inexperienced subjects (n = 8) tested once daily over five consecutive days. There was high intersubject variability and intrasubject consistency, demonstrating little influence of learning and experience. The results are discussed in terms of current ideas on the breakdown effect.

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.

Visual attention in infants: a study of stimulus complexity, habituation, and sex differences.

A modification of Cohen's (1973) method was used to study responses to patterns of varying complexity in 4-mo.-old infants. Complexity in this context was defined in terms of the number of elements and the degree of internal contour contained in a given pattern. The infants were first presented with small red circles on two trials. This was followed by 16 trials of exposure to either 2 x 2 or 8 x 8 or 24 x 24 small red-circle matrix patterns which again were followed by two further exposures (trials) to the small red circle. Response decrement across trials was measured as a function of fixation, i.e., the duration of time the infant was judged to be looking at the stimulus, and latency, i.e., the time it took the infant to orient toward the stimulus pattern. Infants preferred the more complex patterns. All infants showed decrement in duration of fixation across trials for all three types of pattern. Latency of head turning did not change across trials but differed between pattern types. There were not significant sex differences in habituation rates as measured by duration of fixation and latency.

The Poggendorff Illusion and estimates of transverse extent.

The underestimation of transverse extent relative to longitudinal extent, in the Poggendorff Illusion, was tested by varying oblique line orientation, interparallel line distance, and presence or absence of obliques. 20 subjects made estimates of the transverse extent on both a longitudinal and transverse extent. The results indicated that, although underestimation was found for some stimulus conditions, overestimation was found for others. It was argued that even though presence of obliques affected judgmental error the longitudinal-transverse illusion could not form a basis for the Poggendorff Illusion.