Biomedical subjects

D Giaschi

Publications and source records attributed to D Giaschi.

8 recordsLinked to original sources

PubMed1997

Reversible dissociation of sensitivity to dynamic stimuli in Parkinson's disease: is magnocellular function essential to reading motion-defined letters?

A group of 20 control subjects carried out the following visual tests: Snellen acuity; contrast detection threshold for a temporally unmodulated grating and for a temporally modulated grating; speed threshold for recognising motion-defined dotted letters. Normal limits were defined as 2.5 standard deviations from the respective control means. A patient with Parkinson's disease carried out the tests 12 hr after medication was withheld at a time when symptoms were evident ("off" stage), and after administration of medication when it had taken full effect ("on" stage). Confirming previous reports, contrast detection threshold for the temporally modulated grating was much higher during the "off" stage than during the "on" stage, but contrast detection threshold for the temporally unmodulated grating showed little difference. Speed threshold for recognising motion-defined letters did not, however, fall during the "on" stage. We suggest that magnocellular function is not essential for the recognition of motion-defined form.

Contrast Sensitivity↗

PubMed1997

Development of motion-defined figure-ground segregation in preschool and older children, using a letter-identification task.

BACKGROUND: Three-month-old infants can discriminate motion-defined (MD) form, but we do not know the age at which this ability reaches adult levels. Previous psychophysical evidence suggests that different neural mechanisms are involved in the processing of luminance-defined (LD) and MD spatial form in adults. This difference may be reflected in the development of LD versus MD form identification in children. METHODS: We measured speed threshold for identifying MD letters, letter-chart (i.e, Snellen) acuity for high-contrast LD letters and single-letter acuity for high- and low-contrast LD letters. Forty-seven children between 3 and 12 years of age and 20 adult subjects were tested. RESULTS: Development to the adult level was observed as follows: low-contrast single-letter acuity before 3 years; high-contrast single-letter acuity by 5 to 6 years; the ability to identify MD letters by 7 to 8 years; letter-chart acuity by 9 to 10 years. CONCLUSIONS: MD form identification continues to mature in preschool children. LD form identification also matures in this age group but with a different time course. MD letters are not equivalent to low-contrast letters developmentally. Our findings provide further support for the hypothesis that the spatial aspects of MD and LD form are processed separately to some extent.

Adult↗

PubMed1993

The time course of direction-selective adaptation in simple and complex cells in cat striate cortex.

1. Responses of single cortical neurons in area 17 of anesthetized cats were recorded in response to prolonged stimulation with a patch of drifting square-wave grating. 2. During adaptation in the preferred direction, all neurons showed some reduction in response to motion in the stimulated direction and most showed some reduction in the opposite, nonstimulated direction. 3. For complex cells, the time course of response decrement in both the stimulated and nonstimulated directions was exponential, with an average time constant of 5 s. Response recovery was also exponential but significantly slower, with time constants of 8 and 13 s in the stimulated and nonstimulated directions, respectively. 4. For simple cells the dynamics of the adaptation effect depended on the direction of testing. In the nonstimulated direction the time course of the change in sensitivity was similar to that of complex cells. In the stimulated direction during both the adaptation and recovery periods, simple cells showed an initial rapid exponential change on the order of a few seconds that was followed by a more gradual exponential change. 5. During prolonged stimulation in the nonpreferred direction, there was less overall change in sensitivity. For some neurons the change in sensitivity during adaptation and recovery was exponential, with a short time constant for both simple and complex cells and for stimulated and nonstimulated directions. Other neurons showed no change in sensitivity in either direction and a few neurons showed facilitation during the adaptation period. 6. There appears to be a rapid general or nonspecific process, which may be related to contrast gain control, underlying motion adaptation in striate cortical neurons. An additional slow, direction-selective process is revealed when simple but not complex cells are stimulated in the preferred direction. We suggest that this latter type of adaptation is a key feature underlying the perceptual motion aftereffect.

Animals↗

PubMed1992

Motion-defined letter detection and recognition in patients with multiple sclerosis.

Two important distinctions in visual perception are (1) between the detection and recognition of shape (e.g., letters), and (2) between the recognition of shapes defined by a difference in brightness and the recognition of shapes defined by a difference in motion. We report that 6 of 10 patients with multiple sclerosis showed impaired recognition for motion-defined (MD) letters, although the detection of MD letters was normal as were both detection and recognition of luminance-defined letters. We have shown that this was not a function of acuity loss or the loss of ability to detect motion or a general failure of recognition per se, but was confined to a loss of ability to recognize MD letters. The neurological implications of these findings are discussed, and it is suggested that the MD letter test be used by others interested in the central pathology of visual disorders.

Adult↗

PubMed1992

Visual processing of motion-defined form: selective failure in patients with parietotemporal lesions.

The following psychophysical data were obtained from 13 patients with unilateral cerebral hemispheric lesions and 20 control subjects: speed thresholds for detecting and for recognizing motion-defined letters, speed thresholds for detecting coherent motion and for discriminating its direction, and visual acuity for recognizing letters of 96% and 11% contrast. Acuity was between 6/6 and 6/3 for all patients. Four patients showed a selective loss of ability to recognize motion-defined letters, while the ability to detect those same letters was spared, as was the ability to detect coherent motion and discriminate its direction (type I loss). Three patients showed a loss of ability both to recognize and to detect motion-defined letters, while the ability to detect coherent motion and discriminate its direction was spared (type II loss). All seven patients who failed to recognize motion-defined letters had extensive lesions in parietotemporal white matter underlying Brodmann cortical areas 18, 19, 37, 39, 21, and 22. The lesion was in the left hemisphere for three patients and in the right hemisphere for the remaining four. The region of overlap in these seven patients was not invaded by the lesion in any of the other six patients, and none of these six patients showed a loss of ability to recognize motion-defined letters. Three patients showed selective loss of acuity for low-contrast letters with normal Snellen acuity. The lesions in these three patients extended more posteriorly than in any other patient, and their region of overlap was in white matter underlying areas 18 and 19. We conclude that (1) the loss of ability to recognize letters in seven patients was specific to motion-defined letters rather than being a general loss of letter-recognition ability, (2) this visual loss was specific to motion-defined form rather than being a general failure of motion processing, and (3) the visual loss was not produced by lesions that did not involve the localized cerebral region specified above. To explain the existence of type I and of type II loss with sparing of the detection and discrimination of coherent motion, we propose that motion information is processed hierarchically. We further suggest that homologs of the socalled motion and color/form pathways (i.e., areas V1/MT/MST/7a and areas V1/V4/IT) are interconnected to form a distributed system that is important for the recognition of motion-defined form.(ABSTRACT TRUNCATED AT 400 WORDS)

Adult↗

PubMed1989

Suppression of OKN and VOR by afterimages and imaginary objects.

Optokinetic nystagmus (OKN) is suppressed if attention is directed to a centrally placed afterimage superimposed on a moving display. Imagining a stationary object has little or no effect. An afterimage does not provide the retinal slip and misfoveation error signals provided by a stationary object and we have shown that an effective error signal does not arise from occlusion or masking of the display by the afterimage. Although a lack of relative motion between afterimage and moving display could indicate when OKN gain is one, there is no unique relative motion signal associated with a gain of zero. Subjects could partially inhibit the vestibulo-ocular reflex (VOR) in the dark when they imagined a head-fixed object. They could suppress the response more effectively by attending to an afterimage, but the suppression was still only partial. When OKN and VOR were evoked simultaneously, pursuit movements of the eyes could not be suppressed until the vestibular inputs had subsided. We conclude that signals associated with OKN, are fully available to the mechanism that assesses the headcentric motion of objects but that signals associated with VOR are only partially available to that mechanism.

Adult↗

PubMed1989

The less you see it, the faster it moves: shortening the "on-time" speeds up apparent motion.

The apparent motion (AM) created by two spots illuminated in alteration looks faster when there is dark temporal interval (ISI) between the offset of one spot and the onset of the other than when the spots are presented immediately after one another (no ISI), even though the temporal frequency and the spatial separation between spots are held constant. AMISI looks 18.6% faster than AMnoISI at temporal frequencies between 1.5 and 4.5 Hz. Reducing the duty cycle from 0.5 to 0.05 increases the apparent speedup to 30%. This difference in subjective speed is not due to differential saturation of velocity detectors, nor to the apparent spatial separation between spots, nor to differences in the time-averaged luminance of the stimuli. It is the "on-time", the time for which the spot is visible in one position, that determines the subjective speed. The longer the on-time, the slower the spot appears to move.

Female↗

PubMed1985

Adaptation to apparent motion.

A spot alternating between two positions can produce apparent motion (AM). Following prolonged inspection, the AM degenerates into flicker. This adaptation effect was found to depend on spacing and timing; the probability of seeing motion during a 30-sec inspection period declined linearly with log spatial separation (over a range from 0.1 to 1 deg), and with log alternation rate (over a range from 2 to 4.5 Hz). Cross-adaptation, in which subjects were adapted to one alternation rate and tested at another, showed that low alternation rates gave stronger motion signals than high rates did. Adaptation to real motion (RM) strongly suppressed AM, which suggests that AM must be stimulating the same neural pathways as RM. Flickering spots (i.e. in-phase flicker) produced less adaptation than did a spot alternating between two positions (i.e. counterphase flicker), so the adapting mechanism must be responding to relative temporal phase. Embedding the adapting spots in configurations of other spots, which altered the pattern of perceived adapting motion without altering the local retinal stimulation, minimized the adaption, so the adapting mechanism must be responding to the path of seen motion. Adaptation can be used to measure the strength of AM and shows that AM is strongest for small separations, low alternation rates and high luminance contrast.

Adaptation, Ocular↗