Biomedical subjects

K Turano

Publications and source records attributed to K Turano.

11 recordsLinked to original sources

PubMed1995

The optimal motion stimulus.

Contrast energy thresholds were measured for discriminating the direction of a drifting sinusoidal grating multiplied by an independently drifting space-time Gaussian (a generalized Gabor). We argue that the stimulus with the lowest contrast energy threshold identifies the receptive field of the most efficient linear motion filter. This optimal motion stimulus is found to be at 3 c/deg and 5 Hz, with a width and height of 0.44 deg and a duration of 0.133 sec, corresponding to spatial and temporal bandwidths of 1.1 and 2.5 octaves, respectively. The spectral receptive field is aligned more nearly to the Cartesian axes than to the velocity contour.

Contrast Sensitivity↗

PubMed1994

Low vision reading with sequential word presentation.

Individuals with scotomas in the center of their visual fields usually read much more slowly than visually impaired individuals without central scotomas. This study determines the extent to which inefficient eye movements could account for this difference. Using a technique described previously [Rubin and Turano (1992) Vision Research, 32, 895-902] text was presented sequentially, one word at a time at the same location in the visual field. Reading performance for rapid serial visual presentation (RSVP) was compared to conventional text presentation (PAGE) in 14 subjects with dense central scotomas (central field loss, CFL) and 9 without central scotomas (noCFL). Reading rates were faster with RSVP compared to PAGE, but CFL subjects improved less (by a factor of 1.5 +/- 0.41) than noCFL subjects (by a factor of 2.1 +/- 0.38). When reading rate (words/min) was converted to average word duration (msec/word) the reduction in word duration for RSVP compared to PAGE text averaged 150 msec for normal subjects, 171 msec for noCFL subjects, and 250 msec for CFL subjects. We hypothesized that the reduction in word duration was attributable to a reduced number of saccades/word. We confirmed this hypothesis by measuring the number of saccades during RSVP and PAGE reading using a scanning laser ophthalmoscope in four subjects with CFL. All of the subjects made fewer saccades during RSVP than PAGE reading (average reduction = 1.3 +/- 0.5 saccades/word). But even with the reduced number of saccades, CFL subjects required longer word durations than noCFL subjects. These results indicate that inefficient eye movements account for only part of the reduction in reading speed caused by CFL. An additional and potentially more important factor is the limited rate at which peripheral retina can perform the pattern decoding tasks required for reading.

Aged↗

PubMed1994

Visual discrimination between a curved and straight path of self motion: effects of forward speed.

We measured path-deviation thresholds for discrimination between forward movement along a straight and a curved path as forward speed was varied. Stimuli were computer-generated images simulating an observer moving relative to a volume of randomly-positioned dots. Predictions generated from a computer simulation of Rieger's [(1983) Journal of the Optical Society of America A, 73, 339-344] model indicate that the required deviation for the discrimination of a curved from straight path should decrease with increasing forward speed up to a point and then level off. Psychophysical results were opposite to the model predictions; the deviation required to detect a departure from a straight path increased with forward speed. Path curvature remained nearly constant across forward speeds for a constant level of discrimination performance.

Discrimination, Psychological↗

PubMed1994

Visual stabilization of posture in the elderly: fallers vs. nonfallers.

BACKGROUND: Postural instability is one of the important contributors to falling in observers aged 65 years and older. In this study we examine the role of vision in the relation between postural stability and falling, as well as in the relation between postural stability and the fear of falling. METHODS: Community-dwelling adults 65 years and older were administered a questionnaire about their history of falls and fear of falling (N = 185). Postural sway was measured in the same subjects with eyes open and eyes closed. Visual function was assessed by measures of visual acuity and contrast sensitivity. RESULTS: The 29 subjects who reported falling in the last year showed less of a visual contribution to posture stabilization than those who reported no falls. Controlling for age and gender, there is a 2.13-fold increase in the likelihood of reporting falling for a 0.1 decrement in the visual stabilization index when it is measured within the context of reliable somatosensory feedback. Those who reported a fear of falling and those who reported no fear showed similar visual stabilization. Contrast sensitivity was significantly associated with visual stabilization when it was measured within the context of reliable somatosensory feedback. CONCLUSIONS: The visual contribution to postural stabilization is significantly greater in nonfallers compared to fallers, and it is significantly associated with contrast sensitivity.

Accidental Falls↗

PubMed1993

Visual stabilization of posture in retinitis pigmentosa and in artificially restricted visual fields.

PURPOSE: To investigate the relationship between retinitis pigmentosa (RP) progression and the visual contribution to posture stabilization; to examine the extent to which visual-field diameter affects the visual contribution to posture stabilization. METHODS: Posture information was recorded in 35 subjects with well-characterized RP and in 20 subjects with normal vision. Data were collected as each subject stood in a dark environment and as each subject viewed a stationary visual display. In both conditions, somatosensory feedback was concurrently altered. Data were also collected on 10 additional subjects with normal vision wearing field-restricting goggles (visual-field diameters ranged from 26.5 down to 6 degrees). RESULTS: RP progression is accompanied by a steady decrease of the visual stabilization of posture, from normal values at the onset of the disease to the absence of visual stabilization and, eventually, to visual destabilization of posture. Decreasing visual field diameter in the subject with normal vision resulted in a linear decrease of the visual stabilization of posture. However, subjects with RP with comparable visual-field loss showed significantly lower visual stabilization than normal subjects with artificially restricted fields. Moreover, subjects with normal vision with restricted visual fields as small as 6 degrees failed to show visual destabilization of posture. CONCLUSIONS: Most likely, the additional reduction in the visual stabilization of posture shown in subjects with RP, as well as the visual destabilizing effect manifest in the late stages of RP, is caused by anomalous processing of visual information in the remaining visual field.

Adult↗

PubMed1992

Reading without saccadic eye movements.

To assess the limitation on reading speed imposed by saccadic eye movements, we measured reading speed in 13 normally-sighted observers using two modes of text presentations: PAGE text which presents an entire passage conventionally in static, paragraph format, and rapid serial visual presentation (RSVP) which presents text sequentially, one word at a time at the same location in the visual field. In Expt 1, subjects read PAGE and RSVP text orally across a wide range of letter sizes (2X to 32X single-letter acuity) and reading speed was computed from the number of correct words read per minute. Reading speeds were consistently faster for RSVP compared to PAGE text at all letter sizes tested. The average speeds for text of an intermediate letter size (8X acuity) were 1171 words/min for RSVP and 303 words/min for PAGE text. In Expt 2 subjects read PAGE and RSVP text silently and a multiple-choice comprehension test was administered after each passage. All subjects continued to read RSVP text faster, and 6 subjects read at the maximum testable rate (1652 words/min) with at least 75% correct on the comprehension tests. Experiment 3 assessed the minimum word exposure time required for decoding text using RSVP to minimize potential delays due to saccadic eye movement control. Successive words were presented for a fixed duration (word duration) with a blank interval (ISI) between words. The minimum word duration required for accurate oral reading averaged 69.4 msec and was not reduced by increasing ISI. We interpret these results as an indication that the programming and execution of saccadic eye movements impose an upper limit on conventional reading speed.

Adult↗

PubMed1992

Visual resolution of motion ambiguity with periodic luminance- and contrast-domain stimuli.

Visual motion processes were studied with luminance- and contrast-modulated gratings. A sine-wave luminance grating was displaced abruptly back and forth by 3/16 cycle. The display sequence is ambiguous in that each 3/16-cycle phase shift (short-path motion) could just as readily be seen as a 13/16-cycle shift (long-path motion) in the opposite direction. By varying the duration of the interval (IFI) between the two phase positions, the luminance of the IFI, or the spatial frequency of the grating, it was possible to bias the ambiguous percept in favor of short-path motions or long-path motions. A contrast-modulated grating displaced through 3/16 cycle always appeared t undergo short-path motion. Current motion models which incorporate Reichardt-type/energy mechanisms and certain types of auxiliary signal transformations which precede those mechanisms do not adequately explain the effects of IFI intensity on the perceived motion of a sinusoidal grating or the effect of IFI duration on the perceived motion of a contrast-modulated grating.

Contrast Sensitivity↗

PubMed1992

Motion thresholds in retinitis pigmentosa.

Minimum displacement thresholds, or dmin, were measured in 29 subjects with retinitis pigmentosa (RP) and 10 subjects with normal vision. The results showed that RP can affect an observer's ability to judge the correct direction of motion in a random-dot pattern. The majority of RP subjects had elevated dmin. They required a larger displacement to perceive the correct direction of motion. Only 5 of the 29 RP subjects had thresholds within two standard deviations of the mean of the normal-observer distribution. Moreover, three RP subjects were unable to detect the correct direction of motion regardless of the displacement magnitude, and four RP subjects consistently reported motion in the opposite direction at small displacements. The results cannot be explained by abnormal temporal processing or a reduction in the effective luminance. There was a statistically significant correlation (r = 0.72, P less than 0.001) between log threshold and log MAR, consistent with the hypothesis that a reduction in the spatial density of the photoreceptors contributes to the motion-threshold elevation. Motion thresholds also were measured in subjects with normal vision under conditions of simulated "photoreceptor" dropout. The results showed that a random elimination of information from over 25% of the image positions significantly elevates motion thresholds. These results also support the spatial-density reduction hypothesis.

Adolescent↗

PubMed1991

Evidence for a common motion mechanism of luminance-modulated and contrast-modulated patterns: selective adaptation.

Selective adaptation effects were measured with contrast-modulated patterns and sine-wave gratings in order to determine the extent to which the two patterns are processed by common mechanisms. Direction-specific adaptation effects were measured for a contrast-modulated adapting pattern and a test pattern. The contrast-modulated adapting pattern was composed of a sine-wave grating of 8 cycles deg-1 whose contrast was spatially modulated by a sinusoid of 1 cycle deg-1 at one of four levels: 100%, 60%, 30%, or 0%. The results showed that contrast-modulation thresholds for contrast-modulated gratings were raised by 0.3 to 0.5 log units following adaptation to a contrast-modulated grating moving in the same direction as the test pattern, relative to thresholds obtained following adaptation to a contrast-modulated grafting moving in the opposite direction. Cross-adaptation effects were also measured with a sine-wave adapting pattern and a contrast-modulated test pattern. The sine-wave adapting pattern was a sine-wave grating of 1 cycle deg-1 whose contrast was set to one of three levels: 16.4%, 1.25%, or 0%. The contrast-modulated test pattern was a sine-wave grating of 8 cycles deg-1 whose contrast was modulated by a sinusoid of 1 cycle deg-1. The results revealed that contrast-modulation thresholds for contrast-modulated gratings were raised by approximately 0.25 log units following adaptation to moving sine-wave gratings, relative to thresholds obtained following adaptation to a uniform field. Cross-adaptation effects were also obtained with a contrast-modulated adapting pattern and a sine-wave test pattern. The results support the view that signals generated from luminance-domain stimuli and from contrast-domain stimuli are processed by a common motion mechanism.

Adaptation, Ocular↗

PubMed1989

On the mechanism that encodes the movement of contrast variations: velocity discrimination.

Motion can be perceived when a moving pattern is defined by variations in luminance. Motion can also be perceived when a moving pattern is defined by variations in contrast. The central focus of the present study was the isolation and description of the mechanism that is responsible for the processing of the movement of contrast variations. We measured velocity discrimination thresholds to determine whether the coding of the movement of contrast-modulated (CM) patterns is mediated by the same motion mechanism as that which underlies the processing of the movement of luminance variations (sine-wave gratings). The results of the velocity discrimination studies showed that an observer's ability to detect small differences in velocity of both CM patterns and sine-wave (SW) gratings decreased (1) when variations of contrast or luminance approach detection threshold, (2) when the frame duration of an apparent motion display was lengthened, and (3) when patterns were slowed to a standard velocity of 0.75 deg/sec. Three hypotheses were discussed in light of the results. Henning's hypothesis, a two-stage model in which a higher-level system receives input from the output of a lower-level system, was best supported by the results of the present experiments.

Contrast Sensitivity↗

PubMed1985

Discontinuity limits for the generation of visual motion aftereffects with sine- and square-wave gratings.

Visual motion aftereffects (MAE's) were produced with adapting gratings that underwent repeated, abrupt displacements in a uniform direction. MAE's could be generated with sine-wave gratings even when the magnitude of each displacement approached a phase angle as large as 1/2 cycle. The maximum spatial step for generating a MAE with a square-wave grating was less than 1/4 cycle. If a dark pause was introduced between the successive positions (phases) of the adapting grating, MAE's disappeared when the pause was longer than 60-70 msec. The results can be used to define the spatiotemporal-response limits of a short-range motion process (or system of directionally selective motion sensors), and the results also suggest that the individual Fourier components of complex spatial patterns are capable of producing independent signals for direction of motion.

Adult↗